MEarth data release notes ========================= Document version 8.0 2019 June 25 Prepared by Jonathan Irwin This file contains a summary of information that we hope is useful when working with the data, and a description of the contents of the release files (at the end of the document). For additional release documentation, including a more detailed description of how the data were processed, summary tables of objects observed, and finding charts, please see the following HTTP URL: http://www.cfa.harvard.edu/MEarth/Data.html Summary of changes since release 7 ================================== MEarth-North and MEarth-South data from the 2017-2018 season are now included, according to the release schedule. No reprocessing of data from the 2008-2010 or 2010-2011 Northern seasons was carried out for this release, so these light curves have not been re-released. Data release 2 was the last release of data from these seasons. This document has been revised to remove parts of the description specific to these old seasons. Please see the documentation for release 2 for these details. Remaining known issues ====================== This data release contains automatically generated "working copies" of the light curves. These have been subject to minimal human intervention. There are a number of known flaws in this processing, which are detailed below. While the stellar parameters provided in the release materials have been improved compared to early releases, there are still known flaws, especially where a trigonometric parallax is not available. Handling of close multiple sources is in need of improvement. A "blend" flag is provided in the light curve header, but it should be noted that the flag is neither necessary nor sufficient for identifying potential problems, and sometimes flagged objects have usable photometry. Please see the description of the "blend" flag, below. When multiple neighboring sources were identified in the master frame, an attempt is made to generate separate light curves for them. This often produces poor results for the fainter members of very close pairs. Conversely, if a source was unresolved in the master frame (e.g. due to this frame being taken in very poor conditions) only a single aperture will be used, but it is possible for this to shift toward or onto the brighter source in frames with better image quality. Throughout the survey it has unfortunately been necessary to remove the detectors from the telescopes numerous times, usually for repairs. Removal and reinstallation of the detector on the telescope often results in changes to flat-fielding error, so these events require special treatment, especially when they happen during the observing season. A complete list of the occasions when this was done and details of the methods used to account for these changes during processing are given below. Other instrumental features and related information are also summarized below. Instrument description ====================== MEarth uses two separate installations, MEarth-North and MEarth-South. Each is comprised of 8 near-identical telescope/detector systems housed in a single roll-off roof enclosure. MEarth-North is located at the Fred Lawrence Whipple Observatory, Mount Hopkins, Arizona: Latitude: 31d 41' 03.1" N Longitude: 110d 52' 41.0" W Elevation: 2384 m MEarth-South is located at Cerro Tololo Inter-American Observatory (CTIO), Chile. Site coordinates, calculated based on the site survey by Mamajek (2012), are: Latitude: 30d 10' 20.9" S Longitude: 70d 48' 00.5" W Elevation: 2124 m Telescopes are a 0.4m f/9 Ritchey-Chretien Cassegrain design, used on a German equatorial mount. The science detectors are midband coated e2v CCD42-40 in the North, and NIR coated CCD230-42 in the South, operated with thermoelectric cooling at approximately -30C since 2011 September. The pixel scale is approximately 0.76 arcsec/pix in the North and 0.84 arcsec/pix in the South. The filter used from 2011 September onwards is fixed and is made up of 2x 1.5mm thick pieces of RG715 Schott glass built into the camera housing itself. The choice of mounting design has an important impact on the photometry. The telescope must be used on the west of the pier when the target is east of the meridian, and vice versa, with only a 5 degree window during which the target can be observed on the "wrong" side of the meridian before the telescope must be flipped over the pier. Flipping the telescope over the pier rotates the focal plane relative to the sky by 180 degrees, so any off-axis target or comparison star will sample two regions of the detector during the night. This leads to "meridian offsets" in light curves, which require special treatment (see below). We derive the information about which side of the pier the telescope was on from astrometric solutions on the data taken during target acquisition or from the science images as necessary. It is not sufficient to merely assume images with positive hour angle have one orientation, and negative hour angle have another, because it is possible to track slightly across the meridian, as mentioned above. Dedicated flags and an angle measurement are included in the light curves and should be used to detect "meridian flips". Also note that the pointing can be far off in the science images if a flip happened after the target acquisition exposures but before the science image was taken, because there is no way to detect this condition until it is too late. The telescopes are extremely prone to wind shake as used in the present enclosure (which has minimal protection from wind). This results in distorted PSFs in moderate wind, becoming severely distorted in high winds approaching the close-down limit for the enclosure. Due to the design of the building, this affects each telescope differently, and also depends on wind direction. At Mt Hopkins, winds from the east, northeast and occasionally northwest can sometimes also result in extremely poor seeing. This is noticeable in MEarth images even though the intrinsic PSF FWHM is approx. 