Target is not centred after plate solve

[Gerry]

I've only had three sessions with SharpCap (3.0 and 3.1) and my ZWO224MC (on an ES102ED refractor). Lots to learn, as I am coming from using a DSLR camera with BackyardEOS software (with the same scope). Of course, with both setups, Astrotortilla is still doing the plate solving. So far, with SharpCap, when I am able to get plate solving to work, the adjusted FOV after solving does not put the target dead centre, as it did with my old setup. Instead the target is either in the bottom left corner or just out of the FOV. I was expecting the target to be dead centre, as it was when I was plate solving with my old settup. Is there some adjustment that can be made within SharpCap?

[admin]

Hi,

I wonder if this is down to the epoch used by your ASCOM mount? Astrotortilla always solves in J2000. Mounts differ in the epoch that they use for their co-ordinate system, so SharpCap asks the mount what epoch it uses and if it uses JNow then SharpCap converts from J2000 to JNow.

If your mount uses something other than J2000 or JNow (or if it doesn't correctly report its epoch) then you can get this sort of offset. For ASCOM mounts this corresponds to the value of the EquatorialSystem property (see http://www.ascom-standards.org/Help/Pla ... teType.htm).

cheers,

Robin

[Gerry]

Thanks for your prompt reply Robin. I am controlling my HEQ5 mount with the ASCOM EQMOD driver through the Stellarium Scope interface (on a WIN7 laptop), and the epoch was set at JNOW. I changed it to J2000 before my session last evening, and got mixed results - one time the solved target was just off the upper right corner (in contrast to the previous evening where it was in the lower left). But I also had a couple of solves that put the target close to the centre of the FOV. Unfortunately, I'm not aware of what changed (or what I did differently that resulted in the target being centred sometimes and not at other times. Different parts of the sky? The fact that the mount had had more synchs from previous plate solves? I did have the mount reasonably well polar aligned at the beginning of the session (using the HEQ5 built-in polar scope, not the polar alignment routine in Sharp Cap).

[Gerry]

We had clear skies again last evening - three days in a row - doesn't happen that often in Ottawa. Plate solving worked a bit better again - most (but not all) solves put the target in the centre of the FOV. I'm guessing that with the much smaller FOV I have with the ZWO224 in comparison to my DSLR, that I'll have to accept that the target will not always be dead centre after the solve. But I'm definitely feeling more comfortable with SharpCap now. In fact, I've purchased a one year license for the Pro version. Looking forward to exploring all the features of the software.

[admin]

Hi Gerry, glad that you've got things going better now - you are right in noting that the smaller FOV of cameras like the 224 makes the whole goto and plate solving problem much harder.

It's also worth noting that the SharpCap log contains a great deal of info about the plate solving process, including where the mount was pointing before the plate solve, what the solved location was and so on - you may find that a careful inspection of that data might reveal something interesting.

cheers,

Robin