AP Mach1GTO pointing question

To be honest I’m not quite sure where to post this question. It only indirectly involves SGP but I’m hoping other AP mount users can give me some direction.
I set up a new to me Mach1GTO CP3. I use Stellarium for a planetarium. Plate solving within SGP works perfectly and lands the target within a few pixels on the first try. Solve and Sync the mount likewise syncs the mount.

The question involves Stellarium. If I slew to a star from Stellarium the star always lands just west of center. Like every time. Here’s a screenshot of Betelgeuse:

The location of Betelgeuse is exactly where every star lands, again just west of center, when I slew from Stellarium. Plate solving within SGP lands targets dead center.

Any ideas why Stellarium points it off to the west? Could it be a J2000/JNOW issue? I believe I have everything set to J2000.

Hi Joel,

Make sure that you set Stellarium EPCOH to JNow. The coordinates for SGP should always be J2000 and SGP will first query for mount’s EPCOH and all A-P mounts should always return JNow and SGP will convert J2000 to JNow coordinates before sending JNow coordinates to the mount.


Well that’s not confusing at all :slight_smile: I’ll give it a try.
Thanks Peter. I figured you’d have the answer!


This is a bit confusing, and I’d like to make sure that I understand this as well. I have an AP Mach1GTO mount, and I use both APCC and the ASCOM driver for communication between software and mount.

I have noticed a similar issue with Cartes du Ciel. After a slew initiated from CdC my first image is off by a little bit (10’s of arc minutes), and the plate solving centers the coordinates. I assume the solution would be the same here. I need to set CdC to JNow. Is that correct?

Now, if I put coordinates into SGP in order to slew and then center an object as part of my sequence, do those need to be in J2000 coordinates?

Thanks for the clarification.

Best Regards,

In order for both Cartes du Ciel and SGP to agree with each other, it’s best to use same EPOCH. Because SGP parses the mount for EPOCH (J2000 or JNow) via ASCOM driver, it would be best for Cartes du Ciel to use same EPCOH as well.

All A-P mounts returns EPOCH of JNow so it’s best to use JNOW for Cartes du Ciel and J2000 for SGP and SGP will automatically convert to JNow before sending JNow coordinates to A-P mounts.


How to set the EPOCH in Stellarium? I’ve looked everywhere - but the right place.

John C

I don’t think you can set the epoch in Stellarium. If you click on an
object in Stellarium you will notice that both the J200 coordinates and On
Date coordinates are given.

I use Stellariumscope to interface between Stellarium and a Mach1GTO. In
Stellariumscope you set the epoch to JNow.

I just noticed this thread again and thought I’d provide a brief follow up on my previous issues with translating J2000 and JNOW with CdC.

I have since set CdC coordinates for JNOW, and now when I issue a slew command from it, I can confirm that I am pointing much closer at the desired location (as long as I am properly synched). I also confirmed that plate solving in SGP also shows that I’m very close, so I think I have each software talking to the mount properly and making (or not making) its coordinate conversions as needed.

As confirmed by others, I now know the following:

APCC and Astro-Physics Driver coordinates are JNOW.
Coordinates defined from within SGP must be J2000. It will convert as needed.
Coordinates defined from within CdC must be JNOW (current EPOCH).

Other software will have its own requirements.

If I’ve still got that wrong, please let me know! Thanks!

Best Regards,