Telescope mount design

"Dave Nay" wrote in message
...
Hey all!

I am planning to build a new mount for my Orion XT10 Dob. I would like
to do astrophotography in the future, so the requirements for the mount
are of course a little more complicated than the Dob base I have now.
What I would like to do is build an advanced alt-azimuth mount with the
added feature of compensating for the field rotation. Here is the
unique part though.....I want to handle the field rotation by rotating
the entire OTA as the field is tracked. I know that everyone will say
that I can buy/build more traditional mounts like a hefty GEM, or an
equatorial platform, but I want to do something different and creative.
I plan to completely automate and computerize the new mount, and all
axis will be stepper motor controlled. As both a mechanical engineer
and a computer programmer, I know I have the skills to design and build
this mount, but as only an amateur astronomer, I do not know if there is
a flaw in the design concept. Yes, the motion controls will be
complicated.....yes, the math will be ungodly.....yes, it will probably
be expensive (machined aluminum)....yes, there are easier way (why take
the easy route in life?), but WILL IT WORK?

I appreciate any thoughts, opinions (except to tell me I'm crazy) and
suggestions anyone has to offer.

Thanks!
Dave Nay


Going to be tough, would have to make sure the axis on which you turn the
tube is exactly center of the optical plane (which is probablly not the
center of your optical tube). Then the object you are imaging would have to
be exactly centered. As an alternative it may only work if you "stack and
accumulate", stop imaging, turn tube, realign (autoguide), then start
imaging again in short little itterations.

Sounds really interesting, good luck.

Steve Little wrote:
"Dave Nay" wrote in message
...

Hey all!

I am planning to build a new mount for my Orion XT10 Dob. I would like
to do astrophotography in the future, so the requirements for the mount
are of course a little more complicated than the Dob base I have now.
What I would like to do is build an advanced alt-azimuth mount with the
added feature of compensating for the field rotation. Here is the
unique part though.....I want to handle the field rotation by rotating
the entire OTA as the field is tracked. I know that everyone will say
that I can buy/build more traditional mounts like a hefty GEM, or an
equatorial platform, but I want to do something different and creative.
I plan to completely automate and computerize the new mount, and all
axis will be stepper motor controlled. As both a mechanical engineer
and a computer programmer, I know I have the skills to design and build
this mount, but as only an amateur astronomer, I do not know if there is
a flaw in the design concept. Yes, the motion controls will be
complicated.....yes, the math will be ungodly.....yes, it will probably
be expensive (machined aluminum)....yes, there are easier way (why take
the easy route in life?), but WILL IT WORK?

I appreciate any thoughts, opinions (except to tell me I'm crazy) and
suggestions anyone has to offer.

Thanks!
Dave Nay



Going to be tough, would have to make sure the axis on which you turn the
tube is exactly center of the optical plane (which is probablly not the
center of your optical tube). Then the object you are imaging would have to
be exactly centered. As an alternative it may only work if you "stack and
accumulate", stop imaging, turn tube, realign (autoguide), then start
imaging again in short little itterations.

Sounds really interesting, good luck.




Hmmmm.....good point about the axis of the optical path vs. the axis of
the tube. That makes the entire setup a huge collimation project,
although I don't think that is any worse than eyepiece de-rotators. The
axis of rotation on the eyepiece will never be the same as the optical
path there, due to machining tolerances. I don't think the object of
interest would have to be centered, I think the whole field should
maintain correct alignment if the tube is continuously rotated. Like I
said, the math for the motion control is going to be a real nightmare.

Dave

lal_truckee wrote:


Anyway, as a mechanical engineer and a computer programmer, you already
know the KISS principle. Engineering and programming elegance suggest
going after the simpler solutions to the problem.


True, true, true, true. But have you ever wanted to do something, even
though it is completely unreasonable?

Dave Nay wrote:

....yes, there are easier way (why take
the easy route in life?), but WILL IT WORK?

No.

You asked the wrong set of questions -
Could it work? Yes - It's not theoretically impossible
Will it work? No - It's extremely complex with many error paths, and
impossibly tight tolerances.

Anyway, as a mechanical engineer and a computer programmer, you already
know the KISS principle. Engineering and programming elegance suggest
going after the simpler solutions to the problem.

Dave Nay wrote:

Hey all!

I am planning to build a new mount for my Orion XT10 Dob. I would like
to do astrophotography in the future, so the requirements for the mount
are of course a little more complicated than the Dob base I have now.
What I would like to do is build an advanced alt-azimuth mount with the
added feature of compensating for the field rotation. Here is the
unique part though.....I want to handle the field rotation by rotating
the entire OTA as the field is tracked. I know that everyone will say
that I can buy/build more traditional mounts like a hefty GEM, or an
equatorial platform, but I want to do something different and creative.
I plan to completely automate and computerize the new mount, and all
axis will be stepper motor controlled. As both a mechanical engineer
and a computer programmer, I know I have the skills to design and build
this mount, but as only an amateur astronomer, I do not know if there is
a flaw in the design concept. Yes, the motion controls will be
complicated.....yes, the math will be ungodly.....yes, it will probably
be expensive (machined aluminum)....yes, there are easier way (why take
the easy route in life?), but WILL IT WORK?

