Too many holes.... a Starfinder debacle

Well, now I've done it. I put too many ventillation holes in the mirror cell
end of my Starfinder Dob, and the star test now shows a poorly supported
mirror. Not to worry though, I'll fix it.

This is just a caution that anyone considering the Alan Adler fan
modification might want to consider. Turns out there is a point at which the
sonotube becomes sufficiently weak to deform the Meade Starfinder particle
board mirror cell.

As I see it, I have two "good" options here... first, fix it with a (good
and cheap) replacement mirror cell... something I have been considering all
along. The Meade mirror cell bolts into the sonotube in 4 positions, and the
mirror is glued to particle board. I've wanted to replace it to reduce
weight, but I've been equally reluctant as it will affect the balance point.
My concern is that the balance point will move sufficiently far up the tube,
that I will have to replace the base as well. At that point, I'm getting
into enough cost, that I might as well go with option two, order a T-Scope
truss conversion kit from Ed Taychert (I would _love_ to do this).

----------
As a third, "hack" option... interestingly, 3 points make a plane. The idea
that the mirror cell is bolted in 4 points might work okay for other
designs, but I got to thinking that, had the particle board been supported
in only three positions, any "sag" from a weekend area in the sonotube might
not have caused deformation, only miscollimation. I suppose I could try to
prove that theory by mounting the particle board cell by simply driving
three sheet rock screws in at 120 degree intervals and then pulling the
original bolts... but, then, I've never had a better excuse to either
rebuild the sonotube to reduce weight, or just go all out for the truss
conversion.

Stephen Paul

Stephen Paul wrote:
Well, now I've done it. I put too many ventillation holes in the mirror cell
end of my Starfinder Dob, and the star test now shows a poorly supported
mirror. Not to worry though, I'll fix it.

I find it hard to believe that a weak tube could deform that thick Meade
particle board cell, but I suppose it could happen. I know these are
obvious questions, but it doesnt hurt to ask: Did you let the mirror
stabilize before the star test? Have you removed the mirror from the
cell lately? It's good practice to remove and reglue the mirror every so
often because the wood will eventually warp, pinching the mirror. I had
this problem twice with my old starfinder 10" dob. Dont forget to use
stacks of dimes or something else to create a sufficient air gap when
gluing the mirror to the cell.

-tom

Stephen Paul wrote:

Well, now I've done it. I put too many ventillation holes in the
mirror cell end of my Starfinder Dob, and the star test now shows a
poorly supported mirror. Not to worry though, I'll fix it.

This is just a caution that anyone considering the Alan Adler fan
modification might want to consider. Turns out there is a point at
which the sonotube becomes sufficiently weak to deform the Meade
Starfinder particle board mirror cell.


I followed the instructions given in the same article with my 12.5 inch
Starfinder and had NO problems. The 5 exhaust holes are about an inch
in diameter and spaced perhaps a wee bit more than an inch from each
other.

This installation can be seen at:
http://home.earthlink.net/~martinhowell
Once in the site, click on "Installation of fan. . ."



--
Martin

"vic20owner" wrote in message
...
Stephen Paul wrote:
Well, now I've done it. I put too many ventillation holes in the mirror
cell
end of my Starfinder Dob, and the star test now shows a poorly supported
mirror. Not to worry though, I'll fix it.

I find it hard to believe that a weak tube could deform that thick Meade
particle board cell, but I suppose it could happen. I know these are
obvious questions, but it doesnt hurt to ask: Did you let the mirror
stabilize before the star test? Have you removed the mirror from the
cell lately? It's good practice to remove and reglue the mirror every so
often because the wood will eventually warp, pinching the mirror. I had
this problem twice with my old starfinder 10" dob. Dont forget to use
stacks of dimes or something else to create a sufficient air gap when
gluing the mirror to the cell.

Thanks for the feedback Tom.

I keep the scope in the garage which is considerably cooler than the outside
temps these days, so I suppose it could be that the mirror was too cold.
I've had the same results twice now though, and once persistently over a
period of more than a few hours wth the fan running full speed. So, although
I could certainly be wrong, I'm not convinced that it's thermal issues. The
star test looks pretty much exactly like that in Suiter's book, Figure 2.5
on page 27.

In the past, tube currents would have the classic plume on one side, where
this is definitely a triangular shaped pattern the closer you get to focus.
If you go far enough out of focus, the Fresnel rings begin to appear
circular. I first noticed this effect after drilling the three inch hole and
installing the cooling fan. Before the fan, I had no such issues, even
though, and unlike Alan Adler's mod, I've had eight 1 inch holes, about two
and a half inches apart, drilled in the tube for quite a while.

That said, I _am_ basing my conclusion solely on the difference in results
after adding the fan, and fan on or off makes no difference.

Of all that you have suggested, the mirror mounting itself being deformed
seem the most reasonable. Certainly, looking at the sonotube, it doesn't
_look_ like there are too many holes in it. Well, anyway, I ordered a UO
mirror cell today, so this weekend's project will be ridding myself of the
enclosed particle board setup. If for no other reason, than to get better
convection.

Stephen Paul

"Martin R. Howell" wrote in message
ink.net...
I followed the instructions given in the same article with my 12.5 inch
Starfinder and had NO problems. The 5 exhaust holes are about an inch
in diameter and spaced perhaps a wee bit more than an inch from each
other.


As indicated in my other response... I have 8 holes, 2 and a half inches
apart. ;-)