Star Analyser : hopeless on visual only?

[robz (Robert)]

Hi guys.
Tried the S.A. on a 5.1 inch Celestron F5 reflector last night. I used a standard Skywatcher brand 25mm Super Plossl E.P.

The result was not good : I rotated the e.p. through 360 degrees and at best, could get a very(very!) thin spectrum on some of the brighter stars.
Collimation is good as the stars were fairly pin point and yes, I did focus on the spectrum not the stars.

I was under the impression that the S.A. was useable for some visual use, but it was impossible to see any detail in a spectra that was so thin height wise as well as short?

I estimate the S.A. to first element (bottom of e.p.) to be approximately 30mm. Could this be the problem or is there some other factor I am missing here?

Rob.

[sheeny (Al)]

Moving it further away from the EP will increase the dispersion, so that will help to spread the spectrum length ways. Try putting it in the front of a 2" barlow - maybe unscrew the barlow lens and screw it in - provided you can still focus.

To get the spectrum wider for visual use I think there was another grating (rainbow optics maybe?) that came with a cylindrical lens to spread the spectrum across the width. But see how you go without first - because spreading the spectrum in width will also make it dimmer.

I don't think I've ever really tried mine for visual.

Al.

[Merlin66 (Ken)]

Rob,
That sounds like the "normal" initial visual results...
I have a Baader and Rainbow Optics ( as well as an ol' GOTO DV spectroscope) which all use small cylinderical lenses to expand the height of the spectrum and make it much easier to see the defined Balmer lines in the brighter stars.
You can get some small cylinderical lens cheaply from Surplus Shed - or the expensive versions from Edmund Optics etc.
Such is life......

[Rob_K]

Hi Rob - I have a Star Analyser too which I sometimes use visually. I'm not sure what the problem is - perhaps it's one of expectation. The grating alone is never going to give more than a thin spear and it's only going to work on brighter stars. It's never going to give you the detail that you can get by manipulating a long-exposure photographic image.

But that spear can tell you lots of things - which end dominates (red or blue), major absorption lines etc. Try it on a carbon star and see what it looks like. Personally I reckon that using these gratings visually is magic but only if you accept the limitations. Good luck!

Cheers -

[robz (Robert)]

Thanks for the responses everyone.
I guess it's early days and a bit more experimentation required.

I do have a cylindrical lens that came out of an Anamorphic lens. It's a bit on the big side :55mm diameter, but it's not that thick and is lightweight.

Suppose I'll eventually have to get a CCD security camera going with RSPEC software if visual is pretty limited?

[Merlin66 (Ken)]

Robert,
You can use almost any camera to record spectra - from a webcam, CCD to a DSLR.
The "trick" is to get a good distance between the grating and the Chip - around 60mm or so.
Orientate the spectrum horizontally across the chip. Look for and focus on the brightest (1st order) spectrum.
You can use RSpec, VSpec, BASS, ISIS or any of the other programs to process the results.

[robz (Robert)]

Thanks Ken.
I have a 2K native resolution CMOS webcam(got another with less resolution also) that seems pretty good in low light. I was thinking about housing it in a small box and making a Planetary cam. originally, but would it be sensitive enough for spectra work on stars?

I have wanted to get in to astronomical spectroscopy since I was a young teenager and am pretty excited to take the first initial step towards this direction.

Cheers,
Rob.

[Merlin66 (Ken)]

Rob,
Spectra are typically 5 mag fainter than the target star....ie you need a camera and exposure capable of recording 12 mag stars to collect spectra from a 7 mag star....

[robz (Robert)]

Ken, understood.
Better start saving for a camera by the looks of it?

Cheers,
Rob.