2 arcsec, which normally renders them relatively insensitive to seeing variations, and can often be distinguished from wind shake by lower winds and a lack of high image ellipticities, as well as the detailed PSF shape, which tends to be much smoother when the cause of the large images is poor seeing. Images tend to show visible PSF distortions in winds exceeding 15 km/h, and it is likely there are astrometric distortions at even lower wind speeds. The effect on image quality also depends on the nature of the wind with gusty conditions tending to produce a mixture of undistorted and highly distorted frames. Because aperture photometry is used with large apertures, the light curves are remarkably insensitive to these problems, but there is a noticeable trend of large photometric scatter (often caused by a poor zero-point solution) in high wind. These images can usually be detected by examining the FWHM and ellipticity parameters, although the most distorted frames are known to fool the source classification procedure, which can result in no estimates of FWHM or ellipticity being made because it didn't think there were any stellar sources on the frame. This appears in the output as FWHM = -1.0. It should also be noted that the achieved pointing is degraded in high wind. The mounts natively (before correction) show a small amount of periodic error, which can cause image elongation in the y (RA) direction for long exposures. The worm period is 2.5 minutes, so these effects are most pronounced for exposures longer than 30 seconds or so. However, the effects can also show up in high-cadence continuous sequences of exposures taken for transit followup or similar, where it leads to excess scatter in the y coordinate time-series. It is possible for the periodic error to interact badly with the pointing stabilization software loop used during these observations, and this can increase the size of the error since the feedback attempting to fix the worm error occurs out of phase with the error itself. Periodic error correction has been used since 2010 January to address this problem. The periodic error curves are based on measurements of the error averaged over approx. 20-25 worm cycles, using a dedicated set of very high-cadence short exposures on a bright, equatorial star taken during low wind. However, it is found that the solutions need to be updated quite frequently (every few months) as there is some drift in the worm error over time. This has occasionally lagged due to the special conditions (extremely low wind) needed to obtain a good calibration or because it was not noticed quickly. The periodic error can change dramatically when the mounts are re-lubricated, so the calibration is always re-run at these times (annual, at startup after the monsoon). As the mounts have aged, periodic error has grown and became too large to correct adequately on many of the mounts in both hemispheres during 2017. A program of worm block replacements is ongoing to address these performance issues. The original detectors showed residual images ("persistence"), which affects all 2008-2011 Northern data. The time-dependence of the residual images is is found to be well approximated by an exponential decay with a time constant of approximately 20 minutes, although this varies from one telescope to the next and is likely to be dependent on operating temperature. We discuss the effects this has on the photometry and strategies used for mitigation, below, and provide more details of the residual image problem itself on the web pages. During the 2011 summer shutdown, the detector housings were re-built to achieve a lower operating temperature, and a preflash system was added to properly address the problem, so persistence should not affect 2011-2018 Northern data or any Southern data. Data organization ================= The philosophy followed in the data processing has been to start over with a completely separate set of calibration files and light curves whenever major changes were made to the instrument that alter the bandpass of the photometry or other key photometric properties. So far, this has been done on three occasions in the North: 1. September 2008: move from commissioning to the start of the survey. At this time, the full suite of telescopes were bought on-line, and the filter was changed from the RG9 filter used during commissioning to the custom-made RG715 filters for the main survey. 2. October 2010: changed from the RG715 filter to a custom interference I-filter (hereafter called I_715-895). 3. October 2011: CCD cameras were completely re-built in new housings with higher cooling performance and a preflash system. Filter was changed back to RG715, although note that it is not from the same manufacturer, has a slightly different thickness, was used in conjunction with different glass elements near the focal plane, and at a different CCD operating temperature, so the bandpass is not identical. Numerous other small improvements were also made. We refer to these three periods as "seasons", even though 1 and 3 above contain multiple observing seasons (years). The present release concerns data taken in only the last of these periods, so there is only one set of files, named "2011-2018" after the range of calendar years over which they were taken. For data taken in the 2008-2010 and 2010-2011 seasons, please see data release 2. There are no such changes in the South at the time of writing; all Southern data were taken using the equivalent instrument configuration to 3 above. Please note that although quite major changes have been made to the hardware, the same physical CCD chip has always remained associated to the same telescope throughout the survey, and the same set of stars have been observed on each telescope. After the detector housing re-build, the detectors were re-identified using cosmetics (defects) and put back with their original telescope, even though the serial numbers had changed. It was possible to do this unambiguously, so these associations should continue. Other hardware and observational changes ======================================== Active development and improvement of the survey hardware, observational strategy and data processing have continued throughout operations. We have attempted to minimize impact on data taken during a season, preferring to consolidate major changes at the ends of seasons around the times of the summer monsoon when the observatory is shut down. However, hardware failures have meant it has occasionally been necessary to make larger than ideal disturbances to individual telescopes during seasons. This is particularly the case for 2011-2012 in the North. The most important changes that are known or suspected to affect the photometry are summarized in this and the following sections. Changes to individual telescopes ================================ The following table summarizes the dates when a detector was replaced on a telescope after having been removed, with a brief description of the reason for the change. Removal and replacement of the detector is likely to cause changes in flat-fielding error, and sometimes collimation. Each of these events increments the "instrument version number" (see "V" in the light curves) by one, and this triggers the creation of a new per-star magnitude and meridian offset (where data on both sides of the meridian exist) when producing in the light curve, corresponding to the numbered "segment"s (see "S" in the light curve table). North Tel. V 1stnight Description ----- -- -------- ----------- tel01 2 20081001 Start of 2008-2010 season tel01 3 20090901 Changed to operate in focus tel01 4 20091115 Repair (condensation). tel01 5 20101028 Filter changed to I tel01 6 20111011 Rebuilt camera installed, RG715 filter tel01 7 20120101 Camera removed to extract leaf. tel01 8 20120131 Repair (shutter driver). tel01 9 20130613 Jump in photometry tel01 10 20130815 Monsoon, jump tel01 11 20140815 Monsoon, jump tel01 12 20160815 Optics cleaned. tel01 13 20170525 Dirt landed on filter, large dust shadows. tel01 14 20170914 Filter cleaned. tel02 2 20081001 Start of 2008-2010 season tel02 3 20090901 Changed