Questions for the experts!

[Dennis G]

1.Does the length of a (Dob) driven reflecting telescope effect the speed of the drive? As an example: A 1200 mm Dob and a 2400 mm Dob.
2.Does Latitude change the speed of the drive?
3.Viewing a Star low in the Northern sky and viewing a star overhead. Any difference in rotational speed of the telescope?

If we divide 24 hours into into 360 degrees of a circle, what portion of that circle, measured in degrees, does the moon advance in 30 seconds?

[Geoff45 (Geoff)]

Originally Posted by Dennis G

1.Does the length of a (Dob) driven reflecting telescope effect the speed of the drive? As an example: A 1200 mm Dob and a 2400 mm Dob.
2.Does Latitude change the speed of the drive?
3.Viewing a Star low in the Northern sky and viewing a star overhead. Any difference in rotational speed of the telescope?

If we divide 24 hours into into 360 degrees of a circle, what portion of that circle, measured in degrees, does the moon advance in 30 seconds?

1. No. The drive rotates once every 23hr56m4s for any scope in order to counteract the Earth's rotation.
2. No, for the same reason.
3. No, unless you take into account the small effect of atmospheric refraction.

Note added a few days later: I was assuming an equatorial, but of course Dob means Newtonian on a Dobsonian altazimuth mount. However the answers are still "no". Telescope length does not affect drive rate, even if it is an altaz being driven on two axes.

A tad less than 0.125 degrees

[mill (Martin)]

Hi Dennis .
The answer would be no to the questions 1,2 and 3.
The rotational speed will stay the same and is called the sidereal rate whitch is constant.
The last question must be answered by someone else
What already is answered.
I have to learn to type faster

[gary]

Originally Posted by Dennis G

1.Does the length of a (Dob) driven reflecting telescope effect the speed of the drive? As an example: A 1200 mm Dob and a 2400 mm Dob.
2.Does Latitude change the speed of the drive?
3.Viewing a Star low in the Northern sky and viewing a star overhead. Any difference in rotational speed of the telescope?

If we divide 24 hours into into 360 degrees of a circle, what portion of that circle, measured in degrees, does the moon advance in 30 seconds?

Hi Dennis,

You mention you are using a Dob but do not specify whether it is in an Alt/Az
configuration or an equatorial configuration that has the RA axis aligned
with the either the celestial pole or refracted pole (e.g. a Dob mount on
an equatorial platform).

In either case, the focal length of the scope does not alter the drive rate significantly
for the sizes you are talking about.

However, if you are using an Alt/Az scope, the scope will be driven in both axes
simultaneously at continually varying rates and this may well be considered
in terms of a formulation that includes both your latitude and what part of the
sky you happen to be pointing at.

If you have an equatorial mount, unless it has a FOV over multiple degrees,
for all practical purposes it can be driven in the RA axis only. However, the
drive rate is not constant, owing to the effects of atmospheric refraction.

The Argo Navis Digital Telescope Computer we manufacture performs these
calculations 'on-demand' in real-time for various third party servo controllers,
such as the ServoCAT. Argo Navis is Australian designed and manufactured and
is the world's leading telescope computer for these type of applications.

Computation of the Moon's instantaneous tracking rate is reasonably
complex to say the least and is beyond the scope of this very brief response.

However, I hope the above was helpful.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[OneOfOne (Trevor)]

I agree with the triple no answers, but of course the apparent speed of the scope tube would be different. When tracking an object near the pole the tube would appear to move very little as it is swinging in a small arc. If you are following something near your local latitude, ie something that travels high in the sky, the tube would appear to move quite a bit even though it is still moving the same number of degrees. So after half an hour following Antares the scope would appear to have move quite a lot compared to following Alpha Centauri and following Sigma Oct it would appear to just "turn a little".