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IMACS updates

400l/mm grism:

A new grism is available for use with the f/2 camera. Due to a manufacturing error, the ruling on the 400/mm grism is slightly rotated with respect to the prism. This results in a leftward shift of the dispersed images. Observers should be aware that targets more than approximately 12.3 arcmin left of the center (as viewed in the QL tool or in maskgen) will be shifted off of the detector mosaic.

IMACS readout noise modes:

As of July 25th the old "slow" readout mode in IMACS has been removed and the previously known "snail" readout mode has been renamed as "slow". Currently both cameras have only two readout modes: Fast and Slow (which corresponds to previous "snail"). This is part of an effort to only offer configurations with stable noise levels. 


Minimum Exposure times:

In order to avoid shutter delay effects on flux counts, exposure times should be longer than 2sec for both cameras.


Dewar swap: 

As of Dec. 20, 2017 the Mosaic3 dewar is mounted on the f/2 and the Mosaic2 dewar is mounted on the f/4. 

Photometric zero points and grism/grating efficiencies after the swap can be found in the manual. Noise and gain values also updated for both cameras. 

Flexure Control: 

Following change of disperser element, the IMACS piezo-stages are meant to control the flexure of the system by moving the array based on the current rotator position of the instrument.  Prior to initiating an exposure, one should be sure that there are no current "flexure control" error messages on the IMACS message GUI window (usually in the upper left corner of the screen). Sometimes following a change of disperser or a slew to a very different rotator angle, the system takes some time to recover the controlled position. If the system does not recover on its own, then the observer can manually set desired values on the HardHat GUI window (brought up from IMACS GUI -> Options->). Usually just toggling the Flexure Control button ON/OFF fixes the issue. 

It is important that Flexure Control is ON and properly applied before taking images (slits, masks) to measure positions for aligning sources. When positions are measured on images obtained when flexure is not properly controlled, measured detector positions for mask features will vary.  Small flexure offsets are not worrisome but larger offsets MUST be corrected first.  The flexure offsets are noted in microns and recall that the pixels are 15 microns across. 

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