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FrankenFinder

Eyepieces Mount DIY
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#1 Chris Westland

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Posted 19 February 2021 - 12:27 AM

I'd seen earlier posts questioning whether you could convert a straight through finder (e.g., Orion 9x50) into a RACI finder.   Here is the result of my little project, and how I did it.

 

1.  Started with a straight-through 9x50 finder, and unscrewed the reticle / eyepiece assembly and threw them away; then unscrewed the rear plate

2.  Disassembled an old Celestron prism diagonal, and glued the diagonal and finderscope plates together (Gorilla Glue, but I should have used epoxy as GG expands and is messy)

3.  Unscrewed the lens and locking ring from the finderscope tube, and took it to a muffler shop to chop off all but 70mm on the front.

4.  Filed and leveled the cut end of the tube; filed the threads off of the rear plate, and joined the two with epoxy

5.  Cleaned and reassembled the diagonal and front lens.

6.  Inserted a 12.5mm Svbony illuminated reticle eyepiece ($36) in the top of the diagonal, and adjusted optical path on the Moon

 

All told, about two hours worth of work over a couple of days.

 

I've attached before and after pictures.  The SVbony eyepiece has a separate adjustment for the reticle.  Fine focus by screwing the front lens in and out, and firming up the locking ring.

 

In terms of optical quality, it's the best RA finder that I've yet used.  

Attached Thumbnails

  • download.png
  • short2.jpg

Edited by Chris Westland, 19 February 2021 - 11:26 AM.

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#2 SloMoe

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Posted 19 February 2021 - 08:21 AM

waytogo.gif


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#3 Jon Isaacs

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Posted 19 February 2021 - 08:41 AM

A question:

 

An interesting project.

 

Have you measured the effective aperture?  The 50 mm finders are usually around F/3.7.  

 

My math suggests that it probably not operating at the full 50 mm.

 

Jon


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#4 Chris Westland

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Posted 19 February 2021 - 11:17 AM

Have you measured the effective aperture?  The 50 mm finders are usually around F/3.7.  

 

My math suggests that it probably not operating at the full 50 mm.

 

Jon

Thanks for the reminder, Jon.   Yes, the first thing I did was check FL ... it was 190mm (~f/3.8) so you are spot on.   There was also a field stop at the point which I cut off; it was the same location and diameter as the opening on the diagonal.  So everything is more or less where I wanted it.

 

As far as the aperture, on the Moon, I eyeballed the FoV as about 8 Moons, so my back of envelope calcs are: Moon @ .5 degrees x 190/12.5 mag x 8 = 60 degrees.   I might be a little off on my count of # of Moons  undecided.gif

 

How does this sound to you?


Edited by Chris Westland, 19 February 2021 - 04:12 PM.


#5 Jon Isaacs

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Posted 20 February 2021 - 08:13 PM

Chris:

 

The reduced aperture would not affect the field of view, the aperture stop is quite distant from the focal plane. 

 

This is how I see it:

 

A 1.25 inch stop = 32 mm, 70 mm from the 50mm, F/3.8 objective. 

 

In 70mm, an F/3.8 light cone shrinks by 70mm/3.8 =18.4 mm =18 mm

 

50mm-18 mm = 32 mm

 

This means the very center will be fully illuminated but off-axis there will be vignetting.

 

How much vignetting, how visible, that depends on the eyepiece and the field stop.

 

Jon


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#6 emflocater

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Posted 20 February 2021 - 09:52 PM

belushi.gif ...............waytogo.gif

 

Cheers

Don


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#7 alphatripleplus

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Posted 21 February 2021 - 09:01 AM

 

 

In terms of optical quality, it's the best RA finder that I've yet used.  

Nice job and a great description of how you did it. Thanks.


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#8 Chris Westland

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Posted 21 February 2021 - 01:56 PM

Chris:

 

The reduced aperture would not affect the field of view, the aperture stop is quite distant from the focal plane. 

 

This is how I see it:

 

A 1.25 inch stop = 32 mm, 70 mm from the 50mm, F/3.8 objective. 

