spectro-aras.com

Thierry,

Yes, it would a be a good idea to determine what our problem is ... interference and/or fringing. What is the "Astronomy and CCD" reference? I would like to read what it has to say on this subject.

Bernard

Sorry the good tittle is "handbook of ccd astronomy", last edition. I think you already have this book.
http://www.amazon.fr/Handbook-CCD-Astro ... 777&sr=8-1

Thierry,

Yes, and excellent book ... my CCD 'bible' (I have both editions). Obviously I need to read the pages on fringing again!

Bernard

It has been suggested, by Robin earlier and Olivier and others elsewhere, that moving the Lhires main mirror appears to have some influence on the ripple strength. This mirror movement will shift the 'central' position of the spectral line of the CCD, so I am wondering how this could affect the ripples. Any suggestions why this should be?

If a mirror shift does have some influence on Lhires ripple can this also be true for the eShel and, if so, what mirror adjustment could be responsible?

Cheers,
Bernard

I am fairly sure the fringes I see are from the spectrograph and not the atmosphere. They are at constant wavelength positions and do not change with time or location in the sky. Attached is a comparison of eps Aur and Altair taken several hours apart. They match exactly. (I used Altair as a "star flat" to remove them)

Cheers
Robin

I agree that the ripples must be internal rather than atmospheric as I get them in my flats ... but, surprisingly, not sufficiently uniform for the flats to remove them from the images.

Bernard

Yes because there is an atmospheric glowing contribution.
What we saw in Robin spectra is essentially interference fringes due to optic/slit contribution, with regular pattern. But what we saw in the Robin eps aur potassium spectra is certainly due to atmospheric contribution in this very rich of tellurics wavelength domain. Most of the time it is a mix of the two phenomenon and cannot be removed even by professionals.

http://arxiv.org/abs/1201.2336
Very interesting and a simple and more correct than mine explanation of fringes. Add monochromatic interference in spectroscopy and you have all the explanations necessary to understand.

Thanks for the interesting link ... in fact following up on some of the references cited is also productive. It appears that Malumuth et al did a lot of work on the fringing problem, especially relating to the HST. They say the problem is due to internal CCD reflections but also state that "fringing is not significant below ~7000A", which has been said by others (e.g. Howell, etc). Not sure what I should conclude as I certainly have a significant fringing problem at much shorter wavelengths.

Bernard

Bernard - Maybe yours are just plain old monochromatic interference within the coverslip? Andrew