big scope inquiry

After reading about John P's huge 29" Dob he had in the 80's, I got to
thinking about numbers of folks with larger 20" scopes. Any takers step
forward and let us know what you have and how you like it.

Thanks,
Alf

Durring most of 1997 I was wondering, designing, and planning to build
a 20-25" scope. Durring this time, I designed the truss and base and
waited for a mirror to show up on ebay that suited my purposes. It
ends up that a 20"-er showed up first, and I struck a deal with the
(then) current owner. The mirror arrived in Dec of 1998, and I got
started building the various stuff to make a telescope to fit the
mirror in hand. Along-about July the scope was finaly comming together
and was useable in my front yard. And by Aug it was ready for the
annual trip to Ft. Davis Texas.

At first there were the typical bunch of things that lessened the
telescopes efficiency, that got knocked off one by one until the only
thing holding back the telescope was the secondary support. The first
generation was a piano-wire 3 vane suppore that simply vibrated and
almost never stopped vibrating. I tried various tensioning
experiments, thicker wire, and all sortf of things. I finally gave up
and built a 4 vane spider with 0.010 metal more typically used in
flashing of roof seams. This one works really well. The whole
telescope operates like a binocular with no vibration whatsoever, and
holds collimation at all angles.

Another mistake was using silicone glue to glue the mirror to the
mirror cell. This proved to be the source of some subtle errors in the
mirros surface, So I rebuild the cell to have a strap like most other
big DOBs. Still alter, I found that the 27-point cell was build on the
radius for an 18-point cell and had to redesign pats of the lower
support structure. The wrong radii caused some subtle spherical error
in the mirrors figure. This helped make the mirror perform.

After using this scope for 6 years and having it completely debugged,
I am glad I stopped at 20" and did not reach upwards to 25". At 56, I
can still pack and unpack this scope single handely into my SUV. I
would never have gotten a 25"-er down to a weight that could be single-
handledly packed and unpacked. The mirror, cell, base weight in at 72
pounds (50 pound miror) and the rest of the telescope runs the total
up to 111 pounds. About 75% of what a normal DOB weights.

I still ahve my C11 and my 6" AstroPhysics, but hardly use either any
more.

Mitch

On Feb 4, 12:21*pm, MitchAlsup wrote:
Durring most of 1997 I was wondering, designing, and planning to build
a 20-25" scope. Durring this time, I designed the truss and base and
waited for a mirror to show up on ebay that suited my purposes. It
ends up that a 20"-er showed up first, and I struck a deal with the
(then) current owner. The mirror arrived in Dec of 1998, and I got
started building the various stuff to make a telescope to fit the
mirror in hand. Along-about July the scope was finaly comming together
and was useable in my front yard. And by Aug it was ready for the
annual trip to Ft. Davis Texas.

At first there were the typical bunch of things that lessened the
telescopes efficiency, that got knocked off one by one until the only
thing holding back the telescope was the secondary support. The first
generation was a piano-wire 3 vane suppore that simply vibrated and
almost never stopped vibrating. I tried various tensioning
experiments, thicker wire, and all sortf of things. I finally gave up
and built a 4 vane spider with 0.010 metal more typically used in
flashing of roof seams. This one works really well. The whole
telescope operates like a binocular with no vibration whatsoever, and
holds collimation at all angles.

Another mistake was using silicone glue to glue the mirror to the
mirror cell. This proved to be the source of some subtle errors in the
mirros surface, So I rebuild the cell to have a strap like most other
big DOBs. Still alter, I found that the 27-point cell was build on the
radius for an 18-point cell and had to redesign pats of the lower
support structure. The wrong radii caused some subtle spherical error
in the mirrors figure. This helped make the mirror perform.

After using this scope for 6 years and having it completely debugged,
I am glad I stopped at 20" and did not reach upwards to 25". At 56, I
can still pack and unpack this scope single handely into my SUV. I
would never have gotten a 25"-er down to a weight that could be single-
handledly packed and unpacked. The mirror, cell, base weight in at 72
pounds (50 pound miror) and the rest of the telescope runs the total
up to 111 pounds. About 75% of what a normal DOB weights.

I still ahve my C11 and my 6" AstroPhysics, but hardly use either any
more.

Mitch

Sounds interesting...do you have a link to some pictures of it?

