spectro-aras.com

I'm awaiting purchase of the Alpy Calibration module for my L-200, but
I was wondering in the meantime if it would be possible to use MaximDL's
filter, Flaten Background function to make a "synthetic flat" like that shown below:

http://www.astroimage.info/spectra/SpectraSYNflat.jpg

I know this would not be as good as making a "real" flat" but so far
my attempts (about a dozen times using different Halogen lights) have
not successfully yielded decent flats to apply to my spectra data.
Hope the calibration module solves this!

James Foster
Los Angeles, CA

Hi James,

What you show here is modelling the sky background. This is very different from a flat which corrects for localised variations in sensitivity of the sensor (eg due to dirt) and with wavelength (ie the way the QE of the sensor and the efficiency of the grating changes with wavelength)

You first correct the spectrum image, subtracting master bias and dark frames and dividing by the master flat frame. You then subtract the sky background (You could model the sky background in the way you have here but generally it is sufficient to take two regions above and below the spectrum and subtract the average background value in each column from all pixels in that column to produce a flat corrected, background subtracted spectrum image ready for digitising (binning)

Cheers
Robin

BTW what problems are you seeing with your current Halogen flats ? The internal flat lamps are convenient but for me the external lamp flats work at least as well.

Also is the ALPY flat lamp a good match for the L-200? Ideally the light source should look to the spectrograph like the beam from the scope. I believe the size of the white screen and the distance from the slit is chosen in the ALPY to match an f5 beam

Robin

To: Robin,

Thanks for the comments! I'll post some examples of the poorly made external flats and the masters made
by them. Photometric flats are so much easier to make than spectroscopic flats!

James