ISIS question please!

[SteveCuthbert]

Hi
Using my Alpy 600, if I take a series of spectra say 20 fits of the symbiotic star AGDra and each fits includes the background light pollution which I want to subtract. If I move the scope slightly to take a spectrum just of the background light pollution how would I include that removal in the ISIS processing or would I have to remove this unwanted spectrum from each individual fits before processing in ISIS ?.
Thanks
Steve

[Robin Leadbeater]

Hi Steve,

I am not sure why you would want to do this with a slit spectrograph (except perhaps for extended objects where there is no pure sky background in the slit)
Normally you just set the sky background regions above and below the target and let ISIS do the subtraction for you (based on a median in each column in the two regions I expect) With fibre fed spectrographs it is a different matter. There you need either more than one fibre or two exposures subtracting one from the other which is a big (but rarely mentioned) disadvantage with fibre fed spectrographs IMO

Cheers
Robin

[SteveCuthbert]

Ah! I did wonder about that Robin, is it possible to get the sky background removal lines close enough to remove any 'blips' from the unwanted spectra?
Thanks
Steve

[Robin Leadbeater]

Hi Steve,

Yes provided you take into account any geometric distortions ie smile and slant in the lines but this (along with any tilt correction) is part of the data reduction process any way and ISIS can take care of all this for you as well. There is an extreme example of this at the bottom of the page here from Christian Buil of a mag 16 QSO recorded in very light polluted skies
http://www.astrosurf.com/buil/galaxies/spectra.html

Cheers
Robin