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MOD 3C Maintenance/Cleaning

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#1 GeezerGazer

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Posted 15 August 2019 - 01:58 PM

During a recent camping trip where I was frequently changing objective lenses, I noticed that I was getting some black spots on the image.  When I got home, I decided it was time to clean the Mod 3C to rid it of the black specks, seen here, please note that one of the black specks is darker and looks more in focus: 

 

IMG_5211.jpg

 

I removed the front objective, a 1x Envis lens, and cleaned the back of that lens as well as the front window of the NV tube, using an air bulb with very soft lens brush.  I replaced the Envis and checked it again for debris, finding that all of the smaller, out-of-focus bits were gone.  So then I removed the ocular at the back of the housing and cleaned both the rear tube window and the inside of the ocular with the air bulb and lens brush.  Maintenance was complete with no more black specks.

 

In each of these photos there is a slightly brighter center spot.  This is from using the pin hole in the objective lens cover for daytime use in a darkened closet.  The texture seen in these images is the texture on the wall in the closet, out of focus, as the objective lens was still focused at infinity.

 

IMG_5214.jpg

 

There are two points of importance here.  First, if you see debris in the FoV that is slightly out of focus (when the objective is left at infinity focus as normally used for stars) then the debris is likely between the objective and the incoming tube window.  If the debris is well focused, it is probably between the NVD ocular and the outgoing tube window.  Cleaning both exposed surfaces when disassembled is probably necessary... I never actually saw these tiny specks on any of the surfaces, even though I knew they were there.    

 

The ocular on my Mod 3C has very fine threads in what is probably Delrin.  Be sure to turn the ocular counter-clockwise when re-inserting it, until the threads "click" into alignment.  Then gently turn it clockwise to thread it back into the housing to avoid cross-threading.  

 

Pretty simple maintenance... necessary from time to time. 


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#2 11769

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Posted 15 August 2019 - 03:47 PM

Good summary. Unfortunately it's not too hard to cross-thread the ocular lens onto the housing. No damage will occur if it's caught as early as possible. Beyond that, those fine threads cut into Delrin cannot handle much abuse. With the pinhole cover in place, FOD anywhere in the objective lens light path becomes far more noticeable (still fuzzy and slightly out of focus) than if the cover is left off and the device is focused to infinity and looking at a uniformly lit surface. 


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#3 slavicek

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Posted 15 August 2019 - 09:19 PM

Here is my question: how does the debris gets behind the NV ocular?

Second, more serious question: to remove the NV ocular (= the lens you actually look into), you just unscrew it, right?

No debris in mine yet, but it's good to know...


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#4 GeezerGazer

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Posted 16 August 2019 - 11:50 AM

Here is my question: how does the debris gets behind the NV ocular?

Second, more serious question: to remove the NV ocular (= the lens you actually look into), you just unscrew it, right?

No debris in mine yet, but it's good to know...

My guess is that the speck between the ocular and the rear window of my tube was a plastic bit from the fine threads cut into the Delrin.  My ocular has been off and back on, probably a dozen times in 3 years, mainly because of tube wiring issues relating to the gain control.  Tom Lyons in TX, repaired the wiring issues for me last year.  But Delrin surfaces can wear, and the threads for the ocular are cut to tight tolerances.  The threads on the ocular unit and housing unit are very fine, as noted above, and if they are ever cross-threaded, it could certainly cause bits to shear off... another reason to be very careful NOT to cross-thread when re-inserting the ocular.  On mine, the first several turns are very easy, then increased pressure is needed to seat the ocular for the last several turns.  If I encounter ANY resistance during the first 4 turns when re-inserting, I back it out and turn it counter-clockwise until the threads are properly aligned.  Threads on my ocular and housing show no sign of being cross-threaded, but they do show some wear. 

 

The ocular does unscrew from the housing; turn the smooth ring just ahead of the diopter ring to separate the ocular from the housing.  The narrower ring closer to the housing is a stop/lock ring and does not need to be removed.

 

The specs at the front of the tube are difficult to see when using the NVD at night.  But the spec between the ocular and rear window of the tube DID show up in several photos I took the last night of my trip, see two images below with the big speck visible at about 11 o'clock close to the photographed subject.  Visually, specks in front of the the tube, behind the objective lens, are much less noticeable or even invisible, especially if there is a lot of noise in the image from high gain settings when using filters.

 

IMG_5174.jpeg

 

IMG_5176.jpeg


Edited by GeezerGazer, 16 August 2019 - 12:17 PM.


#5 GeezerGazer

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Posted 16 August 2019 - 12:13 PM

Good summary. Unfortunately it's not too hard to cross-thread the ocular lens onto the housing. No damage will occur if it's caught as early as possible. Beyond that, those fine threads cut into Delrin cannot handle much abuse. With the pinhole cover in place, FOD anywhere in the objective lens light path becomes far more noticeable (still fuzzy and slightly out of focus) than if the cover is left off and the device is focused to infinity and looking at a uniformly lit surface. 

This sounds like a user speaking from experience and your advice should be taken seriously!  



#6 GeezerGazer

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Posted 16 August 2019 - 12:27 PM

The NVD ocular acts like a magnifier for the screen at the rear of the NV tube.  The bit of debris that was on the screen, or on the underside of the ocular, was also magnified and clearly seen in the photos.  Debris between the objective and the incoming tube window tends to be out of focus, making them much less noticeable and/or objectionable.  



#7 slavicek

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Posted 16 August 2019 - 06:02 PM

My guess is that the speck between the ocular and the rear window of my tube was a plastic bit from the fine threads cut into the Delrin.  My ocular has been off and back on, probably a dozen times in 3 years, mainly because of tube wiring issues relating to the gain control.  Tom Lyons in TX, repaired the wiring issues for me last year.  But Delrin surfaces can wear, and the threads for the ocular are cut to tight tolerances.  The threads on the ocular unit and housing unit are very fine, as noted above, and if they are ever cross-threaded, it could certainly cause bits to shear off... another reason to be very careful NOT to cross-thread when re-inserting the ocular.  On mine, the first several turns are very easy, then increased pressure is needed to seat the ocular for the last several turns.  If I encounter ANY resistance during the first 4 turns when re-inserting, I back it out and turn it counter-clockwise until the threads are properly aligned.  Threads on my ocular and housing show no sign of being cross-threaded, but they do show some wear. 

 

The ocular does unscrew from the housing; turn the smooth ring just ahead of the diopter ring to separate the ocular from the housing.  The narrower ring closer to the housing is a stop/lock ring and does not need to be removed.

 

The specs at the front of the tube are difficult to see when using the NVD at night.  But the spec between the ocular and rear window of the tube DID show up in several photos I took the last night of my trip, see two images below with the big speck visible at about 11 o'clock close to the photographed subject.  Visually, specks in front of the the tube, behind the objective lens, are much less noticeable or even invisible, especially if there is a lot of noise in the image from high gain settings when using filters.

 

attachicon.gif IMG_5174.jpeg

 

attachicon.gif IMG_5176.jpeg

Yeah, that speck in those photos would be annoying. But as long as I never remove the ocular I should not have this problem.

And as you pointed out, the fact is that any dirt in front of the NV is hardly noticeable, especially since I (and most of others) use NV for nebular objects.


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