Master dark frame

Hi Jack,

Take a number of long exposure darks, at least as long as your longest exposure and with the camera at the same temperature. (I normally try to take 20, using cloudy nights)

Median combine them and subtract the master bias (offset) (ISIS for example can do all this for you) The result is the thermal contribution from the camera, which can be scaled to correct any length exposure

This page describes the master images needed for ISIS (In French but Google translates well enough)

http://www.astrosurf.com/buil/isis/guide_lhires/master/calib.htm

and this page (in English) describes how to generate generate them in ISIS

http://www.astrosurf.com/buil/isis/guide_lhires/tuto1_en.htm

see section 3 – spectral calibration

Cheers

Robin

Hi Jack,

I think Robin and Paul have covered this off well, but I’ll add and re-iterate a few points. It is worth noting that this is the same process as is used for deep sky imaging. The main difference is that you can’t use “sky flats” or just any kind of flat lamp. Instead you need a flat lamp that produces a nice smooth continuous spectrum. The LHIRESIII, ALPY, and LISA all have such a flat lamp built in.

I’ll add that your calibration frames need to be taken at the same CCD temperature, and this should be as cold as possible, whilst ensuring that your CCD can easily maintain this temperature. I find that my SXVR-H694 can hold -20C during the Summer as well as the Winter months.

Best practice is to take of the order of 20+ frames of each type. I was actually taking a new set of darks and bias frames last night. Long overdue as over time the performance of a CCD can change. I try to retake darks and bias frames every 6-12 months. Flats I retake about once a month if I don’t touch the spectrograph. If I change anything on the spectrograph then the flats have to be retaken. This includes focusing the spectrograph collimator lens, but not refocusing the telescope.

I know that John Paraskeva is writing some new calibration routines for BASS. Though MaxImDL has perfectly good calibration routines which work fine for spectroscopy.

Best wishes,

Andy

Hi Jack,

I would just re-emphasize that the author of BASS is re-writing the calibration part of the software. So expect changes to BASS calibration, and I’m not sure where the deficiencies currently lie in the BASS calibration routines. That is why I was suggesting MaxImDL. Once John has finished with the BASS enhancements I am hopeful that will contain useful features.

I usually take a minimum of 20 calibration frames. This reduces the noise when they are combined, which is an unavoidable element of calibration frames.

-5C is OK but if you can go colder then that will reduce the noise in your spectra. Just emphasizing again that the temperature of your CCD when taking the calibration frames must match the temperature when acquiring spectra. So you might need one set of calibration frames at -5C for your older spectra. Then another set at a cooler temperature if your CCD can go colder and hold a stable temperature to within at most a few tenths of a C.

Best wishes,

Andy

Just to add my penny’s worth 🙂 . When I take my darks, flats, flat darks and biases they go into a dated + info directory. For example if I am taking them for the C14 (no focal reducer) using the SBig ST8 then the directory could be 20170126_DFB_C8. This helpful for when I go back to old observations and I want to start from scratch processing them again. I keep all my raw files, all the individual Flats – Darks – Flat Darks – Bias frames, as well the calibrated ones.    

Hi Jack,

The error message or screen shot did not get included in your post.

In general terms, if BASS cannot open it then that implies something went wrong with the calibration process. Before moving to ISIS I carried out bias, dark and flat calibration in MaxImDL, followed by spectrum extraction and wavelength calibration in BASS.

Best wishes,

Andy