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Objective prism

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#1 Marco Prunotto

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Posted 14 June 2019 - 04:10 PM

Hi Everyone,

I’m in search of an prism (7-8 degree) to be used as an objective spectrograph for a 8” apo telscope to make some outreach activity with the classes of stars.

Can someone help me?

Best,
Marco


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#2 mashirts

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Posted 14 June 2019 - 04:34 PM

Here's Edmund's in the US.  25mm seems to be the largest.

 

https://www.edmundop...e-prisms/12456/



#3 TieDyeAstronomer

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Posted 14 June 2019 - 05:49 PM

Hi Marco!

 

Are you looking for a prism that will cover the objective of your 8"? You are unlikely to find one of that size which is of sufficient quality, and if you did it would likely cost a fortune.

 

Have you investigated a Star Analyzer grating? I have one, and it's excellent. I use it for both outreach and science. I was able to show some people at an outreach in the city recently the emission lines of EZ CMa, a ~7th mag Wolf-Rayet star, using my 8" dob and the Star Analyzer 100. And I'm actually reading CN while I wait for some of the spectra I recorded last night with my Star Analzyer to stabilize in PIPP. When it comes to recording spectra, I can get spectra of 7th magnitude stars with only a minute or so of drift scanning in my untracked 8" dob. 

 

 

 

If you are set on using an objective configuration, you might try printing your own large diffraction grating. There is a free software near the bottom of this page which will generate an image to print on transparency film using a laser printer:

 

https://www.coaa.co....e_astronomy.htm

 

I used to use an 8" objective grating printed with the above software before I got my Star Analyzer, and while it did make enough of a spectrum to show the difference between the components of bright double stars to visitors at outreaches (and wow the little kids with rainbows), it is not very efficient. You're blocking out half of the light with the printed lines, and the printed grating directs less light into the spectrum itself than the Star Analyzer, which is blazed to deliver most of the light into one of the first order spectra. I also seemed to notice that the dispersion with the printed grating was lower than the theoretically predicted-- perhaps due to lines blurring together? If you go the printed grating route, you will want to experiment with different wine widths/spacings to figure out the optimum for yourself.

 

I can't recommend the Star Analyzer enough-- it really is a gem of a tool. It took some deliberation on my part initially because of the price compared to what I could print myself, but it's 100% worth it.

Clear Skies!



#4 TOMDEY

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Posted 14 June 2019 - 06:21 PM

Well, Marco ... a good 8-incher is getting up there! To attain full-advantage, it needs good wavefront (sic material and surfaces)... which drives the cost way up, in that size. I've seen such at B&L and Kodak... special-order, always reserved for customers.

 

I'd suggest making it at home, with all the challenges involved in producing a superb plano-plano.

 

Or contact Glass Fab  (Dan in Rochester, NY) or Edmund Optics. I know they are entirely capable of making that for you... and (I'm guessing) the cost would far far exceed that of your telescope... and be worth it!

 

At work, we had Edmund make a bunch of ten inchers for us, but very small angle --- 0.5 degree. My only recollection was they were superb and costly.

 

Closest I've come (at home) is this 4-incher, which is superb... big angle though... ~20 degrees! >>>    Tom

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  • 52 toms objective prism.jpg

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#5 Spectral Joe

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Posted 14 June 2019 - 08:32 PM

Look into making a liquid prism, should be good enough for outreach. Otherwise, I would agree that the Star Analyzer is a good option. Or just a transmission grating before the eyepiece.



#6 BGRE

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Posted 14 June 2019 - 08:40 PM

Should you decide to make it yourself, it will need to be tested in transmission (merely getting the 2 refracting surfaces accurately plane may not suffice), for which you will need an 8" (or larger) return flat and either an 8" (or larger) aperture Twyman-Green of Fizeau interferometer to test it in transmission as it will be used.
The 8 inch apo could be used as the collimator so only the transmision flat (or beamsplitter for a Twyman-Green) is required.

Alternatively in principle its possible to do an autocollimation test first with the return flat only and then with the prism inserted between the return flat and the collimator. Subtract the 2 wavefronts to obtain the prism double pass transmission error.
Do not adjust focus between these measurements. Either a Bath interferometer or a point diffraction interferometer could be used in conjunction with the 8" apo acting as a collimator/auxiliary lens.
NB the return flat position and orientation will have to be adjusted to compensate for refraction within the prism.

Edited by BGRE, 14 June 2019 - 10:03 PM.

