Coulter 13

Hi, got an old 13.1" f 4.5 Coulter red tube Dob. The wood mirror cell
has two more stripped out threaded inserts. I've repaired a couple in
the past, but wish not to fool with it any longer. I've decided to go
with a new mirror cell of some sort. What do you think about the
University Optics 9 point cell? Any others to consider? Also, on the
side of the mirror is a black magic markered circle with -3 inside the
circle. What does this mean? The optics are very good on this scope,
but not spectacular. The mirror coating has a few pinholes, but
otherwise looks okay.

On another note: Had the 13 out a few nights ago trying out my new
Pentax 12mm XF eyepiece. I must say I like this eyepiece a lot. About
the only negative I could see was a slight bit of violet on the edge of
Jupiter at times. Deep sky objects look very nice through this
eyepiece. I like it so well I ordered the 8.5mm.

Jerry Young wrote:

Hi, got an old 13.1" f 4.5 Coulter red tube Dob. The wood mirror cell
has two more stripped out threaded inserts. I've repaired a couple in
the past, but wish not to fool with it any longer. I've decided to go
with a new mirror cell of some sort. What do you think about the
University Optics 9 point cell? Any others to consider? Also, on the
side of the mirror is a black magic markered circle with -3 inside the
circle. What does this mean? The optics are very good on this scope,
but not spectacular. The mirror coating has a few pinholes, but
otherwise looks okay.


The UO 9 point Cells work fine. Heck for (more effort, but less money
the Berrry/Kriege Book "The Dobsonian Telescope" style Cells work pretty
much just as well.
The -3 indicates the Mirror is -3" from the 58.5" that a 13" F4.5
Mirror should be in the focal length were spot on.

Crazy Ed

Thanks for the info, guess I have about an F4.3 mirror then. The mirror
is very thin measuring a little less than 3/4" thickness at the edge. I
suppose the University 9 point cell would be adequate as the original
cell supports the mirror on only three 1" circular Teflon discs. I
would think a 12 point cell would be about the minimum for this thin of
mirror, but can't find anything off-the-shelf like that.

What do you think about the
University Optics 9 point cell? Any others to consider?

If you want more support points and don't want to build it yourself, you
might try:

http://www.tscopes.com/DobParts/

Dennis

Jerry Young wrote:

Thanks for the info, guess I have about an F4.3 mirror then. The mirror
is very thin measuring a little less than 3/4" thickness at the edge. I
suppose the University 9 point cell would be adequate as the original
cell supports the mirror on only three 1" circular Teflon discs. I
would think a 12 point cell would be about the minimum for this thin of
mirror, but can't find anything off-the-shelf like that.


No Problem - Oh yea, seeing as it's a 13.1 the F.L. Is suppose to be
58.95" so -3" results in a F.L. Of 55.95". Not that .45" really makes all
that much difference unless you're using a short travel focuser.

In the "For What It's Worth" category, personally I'd go ahead and just
redo the original mount (after seeing if the "push" screws have a solid base
to push against - I've seen many of the later "Red Tube" Coulters that just
pushed against the wood and not nail heads. No kidding, that's what they
used). Reason being is if you go to a "real" Cell you are going to either
add counter weights or move the Elevation Bearings. If adding Counter
Weights remember to reinforce the Tube Base because the Tubes protection
used to be the Cell. If you move the Elevation Bearings you'll need to redo
the Rocker because the lower portion will be "longer" to correct the weight
change.
Just idle thoughts from someone that's been there.

Crazy Ed

Two of the threaded inserts on the mirror cell have pulled away from
the particle board. It has the nails heads for the push screws to
contact. Two of the threaded inserts in the back plate where the
collimation screws are, pulled away from the wood awhile back. I
repaired them by drilling a 1" hole & filling with hardwood dowel,
redrilling & installing new threaded inserts, seems to be holding okay.
Maybe I should do the same on the mirror cell as well? Getting tired of
messing with it. I realize a new light weight cell will mess up the
balance & create new problems. I see no flex problems with the thin
mirror on only 3 points, but I suppose it's there, star test & images
look good. Seems a layer of carpet glued to the wood mirror cell in
place of the 3 discs might support the mirror better? The tape & large
hose clamp seem to work okay supporting the edge. Wondering if drilling
some extra holes in the back plate would help with cool down?

