Introduction

Why a coma-correction tool for ESI? First, because image quality can benefit from it, and second, because we can. ESI and LRIS-R have some degree of internal coma in their optics. This means that, even though the telescope provides a coma-free focal plane, the detector focal plane has comatic images. However, for imaging the telescope's secondary can be tilted, adding the opposite sense of coma at the telescope focal plane, and canceling the instrument's internal coma. This delivers a nearly coma-free image to the detector.

Instrumental coma manifests itself as a rotator-dependent coma term in the analysis of images. Using the Mira procedure the secondary tilts that would correct for the coma at the detector is measured as a function of the physical rotator angle (not the angle with respect to the sky) and fit with a sine wave. This sine wave can then be used within the coma.tcl tool to predict the optimum secondary tilts at a given rotator angle.

Using coma.tcl

To use coma.tcl, log into kanaha as esieng and set your DISPLAY environment variable. Type "coma.tcl" and the interface should pop up on your screen.

The interface consists of four buttons:

Spectr Set to the last-saved values of the optimal secondary for zero coma at the telescope focal plane.
Imaging Read the current rotator angle and tilt the secondary to the appropriate position for zero coma at the detector.
Reload Read the current secondary tilt as the optimum secondary tilt for spectroscopic mode.
Help Provides some guidance in the use of the tool.

Note above that spectroscopy is best done at the zero-coma condition for the telescope focal plane. This is to get the most light into the slits in that focal plane. Imaging does not really care about the image quality in the telescope focal plane, only at the detector.

So procedurally the OA should run Mira, analyzing the data in the default mode, and sending the secondary tilts. Then click the "Reload" button on the coma.tcl GUI to load those values into the tool.

Imaging

Whenever the observer is about to do some science imaging, go to the field, set to the correct rotator angle, and only then click on the "Imaging" button.

As the physical drive angle changes during a set of observations, the coma correction will slowly get worse. At any time you can pause guiding, click the "Imaging" button, wait for the telescope pointing to correct, and then continue guiding. As a guideline, if the rotator drive angle changes by 60° without correction, the resulting coma will have a magnitude equal to that if you were using the spectroscopic correction rather than an imaging correction. If the rotator is 180° away from where the imaging correction was applied, the coma will be twice what it would be if you were using spectroscopic corrections. In stationary or vertical angle mode the rotator is not physically rotating, so you do not need to worry about adjusting the coma correction.

Spectroscopy

If the observer is about to start spectroscopy, again go to the field and set to the correct rotator angle, but then click the "Spectroscopy" button. Note that the spectroscopic coma correction is not dependent on rotator angle (either physical or sky), so in any rotator mode you do not need to change the tilts as long as you are doing spectroscopy.

Warning!

Pause guiding while tilting the secondary! The telescope control software will change the pointing to compensate for image motion caused by changing the secondary tilts, but it will take several seconds to do so. The guiding software will try to restart guiding on a blank field before the telescope software compensates. So you should pause guiding (which will retain the base x,y coordinates) until the star reappears at the correct place on the guider.

 

Shutting Down coma.tcl

Nothing fancy here, just quit the tool from the title-bar menu.