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#7 BGRE

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Posted 14 June 2019 - 08:46 PM

Look into making a liquid prism, should be good enough for outreach. Otherwise, I would agree that the Star Analyzer is a good option. Or just a transmission grating before the eyepiece.

Another option is perhaps an acrylic prism, at least its easier to work (using hand tools for rough shaping if necessary).
However the blank will need to be more than 1" thick to achieve a 10 degree prism angle.

#8 TOMDEY

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Posted 14 June 2019 - 09:48 PM

Should you decide to make it yourself, it will need to be tested in transmission (merely getting the 2 refracting surfaces accurately plane may not suffice), for which you will need an 8" or larger return flat and either an 8" )or larger) aperture Twyman-Green of Fizeau interferometer to test it in transmission as it will be used.
The 8 inch apo could be used as the collimator so only the transmision flat (or beamsplitter for a Twyman-Green) is required.

Alternatively in principle its possible to do an autocollimation test first with the return flat only and then with the prism inserted between the return flat and the collimator. Subtract the 2 wavefronts to obtain the prism double pass transmission error.
Do not adjust focus between these measurements. Either a Bath interferometer or a point diffraction interferometer could be used in conjunction with the 8" apo acting as a collimator/auxiliary lens.
NB the return flat position and orientation will have to be adjusted to compensate for refraction within the prism.

Yep - that describes the challenge quite nicely! Just the metrology part is not trivial... then getting the material and building the prism... not for the inexperienced or faint of heart! When I was doing my stuff, at least I had access to ~walk-up~ calibrated interferometric cavities at work! So I'd measure my hobby stuff during lunch break, or after hours. Thankfully, management encouraged us to obsess as a hobby as well as at work.

 

On the other hand ... a really good objective prism is certainly a wonderful way to collect myriad spectra all at the same time!

 

My feeling is that making prisms should come after successfully making a few optical-grade planos! Sort of like making a nice 6-inch achromat lens only after successfully making a few objective mirrors...    Tom



#9 KLWalsh

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Posted 15 June 2019 - 07:20 PM

You might want to search for a used aircraft instrument lighting wedge. They’re thin wedges used for front-lighting old needle-and-dial cockpit gauges and even some newer LCD gauges.

The wedges are usually precision components. One or two might suit your needs. BUT, you might have to use an aperture mask and use a 3 inch wedge. Or you might get lucky and find something larger.

And of course, there is the Star Analyzer Grating used for spectral studies, in lieu of a prism.

Edited by KLWalsh, 15 June 2019 - 07:22 PM.


#10 TOMDEY

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Posted 15 June 2019 - 08:35 PM

Ummm... for what it's worth... noticed my original photo up there is deceptive regarding what is the 4-inch clear aperture and the polished face --- These construction lines show it. >>>

 

[My original statement stands... Very good ~diffraction-limited~ prisms are very tough to build!]    Tom

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#11 hamishbarker

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Posted 15 June 2019 - 08:57 PM

or for really cheap - print a 4 line/mm objective grating on transparency film. it's not as good as a star analyser 100 grating, because the film isn't usually optically good, and seems to get some wavyness, probably due to the heat of the laser printer.

 

I used this:

https://www.coaa.co....ating_setup.exe

 

but the spectra from the SA100 are much better due to blazing making a quite efficient first order spectrum on one side.

 

here's a paper about laser printing transmission gratings.

http://aapt.scitatio....1119/1.2768688

 

and here's a paper about objective gratings made of mylar, ruled using a lapped fountain pen nib, on the oschin (palomar) 48" schmidt camera. only amplitude gratings (i.e. black and white lines), best efficiency about 0.22. Obviously prism at efficiency circa 1.0 (minus some reflection and transmission losses) is going to be much brighter.

http://adsabs.harvar...PASP...82.1133V

 

One great technique I have used in outreach is putting a dslr in the focuser with a SA100 grating in the converging beam, and taking circa 15 second exposure with no drive (i.e. not tracking equatorial motion). if the grating dispersion axis is arranged parallel to the dec axis, the long trailed image of the star (zero order) and spectrum make absorbtion and emission features really obvious. I was doing this on NGC6231 (an open cluster in scorpio which I chose at random) when I noticed that one of the stars had a big blue emission line! Turns out that cluster has several Wolf-Rayet stars, and i've been enjoying reading a bunch of papers about the type.


