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In the recent workshop there were discussions about how to chose what star to use as a reference to correct for the instrument response and atmospheric extinction.
In theory any star could be used provided it has a known stable spectrum but it is better to use hot (A or B or O) stars as they have a smooth continuum with few lines so make extracting the instrument response easier.
The reference star should be measured at a similar altitude to the target to minimise the effects of atmospheric extinction. (This is particularly important when observing low in the sky.) Planning the timing of observations can help with this
Where possible it is always best to use a star with a reliable measured spectrum to compare with. (The resolution of the reference spectrum should also be as high as the measurements being made ) The MILES database of stars is an example of reference spectra suitable for use with the ALPY. To help select a suitable MILES star, Paolo Beradi has produced a spreadsheet which lists them with their coordinates. The link to it can be found in this thread on the ARAS forum http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=207&start=30#p2162 . If you are interested this thread includes the history of how use of the MILES database as a soure of reference stars for amateur spectroscopists came about.
Alternatively any A or B star with a reliable spectral classification could be used combined with the generic Pickles spectra for that spectral class but take care it has low interstellar extinction which will redden the spectrum. This is not ideal though as one is relying on the spectral classification being correct and the star being typical of its class. Francois Teyssier has produced a spreadsheet of bright A and B stars with low interstellar extinction potentially useful as reference stars which can be used with the Pickles generic standard spectra. It can be downloaded from his website here http://www.astronomie-amateur.fr/ProjetsSpectro0.html
Both the MILES stars and spectra and the generic Pickles spectra used with Teyssier stars are included in the ISIS database. Note that the latest version of ISIS 5.7.0 has keywords in the fits header to allow you to add what reference star was used and the source of the spectrum (eg MILES or Pickles) It is a good idea to fill these in as it will allow spectra to be corrected should the star chosen or the reference spectrum used be found to be unreliable.
I hope this is useful. comments welcome !
Cheers
Robin
Hi Robin, you mentioned in your post that ideally a reference star should have a low interstellar extinction. In terms of E(B-V) what upper limit would you recommend? Thanks, John
Hi John,
This applies when using a standard star spectra eg Pickles in place of an actual measured spectrum and assuming the published spectral type is correct. It depends how accurate you need to be, particularly at the violet end. Francois Teyssier’s Reference Star Finder spreadsheet default setting gives a warning for stars with E(B-V) >0.05 and an alarm >0.1. If you have ISIS you can estimate how much effect it has using the extinction tool on an A0v Pickles spectrum for example. Attached is the error with E(B-V) = 0.1. I suppose you could even correct for the effect, though that is perhaps pushing the use of stars with published classifications but no published spectra a bit far
If you have a star with a known as measured spectrum like the non dereddened MILES stars for example the extinction does not matter in theory of course, though I would avoid using the MILES stars with very high extinction. Although it was I who first advocated the use of MILES stars back in 2011,
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=191
I have fallen a bit out of love with them as only a few are ideal as reference stars as many are potentially variable.An alternative is the larger MELCHIORS set of spectra which are at much higher resolution so could also be used with higher resolution spectrographs like the LHIRES for example.
https://www.royer.se/melchiors.html
SpecInti software has a script in its toolbox for extracting them in a form readable by the usual software (note there is a typo, a missing _ in the English version of the script)
http://www.astrosurf.com/buil/specinti2/specINTI_toolbox2_en.pdf
and the STAROS campaign website also has an on line extraction tool
https://search.staros-projects.org/Cheers
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