Large format focusers

[Geoff45]

Can someone explain the purpose of large format (2.5", 3", 4") focusers on some refractors or as an aftermarket focuser. As far as I can tell from a cursory check on the web, ccd cameras only come with 1.25" and 2" nosepieces, and eyepieces with diameter larger than 2" don't seem to be around.
Geoff

[Ian Robinson]

I think these are designed to allow the use of large format film and plate cameras, and to ensure zero vignetting.

[Omaroo]

I imagine that given the larger diameter, they are also more mechanically true and stable when racked out.

[Kal]

4" focusers are really for medium format film (6x7cm), while 2.7" focusers will let you use a larger telecompressor without vignetting on a 35mm camera (be it film or ccd/cmos)

[RB]

Exactly Andrew!

[Terry B]

I know this makes sense but I've never found any adapter to connect a medium format camera. I have a pentax645 and it would be nice to try to connect it to a scope but I searched and couldn't find any version of a T adapter or bayonet/screw thread adapter. It's a bit expensive or hard to make one.

[Ian Robinson]

Took about a minute on Google to find :
http://www.astro-physics.com/tech_support/accessories/photo/ada2767.pdf

http://www.handsonoptics.com/gen_takahashi.html

http://www.stellar-international.com/index1.html

Plenty of gear out there that you could use to connect you 6x7 to the scope.

[Kal]

Most of the Astro-Physics stuff (http://www.astro-physics.com/products/accessories/photo_acc/67pfdiagram.pdf), for example, comes with camera adapters to Pentax 6x7 cameras (including the 4" Prime Focus Field Flattener)

For field flatteners, the spacing to the camera must be a certain distance, so I don't think adding a pentax 645 to Pentax 6x7 adapter into the mix would work.

For telecompressor use, you have some versatility in the distance between telecompressor and camera, so you might be able to use one of these adapters.

Or you could just buy a Pentax 6x7 camera body, film stuff is selling for bargain prices now that the digital age has hit.