to operate in focus tel02 4 20091119 Repair (condensation; controller replaced). tel02 5 20101028 Filter changed to I tel02 6 20111011 Rebuilt camera installed, RG715 filter tel02 7 20111223 Repair (shutter driver). tel02 8 20120903 Dirt removed. tel02 9 20160130 Repair (shutter). tel02 10 20160815 Optics cleaned, repair (shutter). tel02 11 20170405 Repair (shutter). tel02 12 20180204 External shutter installed. tel03 1 20080929 Commissioned tel03 2 20090901 Changed to operate in focus tel03 3 20091115 Repair (condensation). tel03 4 20101028 Filter changed to I tel03 5 20111011 Rebuilt camera installed, RG715 filter tel03 6 20120206 Repair (shutter driver). tel03 7 20120621 Repair (shutter driver). tel03 8 20120908 Repair (stuck shutter). tel03 9 20121227 Repair (shutter driver). tel03 10 20130912 Replaced broken rotation adjustment piece; aligned. tel03 11 20160815 Optics cleaned. tel04 2 20081001 Start of 2008-2010 season tel04 3 20090901 Changed to operate in focus tel04 4 20101028 Repair (condensation); filter changed to I tel04 5 20111011 Rebuilt camera installed, RG715 filter tel04 6 20120425 Repair (shutter driver). tel04 7 20120801 Repair (stuck shutter). tel04 8 20140121 Repair (stuck shutter); collimated. tel04 9 20160815 Optics cleaned. tel05 1 20080605 Commissioned tel05 2 20090901 Changed to operate in focus tel05 3 20091115 Repair (condensation) tel05 4 20100208 Repair (stuck shutter) tel05 5 20100525 Repair (stuck shutter) tel05 6 20101028 Filter changed to I tel05 7 20111011 Rebuilt camera installed, RG715 filter tel05 8 20120107 Repair (shutter driver). tel05 9 20121105 Jump in photometry, doughnuts. tel05 10 20130815 Monsoon, jump. tel05 11 20140601 Repair (stuck shutter). tel05 12 20150901 Mount refurbished. tel05 13 20160815 Optics cleaned. tel06 1 20080928 Commissioned tel06 2 20090901 Changed to operate in focus tel06 3 20101028 Repair (condensation); filter changed to I tel06 4 20111011 Rebuilt camera installed, RG715 filter tel06 5 20120108 Repair (shutter driver). tel06 6 20130815 Mount refurbished. tel06 7 20150220 Repair (stuck shutter). tel06 8 20150517 Repair (stuck shutter). tel06 9 20160130 Repair (shutter). tel06 10 20160815 Optics cleaned, repair (shutter). tel06 11 20170405 Repair (shutter). tel07 1 20080605 Commissioned tel07 2 20090901 Changed to operate in focus tel07 3 20091115 Repair (condensation) tel07 4 20101028 Filter changed to I tel07 5 20111011 Rebuilt camera installed, RG715 filter tel07 6 20120417 Repair (shutter driver). tel07 7 20121024 Repair (shutter driver). tel07 8 20130308 Repair (stuck shutter). tel07 9 20130914 Adjusted to correct angle. tel07 10 20140815 Monsoon, jump. tel07 11 20160815 Optics cleaned, repair (shutter). tel08 1 20090106 Commissioned tel08 2 20090901 Changed to operate in focus tel08 3 20101028 Repair (condensation); filter changed to I tel08 4 20111011 Rebuilt camera installed, RG715 filter tel08 5 20120101 Repair (shutter driver). tel08 6 20130514 Jump. tel08 7 20130717 Repair (damaged by thunderstorm). tel08 8 20140914 Filter replaced, aligned. tel08 9 20150815 Monsoon, jump. tel08 10 20160815 Optics cleaned. South Tel. V 1stnight Description ----- -- -------- ----------- tel11 1 20140121 Commissioned. tel11 2 20151118 Repair (shutter driver). tel11 3 20170503 Repair (shutter driver). tel11 4 20170630 Repair (shutter). tel12 1 20140121 Commissioned. tel12 2 20140428 Repair (shutter driver). tel12 3 20140911 Repair (shutter driver). tel12 4 20150423 Repair (power supply). tel12 5 20170612 Repair (condensation). tel12 6 20171019 Repair (shutter). tel13 1 20140121 Commissioned. tel13 2 20150811 Jump in photometry after snow. tel13 3 20160331 Repair (shutter driver). tel13 4 20170505 Repair (shutter). tel13 5 20170630 Repair (condensation). tel14 1 20140121 Commissioned. tel14 2 20151120 Repair (shutter). tel14 3 20160707 Repair (shutter and condensation). tel14 4 20170609 Repair (shutter). tel14 5 20170628 Repair (shutter). tel14 6 20170826 External shutter installed. tel14 7 20171020 Internal shutter replaced. tel15 1 20140121 Commissioned. tel15 2 20140328 Repair (shutter driver). tel15 3 20170608 Repair (shutter). tel15 4 20170629 Shutter adjusted. tel15 5 20170831 Shutter replaced. tel16 1 20140121 Commissioned. tel16 2 20140328 Repair (shutter driver). tel16 3 20150423 Repair (shutter). tel16 4 20170509 Repair (shutter). tel17 1 20140121 Commissioned. tel17 2 20170824 Repair (condensation). tel17 3 20171020 External shutter installed. tel18 1 20140121 Commissioned. tel18 2 20150423 Repair (shutter driver). tel18 3 20170609 Repair (shutter). When detectors were repaired, changes in the shutter shading, non-linearity and gain can result, so these calibrations were regenerated and used to reduce all subsequent data when any evidence for a difference from past behavior was found. The following table gives the approximate reciprocal gain in e-/ADU, readout noise in e-, and saturation level in ADU appropriate for each telescope. North 2011-2018 Tel. Gain RON Sat. ----- ---- ---- ----- tel01 1.8 8.2 48000 tel02 1.7 6.9 41000 tel03 1.5 9.2 46000 tel04 1.7 6.4 48000 tel05 1.8 9.5 53000 tel06 1.5 7.9 48000 tel07 1.4 8.4 45000 tel08 1.4 8.8 51000 South 2014-2018 Tel. Gain RON Sat. ----- ---- ---- ----- tel11 3.2 9.9 50000 tel12 4.2 10.1 48000 tel13 3.9 12.3 51000 tel14 3.7 10.4 47000 tel15 3.3 10.5 51000 tel16 3.8 8.2 49000 tel17 4.6 11.6 49000 tel18 4.0 9.4 47000 Target and field selection ========================== Our target selection procedures are detailed in Nutzman & Charbonneau (2008), and the Northern survey target list was drawn directly from this work. The Southern target list is described briefly by Irwin et al. (2015), and was produced by applying the same procedure to stars drawn primarily from three sources: the set of published trigonometric parallaxes from the Research Consortium on Nearby Stars (RECONS), the Palomar/MSU spectroscopic survey, and the LSPM-South catalogue (Lepine, private communication). Note that the target stars selected for observation were exclusively those with estimated radii < 0.33 Rsol, but see the following discussion for cases where there were other objects in the M-dwarf list present in the field-of-view. It is possible (but fairly unusual) for multiple M-dwarfs to appear within a single field-of-view (approx. 