DeVeny Collimator Focus Estimator

Status: Completed 2023-07-31

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 purpose of this GUI tool is for estimating the values you should input into the LOUI.

For a discussion of the physics behind the collimator focus procedure (including the origin of the equation below), see the DeVeny User Manual. An empirical analysis of collimator focus values from data taken in 2017-2020 yield the approximate relationship:

\[\begin{split}\begin{aligned} \mathrm{collimator~focus}~\approx&~11.0 - 0.08~T_{\rm m} \mathrm{(^{\circ} C)} - 0.14~(\mathrm{tilt} - 25^{\circ}) + 0.7~\delta_{\rm filter}~~~\mathrm{mm} \\ \delta_{\rm filter} &= \begin{cases} \mbox{0,} & \mbox{if no order-blocking filter (REARFILT = Clear (C))} \\ \mbox{1,} & \mbox{if using an order-blocking filter (e.g., GG420)} \end{cases} \end{aligned}\end{split}\]

where \(T_{\rm m}\) is the mount temperature and tilt is the grating tilt angle. The RMS of the fit is \(\sim 0.6\) mm. This equation is coded into the GUI tool.

Unlike other focus procedures at LDT, the DeVeny LOUI focus sequence script does not take the expected focus value, but rather one end of the range, along with the step size and number of steps. This tool is designed to assist you in constructing a range that centers on the value computed from the equation above to most efficiently complete the focus procedure.

After the images have been taken, use the DeVeny Collimator Focus Calculator program to analyze the images and compute the appropriate focus position.

Usage

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

$ deveny_collfocus -h
usage: deveny_collfocus [-h]

DeVeny Collimator Focus Sequence Estimator

options:
  -h, --help  show this help message and exit

When the application launches, a GUI window will appear as in Fig. 3.

Fig. 3 – The `deveny_collfocus` GUI at startup.

If the application is launched from one of the observer computers at LDT (dct-obs1 or dct-obs2), it will have the option to pull the current values for the mount temperature and grating tilt angle from the broker. The current grating angle should always available on the broker, but the TO needs to have the their control software running before the mount temperature will be available (and it may still take up to a minute or so for the appropriate broker topic to update). Load the current values from the broker by pressing the “Get Values from Broker” button.

Whether you load the value from the broker or use this tool at home, you may still manually enter the mount temperature and grating tilt angle in the GUI windows. You will always need to select the rear filter setting you are using.

When you click “Compute”, the bottom half of the GUI is populated with the estimated focus value from the equation above and values to enter into the DeVeny LOUI focus sequence tab. See Fig. 4.

GUI after clicking "Compute" — Fig. 4 – The `deveny_collfocus` GUI after clicking “Compute”. The estimated focus value (with uncertainty) is printed, along with suggested values for use with the LOUI Focus Sequence tab.

Observers should note that the lower limit on the collimator focus motorized stage is 7.75 mm. In warm weather and/or for large grating tilt angles, the optimal focus may lie beyond this limit. Set the collimator focus value at the lower limit and press on.