302 replies to this topic

[Peter Ceravolo]

A small displacement of a Cass secondary can result in a large focus shift. That kind of flexibility is so advantageous that advanced scopes are built with a motorized secondary. It's typical that one is likely to move the focus position out to accommodate accessories that chew up a lot of backfocus. For example a generic 12" f/10 RC secondary is shifted 5mm closer to the primary, the image is displaced about 60mm farther out. What is interesting to note is that the image quality will degrade with increasing image displacement from the design position. Assuming said 12" scope has perfect optics, that 60mm image shift will introduce almost 1 wave of pure spherical aberration. If one focuses the image to minimize the pure spherical one get about a 1/4 wave wavefront. Now 1/4 wave is not too bad, but if you paid for perfect optics...

That kind of degradation will not be discernible if one is imaging, but it will affect visual image quality under the best seeing conditions.

 

The B&C 16" is more forgiving. Only a 1.6mm secondary movement will yield the same 60mm image displacement, and the focused image quality is still better than 1/10th wave.

[Bob4BVM]

A small displacement of a Cass secondary can result in a large focus shift. That kind of flexibility is so advantageous that advanced scopes are built with a motorized secondary. It's typical that one is likely to move the focus position out to accommodate accessories that chew up a lot of backfocus. For example a generic 12" f/10 RC secondary is shifted 5mm closer to the primary, the image is displaced about 60mm farther out. What is interesting to note is that the image quality will degrade with increasing image displacement from the design position. Assuming said 12" scope has perfect optics, that 60mm image shift will introduce almost 1 wave of pure spherical aberration. If one focuses the image to minimize the pure spherical one get about a 1/4 wave wavefront. Now 1/4 wave is not too bad, but if you paid for perfect optics...

That kind of degradation will not be discernible if one is imaging, but it will affect visual image quality under the best seeing conditions.

 

The B&C 16" is more forgiving. Only a 1.6mm secondary movement will yield the same 60mm image displacement, and the focused image quality is still better than 1/10th wave.

This is fascinating Peter, thanks so much for sharing your wisdom and experience with your B&C.

I have learned so much from this thread that enriches my own experience with the B&C i had the good fortune to operate for a few years at PMO here in Oregon.  That scope is a 24" ( at F16, IIRC.)   I wonder how close that scope would track what you say about focus vs secondary position ?

 

Now that I am retired, I should see about getting back on the volunteer staff at PMO.  Thanks for rekindling my interest !

 

CS

Bob

Edited by Bob4BVM, 23 December 2024 - 01:39 PM.

[Peter Ceravolo]

I haven't investigated the issue in detail, but my poking around with Zemax suggests that the lower amplification systems will suffer more, probably because more secondary displacement is needed to move the image. That 24" is probably similar to the 16".

 

However there is a biggly B&C at Western University near London Ontario, a 1.2m f/8 RC that sports multiple foci for research. There's the f/8 RC, a Nasmyth (out the Dec axis) and Coude focus. The secondary is capable of moving a relatively large distance, so much so that there is a movable counterweight in the secondary mount to balance the thick 18" dia secondary on the spider.

 

 

 

It's a very sexy opto-mechanical set up in that the secondary is double sided and can be "easily" flipped; one side for the f/8 and the other for the f/30 Coude. The f/8 and f/30 foci are achieved with the optimal, design spacings, but for the Nasmyth one has to move the f/8 system way out of the optimal spacing to get the focal point out of the dec axis. The spherical aberration introduced is so bad that B&C supplied what I think amounts to a Schmidt corrector plate, four or so inches dia, to place in the beam to minimize the SA. Of course it is not achromatic, so there will be color aberration. I don't think the astronomers used the Nasmyth focus much...