Dob as Astrograph?

[sreckner]

Is there any reason I can't take my 10" Newt out of the Dob mount, put rings on it, mount it on an EQ mount and image using a DSLR?

Thanks.

Cheers,

Steve

[Leveye]

I say no but you may need to re-position the primary mirror slightly closer to the secondary to image at prime focus with it.

[Astronigel]

Or try using a Barlow, that can often bring the focus to your camera.

[sreckner]

Is it a matter of not being able to get the camera's focal plane racked in close enough to focus or out far enough?

[Astronigel]

Not close enough. Moving primary up a little fixes this.

Try a Barlow instead of moving mirror, but of course then your getting different f ratios

[kalasinman]

Obviously you can do this, but that won't make it an astrograph. This term , I believe, requires fast f ratio (f5 or faster) and usually includes light baffles in the scope and sufficient in focus for DSLR and accessories.,

[jsmoraes]

I did it with my 12". I tried move the primary mirror, but I didn't get stable operation. Motif: the mirror on dob mounting is very heavy.
Original:

Modifcation:

 


So, I cuted 35 mm of the OTA and returned to the original screw.

The same for the mounting. I use an EQ6 at the weight limit. I pulled out from the equipment all that was not necessary
Look it:

[sreckner]

Thank you, everyone for your replies; they are much appreciated.

I'm willing to shorten the tube, but how do you determine how much to shorten it. And after shortening, how do you calculate the focal length?

[MattJenko]

Which 10" do you have? I have the Skywatcher 250px (similar to the Orion brand in the US) and I simply put tube rings on and managed to get focus with a DSLR and the Skywatcher 0.9 coma corrector, so at a pretty quick F4.3. Worked fine on a borrowed NEQ6 the one time I tried. It did seem enormous to me though.

[sreckner]

I have a Zhumell Z10.

[happy-kat]

Why not leave it on the dob base, point at the Moon and use your dslr and see whether you run out of enough inward or outward focus distance.

You won't know for sure what you need to do unless you try with what you have first.

[Steve Ward]

More important than tweaking the tube potentially is using a mount that is man enough to carry the very large tube , camera , guidescope + guidecam and associated cabling without

overloading , as well as being well tuned and 'tight' enough to withstand the buffeting that will occur in even the most moderate breeze ... 

More to be gained by getting a nice 'small' , light ED80 refractor for imaging and leaving the 10" dob for visual to look through while the camera is doing it's thing elsewhere ... 

[jsmoraes]

 how do you determine how much to shorten it

It is easy find extensor of 35 mm. You can see the distance between the mark of surface sensor of DSLR and its front. Canon is 45 mm. Cut this 45 mm will put Canon as any other CCD camera. But to find an extensor of 45 mm isn't easy. And you will need it when using eyepieces.

For my set, GSO 305 mm - FL 1500 mm the ideal should be 40 mm. I did only 35 mmm. The focal length will be the same because you didn't change the parabolic of main mirror. The point of focus was moved a little out, only this was made.

You can loose some light information of the border because it can be projected out of the surface of the secondary mirror. But if it happens, it will be very small. And just the area where the coma distortion is more strong. In other words: you don't loose almost not.

[jsmoraes]

where is wrote "... almost not", read: almost nothing.

[sreckner]

The mount I would use is an Orion Atlas with a 40# weight limit; the Z10 weighs 32# without the camera, guidescope, coma corrector and cables. I believe the total will be less than 40#, but a problem with wind did occur to me.

I've thought about getting a refractor specifically for imaging, but I don't know what characteristics to look for. I'd definitely prefer a quality used scope, rather than buying new.