Classical Cassegrain - any ideas?

Robert_T · #1 09-03-2006, 07:34 AM

Hi All, I love my 7inch Tak Dall-Kirkham cassegrain for planetary imaging, an hope one day to move up scale, but this comes at a massive cost ~$10,000 for a 10 inch and nearly $20,000 for a 12inch OTA only.

At those prices I've started wondering about getting the mirrors made locally by someone like Mark S and putting the OTA together myself. I've done this with newtonians before. Any ideas or experience, problems etc, anyone have with DIY cassegrains of the classical or dall kirkham variety? I'd be looking for something in the F12 to F15 range.

I figure if I could get the optics for a 12inch done for ~$3000-4000 and spent another $2K putting it all together I'd still be streets ahead. Might have to come up with some system of focussing via electric movement of the secondary mirror.

Help anyone...

Merlin66 (Ken) · #2 09-03-2006, 08:55 AM

I've built a 12" f5/ f20 Cass in the past. As Ingalls says in ATM bookII, don't!!!
A far easier solution is to make use of the excellent Powermate barlows which have come on the market to achieve the same focal length as the cass. Much less hassle and easier on the pocket.

Moving the primary in a classical Cass screws up the images and introduces abberations! Baffling the system is also a major issue. The mirrors are designed to work at ONLY one spacing to get a back focal length.

Robert_T · #3 09-03-2006, 02:29 PM

Quote:

Originally Posted by Merlin66

I've built a 12" f5/ f20 Cass in the past. As Ingalls says in ATM bookII, don't!!!
A far easier solution is to make use of the excellent Powermate barlows which have come on the market to achieve the same focal length as the cass. Much less hassle and easier on the pocket.

Moving the primary in a classical Cass screws up the images and introduces abberations! Baffling the system is also a major issue. The mirrors are designed to work at ONLY one spacing to get a back focal length.

Stop it Merlin, your depressing me and crushing my dreams

Seriously, thanks for the advice. The sort of planetary imaging I am doing and want to keep doing runs upwards of F40 which is even beyond a 5x powermate. But I know Bird bumps his up more with extension tubes etc so maybe a good option. I've actually got a 12inch F7 newtonian OTA I made some 15 years ago, but it's a challenge for mounting... hmmm, actually maybe I could dob platform something up for that one. I like the eyepiece at the bottom end, but then I like $ too....

cheers,

Miaplacidus (Brian) · #4 09-03-2006, 03:01 PM

Don't know if this is relevant, but I found this link to Cloudy Nights where a guy built his own Cassegrain.

http://www.cloudynights.com/item.php?item_id=1127

Cheers,

Brian.

Robert_T · #5 09-03-2006, 04:12 PM

Quote:

Originally Posted by Miaplacidus

Don't know if this is relevant, but I found this link to Cloudy Nights where a guy built his own Cassegrain.

http://www.cloudynights.com/item.php?item_id=1127

Cheers,

Brian.

Many Thanks Brian, if I do go this path this article is great and tells me much of what I need to know

Merlin66 (Ken) · #6 10-03-2006, 05:36 AM

As an alternative, I've seen arrangements where a star diagonal in the focusser of a newtonian sends the light path via a barlow/ transfer lens back down the outside of the tube so that the eyepiece is close to the dec axis. Makes things a bit easier.

Robert_T · #7 10-03-2006, 08:51 AM

Quote:

Originally Posted by Merlin66

As an alternative, I've seen arrangements where a star diagonal in the focusser of a newtonian sends the light path via a barlow/ transfer lens back down the outside of the tube so that the eyepiece is close to the dec axis. Makes things a bit easier.

thanks Merlin, might be a good addition to get the eyepiece more manageable located on a 12in F7... loss of light and more collimation I expect though

cheers,

Merlin66 (Ken) · #8 10-03-2006, 09:46 AM

You've still obviously got the problem of the secondary size, but a workable solution!