Quote:
Originally Posted by Terry B
You shouldn't need terribly long exposures to get a good S/N. Without guiding the star may not have spent long enough on the slit.
What software are you using to calibrate?
|
Hi Terry,
Yes, I think the star was drifting off the slit fairly quickly. I was having to perform a fairly shoddy process of getting the star on the fully open slit at zero order, then gradually closing the slit and slewing backwards and forwards to try and keep the star on the slit centre, then moving the grating to the required position, taking a neon, then taking lights. I think by the time I got to the last step the star was already drifting off.
I had the slit aligned in RA because we have quite a lot of PE on the mount, but in hindsight the dec drift, which is small but measurable, would take the star off the slit never to return. If I align the slit in dec then at least the PE will take the star on and off the slit.
But really the answer is to start working on guiding and using the societies SBIG rather than my DSLR. Another couple of cans of worms
Calibration was with IRAF It uses a polynomial to fit the wavelength solution and I don't think it works too well when there aren't calibration lines right across the image. I've ended up applying a linear dispersion based on features in the spectrum which gives me a good fit.