Celestron C 9-1/4 alignment issues

Greets--
I have just reassembled my Losmandy GM8 mount, which was shipped from
the factory with .011" end play in the RA axis. In due course, I had to
have a new shaft machined (from 303 stainless, instead of the aluminum
Losmandy uses), and it was machined to fit with .0005" end play on the
RA (polar axis), Needless to say, this helps pointing accuracy and
tracking, removing a wild non-periodic error that it took a while to
hunt down. Now the mount behaves more like an AP mount. ;-)

So,..
I'm fitting my Celestron 9.25" SCT to this mount using a standard
Losmandy dovetail plate...and looking at the OTA, I notice that the
casting to which the radius blocks attach on the Primary end of the scope
is noticeably thinner than the ring which encircles the corrector plate.
The primary end casting is 33" in diameter, the corrector plate ring is
33.25" in diameter. Unless these differences are accounted for in mounting
the OTA to the mounting rail or plate, you are going to have fun getting
this scope to point consistently. I believe it will get very confused when
you move it from one part of the sky to another, especially if your
targets are on different sides of the meridian. To achieve any degree of
pointing accuracy, you have to eliminate cone error. WTF were they
thinking? They could have just machined a little flat on the corrector
ring and this problem wouldn't exist...is that too expensive to implement?

My solution is to shim the radius block on the primary end as required. I
plan on using McMaster-Carr laminated aluminum shim stock, which is very
cool, because you can peel off layers of the shim in .002" increments
until it is the correct thickness. This should get me close to true
orthogonality.

This kind of stuff makes me weary.

Uncle Bob
Fairfax, CA

Hi, Bob. 33" and 33.25" in diameter? Not circumference? Maybe I'm missing something but have you had too much pi?
Anyhow, the difference is 0.0398" or about 1 mm on the radius. Will that make much of a difference in tracking?

How is the viewing across the Bay? Over here in Moraga it has not been too good. However, Mt. Diablo is this weekend.
I can hardly wait to set up my 9.25.

Dan

Uncle Bob wrote:
Greets--
I have just reassembled my Losmandy GM8 mount, which was shipped from
the factory with .011" end play in the RA axis. In due course, I had to
have a new shaft machined (from 303 stainless, instead of the aluminum
Losmandy uses), and it was machined to fit with .0005" end play on the
RA (polar axis), Needless to say, this helps pointing accuracy and
tracking, removing a wild non-periodic error that it took a while to
hunt down. Now the mount behaves more like an AP mount. ;-)

So,..
I'm fitting my Celestron 9.25" SCT to this mount using a standard
Losmandy dovetail plate...and looking at the OTA, I notice that the
casting to which the radius blocks attach on the Primary end of the scope
is noticeably thinner than the ring which encircles the corrector plate.
The primary end casting is 33" in diameter, the corrector plate ring is
33.25" in diameter. Unless these differences are accounted for in mounting
the OTA to the mounting rail or plate, you are going to have fun getting
this scope to point consistently. I believe it will get very confused when
you move it from one part of the sky to another, especially if your
targets are on different sides of the meridian. To achieve any degree of
pointing accuracy, you have to eliminate cone error. WTF were they
thinking? They could have just machined a little flat on the corrector
ring and this problem wouldn't exist...is that too expensive to implement?

My solution is to shim the radius block on the primary end as required. I
plan on using McMaster-Carr laminated aluminum shim stock, which is very
cool, because you can peel off layers of the shim in .002" increments
until it is the correct thickness. This should get me close to true
orthogonality.

This kind of stuff makes me weary.

Uncle Bob
Fairfax, CA

On Wed, 15 Nov 2006 20:03:28 -0800, Dan Spisak wrote:

Hi, Bob. 33" and 33.25" in diameter? Not circumference? Maybe I'm
missing something but have you had too much pi? Anyhow, the difference
is 0.0398" or about 1 mm on the radius. Will that make much of a
difference in tracking?

How is the viewing across the Bay? Over here in Moraga it has not been
too good. However, Mt. Diablo is this weekend.
I can hardly wait to set up my 9.25.

Dan


Brain fade. It strikes the elderly with stunning results. ;-)

Too much pi is right. Circumference is what I meant.
I made a shim of .054" and stuck it under the rear radius block.
As originally configured, the scope was 1min4sec off from the mount's
polar axis. Now it's less, but I won't know how much less until I do an
orthogonality check.
The scope was apparently designed to be used in a fork mount, where this
difference wouldn't be a problem, but when you slap it on a dovetail
plate and an equatorial mount, the error surfaces.
Seeing is soft here too. Can't split epsilon Lyra.
Have fun on Mt. Diablo. Hope it stays dry for you.
Uncle Bob

Uncle Bob wrote:

On Wed, 15 Nov 2006 20:03:28 -0800, Dan Spisak wrote:

Hi, Bob. 33" and 33.25" in diameter? Not circumference? Maybe I'm
missing something but have you had too much pi? Anyhow, the difference
is 0.0398" or about 1 mm on the radius. Will that make much of a
difference in tracking?

