The retrofit of the MODS2 AGw optics in MODS1, in early October 2016, has made observing with MODS2 almost the same as observing with MODS1. This page describes the effect of the AGw retrofit on observation planning, and the remaining differences in both the command set and in the calibration scripts, where exposure times have been adjusted to account for the differences (most likely due to age) between the intensities of the lamps in MODS1 and MODS2


New AGw optics:

In the new MODS1 AGw optics, as in MODS2, the beamsplitter, which directs 50% of the light to the wavefront sensor, covers the entire guide camera field of view, so there is no need to offset the guide star up to the small beamsplitter cube as before, and it will instead remain centered in the field of the guide camera. The upshot of this is that the shadow of the pickoff mirror is now centered on the guide star and will vignette an ~43 arcsec region above and below the guide star. PIs and observers should download the new modsView, v2.1.2, ( and run it with the --shadow; option to confirm that guide stars in older acquisition scripts will not vignette any science targets.



This side-by-side comparison of the output of modsView for the old MODS1 AGw (left) and for the new MODS1 and the MODS2 AGw units (right) for the same target and guide star illustrates the difference. The 6 arcmin x 6 arcmin science camera field of view is outlined by the green box, the 5 arcmin x 5 arcmin cyan box that is displaced downwards depicts the guide patrol field. The guide star is circled in cyan, and the yellow rectangles sketch the shadow of the guide probe — the large upper rectangle (~150″ wide and 86″ high), which surrounds the square guide camera field of view, depicts the shadow of the pickoff mirror, and the lower one (extending off the picture) depicts the arm. Note how the large upper rectangle is centered up/down on the guide star for the new AGw units (right), but is displaced down for the old MODS1 AGw (left).

The bottom line is that MOS observers should be careful about guide stars that are close to the science camera field of view, but on the other hand, they can now select stars near the bottom edge of the patrol field without risk of falling out of collimation.


MODS2 Command Set: The commands used to check the status of or start/stop services and display the latest images and all work in the same way as for MODS1, but are named differently. Below is a table of the equivalent commands for MODS1 and MODS2, with a brief description of their function in the 3rd column.


MODS1 MODS2 Description
mods1 mods2 display status or stop/start services
mods2 status
mods2 start gui
mods2 stop gui
acqMODS –mods1 acqMODS –mods2 run an acq script
execMODS –mods1 execMODS –mods2 run an obs,img, cal or pro script
acqBinoMODS script1.acq [script2.acq] run script1.acq on both MODS [or script1.acq on MODS1 and script2.acq on MODS2]
execBinoMODS script1.obs [script2.obs] run script1.obs on both MODS [or script1.obs on MODS1 and script2.obs on MODS2]
modsDisp mods2Disp automatically display last MODS blue and red channel images


MODS2 Calibration Scripts:

The lamp settings and exposure times used by the internal calibrations scripts in ~modseng/modsScripts/modsCalib/ were determined for MODS1, and these have been found to yield count levels that are too high in some cases with MODS2. This has necessitated the creation of a set of internal calibration scripts specifically for MODS2. These MODS2 scripts may be found on the obsN machines in ~modseng/modsScripts/mods2Calib/. Dual grating mode scripts are those which end in (The scripts which end with the were created to use only the red-channel of MODS2, since only it was available in November 2015 when problems with the SX AdSec forced it into use.) The MODS1 scripts are still in ~modseng/modsScripts/modsCalib/.


Still the calibration script generated by mms2obs would need to be edited to reduce the exposure time of the Ne+Hg[Ar] comparison lamp from 1 to 0.5 sec. These changes are made in the script (and which should be copied to your working directory and edited only to replace the mask ID with the mask ID for your specific mask.


MODS2 Script Name Description of Calibration Script internal dual imaging flats at riz Comparison Lamps in Dual Grating mode Thru-Mask Flats and Comparison Lamps in Dual Grating mode Pixel flats in Dual Grating mode 0.6″-1.2″ Slit flats in Dual Grating mode 5″ Slit flat in Dual Grating mode