26x26 arcmin). For the Northern survey, we observe a single field in such cases, with the position adjusted to contain as many of these M-dwarfs as possible (any left over are assigned their own field as normal). This was done using the full "LSPM-North M-dwarfs" list, i.e. without the < 0.33 Rsol criterion. Exposure times were set to avoid saturation on the brightest M-dwarf in the field with estimated radius < 0.33 Rsol, and only fields containing at least one M-dwarf meeting this estimated radius criterion were observed. This was not done in the South due to concerns about off-axis defocus (due to the curved focal surface of the telescope), and increased flat fielding error at the edges of the detector, which both appear to be more pronounced than in the North. Instead, each target star is observed in its own field, but apertures are still placed on all stars meeting the 0.33 Rsol criterion and light curves generated. In the North, field centers chosen for each target field were dithered within a 1x1 arcmin box about the expected positions of the target M-dwarfs in order to mitigate the effects of persistence. This procedure tries to ensure each target never "sees" the persistent image of a previous target in the photometric aperture, which could severely disturb the photometry (especially since the other targets under observation might change during the night or from night to night). This was not necessary in the South; the primary target of each field is always placed at the center of the detector, in an attempt to minimize flat fielding error introduced at meridian flips. We try to return a given target to the exact same detector pixels every time it is observed on a given side of the meridian. This means that the target does see its own persistent image. We have found that this can lead to ramps (mostly for the highest cadence continuous observations, such as transit follow-up), and magnitude offsets depending on the cadence for objects where the cadence has been changed during a season. It is obviously extremely undesirable to attempt high-precision photometry using detectors with residual image problems, and this was properly addressed by implementing a pre-flash system at the 2011 summer shutdown in time for the start of the 2011-2012 season. The South has always used the pre-flash. In the North, apertures are placed on all objects in the "LSPM-North M-dwarfs" list and light curves extracted. This means there is occasionally usable survey photometry for an earlier star than our notional cut-off. Observational strategy and cycling of the targets list ====================================================== The observational strategy has been summarized in several publications (Nutzman & Charbonneau 2008; Irwin et al. 2009; Berta et al. 2012; Irwin et al. 2015), but we repeat the important features here. We also provide time-lapse movies of selected nights on our web pages, which illustrate many aspects of the discussion visually. Except for cases of bad weather, or where adjustments are made by the operator, targets are observed for the entire time they are at a zenith distance of less than 60 degrees (approximately) and the Sun is more than 10 degrees below the horizon. All targets are assigned a scheduling priority, which are mostly the same, but this can be used to ensure a given target is observed at the desired cadence when the schedule becomes busy during the night (e.g. due to realtime triggers in later seasons). Any target within 40 degrees of the Moon at full Moon, scaling down as the fraction of lunar disk area illuminated away from full, was not observed on the night in question. Target observations are sometimes interspersed with other observations, e.g. follow-up activities, and in later seasons, realtime triggers, both of which can cause lengthy interruptions, but occur infrequently. Otherwise, we aim to return to each target every 20 minutes (in practice, the achieved figure tends to be closer to 30 minutes). This is called a "visit" below, to distinguish it from an individual exposure - multiple exposures might be taken at each "visit". We attempt to maintain the same set of "well-observed" targets for as long as possible in a given season in order to prevent small amounts of data being accumulated on many targets. Exposure times were modified as infrequently as possible for stability reasons, due to the large shutter shading corrections and in earlier seasons, concerns about persistence. This was done annually during summer shutdown, and the quality of the exposure time estimates has been progressively refined throughout survey operations. Mid-season changes were usually reserved for objects where saturation was found to be a problem. Starting in 2009 September, we computed the exposure times as discussed in Nutzman & Charbonneau (2008) to achieve a 3 sigma detection of a 2 Earth radius planet in each "visit", given the assumed stellar properties (see below), although some exceptions were made for bright, late-type objects, where we simply continued to gather as many photons as possible before saturation (the observing time for these objects is dominated by the slew and field acquisition overheads so the extra time spent integrating comes at negligible cost). Telescopes were operated in focus, and it was found that it was often not possible to obtain enough photons before saturation in one exposure, particularly for earlier-type targets, so visits for these objects were split into multiple sub-exposures to make up the required total integration time. The exposure time computations try to account for all of the noise sources included in our noise model (see below, under e_Mag). Exposures longer than 120s were found to exhibit poor tracking (there are no auto-guiders), so we also split these to keep the maximum single-exposure time below 120s. These groups of observations are presented individually, rather than stacking or binning them, because this would result in loss of information. The scatter within a visit can be a useful noise estimator, and having all of the data permits the use of robust (outlier-resistant) averages. It is not possible to observe all survey targets on the same night at the desired cadence. In practice, approx. 120-240 targets are observed at the 20 minute survey cadence per night (the number of targets observed per night depends on the length of the night / time of year, but has also decreased each survey year; there are a number of reasons for this, including increases in the time per visit due to increasing numbers of sub-exposures or more realistic exposure times, and poor scheduling decisions in the initial implementation leading to the targets with the shortest time taken per visit tending to be observed early in the survey). In order to complete the survey (which intended to target 2000 stars), we must stop observing targets after sufficient observations have been obtained to reach the phase coverage needed to detect planets in our desired size and orbital period ranges (see Nutzman & Charbonneau 2008). The implementation of this has varied, but real-time transit detection performing at the level required to reach habitable zones for small planet sizes was not available during many of the early seasons, so observations were extended to partially compensate for this. Except for cases where the data were clearly unusable (which were removed immediately), this "cycling" of the targets list occurred annually during the summer shutdown period. Targets with phase coverage deemed sufficient at this point were dropped, allowing new targets to take their place, although exceptions were made for objects of particular scientific interest, which therefore have much longer time-series. Many of these criteria were quite subjective. A change in observational strategy was made at the start of the 2011-2012 season in the North only, to obtain