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[ATM] Corrector/reducer for a fast Newtonian
My current ATM project (a 10" Mak, using Intes-Micro optics) is
progressing nicely, so I'm starting to think about the next project: a
large, very fast corrected Newtonian for an autonomous imaging
telescope.
Here are my ideas at present:
The primary will be a 28" f/3.67 mirror from Kennedy Optics. The
initial camera will be an SBIG ST-7XME or ST-8XME depending on
available funds; at some point I may use a larger camera (up to 35mm
format) for situations where field of view is more important than
sensitivity.
The telescope structure will be a heavy-duty, Serrurier-style double
truss, using materials chosen to make it as athermal as possible.
Steel and aluminum might be acceptable if I use a pair of X-trusses.
The mirror will be ventilated with fans blowing across the front and
out the back. The mirror box will probably be oversized, and made from
metal with an insulator on the inside.
We're planning to construct either a horseshoe-yoke mount, or a
split-ring equatorial, using a Mel Bartel servo drive system. The
mount will probably be welded steel plate. Luckily, a friend and
fellow amateur astronomer has a machine shop.
(You probably want to ask: why am I not building a motorized dob with
a field derotator? Well, my friend with the machine shop wants to
build a big equatorial mount. We have our own fun...)
It looks like an 18 point cell will work; the substrates are 2.08"
thick at the edge. I'm not sure how to implement lateral support,
especially as this 'scope will be equatorially mounted. I've looked at
the counterweighted astatic support, but I don't quite understand the
motivation for that.
I know of one design problem that I don't know how to solve: wide
field imaging performance. A coma corrector is necessary of course,
but the pixel scale (with 9 micron pixels) is 0.6"/pix with the extra
x1.15 factor from a Paracorr. That suggests 2x2 binning, which would
render the field of view awfully small with an ST-8XME (never mind an
ST-7XME!). There isn't room to add a focal reducer between a Paracorr
and the focal plane, and I'm not sure how well a focal reducer would
work with an f/4.2 incoming beam anyway.
It is possible to design a combination focal reducer/coma corrector. I
found one design online at
http://www.astro.hr/mace2002/Report/MACE2002_final.PDF - look on pages
63 and 67. This could be used, but it appears that fabrication would
be difficult and/or expensive. The design is:
Five lens focal reducer, designed for a 1m f/3 paraboloid:
(Design from a paper by Zeljko Andreic)
SURFACE RADIUS THICKNESS APERTURE RADIUS GLASS SPECIAL
OBJ -- 5.6743e+19 8.4186e+17 AIR
AST -5.8830e+3 -2.7197e+03 361.897150 A REFLECT *
2 -320.748335 V -6.878973 70.000000 BK7 C
3 -202.021812 V -70.677574 70.000000 AIR
4 -310.258760 V -14.475886 65.000000 F6 C
5 4.3789e+03 V -50.665601 65.000000 AIR
6 -443.950052 V -18.094858 53.000000 BK7 C
7 1.7508e+03 V -5.066560 53.000000 SF11 C
8 -144.758860 V -14.475886 53.000000 LAFN28 C
9 -2.8344e+03 V -49.830000 53.000000 AIR
10 -- -- 36.000000 BK7 C
11 -- -- 36.000000 AIR
IMS -- -0.001395 35.780712 S
(I don't understand why the size of the primary is 28.4" rather than 1
meter - this differs from the other examples in the paper. And I don't
understand surfaces #10 and #11; I guess that just defines the focal
plane.)
This appears to offer less correction than a Wynne design but it's not
too bad over a ~ 1 degree diameter field. Also, the spot diagram is
nicely circular which is good for astrometry.
I do need a reasonable FOV in order for the autonomous imaging to work
- large enough so that when the mount slews across the sky to some
known location (a bright star), it will be on the chip. I don't want
to write telescope control softare that takes images in a search
pattern! And I dislike the idea of wasting photons, so why waste area
in the focal plane by having a limited FOV?
For the secondary, I'm tentatively thinking 6" to fully illuminate a
43mm circle (for 35mm format, in case I strike it rich and get an
STL-11000M). The 21% obstruction is very acceptable. I could easily
tolerate a larger secondary obstruction, for use with a focal reducer
and large camera.
I don't want to cut corners on this 'scope, though it would be okay to
start with a binned ST-8XME/paracorr and try to construct a
corrector/reducer as a later project.
So, any comments, criticisms, or suggestions?
--
David Whysong
Astrophysics Group
University of California, Santa Barbara
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