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Star Analyzer with and without a prism

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#1 cardanoc

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Posted 13 November 2019 - 10:55 AM

Greetings

For a long time I wondered about the benefits of using a prism along with the Star Analyzer.  I decided to set up an experiment in my basement.
I have a 50ft dark basement.  At one end, I place a neon test bulb. It was wrapped in foil to trap the light; however, a small hole was added to simulate a star.
At the other end of the basement, I set up Astro Tech 72mm fl ED refractor in an uncollimated arrangement.  I used an ASI 174MM (monochromatic) camera. I attached a Star Analyzer (100l/mm), with and without a prism, and took many FITS images.  Flat fields and dark frames were also taken.

 

The images were processed with Astro Art. 

 

The only processing done was to average all frames and correct for flat/dark fields.
I use the 3.8 degree prism from Paton Hawksley.
All focusing was done with RSpec.
I have attached my results.
These differences are obvious.   The prism definitely makes a difference.

 

Enjoy 

 

Conrad Cardano 

Rhode Island

Attached Thumbnails

  • ASI_50_final.jpg
  • ASI_GRISM_50_final.jpg

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#2 descott12

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Posted 13 November 2019 - 11:36 AM

That is quite a result! I was just reading about that prism yesterday. I read that it is harder to use/calibrate as the response is non-linear. Is that true??



#3 j.gardavsky

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Posted 13 November 2019 - 11:50 AM

Yes, the prism cleans up the spectrum.

 

Back in the days, the SOPRA Paris has designed and manufactured the grating/prism "double" monochromators for their spectroscopic ellipsometers.

 

Best,

JG


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#4 cardanoc

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Posted 13 November 2019 - 11:52 AM

Hi

 

Yes, this is true.

 

With a prism, the dispersion of the spectra across the chip is not linear.

 

I always have the star in the field of the chip to help me with the calibration.

 

I try and have as many lines as possible to help me construct a good calibration curve.

 

Conrad

 



#5 Organic Astrochemist

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Posted 13 November 2019 - 01:19 PM

Thanks for the comparison data, but I'm not sure it tells the full story. The Neon lines you have observed are all towards the red end of the spectrum.

 

Consider this spectrum of an A2V star, 29 Cyg, without the prism and notice how many Balmer lines I could see, probably to about H-zeta. The instrument response isn't perfect but I could probably improve that.

29 cyg with A2V.png

 

Here's another A2V star, 26 Peg, with the prism. I think my blue performance has been severely degraded. I think I will probably remove the prism.

26 Peg with A2V.png


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#6 cardanoc

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Posted 13 November 2019 - 01:28 PM

Hi

 

That's very interesting.

 

According to the literature, adding a prism benefits most in the red end of the spectrum.


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#7 CPellier

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Posted 13 November 2019 - 03:16 PM

I'm also using the 3,8° prism since a few weeks and I do agree that it noticeably improves the sharpness of the spectrum, as well as the spectral resolution, even if, of course, this does turn the SA into an ALPY600 wink.gif. Indeed for me the improvement is noticeable in red and infrared, but the problem with the red sharpness of the SA spectrum alone comes from the chromatic coma that is corrected by the prism, since often the spectra will be focused in green light.

I did not noticed any degradation in blue light light, although the differences are less important the spectrum is still sharp. I'm using a 305 mm Newtonian. Here is Beta Aquarii (Sadalsuud) in red (SA alone) and blue (with the prism) ; these are uncorrected ADU spectra recalculated as if total exposure was 1 second, with the infrared part.

The only drawback is that calibrating the spectrum with the prism is way more complicated!

    

Attached Thumbnails

  • sadalsuud_sa_prism.PNG

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