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  #21  
Old 20-08-2019, 12:17 PM
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LewisM
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It does, but I wouldn't be putting coma correctors or, in my case, CCD cameras in the oven to get the offending t-thread ring off
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  #22  
Old 21-08-2019, 09:37 AM
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The_bluester (Paul)
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The heat will just be expanding the aluminium a little. In theory given the female thread should be larger diameter than the male thread it should open up the clearances just a touch. I feel the pain on this one, I have two adapters that I have been using together and they are always ridiculously difficult to get apart again. You screw them together with two fingers and it is just about a hammer and cold chisel job to separate them again. I had to resort to pressing them between two pieces of hard'ish rubber and twisting in opposite directions to get them apart. And that is with a delrin spacer between them.

The biggest cause of the issue IMO is production tolerances. I recently got a part modded by Joshua (Getting a filter thread turned into an adapter he previously made for me to fit a reducer/corrector to my ED72, something I should have thought of the first time) and ended up having to send the actual filter I wanted to use, measuring everything up showed that while I had a handful of 2" "Filter thread" (M48) parts from various manufacturers, no two of them were actually the same size and it was by big enough margins that the part nearest the average diameter was what had to go with it as the template.
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Old 21-08-2019, 09:53 AM
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Merlin66 (Ken)
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While designing and manufacturing the Spectra-L200 spectrograph I needed to incorporate T thread attachment for various accessories.
There is NO recognised working standard for an astronomical M42 x0.75mm T thread!

I ended up using a Baader T thread spacer as a "master" after having numerous complaints about "your T thread doesn't fit my xxxx"

A real PITA!!!

Last edited by Merlin66; 21-08-2019 at 10:25 AM. Reason: added info
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  #24  
Old 21-08-2019, 09:59 AM
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Hmmm..

M42x0.75 should be standard ISO metric thread.
https://en.wikipedia.org/wiki/T-mount
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  #25  
Old 21-08-2019, 10:23 AM
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Merlin66 (Ken)
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Bojan,
Yes....but I don't think any/ many of the manufacturers of threaded adaptors use an ISO gauge to verify their thread.
It's a bit like the wild west when it comes to astronomical adaptors!!!
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  #26  
Old 21-08-2019, 10:46 AM
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A big example is Skywatcher parts. I had a little ST80 at one point, bought secondhand on a whim and eventually sold to someone who wanted a guider. The fit of the Skywatcher supplied compression fitting adapter in the focuser drawtube was astonishingly loose. Then I bought the Evostar 72mm this year to use with my ASI294 and the supplied compression adapter was just as bad, you had to be really careful in fitting it as it was absurly easy to cross thread. If I made threaded parts which were that poor a fit in year 8 metalwork (Which included some fitting and turning type stuff) I would have been sent back to the lathe to do it again!

Joshua now has dimensions for an adapter that fits my ED72 drawtube really nicely but my worry is if someone else was to get him to make one, it might fit as nicely as the one he made me, or it might not!
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  #27  
Old 21-08-2019, 12:49 PM
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Yes...

It all boils down to quality control (or lack of it).
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  #28  
Old 23-08-2019, 08:48 AM
Sunfish (Ray)
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Baader, on their website, publish a discussion about thread sizes for filters and suggest that all standard thread sizes manufactured are not exactly the same and a pretty loose standard exists. As we know , 0.75 tpi does not often measure at that exactly. So they have created their own standard which roughly matches all the known brands so the filters will fit every manufacturer except perhaps the worst. I have a set of three t2 celestron identical new OAG adaptors and one will fit a chinese t2 / m48 adaptor and the others will not go past a few squeaky threads.


I wish I was game to try the boot polish but knowing me I will have boot black every where. Perhaps a different colour and then I could see if it was escaping the thread.
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  #29  
Old 23-08-2019, 08:57 AM
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Another point I wonder about is the thickness of the anodising coating. Anodising coating must come many different thickness and applied after many different chemical processes , very much like the different grades in window coatings from just a few microns to 20 or more microns. Does that make a difference ..?
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  #30  
Old 23-08-2019, 09:07 AM
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Not if they are designing and manufacturing things properly. They should know that the final finish will add “A” thickness per surface and design the machining of the part to be “Ax2” undersized before coating it. With a small tolerance for variations in machining and final thickness of the coating.

Joshua on here is more or less needing to work the out part by part for bespoke bits, but when they turn them out in batches hundreds strong!
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