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- This topic has 7 replies, 2 voices, and was last updated 5 years, 3 months ago by John Coffin.
6 February 2018 at 12:53 pm #573947
I have captured a spectrum of Merak using a Canon 450D which has had a full spectrum modification, an Alpy and an Equinox Pro 80. I’ve converted the CR2 files to FITS files in ISIS.
As far as I can tell I’ve followed the instructions on the ISIS website for processing a spectrum from an Alpy with calibration module. I’ve made master flats, offsets and dark frames. Everything goes well in ISIS until I use the calibration assistant. I think I’ve correctly identified the H alpha line, but the RMS is repeatedly around 20.
The spectrum is tilted about 3 degrees so I’ve had to use a large binning zone of 180. The software does produce a nice looking raw spectrum but it is not wavlength calibrated. I can’t post the FITS files because they are too large but I‘ve made a Word document with some screen captures.
I can process the spectrum in RSpec, but in the long run, I think ISIS has more flexibility. It also stacks the subs very nicely.
Can anyone advise me? Is this a known problem with processing DSLR images?
Attachments:6 February 2018 at 1:50 pm #579068
Are you using the calibration assistant method described here ?
If the rms errors are large in the wavelength calibration. It is usually because ISIS is not finding the correct lines in the lamp spectrum. To get this to work correctly the entered pixel size is critical. Because of tolerances in the ALPY this is not identical to the actual pixel size. You can calculate the pixel size to be entered by measuring the distance in pixels between H alpha and H beta in a hot star. The pixel size is then 3123/no of pixels. You may still need to do some fine tuning though to get ISIS to lock onto the lines correctly. Also the lamp spectrum needs to be well exposed and in good focus.
If you are having problems it can be worthwhile taking a step back and not using the calibration assistant but following the instructions here which has more information on the automatic wavelength calibration function and includes information on troubleshooting problems (method is 2 for the calibration module but the whole thing is worth reading to understand what is going on)
If fact I generally use this method rather than the calibration assistant
Once set up, I find the automatic calibration reliable and much easier than finding the lines manually but it can be fiddly initially to get the pixel size correct.
Robin6 February 2018 at 2:15 pm #579070
Note that the calibration assistant uses both calibration lamp lines and Balmer lines, which is explained in more detail in method 3 of the above link, so both sets of lines have to be clear in the spectra and identified correctly by ISIS. Checking the individual errors for each line can help with finding which lines are causing the problem.
Robin6 February 2018 at 2:31 pm #579072
Having accurate geometric corrections can also influence how easy ISIS finds locating the lines. You mentioned a 3 deg tilt but the screenshot shows 1.5 deg. Is this correct? Also is the smile correct? (measured using a strong line in the lamp spectrum)
Robin6 February 2018 at 3:00 pm #579073
I’ve just realised that you have to use a Barlow lens with a DSLR dont you because of the back focus? In that case the 5.2 um pixel size entered in the general tab will definitely not be correct.
Looking at your screenshots Ha alpha is at 3383 and H beta at 1623 so the effective pixel size will be 1.774um which ties in with the 3x magnification factor quoted for the DSLR Barlow in the ALPY instruction manual. I suspect this is the main cause of the problem
Robin6 February 2018 at 4:48 pm #579074
Thanks Robin, that’s really useful advice. I do have the Alpy barlow. I changed the pixel size as you suggested and it worked. RMS now 0.12. John6 February 2018 at 5:41 pm #579077
Brilliant ! Once you get used to it, it is features like this that make ISIS so nice to use.
Robin7 February 2018 at 8:27 pm #579082
Thanks everyone for all your help. Here is the final calibrated spectrum of Merak captured with a full spectrum modified Canon 450D and an Alpy. I was using a 500 mm focal length refractor and used a QHYCCD Polemaster as an electronic finder. It has a huge field of view (11 x 8 deg) but showed stars down to magnitude 8 surprisingly.
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