Flat reduction -- Ymax and Ymin

[EdWiley]

For my own benefit I am working on a document summarizing Christian's "How to Make a Masters" document. All is very clear except some options in the flats reduction. I did not find information about three options.

Vertical gain correction
Coordinate Ymax for validity zone
Coordinate Ymin for validity zone

Can someone point me to information? is the option critical? Are there defaults for Ymax and Ymin? I am using the ALPY600 calibration module for the flats. Perhaps I need to measure the flats and then put in the values?

[Robin Leadbeater]

Hi Ed,

This is how I understand it.

Y max and Y min define the region along the slit where the flat is properly illuminated. For the ALPY it is very obvious as the slit is shorter than the vertical field of view so you would make Y max/min just inside the illuminated region. (It is defined so you do not include the dark areas in the calculations)
(It also defines the region along the slit where you can place a star and where you can place the zones for background subtraction)

Vertical gain correction removes any variations in the flat intensity in the vertical direction. I am not completely clear on the advantages of doing this (perhaps helps the calculations in some way eg lower noise?)
If you have a perfect flat light source in front of the telescope with even illumination in the vertical direction you could omit the correction and this would allow you to do absolute flux calibration regardless of the position of the star along the slit (useful for example if you are doing measurements at different positions in extended objects) In general however our spectroscopic flats (including those generated by the calibration module) are not uniform enough to do this so you have the option to force the flat to be uniform in the Y direction.

Cheers
Robin

[EdWiley]

Thanks, Robin. I brought up the flat and measures Ymax at 762 and Ymin at 308. So I will try those values. Now I have another problem and will post it separately.

Ed

[Andrew Smith]

Ed, Robin, I think Robin is correct about the "vertical gain correction" factor. I think it was initially introduced to cover for the fact that the LISA calibration unit produces anything but a flat field in the vertical direction. I did discuss this with Shelyak at the time I bought my LISA but was told it was ok. In my view using the "vertical gain correction" overcome the issues that a non flat field vertically will produce but at the expense of the flat field not doing its job on image defects and pixel response variations.

I use an externally illuminated screen now for flats. I also crop my images at the time of capture to reduce download time, file size etc. as well as avoid the need for Ymax & Ymin.

Regards Andrew

[Olivier Thizy]

Andrew,


Christian talk about it here:
http://www.astrosurf.com/buil/isis/quic ... lat_us.htm

Basically, do not select the "Vertical gain correction" except in very peculiar situation... Hum, I have been activating this option so far (don't remember why), will tick it off from now on...


Cordialement,
Olivier Thizy
Vous ne verrez plus des étoiles comme avant !
http://www.shelyak.com/en/

[EdWiley]

Thanks Olivier, that makes it easy.

Ed