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
Robin
