----- Original Message -----
Sent: Saturday, February 02, 2002 4:04
AM
Subject: Re: ATM Mirror box and rocker
box height issues
Weight*fulcrum is the issue. Having a
lighter primary mirror (20lb vs 33lb)
makes a difference.
Suppose for the sake of discussion that the
center of gravity of the mirror
box
and secondary cage are 48 inches apart and
that the poles (8 of them) weigh
1 pound each. Let's also say that the
mirror is 8 inches below the CG of
the
mirror box. Furthermore, assume
the mirror box weights 20lb and the
secondary cage (with a heavy eyepiece) weighs
12lb.
offset
weight object
0
20 mirror
8
20 mirror box
32
8 poles
56
12 secondary cage
Using Excel's goal seeker to find the center of
gravity of the combined system
we find it to be 18.13" above the center of
gravity of the mirror. If we assume a
33lb mirror instead, we get 14.90", or about a
3.23" difference. That's close
to the 3.5" inches you speak of.
If you want to lower the pivot point, you can add
more counterweight (physical or
spring tension) to the mirror box. However, having a higher rockerbox can
be viewed as a good thing, since it means you don't have to stoop as low when
you observe close to the horizon. It
doesn't change the overall height of the
viewfinder at zenith.
Jeff Anderson-Lee
----- Original Message -----
Sent: Friday, February 01, 2002 1:03
AM
Subject: ATM Mirror box and rocker box
height issues
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