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Recycling instrument responses

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k.gurney's picture
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Joined: 08/01/2016 - 18:58
Recycling instrument responses

I have been taking a  few spectra recently with my Alpy600 and trying to get targets between cloud coverings...

On one occasion, I failed to capture a MILES reference star for instrument response (IR) calibration - the cloud beat me to it. However, I started to think about recycling a previous night's IR, as the target and  MILES star were the same, and the observation time was roughly the same too. My understanding is that you are 'calibrating out' idiosyncracies of the apparatus and the effects of the atmosphere. If the altitude at observation is roughly the same, with the same airmass (especially near zenith, where it doesn't vary varies  much) then surely this is OK? The IR, is after all, supposed to be a smooth, low spatial frequency function (over the spatial extent of the sensor) and is obtained in a fairly crude way (where shall I put the fitting slider in ISIS!); so small variations in the IR sectrum aren't going to be relevant.

The same applies to the use of the MILES star in wavelength calibration. Thus, I have recently taken to using the 'predefined  dispersion equation' method in ISIS using the MILES star as its (Balmer line) reference. This gives much better calibration at the blue end where the Alpy lamp is a bit ineffective (you stilll need a calibraion lamp frame to 'lock' the position of the polynomial function). So, assuming the dispersion doesnt change much over a couple of nights (why would it if the whole Alpy-camera assembly has not been undone?) I wonder if you could still take advantage of the previously found value? I could imagine temperature variations might intrude here, but I am not comparing a winter's night with last week :)

Kevin

Robin Leadbeater's picture
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reusing calibrations

Hi Kevin,

Concerning instrument response/atmospheric extinction, Christian Buil has been moving away from the reference star at the same air mass method recently.

http://www.astrosurf.com/buil/instrument_response_us/

Essentially he is separating out the true response of the instrument which he reuses, from the atmospheric extinction which he models. Although this is similar to the technique pros use,  I am not totally convinced about this for my atmospheric conditions so am currently still using the old reference star as similar air mass method, though I do admit to reusing instrument responses quite freely for H alpha at high resolution with the LHIRES where the atmospheric extinction effect is small.  David Boyd was at OHP this year so perhaps he has some updates on this new method?

I think your method of reusing the dispersion equation should work well with the ALPY provided the temperature does not change too much as it seems very stable to me. I would suggest still taking a lamp spectrum and using a lamp line as a single anchor point to avoid any problems with overall shifts. You could also use this lamp spectrum to make sure there has been no change in dispersion eg due to temperature by checking the position of second line in the spectrum

Cheers

Robin

k.gurney's picture
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Joined: 08/01/2016 - 18:58
Thanks Robin,

Thanks Robin,

Yes, I see there is an 'Auto atmosphere' button in the general tab in Isis but sparse documentation...

So, I think I'll recycle IR data with care... I think if you separate the instrument  components (camera and Alpy) and are working at quite different temperatures, you might want to be sure to use new IR data, but otherwise it looks OK. In addition I have discovered the comment field in FITs editing in BASS! (Homer Simpson moment here...). Suitable use therein should make it clear.

Cheers

Kevin