Cassegrain Focussing Query

[BlueAstra]

I have a small Newtonian Telescope that has 'fixed' primary optics and is focussed by moving the eyepiece forwards and backwards. I've been looking at some larger Cassegrain type telescopes as an upgrade, and have noticed that they are often focussed by a knob that moves the main mirror forwards and backwards.

Why is it done this way? Shurely it is less complicated (with less image movement and potential for main mirror tilt) to use the same type of focussing as in the Newtonian, where the eyepiece is just moved backward and forward?

I'm sure I'm missing something obvious and will kick myself when I hear your answers!

[John]

That's a very good question and you are quite right that the moving primary system does introduce the possibility of image shift and "mirror flop". Generally the system is well implemented these days but they can be issues with older cassegrains or ones with large primary mirrors. Many people fit a conventional focusser (usually a crayford) to their cassegrains to overcome this. A mirror lock is another solution which is used.

John

[Whippy]

AFAIK, the problem with the 'moving eyepiece' method of focusing is that generally you have a limited amount of drawtube travel which can cause problems, especially when attempting imaging. The moving mirror way has a lot more travel. There are some Cassegrains that do have a crayford as standard, the Intes MK67 Mak-Cass is one that springs to mind.

HTH

Tony..

[philsail1]

Yes, I would echo Tony's comments there. It seems that even if you add a Crayford focuser to an SCT scope, you will still need (under certain circumstances) to use the scopes conventional focuser to reach focus successfully - then use the Crayford to "fine" focus.

The main benefit of adding a Crayford to an SCT, is that you can achieve a precise focus much more easily, and without any mirror shift. Also, Crayfords usually have an index scale on the focus tube which you can use to "guage" (and note)the position of focus for a variety of different eyepieces, imagers, binoviewers, etc.

Regards,

philsail1

[BlueAstra]

My Newtonian probably has about 25mm or so of focus travel. Both types of telescope have fixed focal planes, so I would have thought that would be enough for a Cassegrain. I can only think that most Cassegrains have longer focal lengths and F/N's, so the depth of focus may be greater, requiring more travel for the same defocus effect, so to speak. However, a focal plane is a focal plane, and whatever sophisticated focus mechanism you can apply to a Newtonian (or even a straight refractor) to reach it you could apply to a Cassegrain.

[Merlin66]

You're 99% correct.

The SCT optics ARE designed for a specific distance iro back focus, BUT one of the strengths of the design is that you can vary this by up to 100mm without affecting the image quality.

This allows much more freedom when attaching accessories; you can easily fit filter wheels, adaptors, cameras etc etc to SCT which would not be easy on conventional newtonians.

I have spectroscopes which easy fit the 12" LX200 but give me MAJOR headaches to fit to other scopes!

[philsail1]

Yes, I would agree with you "Merlin."

I can use my "Binoviewers," easily achieving focus and retaining the image quality with my Celestron C8 SCT, but I have to remove the section of the focuser to use them on my Orion Optics 8" Newtonian (the quality of the image is retained on the Newt as well).

Regards,

philsail1

[BlueAstra]

OK, I see it can be useful for locating accessories. But doesn't this mean that the secondary is consequently oversize to prevent vignetting as you push the focus out? Or is some loss expected?

[Merlin66]

The secondary mirrors in ALL SCT's are oversize!! That's one of the down sides....

[ollypenrice]

I have just put a Revelation Crayford on our 10 inch and found it a joy after all the image lurching ('shift' is an entirely inadequate term!!) that I had grown to detest. I bought it nearly new from this forum and find it fine. I'm not sure why one would spend a great deal more on a Moonlite unless the fixed focal reducer on the Revelation is an issue on some setups. Recommended.

Olly