 

In 70mm, an F/3.8 light cone shrinks by 70mm/3.8 =18.4 mm =18 mm

 

50mm-18 mm = 32 mm

 

This means the very center will be fully illuminated but off-axis there will be vignetting.

 

How much vignetting, how visible, that depends on the eyepiece and the field stop.

 

Jon

Thanks for the explanation, Jon.   32mm (~1.25") is exactly the same diameter as the opening on the diagonal, which is at the same position (70mm) as the original 1.25" field stop was.  So I suspect I am 'on the bubble' as far as vignetting.   I'll check this out tonight.



#9 Jon Isaacs

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Posted 21 February 2021 - 06:00 PM


 


 

Thanks for the explanation, Jon.   32mm (~1.25") is exactly the same diameter as the opening on the diagonal, which is at the same position (70mm) as the original 1.25" field stop was.  So I suspect I am 'on the bubble' as far as vignetting.   I'll check this out tonight.

 

I'm sure there's vignetting, whether it's visible, hopefully not. The eye is remarkably insensitive to vignetting. 

 

 

Chris:

 

I'm sure there will be vignetting but whether you'll see it is another question, the eye is remarkably insensitive to vignetting. 

 

I see a 12.5 mm Svbony illuminated reticule eyepiece with a 40 degree AFoV.  That implies a 2.63° TFoV at 15x based on a 190 mm focal length.  

 

For a 50 mm finder, I think 5° and 6°.

 

Jon



#10 Jon Isaacs

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Posted 24 February 2021 - 08:20 PM

I was just looking at a 9x50 Orion finder and I remembered that Amici prism diagonals of the type used here typically have clear apertures of about 20mm.  The analysis here is based on a clear aperture of 1.25 inch (32mm) but it's very unlikely that the actual aperture of the prism is anywhere near that large.  

 

I would be interested in an actual measurement of the working aperture of the Frankenfinder.  There are a number of ways to do this.  A popular one is the flashlight test. 

 

https://www.cloudyni...t the objective.

 

Jon


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#11 Rutilus

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Posted 25 February 2021 - 08:13 AM

I recently built a very similar finder scope to the OP.  I also used a Celestron Amici prism diagonal and it has an aperture

of 22mm. Using a 12mm illuminated reticle eyepiece (which requires 10mm more in-focus compred to the standard 

12mm Plossl I have). I get 44mm with the flashlight test.

 

Using standard Plossl eyepieces I get 48mm with the test, which is the same measurement that I got 

when drawing the system out on paper.  While Chris will neeed to do the test with his finder, I hope this information

is of use for anyone thinking about building such a system.



#12 Rutilus

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Posted 26 February 2021 - 04:54 AM

Chris - I've just noticed in your photo that the star diagonal looks more like a standard Celestron 90 degree

diagonal which will have a left to right reversed image. Have Celestron changed the design of the case in

recent years?  Such a diagonal in my build works the opposite way to the Amici prism and requires 10mm of

out focus.

Doing the flashlight test with my 12 illuminated eyepiece gives a diameter of 48mm.



#13 Jon Isaacs

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Posted 26 February 2021 - 08:16 AM

Chris - I've just noticed in your photo that the star diagonal looks more like a standard Celestron 90 degree

diagonal which will have a left to right reversed image.

 

Good point.. 

 

I assumed Chris was using a RACI diagonal but you are probably right, it's probably a prism type star diagonal.

 

Jon



#14 earlyriser

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Posted 26 February 2021 - 08:26 AM

Chris - I've just noticed in your photo that the star diagonal looks more like a standard Celestron 90 degree

diagonal which will have a left to right reversed image. Have Celestron changed the design of the case in

recent years?  Such a diagonal in my build works the opposite way to the Amici prism and requires 10mm of

out focus.

Doing the flashlight test with my 12 illuminated eyepiece gives a diameter of 48mm.

I imagine that one would get used to the reversed image after using it for awhile.  




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