observations of all MEarth targets at a cadence of approximately 10 days (in addition to the sample being actively searched for transiting planets at high cadence, described above). These data were primarily intended for astrometry, but may also be useful for long-term monitoring of the photometric behavior of the target stars. A further change was made to the observational strategy at the start of the 2012-2013 season in the North only, to observe each planet survey field on two telescopes simultaneously in order to improve sensitivity to shallow transits. This means there are two light curves for each planet survey target. Please refer to Berta et al. (2013) for more details of this strategy and the motivation for the change. Targets were re-prioritized at the same time, and many of the most favorable targets observed earlier in the survey were re-visited using the new strategy to take advantage of the improved sensitivity. When analyzing these data, it is important to note that not all systematic effects are independent between the telescopes. Data processing =============== For full details of our data processing procedures, please refer to the documentation on the release web page. The light curve files present aperture photometry derived using a series of concentric, circular apertures, starting from a "base aperture radius" of 4 pixels for Northern data in 2011-2018, and 3 pixels for Southern data. Larger apertures are also computed at factors of two in area, i.e. radii of sqrt(2)*r and 2*r, and (for the South only), 2*sqrt(2)*r. Sky background estimation was performed using a circular annulus between 6*r and 8*r. Light curves are generated for all of these apertures, and the one showing the smallest RMS scatter is chosen. For most targets, this is the 2*r aperture. The methodology followed in producing differential light curves is mostly standard (e.g. Honeycutt 1992), with one important exception. In order to avoid diluting the empirical inverse variance weights used to solve the individual frame zero-points, we account for the various "instrument versions" and the "meridian flips" by deriving a suite of N_S per-star magnitudes (or equivalently, one magnitude and N_S-1 offsets). These are called "segments" in the discussion in this document, and each distinct "S" value in the light curve tables (see below) was allowed its own set of per-star magnitudes for every star on the frame. These are all tied to the reference (master) frame, so in doing so the meridian offsets and other sudden flat-fielding changes at instrument modifications are removed, on assumption of a constant intrinsic stellar magnitude. It is important to be mindful that this has been done for some applications of the data (i.e. where the assumption of a constant stellar magnitude is not correct). In practice, these coefficients are always re-fit in any use of the data where this is important (e.g. the variability searches). Known systematics and techniques for their mitigation ===================================================== The dominant systematic effect that is (deliberately) not corrected in the MEarth light curves is due to "second-order" (color-dependent) atmospheric extinction, resulting from telluric water vapor absorption redward of 9000A in the bandpass of the RG715 filter (combined with CCD quantum efficiency). This effect appears in the differential photometry due to the extreme spectral mismatches between the targets and comparison stars, combined with the nature of the targets (M-dwarf spectra rise steeply through the bandpass toward the red end, which is where the problems are). In order to address this effect, the observations of all M-dwarf targets are combined to make a second, lower cadence (binned) comparison light curve, which we call the "common mode". We then use this to correct the data for the individual targets. This "double differential" photometry procedure is found to work well, but with one complication. Due to differences in the target spectral types (and in some fields, the comparison stars, e.g. at low Galactic latitude) it is necessary to scale this "common mode" by a factor which varies from object to object. It has proved difficult to calculate this quantity a-priori, so instead we fit for it using the target light curves to derive a multiplicative coefficient applied to the common mode before subtracting it (for the equations, see the description of the "null hypothesis model" below). This coefficient is usually well constrained, but needing to derive it empirically by fitting the light curve brings with it the highly undesirable feature of necessitating making some kind of assumption about the intrinsic photometric behavior of the target. The present implementation of the "common mode" derivation takes a simple median of all target M-dwarfs in 0.02 JD (approx. half-hour) time bins, which appears to be adequate, although it should be noted that this procedure could potentially be improved, e.g. by accounting for the different target spectral types. We give the "common mode" itself in the original bins as a separate data file (commonmode.txt), but the light curves include the "common mode" interpolated to the dates of observation for straightforward application. We have also detected some systematics correlated with the FWHM of the images. This is not unusual for light curves derived from aperture photometry, and is thought to result from contamination of the aperture fluxes by neighboring stars in crowded fields. While it is not completely correct to do so, we address this issue by also performing a "decorrelation" against FWHM in light curves where there are strong trends. Finally, we repeat the mention of the "meridian offsets". The light curve generation allows a meridian offset for each star in each "instrument version" (these appear as new "segments" and are numbered with unique values in the "S" column, below), so this effect is already somewhat mitigated, but the process makes the assumption of no stellar variability, which is likely to be false for most targets, and in some cases the meridian offset removal can corrupt astrophysical signals. Therefore, due to the treatment made in processing the light curves, it is necessary to re-fit these "meridian offsets" when modeling the long-term stellar behavior, e.g. variability. Since the flat fields also change slightly over time, for some uses of the data it may be necessary to consider separate meridian offsets for each night, e.g. as done in Berta et al. (2012). Other "external parameters" are included in the file, and many of these are helpful in identifying systematics. We note particularly that large pointing offsets can introduce systematics caused by flat-fielding errors, and in many analyses we perform on the data (particularly automated transit searches) we filter out points with large pointing offsets (see the X and Y columns) in order to avoid false alarms. Release files ============= Each season (as defined above) has its own directory in the release. Within each of the directories, there is one light curve file per star per telescope. No attempt has been made to merge data taken on the same star with multiple telescopes. Data files are named: LSPMJhhmm+ddmm_telnn_yyyy-yyyy.txt (North) or 2MASSJhhmmssss-ddmmsss_telnn_yyyy-yyyy.txt (South) A change in naming scheme was necessary for the South as not all of the Southern stars have LSPM-South identifiers, and even when available, the LSPM-South identifier system is not in common use. The components of the filename (separated by _ characters) are as follows: LSPMJhhmm+ddmm is the designation of the star in the Lepine-Shara proper motion catalogue (LSPM-North; Lepine & Shara 2005) used to select the MEarth-North targets. Please note that the LSPM designation is always used, even when the star has other, more common, names in the literature. Note that the LSPM designations sometimes include additional letters after the abbreviated position. 