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Dear All,
I would really appreciate some advice on where my
current scope design seems to be going astray. The height of my
mirror box (and consequently the rocker box) seems a little on the high
side. The scope I am constructing is virtually straight out of Kriege and
Berry's (excellent) book.
Specs of the scope which is are as
follows:
Mirror
Diameter: 16"
Focal Ratio:
F4.5
Mirror
Weight: 20
pounds
Secondary Mirror
3.1"
The secondary cage is virtually exactly per the
above book (although approx 0.5" larger I.D. @17.5") and weighs (by my
calculations) 12 pounds. Height of the secondary cage is by the book at say
11". The spider and secondary holder are standard Novak units.
Plywood is Finnish Birch @ 15mm thick. I also made a totally standard 27pt
Kriege mirror cell with heavy walled 1" tube and 1.25" wide 0.25"
thick side rails (am aware that this is overkill and I could
theoretically get away with 3 - 6 pts :-))
Notably heavy items include supposedly standard
issue items for obsession like the following:
ngcdx3
focuser
1 lb
allowance for
eyepieces
1.5lb
Obsession upper cage pole clamps
1.79lbs (direct from Obsession - pretty
meaty methinks...)
The mirror box is made of 18mm (say 3/4") Finnish
birch (guessing about 13 - 15 ply) - I am supposing this is similar to FinnPly
(although it doesn't actually go by that name in Australia).
I calculate the mirror box will need to be 18" high
which is at least two inches higher that the above book suggests.
Interestingly, I did a few 'what-ifs' with the
torque calculations and determined that if my mirror was 2" thick @ f5.0
(mirror 33lbs per Nova optics - with more torque at the secondary cage as an
offset) Unlike my current 1.37" f4.5 (20 lbs) the mirror box would
still need to be about 18 inches. I 3d modelled the scope and determined that
the measurement from the ground to the top of the mirror box (excluding
bearings) is 23.5" (Compare this with an 18" f4.5 obsession per the Obsession
website at only 21" for the same dimensions.) I therefore extrapolate that the
bottom half of my telescope is approximately 3.5" too
high???.....
Can anybody shed any light on the above
- Given the relatively standard 'Obsessionesque' components and design
of the above scope can anyone work out how it is that Obsessions manage to keep
their rocker boxes so low whilst mine seems disproportionately
high.
Many Thanks
Andrew Hincks
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