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David is this what you need http://www.astrosurf.com/prostjp/Dispersion_en.html
This may also help http://adsabs.harvard.edu/full/1996PASP..108.1051S and references it contains.
Regards Andrew
Hi Robin, you are right there are probably too many, unknown or uncontrolled, variables to be sure.
Regards Andrew
I suspect it is an effect of any misalignment of the slit with the parallactic angle having less effect with the wider slit.
This may be relevant http://www.eso.org/sci/facilities/paranal/instruments/vimos/inst/atmos/atmos_paper.pdf
Regards Andrew
I don’t disagree with what you say Robin. If you have a technique that delivers the accuracy you need for your objectives e.g. you SN matching work then absolutely fine. The difficulty I have is when corrected spectra or even flux calibrated spectra are presented with no estimate of their errors in the continuum level. (I don’t think the BeSS standard requires it.)
If you are using your own data I am a great believer in doing the minimum processing consistent with your goals to minimise added noise and potential artifacts. As we have commented before it is different when adding data to a general use database.
Regards Andrew
I think this highlights the difficulties of doing low resolution spectroscopy at ground level. I did see your post on the ARAS site David but could find no references to your question via various Google searches. It seems planes are not an issue for the professionals.
I think in addition to the issues covered above if you want to get an accurate continuum you need to operate with a wide slit. That is one where the slit is significantly greater that the seeing disk so as to admit 99.99% + of the incident star light. Thus your spectral resolution will be seeing/guiding limited.
Obviously, this will reduce your spectral resolution compared to a narrow slit where it defines the effective resolution (all else being equal). So you trade off photometric accuracy for resolution
I think your science goals will dictate which is needed and if it is both then two set of observation may well be needed.
Regards Andrew
PS see here http://www.astrosurf.com/buil/alpy600/photometric_slit.htm
Were the spectra taken at a similar air mass? The instrument response includes the effect of the atmosphere as well as the instrument.
Regards Andrew
I would agree with all Robin has said above. I would,however, caution that you need to check what your processing is doing what you expect and not adding any anomalies.
While I have not done this yet I am considering taking the mean of my bias frames and subtracting it, rather than the bias frame, from the other frames (targets, flats, darks). The logic is that for CCD (but not CMOS) the bias or offset is not a function of the pixels but of the output electronics. It should, at least in the short term be constant, which is estimated by reading out an unexposed frame. Some amateur cameras and professional ones have an over-scan region for this purpose as it avoids any residual dark current light leaks etc.
If this is a valid approach, and I think it is, it avoids adding in noise during the bias removal. Thoughts welcome.
Regards Andrew
Andy, I am not trying to make a big issue of this. I agree any spectra going into a general database needs to meet it’s standards and the BeSS standards is fine.
When I did my dark v bias comparison I used a region without hot pixels as they would be rejected in a standard statistical test when calculation the mean and variance of a sample.
The examples I gave were just to point out different style of instruments have different characteristics and the need to think about what processing is relevant. (Another example is to flat field a slitless spectra or not.)
Regards Andrew
As I understand it (and I may well be wrong) in stellar classification the spectra are rectified and this is built into the definition of equivalent width (see The Classification of Stars Jaschek and Jaschek).
The issue of what processing spectra should have depends I think on what scientific purpose the spectra is to be put and to a degree what type of instrument it was captured. BeSS had a particular goal and hence it’s rules.
As an instrumental example short exposures (<10min) I can see no statistical difference between a bias frame and a dark from for my SXVR H694 at -10deg so why use a dark frame and increase the noise why not use just a (constant) bias correction? Similarly for a fiber fed spectrograph where the spectra is always on the same pixels why flat field if you use a standard star for instrumental correction?
Just some random thoughts.
Regards Andrew
I think the simple answer is yes. Not only the atmosphere but also any surrounding shell or ring of gas which is normally mostly hydrogen. It is not always the case but there has been a strong focus on Be stars where the hydrogen alpha line has been the main focus as it shows the most variation.
Other lines are important for radial velocity measurement and in spectral classes where hydrogen lines are very weak or absent.
Regards Andrew