2MASSJhhmmssss-ddmmsss is the full 2MASS identifier for MEarth-South targets. The 2MASS designation is always used, even when the star has other, more common, names in the literature. The 2MASS identifiers are never shortened or abbreviated, and never have additional letters added. telnn telescope number (nn = 01 to 08 for the North, 11 to 18 for the South). Telescopes are identified by a running number, following the layout of the telescopes in the building. yyyy-yyyy season (see above). Finding charts are named: LSPMJhhmm+ddmm_telnn_yyyy-yyyy_aperture.png or 2MASSJhhmmssss-ddmmsss_telnn_yyyy-yyyy_aperture.png these reside in a separate directory, and show the location of the photometric aperture and sky annulus (where applicable) on the MEarth master image, and the first and second epoch Schmidt plate scans. In the North, the scans are from the Digitized Sky Survey (DSS; accessed via the ESO archive), using the red (R_F) plate wherever possible for the second epoch (there are a few cases where the blue (B_J) plate was used because the red was unavailable). For the South, the first epoch is from the SuperCOSMOS scans of the first epoch Palomar Observatory Sky Survey (POSS-I) plates wherever possible (roughly, declinations above -21 degrees) as the baseline is often substantially better than the first epoch DSS. The second epoch is taken from the DSS as in the North. The charts are intended for assessing which sources contribute light inside the photometric aperture, but can be useful for other purposes, such as assessing the quality of the master image (they are probably too small to be used as conventional finding charts, however). The charts were generated with SAOImage DS9. The astrometric registration used in this version of the charts relies on the world coordinate system (WCS) in the DSS FITS headers, which we find can sometimes be off (particularly poor quality MEarth master images can also be off). This problem is usually quite obvious to the eye, and it is rare for it to be large enough to impact use of the charts. NOTE: the DSS is subject to copyright, and acknowledgments are requested in any publication based on DSS data. Please see: http://archive.stsci.edu/dss/acknowledging.html There are also some other "global" files included in the release: README.txt This file. commonmode.txt The "common mode" in the original 0.02 JD bins. summary.txt Summary information on the light curves (see below). Light curve file contents ========================= The light curve files are space-separated, formatted ASCII, with newline (line feed) record terminators. Each record presents one photometric measurement. This format has been used to ensure human- as well as machine-readability and maximize compatibility across different software platforms. In the files, # is used as a "comment" character. The files start with a header (all as "comments", i.e. with # at the start of the line) containing a set of name = value pairs with information about the target drawn from the tables of Lepine & Shara (2005), Lepine (2005), and Nutzman & Charbonneau (2008) for the North; and RECONS, Stauffer et al. (2010), Reid et al. (1995), Hawley et al. (1996), and the LSPM-South (Lepine, private communication) for the South. Some parameters derived from the light curve are also included. See below for a full description of the quantities in the header. Two lines then give the column titles, also in a comment. The table follows these lines until the end of file, and is guaranteed not to contain any comments. The table uses space separated, formatted ASCII records, with each record terminated by a newline character (line feed). There are no blank columns, and the column spacing is also consistent throughout all files in the release, so extraction of bytes within each record (as with the common CDS/Vizier ASCII catalogues), or splitting on whitespace, such as the default behavior of the awk(1) utility, will parse these records into columns correctly. The files can also be read in popular spreadsheet programs, by setting the record delimiter to space, and switching on the option to treat multiple consecutive delimiters as a single delimiter. The following columns are present in the light curve table: Column Format Unit Description BJD D14.6 days Barycentric Julian date of mid-exposure, in the TDB time-system. The full JD is given without truncation or subtraction to avoid ambiguity, but please note that as a result, it is essential to read these in double precision if they are left formatted as-is. Timing accuracy is only approx. a few seconds, due to limitations of the data acquisition software and the network time protocol implementation on the Windows computers used to perform the data acquisition. Mag F11.6 mag Differential magnitude. e_Mag F11.6 mag Uncertainty in the differential magnitude. Computed using a standard CCD noise model, which includes Poisson noise in the target photon counts, sky background noise (this is estimated empirically, so it includes readout noise), scintillation noise using the formula of Young (1967), and the uncertainty in the magnitude zero-point correction (derived from the comparison stars) that was applied to produce the differential light curve. This noise model is known to underestimate the uncertainty for data taken in poor conditions, e.g. thick clouds. See below for information useful to detect these and other conditions that can degrade data quality or lead to systematic errors. tExp F7.3 sec Exposure time. DMag F7.4 mag The magnitude zero-point correction that was applied to the frame to produce the differential light curve. This has already been applied in the "Mag" column. It is included for the purpose of detecting frames with large light losses, e.g. due to clouds, which might need to be treated with suspicion or possibly discarded, depending on the nature of the analysis. More negative numbers correspond to less light, i.e. greater losses. Typically, we find that points with DMag < -0.5 should be treated with suspicion. FWHM F6.3 pix Full width at half maximum estimated from the stellar images on the frame. This quantity can be spurious in cases of extreme image distortion due to wind shake, or if no stars were seen on the frame. A