Re: Mount query in J2000 or JNow?

Howard Hedlund

The mount does not use epochs.   It has no clue regarding which epoch is being used and cannot respond to an epoch query.   No epoch conversions are done by the mount.   It accepts whatever coordinates are fed to it, and spits those same coordinates back out.   The GTOCP3 is simply a servo control.   It does not have a concept of the sky, but does basic calculations to go from point A to point B.   The GTOCP3 depends on the controlling software to send the correct coordinates.   This is to avoid double precession calculations.  


·         The keypad takes the catalog coordinates in its database (J2000) and precesses them to JNow before sending them to the mount.   Therefore, GoTos done from the keypad are precessed to JNow.

·         Most other software programs can be set to either send the catalog J2000 coordinates or to send precessed JNow coordinates.

·         The most critical thing is that you aren’t mixing up the epochs.  


Mag. 7 Skies!


Howard Hedlund

Astro-Physics, Inc.

Phone: 815-282-1513

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From: ap-gto@... [mailto:ap-gto@...]
Sent: Wednesday, April 08, 2015 5:38 PM
To: ap-gto@...
Subject: [ap-gto] Mount query in J2000 or JNow?




If a software like Sequence Generator Pro (SGP), Maxim, The SkyX, etc. queries the mount for Epoch, what will GTOCP3 or A-P V2 ASCOM driver respond: J2000 or JNow?

I am currently using SGP along with latest A-P V2 ASCOM driver and SGP just made an upgrade and said the following:

"All coordinates in SGP are assumed to be J2000. This is always true for:

  • Framing and Mosaic Wizard
  • Elbrus
  • (when using Nova)

You should set Pinpoint to J2000 and if you're manually inputting coordinates into SGP those need to be J2000 as well. Also if you're using ANSVR that should also be set to output J2000.


We may change this behavior later on. But we believe in incremental development (no need to spend weeks developing something if that's not the right solution). So we put this out as an initial fix and may adjust later depending on feedback.


SGP will read the epoch from the mount and translate the coordinates into that epoch. So if the mount is J2000 it will stay J2000. If it is JNow they will be converted."



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