Which Star Analzyer

[MattJenko]

Hi,

I am going to get myself a Star Analyzer in the next few days, but wondered which was the one to go for. I understand the 200 is for a filter wheel and meant to be closer to the sensor, but is this a compromise to allow use in a filter wheel and a 100 at proper spacing would actually be better? I can do either option, just wondered what would be objectively better.

Cheers

Matt

[Gasman]

Hi Matt

I`m a big fan of the Star Analyser and had the 100 for a good few years. Its an excellent little tool for cutting your spectro teeth on . Never tried the 200 as I progressed to an Alpy 600 after my Star Analyser. I have heard the detail might be a little better with the 200 but can`t vouch for that hopefully a 200 user will come on and comment. I gather Robin was involved with its development??.

cheers

Steve

[Merlin66]

The resolution of both the SA100 and the SA200 when used in a telescope ("converging beam") is about the same.

The greater dispersion of the SA200 is better suited, as you say, to shorter distances between the grating and the CCD.

[robin_astro]

Hi Matt,

In general if you are using it between telescope and camera and you can get the optimum spacing, the SA100 works slightly better giving better resolution on average over the wavelength range but it depends to some extent on the setup you plan to use. Can you give me more details?

You can also check using the on line calculator

http://www.patonhawksley.co.uk/calculator/ 

and you can  find more details on the SA200, comparing it with the SA100 on my website here

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

Cheers

Robin

[robin_astro]

Matt,

I have just looked at your astrobin image of Jupiter. This setup (without the Barlow) should work nicely with the SA100.  You can see the sort of thing you can do with a roughly similar sort of setup by Jim Ferreira here

http://www.lafterhall.com/spectroscopy.html

Cheers

Robin

[MattJenko]

Thanks Robin. I am getting close to pulling the trigger on a mono Atik 414Ex, so a smallish mono CCD chip. I am getting a filter wheel for LRGB filters for now and can get an SA200 to put in as well so that the whole unit can remain attached to the scope for both imaging and doing some spectroscopy learning. That way I don't have to replace anything and redo flats etc as it will all be in a single unit. I have no problem with having a separate setup for an SA100 though, it just means I need to be careful when swapping between spectro work and imaging. The small chip means I don't have to worry about flatteners and spacings too much either if that is a consideration.

Thanks

Matt

[robin_astro]

Hi Matt,

In that case I would probably go for the convenience of the Star Analyser (probably the SA200) in the  wheel but it could be  worthwhile checking the spacing you will have with both  SA100 and SA200 using the calculator as a small refractor does not need much distance because of the small  star image. 

The 414EX should be an excellent camera for spectroscopy. I use a 314L+ which is very good for this sort of work and the extra QE across the range of the 414 should make it even better. The only worry I had concerning a potential increase in ripples in the spectral response (which are difficult to deal with in slitless setups like the Star Analyser) appears to be unfounded.

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1068

Robin

[MattJenko]

Thanks again. Will order a SA200 at the weekend - excellent.

[nytecam]

Robin - perhaps you could clarify the 'geometry' of the image projected via the grating onto the sensor please? I've always assumed it's cylindrical so that zero order and spectrum are incline across the sensor and don't come to common focus. Does changing from 100 grating to 200 effect this issue if real? TIA

[Merlin66]

No, is the same effect - same aberrations....

I use the ol' Baader 207 l/mm....

In the converging beam there are always limitations. The 200l/mm just allows closer spacing (a la filter wheels etc.) between the grating and the CCD chip.

[MattJenko]

My SA200 has arrived, along with a SX filter wheel and a 414ex. Far too many new things to learn to expect any sensible output anytime soon, but can't wait to get practicing! Thanks for the advice.

[robin_astro]

Robin - perhaps you could clarify the 'geometry' of the image projected via the grating onto the sensor please? I've always assumed it's cylindrical so that zero order and spectrum are incline across the sensor and don't come to common focus. Does changing from 100 grating to 200 effect this issue if real? TIA

Hi Maurice, Ken

For the same linear dispersion (ie length of spectrum) the shift in focus along the spectrum due to field curvature is less for the 100 l/mm grating. Other aberrations due to the converging beam are also less for the smaller dispersion angle of the 100 l/mm grating which makes the 100 l/mm more tolerant of fast scopes. There is a direct comparison of actual resolution measurements using 100 and 200 l/mm gratings on my website here.

http://www.threehillsobservatory.co.uk/astro/spectroscopy_16.htm#SA200_or_SA100

This is why, where the larger required  distance can be arranged conveniently, I recommend using the SA100 over the SA200 .

In Matt's case the SA200 gives the advantage of  a common imaging/spectroscopy system using the grating in a filter wheel without having to rebuild for spectroscopy.

The differences are not dramatic between 100 and 200l/mm but higher dispersion gratings eg 600l/mm give very distorted spectra when used in the converging beam. 

Robin

[robin_astro]

The attached sketch shows the effect on field curvature of doubling the angular dispersion for same linear dispersion. (The dispersion angles are exaggerated here to show the effect. The actual angles for the SA100/200  are  only a few degrees)

Robin