Jerry Young wrote:

This time I'll answer between the lines so I don't miss anything.

Two of the threaded inserts on the mirror cell have pulled away from
the particle board. It has the nails heads for the push screws to
contact. Two of the threaded inserts in the back plate where the
collimation screws are, pulled away from the wood awhile back. I
repaired them by drilling a 1" hole & filling with hardwood dowel,
redrilling & installing new threaded inserts, seems to be holding okay.

Was the damage from over tightning or was the particleboard breaking up?
If the latter it's possible you have moisture damage and repairs might be
futile........... But the Dowel solution is a Good one!

Maybe I should do the same on the mirror cell as well? Getting tired of
messing with it. I realize a new light weight cell will mess up the
balance & create new problems. I see no flex problems with the thin
mirror on only 3 points, but I suppose it's there, star test & images
look good. Seems a layer of carpet glued to the wood mirror cell in
place of the 3 discs might support the mirror better? The tape & large
hose clamp seem to work okay supporting the edge. Wondering if drilling
some extra holes in the back plate would help with cool down?


If you choose to go the Carpet route use that "Astro Turf" style stuff
with a more or less open base/backing cut to match the Mirror diameter.
Drill 4 or 5 1/2" holes that go from outside to where the base of the
mirror is now and paint the exposed holes (After removing the Mirror , clamp
and tape! ;-) ).
To support the Mirror cut 2 1" dia. Dowels in half that are ~2.5" long
(should be the length from the base of the Mirror support disk to the top of
the Mirror) then drill ~1/4" holes and countersink each on the flat side
where the center of the Mirror will be. Mount the 1/2 dowels with a screw
and glue/epoxy whatever. Paint the dowels to protect them. Place the Carpet
and mirror in place and to retain the Mirror inject RTV (I use the Mirror/
Bathroom stuff) into the holes you drilled and countersunk in the upper part
of the dowels and give it at least a day to setup.
If done right you will now have a "million point" Mirror Cell!

Time for me to get some Sleep, but if you've got more questions you think
I might be able to help with feel free to ask away.

Crazy Ed

If you're wearing out the T-nuts then you're adjusting the scope too often
or there are some real problems eslewhere on the scope.
If the cell is screwed to the side of the tube, you should make a glued on
pad (you can additionally screw it to the tube) upon which to set the cell
so that the bolts holding it onto the pads go up the tube. This will not
only keep the cell from moving about up and down the tube but also allow for
the removal of the primary and replacement without having to recollimate the
scope.
I'd just do what is necessary to make the scope collimation free and keep my
hands off of the next set of T-nuts.

--
Why do penguins walk so far to get to their nesting grounds?

The threaded inserts are fine, they're just pulling out from the
particle board. Now, since you mention it, I bet the particle board is
weak from exposure to moisture, never thought of that. Two on the back
of the scope were pulled out when I bought it & I repaired them. Now
two more have pulled away from the mirror cell just recently. It's a
combination of weak particle board & over tightening, I guess. I bought
a new Coulter 10" around 15 years ago or so & one collimation threaded
insert was pulled out from the wood when I opened it from the box brand
new. The 10" had a much thicker mirror, I guess the quality varied
quite a bit.

Bob May wrote:

If you're wearing out the T-nuts then you're adjusting the scope too often
or there are some real problems eslewhere on the scope.
If the cell is screwed to the side of the tube, you should make a glued on
pad (you can additionally screw it to the tube) upon which to set the cell
so that the bolts holding it onto the pads go up the tube. This will not
only keep the cell from moving about up and down the tube but also allow for
the removal of the primary and replacement without having to recollimate the
scope.
I'd just do what is necessary to make the scope collimation free and keep my
hands off of the next set of T-nuts.


Fun that there are so many ways to get the Hair off a Cat! ;-)

Crazy Ed