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How to improve these SA100 results?


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Hi Guys,

Following on from our last conversation, the DIY spectrometer went totally wrong!! So I'm now testing a Star Analyser 100.  Here are some results and I don't really feel I've got the best out of them.  Could you guys comment, please?  The set up is a SW80 with a DSI III pro - mono.  Sensor has 1360 x1024 by 6.45 μm pixels.  It also has some marks on the sensor.  I set it up at both 30mm distance and 50 mm from SA100 to the sensor.  These two are Bet-Cas and Del-Cas at 50 mm:

Bet-Cas

image.png.bc1e268e442e9e8161cf3ad7c1f2b125.png

Del-Cas

image.thumb.png.8ddd4d9dc7d05b7cf061531f88dd8d84.png

Del-Cas (RSpec binned)

image.thumb.png.90985fdd54641bed6930bb04662c1471.png

Potentially I worry that I am 1) out of focus, 2) Camera is too pixelated, 3) SW80 has too great a chromatic aberration, 4) the 50 mm distance to the sensor is too great, 5) maybe I need some flats or 6) All the above.

I've been driving with the C8-N and imaging with the SW80.  I suppose I could switch the cameras round and drive with the SW80 and image with the C8-N.  That would deal with the chromatic aberration.

Just before the clouds rolled in I took out the spacer and focused again.  It's just a random piece of sky in Taurus at 30 mm from the sensor.

 

I'd love to know what you think.

Regards

Steve.

image.png

Edited by SteveBz
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Hi Steve,

50mm spacing is a fine starting point for your sensor

There are a few obvious issues

1. Yes you have severe chromatic aberration which is varying the  focus along the spectrum and causing the fishtail effect.  Fast achromatic refractors are not good for spectroscopy. The Newtonian will be much better.

2.  It looks like you have some sort of Bayer pattern in the image. Does your software think it is a colour image ?

3. Your first two spectra may be over exposed. Err on the underexposed side to start with  which makes it easier to see and focus on the features in the spectrum. eg see the faint spectrum of a cool star at the top left of the wide field image which shows molecular bands before it goes way out of focus into a broad fishtail at the IR end

Cheers

Robin

Edited by robin_astro
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20 minutes ago, robin_astro said:

see the faint spectrum of a cool star at the top left of the wide field image which shows molecular bands before it goes way out of focus into a broad fishtail at the IR end

 

stever_SA100.png

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9 hours ago, robin_astro said:

 

stever_SA100.png

You always give such thorough and great answers.  This answer especially.   So my plan of action should be:

1 - Revert to 50mm.

2 - Switch the scopes (sadly, if I do this, I have to decommission my Orion Thin OAG).  

3 - Investigate the DSI III camera driver (there has indeed been quite a long discussion about it recently about NOT debayering the colour version, I wonder if they've got the 'not' in the wrong place).  I can get rid of the debayering problem by binning, but that reduces the resolution of the image, however, it buys me time while I do the other actions.

Thank you as always.

Regards,

Steve.

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4 hours ago, SteveBz said:

sadly, if I do this, I have to decommission my Orion Thin OAG

Most people normally run the Star Analyser without guiding, taking short exposures and stacking them but the distance from grating to sensor is not that critical so this might allow you sufficient leaway to mount the grating before the OAG say and you can still guide on a field star zero order if you want. You can use my calculator hosted on the RSpec website to explore the range.

https://www.rspec-astro.com/calculator/

(Closer gives a more concentrated but lower resolution spectrum for fainter objects while a larger distance gives more resolution up to a point for brighter objects. Avoid going too close and getting warnings but you can increase the distance until you get a warning about problems fitting the spectrum in the field.)

Cheers

Robin

Edited by robin_astro
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1 minute ago, robin_astro said:

Most people normally run the Star Analyser without guiding, taking short exposures and stacking them but the distance from grating to sensor is not that critical so this might allow you sufficient leaway to mount the grating before the OAG say and you can still guide on the field star zero order if you want. You can use my calculator hosted on the RSpec website to explore the range.

https://www.rspec-astro.com/calculator/

(Closer gives a more concentrated but lower resolution spectrum for fainter objects while a larger distance gives more resolution up to a point for brighter objects. Avoid going too close and getting warnings but you can increase the distance until you get a warning about problems fitting the spectrum in the field.)

Cheers

Robin

Yes, and I didn't guide either.  But If I switch back and forth between imaging and spectroscopy, I have to set up and align each time.  I was trying to have two parallel scopes and be able to choose on the night whether to image or take spectra.  If I have to switch over each time, then it takes half the evening (ie half the available viewing time) and we have so little time at the moment, it's a real heart-break to waste that time. 

Maybe I could find a cheap Newtonian like one of those little 76mm ones and mount it in place of the SW80 and still run parallel scopes.  In fact I do have a 114mm scope of 500 mm that might not be too heavy.  That's an F/5.  I'll look into it.

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