CAPSCam Users Manual

CAPSCam calibration

Dark Frames
The dark contribution is negligible, even to 600 sec exposures. However there is some significant (but stable) bias component, especially using the Guide Window.

Dome Flats
For Full Frames (no GW), flat fields can be taken in the standard way, e.g., 10 sec exposures with the lamp at 300 V yields about 10000 counts per pixel.

Dome Flats and the Shutter
If Dome Flats are taken using the fast GW reading modes (exptime < 1 s), a significant structure is apparent in the images due to the finite shutter open/close time. This generates a six-petal flat-like structure that can be calibrated out as a flat effect, but it strongly depends on the GW exposure time. The six-petal shape can also be seen in some images on the sky. Fully removing this structure requires a number of additional calibration frames.

Taking Data
Center the GW on the target star at the same position at the center of the image and with the same size, i.e., at 1024 1024 64 64. If this is not possible (e.g., you need a larger GW, or have to move very bright object out of the FOV), you will need Dark Frames for each GW configuration. Putting the GW at the central position minimizes the astrometric effect of the petal like structure, even if it cannot be totally removed.

The recommended set of calibrations per night of observation would then be:

Full Frame mode:
Dark Frame FF: 10 x 10 sec exposure
Dome Flat FF: 10 x 10 sec exposure (lamp @ 300 V)

Guide Window mode:
Dome Flat FF+GW: 10 x 10sec FF/ 0.2 sec GW (lamp @ 300 V)
Dome Flat FF+GW: 10 x 10sec FF/ 1.0 sec GW (lamp @ 300 V)

Guide Window mode with all different GW positions used during the night**
Dark Frame FF+GW: 10 x 10sec FF/ 0.2 sec GW

* Can be done at the end of the night
** Required if the GW exposure time is < 1 sec (under investigation). The GW can be positioned at any place, but the best location is 1024 1024 64 64. IF THE PETAL-LIKE STRUCTURE DOES NOT APPEAR, IT PROBABLY MEANS THAT THE SHUTTER IS NOT WORKING.