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What altitude for reference star?

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John Coffin's picture
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What altitude for reference star?

I have started using Paolo Berardi’s very useful Miles star finder http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=941

I was wondering how to choose the best altitude for a reference star when the target star may have changed in altitude by 4 or 5 degrees within an hour. For example, AG Dra last night was at 41 deg when I started the sequence of 6 x 600s exposures and at 37 deg at the end. I chose HD 192640 in the end because it was at 37 deg.

Any ideas? Thanks.

Robin Leadbeater's picture
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Flux calibration error with altitude

Hi John,

The error in calibration for a given altitude difference depends on altitude.  The best match would be to the mid point altitude but in practise except at low altitudes the error is small for 5-10 deg difference in altitude.  For more details see my recent post here

https://britastro.org/comment/5536#comment-5536

The ARAS forum link there gives links to Francois Teyssier's spreadsheet for choosing reference stars which includes MILES and other potential  stars and  takes the error into account.   There is also a  chart linked from the ARAS post showing  error relative to altitude.

Cheers

Robin

 

David Boyd's picture
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Atmospheric correction

To mitigate this problem, if I am recording the spectrum of a target at low altitude (close to 30 deg) I will look for a reference star as close as possible in altitude to my target and then record spectra of the reference star both before and after the set of spectra of the target. I compute response profiles from both sets of reference star spectra and take the mean of these to use in correcting the target spectrum. Although not perfect, this is better than choosing just one of the response profiles. It is instructive to see how much the response profile can change over an hour for a star at low altitude.

David

Robin Leadbeater's picture
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focus shift

Hi David,

This is a good technique I sometimes also use if I suspect there has been a shift in focus during a long run.  My setup suffers from chromatic aberration which can affect the response depending on focus due to selective sampling of different wavelengths by the slit as here for example.

https://britastro.org/comment/5542#comment-5542

Robin

John Coffin's picture
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Error relative to altitude

Thanks for your helpful advice. The  chart relating error to altitude was very useful Robin. For other readers this is the final link to it.

http://www.astronomie-amateur.fr/Documents%20Spectro/Ref_013.pdf

I must try and find my targets before they get too low in the sky!