|
The mirrorbox is 19.3 inches high and 25.9 inches wide
(outside dimension). It is made of 0.7 inch thickness baltic birch
plywood. The mirrorbox is strengthened with four corner braces. The
altitude bearings are made of two layers of 0.7 inch thickness baltic
birch plywood and attached to the mirrorbox with carriage bolts and
locknuts. I used formica on the edge, which runs on teflon pads. De
sides and bottom of the rockerbox are also made of two layers of 0.7
inch plywood. I have glued a formica layer against the bottom. The
groundboard, with three teflon pads for the azimuth bearing is Y-shaped
(see photo). To move the
mirrorbox-rockerbox assembly around, two wheelbarrow handles can be
attached to the sides of the rockerbox as this picture will show.
A large mirror needs a floating cell to prevent it from
deforming. I made an 18 points cell. A 12 points cell would have been
good enough, but an 18 points cell is a bit more forgiving. According
to PLOP the deformation caused by an 18 points cell for this mirror is
about 1/580 lambda RMS, which is very good, given the fact that 1/128
lambda RMS is still acceptable. The cell and tailgate are made of
stainless steel. Three pictures of the mirror cell are here.
Silicone adhesive (RTV)
At first I didn't use a lateral support, but had the mirror glued to
the 18 support points of the mirrorcell with 0.125 inch thickness,
about 0.8 inch wide silicone blobs. The startest showed a fairly large
amount of astigmatism. I couldn't check if the astigmatism was in the
mirror, because I couldn't rotate the mirror. So I decided to cut the
sillicone and installed a sling (made from an old car safety belt).
The sling
The astigmatism was gone (though I sometimes saw some when pointing to
very low altitude). Because of the stretching of the sling and because
the mirror did move sideways while using an equatorial platform, I
decided to make made double metal sling,
an idea of Nils Olof Carlin (see linkspage).
The double sling
This doublesling gave less astigmatism then the RTV, but sometimes I
still saw some. The reason could be the fact that the way the slings
are attached to the mirrorbox grip the mirror too tight. In Nils Olof's
opinion the problem is probably, that the upper attachments of the
slings, to the adjustment bolts, are too close to the mirror. To get
rid of the problem I made piano wire edge supports.
Piano wire supports
This page shows how I made
the piano wire supports and
here is a photograph of one of the supports in my 20 incher. I
never saw astigmatism since I use these supports, they do not stretch
or deform and the mirror cannot move sideways. The supports are placed
at 90 degrees, at the lower corners (when pointed at the horizon) of
the mirrorbox. They touch the mirror's edge at the plane of the center
of gravity of the mirror and point to the center of the mirror. I have
the idea of this type of lateral support from Frederic Gea's homepage.
There's a link to his very interesting pages on my linkspage. There's
also a link to Nils Olof Carlin's article about edge supports. His
study shows, that piano wires, if placed accurately, work very well.
They are even slightly less critical to accurate placement then a
sling. As I see it, given the need for very accurate placement, a big
advantage of using piano wires as compared to a sling, is that piano
wires stay in the correct position and a (traditional) sling does not.
Rollerbearing supports
The piano wires work very well, but there is one (minor) problem. When
I collimate the primary when collimation is relatively far off, the
mirror tends to 'hang' a bit in the piano wires. Pointing the scope in
vertical direction lets the wires 'jump' to neutral position again,
after which the collimation can be finetuned, so it's no big deal. But
after I motorized my telescope (Mel Bartels' stepper system), pointing
the scope up was a bit more work then it used to be and I decided to
look for another solution. A discussion about edge supports on the
Torus mirror mailinglist and a picture of a 'rollerskate bearing' edge
support from Nils Olof Carlin in the photo section of this list, made
me decide to make rollerbearing edge
supports.
The two 1.8 inch wide, 25.2 inch outside diameter rings of
the secondary cage, are made of 0.6 inch thickness baltic birch
plywood. The 12 inch long struts are made of 1.2 inch outside diameter
(0.059 inch wall thickness) aluminium tubing. I couln't find the right
type of threaded inserts to attach the rings to the struts. I glued 0.5
inch long wooden dowels in the struts and glued struts and rings
together. I tried very hard to find kydex in The Netherlands, but I
didn't even meet anyone who had ever heard of it. Instead of kydex I
used thin plastic, attached to the inner rim of the rings with
two-sided tape. On the inside of the secondary cage the plastic
opposite to the focuser is covered with very flat black towelling. The
two baffles opposite to the focuser (one on the upper ring and one
below the lower ring) are made of some kind of foam, sometimes used as
camping matrasses. The baffles, as shown in this picture are also covered
with black towelling. The baffles are attached to the rings (the lower
baffle also to the truss) with velcro.
To cool the mirror down as quickly as possible I installed
two 4 inch 12V fans: one blowing against the bottom of the mirror and
one blowing across the surface. Both fans have their own on-off switch
and potmeter, as shown in this picture.
To prevent vibrations to show up in the eyepiece I glued (with
two-sided tape) strips of a 0.4 inch thickness mousepad between the
fans and the wood, as shown in this
picture. I haven't noticed any vibrations in the eyepiece and,
though the fans can be heard when running full speed, they are not
noisy at all.
A good 2 inch focuser is very expensive in The Netherlands.
Therefore I made my own Crayford focuser, using very good information
from other websites. The base is made of 0.6 inch plywood. The arc to
which the four bearings are attached is made of two layers of 0.8 inch
plywood. The drawtube is moved by a 1/4 inch threaded rod, covered with
a piece of rubber hose. The knobs to move the rod are made of
aluminium. The inside diameter of the drawtube is 2.28 inch. Someone
else made the 1.25 inch adapter for me and the whole thing is anodized.
Here is a photograph of
the focuser. And here
is a large photograph.
|
