IMACS – Updates

IMACS – Updates

  • Issues with the MACS collimator

UPDATE: as of April 15, 2022, the collimator has been serviced and the issue has been fixed.

Example of an IMACS image affected by the elongation artifact

On March 25th, 2022 we became aware that deep IMACS images were showing elongation of the stellar profile of saturated stars in most parts of the field. Based on pinhole images the effect also appears in not saturated images but it is more subtle. 

For the f/2 camera, the chips that are mostly affected are 1,2,3 and 4. The effect is worse in the edge of the mosaic, of the order of 10%, decreasing to 1% in the areas that are less affected. 

For the f/4 camera, all chips seem to be affected, with the intensity decreasing from right to left (chips 3,4,8,7). Here the ratio has a smaller gradient with an average of a 3% effect through the mosaic. Chips 1 and 6 seem to be the ones affected less. 

This artifact should have marginal effect on spectroscopic programs. 

The effect on imaging programs depends on the scientific goal. For example programs that rely heavily on PSF analysis and deconvolution might be unfeasible. 

The root cause of this issue has been traced back on the collimator optical elements and the instrument team is working on a solution. 

  • Flexure Control: 

As of October 2019, the IMACS GUI has been modified in order to keep flexure control OFF by default. It is possible to turn flexure control ON, but there are rotator angle ranges where the piezo-stages cannot control the flexure of the system properly, resulting to error messages. In the “control OFF” configuration, we expect an offset of <1.5pixels in the x direction and <1pixel in y for the f/2 camera, throughout the rotator range. 

If the observer notices larger motions, they should mention it in the relevant section of the night report. 

  • Chip#4 on f/2 camera: 

As of September 2019, we are experiencing intermittent loss of signal (0 counts) in chip#4 of the f/2 camera. We believe that this is due to problems inside the dewar. When this happens, currently the only options are to either change field (since by changing rotator angle, chip#4 might become responsive again), or keep obtaining data sacrificing the information on chip#4. 

  • 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. 

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