I’ll combine my replies to Robin and Andy here…
Andy said “ As I understand it you are moving the spectrum down the slit,”
No – I am changing the angle of the main mirror using the set screw on the bottom. I try and use a central slit position at all times. As you swing the mirror back and forth, you get the images I showed.
On grating holder… Today I tried the screws – they were as tight as they could be… Mmm.. (Thanks to Andy for posting the diagram of which screws to adjust on http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=997&start=20)
I took this opportunity to adjust the grating for uniform image placement over wavelength range. It wasnt bad before – a change of about 10% of vertical sensor size in going from Ha++ to zero-order image – but I got it down to 5%. They were tiny adjustments…To do this I used the Shelyak recommended hi-tech approach – piece of card with a gap in it, placed over the mirror slit – but it works for this. At the end, I just cinched the screws tight and didn’t drive them all the way home.
Robin noted that the total flux should stay constant. Logically it should, so let me explain what I am measuring. I am taking the final summed image from Isis, then just doing a windowed stats measuremnt of the spectrum image. But I think Andy also reported a change in total flux with this kind of manipulation…?
I am waiting for abreak in the clouds to try all this with a real star… It could be my slightly mis-oriented prism (and/or some overtightening of the grating clamp) was causing some artefactual thing here…