I will reply to all posts here to avoid having to chase up and down the thread. (I have to say I don’t like this type of forum layout. I much prefer the more common layout like Stargazerslounge etc which are linear and you can quote)
Reply to #8
>(i) The spectrum gets wider (more height in Y-axis) as it goes down the sensor.
Yes to me this is unexpected but we need confirmation if it is normal. No clear skies here but I will dig out my artificial star or perhaps replace the slit for a hole and see what I get. I think we need to check this first before adjusting anything else.
>(ii) As well as this, the overall integrated flux gets bigger – an increase of around 50% from the two extremes explored here. I am assuming the final Isis image has removed the background and so the flux is signal, rather than background+signal. (You might expect total flux to increase just because its a bigger area…)
No the total flux should stay the same. (The light from the star is just being spread over more rows). The flux may be changing as the mirror vignettes the return beam
>(iii) However, the spectrum is not so good in the larger-flux, lower-in-image version. In particular, it doest work well with the optimal binning option
Optimum binning selects particular rows to maximise signal/noise. For flux measurements, turn off optimal binning (and just set the binning zone to just include the full height of the spectrum (stretch the image to see the full extent) and subtract the background setting the background zones away from the spectrum above and below
Reply to #14
>By the way, do you know how Isis calculates R? It seems to do it from the image/spectrum and not from parameters as such.
I think it calculates it from the lamp spectrum. It can give variable results though. I prefer to measured the FWHM directly off the lamp spectrum. R = wavelength/lamp line FWHM
Reply to #23
>I have not tried loosening the screws on the grating holder – it sounded scary 🙂 But maybe I should try and re-tighten to get a feel for how this works
I recommend we clarify the widening with mirror position first before touching the grating.