Adapters, threads, and general confusion.

[Garbz (Chris)]

Hey all,

So some of you lovely forum folk have pointed out to improve performance of my field flattener I need to change the distance between the flattener and my DSLR sensor. Currently I measured it at around 109mm. The correct distance apparently is somewhere closer to 85mm and as such I'm getting quite bad distortion (coma?) at the edges of my frame.

My reducer has the same thread as the back of my Celestron C8 SCT. It's a 2" thread, and that's about all I know. Currently I'm using the Celestron T-adapter which adapts the 2" SCT thread to a T-thread and then a Nikon T-mount adapter sits on the back of that. The Celestron T-adapter is too long.

Currently I'm looking at spacer products like this one here:
http://myastroshop.com.au/products/d...p?id=MAS-005I2
Apparently they are T-thread to T-thread of various sizes, and then in the detailed description they say M42 which is not technically the same as T-thread so there's the first facepalm but I'll let that slide.

The other product I'm looking at is this one here:
http://myastroshop.com.au/products/d...p?id=MAS-029G4
It says 2" to M42x0.75 (T-thread).

Now is the 2" visual back of SCTs a standard thread? Or in this case the 2" back of the Meade f/6.3 reducer?

Will these two products fit or do I need to guess and hope for the best with my adapter selection? It's quite difficult to find details on the various threads used in this equipment.

Whatever my solution is, it can't exceed 39mm as my mount to sensor distance on my DSLR is around 46mm, and I'm targeting 85mm.

[peter_4059 (Peter)]

Chris,

The 2" SCT is normally 2" 24TPI and the T thread is M42x0.75mm pitch. If you bring your current adaptor tomorrow I'll bring my thread gauge and we can check what you have.

Peter

[Merlin66 (Ken)]

There are short versions of the SCT>T thread adaptors available...then you add the necessary T thread extension pieces.....
(Are we taking about standard Celestron reducers ie x0.63 and standard SCT's?? If so, the design distance is 110mm from the reducer to the CCD plane....)

[Garbz (Chris)]

Peter, I'm down in Melbourne till Wed.

Ken, It's a Meade f/6.3 reducer on the back of a Celestron C8 SCT. I do have distortions at the edge of the field http://www.flickr.com/photos/1009024...in/photostream with the roughly 109mm spacing. The user of the Celestron T adapter would explain the spacing distance then.

Meade have a T-adapter as well but I have been unable to find any info on the length.

[Merlin66 (Ken)]

Hmmm
Sorry to say, that may be as good as it gets.....

[Garbz (Chris)]

Really? I've had comments that say it should be better and to start tweaking. Currently the field is not really any flatter than the telescope at f/10 without the corrector... only wider.

[Merlin66 (Ken)]

In the Ol' days when doing astrophotography I changed from the standard Celestron/Meade reducers to the Optex Next Gen version.
http://www.optcorp.com/product.aspx?pid=3531
This produced much better and consistant results.
Just my $0.02