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First light Star Analyser 200 vs. SA-100

dslr beginner catadioptric
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#1 mwr

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Posted 07 December 2019 - 09:41 AM

I'm using the SA-100 for low resolution spectroscopy.  The achievable resolution is, among other factors, dependent on the grating to sensor distance. However, due to the limited back-focus capacity  of my system (f/9.4) I could never get to the optimum distance that lies at 23 cm. In such a situation, the use of the SA-200 is recommended by Robin Leadbeater: http://www.threehill...optimum_spacing

 

I have now replaced the SA-100 by the SA-200 and the results look quite good:

 

The first test object was the quasi-stellar planetary nebula IC 2149 in Auriga:

 

Folie1.JPG

 

10 Frames (300 sec. each) were stacked with Fitswork and analysed using RSpec. FWHM was determined for H beta and the results were compared (SA-100 vs. SA-200):

 

Folie2.JPG

 

The resolution could be considerably improved using the SA-200. The [O III]-lines are now clearly resolved.

 

The second test object was Capella (alpha Aurigae). 50 Frames were stacked (250 msec each) and the spectrum was rectified. Using the SA-200 most of the famous Fraunhofer can be resolved:

 

Folie3.JPG

 

A comparison with a spectrum of a G8 III standard star that was filtered to the approx. same resolution (Gaussian filter using VSpec) shows a good agreement:

 

Folie4.JPG

 

Resumée:

 

In my situation the purchase of the SA-200 was a good decision. Its usage is, indeed, highly recommended if not enough back-focus is available.

However, there is still room for improvement: An excellent spectrum of Capella using the SA-100 (with better optics and a more experienced user) can be found here:

 

https://forum.vdsast...2&t=4126#p25273

 

 

 

 

 

 

 

 

 


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#2 Organic Astrochemist

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Posted 07 December 2019 - 12:44 PM

Awesome results.

I really think it is unfortunate that for some people fitting the star analyzer into a filter wheel has been an important priority because, as you illustrate, a greater distance to the sensor really affords a much richer spectrum. And I think that really should be the point of spectroscopy.

 

Of course, not everyone has a camera as big as yours, but I think most people should try for as great a dispersion as possible. 

 

PN concentrate their light into a few lines so one can observe fainter objects. I'll be curious to see your limiting magnitude on stars. I live in a horribly light polluted city. When the sensor distance was ~14 mm I could easily see spectra for mag 10 stars, but now at about 100 mm the sky is really bright with mag 7 stars although I haven't tested the limits. I think that being able to change the distance to suit the target is a great advantage of the star analyzer.



#3 robin_astro

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Posted 08 December 2019 - 10:41 AM

 

I really think it is unfortunate that for some people fitting the star analyzer into a filter wheel has been an important priority 

Yes but the Star Analyser is initially there to get people to try spectroscopy for the first time and for many people, at least when starting out with spectroscopy, the reality is imaging is typically their priority not spectroscopy and they are looking for a way to try spectroscopy with minimal cost and disruption to their existing setup. If the bug bites of course then a dedicated setup optimised for the type of spectroscopy you want to do is the way to go. (Be it examining the spectra of  bright stars in detail in which case high dispersion is an advantage) or recording  fainter targets eg novae,supernovae and active galaxies to measure their cosmological redshifts etc in which case low dispersion is preferred.

 

Cheers

Robin



#4 descott12

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Posted 10 December 2019 - 10:18 PM

My understanding is that the SA-200 will increase the dispersion and this will widen the spectrum and possibly separate two overlapping peaks. Correct?

But is it actually higher resolution or is it simply stretching the spectrum?

 

My dispersion with my current setup is 3.65 A/pix which I believe is pretty good but maybe not. Is there an "ideal" dispersion value to shoot for?

 

Also, the depth of the absorption lines is more dependent on good focus. Is this true?

I was going to experiment with a greater distance and see what that did but I feel my primary problem now is that I am not getting very deep absorptions even though I have fiddled with the focus quite a bit.