value <= 0 indicates that no estimate could be made, usually due to there being too few sources classified as being stellar on the frame. It is essentially impossible for the FWHM of the images to be less than 1.5 pixels, so any such value should be treated as spurious. Ellip F5.3 - Ellipticity of the stellar images on the frame. This quantity can also be spurious in cases of extreme image distortion due to wind shake, although is more reliable than the FWHM. A value of exactly zero in conjunction with FWHM <= 0 indicates that no estimate could be made (see FWHM, above). Airmass F7.5 - Airmass at mid-exposure. This quantity is computed for the position of the target star, as measured from MEarth astrometry. X F9.3 pix Measured X, Y pixel coordinates of the target. Y F9.3 pix Angle F7.2 deg Angle relative to the reference (master) frame (estimated using the comparison stars). These quantities are intended for detecting pointing errors, and similar issues, which might introduce systematics due to flat fielding error into the photometry. The angle can also be used to detect a "meridian flip", where it changes by approx. 180 degrees when the frame in question was taken on the opposite side of the meridian from the reference frame. Sky F8.2 ADU Local sky background level used in the aperture photometry. Peak F5.0 ADU Peak pixel counts in the object, including sky. Used for detecting saturation and/or identifying points where non-linearity might be a concern (see under "Changes to individual telescopes", above, for a table of saturation levels appropriate for each detector). S I2 - "Segment number". Identifies which points of in the light curve were solved together with one set of per-star magnitudes in order to produce the light curve. This numbers from 1 in each light curve. Points with the same value in this column are essentially those believed to share the same flat-fielding error. V I2 - "Instrument version number". This integer (numbered starting from 1, over the whole lifetime of the telescope) is incremented every time the detector was removed and replaced on the telescope, or other changes that are likely to cause offsets or systematics in the photometry were made. The software uses separate per-star magnitudes on either side of the change to account for this. R I1 - Realtime status flag. Non-zero if the data point was taken in response to a real-time trigger. Values are currently 1 or 2, corresponding to two stages of the real-time detection process: confirmation (R=1), and high-cadence followup (R=2). F I1 - Flags. The following values are used, combined with a bitwise OR operation: 2 Aperture contains known bad pixels. 4 Possible saturation detected. F != 0 means the magnitude may be corrupted, but not necessarily. Usable measurements can still be obtained (depending on the requirements) when either flag is set. CM F9.6 mag Common mode, interpolated to the time of observation. Corr_Mag F11.6 mag Differential magnitude with a simplistic model applied to correct for the common mode, "segment" zero-point offsets, and FWHM correlated systematics. This provides a very simple-minded "detrended differential magnitude" with the major known systematic effects removed. However, it assumes the star is constant, which is likely to be false for a large majority of the targets, and is intended only to get up and running quickly (e.g. for making a cleaned plot of the data). We strongly advise against using "Corr_mag" for scientific purposes. This is particularly true for studies of stellar variability. The following parameters are included in the header. Any parameter that is not available (null) is excluded. Parameter Unit Description telescope - MEarth telescope that observed this light curve (see above). season - Season the light curve was observed in (see above). filter - Filter used. This can take two possible values: RG715 the RG715 long-pass filter I the "I_715-895" interference filter aperture pix Photometric aperture radius used to generate the light curve. deblend - Flag to indicate that the detection of the target was de-blended. Can indicate the presence of a nearby star that may contaminate the aperture photometry. Note however that it is still possible for this to occur even when the flag is not set, if the source detection software could not resolve the blend in the MEarth master image. It is also sometimes possible to obtain entirely usable (or indeed, relatively uncontaminated) photometry when the flag is set. It is recommended to examine the finding charts to assess the contamination. lspmname - Lepine-Shara Proper Motion catalogue (LSPM) North identifier (Lepine & Shara 2005). The following cross-identifications and parameters are also from this catalogue, except where noted. gliese - Gliese (1969) or Gliese & Jahreiss (1979) catalogue of nearby stars number. Retrieved by cross-matching (via the 2MASS identifications) with the catalogue of updated positions for the Gliese stars by Stauffer et al. (2010). These identifiers have been manually verified wherever there was any ambiguity, and are believed to be correct, but there may be errors remaining in cases of close multiples. lhs - Luyten Half-Second catalogue number. See the revised LHS (Bakos et al. 2002), which was used to assist in the recovery of these identifiers for the Southern stars. nltt - Revised New Luyten Two Tenths catalogue number (Salim & Gould 2003). Note that recovery of these identifiers may be incomplete for the Southern sample. hip - Hipparcos catalogue number. tycho - Tycho-2 catalogue identifier. Note that recovery of these identifiers may be incomplete for the Southern sample. ascc - ASCC-2.5 catalogue number. Not available for the southern sample. ucac - UCAC-2 catalogue number. Not available for the southern sample. twomass - Two-Micron All-Sky Survey all-sky data release identifier. usnob - USNO-B1.0 catalogue identifier. Not available for the southern sample. kic - Kepler input catalogue number. ra hr Right ascension and declination in : dec deg delimited sexagesimal (base-60) format. Positions are equinox J2000 and epoch 2000.0, see "aflag" for the source of the astrometry. pmra arcsec/yr Sky-projected absolute proper motion. See pmdec arcsec/yr "aflag" for the source of the astrometry. aflag - Astrometry source flag. For the North, this is taken from from Lepine & Shara (2005) and uses the following values: T Tycho-2 catalogue A ASCC-2.5 catalogue S 2MASS position and stated proper motion O Other source For the South, we have added two additional values: H Hipparcos catalogue R RECONS plx arcsec Trigonometric parallax and uncertainty. e_plx arcsec r_plx - Reference for trigonometric parallax, given as an ADS bibliographic code. distmod mag Adopted distance modulus. s_distmod - Source of the adopted distance modulus. The following values are used: T Trigonometric parallax. P Photometric distance estimate. To identify the source, see light curve file headers. If any of the vmag, rcmag, icmag and r_vrimag quantities are given, these were used to compute the distance modulus using the Henry et al. (2004) relations. Otherwise, values