Re: Flat frame gradient removal and optical axis
Posted by
karenacollins on
Oct 20, 2013; 4:58pm
URL: http://astroimagej.170.s1.nabble.com/Flat-frame-gradient-removal-and-optical-axis-tp116p117.html
Gradient removal can be disabled by
deselecting Data Processor->Preferences->"Remove gradient
from calibrated raw flats when creating master flat". This should
solve the problem.
Karen
On 10/20/2013 12:15 PM, froesccn [via AstroImageJ] wrote:
It appears the AstroImageJ data processor plugin runs
a gradient removal when generating the master flat. I noticed this
when processing a data set that had been cropped from a much
larger field of view. The center of vignetting was actually off
axis, but forcibly shifted to the image center by the data
processor. It would seem that this makes sense only when the
optical axis is actually in the center of the image, otherwise it
can introduce almost arbitrarily large errors.
I wondered whether the assumption is hard-coded or whether there
is a FITS header keyword that should be set to indicate the
optical axis. If not it might be useful to make the gradient
removal optional, and/or to have a warning when the estimated
gradient is much larger than what could be expected from a
reasonable flat frame. The cropping use case is probably exotic,
but I suppose there could also be some cameras with multi-chip
configurations where this might be a problem.
BTW I don't need an urgent fix, just curious. I'm processing data
using my own code for my study and used AstroImageJ results as a
baseline for comparison.