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A New Solution to the 90 Degree Diagonal Problem

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#1 chemisted

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Posted 02 January 2021 - 12:11 PM

Old Nikon manual focus lenses are a favorite of NV device users especially for wide field views of emission nebulae.  Viewing at the zenith with the longer focal length lenses can be challenging, however, and at least one previous post has attempted to solve this problem (LINK).  My new approach takes advantage of using an eyepiece with the lens in an afocal observing setup (LINK).  I am using a Nikkor 300mm f/4.5 lens with 2" adapter in combination with the Russell Optics 85mm Super Plossl.  Because of the exceptionally long eye relief of this lens I can screw on my Baader BBHS T-2 diagonal to the top of the lens and still have plenty of eye relief left to interact with the NVD Micro.  I have only tested this during the day thus far but the 85mm works so well with my TV-140 flat field telescope I think it will work well in this new mode.  Since the 90o turn happens after the eyepiece and before the device the 'feel' to this arrangement is very much like normal use of a refractor.

 

This setup provides ~6.7 degree FOV at f/1.4.  Nikon made about half a dozen different 300mm f/4.5 models over the years and they are plentiful on the used market.  I see no reason why even longer focal length lenses that are f/4ish could not be used as well if one is willing to put up with greater length and weight.

 

I will update this post when the skies cooperate and I can report on the FOV in a nighttime setting.  Sometime during the coming week looks promising in that regard.


Edited by chemisted, 02 January 2021 - 12:15 PM.

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#2 chemisted

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Posted 07 January 2021 - 09:23 AM

Last night I could see stars so I gave this arrangement a try.  It came to focus easily.  My target was the Flaming Star/Tadpole nebula region (IC405&410) as the nebulosity is extensive there and there are plenty of stars.  Those stars were sharp and distortion free throughout the FOV.  The nebulae were easy even though it wasn't a perfect night (thin clouds present elsewhere and may have extended into my viewing field).  I followed this scene to the zenith with relaxed comfort from my chair.

 

Will I be using this setup in the future?  No, I won't.  With a magnification of 3.5X everything was on the small side and I have alternative telescopic views that are much better.  I carried out these experiments to show that the long eye relief of the 85mm lens does make this approach feasible.  My 300mm f/4.5 would probably cost about $50 on the used market which may be an attractive option for some.  There is only so much its 67mm aperture can do, however, and given the incredible variety of short focus telescopes out there it may be that that avenue is a preferred one for others to travel.


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#3 GeezerGazer

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Posted 20 January 2021 - 02:42 AM

It's certainly an interesting arrangement.  Using a camera lens as the prime objective for an eyepiece, used in afocal with NV.   Did you see any vignetting with this system?  

Ray



#4 chemisted

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Posted 20 January 2021 - 08:00 AM

Because of the T-2 connection between the camera lens and eyepiece the effective field stop is ~35mm.  That is how I calculated the TFOV (57.3X35/300=6.67o).  The image in the device eyepiece was small because of this.  If I had used an arrangement like that devised by Gatorengineer that allows the full field stop of the eyepiece to be used (46mm) then the TFOV would be almost 9o with this 300mm lens.  I think this approach would work with a much longer focal length camera lens that is about f/4.  I do not have one to try but in another thread Gatorengineer brought up a Pentax 67 600mm f/4 lens as a possibility and I think with a full 2" adapter that might work out well.



#5 chemisted

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Posted 20 January 2021 - 08:38 AM

I am also investigating this approach with my Celestron 45mm Plossl.  It has a long eye relief (though not nearly as long as the 85mm) that just might allow me to do the same afocal approach.  It requires my adapting a 0.965" diagonal that should have a short enough path length to do the job.  I have really liked the views I get with this Plossl and both my 200mm and 300mm Nikkor lenses and being able to use them both with diagonal viewing would be super fun.



#6 chemisted

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Posted 17 February 2021 - 03:36 PM

I acquired an old 0.965" diagonal through CN classified and did a little with it using the 45mm Plossl.  The approach looked promising so then I decided to contact Gary Russell to see if he could make me a 1.25" 50mm Plossl with 1.25" filter threads cut in the top.  I felt this would provide a faster focal ratio (f/2.4) and might give an even longer eye relief for a better working distance with my diagonal.  All of this worked out as planned and here are the details using the RO 50mm Super-Plossl XLC.

 

The diagonal was a prism and described as a Celestron although there were no markings on it.  I removed both the nosepiece and the eyepiece holder.  Under each was a 19mm clear aperture insert.  I discarded the one at the Plossl side and retained the one at the exit to the ENVIS lens. First studies with the 45mm had shown the prism to be unacceptable so I removed it and replaced it with a mirror that I pilfered from another piece of equipment.  For a solid connection to the 50mm I glued a 1.25" retainer ring from a filter.  The filter shell I cut in two and glued the side with female threads to the ENVIS side of the diagonal.  With these alterations completed I screwed the RO 50mm Super-Plossl directly onto the diagonal and using a male/male gender changer attached the NVD Micro to the other side.  After adding the 12nm H-alpha filter to the eyepiece I slid it into the 300mm Nikkor and I was ready to go.  Daytime tests had shown that I had nearly 20mm of eye relief coming out of the diagonal so there was absolutely no worry about a good interaction with the Micro.

 

Last night was absolutely clear but very cold.  I am quite pleased with the performance of this setup.  The 5o FOV has very nice star shapes throughout and, while there is mild vignetting in the outer 15-20%, the views are to my liking.  I cruised from Heart & Soul through the Rosette to Horsehead/Flame with all very sharp and nicely defined.  The wind picked up and I was done.

 

This is another carry-it-out with one hand arrangement and, as such, will probably get some use.  I do have to do a head-to-head comparison with the competing ST-80 setup as that was surprisingly good as I mentioned in another thread.  With both I have a comfortable seated view of the night sky with essentially no setup time at all.


Edited by chemisted, 17 February 2021 - 04:38 PM.


#7 chemisted

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Posted 20 February 2021 - 08:01 PM

With one simple change I have eliminated the vignetting that I observed my last time out.  It seemed likely to me that I had the diagonal too close to the eyepiece so I inserted one of my 1.25" filter shells between the 50mm Super-Plossl and the diagonal.  A quick test tonight showed an excellent FOV without the annoying edge darkening.  I will try to give a viewing update when I have a clear, moonless night.




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