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Instrument response with Lhires

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k.gurney's picture
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Instrument response with Lhires

I thought I would ask this in a separate thread as its a matter of principle and nothing to do with tweaking the kit.

So, with the Alpy, we are enjoined to take the spectrum of a nearby reference star (typically in MILES database) and use this to obtain an instrument response (IR). I can see this works for low resolution, because it makes sense to talk about the  continuum response  (smooth curve, ignoring line features). However, with high resolution, its quite often the case that the bulk of the spectrum is a line feature (I am thinking of Vega's Ha line, for example).

So, how do we define the continuum respone in this case? And should we be trying to replicate the IR calibration step using a companion reference spectrum, as per low res (Alpy etc)?  Could you get away with some stock spectra you have taken at different air-masses? Or just rectify the spectrum and obviate the problem at source, as it were?

Cheers

Kevin

Robin Leadbeater's picture
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High resolution instrument response

Hi Kevin,

You can do an instrument response in the usual way but you need a high resolution spectrum to compare with so you can match profile including the line. You can see an example of that about halfway down on Christian Buil's page on reducing LHIRES spectra here 

http://www.astrosurf.com/buil/isis/guide_lhires/tuto1_en.htm

and also specifically covered on this page

http://www.astrosurf.com/aras/tutorial5/note1_us.htm

Fortunately unless you are working at the far blue end, atmospheric extinction does not have much effect over the narrow wavelength range so you can use stars which might be some distance from the target (or even at a pinch instrument responses taken on different nights)

Two good sources for high resolution spectra of bright stars are the UVES bright stars 

 http://www.eso.org/sci/observing/tools/uvespop/bright_stars_uptonow.html

and provided you are working above 4000A,the ELODIE 3.1 list (spectra from the ELODIE archive selected for quality.) See here on ARAS for more background on this set of stars

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1369

Both of these are available in the built in ISIS database (you have to load the ELODIE 3.1 star list)

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1864

Is it necessary/worth it rather than just rectifying the spectrum for small wavelength ranges?  Possibly not, it depends on the application but BeSS recommends it. 

Cheers

Robin

Robin Leadbeater's picture
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Doing instrument response/extinction correction like the pros

It is worth noting that the technique of using a reference star near in elevation to the target is technique developed to make it easier for amateurs. Because professionals have stable setups and know their atmospheric conditions, they tend to use a standard instrument response measured infrequently using precisely measured spectrophotometric standard stars, combined with a measure of extinction on the night together with an atmospheric model. ISIS does have the tools to do this though. 

http://www.astrosurf.com/buil/isis/guide_response/method.htm

k.gurney's picture
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Hi Robin

Hi Robin

Once again - thanks for a full and detailed response!

I had tried with a MILES star comparison but, as you say, it should really be done with something at high res. Interstingly though, I did get an 'instrument response' which looked typical of that shown on CB's page.

If I were to take spectra some of the ISIS database reference stars, could I go back and make IR's to retrospectively apply them?

Cheers

Kevin

Robin Leadbeater's picture
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IR from different days

You could try measuring the IR using a high reference resolution spectrum  and see how it compares with you MILES IR.(Unless I am working low to the horizon or at the blue end just use a bright star like Vega, Altair, Regulus for example which can be recorded quickly.) If it is significantly different, it might be safer to just rectify the spectra you have already taken. (You can still submit them the BeSS setting the appropriate flag in the fits header.

To be a bit controversial (and this is just my personal view) I think for relative flux calibrated H alpha spectra, most of the time IR correction of a narrow wavelength range at high resolution is a waste of good observing time. (And may even lead to increased variability, though I need  to quantify this). If you use a flat, You are already getting rid of all instrument affects as they divide out, leaving the flat lamp spectrum  and the atmospheric extinction which hardly varies across the range.  If you are using ISIS, this even removes the small slope due to flat lamp spectrum, assuming a black body at 2750K I believe so the  IR ends up being  effectively a horizontal flat line. In the projects I have been involved in where narrow range spectra have been used, the first step  in analysing the data has been to rectify all the spectra first in any case.

Cheers

Robin

k.gurney's picture
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Robin said: " I think for

Robin said: " I think for relative flux calibrated H alpha spectra, most of the time IR correction of a narrow wavelength range at high resolution is a waste of good observing time."

Yes - this had been my assumption, albeit based on a hunch rather than any evidential basis...

Uisng 'precedent and custom' as the criterion, my encounters with   the litertature  seem to show rectified data in almost all cases.

Cheers

Kevin

Robin Leadbeater's picture
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Typical H alpha instrument responses

I quickly  pulled out a couple of instrument responses at H alpha (using a 1200 l/mm grating so covering a wider wavelength range than the 2400 grating.) They are flat within +-1.5%

andrew.j.smith1905's picture
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IR correction.

I agree with Robin's comment in #5 . You can easily add noise or worse systematic error from the reference. 

I simply use the W lamp to remove the blaze from my echelles spectra and then rectify just those orders I wish to measure. The echelles has a more complex set of IR issues than the Lhires III. 

Regards Andrew