Primary Mirror (self made):
the moment my 14" and my 20" newtonians have to share one finderscope.
is not a real problem, as it is very seldom that I'm using both
telescopes at the same time (in fact I haven't done it yet). This
drawback give me a good arguement to build another telescope. As I
would like to start working with CFK this gaves the opportunity to make
first experiences at the same time.
"Reiterlein", the existing 4.1" (106mm ) newton finder uses an 1 1/4" focusser (in fact it is just a tube with 1 1/4 " free aperture). The new one should get a 2" focusser to allow maximum field of view. I would like to admire widefield objects as a whole in future with this instrument.
Optics:The "blank" is second hand. It was working in a cheap chinese telescope befor a guy had the idea to clean it rubing down with a normal towel. I got it for few Euros. As it had f/7 focal ratio I'd to start with rough grinding. I build a sandwich tool made of stoneware tiles as I did it in the past. You can find a detailed description building this kind of tools (unfortunately only in German at the moment) here . As the blank did had a precurve (and the tool did not) I "preginded" the tool with an angle grinder at the outer radius to save some time. It took me aprox. 100 minutes to finalise rought grinding. After another 250min (net) I declared the mirror ready. Due to realtively "soft" glass (plate) I made fast progess, both with grinding and with polishing.
I took 0.03" thick sheet metal for the
"arms". I the middle you can see aluminium square tube piece (length
0.6"). How to fix the arms at the upper cage? I glued scrue nuts at the
side of the arms with some epoxi resin. As there are only minor forces
this smal unit, it is stable enough.
The 38mm (1,52”) diagonal
is silicone glued with 3 blobs of 2 mm (1/10") thickness directly onto
to the diagonal al-sheet, that I cut with a simple mechanical fret saw
out of a aluminium metal sheet and bended it to a 45 degrees angle.
obstructtion is 30% - thats a lot, but as this telescope should
primarly show wide fields at low power I gave priority on large
iluminated field of view. Nethertheless I'm sure that I can enjoy
magnification up to 150*; not bad for a finderscope
To protect the secondary mirror against dust
and touching, I cut a eyepiece capsule 4 times (one for each arm of the
spider). At the top of the cuts I drilled small holes. It is very easy
to screw and unscrew the "mirror protector".
Cage and Focusser:
Basis for building the secondary cage are
two rings made of baltic birch plywood, inner diameter aprox. 6", 0.4"
thickness. I coverd it with 0.03" airplane plywood before it was
laminated with Carbon fibre. From stiffness point of view laminating
was not neccessary, but I think it looks nice.
I did my first experiences with carbon fibre
and laminating. It took much longer as expected until epoxi resin
soldifies; I turend the unit more than two hours to avoid that the
liquid epoxi resin will drop from the cage down to earth.
I use the same foucusser as for most of my telescopes, it's a Kineoptics HC-2 helical bougth from Joe LaCour. As all the others it performs very well. It is one of the most lightweight focusser I know, this for reasonable cost.
Same principle as with URSUS, my 20" Newton.
The tubes (0.4" carbon fibre tubes) will be fixed by two with
smal brakets made of aluminium. Key-Hole
houses the skrew bolt head. Clamping will be reached with srewing the
flat knurled nuts.
truss clamping and mirror cell:
The mirror cell is made of 0.018"
carbon fibre plates, cutted and glued with epoxi resin. Sawing was very
difficult, as my saw blades were not stiff enough for this material. At
the end it was more melting and rubbing than sawing.
The mirror cell is made of 0.018" carbon fibre plates, cutted and glued with epoxi resin. Sawing was very difficult, as my saw blades were not stiff enough for this material. At the end it was more melting and rubbing than sawing.
It is possible to use this newton with a
simple photo-tripod. Mounting will be realised with a bolt already
fixed at the mirror cell. this bolt will be screwed at the basis plate
of the tripod. With some smaler adjustments it is possible to use this
combination up to mid-power magnification (100X), for high-power a
"real" mount will become neccessary.
As I still have a
13.3" (340mm) selfmade
mirror in the basement, my next plans are to build a telescope around -
CFK should become the favourite basic material for it.