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Extended spectra - quick question...

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Anthony Rodda's picture
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Joined: 01/09/2014 - 14:15
Extended spectra - quick question...

...Or not.

If I'm taking spectra of an extended objects such as Nebulae, etc do I take a 'blank' sky spectra to subtract my local light pollution,etc.?  If I did, would this by definition, contain a 'flat' (in the form of a sky-flat)?

How would I apply a reference spectrum?

Regards

T

Robin Leadbeater's picture
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Extended objects

Hi Tony,

If the nebula extends beyond the length of the slit  and you want to produce a flux calibrated spectrum then yes, you need to take a separate sky background spectrum image of the sky near the nebula under as near as possible the same conditions and with the same exposure time. I would then preprocess them all with bias, darks and (halogen)flats as usual then subtract the nebula image from the sky image to produce a final image ready for extracting the spectrum profile as usual (without any background subtraction). The instrument response calculated from a nearby reference star is applied in the normal way.

Cheers

Robin

Anthony Rodda's picture
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Thanks Robin

That makes sense, I think I understand now.  To paraphrase - process both target and 'sky' in the usual (separate) ways, with 'sky not removed' ticked in ISIS to produce two spectra using a nearby reference star, then mathematically subtract sky profile from target profile (in ISIS tab 5 "Profile").

Regards

T

Robin Leadbeater's picture
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Extended Objects

 Hi Tony,

I need to think about doing it that way as I think ISIS normally does some internal rescaling rather than outputting in ADU counts so the final spectra may not subtract correctly. (you would need to test this)  What I was proposing is to produce a single pre processed spectrum image manually with the dark, flat correction and the sky image subtracted already, ready for processing in ISIS without any further dark, flat bias correction or background subtraction to produce the spectrum profile but ISIS may then complain if it is not given any master dark or bias frames. 

It seems this might not be as straightforward in ISIS as I thought. Perhaps putting the question on ARAS might be a better bet. Francois Teyssier there would know how to do it in practise.

Cheers

Robin

Anthony Rodda's picture
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Thanks (again) Robin

Very good point.  If I set the ISIS "Settings" >> "Spectral Domain for profile scaling" boxes to zeroes the output is in counts per second (or ADUs, I'm not sure).  I only know this because I've been playing around with Flux Calibrated spectra.

I think that that means I can follow your method (?).

Regards

Tony

Robin Leadbeater's picture
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ISIS output in ADU

Hi Tony,

I forgot about that. In fact it was me who asked for it !

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=516#p1829

It was added in v5.1.3

http://www.astrosurf.com/buil/isis/new/release.html

Cheers

Robin