Hi folks,
I took a recent spectrum of Gamma Cas H alpha and for some reason the EW turned out to be way way too low. This was after a good spectrum was taken just a week or so earlier. I am scratching my head trying to figure out what I could have done during the processing to cause it. First time I have ever seen this happen. It's a very easy target, only a second magnitude star and the signal to noise is great, resolution (which I understand does not affect the EW) great. I tried retaking my darks and bias frames, reprocessing with and without instrument response, recalculating instrument response, using a different comparison star, trying different reference regions of the continuum and even trying different versions of the processing program. No changes noted thus far. So while I am waiting for better weather to try again, I was hoping someone could tell me potentially what processing steps if gone awry could affect the EW measurement adversely. I attached a file showing the calculated difference (black arrow) between the good spectrum and the recent one that was way off. The difference should be negligible which it clearly is not. Thanks, Dave Doctor
Perhaps the difference is real. Gamma Cas H alpha is variable. eg
http://www.spectro-aras.com/forum/viewtopic.php?f=14&t=714
How do your EW compare with other spectra in BeSS ?
http://basebe.obspm.fr/basebe/
Cheers
Robin
If it is not real, then either the continuum is too high or the emission line is too low. Here are some other things you can check:-
Saturated pixels in the emission line ?
Spectrum binning zone does not includes full width of spectrum
Sky background binning zones are contaminated
Cheers
Robin
Also check how your response using your comparison star looks.(Look for residual contamination from H alpha in reference star in the response) If you used a flat, the response should be almost a flat line over such a small range at this wavelength. It should also be almost unchanged over a long period provided you are using the same setup and flat lamp. Try without response correction to check if the difference is still there.
Cheers
Robin
Yes I checked with and without response and no difference meaning the "difference" was still there. There could be an issue with saturated pixels in the emission. A rough estimate looking at image stats of a subframe shows that the max is extremely close to saturation. So certainly exposure could be reduced next time.
Dave
One final item to add to the check list is the cosmic ray filter if you are using one. If the threshold is set too aggressive, it can "heal" pixels containing real data in high contrast areas, particularly if the spectrum is close to being undersampled. If you are using ISIS, check the intermediate @map images which show which pixels are being healed.
Cheers
Robin
Thanks Robin. The filter is set to 15 which I thought was pretty low. Yes, in ISIS I have not previously saved the intermediate files but will look at them. Thanks again!
Dave
Thanks Robin. I was told the difference is not real. BeSS looked at it and concluded the same thing ,that despite the variable nature it isn't that unstable to exhibit such a large change in that short a period. The comparison spectra there did not show it. Also those actively researching this system currently have told me my spectrum is a definite outlier and not consistent.
Regards
Dave