ATM 18" edge support: Sling or 2 wires? ... so I used both.

[Ed Taychert <elt@irony.com>]

I've had first light with my 18 incher and have some
comments on using the cell I made. There were/are some
issues; I think they can be corrected.

I share my notes, not because I'm some sort of expert on the
matter, but the opposite ... perhaps someone else will save
some time by reading of my mistakes in the archive ...

Here's a picture of the cell -
http://www.irony.com/Ed/astro/18inch/SlingWire.jpg

1) I've written of making the carbon fiber covered
1/4 plywood triangles before. These worked very well.
I had some concern about the skinny connecting bar
flexing under the weight of the 30 pound mirror.
If it does, it's not eye-visible so I think it
would have to less then 1/16 inch. But ...

2) The mirror hits the screws in the center of the
triangles. I used two layers of "medium duty" felt
pads (because they felt firmer than the heavy duty
pads) and test piece of plywood cleared everything.
But the pads compress under the weight of the mirror.
That (and if there is any flex from the connecting bar)
turns the cell into a 6 point cell.

I didn't realize this when I was doing the Ronchi test
on Polaris ... the overall shape of the lines was straight,
but the width of them varied in and out across the length
of the lines. I'm an utter neophyte at Ronchi testing, but
the waviness of the lines and the surface deformation
calculated by plop.exe for a six point cell look similar.
I'm hopeful that that lines will clean up with I fix the
support. My plan is to not worry about any slight bend in
the crossbars, if any is present, but to replace the felt
with harder and thicker "Magic Sliders."

3) I found that the 2 wire edge support mechanism is easy
to build and easy to adjust. Since the lower ends attach
to the mirror box, I had no idea how to make that work
when the tailgate was lowered; I wanted the edge
support system to more or less stay in place.

My solution was to make a non-adjustable sling for the
mirror as well. The sling holds the mirror a quarter
inch low, then the turnbuckles can be connected
and the mirror raised into position.

The pictures shows three slots cut into the webbing to
accommodate the wire cabling used as the primary support.
(Also, the webbing was pre-stretched with 40 pounds before
installations... I hope it more or less keeps its
current length!)

One problem I had was that I used black electrical tape
in three places to hold the sling on the mirror's edge.
I had taped it so tightly that the wire would not move
under the tape. Apparently, this stressed the mirror a
bit when I tightened the turnbuckles because imaging
improved when I released the tape... I like the tape
to be on there for transport and the like; I just have
to remember to release it before use.

  ---

So I add cell work to the list of things I have to do to
finish the scope ... I wrote of the "clampless clamps"
in a previous note -
http://www.irony.com/Ed/astro/18inch/ClamplessClamps.jpg
As suspected, the upper pins are a bit flimsy. They did
hold up for the long weekend that I had the scope at the
Black Forest Star Party (over 500 folks last weekend!)
but they are already bent this way and that. The scope
moved very well but it did have about an arc-minute of
springiness ... I'm hoping that fixing the top fasteners
will eliminate that, though I'll have much more time
with the concept before I draw any conclusions.



... and thank you to all the people that I stole ideas
from!

  - Ed.