Spectra processing

Forums Spectroscopy strange Hb in Be (48 Per)? Spectra processing

Andy Wilson

Hi Andrew,

You raise some important and interesting points, possibly worth their own forum threads.

I agree the processing of a spectrum should depend on the scientific use to which it is intended to be used. When submitting to databases though, it is my opinion that it is best not to rectify the spectrum unless necessary. The reason for this is that a researcher can always rectify a spectrum should they wish to do so, though they cannot unrectify it. Note that when a rectified spectrum is submitted to the BAA Spectroscopy Database then the BSS_NORM fits header should be populated.

I have no strong opinion on whether or not to rectify a spectrum when no response correction has been applied.

On the dark vs bias front, I would expect to see differences between dark and bias frames with amateur CCDs. I also use an SXVR-H694 and I see a big difference between my bias and dark frames.

For the following stats I had the CCD set to -20C with 600 second darks, with statistics computed by MaxIm DL from single raw bias and dark frames.

                              Bias          Dark
Mininum                  882          1006
Maximum                1315        56522 (due to hot pixels not cosmic ray hits)
Average                  1084.580  1133.068
Standard Deviation  15.960      211.396

I also advise to flat field as know of a couple of reasons why the response correction from a reference star would not correct for all variations in the CCD.

First, when using a reference star to create an instrumental and atmospheric response curve, then there is a degree of smoothing to remove small scale variations in the curve. This is to give the bulk wavelength dependency. So by its very nature it does not remove the individual pixel to pixel variations.

Second, the target star and reference star on the CCD need to be exactly aligned so that the light path through the spectrograph illuminates the CCD pixels in exactly the same places. This is nearly impossible with a slit spectrograph, but I am not sure if a fibre fed system would get around this.

I am happy to be proven wrong if my understanding is at fault. Also please excuse this lengthy post but I thought it worthwhile for these important topics.

Kind regards,