Dome/Sky Flat Fields - Imaging

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Flat Field Images

Dome and Sky Flat frames in the imaging mode are controlled by two different templates which differ in only one aspect: the telescope pointing remains fixed (on the screen) for dome flats while it is jittered between sky flat frames (by a user defined amount) to ensure that any bright star in the field is not imaged on the same pixels on the CCD from one frame to another. The telescope returns to the intial position after each sequence of sky flat frames.

One only needs to define the required number of counts in the template. The procedure takes a 1 second exposure and calculates the peak counts (after subtracting the bias and excluding pathological pixels). It then calculates the exposure necessary to achieve the required counts. If the required time is less than 1 second or more than 300 seconds the procedure returns an error and aborts the sequence. However the exposure time varies across the CCD by about 10 msec due to the non-zero time taken by the shutter to close. We recommend that observers take frames which have been exposed for at least 3-5 seconds.

You find here a discussion of the different types of imaging flats.

Dome Flats

Dome flats must be completed during the afternoon calibration session between 4.30 PM (local time) and sunset.
The illumination source consists of an array of lamps of staggered brightness values. Different filter/binning combinations require different settings of the lamps to achieve the required flat field level within a sensible amount of time (5 - 75 seconds). One can string together all the templates, with different filters and binning modes, in the same OB. However it would be more practical if one  arranges the templates in an ascending (or descending) sequence of the required lamp brightness.  Narrow band  and U-band filters require the brightest illumination while R- and i-bands require the least.

Note that for most instrument set ups the required lamp and exposure time settings are already stored in a look up table, and so are completely automatic for the user. Only when using unusual combinations of optical elements, or asking for a level other than 40,000 counts will the times and lamp voltages have to be adjusted manually.

jP2PP Panel with Typical Parameters   (click for panel)

The above example shows an OB comprising a sequence of

  • U-band dome flat  frames at  40,000 ADU  level with normal readout and  2x2 binning  followed by
  • R-band dome flat  frames at  40,000 ADU  level with fast readout and  2x2 binning  and finally
  • i-band dome flat  frames at  40,000 ADU  level with normal readout and  1x1 binning.
The FITS image output by the template is named  EFOSC_FlatIma.#.fits

Parameter Values   One does not have to (in fact, should not) change the value of any parameter other than:

  • Filter:  See the Filter Set for a list of the available choices
  • Intensity level: 40,000 ADU is both practical and sufficient. This level also ensures that even the brighter pixels  on the CCD do not saturate.
  • CCD readout speed: normal , fast  or  slow.
  • CCD binning: 2x2  or  1x1
  • Number of exposures: 5  frames of the above recommended 40,000 ADU level is sufficient

Sky Flats

Skyflats are taken with the telescope pointed towards an empty field. The co-ordinates of the empty fields are stored in the telescope database and the operator will leave the telescope tracking one of these fields after the dome is opened a little before sunset.

One has about 15-30 minutes during each of evening and morning twilights and this may be insufficient for getting all the required sky flats.In particular,narrow-band filters  and/or   unbinned frames  and/or  slow readouts require long exposures. For e.g, a  B-band unbinned sky flat sequence (5 frames of 40,000 counts each) hogs the entire twilight period.

During the evening twilight the temporal sequence of filters should be narrow-band filters followed by broad-band filters and within these groups the bluer filters should be observed earlier. The sequence should be reversed during the morning twilight.

Depending on the filter and binning used one can (should) start observing from  sunset (narrow band filters, unbinned blue filters). The sky will be too bright for others for another 10-15 minutes.

Only the required intensity level has to be specified. The procedure takes a test exposure and calculates the duration of subsequent exposures from the algorithm of Tyson & Gal (1993, AJ, 105, 1206).

jP2PP Panel with Typical Parameters   (click for panel)

The above example shows an OB comprising a sequence of

  • 5 U-band  sky flat  frames at 40,000 ADU  level  with  normal readout  and  2x2 binning  followed by
  • 5 B-band  sky flat  frames at  30,000 ADU  level  with  normal readout  and  2x2 binning  followed by
  • 5 V-band  sky flat  frames at  20,000 ADU  level  with  normal readout  and  2x2 binning  followed by
  • 5 R-band  sky flat  frames at  30,000 ADU  level  with  normal readout  and  2x2 binning  and finally
  • 5 i-band  sky flat  frames at  20,000 ADU  level  with  normal readout  and  2x2 binning
The above OB can be fully executed during a twilight period most of the year. At times one may have to reduce the R band counts to 20,000 ADU as well.

The FITS image output by the template is named  EFOSC_FlatSkyIma.#.fits

As an example the following image is given, which shows a V-band sky flat at 30,000 ADU level with normal readout and 2x2 binning. Click on the image to get a larger version:

Parameter Values   One does not have to (in fact, should not) change the value of any parameter other than :

  • Filter:  See the Filter Set for a list of the available choices
  • Intensity level:  20,000 -- 40,000. A value greater than 40,000 may lead to the saturation of some of the brighter pixels on the CCD.  The appropriate value from the above range  will have to be chosen taking into consideration the duration of twilight and the number of sets of flats (different filters etc) required.
  • CCD readout speed: normalfast  or  slow .
  • CCD binning: 2x2  or  1x1
  • Number of exposures: 5   exposure  in the above ADU range are sufficient.
  • Telescope offset: The default values are fine but the observer may change these values according to his/her tastes
 
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