There are some scripts available which can make easier INGRID linearity measurements:

1. Preliminaries: lamp setup & filter selection
   
   • If possible, put the telescope in an appropriated position for taking flat fields (e.g.: DOME 36, AZI 298, El 45). Remember that if INGRID is in GRACE, it will be necessary to set the tertiary mirror to the GRACE focal station while the telescope is at zenith.
   • Open the primary mirror petals.
   • Turn on the flat field lamp, which is normally on the WHT dome walkway.
   • Find the combination of filters-position of the lamp, which gives you some ~500-2000 ADU/s at the INGRID detector. If itīs too bright, it might be necessary to point the flat field lamp to the floor or even place it on the observing floor, but always try to keep an uniform illumination at the telescope pointing.

   In case the telescope can not be moved, it is also possible to check the linearity by using the thermal background emission of the dome and telescope as a light source in the K band. Just try to find the k-band filter which gives you the appropriated count rate (possibly k, ks, kcont, brg, ...). If there are strong ambient temperature changes during the tests, the data could be compromised, but we do not have a constant source of illumination when using the lamp either. We will take our data in a specific way (described below) in order to avoid illumination change effects while the linearity data is being taken.

   The configuration of telescope position, dome illumination and filter must be kept fixed through the linearity test, otherwise the data will be useless.

2. Get the darks and flats
   
   • On taurus, copy the shell script linearity_in (available here) in a suitable place (e.g. /home/whtobs/scripts/).
     This script will first take a series of darks with increasing integration times, then a series of flats with the same integration times and the selected filter in the beam, and finally another series of darks.
     Two flats will be taken for each integration time. The script will take two sub-series of darks: the exposure time will be increasing through the first and decreasing though the second. Moreover, the two series will be mixed up. This will average out any possible influences of the lamp intensity or temperature variations on the general trend of the linearity curve.
   • Give execution permission to the script: chmod u+x linearity_in
   • Edit the script and update the factor corresponding to the filter selection (first lines of the script). This will ensure that we take images over the whole dynamic range of the INGRID array.
     Let count_rate be the number of ADUs per second that you get with the selected filter, then the factor must be the one that makes:
     22 x factor x count_rate ~ 30000
   • Run the script; i.e. type: SYS> /home/whtobs/scripts/linerity_in <filter>
     where <filter> must be replaced by the filter you have selected for doing the test (e.g. SYS> /home/whtobs/scripts/linerity_in brg).
     Depending on the count rate, the script will typically take from ~15 to 40 min to finish.

3. Analyze the results
   
   • Move to the directory where the data has been stored (e.g. cd /obsdata/whtc/20040120).
   • Create a list containing the file names of all the images (darks and flats) taken in the linearity test (in the order in which they were taken); e.g. type:
     ls r655*.fit > list,
     then edit the file list and delete any file names which does not belong to the linearity data set.
   • Open an IRAF session (open an xgterm and then type ncl or cl)
   • Change to the directory where you have the data and the list of files:
     cl> cd /obsdata/whtc/20040120
   • Start the INGRID quicklook package: cl>ingrid_ql
   • Type: in>epar ilinearity, and update the parameters: e.g. files = @list, outtab=linearity_20040120, section = [*,*] or [150:450,150:450], or any other image area for which you want to obtain the results.
     Type :q after finishing updating and wait a few minutes...
   • An xgterm will appear which shows your linearity data. There are many useful options in this task to examine the results (e.g. 'x' plot gain versus ADU, 'f' flux versus ADU, etc).. Type ? for a list.
   • Compare with previous measuremnets and report and document any important changes. Good luck!