determining sidereal rate for tracking

[alistairsam]

Originally Posted by gary

Indeed, it is a result of the FOV.

As you would have read, the focal ratio is the focal length divided by the aperture.
Longer focal length instruments have smaller FOV's. So a scope with a long focal
length and large (slow) f/ratio works fine for photographing bright objects such as
the Moon and Jupiter as exposure times can be kept short. For extended objects,
the better choice is a scope with a smaller f/ratio (faster) as the imaging time
for these types of objects becomes smaller.

Hi,

does the size of an object vary with focal ratio when the prime focus
method is used? for example, would a planet vary in size between an F/4 and F/8 when viewed through a camera at prime focus?

Is there any information that details how the exposure time can be calculated from an F4 to F6, eg, if a 30 min exposure is considered minimal to achieve reasonable detail for a DSO with an F/4, is there a way to calculate how much longer the exposure time would be with an F/6? I'm just trying to understand the difference and then invest in a shorter focal length mirror as I'm using an F/6.

on the topic of polar aligning, how important is it to get the alignment accurate for tracking, and what is the best way of aligning, would a digital compass be more accurate than a conventional compass?
are gyroscopes used for alignments ? reason I ask is because there are several semiconductors with gyroscopes built in and it would be an interesting application if adapted.

[alistairsam]

Hi,
I worked out how to track with an equatorial mount using the sidereal rate, but how do you calculate tracking rate for altitude and azimuth axes on an alt/az mount like a dob?
I know there will be field rotation, but if there is a field de-rotator, can alt/azimuth tracking provide sufficient accuracy for a 30 minutes or more?
these days, quite a few dob mounts come with tracking.
is it still 360 deg in 24hrs for the alt and azimuth so both move at the same rate? i don't presume this would be the case as the motion of the star is not horizontal on the horizontal plane, its actually inscribing a circle, so there would be a function of the radii I think?
any pointers?

[Damienandwendy (Damien)]

I have been looking at Alt/Az tracking over the last week or so. Unlike an equatorial mount, objects viewed through a scope on an Alt/Az mount will have different rates based on their position in the sky in relation to the celestial pole as they rotate on a circular path around it.

I have a SKywatcher AZ3 mount with slow motion screws and am just looking at tracking something for a maximum of 15-30 min, ie find it turn on tracking ..... show the wife and kids .

The best I have been able to figure out at the moment is to have a look at a star chart program like Stellarium, work out the change in Alt/Az over an hour and set the rate on my drive circuit to match. I am currently looking at using a stepper with a 1350:1 reduction drive attached to it and a basic drive circuit to get things moving.

I haven't even looked at what sort of errors I could tolerate with 40-60x magnification on my 80mm refractor, but I guess field of view for the given eyepiece will be a key ingredient.

cheers

Damien

[alistairsam]

Hi,

there must be some formula out there for alt/az tracking as the auto tracking dobs that come out these days are pretty good at keeping planets in the FOV for an hour and it'd be interesting to see how they do it.
the formula or algorithm might be quite complex as it would depend on the position at any point in time, distance from the pole and/or distance from the celestial equator, alt rate and az rate would vary differently at different points in the sky.
havent found any methods so far though.
would you have details on the geared stepper you're looking at?

[Damienandwendy (Damien)]

Yup, quite complex calculations in there, especially when out of school for 20 odd years.
The steppers I have were purchased from Oatley Electronics about 8 years ago. The details from the label are:
C35M048A03-X
12VDC 155ohm
7.5 degree Ratio 1350:1

Hope that's enough to go off as that's all that's printed on them.

cheers

Damien