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Up: CGS4 Handbook 

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CGS4 uses three easily obtainable array readout sizes. The default is the 256 x 178 array which is used for most observing modes and has a minimum exposure time of 0.12 seconds.  It is named CGS4_long (where long indicates use with the long focal length camera).

The second array is the 256 x 84 array (CGS4_short), which also has a minimum exposure of 0.12 seconds. This is used when the short focal length camera is installed in CGS4, but can also easily be used with the long camera if desired.

However, there are occasions when a shorter exposure is required, for instance when observing extremely bright stars or when working in the M band. Another, much smaller, subarray exists with size 256 x 32 (CGS4_5mic) and offers a minimum exposure time of 0.016 seconds.

The following explains how to load and use the subarray from within the CGS4 aquisition software:
 

Selecting the subarray

1. From within the Spectroscopy  menu , choose Set_Alice using the  cursor and <Enter> key.
   
   
2. Select Set_readarea_&_type using the cursor keys and <Enter>.
   
   
3. Select CGS4_5mic as the readout area, and Deepwell as the readout type using the cursor

keys, and then use PF1 twice to exit back to the Spectroscopy menu.


4. Take a bias using Set_Single_Observation (you must take a new bias frame in order for bias subtraction to work with the subarray). Remember to take a new flat and arc as well at the wavelength setting of your intended observations, as the DR will refuse to use flats and arcs taken with a different readout size.
   
   
5. In the data reduction software, you need to specify fpa46_5mic as the bad pixel mask.

Using the subarray

1. Because the subarray is not centred at the same position as the larger arrays, the peakup row needs to be changed. Currently, the row to use when observing with the subarray is 128. This translates to row 18 on the data reduction side. Therefore you need to change the plotting rows in CGS4DR if you wish to display your extracted spectra.
   
   
2. The small size of the subarray precludes nodding large distances, and you will have to change the size of your nod (and chop size if you are chopping as well). You have two choices. You can either change the nod size in the quad_slide exec you are currently using, or use a system exec called quad_slide_subarray. This latter exec uses a slide of  4.88 arcseconds, or 8 rows.  The 32 rows of the subarray correspond to 15.92 arcseconds on the sky.
   
   
3. To return to the larger array, repeat the above process, except this time choose CGS4_long as the readout area, and Standard as the readout type. There is no need to take a new bias, as the data reduction software will automatically choose the correct bias you took when running the array_tests exec at the beginning of the night.

NB

Last Modification Date: 1997/1118 - Last Modification Author: Tom Kerr
Tom Kerr (tkerr)