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IMACS updates

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. 

Preliminary photometric zero points can be found in the manual. Efficiencies do not correspond to the new configuration and will be updated as soon as possible. Also noise on Mosaic3 is higher by a factor of ~1.5, while for Mosaic2 remains at the same level. 



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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