BAA spectroscopy database

This is great timing...

I have recently been investigating the purchase of a Star Analyser and RSpec. I'd like to classify RV Tauri spectra over their pulsation range.  I have a 12" scope @ f5.4 with a STT6303me and FW8-STT Self-guiding filter wheel.  I've used the calculator and determined that the SA200 is the better choice.  I'm just not sure I can get meaningful detail sufficient to classify at sub-levels. Could you offer any advice?

Thanks for your help

Mke

Spectral classification is normally done at a resolution of 5-10A,  compared with a typical resolution of a few tens of Angstrom for Star Analyser setups so the spectra you get are pretty broad brush.(Good for identifying broad emission lines in Novae/Supernovae for example but less good at teasing out changes in finer features).  Obvious changes can be seen such the the change in continuum shape through the temperature range, the Balmer absorption lines disappearing from A to F or molecular bands developing from K  to M for example.  

This set of spectra by Jim Ferreira gives an idea of what can be acheived with a Star Analyser across the spectral classes.

 http://www.lafterhall.com/epsilon_cassiopeiae_B2pvar_25sep2012_10spec_i…

Though this is with a small aperture giving ~20-30A resolution.  Your resolution is probably going to be less than this due to the larger star image size of your larger aperture. In a star image size limiting situation which I suspect yours will be (depending on your seeing and how far the filter wheel plane is from the camera sensor), the RSpec calculator can be used to roughly estimate the resolution ( The dispersion in Angstrom/pixel x the star image FWHM in pixels)

Robin

Hi Brad,

The short answer :-  I would say  using a focal reducer, combined with an aperture stop would probably squeeze the most resolution out of your setup but it is tricky to say exactly where the optimum point might be.  

Now for the long answer ;- 

There are various competing effects on resolution with these grating in the converging beam setups. and how to optimise these depends on the particular configuration.

 Ignoring for the moment the aberrations due to the converging beam configuration, we can increase resolution either by:

decreasing the star image size by reducing the focal length (or by moving to a better observing site ! )

or by

making the spectrum longer by increasing the distance between grating and camera sensor or by using a finer grating.

But now we need to consider the aberrations caused by the geometry of the setup which will ultimately limit the resolution. These are:

The change in focus along the spectrum due to field curvature. This is worse the longer the spectrum is and the greater the dispersion angle. Minimising this effect therefore favours coarser low dispersion gratings (ie the SA100 over the SA200) and shorter spectra (which needs a smaller star image size)

and

wavelength dependent coma which is  due to the beam converging rather than being parallel at the grating. This is worse for low focal ratios so works against our trying to reduce the star image size by using a focal reducer. It is also worse for finer, higher dispersion the gratings.  

The result is that large aperture scopes (which need a low focal ratio to give a small star image size) when combined with a short distance between the grating and sensor such as found in filter wheels (which need a higher dispersion grating to spread the spectrum out enough) are the most challenging configuration to give good resolution.

So to get back to your original question -   Using an off axis aperture stop combined with a focal reducer (thus reducing the focal length, but not dropping the focal ratio) is probably going to be your best way to get more resolution. (You are effectively making your big scope look like a smaller one!) I would not advise going below ~f5 though and always check that you are not undersampled (ie make sure the star image is not smaller than 2 pixels )

Another intereresting configuration I have seen using a focal reducer is to place it after the grating, thus keeping the grating in the less converging beam so keeping the chromatic coma in check but still getting the benefit of the reduced star image size, even at full aperture. This does need space between grating and camera which is not normally available in a typical standard imaging setup though. 

Robin

Field curvature and chromatic coma can also be reduced by combining the grating with a small angle wedge prism  (making a grism)  These are quite common in professional  setups where multiple grisms and filters are selectably placed in the beam (though normally in a parallel beam) to produce a low resolution spectrograph combined with an imaging system but a "grism star analyser" would be too thick to fit in a typical amateur filter wheel.

Robin

I believe this page shows the "optimization" Robin is discussing:
 http://www.astrosurf.com/buil/staranalyser2/evaluation_en.htm

James