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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

 

~ click on ~ >>>

Attached Thumbnails

  • 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.


#4 devoldev

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Posted 20 July 2020 - 12:51 AM

in the last years I have tried repeatedly using objective gratings; particularly old two-star filters for photography by Cokin or by Hoya; they must be OLD because the modern one are non regular diffraction gratings; the OLD filters are simply regular diffraction gratings of about 200 l/mm; they may be found used on the market; however they are filters with a diameter until 80 mm and 200 l/mm lines or even 250 with some Hoya; the results are nice and interesting on brilliant stars; at the moment it is possible to ask for larger gratings to Holographix; but I don't know the price. You may use a 135 mm objective with a Cokin mount; an interesting trick is to do the spectra without any astrotracking but simply putting the camera with the filter perpendicular to the star pathway; in this way the spectrum is "written" on the camera in a 1-2 minutes B-type exposure. we may compare our results if you are interested


Edited by devoldev, 20 July 2020 - 12:59 AM.


#5 robin_astro

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Posted 21 July 2020 - 06:00 AM

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.

The problem with cheap plastic gratings and those used for photographic effects is they are usually very inefficient (Deliberately so in the case of those for photographic effects.) with little of the light ending up in the spectrum you want which limits you to the brightest objects.

 

For example with the Cokin #40 240 l/mm grating only 2% of the light is in the spectrum

 http://www.astrosurf...NGS EFFICIENCY 

 

For astro work look out for blazed gratings like the Paton Hawksley Star Analyser and some of their other educational grating. 

 

You can see some objective grating setups on here on my website using Star Analysers

 

 

http://www.threehill...troscopy_17.htm

http://www.threehill...troscopy_14.htm

 

 

Cheers

Robin


Edited by robin_astro, 21 July 2020 - 06:01 AM.


#6 robin_astro

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Posted 21 July 2020 - 06:08 AM

Hi

 

 

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

 

 

It depends how you define "better". Yes it is higher resolution because of the aberrations in the converging beam setup but you are still limited to bright objects. The same grating behind an 8 inch telescope can measure objects 15 magnitudes (1 million times) fainter than Betelgeuse

 

Cheers

Robin



#7 robin_astro

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Posted 21 July 2020 - 06:13 AM

 I chose the 100 lines/mm grating to give me a wide field of view. 

 

 

In an objective grating setup the field of view does not depend on the grating only on the focal length of the lens. In fact for a given dispersion (length of spectrum) a finer grating with a shorter focal length gives a greater field of view.

 

http://www.threehill...troscopy_17.htm

 

 

Robin

 

(link corrected)


Edited by robin_astro, 21 July 2020 - 06:15 AM.


#8 robin_astro

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Posted 21 July 2020 - 07:43 AM

Just realised this is a resurrection of an old thread, apologies if these points have already been covered

 

Cheers

Robin




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