Question about the Antares/Orion twist lock adapter

[Black Knight]

Like this: http://www.rothervalleyoptics.co.uk/2inch-1-25inch-twist-lock-adaptor-with-t-ring_d2911.html

I understand how potentially useful these might be for centring 1.25" eyepieces or whatever in the focuser, but don't these adapters simply transfer the off-centre problem to the host 2" adapter on the focuser? That is, if the twist lock adapter itself is mounted off-centre because of mounting screws on the 2" focuser, then won't a 1.25" eyepiece still be off-centre, even if it is perfectly centred in the twist-lock. If you see what I mean .....

Or am I missing something about how they actually work?

[umadog]

You're right, but the degree to which the problem occurs depends on how well the adapter fits in the focuser. Off-centering can only occur if there's slop. This is one reason why it's worth paying more for good 2" collimation tools: one reason why they're more accurate is because there's less slop and they fit well into the focuser. That said, I think I remember reading that the tolerance for being centered is fairly lax. i.e. you can be a little off (perhaps 2 or 3 mm) and it's ok. That would make sense, actually, since larger scopes have their mirrors sit in a sling and so can slide from side to side across the cell by several mm. This doesn't seem to lead to any performance issues.

[John]

That was my question when these adapters first hit the market - how do you make sure that adapter is centered ?. Baader make a 2" centering adapter which is compatible with some leading brands so that's one answer I guess.

Umadog has it right though, mechanical precision is the real solution. That costs a few bob though.

[Black Knight]

Thanks for this guys. I've never actually seen one of these things, so I didn't know if I was missing something fairly basic! . Umadog, it's interesting what you say about this not leading to performance issues. Are you talking specifically about collimation, or in general?

[RikM]

I use the Orion version of this thing in my Dob. My reason for purchase was that my fingers don't work so well in the cold, so tiny little thumb screws are a pain. What I have done is to standardise on 1.25" eyepieces, then I have screwed the SCC adaptor into the 2" focuser and just leave it there. I collimate using a Cheshire / sight tube in the SCC adaptor, so even if there is a bit of off-centre of the adaptor in the focuser, I have compensated for it.

It works very well and I would have no hesitation in recommending one.