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Focus not engaged

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#1 rld

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Posted 23 October 2019 - 01:38 PM

Last night I ran into a strange problem - my field model Q 3.5 would not respond to focus adjustments in either eyepiece mode or barlow mode. It worked fine for the first 20 minutes and then felt as something disconnected as I focused on Vega. The outside temperature was 15 degrees colder than inside where the scope was stored. At first, I thought it might have been because the scope was pointed almost straight up to find Vega. So this morning I took it out on my patio and tried it both horizontally and vertically - no problem for about 20 minutes, then no connection between the focus knob and eyepiece or barlow focus. The difference in temperature was about 20 degrees this am. I decided to let it further cool down and when I went out about half an hour later, everything was working fine again. Are there some very fine tolerances between the focus knob and the mirror that a difference in temperature would cause them to disengage?



#2 Loren Gibson

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Posted 23 October 2019 - 01:57 PM

If the focus knob on your field model is like that on the Standard, you have a knob that slides onto the shaft. The knob has a set screw that needs to be tightened sufficiently in order to make sure the shaft turns with the knob.

 

Perhaps that setscrew is not tightened quite enough, resulting in not consistently turning the shaft with rotations of the knob?

 

Just a guess. I'm not sure what else would result in the focusing working sometimes but not others. Those who have dissected the Q and know its innards may have other ideas.

Loren



#3 Loren Gibson

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Posted 23 October 2019 - 01:59 PM

Follow-up: The setscrew in the focus knob is probably takes a Bristol spline wrench, not a hex-head ("Allen").



#4 rld

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Posted 23 October 2019 - 02:22 PM

Loren:

Thank you for that insight. However, the focusing knob and the shaft screwed all the way into the rear housing of the scope and then all the way back past normal infinity focus. That would indicate to me that the set screw is fully tightened. I'm thinking it must be something in the connection between the shaft and the mirror.

Bob



#5 Optics Patent

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Posted 23 October 2019 - 03:58 PM

On the chance that your focus shaft has detached from the mirror thimble assembly, do NOT let the scope point downward. The mirror may be loose and crash into the corrector.

First test: with scope pointed to the zenith, observe the mirror as you adjust the focus knob the full range of adjustment. Does it appear to move significantly and move throughout the full range of rod adjustment? If the rod is detached it may push up to a safe limit but the focus won’t pull it back all the way because of a spring behind the mirror. Note that the corrector will distort your depth perception.

Also if detached from the mirror the rod may fully unscrew without resistance at the rear limit.

Was this operated at very close focusing distances recently or potentially abused by someone trying to unscrew the focus shaft?

Have you opened the axial port to verify that the prism and Barlow are operating normally?

Edited by Optics Patent, 23 October 2019 - 04:01 PM.

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#6 rld

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Posted 24 October 2019 - 12:43 PM

Ben:

 

Positioned pointing to the zenith, the mirror appears to move significantly and does so throughout the full range of rod adjustment. This test was done indoors with a room temperature of 70 degrees. The rod met full resistance when screwed all the way back.

 

This summer, I did do some bird watching using the near focus. I don't know about any prior abuse since I purchased this scope in May and have only used it in warm weather. The night before last was the first time with it out in significantly cooler temperatures.

 

The prism and barlow appear to be working properly. I do notice that the arm that moves the Barlow protrudes slightly into the top of the axial port. I'm assuming this is normal.

 

Is it possible that the mirror cools faster than the thimble that goes through the middle and causes it to bind until the entire scope reaches a temperature equilibrium?

 

Or more likely, am I missing something in the operation of the Q?

 

Bob



#7 Optics Patent

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Posted 24 October 2019 - 01:06 PM

All good news. Is the failure to focus persisting?

If it’s intermittent or only when cold try viewing for mirror movement over the focus range when you can’t focus

Almost every time I have a focus problem it’s an optical setting or just not finding the range. Maybe a clear target like the moon or a streetlight you can be sure your aimed at would help verify.
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#8 BillHarris

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Posted 24 October 2019 - 01:43 PM

Ben, is it really possible for the focusing screw to disconnect from the mirror thimble arm at it's far travel (at the nearest focus setting)? It seems that there ought to be a "stop" to prevent it from unscrewing.

I ask this because I do bird photography too, and I'd be concerned about racking in very close focus. I'll also need to look at the disassembly videos to verify my understanding of the details.

#9 Optics Patent

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Posted 24 October 2019 - 01:46 PM

No. The shaft has a little dime sized Teflon disc with a central hole that is attached to a little protrusion with a nail head at the end of the shaft. That disc pops on and off and strong focusing force at closest focus (shaft protruding max) could pop off the disc rendering the shaft only able to push and not pull. And permitting the main spring to push the mirror forward without restraint.

#10 rld

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Posted 24 October 2019 - 02:29 PM

Thank you, Ben. I'll give it a try tonight. We've had a cold front move in with snow. That should be a pretty good test.



#11 Optics Patent

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Posted 24 October 2019 - 02:52 PM

Be sure to use the snow shield. ;-)

#12 BillHarris

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Posted 24 October 2019 - 03:49 PM

So approach the end stop gently and with caution. I need to run it all the way to that stop and see what it feels like when it's reached.

#13 rld

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Posted 25 October 2019 - 10:43 AM

Ben:

No problems last night. It must have been some kind of operator error. wink.gif



#14 Steve C.

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Posted 25 October 2019 - 03:07 PM

I ran into something similar a while back. I thought the focusing knob had gotten broken or something. The problem turned out to be that there's a very narrow range between slightly out of focus and focused perfectly. It's real easy to spin the knob fast and zip right past the prime spot. 


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