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A new way to reach focus: Eyepiece projection!

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#1 Astrojensen

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Posted 01 December 2014 - 03:28 PM

Hi all

 

Yesterday evening, while on work, I suddenly had an idea. I don't know why it came out of the blue, like that, but I suddenly realized that there would be a way to reach focus with a binoviewer on ANY and ALL scopes, no matter what their design:

 

Eyepiece projection. 

 

Yup, that's right. Use an eyepiece in front of the binoviewer, to project its image into the binoviewer, where you then observe the image with normal eyepieces. Would it work? I was sure it would work, but I wasn't sure about how high the magnification was going to be. It would obviously vary with the focal length of the eyepiece used for projection. I also wasn't sure about the image quality. 

 

As soon as I came home from work that evening, I set my 6" f/8 newtonian up and started to experiment. I have two 40mm TS Easyview eyepieces that are made for projection and afocal photography and they have integrated T2 threads, so they can attach directly to a Baader Maxbright binoviewer. The T2 thread is exposed by removing the rubber eyeguard and can slide up and down by about 25mm.  Nifty. I threaded one to the front of the bino and put another into one of the eyepiece holders, put it into the focuser and...

 

It worked! And very well, actually! I compared with a 2.6x GPC, the lowest magnification GPC with which I can reach focus, and the magnification factor with the 40mm eyepiece as close as possible to the binoviewer, was around 2x!! if I extended the working distance, I could increase it to a little more than the 2.6x GPC, so maybe 2.7x - 2.8x. Yes, it worked as a zoom!! And the image was correctly orientated, like in a binocular!   

 

I spend quite some time comparing this setup with the 2.6x GPC and the GPC won slightly on brightness and was a bit sharper as well, but the image in the projection setup wasn't bad at all. Interestingly, there were noticeably less reflections, when the Moon was close to the field stop. The view was slightly soft and defocused near the field edge when I used 25mm Zeiss OPMI eyepieces in the binoviewer, but otherwise it looked really nice. Still, the GPC was always slightly sharper. 

 

I tried removing one of the lenses in the projection eyepiece, to change it into a ~80mm eyepiece and while the image was now MUCH less magnified, it didn't look very good at all. I attribute some of this to the simplicity of a single lens. I also had to use around 6" of extension tube to bring it to focus... 

 

I am pretty sure, that a dedicated projection eyepiece could be made, that would essentially offer a 1:1 projection scale, allowing the same magnification on the binoviewer that you would normally get with a single eyepiece, all without any problems of reaching focus. It would work in a coma corrector, too! Alternatively, coma correction could be designed right into the projection device! A problem might be, that it needs to be at least the same f/ratio of the scope, or faster, lest there will be vignetting. This is easy to achieve in a f/8 scope, but an f/3 is too fast to fit the light cone into a normal binoviewer, so it won't work with a 1:1 projection either. Here a barlow or Powermate remains the only viable solution. 

 

I still think this has something going for it and will merit further investigation.

 

 

Clear skies!

Thomas, Denmark



#2 Mark9473

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Posted 01 December 2014 - 03:53 PM

Interesting ideas, Thomas.

A "dedicated projection eyepiece that offers a 1:1 projection scale" sounds a lot like a relay lens.

Siebert used to list a zero (I believe he meant 1) magnification OCA on his website, which is exactly that I believe. Have you ever tried it? The link from his homepage now seems to point somewhere else.



#3 Astrojensen

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Posted 01 December 2014 - 04:50 PM

Hi Mark

 

No, I haven't tried Siebert's 1x OCA. 

 

 

A "dedicated projection eyepiece that offers a 1:1 projection scale" sounds a lot like a relay lens.

That's essentially what it is. 

 

Edmund has a 1:1 relay lens with 100mm focal length and 22mm clear aperture, f/4.54, in a housing with 30mm outside diameter and 34mm long. This sounds very close to the dimensions of the lens stack in the 40mm TS Easyview... The downside is the price, $150... 

 

I'll see if I can find some cheaper lenses to experiment with, before taking the big plunge. 

 

 

Clear skies!

Thomas, Denmark



#4 Jawaid I. Abbasi

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Posted 01 December 2014 - 06:23 PM

Thomas,

Can you post a picture. This is interesting and I would like to do it too but I still not clear how did you do.

Thank you



#5 doug mc

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Posted 01 December 2014 - 07:27 PM

This might be slightly of subject. Last night I dismantled my William Optic 1.6x ocs to place some flocking strip into the locking ring where I notice some reflections when observing the moon. I discovered that it is made up  with two sets of lenses. So it is not a standatd barlow that I though it was. 



#6 faackanders2

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Posted 01 December 2014 - 11:06 PM

If you have a reflector, Denkmeier used to make a reducer power switch which in conjunction with the low power of the dual power switch and Newtonian OCS resulted in 1X equivalent barlow for a great bonoview finder.

 

Using the new A45 Multipurpose OCS (and required infocus extension to focus) this same configuration results in 0.88 barlow equivalent.  Even though the TFOV was significantly larger objects like the Helix, North American, and Viel were harder to see than with the higher power Newtonian OCS.



#7 GlennLeDrew

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Posted 02 December 2014 - 02:30 PM

Whichever lens or system is used, it's to be preferred that it be designed around the two nearby conjugates present in a relay application. More so where faster f/ratios are at work. An eyepiece is designed so that one conjugate is its own focal length and the other is optically distant, perhaps infinity by design. Pushing the former out and pulling the latter in will introduce some spherical aberration, although this may be of import only at smaller exit pupils.



#8 Astrojensen

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Posted 02 December 2014 - 02:35 PM

Aha! Thanks for the info, Glenn. 

 

 

Clear skies!
Thomas, Denmark




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