AstrOmatic forum
Skymaker for IR-filters - Printable Version

+- AstrOmatic forum (
+-- Forum: AstrOmatic software (/forumdisplay.php?fid=3)
+--- Forum: SkyMaker (/forumdisplay.php?fid=19)
+--- Thread: Skymaker for IR-filters (/showthread.php?tid=495)

Skymaker for IR-filters - Jennifer Piel - 11-07-2009 19:07

Hi all,

currently, I am trying to create a fits image with Skymaker according to an exposure of a galaxy cluster taken with HAWK-I@UT4 at Paranal in the Ks-filter. The parameters, Skymaker requires for the simulation are as follows:

CWL=2.146 microns

My problem is, that due to the high skybackground, skymaker seems not able to create a fits image, as all pixels are saturated (in contrast to the real image). I tried to enlarge FULLWELL for several orders of magnitude, but nevertheless, only the brightest simulated objects such as large stars are visible.
Do you have any suggestions what I need to change in the parameters or is skymaker not able to produce fits-images in the IR?


Jennifer Piel.

RE: Skymaker for IR-filters - Emmanuel Bertin - 11-07-2009 20:55

Hi Jennifer,

thanks for posting your question on the forum. SkyMaker has no problem producing images in the IR (actually, simulating NIR images was the main motivation for starting writing SkyMaker at the time). If the background surface brightness, the gain, the exposure time and the magnitude zero-point ("per second of exposure") are correct, then the result should give you a very accurate idea of what the image depth would be with a real instrument.
To get rid of the full well limit, you might either
  • set it to 0 (equivalent to infinite, or a very high number as you did)
  • or do as if the exposures were split and averaged, which has the advantage of allowing realistic values for the well capacity and the saturation level: for instance to simulate a 1 hour exposure as an average of 120 times 30-seconds exposures for an instrument with a gain of 1e-/ADU you just need to set:
    GAIN 120
When measuring the completeness magnitude limit afterwards, remember to set MAG_ZEROPOINT in SExtractor to the original ZP "per second" + 2.5 times log_10 exposure time (30s in the example above).
- Emmanuel.

RE: Skymaker for IR-filters - Jennifer Piel - 11-11-2009 11:09

Hi Emmanuel,

thanks for your fast answer. The image really improved with your suggestions. But the best result I achieved, when I changed the magnitude system from AB to Vega in the run of Stuff I did before for creating a catalogue. I was not aware, that actually this filter of my data set has a different magnitude system, than the other images, leading me to a question I'm going to pose in the Stuff forum.