10" PDS spacing

[Frank the Troll]

I have a 10" dob not being used, and want to use it for prime asto photography

What spacing do I need between the secondary & primary to make this for example a SW 10" PDS

Cheers

Frank

Edited February 10, 2020 by Frank the Troll

[Guest]

Hi Frank, I don't know the answer to this question but - I have an SW 8" PDS so I wondered if you could work it out from that. Presumably what you need is to have the prime focus at the same distance from the outside of the tube. So if I measure the distance from primary to secondary on mine we should be able to work that out. Does that sound right?

There is one complication which is to do with the size of the secondary. As I understand it when SW designed the PDS version of the earlier scope they shortened the distance between primary and secondary which would put the prime focus further out from the side of the tube. I assume to allow for hanging cameras and stuff on it. However at the same time they made the secondary mirror slightly bigger. I think the way it works is that since you are cutting off the cone of light from the primary closer to the primary the secondary would have to be correspondingly bigger.

If this seems reasonable then I could measure my 8" PDS tomorrow.

Cheers

Steve

 

[Frank the Troll]

Hi steve, not sure that would help, ideally I need someone to measure  the 10" PDS  to find the difference too mine

Cheers

Frank

Edited February 10, 2020 by Frank the Troll

[alacant]

On 09/02/2020 at 17:49, Frank the Troll said:

prime asto photography

Hi.

Do you mean that you cannot currently bring your camera to focus?

We have an old blue tube SW 10" and it focuses fine with a DSLR. If you did bring the mirrors closer, you would also need a larger secondary to intercept the now increased diameter light cone. FWIW, we replaced the original 58mm secondary with a 63mm when we discovered the original was missing light around its edges.

Cheers and HTH.

Edited February 10, 2020 by alacant

[Frank the Troll]

I will have to check that out, not put a camera to it yet, I used a pre-drill tube from Orion, so thouhgt it would be different from a SW 10" PDS

Mirror is an F4.7 Its being wasted so have decided to mount it on the EQ6

 

Edited February 10, 2020 by Frank the Troll