On 7/10/20 1:12 PM, Chris Albertson wrote:
On Fri, Jul 10, 2020 at 10:33 AM andy pugh <bodge...@gmail.com> wrote:
Sprockets for T2.5 belts are available with 10 teeth, so that would be
a 2.3m table for the 300:1. Which is also probably too big. I suppose
that the design criterion is that one stepper step should be less than
the angular resolution of the telescope. So what is that, in this
case?
We don't know the size of this 'scope but let's assume about 12"
diameter.
then it is on the order of 1/2 arc-second
If you're talking about my telescope, it's an 8"
You need to move at 1/2 the resolution otherwise you can see the
steps. So
let's say each step would be 1/4 arc-second
Stars move 15 degrees per hour or 15 arc-seconds per second. So he
needs
to step at about 60 Hz.
That sounds about right, but I am planning on using a worm gear too..
so the motor can continuously spin.
(I also heard from people that built those platforms and experimented
with them, that some "shaft couplers"
reduce vibration quite a bit
Let's assume 1/64th micro steps these are roughly 100 arc-seconds per
step. He needs about 400:1 gear reduction.
This is getting unreasonable because the telescope will NEVER achieve the
theoretical resolution because of the atmospheric blurring Lets say
we do
1 arc-second steps and 100:1 gears.
Yup, and then there's wind etc too, and telescopes are also good
detectors for picking up vibrations from the ground.
The traditional professional-level method is to use very nice and very
expensive brass wheels and stainless steel worms and even more expensive
bearings and shafts. But nowadays with computers, we can place a camera
such that it looks at the image of a highly magnified star then we
measure
the X,Y location of the image on the CCD sensor and use a PID feedback
loop
to keep the star image from moving. So the servo loop tries to keep
drift
to zero. They measure drift with units called "mas" for milliarcseconds.
I am actually trying to see if I can build something like that too
But this is a "Dobsonian" scope and the goal of these is to be cheap and
portable, exactly the opposite of a professional level scope. Most of
these use a door hinge as the axis that is roughly aligned by eyeball
with
"north" and the drive is a hardware store threaded rod turned by a DC
geared DC motor and the operator sets the tracking speed by eye and hand.
These things reduce the drift to what can be tracked with the eye.
I read about barndoor equatorial platforms, those are mostly used for
cameras. Also wouldn't really work with a dobsonian mount, since the
telescope would start tipping. The actul equatorial platform runs in a
"cone", with elliptical tracks on a bearing. You'd turn one of the axis
in a bearing and move the whole platform around the center of gravity
(or close) of the telescope. At least that is what most of the plans say
to do.
It seems our OP is trying to do better than the common door hinge and
threaded rod tracker but still wants a 'scope that fits in the back of
his
car and costs less than the car. So sub-arcsecond pointing is going
to be
hard.
Yes definitely. A dobsonian telescope has a newtonian tube, also called
light buckets, they can collect a lot of light, however because of their
size, having a good stable equatorial mount is difficult, that's why
they came up with the dobsonian mount. That mount works very well for
"direct" star gazing, you just move it when you need to. Looking at
Saturn is a bit of a challenge that way already though. The equatorial
platform seems to work really well and one can track something for
approx. an hour or hour and a half.
The barndoor platform seems to work well when you look at stars near
polaris, or opposite of it, but other directions make it just
unpractical because of "balancing issues"
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