Classical Cassegrain with a Naysmith focus....stupid idea or not?

[catburglar]

I been doing doing a bit of Googleing and have come across a potential project that might well be a stupid idea..... please talk me out of it....

I've got a celestron c6 and I find the focus is a bit soft. It's particularly noticeable on the planets. The scope is well collimated and cooled, and there's been some decent seeing, so this got me thinking. Is this because of the central obstruction (>35%) causing a reduction in contrast? The C6 has the largest CO of all the Celestron SCT's.

 

So I started to consider a good planetary scope.....a skymax 180 was the first thing I thought of but it didn't stop there. How could I get more aperture, less central obstruction and maintain a manageable OTA?

I stumbled across the Oldham Optical website. There's some discussion about various optical designs and a spreadsheet that allows you to enter some of the key design parameters and it will calculate the others.

It turns out that I could take the mirror from a 10 inch Sky-Watcher Quattro F4, add a custom made secondary and turn it into a 10 inch F15 classical cassegrain with a naysmith focus  that exits through the altitude axis of a dob mount, in an OTA less tham 1m.

 

With 3700mm of focal length and <25% obstruction, it'd be a dream planetary scope- or would it? 

 

 

 

[dweller25]

Found this.....

It doesn't answer your question but it's interesting

[Peter Drew]

Doable but not exactly straightforward. The manufacture of the required hyperbolic secondary would not be cheap, the elliptical flat (tertiary mirror) would have to have its own spider mount and baffle tube to reduce sky flooding or the contrast would suffer and three reflections would result in light loss anyway. Oh, and did I mention collimation?. A better system would be to use a standard classical Cassegrain and incorporate an erector transfer system as per a Dall-Kirkham-Dall. The erector transfer assembly picks up the focal point way ahead of the primary mirror permitting a much smaller secondary and a stop accurately placed in the transfer assembly completely eliminates sky flooding, you would  of course need a perforated primary.

[dark star]

Hi, could you give some more information on the erector transfer assembly? This would go in front of the secondary mirror?

I have a 10 inch f/4 mirror with a hole in it and am planning to have a go at making a secondary for a classical cassegrain when I have finished the 20 inch mirror for a dobsonian I am currently making (which won't be for a while!).

David

[Merlin66]

The cass secondary is not too difficult to polish and test if you use the magnifier lens/ Gaviola test.