Hectochelle Focus

Using the focus selection in SPICE under standard ops, take 5 exposures in RV31 at 2x2 binning, 120s each, with a central focus position (say the most recent focus), and focus separation of +0.05 units.

In 2019, we installed a new focus analysis routine in iraf, called "chelle_focus." So, in the iraf window, type "chelle_focus". The program takes about 15s per exposure, and will produce a plot at the end, and list the best focus. Make sure you change the focus back to that value.

To run the focus analysis program on a different date, you must type:
chelle_focus "year.monthday".

The quotes help iraf to parse the date.
Below is the old method of focusing.
Focus chelle using RV31 1x1 comps. View images using "rotate 270" and "invert X and Y." This puts blue on the left, red on the right, and quicklook extraction goes from bottom to top.

At correct focus, PSFs dead center in the image should be tight vertical ovals with no tilt visible. If the images are tilted like "/" then the focus value needs to be increased.

At correct focus the PSFs in the upper left corner will display a pronounced apostrophe shape. Images in the lower left corner will have a pronounced smear.

There is quite a bit of curvature across the individual apertures and the Th-Ar line PSFs will be tilted in other locations on the image. The 3 positions to check (dead center, upper left, lower left) have very consistent and distinct PSFs, holds true for all filters. Focus value determined from RV31 is saved to file and other filters are offset from this.

Chelle spectrograph focus is temperature dependent but slow to change and is very stable over a period of weeks or more. A change of a half-step (0.05) is visually noticeable. The focus value typically changes by one step (0.1) every other month, but will change by a half-step during a run if there are large changes (10 degrees) in ambient temperature. Direct relationship, increasing temp will require an increase in focus value.

Check focus at start of run by visually inspecting RV31 1x1 comp image and bump focus if necessary. There is usually no need to run a full focus sequence at the start of a run. Spice has a built-in routine to take a series of indiviual exposures moving the focus between exposures. The focus analysis routines will work to analze a sequence (./FOCUS.sh X) and also load the images in ds9 (display_focus X) where X is the number of exposures taken.

Curiously there is a historical creep in the focus values. For all of 2009 the focus was around -2. In 2012 the value was around -1. In 2013 the value was around -0.75. In 2014 the focus value is around -0.5 In 2007 and 2008 the focus varied from +1 to -2, this was before the stage homing routines were re-defined. See plot at the bottom.

Here are some images showing PSF's at the 3 locations on the ccd to check. The default linear zscale hides the fine detail but highlights the tilt. Linear minmax scale shows more fine detail. Best focus in the center defined here as 0, with two steps of 0.1 in both directions shown.

Dead center RV31 1x1 ThAr rotate 270 invert XY:

Dead center zoomed in:

Upper left corner:

Upper left corner zoomed in:

Lower left corner:

Lower left corner zoomed in:

History of focus: