Star Analyser direct or SA grism

[Andrew Smith]

Robin and others - I notice that Robin continues to just use a SA driect in the converging beam without the improvments of a wedge prism and/or a UV/IR cut filter as popularised bt Christian Buil. I was jus wondering why.

Thanks Andrew

[Ken Harrison]

There's minimal (<10%) improvement using a grism with the SA100.
It's benefits kick in with gratings >200 l/mm due to the increased dispersion. (In Christian's example the benefit is in the 2nd order ie equivalent to a 200 l/mm in 1sr order)
Adding a UV-IR filter just limits the wavelengths recorded, and as Christian says, suppresses the 1st order overlap on a 2nd order spectrum.....

[Robin Leadbeater]

It is popular with some users. In my particular case I cannot fit a wedge prism in the filter wheel.
but there are a number of other reasons I do not personally recommend it for the SA100, particularly for beginners

The alignment is critical. If you do not get it right you can make things worse.
Focussing becomes more difficult due to the astigmatism introduced
The wavelength calibration becomes significantly non linear
The zero order is no longer a reliable calibration reference point (The zero order becomes smeared out into a short spectrum, the centroid of which varies with the spectral type)

In practise the improvement appears to be marginal with low dispersion angle systems like the SA100 except perhaps when working in the far red/IR. With the possible exception of Christian's results, the best results I have seen published using the Star Analyser are without a wedge prism (Care with focus, good seeing and keeping the star image size small by limiting focal length are much more important.

Cheers
Robin

[Robin Leadbeater]

I only filter the Star Analyser when working beyond 7500A where there is a risk of overlap from the 2nd order spectrum. In this case I use a red filter with a sharp cut off at 6100A.
http://www.threehillsobservatory.co.uk/ ... poster.pdf
This has the advantage over a more traditional green/yellow order filter that it also cuts out much of the artificial light pollution as well.

Cheers
Robin