APT Plate solve and synch

[Guest]

If I take a picture with APT I can plate solve it and that works fine. As I understand it the coordinates from plate solving are in J2000 coordinates. If I click on synch then APT updates the coordinates in eqmod to be equal to the plate solve coordinates. The problem is that the telescope now has the wrong coordinates because the coordinates from the plate solve are J2000 but the telescope is actually pointing at the apparent coordinates which will be different. Is there any way around this?

[Merlin66]

You can re-set the ASCOM driver ( I assume you use something like CdC?) to J2000, this would then match your APT coordinates.

 

[Guest]

Thanks Merlin,

I posted a question about this on the eqmod forum. Chris Shillito replied that it eqmod doesn't care which coordinate system is used and in fact he said that it's better to leave it as 'none' and just make sure all the other software is using the same coordinate system.  

I use CdC. There are a few mysteries for me. For example if I take a picture of a star field that image is effectively Jnow in the disposition of the stars. PLate solving works fine but plate solving is using a J2000 catalogue. Until I started looking into it I didn't realise that there was a significant difference between J2000 and Jnow coordinates.  The star I was looking at S Cas had a difference of 400 arcseconds between the two.  That's about 8% of my field of view.  I don't know if the difference is mainly due to precession or proper motion. I suppose if it's mainly due to precession then all the stars will move together so platesolving will still work but you end up with the wrong apparent coordinates which is what we are really dealing with when we take a picture.

I know it's not a big deal and I know how to change the coordinate system on cdc so that they match but it still bugs me. It seems a bit flakey.  Suppose you took a picture, plate solved it so you know the J2000 coordinate of the centre of the field. But suppose there was some new object in the image. You could give the j2000 coordinates of that object to someone else but if they sent their telescope to those coordinates they'd go to the wrong place.

Cheers

Steve