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Objective Gratings

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#1 cardanoc

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Posted 14 January 2020 - 04:39 PM

Hi

After reading Uwe Zurmühl article "Objective Gratings for Amateurs" in Spektrum 55, I decided to try an objective grating and stopped spending my time on a slitless spectroscope.

 

While the Star Analyzer 100 is readily available, it is small and would only be useful on the brightest stars.  I decided to try the 45mm format grating from Paton Hawksley.  I chose the 100 lines/mm grating to give me a wide field of view. 

 

I built a slide holder and connected it to the front of my 72mm f/6 refractor.  According to TransSpec V3.2, by Ken Harrison, I should get a resolution of R = 2000.  This is ten times greater than my converging beam setup using the Star Analyzer 100.

 

I was very pleased with the results.  I used Menkalinam, an A2V star, for focus and calibration. With all of the discussion of Betelguese, I wanted to take a spectra of it.  I took thirty 40 second exposures (along with dark frames) using an ASI 174mm camera.  I stacked the frames using Astro Art. 

 

The attached picture shows my Betelguese spectra.  This is far better than any spectra I ever took with my converging beam setup.

 

I made some inquires with several manufacturers about getting a larger 100 lines/mm grating (i.e., 75mm square).  Sadly, the costs went from $500 to over $1000!

 

 

Attached Thumbnails

  • Betelguese.jpg

Edited by cardanoc, 14 January 2020 - 04:41 PM.

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#2 TOMDEY

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Posted 14 January 2020 - 04:54 PM

Cool! Like all things optical, grating cost goes up fast with size... because the precise phase-relationship across the entire format must be maintained to achieve the theoretical spectral resolution. I have this nice objective prism... and haven't even tried it yet!    Tom

 

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  • 104 original Fraunhofer objective prism 1899 and Toms.jpg


#3 StarBurger

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Posted 14 January 2020 - 05:11 PM

From a spectroscopy ignoramus: I have to ask if the "plastic" copies of a ruled grating will do the job as well. 

I guess I am answering my own question- if they were any good we would all have them?

Just the same the spectrogram of Menkalinam is very impressive. Well done.


Edited by StarBurger, 14 January 2020 - 05:13 PM.



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