#5 robin_astro

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Posted 11 December 2019 - 05:11 AM

Hi Dave,

 

With slitless systems like the Star Analyser the resolution firstly depends on the size of the image of the target relative to the length of the spectrum so anything you can do to make the target smaller will help (eg good focus, seeing, collimation shorter focal length etc).  Planetary nebula, even small ones are often more fuzzy than stars so can benefit from higher dispersion than stars. Eventually as you increase the dispersion though other aberrations come into play and limit the resolution. The lower the telescope focal ratio and the longer the spectrum the worse they become. At what point these aberrations limit resolution depend on the setup and although there are theoretical calculations, in practise it seems they are pessimistic and users are getting some good results using higher dispersions than theory suggests would be beneficial. Having said that 3.65A/pixel would normally be more than enough to give good results. Can you remind me of your setup, in particular what is the size (FWHM) of an in focus star image with your setup ?

 

Cheers

Robin


Edited by robin_astro, 11 December 2019 - 05:13 AM.


#6 descott12

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Posted 11 December 2019 - 08:20 AM

Hey Robin,

I am using by Evolution 8 SCT at f10 with a 178 MM (2.4 um pixels) camera.

I had to use the calculator to figure out what distance I am using based on my dispersion. My dispersion is 3.65 so my total distance must be about 70 mm and the calculator says the FWHM is 11.4


Edited by descott12, 11 December 2019 - 08:30 AM.


#7 robin_astro

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Posted 11 December 2019 - 10:04 AM

Hey Robin,

I am using by Evolution 8 SCT at f10 with a 178 MM (2.4 um pixels) camera.

I had to use the calculator to figure out what distance I am using based on my dispersion. My dispersion is 3.65 so my total distance must be about 70 mm and the calculator says the FWHM is 11.4

OK, the calculator works out the star image size based on perfect focus and the seeing (assumed to be 3 arcsec. If you know yours is better or worse you can fill in the appropriate number.)

 

The theoretical best resolution is the FWHM (pixels) x the dispersion (A/pix) = 42A in this case which is average for the Star Analyser but  if your star image is bigger than that (worse seeing, poor focus) you will lose resolution. 

 

To increase the resolution of your setup you can either reduce the size of the star image or increase the distance.

 

If you have a focal reducer this will reduce the size of the star image. (A common way to use the Star Analyser with f10 SCT) This would increase the theoretical best resolution to 26A

 

Your camera sensor is large enough to allow a larger spacing and still fit the zero order and spectrum. If you increase the spacing to 100mm, this would increase the theoretical resolution to 29A

 

You could even combine  the two and get a theoretical resolution of 19A, though the other aberrations might start to come into play then

 

Cheers

Robin



#8 descott12

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Posted 11 December 2019 - 10:28 AM

OK, the calculator works out the star image size based on perfect focus and the seeing (assumed to be 3 arcsec. If you know yours is better or worse you can fill in the appropriate number.)

 

The theoretical best resolution is the FWHM (pixels) x the dispersion (A/pix) = 42A in this case which is average for the Star Analyser but  if your star image is bigger than that (worse seeing, poor focus) you will lose resolution. 

 

To increase the resolution of your setup you can either reduce the size of the star image or increase the distance.

 

If you have a focal reducer this will reduce the size of the star image. (A common way to use the Star Analyser with f10 SCT) This would increase the theoretical best resolution to 26A

 

Your camera sensor is large enough to allow a larger spacing and still fit the zero order and spectrum. If you increase the spacing to 100mm, this would increase the theoretical resolution to 29A

 

You could even combine  the two and get a theoretical resolution of 19A, though the other aberrations might start to come into play then

 

Cheers

Robin

Thanks Robin.l was going to try increasing the distance and I was also thinking about getting a reducer anyway so what's a  few hundred more dollars! This hobby is definitely a money pit!!!




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