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Visual spectroscopy with 120mm refractor options

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#1 spkerer

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Posted 07 February 2021 - 08:02 PM

I'm a visual observer.  What options - if any - do I have to do visual spectroscopy?  I've read through what I could find but I can't get a good idea of whether its a viable option to do visual spectroscopy with a 120mm f/7.5 triplet refractor.  I'm not interested in hooking it up to a camera or adding any electronics to my setup.

 

So... what options do I have if I want to be able to visually observe the spectrum of targets?

 

Thanks.



#2 John Harrington

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Posted 07 February 2021 - 08:38 PM

Spkerer, for purely visual spectroscopy, a good strategy would be to look for a used Rainbow Optics spectroscope, which combines a 200 lines/mm transmission grating with a cylindrical lens to spread the spctrum's width for easier observing.  The Rainbow Optics unit (designed by Jim Badura) is very efficient (i.e., puts the vast majority of light into the 1st spectral order), but with 120mm of aperture you probably won't be able to go much deeper than about the 4th magnitude.  There's a good review of the Rainbow Optics unit at:

http://users.erols.c...es/starspec.htm

 

Start by observing some nice, bright A-type stars like Altair or Vega, which should clearly show the Balmer absorption lines for hydrogen.  You could then try observing a cool dwarf star like Antares, which should show absorption lines created not by atoms, but by molecules like TiO.  

 

Spectroscopy can be very addictive!  I started with (and still enjoy) the Rainbow Optics model but am looking forward to acquiring a more sophisticated slit spectrograph from Shelyak.  For a good overview of amateur spectroscopy and its applications, you could view the recent AAVSO spectroscopy seminar online at:

 

https://www.youtube....h?v=fDJ2M3LlAAw

 

Clear skies,

 

John



#3 John Harrington

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Posted 07 February 2021 - 08:46 PM

PS:  If you do acquire a Rainbow Optics spectroscope, make sure to get the "visual and CCD" version that includes the cylindrical lens.  To use the spectroscope, simply screw the grating into the telescope end of your 1.25" diagonal, focus with your eyepiece, and then attach the cylindrical lens to the top of the eyepiece.  That will ensure sufficient separation between the grating and cylindrical lens to show absorption lines.  (Just screwing the grating into one end of an eyepiece and attaching the cylindrical lens doesn't give enough separation to really show absorption lines well.)



#4 Pete W

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Posted 07 February 2021 - 08:49 PM

Like John I've used a Rainbow Optics spectroscope for just visual use.  The diffraction grating threads onto the barrel of a 1.25" eyepiece and a cylindrical lens slips over the top of the eyepiece to extend the streak spectrum into a bar.  The eyepieces have to fairly small like older plossls or orthoscopics so that the cylindrical lens can slip on top.

 

I don't think they sell them anymore, but as previously mentioned you might be able to find one in the used market. 

 

I've used mine with an 8" newtonian.  Main sequence A-class stars will show hydrogen absorption lines and cool M-class stars show some bands.  Once you get fainter than about 4th magnitude the spectrum no longer appears in color.  Even faint Wolf Rayet stars will reveal some emission lines.  You can find a few in Cygnus, Canis Major and Scorpius.  By far the most impressive visual spectrum belongs to Gamma Velorum - the brightest Wolf Rayet star - but it may be too far south for you.



#5 Enkidu

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Posted 11 February 2021 - 03:02 AM

I spent some time failing to track down a used Rainbow Optics unit a while back. I ended up with a Star Analyzer 200 and a mounted cylinder lens from Thor Labs over the eyepiece, with a refractor and a ~2mm exit pupil, in good seeing. Cold winter nights perhaps. The target advice above is good.



#6 spkerer

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Posted 11 February 2021 - 09:10 AM

Thanks for all the responses.  Looks like a Rainbow Optics unit may be hard to come by, so I'm not sure its worth pursuing too hard.

 

I was curious about the Star Analyzer 100 or 200 for visual use - but I couldn't get a good idea about whether it would work for visual.  Everything I found about it seemed focused on some form of imaging setup with them.




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