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Goodbye C-Mount

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#1 DavidWasch

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Posted 15 May 2020 - 11:29 PM

The C-mount’s 22mm aperture limits the NV sensor’s fully illuminated field to a focal ratio of 3.6 or higher (see Eddgies description at this link:https://www.cloudyni...4#entry10134960). That can be mitigated by utilizing a lens with a rear element that extends back into the NV housing. The ENVIS lens does this, but it is expensive, as are the afocal magnifiers for the ENVIS-- and they still introduce vignetting.

 

I purchased a Night Owl Cyclop lens cheaply on Ebay. It is an 80mm f/1.7 that I wanted to use without significant vignetting. In order to do that, I had to eliminate the c-mount restriction. So, I went at it with a Dremmel:

 

Cut C-Mount Adapter
 

Obviously, I have some painting touch up to do, but it was good enough as-is for testing. It goes back on the PVS-7 without the front section. With the addition of the adapter that I describe below,  I can confirm that I am able to utilize even my 50mm f/1.2 with minimal vignetting.

 

Image from Pentax A 50mm @f/1.2

 

I would recommend this mod to other brave PVS-7 owners willing to cut up their expensive c-mount adapter. I do lose the ability to use cctv lenses. But, for about $75 invested, I

  • can use my favorite (and inexpensive) camera lenses will minimal vignetting,
  • can have more control over placement of filters and focal reducers
  • don't need to  purchase a c-mount to filter adapter that is otherwise needed, and
  • can operate the PVS-7 at the fast focal ratios (ie. f/2.8) of the astrograph I hope to one day purchase without vignetting

 

Total Adapter

Edited by DavidWasch, 16 May 2020 - 04:29 AM.

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#2 DavidWasch

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Posted 15 May 2020 - 11:31 PM

Here’s how I did it:

 

Adapter Setup

 

I have a Pentax camera with some lenses I’d like to try for NV, so I decided to go with that. Lens mounts for the major camera manufacturers (Cannon, Minolta, etc.) are easy and cheap to get in the form of macro extension tubes. Here’s a picture of the one I picked up:

 

macro tube

 

The top section comprises the mount adapter and is only a couple millimeters thick. It’s threaded at M60, but rather than use step down filter rings, it was far easier to just glue it to the next part. I used E6000 adhesive, but I plan to reinforce with JB Weld to make the bond extra solid.

 

The next link in the adapter chain is a focusing helicoid, but another option is an m48 variable extender. The point is that the spacing between the lens and the sensor needs to be adjustable so infinity focus can be obtained. Pentax flange to image plane distance is 45.46mm.
At a 1.4 the depth of focus is only .0062mm, but that level of accuracy isn’t needed so long as infinity focus exists *somewhere* in the focusing range of the lens. This adjustable section ensures that it will.

 

Helicoid Focuser

 

The next adapter is just a threading adapter that changes the M42x.1mm photographic pitch on the heliciod to M42x0.75mm pitch used by telescopes (aka T-Thread, aka T2 thread).

 

Finally, I have the section that clamps on the nosepiece of the PVS-7, a simple 2” eyepiece holder with a T thread at the other end. I purchased a Blue Fireball adapter that is 40mm long. Perhaps a better option would be the Baader Clicklock to M48 (aka filter thread), but I didn’t want to spend that much on an experiment. I may switch to it in the future.

 

Blue Fireball 2" Eyepiece Adapter

 

The 2” eyepiece adapter can use a cheap ring adapter to substitute for the filter holder normally purchased through ScopeStuff. The eyepiece adapter can provide more flexibility because it extends only 15mm beyond the tube’s sensor. So, I can position a focal reducer much closer to the imager to fine tune the amount of focal reduction I’m shooting for.


Edited by DavidWasch, 16 May 2020 - 03:53 PM.


#3 a__l

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Posted 16 May 2020 - 01:17 AM

I am wondering how you center and align axis the ScopeStuff adapter on the PVS body?



#4 DavidWasch

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Posted 16 May 2020 - 03:01 AM

I don't have to se the ScopeStuff adapter. It's not needed because the Blue Fireball adapter has all the threading I need.



#5 a__l

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Posted 16 May 2020 - 03:27 AM

This adapter and body pvs. There are no threads, two screws and brass ring.

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  • pvs.jpg


#6 DavidWasch

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Posted 16 May 2020 - 04:26 AM

Ah. Thank you.

