Olivier,
good question. Because nobody could provide an answer at OHP, I tried to find a acceptable answer by myself during observation at the OHP.
My personal opinion: Reference stars to calibrate the instrument response should be bright enough. There are two reasons for choosing stars around 3rd magnitude and above: Signal and signal. That means two things: One should require a good quality signal, but not too long exposures which will mean wasted time. Another aspect might be an almost similar air mass for the calibration standard and the target to observe. If this condition is not fulfilled and stars are too far away from each other with different airmass, it will be a mess to calibrate the whole observation site over years.
Calibration of the observing site is, what astronomers did and continue to do with spectro-photometry and integral photometry. Calibration of the site is a very time consuming task by itself. I don't know if one ever tried this for any typical suburban site, like those amateurs are using. I guess weather conditions vary much more on a suburban site, than at a typical, carefully selected location of an astronomical observatory. For me there will also be different areas of sky brightness and directions where large cities may increase seeing and air pollution. Usually, I try to avoid these directions of sight for astronomical observation.
So, my conclusion is: Having no calibrated site, air mass of reference and target should be similar. The reference standard should not be too far away. Ideally the reference should have similar declination to the target. In this case one might try to do alternating observation of target and reference.
To have indicators about magnitude and selection of references stars:
Steve Shore suggested HD 217086 for reference, a young stellar object of magnitude V=7.66. For my 8" Cassegrain, a star of that magnitude looks too faint as reference. For 30 sec exposure of a Canon DSLR and 8" Cassegrain with Alpy, the signal of HD 217086 will be as low as CH Cyg. Means, you don't even find it on the screen of the camera from a single exposure, but there will be enough signal for stacking 10-50 spectra.
Finally, and more intuitively, I selected the only two bright stars listed on "ESO RA ordered list of spectrophotometric standards", that might be good:
- HR 7001, alpha Lyr
- HR 153, zeta Cas
Zeta Cas should be good reference for other spectroscopic targets in this region as well, like gamma Cas. Unfortunately, there are not many bright stars listed with declination similar to VV Cep found on ESO list. Alpha Lyr is a bit far away from VV Cep. But, for my site it will be around zenith these months, good for instrumental calibration with air mass close to one.
Summary: I would suggest these additional criteria for spectrophotometric reference stars are:
- Brightness. Example: Magnitude <V=4 for Alpy + DSLR and 8" telescope.
- Air mass could be fulfilled with similar RA and DE, probably alternating observations
- Suggest one reference with DE=90-[altitude of site location] to calibrate instrument at air mass close to one.
Quick Reference:
http://www.eso.org/sci/observing/tools/ ... anlis.html
Perhaps, we could compile a list with additional reference targets.
Thilo