Trying out ISIS?

[Gasman]

Hi

I like to try out different software packages especially regarding spectroscopy with an Alpy 600 and been trying out some of the online tuts for ISIS but a bit unsure about a few things, maybe its the French-English translation or maybe its just me but one of the tuts  describes a method using a Lisa with the  Alpy600 where a reference image such as Vega is used as well as a Neon calibration spectrum but I`m not sure why the need for both?. Also I`m unsure how to actually calibrate a Neon spectrum as the `Calibration` screen seems to expect a spectrum with an Halpha line shown and simply asks for the x coordinate where the Halpha occurs?. Other packages like Rspec and BASS allow you to calibrate a spectrum by simply clicking on a peak and input the relevant wavelength. I`m sure I`m just missing it somewhere, might be a bit cheeky and ask Christian if he has a spare 5mins to knock up a youtube describing the method

cheers

Steve

[robin_astro]

Hi Steve,

This page perhaps better explains better the different suggested ways of calibrating  the ALPY and the improvement you get by combining lamp and A star spectra.

http://www.astrosurf.com/buil/isis/guide_alpy/resume_calibration.htm

The main reason for combining both an A  star and the lamp is to get better accuracy by wider wavelength  coverage of the reference lines.  (You can only rely on a non linear fit between where you have measured points)

ISIS only uses the approximate position of H alpha line or the bright neon line (and the pixels size) as a reference  to go and find all the other lines (star and lamp depending on which mode you select) automatically. It does not actually use the clicked value in the calculations, it finds the lines itself and measures their positions (like the pro software does) the calibration procedures that include the star spectrum assume that you have taken an A star reference in any case for instrument response correction so you will always have a star spectrum to use rather than having to identify a line in your unknown target (which is begging the question somewhat)

ISIS works best if you follow a set procedure for every spectrum you take (In fact it effectively forces you do follow best practise) but you can if you want also set it up to calibrate using any set of reference lines. It is just that the method described once set up allows a high degree of automation and repeatability between results. 

Cheers

Robin

[Gasman]

Thanks for that Robin, I am working through your translated tut!. I`ve been trying Isis for a couple of evenings and I think its a powerful piece of software albeit not very intuitive. For instance I have a stack of 20 fits files each one requiring a 0.7 degree rotation to get the spectrum horizontal (as much as I try I can`t get the slit bang on horizontal!!) , now do I enter 0.7 in the `tilt` box for the calib image (ie the first one of the stack) but if I do then I can`t set the graticule bang on the spectrum centre as its off horizontal so do I then rotate the calib image beforehand using such as Astroart and then use that for the calib image??. I assumed all the remaining images will then be rotated by 0.7 degs but that doesn`t seem to be the case or does all the images need to be rotated first before using Isis ?

Thanks again

Steve

[Gasman]

Hi again Robin

I think the penny has finally dropped so bleedin obvious!!. I couldn`t grasp why I had a nice Neon spectrum but ISIS was giving me an RMS of 40+ each time I tried to use it for calibration.

It turns out that I was selecting the `Alpy 600 with calibration module` and so ISIS expected Argon lines as well as Neon which is in the calibration lamp and of course I was only giving it Neon lines DOH!!.

The Neon lamp I was using is a nitelite which is fairly bright and needed only a second or two exposure but the Argon/Neon (Relco type) bulb is pretty faint needing longer exposures hence the `30 - 60 seconds` suggested in the tutorial.......all part of the fun(?)   Need a clear night now!

cheers

Steve