How is the viewing across the Bay? Over here in Moraga it has not been
too good. However, Mt. Diablo is this weekend.
I can hardly wait to set up my 9.25.

Dan


Brain fade. It strikes the elderly with stunning results. ;-)

Too much pi is right. Circumference is what I meant.
I made a shim of .054" and stuck it under the rear radius block.
As originally configured, the scope was 1min4sec off from the mount's
polar axis. Now it's less, but I won't know how much less until I do an
orthogonality check.
The scope was apparently designed to be used in a fork mount, where this
difference wouldn't be a problem, but when you slap it on a dovetail
plate and an equatorial mount, the error surfaces.
Seeing is soft here too. Can't split epsilon Lyra.
Have fun on Mt. Diablo. Hope it stays dry for you.
Uncle Bob

For all their praises the c9.25 is still a mass market scope. Thus
the issue? Just be glad it's as good as it is optically, for what it is?
I assume yours is. You could upgrade.

kw

"Uncle Bob" wrote in message
news
On Wed, 15 Nov 2006 20:03:28 -0800, Dan Spisak wrote:

Hi, Bob. 33" and 33.25" in diameter? Not circumference? Maybe I'm
missing something but have you had too much pi? Anyhow, the difference
is 0.0398" or about 1 mm on the radius. Will that make much of a
difference in tracking?

How is the viewing across the Bay? Over here in Moraga it has not been
too good. However, Mt. Diablo is this weekend.
I can hardly wait to set up my 9.25.

Dan


Brain fade. It strikes the elderly with stunning results. ;-)
I have been able to achieve this all my life... :-)

Too much pi is right. Circumference is what I meant.
I made a shim of .054" and stuck it under the rear radius block.
As originally configured, the scope was 1min4sec off from the mount's
polar axis. Now it's less, but I won't know how much less until I do an
orthogonality check.
The scope was apparently designed to be used in a fork mount, where this
difference wouldn't be a problem, but when you slap it on a dovetail
plate and an equatorial mount, the error surfaces.
Seeing is soft here too. Can't split epsilon Lyra.
Have fun on Mt. Diablo. Hope it stays dry for you.
Uncle Bob
Did you 'double check' the thickness of the blocks supplied with the
rail?. On my C11, one block was 0.500 thick, and the other just on 40 thou
thicker, to compensate for this difference. The 'measured' difference was
42 thou, so this was nearly 'spot on'.

Best Wishes

On Thu, 16 Nov 2006 10:57:01 +0000, Roger Hamlett wrote:



Did you 'double check' the thickness of the blocks supplied with the
rail?. On my C11, one block was 0.500 thick, and the other just on 40
thou thicker, to compensate for this difference. The 'measured'
difference was 42 thou, so this was nearly 'spot on'.

Best Wishes

Hi Roger-

I single-checked the distance from the bottom of the radius to the bottom
of the block with a dial caliper and got a nominal .500. But now that you
bring it up, I will check them again.

Thanks,
Uncle Bob

On Thu, 16 Nov 2006 04:06:26 -0600, Bull Feathers wrote:

For all their praises the c9.25 is still a mass market scope. Thus
the issue? Just be glad it's as good as it is optically, for what it is?
I assume yours is. You could upgrade.

kw

Indeed, they are not perfect, but they are pretty damned good.
It star tests very well. Not as good as my Intes MN-66, but it has a lot
more back focus and a great deal more light gathering power.
It will be a dark day when I part company with this scope (or the MN 66).

I think for a mass produced telescope, it ranks among the very best.

Clear Skies and good seeing!
Uncle Bob
Fairfax, CA

On Wed, 15 Nov 2006 20:03:28 -0800, Dan Spisak
wrote:

Hi, Bob. 33" and 33.25" in diameter? Not circumference? Maybe I'm missing something but have you had too much pi?
Anyhow, the difference is 0.0398" or about 1 mm on the radius. Will that make much of a difference in tracking?

How is the viewing across the Bay? Over here in Moraga it has not been too good. However, Mt. Diablo is this weekend.
I can hardly wait to set up my 9.25.

Dan

It works out to about 7 minutes of arc. It shouldn't affect tracking,
but it would make a bit of difference with the accuracy of go-to. How
excited Uncle Bob needs to get about this depends on how much the
optical axis of the telescope deviates from the mechanical axis, and
how much the centers of the end rings deviate from either axis.

Bud
--
The night is just the shadow of the Earth.