CCD Sensitivity

[Pig]

Just a quick question .... I have the feeling it has some relevance
How will the sensitivity/ bias of the chip effect the spectra ? IE I would think chips vary quite a lot and this variation may influence the spectra, if so, how do you go about adjusting / allowing for this ?

[robin_astro]

Yes many things affect the shape of the spectrum (the main ones are the response of the grating and the camera sensor but the telescope and spectrograph optics also have an effect) The absorption of the atmosphere (which is greater in the blue and depends on how high in the sky your target is) also affects the spectrum.  All these are usually corrected for by measuring a "standard" star (so you know what the spectrum should look like) at roughly  the same location in the sky as your target (so the affect ofthe atmosphere is the same).  If you then divide the standard star spectrum you measure by what it should look like, the result is the "instrument response"  You can then apply this to the spectrum of your target to correct it to produce the true spectrum. You can see the steps using the Star Analyser for example in a couple of presentations  I  have given at BAA workshops.  (Using Visual Spec and ISIS software here but other software like Rspec  or BASS  can be used.

http://www.threehillsobservatory.co.uk/astro/spectroscopy_10.htm     slides 31-48 

http://www.shelyak.com/dossier.php?id_dossier=36     Low resolution slitless spectroscopy - observing a fast transient of a T Tauri star, Robin Leadbeater (3MB)

Cheers

Robin

[Pig]

Thank you Robin .... I will check the links later.
I have tinkered with RSpec comparing references against my captures thus I believe I am on the right track  I have seen the instrument response feature also and will give this a go, I just need a gap in the clouds now.