Cassegrain Optics Question for the Experts

[glend]

I have been reading Mike Lockwoods comprehensive article on Cassegrain Optics, with commentary on some various design types. Article is here, scroll down to (Cassegrain Design Equations).

http://www.loptics.com/ATM/index.html

On the Design Equation page , Mike talks about many aspects but my question relates to mirror separation distances (d in the equations), and the role of separation in determining focal length, correction, and illuminated field of view.
My question is:
In a Classical Cassegrain, focal length is finely tuned and can be shortened or lengthen by altering (d) and is inversely proportional (ie, if you reduce the distance you can increase the focal length, and vice versa. The placement also affects Over and Under Correction of Spherical Aberration.
Is there a way to visually, or through bench testing of the scope (with normally available tools, like a Cheshire and circular grid target (easily made) to confirm full illumination (visability) of the primary field of view?


Mike suggests (in section D, on Size of Illuminated Field), we should be able to move our eye off to the side to see the edges of the primary, or not, as an indicator of full secondary illumination by the primary. I know through recent discussions (on Cloudy Nights) that the new GSO Classical Cassegrains, do not use the entire primary diametre, as they use a reduced secondary size ( meaning smaller central obstruction) to achieve a spot size of 15mm, to give optimal planetary performance.

My second question is: If I slightly alter primary tilt to tune out coma, (and there Is not much change they could make to separation), (note that in this design the primary mirror can be considered as fixed, the small adjustment and lock screws are for tilt and are not there providing a range of movement like that available in a typical Newt design).

If I do seek to tune out any residual coma, would I be changing spot size, affecting correction, and performance in some substantial way?


I am trying to gauge how my collimation setup, as i fine tune my scope, may impact on performance.
Thanks for your thoughts.:question:

[multiweb]

Visually I couldn't tell how mirror spacing affects field illumination but if you stick a CCD at the back, shortening the distance between primary and secondary will increase vignetting. Having said that if the spacing primary/secondary is shorter than recommended you will see increased spherical aberration in your in/out of focus airy discs. You lose the symmetry and some rings get brighter so the light is no longer focused at the same point.

[multiweb]

Minute adjustments in the primary tilt can have a big influence on collimation. In my experience close enough is good enough. Where possible I've always avoided to change the primary tilt usually set at the factory.

If I had to do it then it was always a mechanical adjustment, not optical.

Tell tale of a misaligned primary is usually astigmatism and oblong on axis stars.

[glend]

Thanks Marc, at this stage the scope is visual only, for the Encke Gap Challenge, etc, so I actually don't have a camera to stick on it (other than a DSLR), and I am wary of touching the primary adjustment, beyond normal tilt control to get an evenly lit ring all around my Cheshire, and final tuning with star testing.
As the scope uses a parabolic primary, and hyperbolic secondary, the risk of Spherical Aberration cannot come from the mirrors, other than positioning imho, and I have not noticed uneven ring brightness in my star testing to date.

[glend]

Marc, I have checked both mechanical and optical alignment, and did use the reflected donut technique to adjust slight tilt anomaly. I have not noticed any astigmatism, and star shapes are round at focus, but I do still have a little coma to tune out. I agree it should not be touched, normally, but it is unclear to me what GSO does at the factory in regards to setup. I doubt they test it through the focuser, as they are known to have a slight tilt induced in that mechanical alignment, due to the way they do the threads on the mirror back plate (discovered originally by RC owners). Still it looked ok on first star testing, but showed significant concerns when bench tested, like primary tilt, off centre spot laser, etc.. I have faith in GSO mirror engineering, it is the mechanics that concern me. Sort of like they put together a couple of mechanical tweaks to get the optimal optical result, but they don't stand up to mechanical scrutiny.

I will continue to star test as conditions permit, but I was stopped last night because of sky conditions prevented higher power testing. As far as I managed to get using low power EPs it was ok, but I admit that my one old, good eye, maybe not the best judge.

[Stefan Buda]

Glen, it would be very easy to answer all those questions if they were spread out a bit.

You can use the coma, inherent in the design of the CC, to achieve good colimation by making it (the coma) symmetrical across the field, in every direction. For that you can use high power eyepiece and adjust the tilt of the mirrors until you get a round doughnut in the center of the field. A round doughnut indicates absence of coma and it is possible to achieve only on the optical axis. If the optical axis is in the center of the field, the colimation is done.
Easily said but a bit harder to do. My approach would be to use a laser to get the secondary pointing at the center of the field through retro reflection and then adjusting the primary to eliminate axial coma.

Mirror separation is a different matter. A small change in separation can move the focal surface (it is not a plane) something like 10x more, depending on secondary magnification. The focal length will not change much but spherical aberration will kick in, on either side of optimum spacing. This is also easy to see with a high power eyepiece. Just look at the size of the central obstruction in the slightly defocused star doughnut, inside and outside of focus. This is an incredibly sensitive test for spherical aberration.

[glend]

Just came in from some testing, coma is no longer present with any low to mid range EP, but conditions did not favour going above 200x at this time. I did have to tweak the primary slightly to get rid of the coma that was still there after the bench setup. So I am confident the primary is good now. Focuser positions are where I expected, within a mm or two of original recorded position.
Star testing shows good Airy disk with centred secondary shadow, no astigmatism, nor spherical aberration. I need to get one night of very good Seeing to finish off testng above 200x, but it is looking very good at this point. And my star testing result also ties in nicely with my bench test setup,