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Focuser question

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#1 Quinnipiac Monster

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Posted 12 October 2019 - 03:16 PM

Hi all,

 

A few nights ago I dropped the screw that keeps my eyepieces in place, and never found it again. Nothing I have in house screws in, does anyone know what the focuser's brand is, and what kind of threading it uses? I bought it second-hand at Stellafane (see picture), it seems to be nicely manufactured and it works great.

 

Thanks!

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Edited by Quinnipiac Monster, 12 October 2019 - 03:18 PM.


#2 Quinnipiac Monster

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Posted 14 October 2019 - 05:15 PM

All right, I found what it is. Apparently, an early JMI NGF-mini, the version for Newtonian telescopes. Still looking for a replacement eyepiece screw, but meanwhile there's another problem I've noted recently. At times, the motor would just not work; you press the buttons on the controller, nothing happens.

 

The behavior seems to be rather erratic, in the course of an observing session the focuser would work at first, then nothing for a while, then on again. I tried a different controller (I have a spare one), same same. After disengaging the motor, I  found that the hand knob is very hard to turn, is it possible that the tension screw is just too tightened and the motor has a hard time turning it?

 

A JMI instructions sheet for a similar focuser I found online (http://www.physics.c.../ngfs_instr.pdf) stresses that everything else is factory tested and should never be touched, as pusher bearing alignment (for example) is extremely critical. 


Edited by Quinnipiac Monster, 14 October 2019 - 05:18 PM.


#3 25585

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Posted 14 October 2019 - 05:50 PM

Best thing is to buy small assorted bolts and try them out for fit .



#4 Jon Isaacs

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Posted 14 October 2019 - 09:39 PM

It's either a 6-32 or 8-32 screw, probably a 6-32. A nylon thumbscrew is nice because it doesn't Nick the eyepieve or adapter.

 

Jon


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#5 kfiscus

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Posted 14 October 2019 - 09:45 PM

Check for a loose connection somewhere since you've already eliminated the controller as being the problem.



#6 Quinnipiac Monster

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Posted 15 October 2019 - 07:40 AM

It's either a 6-32 or 8-32 screw, probably a 6-32. A nylon thumbscrew is nice because it doesn't Nick the eyepieve or adapter.

 

Jon

Thanks! :)



#7 Myk Rian

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Posted 15 October 2019 - 08:09 AM

OR, a  4 or 5mm. Are those hex screws sae or metric?

 

I go through all the hardware stores' fasteners section looking for sae and mm thumbscrews.



#8 Redbetter

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Posted 16 October 2019 - 12:52 AM

The 2003 vintage JMI on my Obsession has 6-32 screws for the 2" and for the 1.25" adapter.  I checked them a few moments ago.


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#9 Quinnipiac Monster

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Posted 19 October 2019 - 12:35 PM

OR, a  4 or 5mm. Are those hex screws sae or metric?

 

I go through all the hardware stores' fasteners section looking for sae and mm thumbscrews.

Good question. Home Depot doesn't have anything that works apparently, the smallest screws I could find are 6/32 and they are way too large. 



#10 Quinnipiac Monster

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Posted 19 October 2019 - 12:37 PM

The 2003 vintage JMI on my Obsession has 6-32 screws for the 2" and for the 1.25" adapter.  I checked them a few moments ago.

They must have changed bawling.gif They don't fit, the hole is much smaller. This is a first generation JMI focuser, probably around 30 years old. 



#11 Jon Isaacs

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Posted 19 October 2019 - 12:44 PM

Good question. Home Depot doesn't have anything that works apparently, the smallest screws I could find are 6/32 and they are way too large. 

 

I wonder if home depot's screws were properly sorted. 6-32 is tiny.

 

Jon



#12 Quinnipiac Monster

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Posted 22 October 2019 - 10:29 AM

I wonder if home depot's screws were properly sorted. 6-32 is tiny.

 

Jon

This focuser's thumbscrew is tinier yet. I don't know why, but 6-32 won't fit. True that most (all?) other focusers seem to have bigger thumbscrews, at least the recent ones.


Edited by Quinnipiac Monster, 22 October 2019 - 10:31 AM.


#13 Chucke

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Posted 22 October 2019 - 02:11 PM

I find that the screws at HD are frequently missorted.  Try McMaster-Carr.


Edited by Chucke, 22 October 2019 - 02:11 PM.


#14 Quinnipiac Monster

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Posted 10 April 2020 - 09:24 AM

All right, just in case anyone else has a (very) early JMI mini. No threading I have tried worked. I ordered an assortment of thumb screws from McMaster-Carr: 6-32, 6-40, 4-40, 4-48, nothing. Whatever threading they used at first (perhaps, metric?) JMI almost immediately switched to 6-32, thankfully. 

 

In the end, I re-threaded the hole as a 6-32 and was done with it. It took minutes, and did not even have to take the focuser off of the telescope. It was indeed the first suggestion I got, and I should have listened to it. 




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