Confused about RA motors

[Samop]

Hi. 

I'm going to put an RA motor on my birthday present list this year as my medium/long term plan is to do some astrophotography.

I'm confused.

Reading up about these motors it ssems as simple as(and I'm paraphrasing here) align the mount, point the scope at your chosen object, press the 'go' button and that's it.

But my limited common sense tells me that objects near the horizon will apparently move faster than things near the centre, so surely the motor would need to run at different speeds depending upon where you're looking.

What have I misunderstood?

[malc-c]

Details of the scope / mount and proposed motor option would have been  helpful, but basically yes you have misunderstood how a GEM works.    

The motor and gearing will rotate the RA at a constant rate (Sidereal) and it won't change.  When viewing items near the celestial equator to the East, the position of the DEC axis is such that the effects of that rotation means that the scope's angular movement is more notable than it is at the pole.  If you place the scope in the normal default home position (pointing at the North pole) and engage the RA drive, or if you have placed the scope following a target one or close to the celestial equator, the RA drive will still run at the same rate to rotate the RA axis at 15 degrees per hour to match that of the Earths spin thus keeping the target in view.

[Cornelius Varley]

14 minutes ago, Samop said:

But my limited common sense tells me that objects near the horizon will apparently move faster than things near the centre, so surely the motor would need to run at different speeds depending upon where you're looking.

What have I misunderstood?

The Earth rotates at 15^o / hr, no matter where the target is. The RA motor compensates for the rotation by rotating the mount at the same rate, thereby keeping the target centred. Targets further from the NCP may appear to move faster than those closer to the NCP but are still rotate at 15^o / hr, they just move a greater distance but at the same rotational rate. See photo

 

 

[Samop]

Ahh, thanks, that makes sense.

I should have said, ĺ I have Skywatcher Explorer 150p with the EQ2-3 mount. I've had it 6 weeks and there hasn't been a single clear sky opportunity to use it yet!

Doing some further research it seems that the "standard" motor for this mount has 3 preset speeds. Why would you need these?

[Cornelius Varley]

Just now, Samop said:

Doing some further research it seems that the "standard" motor for this mount has 3 preset speeds. Why would you need these?

The preset speeds are Solar, Sidereal and Lunar. Both the Sun and Moon each track across the sky at different rates to the stars, so need slightly different tracking rates. There should also be a switch to change from Northern to Southern hemisphere, depending on where in the world you are and also buttons to give faster slewing rates. It would be easier/quicker to disengage the drive and manually slew the mount to the new target, reengage the drive and then use the handset to centre the target.

[Samop]

It's alll making sense now. 

Thanks for taking time to explain.