Design, construction, tuning of spectroscopes
Information and discussion about softwares (telescope remote, autoguiding, acquisition, spectral processing ...)
Andrew Smith wrote:
As an extreme test it might be interesting to deliberately add a severely vignetting field stop before the grating.
Thinking about it, and taking it to the extreme, this is what a slit spectograph is. A flat taken without the grating would definitely not do the job in this case
Robin
Quite right but the slit also cuts out the contribution to the FF from the rest of the field which is the worry I have with with the SA FF!
Robin - The answer is in your comment about a slit spectroscope. What you want is the FF that would have been created has a slit been in place at the location of the zero order image but this is imposible! So the question is what obtainable FF best approximates this ideal?
I think we've all had similar thoughts.....
Conventional flats taken without the grating.....these would allow correction of the usual suspects....
Nothing changes when we add a grating.....the camera is still in the same position, at the same focal point (albeit it may be slightly different to focus the spectra) and covers the same FOV.
When a grating is added I think effectively two things happen...
1. For the zero order part of the image the grating acts as a transparent (we hope) filter and the light distribution would be unchanged - unless the grating it'self causes additional vignetting. This "part" of the image would be corrected by the flats. Affects the sky background?
2. "Overlaid" on this zero image is the series of spectra produced after the grating and superimposed on the "zero order image". I'm assuming the quality of the spectrum is not, or minimally, affected by the zero order star position in the FOV.
If this is correct, or at least an acceptable argument, then applying flats would only improve (?) the sky background and have no impact on the spectra....
(Howell in his " Handbook of CCD Astronomy", p159-165 discusses slit-less spectroscopes and raises the difficulty of applying spectral flats. BTW he also quotes a further reference - R W Pogge, 1992, Astronomical CCD Observing and Reduction Techniques, "ASP Conference Series, Vol#23 Ed Howell" (recently reprinted) - has anyone got a copy? If so is it any good?)
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
"Imaging Sunlight - Using a digital Spectroheliograph" - Springer http://www.astronomicalspectroscopy.com
