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"Pinched" spectra in RSpec with SA-100

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#1 descott12

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Posted 24 July 2019 - 10:14 PM

Hello All,

I have had pretty good luck with my SA-100 using my Evolution 8 SCT. However, it is out for repairs so I tried using my 102 mm f6.5 refractor and I got some strange looking spectra. I am not sure if it is due to the refractor or not. As you can see, the spectrum is pinched around the green/yellow wavelengths. And this pinching was even apparent when just looking thru a higher power eyepiece.  I don't remember seeing that with my other scope.

 

Any idea what is going on?

 

 

EDIT: This was Vega (almost straight up overhead) with an ASI 178 MM camera. I realize my focus is not good but I didn't get far enough to fine tune it as I was trying to figure out where the pinch effect was coming from.

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Edited by descott12, 25 July 2019 - 06:14 AM.


#2 robin_astro

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Posted 25 July 2019 - 10:02 AM

Hi Dave,

 

This is caused by severe chromatic aberration in the telescope optics. (The focal plane depends on wavelength so with an achromat for example you only get 2 wavelengths in perfect focus. In this case at the "pinch point" and the far red end).  It will affect the resolution along the spectrum but should not affect the overall shape provided you include the full width in the binning zone. The dip in the spectrum at the pinch point in this case is probably due to overexposure and saturation where all the light is concentrated on a small number of rows of pixels. You could probably play with the focus to better average it out but ideally a reflector (or SCT) or a good quality apochromat would be better. 

Cheers

Robin


Edited by robin_astro, 25 July 2019 - 10:04 AM.


#3 robin_astro

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Posted 25 July 2019 - 10:09 AM

See also the FAQ #9 in the Star analyser manual

https://www.patonhaw...y.com/resources


Edited by robin_astro, 25 July 2019 - 10:11 AM.


#4 descott12

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Posted 25 July 2019 - 10:26 AM

Robin,

As always, thanks for the information. By the way, the doc in the link is very helpful. Not sure how I missed that...

This is my first refractor so I have never experienced chromatic aberration before.




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