were taken from Table 3 of Lepine (2005). S Spectrophotometric distance estimate (for the North, see Table 3 of Lepine 2005 for references; for the South, the spectrophotometric estimates are all from the Palomar/MSU survey; Reid et al. 2005; Hawley et al. 2006). L Distance modulus based on estimated V-J colour. For the North, these were taken from Lepine (2005), and for the South, were calculated using the same method, but with photometry supplemented to use more reliable V-band magnitudes from APASS rather than photographic estimates, where available. The reliability of these estimates varies depending on the quality of the V-band photometry adopted when doing so. Information to assess this is given in the headers of the light curve files. Note that many of these distance moduli were derived using photographic photometry and are highly uncertain. bjmag mag Photographic magnitudes from USNO-B1.0, rfmag mag SUPERBLINK (Lepine & Shara 2005; in the inmag mag latter case only the R_F magnitude is given), MSOUTH (Winters et al. 2015), or SuperCOSMOS. jmag mag 2MASS J-band magnitude, "combined" e_jmag mag uncertainty, and "qual" flag. Please see the q_jmag - 2MASS explanatory supplement for a detailed explanation of the flags. hmag mag Likewise for 2MASS H-band. e_hmag mag q_hmag - kmag mag Likewise for 2MASS Ks-band. e_kmag mag q_kmag - vest mag Estimated V-magnitude. s_vest - Source of the estimated V magnitude. T Tycho-2 catalogue A ASCC-2.5 catalogue K Koen et al. (2010) R RECONS P PMSU or APASS U Estimated from the photographic magnitudes (bjmag, rfmag, inmag). If s_vest=U, vest and vmjest can be highly uncertain and may have substantial systematic errors, particularly for red objects. vmjest mag Estimated V-J colour. vmag mag Optical magnitudes in the Johnson-Cousins rcmag mag system. icmag mag r_vrimag Source of optical magnitudes. spectype Optical spectral type and reference. r_spectype mass Msol Estimated stellar mass and radius. Please radius Rsol check the "s_distmod" flag (above) to assess the reliability of the distance assumed in computing these values. Based on the methods used in the Nutzman & Charbonneau (2008) paper, but with an updated compilation of trigonometric parallaxes and an updated mass-luminosity relation. NOTE: some modifications were made to the methods described in the Nutzman & Charbonneau (2008) paper when generating the values shown here. Trigonometric parallaxes were required to be measured to better than 33% in order to be used. The double exponential ("forward") form of the K-band mass-luminosity relation from Benedict et al. (2016) was used rather than the Delfosse relations used in previous releases and the original paper. The Bayless & Orosz (2006) mass-radius polynomial was used to estimate the stellar radii from the masses. The following parameters show the "null hypothesis" model (corresponding to no variability) from our variability search software. This was also used to compute the "Corr_Mag" column. The model is as follows: Mag = null_dc[S] + null_cm * CM + null_fwhm_k * (FWHM - null_fwhm_0) where Mag, S, CM, and FWHM refer to the columns in the light curve table, and the square bracket notation refers to an array of values (indexed here by S, which numbers from 1). The parameters null_chidof, null_nfit, and null_nparm give the chi squared per degree of freedom (assuming the uncertainties in e_Mag), number of light curve points that were fit, and the number of parameters that were fit, respectively. Common mode file contents ========================= The following columns are present in the common mode file: Column Format Unit Description JD D10.2 days Full Julian date of the start of the bin in the UTC time-system. (NB: not Barycentric) CM F8.5 mag Differential common-mode magnitude. Currently this is derived simply by taking the median differential magnitude in the bin. sigma F8.5 mag Scatter in the common-mode bin using a robust MAD (median absolute deviation) estimator, scaled to Gaussian equivalent standard deviation (i.e. 1.48*MAD). nmeas I4 - Number of measurements in bin. nobj I4 - Number of independent objects in bin. tamb F6.1 C Ambient temperature. humid F6.1 % Relative humidity. press F6.1 hPa Ambient pressure. skytemp F6.1 C Sky-Ambient temperature from cloud sensor. NOTE: the last four columns are -999.0 if not available (e.g. due to failures of the various weather sensors) and are averages of the instantaneous measurements current at the start times of each exposure included in the bin. When pressure is available, the tamb, humid and press measurements are from the Vaisala WXT510/520, otherwise tamb and humid are from the cloud sensor. The cloud sensor is considered a critical device for observatory operation, so the skytemp measurement is normally available. Summary table contents ====================== The following columns are present in the summary table. Note that many columns are simply repeats of header information, so see also the more verbose descriptions, above. Fields are entered as NULL when not available. There is one row per light curve. If a star was observed on multiple telescopes, there will be multiple rows for it. Column Format Unit Description LSPM_Name A15 - Lepine-Shara Proper Motion catalogue (LSPM) identifier (Lepine & Shara 2005). Gl/GJ A7 - Gliese (1969) or Gliese & Jahreiss (1979) catalogue of nearby stars number. LHS A5 - Luyten Half-Second catalogue number. rNLTT I5 - Revised New Luyten Two Tenths catalogue number (Salim & Gould 2003). HIP I6 - Hipparcos catalogue number. 2MASS A16 - 2MASS all-sky release identifier (should be prefixed with "2MASS J" to obtain full identifier). RAh I2 hr Right ascension and declination given as RAm I2 min space-separated base-60. Equinox J2000, RAs F6.3 sec epoch 2000.0. DEn A1 - DEd I2 deg DEm I2 am DEs F5.2 as PMRA F7.3 as/yr Absolute proper motions. See above for notes PMDEC F7.3 as/yr regarding the RA proper motions. Pi F6.4 as Trigonometric parallax, uncertainty, and ADS e_Pi F6.4 as bibliographic code for reference. Ref A19 - SpT A6 - Optical spectral type and reference. Ref A19 - B_J F4.1 mag Photographic magnitudes. R_F F4.1 mag I_N F4.1 mag J F6.3 mag Infrared magnitudes from 2MASS. Upper limits H F6.3 mag are denoted by a leading < symbol, and one Ks F6.3 mag less decimal place is printed to make space. Mass F4.2 Msol Estimated stellar mass and radius. Rad. F4.2 Rsol S A1 - Source of the distance modulus estimate used to obtain the mass and radius estimates. See above. Tel. A5 - MEarth telescope used to observe the light curve. Exptime A16 sec List of exposure times used in the light curve. If there were multiple exposure times, they are all given, ordered from shortest to longest and separated by commas. Nmeas I5 - Number of measurements (exposures) in the light curve. Nni I3 - Number of separate nights on which observations were obtained. Start I8 date Nights on which the first and last exposures End I8 date in the light curve were taken. Uses the common "night of" notation written yyyymmdd - the date on which the start of the night occurred, in local time.