Crayford focuser vs. rack and pinion

[Clayton E. Cramer]

I am about to rebuild my 8" f/7 to be a little lighter (so I can replace my
Cave mount with a Losmandy GM-8),
and to use a 2" focuser. I am also looking to go for a lower profile
focuser, so that I can get the diagonal a bit
smaller. Can someone explain to me the pros and cons of Crayford focusers?

It appears that the big wins for the Crayford are much lower racked in
height, and zero image shift while changing
focus.

I don't see any 2" rack and pinion focusers that have 1.5" racked in
heights--but there are plenty of Crayford
focusers that are in the 1.5" to 1.6" range. On the down side, along with
low profile you get your choice of
either relatively small travel (a problem for doing astrophotography, where
you need a couple of inches extra
travel to get the focus inside the camera), or a lot of penetration into the
tube when you have the focuser all the
way down (probably not a big problem, since the tube is 9.75" OD, so the
first 0.875" isn't even intruding on
the mirror, and even a little intrusion probably won't affect the image
much.

[Howard Lester]

"Clayton E. Cramer" wrote

where
the

I have the JMI NGF-DX3 2" focuser. In order to accommodate more
"out-travel", I believe you can get an accessory extension tube from JMI.
The better method is to make additional holes in the tube for your mirror
mount, spacing them an inch or so closer to the focuser end of the tube. You
would re-mount your mirror cell in those holes when you want to use your
camera. That would be my preferred method as opposed to the extension tube.
Using the extension tube takes the eyepiece further from the secondary,
reducing the sizes of your 100% and 75% illuminated fields to some degree. I
can elaborate more (or confuse you more) if you like. You can figure this
"illuminated field" stuff out yourself by using the NEWT program, seeing
what happens to them when you change the height of the focal plane above the
secondary.

Howard Lester




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[Bob May]

You also forgot the zero backlash of the Crayford focuser. The shaft of the
focuser know bears directly on the focusing tube and moves the tube by
friction rather than the R&P focusers where there is a rack and pinion
between the two items with the concomittment slop inherent in a gear system.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works evevery time it is tried!

[Howard Lester]

"Bob May" <bobmay@nethere.com> wrote in message
news:1072122291.579843@news-1.nethere.net...
the
system.

The word is "concomitant."

Engineers... *sigh* Some SHOULD be committed. ;^)

Howard Lester

[Jan Owen]

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"Howard Lester" <hlester@mmto.org> wrote in message
news:bs7opi$qea$1@oasis.ccit.arizona.edu...
of

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