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 Persistent sky lines when extracting faint spectra with apall
   
cwbullivant
 10/18/2014 07:14AM (Read 1724 times)  
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Registered: 04/11/2014
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For the last few months I've been working with spectra taken at the Bok telescope at KPNO, and one particular data set has been especially difficult to reduce properly. The spectra for this night are generally fainter than the rest of the data that I've worked with, and I've yet to find a way to consistently remove two particular sky lines completely without significantly worsening the overall quality of the spectra (I've found ways to manipulate the background fit that eliminate half or all of the sky line, but seem to make the spectrum much noisier). One is the OI line at 5577Å, and I haven't conclusively identified the other just yet.

I've asked my research advisor for assistance, but he doesn't make significant use of IRAF (preferring IDL), and I'm not familiar with IDL beyond the basics of the basics.

I think that some of the problem is in the reduction technique; since all of my objects are extragalactic (mostly Type II supernovae), I was told I needed to use the background fitting in apall to subtract out the background galaxy, and therefore to use a very close, very tight background sample (usually -10:-x,y:10, where x and y are the lower and upper limits of the aperture). And in the background trace, the advice was to get a trace that was approximately linear, using either a 1st order linear spline, or a second order Legendre polynomial. Doing it this way unquestionably gets me a better result than trying to get a trace that actually approximates the data points, but it still doesn't work for the sky lines.

Is there another routine I should be looking at, or do I just need to adjust the technique?

 
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