DeVeny Collimator Focus Calculator

Status: Completed 2021-11-19

Overview

The internal optics of the DeVeny Spectrograph are focused by pistoning the collimator mirror (a separate task from focusing the telescope onto the spectrograph). The DeVeny LOUI includes a tab for performing a focus sequence (images of arc lines interleaved with collimator focus moves). The DeVeny Collimator Focus Estimator tool is used to estimate the focus value and sequence range needed.

Once the sequence of focus images has been collected, this tool will analyze the arc lines in each frame to calculate the optimal position at which you should set the collimator. See Usage for details of the command-line options for use with the tool. When run, the following processing steps are applied to the focus frames:

  1. All frames are read in, and the middle frame is inspected to find appropriate spectral lines for analysis. (This frame is what is shown in Fig. 5, theoretically the frame closest to focus, especially if deveny_collfocus was used to generate the sequence range.)

  2. The marked lines are then identified in all the other frames, and the FWHM for each is computed using scipy.signal processing routines.

  3. The FWHM as a function of collimator position is computed for each line identified, and a parabola is fit to the plot (see Fig. 7).

  4. For arc each line, the minimum (red lines in Fig. 7) and optimal (blue lines) focus values are computed. See the DeVeny manual for a discussion of astigmatism and why the two values are not the same.

  5. Finally, the optimal focus value is plotted as a function of CCD column position (Fig. 6) to find the overall optimal collimator focus value to use.

An example terminal output corresponding to the figures is shown below:

================================================================================
  DeVeny Collimator Focus Calculator

 Processing center focus image /deveny/20210520a/20210520.0177.fits...
  Background level: 2351.1   Detection threshold level: 2451.1
 Number of lines found: 39

 Processing arc images...
100%|████████████████████████████████████████| 10/10 [00:00<00:00, 40.87frame/s]

  Median value of all linewidths: 3.03 pix
================================================================================
*** Recommended (Median) Optimal Focus Position: 10.49 mm
*** Note: Current Mount Temperature is: 18.0ºC

  Plots have been saved to: pyfocus.20210520.040659.pdf

In both the terminal output and the plots, the current mount temperature is noted so that the observer can judge if the collimator focus needs to be revisited during the night based on temperature changes. The collimator focus temperature term is approximately \(-0.08~{\rm mm/C^{\circ}}\), meaning that a temperature decrease of \(5 {\rm~C^{\circ}}\) will cause a \(0.4~{\rm mm}\) increase in the optimal collimator focus position.

Arc line plot

Fig. 5 – Example of page 1 of the output PDF, arc line identification plot.

Optimal focus versus line position

Fig. 6 – Example of page 2 of the output PDF, optimal focus versus line position plot.

Individual line focus curves

Fig. 7 – Example of page 3 of the output PDF, individual line focus curves.

Usage

The script usage can be displayed by calling the script with the -h option:

$ dfocus -h
usage: dfocus [-h] [--flog FLOG] [--thresh THRESH] [--nodisplay]

DeVeny Collimator Focus Calculator

options:
  -h, --help       show this help message and exit
  --flog FLOG      focus log to use (default: last)
  --thresh THRESH  threshold for line detection (default: 100.0)
  --nodisplay      DO NOT launch Preview.app to display plots (default: False)

This command-line tool must be invoked from the /deveny/<UTDATE>/focus directory on the observer machine (dct-obs1 / dct-obs2) so that it can find the focus index files created by the DeVeny LOUI. To run the tool on the most recent focus sequence taken, simply run the routine with no options.

If more than one focus sequence was taken, the tool can analyze a particular sequence by using the --flog optional input, where the file log has the form deveny_focus.<UTDATE>.<UTTIME>. For instance, to produce the plots in Fig. 5 - Fig. 7, you would:

$ cd /deveny/20210520/focus
$ dfocus --flog deveny_focus.20210520.040659

If you want to increase the threshold for detected lines above the default 100 DN over background (to decrease the number of lines detected), use the --thresh optional input.

By default, the tool tries to launch Apple’s Preview App (the observer machines at LDT are iMacs) to display the plots shown in Fig. 5 - Fig. 7. If running on macOS, and you desire to not display the plots, use the --nodisplay option. If this tool is being run on a different operating system, it will simply bypass this step.