I appreciate any thoughts, opinions (except to tell me I'm crazy) and
suggestions anyone has to offer.

Thanks!
Dave Nay

Hi Dave,

Yes, it will work and it will have one small advantage, the spider will not
appear to rotate during
the exposure.
The azimuth axis will then carry the weight, and more importantly for
control bandwidth/tracking accuracy
the moment of inertia, of the elevation and tube rotation axis. A further
complication is that unless you have
an auto guider, the optical alignment with respect to the tube rotational
axis will cause objects to make an arc
as they are tracked. The big advantage of a traditional elevation over
azimuth mount is that the gravity loading
is a function of elevation only. Big telescopes use this to their advantage
to simplify mirror support.
Additionally offset loads do not require precise 6 degree of freedom balance
as a instrument at a newton focus
does not need to handle the varying load due to tube rotation. Your design
will require a complete balance
solution. If you do use an auto guilder it will require you to process the
guide signals with coordinate rotation.

Stepping motors work well for the control of loads that do not require a
large speed range.
The alt/az mount requires an infinite speed range if you track through
zenith and so there is always
a blind spot of some diameter at zenith. If you gear to have 0.1 arc second
steps and your max. speed
is 1 degree per second then you will need to step at 36 Khz. you might want
to look into micro-stepping
drives as well. You will need to come up with a a cable wrap so that wires
will not get caught and
if you are worried about to many turns winding up the cables that produce
disturbance torque's,
a 540 degree limit switch in azimuth.

To build such a device it would seem to me, would be driven more by the
unique design and not
the astrophotography. This is not a bad thing if you like building
telescopes.

A fellow unique telescope builder,
Dan

Steve Little wrote:

"Dave Nay" wrote in message
...
Hey all!

I am planning to build a new mount for my Orion XT10 Dob. I would like
to do astrophotography in the future, so the requirements for the mount
are of course a little more complicated than the Dob base I have now.
What I would like to do is build an advanced alt-azimuth mount with the
added feature of compensating for the field rotation. Here is the
unique part though.....I want to handle the field rotation by rotating
the entire OTA as the field is tracked. I know that everyone will say
that I can buy/build more traditional mounts like a hefty GEM, or an
equatorial platform, but I want to do something different and creative.
I plan to completely automate and computerize the new mount, and all
axis will be stepper motor controlled. As both a mechanical engineer
and a computer programmer, I know I have the skills to design and build
this mount, but as only an amateur astronomer, I do not know if there is
a flaw in the design concept. Yes, the motion controls will be
complicated.....yes, the math will be ungodly.....yes, it will probably
be expensive (machined aluminum)....yes, there are easier way (why take
the easy route in life?), but WILL IT WORK?

I appreciate any thoughts, opinions (except to tell me I'm crazy) and
suggestions anyone has to offer.

Thanks!
Dave Nay


Going to be tough, would have to make sure the axis on which you turn the
tube is exactly center of the optical plane (which is probablly not the
center of your optical tube). Then the object you are imaging would have to
be exactly centered. As an alternative it may only work if you "stack and
accumulate", stop imaging, turn tube, realign (autoguide), then start
imaging again in short little itterations.

Sounds really interesting, good luck.

The optical axis and the rotational axis only have to be parallel. They
do not have to coincide if the whole tube is rotated. A conventional
field derotator must have the rotational axis very near the optical axis
OTOH.

Chuck
--
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons

I appreciate any thoughts, opinions (except to tell me I'm crazy) and
suggestions anyone has to offer.


My suggestion. If you have never done any astrophotography, find someone near
by that is an accomplished astrophotographer. Go out and spend a night with
him/her and see what sort of task it actually is to do astrophotography with a
quality EQ setup.

It also seems to me that you have added at least one variable here, besides the
correction for RA and Declination errors, you will also have to compensate for
field rotation errors. Doing this by hand guiding would be quite fun and
building an autoguider to handle the rotation in addition might require some
addition information, not sure on this one. It may be that you can ignore the
field rotation velocity error, it may be sufficiently small.

It seems to me that when guiding on a single star there is not enough
information to determine a single correction. If one is guiding on single star
with an EQ mount, then there is only one solution, but adding a rotation
variable would mean to me that you need two stars to guide.

-------

But my main suggestion is to get out there and see how it really works if you
have not done so. It is easier to consider the realities of a new scheme when
one is aware of the peripheral issues that are not so obvious.

Those guys and gals that can setup and produce pinpoint stars on a long
exposure have some real skill.

jon