PNS MANUAL: Taking dome flats
Taking dome flats (updated December 2009)
There are lamps in the WHT dome for dome flats, which can be switched on from the dome.
The adopted procedure is:
- Point the telescope at zenith position
- Make sure CCDs readout parameters are OK (window, readout speed).
- Check the the mirror is OUT.
- Check that autoguide is out of the FOV: AG Radial>10000 on the control
- From the PN.S laptop select MASK OPEN and set the exposure time to 30 sec
- Switch on the flat lamp, minimum power, all three additional lamps on to
avoid light gradients.
- From the DAS type: expose 30 'Dome flat' . As the CCDs start integrate, click EXPOSE on the laptop
- Repeat as often as desired for different rotator positions (suggested: 0,
45 and 90 degrees). In this way the error due to correct for falt fielding
a science image taken at a different PA from (0, 45, 90) is less than a few
- Domeflats cannot be done for the Halpha (lights too bright for unsaturated
Taking dome flats (OLD NOTES)
There is an incandescent lamp in the WHT dome for dome flats. We found that pointing the
telescope at 45 degrees elevation at a blank bit of the dome, and illuminating this from below with the lamp, gave rather uniform illumination, certainly sufficient for measuring the pixel-to-pixel variations in the flat field.
As the shutter is rather slow, the exposure times have to be controlled by swithing the lamps on and off while the shutter is fully open, then closing the shutter, and reading out.
This requires a fit person, a lazy person, and two synchronized watches.
The procedure is as follows. It assumes a 20-sec integration time.
If walky-talkies are available a less energetic approach is possible!
- Make sure CCD readout parameters are OK (window, readout speed).
- Set up the dome flat lamp as described above, and switch it off
- Lazy person sits in front of controls. Energetic person put on sneakers.
Both synchronize watches and agree on lamp switchon time (1 minute into the future is OK).
- Energetic person goes to lamp in dome.
- A little (< 20 sec) before agreed time, lazy person issues expose 60 "Dome flat" on DAS, and expose 60 on PNS PC.
- At agreed time, energetic person switches on dome lamp, for a total of 20 secs, then switches it off again.
- Repeat from step 2 as often as desired.
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Last updated 19-07-2001
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