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#12 BGRE

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Posted 15 June 2019 - 08:59 PM

Another option is to assemble an "eyepiece" containing a direct vision prism preceded by a field lens to recollimate the light before it enters the prism.
However such a device works best on axis and off the shelf direct vision prisms are only available in small sizes.
available in small sizes if at all.
Perhaps one of the Surplus shed direct vision spectroscopes could be used with the slit removed??

A larger dispersion prism is needed if the prism is located in the eyepiece exit pupil to achieve the same results as an objective prism.

Edited by BGRE, 15 June 2019 - 09:00 PM.

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#13 hamishbarker

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Posted 15 June 2019 - 09:00 PM

baader planetarium sells 140mm 7 degree wedge  flint glass objective prisms. it says 5.5degrees perhaps this is the deviation angle?   420 Euro.

 

https://www.baader-p...)---140-mm.html



#14 BGRE

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Posted 15 June 2019 - 09:09 PM

They probably meant 5.5 inches rather than 5.5 degrees. A 7 degree F2 prism wont have a deviation of 5.5 degrees.

#15 gregj888

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Posted 16 June 2019 - 01:06 AM

Marco,

 

Not sure what you are doing, but would this help?

 

http://astro.physics...m-ajp-jan03.pdf



#16 Oregon-raybender

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Posted 16 June 2019 - 02:07 AM

Since this is a class project or out reach. I would suggest a simple plastic transmission grating over a camera system. Instead of adding one on front of the scope with a limited FOV. Using a camera you can image a wide angle of various stars and also street lamps, to show the spectra. Image of Orion shows several stars that clearly are different. Color cameras do a wonderful job and is easier to use than a scope. Have the students cover the sky, the results would be interesting. Also the displaying the data is in minutes.

 

Starry Nightswaytogo.gif


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#17 KLWalsh

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Posted 16 June 2019 - 08:25 AM

Just another reminder of an alternative to using a prism is the Star Analyzer grating. I have one and it, and the software written for it, are great.

https://www.rspec-as.../star-analyser/

Edited by KLWalsh, 16 June 2019 - 08:25 AM.

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#18 BGRE

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Posted 16 June 2019 - 08:34 AM

That works best if its located in a pupil plane.

#19 Marco Prunotto

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Posted 16 June 2019 - 01:42 PM

thank you all for your suggestion. I think I will go for the Baader 7 degree 140mm prism.

Thank you all for your inputs

Best,
M


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#20 hamishbarker

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Posted 17 June 2019 - 02:19 AM

SA100 is RRP 124 Euro, baader prism 425 Euro. SA100 is linear dispersion, prism is nonlinear. Baader may be higher resolution than the circa R=70 of the SA100, but I don't know how to calculate R for prisms (need to look it up). But for outreach is that really going to be the critical thing?

 

Dispersion for the 7 degree prism spectra might be be higher or lower than SA100, depending on focal length. For the same dispersion, the prism will be brighter (about double).

 

I think for outreach, SA100 is much better. It's much cheaper (3 outreach SA100s or one prism?) and lighter, less likely to get damaged, and unless the outreach location is so light polluted that the extra brightness is absolutely critical, still going to show the different spectra sufficiently brightly and clearly (at least for O, A and B and WR stars down to say 4 visual magnitude and of course much lower for imaging) . 

 

Am I missing some other vital criteria? Is resolution of spectra from the prism going to be much much better? I guess for survey work, only one spectrum per star (no -1 order , zero order or 2nd order spectra ) so the fields are less crowded. But then, the zero order images make it much easier to follow back to which star belongs to each spectrum.

 

I've also used my SA100 on binoculars, taped as an objective grating over one of the objectives, it makes for interesting viewing with one side of the binoculars dispersed, and the other not.

 

I have also previously purchased a big square of 1000lpmm plastic grating (150mm square) on ebay like this,

https://www.ebay.com...pMAAOSw-ttcoL6r

 

but without the case, I think it was about 15 dollars. i have cut a few objective gratings out of it and stuck them on various camera lenses and still have 75 x150 left. Will someday also try using it in front of the 200f6 newtonian, aimed at a ball bearing or chrome rod in sunshine against a black cloth to see if I can view a really high dispersion solar spectrum with minimum equipment.


Edited by hamishbarker, 17 June 2019 - 03:59 AM.

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#21 BGRE

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Posted 17 June 2019 - 09:16 AM

Prism resolution = t*dn/dlambda
where t is the difference between the prism thickness at the top and bottom of the beam.


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