Very interesting work Christian, there are parallels here between trying to do absolute and differential photometry.
It also occurs to me that while acquiring spectra is possibly simpler with low resolution spectographs like the Alpy and LISA (and so attractive to new comers to spectroscopy) the processing of the spectra, to obtain scientifically valid results, is in fact much more testing and requires considerable understanding and skill, albeit simplified by using ISIS.
I have added an important annex about spectra reduction by using direct atmospheric transmission (sorry in french for the moment, but many illiustrations are presents).
The key is the evaluation of a true constant Instrumental Response, independant of atmosphere. See
This is really useful now we are doing more science at lower resolution (with Alpy LISA etc)
An additional problem I often find when using generic (eg Pickles) spectra for reference stars is that the actual spectra can be significantly different from the Pickles spectra for a number of reasons.
1. The catalogued spectral classification is disputed or inaccurate
2. There is significant interstellar redening
3. The metalicity is not the same as assumed by Pickles
4. The star is peculiar in some other way
As a result, I now avoid Pickles and use actual (eg Miles) spectra of the reference star where possible
Andrew Smith wrote:Very interesting work Christian, there are parallels here between trying to do absolute and differential photometry.
It also occurs to me that while acquiring spectra is possibly simpler with low resolution spectographs like the Alpy and LISA (and so attractive to new comers to spectroscopy) the processing of the spectra, to obtain scientifically valid results, is in fact much more testing and requires considerable understanding and skill, albeit simplified by using ISIS.
Regards Andrew
Hi Andrew,
I agree. High resolution is hard to measure but easy to reduce. Low resolution is the opposite
I think we have to be a bit careful here not to confuse beginners though. Although the total effect of the atmosphere is large, we are generally taking about much smaller differentials between for example the conditions when the reference and target star was recorded. For example in Christian's first plot, if the two spectra at 59 and 34 deg elevation were both normalised (ie rescaled) at a common wavelength of say 5500A, the difference in the continuum above about 4000A would be small. For a beginner, this is probably sufficient. It is much more important that beginners do the simple things correctly like dark and background subtraction which can have a much bigger effect on instrument response.
I agree. High resolution is hard to measure but easy to reduce. Low resolution is the opposite
I think we have to be a bit careful here not to confuse beginners though. Although the total effect of the atmosphere is large, we are generally taking about much smaller differentials between for example the conditions when the reference and target star was recorded. For example in Christian's first plot, if the two spectra at 59 and 34 deg elevation were both normalised (ie rescaled) at a common wavelength of say 5500A, the difference in the continuum above about 4000A would be small. For a beginner, this is probably sufficient. It is much more important that beginners do the simple things correctly like dark and background subtraction which can have a much bigger effect on instrument response.
Cheers
Robin
Sorry Robin I was not trying to confuse. I agree with you using a reference star recorded close to the same air mass as the target and calculating the relative IR is simplest for the beginner and is in my view similar to doing differential photometry.
Christian don't worry about the French I find translating the page with BING is near enough and when it gets it wrong it often adds a little humour!
Comment avez-vous fait le flat correction? Si vous avez fait une correction classique puis la réponse de l'instrument que vous avez calculé est le spectre de la flat lampe et ne dépend pas de l'instrument. Cette «réponse de l'instrument" va changer avec les modifications du spectre de la flat lampe (âge de la lampe ou le changement de tension par exemple), cela pourrait être corrigées pour en comparant chaque flat avec le «master flat» utilisée pour calculer la "réponse de l'instrument"
How did you make the flat correction? If you made a conventional correction then the instrument response you have calculated is the flat lamp spectrum and is not dependent on the instrument. This "instrument response" will change if the flat lamp spectrum changes (lamp age or voltage change for example) This could be corrected for by comparing each flat with the "master flat" used to calculate the "instrument response"
J'utilise effectivement une lampe tungstène pour réaliser le flat-field. Bien sur il faut quelle soit stable. C'est vraiment le cas avec la lampe qui épuipe le système de caibration de Alpy 600. J'ai bien vérifié cela et c'est un point important.
Il faut être vigilant, et faire des contrôles de temps en temps.
Thanks Christian for your interesting study. However, I do not understand how this new study is related to the previous one : http://www.astrosurf.com/buil/extinction/calcul.htm
in which you proposed a procedure to account for the absorbing effects of the atmosphere. What makes the recent procedure superior to the old one ?
Also, I fully agree with Robin who claims, in his post dated Sun May 12, 2013 9:13 am : "If you made a conventional correction then the instrument response you have calculated is the flat lamp spectrum and is not dependent on the instrument" (more precisely I think your instrument response should show the reciprocal of the black body emission of your tungstene lamp). Do you apply a special processing to your flat prior to applying the correction?
Patrick
Am I right in concluding that whenever you divide an observed spectrum from a slit spectroscope with a flat (as in almost any normal spectral processing) you in effect remove the response of the "instrument" and leave the convolution of the targets spectra with the spectra of the flat lamp?