 

The adapter with the brass ring and screws attaches to the piece I cut the c-mount off of.

 
Cut C-Mount Adapter
 
This goes back on the PVS-7. It holds the intensifier tube in place and provides the 2" structure that the Blue Fireball holds on to. I clarified the text in original post.
 
Hope that helps.

Edited by DavidWasch, 16 May 2020 - 04:30 AM.


#7 Eddgie

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Posted 16 May 2020 - 03:42 PM

For those that have not owned the PVS-7 with C mount it might not be clear what you have done, but I can see how the configuration works. 

 

 Very clever and a very good result! I am impressed by your ingenuity.



#8 a__l

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Posted 16 May 2020 - 06:25 PM

Ah. Thank you.

The adapter with the brass ring and screws attaches to the piece I cut the c-mount off of.

This goes back on the PVS-7. It holds the intensifier tube in place and provides the 2" structure that the Blue Fireball holds on to. I clarified the text in original post.
Hope that helps.

 

How different are the adapter’s inner diameter and nose outer diameter? The fact is that even standard focusers and eyepieces can be skewed.
Typically, the skew in this case is eliminated by pressing two planes from two devices. But you do not have a second plane on the body PVS.
We are waiting for a test frame of stars fields.



#9 DavidWasch

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Posted 16 May 2020 - 06:58 PM

How different are the adapter’s inner diameter and nose outer diameter? The fact is that even standard focusers and eyepieces can be skewed.
Typically, the skew in this case is eliminated by pressing two planes from two devices. But you do not have a second plane on the body PVS.
We are waiting for a test frame of stars fields.

That is a great point, and one I've been thinking about. The 2" eyepiece holder does fit fairly snugly with the existing nose piece, and is set against the mounting stalk at the top of the housing. There seems to be no play once the holder is in place. BUT - it is not leveled against the housing on all axes.

 

Proof will be in the pudding, or starfield, as they say. High clouds tonight, but they might be thin enough for me to run a test. As soon as I have some pics to share, I will post them.

 

If I don't have sharp focus across the field, I could try two options to resolve the issue: 

 

1- substitute a Baader Clicklock for the Blue Fireball eyepiece clamp, which should be more sturdy (I was thinking of getting it anyway), and

2- adding a ring section to the front of the housing so I have more contact between the housing and adapter.

 

I'm optimistic that it will work fine as is, however. The connection between adapter and housing really just has to handle the weight of the PVS-7 housing effectively, since the adapter is either held by hand, or by a telescope focuser. I should know for certain soon.



#10 DavidWasch

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Posted 16 May 2020 - 08:28 PM

Hazy enough tonight that it was hard to tell focus with the lens that came with the PVS-7. Wednesday night seems like my next good night. I check back here then.



#11 Souldrop

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Posted 16 May 2020 - 10:01 PM

Looks good! I may have to copy it at some point. Thanks for the write up!



#12 Jeff Morgan

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Posted 16 May 2020 - 11:20 PM

Wow, clever. And brave wink.gif



#13 cnoct

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Posted 19 May 2020 - 08:12 AM

bawling.gif

 

That adapter is what ITT used on their NQ5000 system, one of their few factory c-mount systems.

 

It was a coveted adapter at one time, though you gotta do what one's gotta do. 



#14 a__l

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Posted 20 May 2020 - 02:48 AM

I have other NV body with an inner hole diameter of 28 mm. So I made 3 new adapters.
M42x0.75 for 2" nose (photo on the left), M42x0.75 (center) for bayonet mount (additional M42 rings are needed for the required rear length for the lens specification) and under C-mount with a distance to the sensor of 17.5 mm, as well as for 1.25" nose.

Attached Thumbnails

  • Ring1_.jpg
  • Ring2_.jpg

Edited by a__l, 20 May 2020 - 02:49 AM.


#15 DavidWasch

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Posted 20 May 2020 - 07:13 AM

That looks great! When you say 'made' what exactly do you mean-- did you machine the nose part?

 

What body provides the 48mm hole?-- that's the monocular I would look for after for PVS-7.



#16 a__l

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Posted 20 May 2020 - 05:18 PM

That looks great! When you say 'made' what exactly do you mean-- did you machine the nose part?

 

 

No. I was guided by standards T2 and C-mount, which use various devices (noses, lenses). In this case, I made 3 adapters (from bronze, T2 - male and famale). On a lathe. For my NV, I did not find on sale the adapters that I needed.




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