Focal reducer threads

[jacko61]

I wasn't sure where to put this but here goes anyway.  I have a Celestron C8 that I've been using a Meade focal reducer on for a couple of years. Never had a problem threading it to the scope or threading the celestron visual back to the reducer.  I've just acquired a 2inch baader clicklock which screws to the scope with no issues. However, when I try to screw it onto the focal reducer, the threads bind up after no more than a turn and a half. I've inspected the threads on the scope, FR and clicklock and none are damaged in any way. Can anyone explain why they're so tight when the threads must be the same?  Would a smear of grease on the clicklock threads help?

[fifeskies]

Sounds like the threads are possibly NOT the exact same, there are differing thread pitches for the same diameter.

Some are 1 thread per mm , some 1 thread per 0.75mm (and a few other odd sizes sometimes),

Typically they go on a turn and then bind when mismatched.

Occasionally they will engage one way but not the other ,  (e.g. smaller pitch might go in a female bigger pitch but not the other way round if pitches are very close , eg a 0.6mm and a 0.75mm pitch mismatch. Especially if thread manufacturing tolerance is poor.

Never "encourage" them to screw in as they will be hard to separate again and leave the threads distorted.

 

DONT smear grease, it will only end up on the glass elements as smudges if you do and is a pain to remove.

(specialist graphite powder is better but I don't even recommend that, properly clean matched threads should screw in with minimal effort)

 

All is not lost however if they are differing pitched threads as there are a selection of adapters that have 1 pitch on a side with the other pitch on the other side , allowing you to convert to join parts together.

Also for adapting 50mm to 48mm , etc etc.

 

this is handy to read through    Astronomy Threads Explained (agenaastro.com)

 

 

various styles exist but they mostly look like below (this one also adapts diameter)

 

 

 

Edited January 19, 2021 by fifeskies

[jacko61]

Thanks for that. Very interesting read. All sets of threads measure at 24 TPI with my gauge so I'm not sure what's going on. The Baader website states a 2" -24 thread on the clicklock, the celestron thread is 2" -24 and the meade  has a standard male 2"-24 sct thread on one side and a standard sct female thread on the other so there shouldn't be an issue with the fit. 

Graeme

 

[fifeskies]

 

On 19/01/2021 at 17:40, jacko61 said:

Thanks for that. Very interesting read. All sets of threads measure at 24 TPI with my gauge so I'm not sure what's going on. The Baader website states a 2" -24 thread on the clicklock, the celestron thread is 2" -24 and the meade  has a standard male 2"-24 sct thread on one side and a standard sct female thread on the other so there shouldn't be an issue with the fit. 

That does sound strange as they should be just connecting if that is the case, perhaps one of the elements has been stressed out of being exactly circular and is very slightly oval.

If you want to try some "thread lubricant" graphite powder might be worth a try , only use a touch however.

RS PRO Lubricant 50 g Graphite | RS Components (rs-online.com)

I have not used this myself but have chatted to others who use it for camera screw fit lenses and say it works well as long as you are very frugal with it.

 

[jacko61]

Sorted. There's a very slight ripple in the threads on the clicklock.  Ran a thread cleaning tool round it a few times and rubbed a 4B pencil into the threads. Ran the reducer into the click lock a few times to check it's threading the full length now and cleaned it all up again. There's still a tiny bit of binding in a couple of spots on each rotation but at least I don't have to shell out for an adaptor.