Lunar drift while tracking at a lunar rate.

[Guest]

On an equatorial mount if you assume that the polar axis is set up perfectly and also that the mount is set to track at a lunar rate. What kind of drift would you expect to see on the lunar image?

[CraigT82]

None!

[Guest]

 I thought the moon moved in both RA and DEC. I also thought that most mounts only track in RA. So that would mean that the moon would drift in DEC even if you selected Lunar rate. I'm not sure about this. Anyway I was trying to get an idea of how much it would drift in say an hour. I think it's probably quite small.

Edited April 4, 2020 by Guest

[CraigT82]

I'd assume so, isn't that what the lunar setting is for? To engage the dec? Never had a heq5 though (or any synscan mount) so might be wrong. 

I think the lunar rate motion varies day to day though so you even so you would probably would get some drift which varies day to day, but it cant be much.

Edited April 4, 2020 by CraigT82

[CraigT82]

Just to add. I usually image at just under 5m FL and don't really see any drift over a 5min capture if I'm well aligned, and I've only got a syntrek mount with no lunar setting. 

[symmetal]

The moons orbital plane is at an angle compared to the Earth's equatorial plane so will slowly move 1 cycle up and down in declination taking approximately 1 Earth day to complete, if viewed using an equatorial mount. In the diagram below the moon's orbital plane is inclined 23.44 + 5.14 = 28.58 degrees to the Earth's equatorial plane, which your eq mount is aligned to. This periodic declination cycle won't be exactly 1 Earth day long as the Moon also orbits the Earth every 28 days or so. 🙂

Alan

Edited April 4, 2020 by symmetal

[Guest]

Thanks everyone,

I'm fairly sure that my mount HEQ5 Pro only tracks in RA but I'd be happy for anyone to confirm that or not.

From what I understand the moon changes DEC by about 10 degrees over about 14 days. The rate of change is not constant so it will drift faster or slower depending on the part of the cycle it's in. But if you average it out that's about 1.8 arc minutes per hour.

[symmetal]

6 hours ago, woodblock said:

I'm fairly sure that my mount HEQ5 Pro only tracks in RA but I'd be happy for anyone to confirm that or not.

Yes, EQ mounts just track in RA. If accurately polar aligned there would be no Dec movement when imaging DSOs so there is no need for Dec tracking. Solar system objects will drift in Dec but would be very slow for the planets but faster for the Moon as seen.

Selecting Lunar or Solar tracking on the EQ mount just changes the speed of the RA tracking to match the speed of the Moon or Sun's momement in RA.

Alan

Edited April 5, 2020 by symmetal

[CraigT82]

5 hours ago, woodblock said:

But if you average it out that's about 1.8 arc minutes per hour.

Ah that is faster than I thought it would be. 

I'm still not sure if the lunar tracking mode activates the Dec motor though. Should be easy enough to test with a multimeter, or even just an ink dot on one of the dec spur gear teeth

[Guest]

I've seen some drift when I've been imaging the moon but I think that was more down to poor polar alignment. The exposure times are very short so there won't be any drift during the exposure but I've had to adjust the position of the image every 5 minutes or so to bring the target back to centre. Any remaining drift between exposures can be taken out by aligning during processing. PIPP will do that.