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Spectra calibration when using simple gratings such as the SA 200

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Spectra calibration when using simple gratings such as the SA 200

I have a couple of questions about how to properly take flat fields and how to sutract the sky background spectrum from the one of the star I want to image. In the AAVSO guide I have read that slit spectrographs have an internal lamp that can be used for taking flats but I am planning to use the star analyser 200.

As regards the sky background spectra should I use and area of the frame with no stars and then subtract it from the spectrum of the star?

Thank you for your help


Robin Leadbeater
slitless spectra - flat field and background subtraction



Spectroscopic flats

Flat correction is useful for slit spectrographs. *  but the same technique does not work with slitless systems.  The problem is spectroscopic flats are a mixture of position and wavelength dependent effects. With a slit spectrograph the position of a particular wavelength in the flat is fixed and defined  by the position of the slit so a conventional flat taken through the spectrograph can be used but in a slitless system any particular wavelength can end up at any location across the flat image and it is not straightforward to untangle this interraction 

Professional slitless systems also have this problem and the solution for them is to build up a 3D flat ie a separate "flat" for each location in the image. This is impractical for amateurs so the advice I generally give now is:-

Take a flat (eg of the twilight sky at the zenith) with the grating in place just to  check for any potential issues, particularly if small scale eg dust and try to remove these or locate your spectra away from them. 

Keep the optics as clean as possible to minimise dust donuts and place your reference star and target at the same location in the field. The instrument response will then take care of any large scale flat field  issues, eg vignetting

If you want to explore the potential errors due to not taking a flat or you are forced to measure spectra at different positions in the field then I can suggest taking spectra at different locations and seeing how much the spectrum changes  for your particular setup.  (You could even if you want produce different instrument responses for use in different areas of the field)

* Note that with slit spectrographs, as well as removing conventional flat defects, doing a flat field correction is useful for removing any small scale features in the instrument response (eg ripples in the camera response).  Unfortunately with slitless systems, these cannot be removed using a flat so must be included in the instrument response measured using a reference star. 

Sky background subtraction.

Typical spectrum processing software used by amateurs allows you to select a zone covering the spectrum and also zones above and below the spectrum where the sky background is measured and subracted from the spectrum (If you are familiar with aperture photmetry, this is the spectroscopic equivalent.) Take care in crowded fields though to make sure the zones you select are free from background stars and spectra. You can see an example of zone selection in a crowded field here on my website.




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