Focal plane location

[The Warthog]

If I were building, say, a 12" Dob, in designing the location of the Primary and Secondary mirrors, where should the focal plane of the mirror be located, with respect to the side of the tube? Should it be inside or outside the tube, right at the tube, etc? I am condidering a helical focuser, but may go for a Crayford.

[Kaptain Klevtsov]

Where the field stop is on the eyepiece, more or less, ish. So outside the tube.

If a DSLR will focus on a Newt., then the focal plane of the mirror must be where you oput the focal plane of the camera. If that works for commercial designs, it must be a close starting point to work from?

Kaptain Klevtsov

Kaptain Klevtsov

[Merlin66]

I'd go for a focus about 120mm or 150mm outside the tube.

This would allow you to get a DSLR etc to focus.

If you can get a measurement of the height of the focuser racked in, add at least 60mm to this dimension.

[davew]

Hi Warthog,

Take a peek at - Oldham Optical Web Site

They have a spreadsheet and a few tips on telescope design. All good stuff.

Dave.

[The Warthog]

Thanks, guys. Jonathan, your comment about the camera explained a lot, and that's a great website, Dave. I'll have to get going on it, while my wife isn't looking. I was given a 11½ inch mirror and a secondary by a generous friend, and have only to put the tube, cell and focuser together to have a very nice Dob. As it's going to be a Dob, I'm not so worried about attaching a camera.

[Kaptain Klevtsov]

As it's going to be a Dob, I'm not so worried about attaching a camera.

I'm surprised, even a Dob. can snap the moon WH, stay away from the darkside....

Kaptain Klevtsov

[The Warthog]

I'm surprised, even a Dob. can snap the moon WH, stay away from the darkside....

Kaptain Klevtsov

The darkside? Oh, you mean astrophotography! Still, I will snap some pictures of the Moon with that long focal length.

[davew]

Hi Warthog,

Given a mirror set eh ? Nice. Just one thing, do you know the focal length to within a few millimetres ? If not, don't ask me how to find it when you have a load of bits all over the garage floor. Someone will know however.

Dave.

[The Warthog]

The focal length is 71.64 inches. My friend said that it performed well on DSOs, but since the outer edge is a little out of true he used to mask it down to 9" for planets and the Moon, making it a 9" f/8 scope, where it performed brilliantly, he says.

I like the fact that it is f/6.24 as it will be a little more tolerant of less expensive eps than the usual f/4 that you get in this size. And, it's still short enough that I don't need a stool to stand on.