Thanks

See he Taken at the north east end of the Suites at Prude Ranch in
Ft. Davis Texas near sunset.

http://img265.imageshack.us/my.php?i...ope0001uw5.jpg

I had a 21" motor driven Coulter Dob at one time. The entire scope was
custom made by Coulter in 1989. I had established a repoire with Coulter's
owner at several star parties, expressed my interest in a motor driven Dob
bigger than 17", but less than 29", and had one on my doorstep 9 months
later. This was probably only one of half a dozen Coulter scopes that were
ever custom made. These used hand made mirrors from an outside source, so
optically they were much better than the standard Coulter. I could easily
split difficult doubles and regularly saw intricate detail in Jupiter's
bands and storms on Saturn. The coma others have spoken about was
non-existant in this scope. The drive was about as good as possible at the
time. I never did astrophotography, but I would estimate a good 3-4 minutes
before drive corrections were needed. This scope would have made an
awesome planetary imager for sure. The best part of all this was the price-
only a couple of hundred $ more than the 17" they had at the time. Lots of
folks I met at sp's after acquiring this scope said they felt cheated by
Coulter when comparing. Like I told them though, I happened to be at the
right place at the right time. Lots of these "extras" were planned on being
incorporated in future Coulters, but by then of course the owner died and
Coulter eventually did also.

Ten years ago, I sold the scope, which I seriously regret now. I was in
financial need at the time and was offered roughly 3x more than I paid for
the scope to part with it, so I took the offer. However, since it was
custom made and a rarity, I'd hate to ponder what it would be worth now as a
true collector's item.

Chase

After reading about John P's huge 29" Dob he had in the 80's, I got to
thinking about numbers of folks with larger 20" scopes. Any takers step
forward and let us know what you have and how you like it.

Thanks,
Alf

On Feb 7, 7:16 am, "Chase List" wrote:
I had a 21" motor driven Coulter Dob at one time. The entire scope was
custom made by Coulter in 1989.

-edit-

The coma others have spoken about was non-existant in this scope.

???

Did the mirror have a long focal ratio or a coma corrector?

wrote in message
...
On Feb 7, 7:16 am, "Chase List" wrote:
I had a 21" motor driven Coulter Dob at one time. The entire scope was
custom made by Coulter in 1989.

-edit-

The coma others have spoken about was non-existant in this scope.

???

Did the mirror have a long focal ratio or a coma corrector?

Nope. It was f/4.5 as was most if not all of their other mirrors. I may
have been a little too precise originally though: not completely
non-existent coma, there was a small amount but not like the original
Coulter mirrors.

Chase

On Feb 7, 8:18*am, "Chase List" wrote:
wrote in message

...

On Feb 7, 7:16 am, "Chase List" wrote:
I had a 21" motor driven Coulter Dob at one time. *The entire scope was
custom made by Coulter in 1989.

-edit-

The coma others have spoken about was non-existant in this scope.

???

Did the mirror have a long focal ratio or a coma corrector?

Nope. *It was f/4.5 as was most if not all of their other mirrors. *I may
have been a little too precise originally though: not completely
non-existent coma, there was a small amount but not like the original
Coulter mirrors.

Chase

Coma in a parabolic mirror is always the same for any given focal
ratio, regardless of make or correction state. It is an inherent fixed
feature of a parabola. One cannot make it more or less with a single
surface reflection (a diagonal does not change the inherent nature of
the parabola).

Rolando

Chase List wrote:
These used hand made mirrors from an outside source, so
optically they were much better than the standard Coulter. I could easily
split difficult doubles and regularly saw intricate detail in Jupiter's
bands and storms on Saturn. The coma others have spoken about was
non-existant in this scope.


You realize that coma is a function of f-ratio ? And that it doesn't
matter who made the mirror. If it is anything faster than f/8 there will
be coma. And it will noticeable. And purists will tell you that even f/8
mirrors have coma.


Thanks


Bill

--
William R. Mattil

http://www.celestial-images.com

On Feb 7, 7:18*am, "Chase List" wrote (with some snipped):

. . . not completely
non-existent coma, there was a small amount but not like the original
Coulter mirrors.

To add to what others have stated about coma: Some people (including
myself and at least one magazine telescope reviewer) have been fooled
into blaming a primary mirror's coma for poor edge-of-field
performance when the more generous contributor resided within the
eyepiece(s).

Bill Greer
To sketch is to see.