SCT > T2 adaptor wiggle

[badhex]

Hl all, 

Some time ago I bought a set of Bresser T2 extensions of which an SCT adaptor was part at one end. I hadn't specifically bought them for that but figured it would come in handy at some point, and now that day has arrived as I want to replace a Mak visual back with compression rings. 

I knew it comprised of a lipped T2 tube, with a knurled outer ring to screw onto the SCT thread. Presumably this is to allow imagers to slightly undo the ring and rotate the T2 tube for cameras etc. Upon further inspection, I've now found that there is some significant lateral play between the two parts, see video below. 

It's possible to screw down the ring so that the inner T2 tube doesn't move, however the T2 tube is unlikely to be centred correctly, which I would have thought introduces collimation issues. 

What's the best move here? Use a very thin O-ring to try and centre the tube, or just buy an SCT > T2 adaptor which is all one piece? 

 

[globular]

I have the same one - with the same issue.

I've stuck with it because;

a) it only adds 7.5mm and I can't find a better one with such a short connection.

b) I only use it occasionally to give my lowest power views with my binoviewer.  (More often than not I want high power with the binos - and with the GPC I have enough back focus to use my usual visual back.)

c) I get it nearly tight then give it a wiggle to try and make sure it's centred before final tightening. Not perfect - but not noticed any issues in my visual only use case.

Sorry I don't have a suggestion - but please update the thread with your solution when you find one - I might copy you.

[badhex]

2 hours ago, globular said:

I have the same one - with the same issue.

I've stuck with it because;

a) it only adds 7.5mm and I can't find a better one with such a short connection.

b) I only use it occasionally to give my lowest power views with my binoviewer.  (More often than not I want high power with the binos - and with the GPC I have enough back focus to use my usual visual back.)

c) I get it nearly tight then give it a wiggle to try and make sure it's centred before final tightening. Not perfect - but not noticed any issues in my visual only use case.

Sorry I don't have a suggestion - but please update the thread with your solution when you find one - I might copy you.

Thanks, it's a bit rubbish that it does this! I'm also visual only. If I come across an answer I'll report back. Best I can think of is an o-ring to centre it. I have seen a short SCT > T2 adaptor somewhere, if I find it again I'll post here. 

[globular]

There is a baader one that might be better at centreing... I don't know for sure though.  It's 10mm optical length vs 7.5mm for the bresser.

https://www.firstlightoptics.com/adapters/baader-sct-t-adapter-ultrashort-t-2-male-2-sct-female.html

 

[globular]

1 hour ago, badhex said:

Best I can think of is an o-ring to centre it

I've measured with callipers and an o-ring with width 0.5mm should do it - in theory.
I happen to have some too - but it's nigh on impossible to get it to stay on the small lip.  

May be some 0.5mm metal tape stuck on around the perimeter would work? 
I've just tried with masking tape and it works quite well.

Edited October 29, 2023 by globular

[Peter Drew]

As long as you can tighten the adapter, a small amount of decentring shouldn't upset collimation, tilt due to sag if the item did not tighten, would.  There may be a physical detail like a chamfer which centres the unit once tight.    🙂   

[globular]

7 minutes ago, Peter Drew said:

There may be a physical detail like a chamfer which centres the unit once tight

There should be, but there isn't.  Maximum miscentreing is about 0.5mm, so not that bad - but is there.

[badhex]

14 hours ago, globular said:

I've measured with callipers and an o-ring with width 0.5mm should do it - in theory.
I happen to have some too - but it's nigh on impossible to get it to stay on the small lip.  

May be some 0.5mm metal tape stuck on around the perimeter would work? 
I've just tried with masking tape and it works quite well.

Good work with the tape, thanks Glob. I'll have a look if I have any knocking about which could do the job. 

14 hours ago, Peter Drew said:

As long as you can tighten the adapter, a small amount of decentring shouldn't upset collimation, tilt due to sag if the item did not tighten, would.  There may be a physical detail like a chamfer which centres the unit once tight.    🙂   

Thanks Peter, good to know that a small amount of decentring is hopefully okay. Regarding the chamfer, I'd actually expected such a detail but as Glob says, sadly it has not been implemented.

Finally, I did find this:

https://www.firstlightoptics.com/adapters/astro-essentials-low-profile-sct-2-to-t2-m42-adapter.html

The threads are 5mm each, and the overall length is stated at 11mm, which suggests an optical length of 1mm. I'm assuming it's probably a bit more than that due to tolerances etc but looking at the pictures, not much more - maybe 2-3mm at most. 

I've messaged FLO for more detail, will let you know.

Edited October 30, 2023 by badhex
massive picture size!

[globular]

That looks promising.

Fine on its own if you don’t need rotation. Or use with an sct lock ring if you do need rotation.  Something like this…
https://www.firstlightoptics.com/adapters/baader-2-nexstar-locking-ring.html
(but overall back focus will change slightly at different angles)

[globular]

I've had another play to find a solution to the wiggle with the Bresser adapter that is more robust than the tape prototype.

I found a plumbing washer (8M/45/52)....

... that snaps onto the adapter ...

I then trimmed it to leave 5 tabs sticking out about 1mm ...

Which fits snugly inside the threaded part and keeps it central (while still allowing rotation as it doesn't jam)...

The wiggle is eliminated and being plastic it should be fairly robust in the cold damp conditions it'll be used in, so it should last a good while.

It doesn't change the optical length - but the thread screws on a millimetre or two less than before.  Still enough to feel safe and secure though.

[badhex]

57 minutes ago, globular said:

I've had another play to find a solution to the wiggle with the Bresser adapter that is more robust than the tape prototype.

I found a plumbing washer (8M/45/52)....

... that snaps onto the adapter ...

I then trimmed it to leave 5 tabs sticking out about 1mm ...

Which fits snugly inside the threaded part and keeps it central (while still allowing rotation as it doesn't jam)...

The wiggle is eliminated and being plastic it should be fairly robust in the cold damp conditions it'll be used in, so it should last a good while.

It doesn't change the optical length - but the thread screws on a millimetre or two less than before.  Still enough to feel safe and secure though.

Oh nice, look like that works well!

I've also just found a very cheap single piece SCT > T2 adaptor on ebay - £4 delivered! TBH I am not sure how good the quality will be, but I guess for £4 it's worth a punt. I would normally spend a few extra quid and support a proper astro dealer, but I'm totally skint at the moment so looking at alternative options in the short term. I'll let you know how I get on, happy to PM you the link if you are interested.

[globular]

3 minutes ago, badhex said:

I'll let you know how I get on, happy to PM you the link if you are interested.

Thanks Joe.
I'll stick with my diy solution - I need rotation for my use case.
If the ebay one is no good then drop me a PM with your address and I can post you a couple of diy trimmed washers. 

[badhex]

23 hours ago, globular said:

Thanks Joe.
I'll stick with my diy solution - I need rotation for my use case.
If the ebay one is no good then drop me a PM with your address and I can post you a couple of diy trimmed washers. 

Awesome, thanks! I'll give you a shout if this adaptor doesn't work out. Yes, rotation is not required for me as I'm only adding a low profile EP holder to the T2 end.