WFI Calibration

Calibration Procedures

This page is designed to give you a bit more information on how you create and execute Calibration observing blocks. See the notes of the Standard Calibration Plan to determine whether you need to create your own OBs for Service Mode observations, or whether your calibrations are included as part of the Standard Calibration Plan.

Biases and Darks: WFI_cal_Darks

Bias and dark current frames are taken with the same template
  • exposure time = 0 seconds for biases
  • exposure time > 0 seconds for darks
Typically one needs about 10 biases/night. Darks are not really needed.
Darks and Biases are generally taken in the afternoon, or left running after a night of observations, with a dark dome and the mirror cover closed.

The WFI CCD page has more information and a history describing the RON, Gain and Bias levels for each of the WFI chips.

Click here to download a standard Bias OB. You can then import this directly into your P2PP.

Super Flatfields

Where available, superflats, made from the actual science images, provide the best flat fielding. However, it will not be possible to make super flats in the following cases, and other types of flats will have to be taken instead :

  • highly crowded science frames
  • un-dithered images of a single field
  • when targets in different frames are positioned on the same area of the CCD
  • bands in which fringing is significant (R, I and z bands)
  • bands in which the sky background is low (very blue bands, e.g. U band, and narrow bands)
  • only a few science images are available for constructing a super flat
Since super flats are made from the science frames themselves, they will not be discussed further.

Sky Flatfields:WFI_img_cal_SkyFlat

Twilight sky flats provide the next best alternative for flatfielding your data. These are generally taken in the evening and/or morning twilight periods, depending on the condition of the sky. For evening twilight, the telescope is pointed to a blank field ~1h East of zenith, for morning twilight it is pointed ~1h West of Zenith. In service Mode at least, only 3 flatfields in each filter are taken. This is the bare minimum in order to make a decent flatfield but also allows us to maximise the number of filters we can calibrate each night.

Within the template, one needs to define:

  • Filter: Which filter you wish to calibrate
  • Number of Exposures: How many flatfields you would like in this filter
      Note: this is capped at 3/filter in Service Mode
  • Intensity Level: How many counts do you want in your flatfields?
      Note: default is 20000 counts for Service Mode observations
Once started, the procedure:
  • takes a 1 second exposure and reads out 1 of the 8 WFI chips
  • calculates the peak count level in that exposure
  • calculates the exposure time necessary to reach the required number of counts
    • If the required time is < 0.5 seconds, it will tell you the sky is too bright and ask whether you want to abort the OB or keep trying
    • If the required time is > 0.5 seconds, it will display the exposure time it plans to use and asks whether you wish to continue or abort
  • takes the requested number of exposures, offsetting the telescope in between each one so that the stars do not lie on the same pixels and can be averaged out
  • telescope returns to original position

One has 15 - 30 minutes in each of the twilights to collect Skyflats. This is enough time to complete 3 to 5 filters (depending on the season and the specific filters you are using).

Click here to download a standard Skyflat OB (for the V/89 filter). You can then import this directly into your P2PP, duplicate it and edit the duplicates to make Skyflats for other filters.

Dome Flatfields: WFI_img_cal_DomeFlat

Dome flats are the insurance frames, to be used when all else fails. Domeflats are taken during the afternoon calibration session between 4.30 PM (local time) and sunset. The illumination source is a halogen lamp whose brightness is controlled by applying different voltages. Each filter has its own voltage setting (which the support astronomers and telescope operators know) in order to achieve the required flat field level within a sensible amount of time.

The template for Domeflats is almost the same as for Skyflats. Within the template, one needs to define:

  • Filter: Which filter you wish to calibrate
  • Number of Exposures: How many flatfields you would like in this filter
      Note: this is capped at 3/filter in Service Mode
  • Intensity Level: How many counts do you want in your flatfields?
      Note: default is 20000 counts for Service Mode observations
Once started, the procedure:
  • takes a 1 second exposure and reads out 1 of the 8 WFI chips
  • calculates the peak count level in that exposure
  • calculates the exposure time necessary to reach the required number of counts
  • takes the requested number of exposures
We recommend that you create separate Domeflat OBs for each filter you intend to use. This allows us to adjust the voltage of the lamp (which requires a drive down to the telescope!) and change the sequence of domeflats based on the current voltage setting.

Click here to download a standard Domeflat OB (for the V/89 filter). You can then import this directly into your P2PP, duplicate it and edit the duplicates to make Domeflats for other filters.

 
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