Sharpcap 4 beta plate solving not centering correctly

[Moorefam]

I plate solve with All Sky Plate Solver and each time I use it it plate solves within a minute but does not centre the object in the centre of the the screen. it lies roughly 2/3 down from the top and 2/3 to the left consistently. I have all the recommended files downloaded and it has found All Sky Plate Solver. I have input scope focal length chip details etc, time is correct, lat and long OK. I would love to know how to fix this or apply an offset so I can add more subs on another occasion. If I plate solve a second time there is very little change if any.

[Menno555]

The result of the plate solving depends not only on SharpCap but also on your mount.
If SharpCap gives a command to your mount to go to a certain position but your mount is a bit off, then it will deviate.

What you describe happens to my Ioptron CEM25EC mount too sometimes. To solve that at that moment I only can do 1 thing: let SharpCap plate solve and give the command to the mount. Then on my mount, with the handcontroller, I can "Sync to target", meaning I tell the mount to go to a certain star, the mount slews to where it thinks the star is and then I have to manually center that star and thus giving a correction on the coordinates.
When I do a plate solve action again after that, the star is almost in the middle.
I don't know if your mount has that same option though.
But this is a solution for that moment. If your mount is off with for example polar alignment, then it also can cause it to happen.
Or if your DEC and RA axes are not leveled. Or when the balance with the counterweights is off.

Menno

[chongo228]

This subject has come up recently on Cloudynights. I think there are currently two or three active threads on it.

Something you are using is set to Jnow vs J2000. If you're using EQmod that's probably your issue....set it to J2000.

[admin]

Surely set EQMOD to JNOW?

The plate solver always gives results as J2000. EQMOD has no behaviour change internally if you switch it between J2000 and JNOW, but does report JNOW as its co-ordinate system. If SharpCap detects the mount is JNOW it will perform a J2000->JNOW conversion on the plate solving co-ordinates before sending them to the mount. You should also set up your planetarium program to send movement commands to JNOW co-ordinates.

cheers,

Robin

[admin]

BTW...

Very close to the top of my list of things to do is a 'click to center' option that can be used after plate solving - that should help massively with one small extra user action.

Cheers, Robin

[Moorefam]

Thanks everyone and I will try Cartes du Ciel in JNOW. However what I don't understand is this. Suppose I centre Mars in my Sharp Cap window. Then plate solve. if true to form it will find an error of position and move Mars 2/3 to the left and down. You would have thought that when comparing the image generated in Sharp Cap 4 beta with the relevant star map then it would not move the scope image taken as it fitted exactly over the star map. That assumes that the software only changes the scopes position relative to where it is currently pointing. However if it accesses where the mount thinks it is according to its RA and Ascension encoders, and they are slightly off then it will move the scope. Should I disable the encoders I wonder? I hope this makes sense.
Something else. I am happy the my polar alignment is pretty good and the mount is level. However when I take an image of the pole star it isn't central presumably because of cone and other errors. Should I adjust the scopes position so it is pointing at the pole star?

[admin]

Hi,

pick a star – perhaps Rigel. The RA and DEC positions of this start today are slightly different from those in the year 2000 due to the gradual change in orientation of the Earth's axis (things like procession, nutation, etc). The coordinates today are what we call JNOW. The coordinates back in the year 2000 are the J2000 co-ordinates. There is a mathematical relation that allows you to convert backwards and forwards between these, but the software needs to know whether to apply this or not.

EQMOD never applies the conversion, so it needs to be sent coordinates as JNOW values otherwise it will end up pointing at slightly wrong place.

Plate solving applications always produce results in J2000 (at least Astap and Astrotortilla/Ansvr/ASPS do).

SharpCap knows to do the conversion if the mount Ascom driver reports that it uses JNOW as its coordinate system – making the configuration change in EQMOD will cause this to happen.

If Cartes-du-Ciel sends commands in J2000 then you still have a problem - it will send the J2000 co-ordinates for Rigel. SharpCap will plate solve, convert the co-ordinates to JNOW, sync, go back to the initial co-ordinates that CdC sent and end up with Rigel off center, since it has gone to the co-ordinates for RIgel back in the year 2000.

it's all a bit of a mess to be honest - the problem is that all the pieces of software in use have to be set up consistently or it won't work.

Robin

[Moorefam]

Robin,
I set for JNOW tonight and disabled the encoders and objects now are dead centre so one of these works. I should now re-enable the encoders and see what happens and check that it is setting for JNOW that did the trick. Thanks for the explanation.