Telescope Project

TELESCOPE PROJECT:

WHO I AM: My background is engineering, 60 years old. I am far to
being an expert in astronomy but have been interested for years in
this field of activity and rather technically oriented.
In view of my retirement, I plan to Design/Build a telescope having
the following characteristics:

SCOPE TYPE : Classical Cassegrain, f/4.4, equipped with a SBIG CCD
auto tracking Camera ST9XE 512 x 512 Pixels, 20m, operated at 40m (Bin
2 x 2), for up to some 20 Min. integration time, giving a Pixel scale
of 2.5 Arc-Sec, and a field of view of 10.8 Arc-Min.

PRELIMINARY DESIGN CRITERIA:

PRIMARY MENICUS MIRROR: Dia 750MM (30In), 63MM Thick (2.5In), f/3.0,
with or without central hole, depending on the difficulty to figure
such a mirror with a hole. Coude arrangement with flat diagonal
mirror to direct the beam through an hollow declination axis could be
considered. Blank material to be Pyrex or BVC.
SECONDARY MIRROR OBSTRUCTION: 0.20
FIELD FLATTENER: May be required as a consequence of field curvature.
TYPE OF TELESCOPE MOUNT: Equatorial, Horseshoe, similar to the one
built by AutoScope for Lawrence Berkely Laboratories.
DRIVE SYSTEM: Open loop with stepper motor, 2 steps precision belt
reduction plus friction drive, for both R.A. and Dec. axis.
DRIVE SYSTEM ACCURACY: Pointing: 10 Arc-Min; Tracking: 10 Arc-Sec
over a period of some 20Min.
MAIN MIRROR SUPPORT: Closed mirror cell with 9 or 18 support floating
points.
TYPE OF OPTIC ENCLOSU Open, Secondary mirror supported by quadruped
truss. (this choice is a compromise as it is believed that a closed
tube system, normally necessary for the expected environmental
conditions of the observatory site, would be too difficult and too
costly to build, too heavy and consequently not stable enough.
END USE: Astrometry, more specifically high magnitude Main Belt
Asteroid search and orbit determination. I realize this system will
not be at high professional level but it is important that valuable
scientific data be obtained from it.
SITE LOCATION: Canada, province of Quebec, Saguenay Region, in mostly
unpopulated low mountain environment at approx. 70° North Latitude, at
some 250M (800Ft) from the Saguenay River and 80M (260Ft) above sea,
medium humidity environment, on a rock escarpment with very few small
buildings around, far from any light pollution and vibration. There
is a 30M (100Ft) direct access to a control room located on the 4 Th
floor of my residence through a 9M long (30Ft) foot bridge at 9M
(30Ft) above ground. Seeing is presumably poor to medium with rare
good conditions.
HOUSING: Roll-off roof remotely operated from the control room.
PROJECT PLANNING: I plan to personally Grind/Polish/Figure/Test the
primary mirror. A Hindle/Draper type machine is to be used for
Grinding/Polishing/Figuring. I also plan to design and build the
whole telescope. The first manufacturing step to begin with as soon
as possible (after site seeing evaluation) is the Design/Construction
of the Mirror grinding machine.
An heated and air conditioned garage space on the 1 St floor (10M x
7.5M x 4.1M), is available for mirror work with an hydraulic elevator
service (1.2M x 0.9M x 2.1M, 350Kg capacity) from basement to control
room and observatory level.
Extensive use of available software will be provided for Raytrace,
mirror floating point support optimization, finite element analysis …
I can have access to modern industrial manufacturing facilities
(casting, machining, welding) and I currently use CAD systems
(AutoCad, SolidWorks …)
The computer system I am using now is Windows Me, Pentium III, 1 GHz,
512 Mo RAM, 2 Go Hard Disk. It could be upgraded whenever necessary.
Timing is not a reel concern as this project will eventually be close
to a full time retirement activity. I can allow an annual budget of
some 20,000$ for the supply of Material/Components.

PURPOSE OF THIS MESSAGE: I would like to share information in view of
this project. Eventually, it would be highly interesting to also
share expertise and exploit this observatory facility for astronomical
projects through remote access.
THE VERY FIRST STEP TO BEGIN WITH IS THE EVALUATION OF THE YEAR ROUND
SEEIN POTENTIAL OF THE OBSERVATORY SITE AS THIS MAY HAVE A DIRECT
IMPACT ON THE WHOLE PROJECT. DOES ANYBODY HAVE AN IDEA ON HOW TO
PERFORM THIS TASK ? A 3 In REFRACTOR, FLUORITE OBJECTIVE, ZOOM
EYEPIECE 20X-60X IS AVAILABLE (LEICA APO-TELEVID 77) SHOULD IT BE OF
ANY VALUE.
It would be highly appreciated to receive opinions and guide lines on
the whole project concept in regards to the end use and the
characteristics of the scope, in relationship with the probable seeing
characteristics of the site. In other words:
1. what can I expect from this system to detect Main Belt Asteroids
with enough positioning accuracy to allow orbit calculation ? Is
magnitude 18 realistic ?
2. Is the field of view large enough to allow guide star acquisition
and tracking ?
3. Is the combination aperture/focal ratio/field size/CCD
detector/Paraboloid main mirror, in association with the seeing
optimum for an acceptable Signal/Noise ratio, or a smaller
aperture/spherical main mirror, without field flatner, with similar
Pixel scale be just as good ?
4. What maximum mirror surface error can be tolerated (1/4, 1/8, 1/12,
1/20 wave ?). For the type of arrangement contemplated what is the
minimum demand on the mirror support system (3, 9, 18, 27 floating
points ?)
5. What should I target as maximum moment of inertia for a stable and
controllable scope ?
6. What should be considered as minimum stiffness for the scope
structure, and for the backlash mechanism of the drive system,
pre-loading of bearings … ?


Rejean