Help for a newbie?

[Dr Paul Leyland]

Would anyone be willing to help out a complete newbie at spectroscopy?

I fitted a Star Analyser 200 to my 0.4m scope and snapped some images of Arcturus. No particular reason for that star other than I used it for focusing. The first thing I discovered was that the grating is at about the worst possible orientation with respect to the CCD. That will be fixed before the next attempt. The attached image, a 200ms exposure, shows what I mean — note the angle of the edges of the frame! Second, a subsequent browse around the web suggests that Arcturus has a relatively featureless spectrum and I may have been better off with Vega or Antares for a first attempt. There is an absorption feature towards the blue end but I don’t see much else

Any suggestions for good targets on which to practice and what relatively easy to use software I should consider installing? Linux, or architectural neutral or Windows software which runs under WINE is preferred. Is stacking multiple images a good idea, as it is generally in astrometry and photometry and, if so, how to do it? My approach to stacking so far is to plate-solve and then stack on the images’ WCS but that may not work so well on spectra, though I can give it a try in a more crowded field.

Thanks,

Paul

Attachments:

1. 

   arcturus.png
   

[Eric Watkins]

Hi Paul,  I’m in a similar position.  I have just bought a 100 line star analyser which I’d like to try out on my 16 Newt.  So would be interested in the advice you get.

Regards,

Eric

[Robin Leadbeater]

You ar in the right place, I developed the Star Analyser

P Cygni is an excellent test target  It shows nice clear emission lines of H and He, ideal to focus on (focus on the features in the spectrum, not on the zero order star image, noting that the field curvature, particularly with long spectra means that the focus will vary along the spectrum to some extent

http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm#SA200_filter_wheel

16 inch is a big aperture for a slitless converging beam setup though unless the seeing is exceptional. The resolution is at best defined by the size of the star image relative to the length of the spectrum. The problem is as you increase the length of the spectrum the aberrations of the simple setup get worse and if you try to use a lower focal ratio to keep the star image small, the aberrations also get worse due to the steeply converging beam so these setups work best on modest apertures. give it a go though and I can help steer you to get the best out of  the setup

don’t worry too much about software at this stage, concentrate on getting a clear image. It is a low resolution “broad brush” type of device and most of the interesting features the Star Analyser are immediately visible. This is an example of the sort of features you should be able to pick up. 

https://britastro.org/observations/observation.php?id=20201216_234948_8cabda965bfe692f

Cheers

Robin

[Robin Leadbeater]

As a rough guide to what setups work I developed a simple calculator which is hosted on the Rspec website. (The Rspec software there is good for beginners using the Star analyser but you will  outgrow it  as you progress to more research based spectroscopy)

https://www.rspec-astro.com/calculator

There is a tutorial on the Star Analyser on the site here

https://www.britastro.org/downloads/15701

and if you are interested in its development, I talk about it here

https://britastro.org/video/11250/12234

Cheers

Robin

[Robin Leadbeater]

Yes it is standard practise to align and stack shorter exposures with slitless spectra (on the zero order image) as it avoids guiding, effectively using the same techniques as planetary imagers to select the sharpest images (but no further processing eg sharpening etc) modern CMOS also means the noise penalty is lower than with CCD 

Cheers

Robin

[Dr Paul Leyland]

Thanks, Robin, for getting back to me so quickly.

I used the calculator to determine that with my equipment the SA200 was the better bet. Your advice: “On telescopes with a larger aperture and a longer focal length, the dispersion of the SA100 can be insufficient to achieve the best results.” tallied well with my set-up — a 0.4m f/6.5 Dilworth with a SX imaging train. Plugging the relevant numbers into the calculator seemed to bear out that advice.

I will check out the other links too.

[Robin Leadbeater]

Sorry for any typos, I am on holiday and using a portable device. Normal service should be resumed next week.

[Dr Paul Leyland]

Thanks. I will give that one a try when we have a decent night here. A calima has filled the sky with very fine dust 8-(

Would an off-axis stop help for bright objects, do you think? It would reduce the sensitivity and I generally dislike throwing away photons.

If I get anywhere I may consider using a more conventional spectrograph, but that is serious money to spend on a whim, hence the SA.

[Robin Leadbeater]

If you are limited  in how far away you can mount the grating then the SA200 can give you the dispersion you need for useful resolution but it does not quite match the SA100 at double the distance. The field curvature is also more severe and you can see this in your spectrum as it widens towards the red end. Getting the best result is a compromise with what is a primitive setup but think of it as good inexpensive way to “cut your teeth” before upgrading to a “proper” spectrograph if the bug bites

[Dr Paul Leyland]

Yes, I am very limited  as to how far I can mount the grating, unfortunately.

[Robin Leadbeater]

The effect of an off axis stop is an interesting one. The star image size will remain the same  (assuming seeing limited) so the maximum theoretical resolution is unchanged. The defocussing effect of field curvature should be less due to the increased depth of field with the higher focal ratio and the chromatic coma should also be less with the less steeply convergent beam. I did suggest this to someone recently though in a similar situation and they reported no obvious improvement.  Best approach I would say is to see what you can get on the existing setup and then try a stopped down aperture. (What spacing do you have currently?)

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