William Optics GTF-81 Focuser with Atik 460

[mikeb]

I have a GTF-81 which I have been using with my Canon T3i DSLR.I had to buy the 2" extension for the Canon to get it to focus which it does at 12.5cm  given that the camera and extension represent a distance of 10cm the focuser being 2.5cm.  This implies that it is not possible to come to focus with this tube without an extension of some kind.  I have recently purchased an Atik 460 camera and have found it impossible to get focus without a diagonal (or extension tube).  With the back focus of the Atik being 1.3 cm it would imply that I need to rack the focuser out to 11.2cm to gain focus.

I was talking to a friend who has a Mergaz with a field flattener.  He has never had to use an extension tube to get focus on either his DSLR or CCD camera.  Am I doing something wrong?  Is there something wrong with the OTA or is this normal for the GTF-81?

[Chris]

I don't have a GTF-81 but it doesn't seem to have a problem.

The standard for a T adaptor is 55mm from the T adaptor to the focal plane.
Then you have a 50mm extension and the focuser is racked out 25mm.

This gives a focus position of 55 + 50 + 25 = 130mm behind the focus zero position, this seems perfectly reasonable.
Replacing the DSLR with the Atik would mean that you would need to move the focuser out to 67mm, or add more extensions.

Your friend's Mergaz with a field flattener is a totally different optical system.  The flattener will have it's own specification for the distance from the flattener to the focal plane, this could easily be 55mm so the DSLR screws directly to the flattener.  The flattener will probably need to be moved inwards to achieve focus and so may not need an extension.

Chris