CPWI and Sharpcap RA/Dec Do Not Match

Using SharpCap's Polar Alignment feature
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curtismacc
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CPWI and Sharpcap RA/Dec Do Not Match

Post by curtismacc » Fri Nov 15, 2019 7:14 pm

I just got a CGX-L mount and have been trying out the Celestron/PlaneWave CPWI mount control software. I also use Sharpcap to control my camera and collect images. Since SC can also connect to the mount and control the mount GoTo, Polar alignment and Plate solve I have connected SC to the mount through the CPWI software without an issue using the CPWI driver under Hardware/Mounts in SC. I can move the mount with either the CPWI or SC Up/Down/Left/Right buttons without any problem.

What puzzles me is that CPWI and SC both display the mount's Alt/Az and RA/Dec coordinates but while the Alt/Az coordinates are exactly identical between CPWI and SC, the RA/Dec coordinates are consistently off by ~30min in RA and ~7deg in Dec. It doesn't matter if I connect CPWI to the mount through the HC or the mount USB connection, the offset is there and similar in size. This offset is pretty repeatable if I shut down both SW and re-start them and if I shut down the PC and re-start everything including the mount. CPWI indicates the RA/Dec are J2000 coordinates. My understanding is that SC gets the coordinates from the Ascom driver, in this case CPWI, so I cannot understand why they would not agree exactly. I used an on-line calculator to convert the CPWI J2000 coordinates to JNow. In that case the RA/Dec are now nearly identical.

Does SC display the RA/Dec in RNow or J2000?

I have attached screen captures showing what CPWI and SC report first for the connection through the HC and second for the connection through the mount USB port.

Regards,
Curtis
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CPWI vs Sharpcap RA-Dec through USB.png
CPWI vs Sharpcap RA-Dec through USB.png (264.01 KiB) Viewed 194 times

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Re: CPWI and Sharpcap RA/Dec Do Not Match

Post by admin » Sun Nov 17, 2019 5:14 pm

Hi,

Actually, SharpCap displays the value that the mount reports, which may be J2000 or JNOW depending on what the author of the mount's Ascom driver has chosen. It looks in this case as though the author has decided to report the coordinates by the Ascom driver in JNOW coordinates. The only time that SharpCap makes a distinction is when you use the plate solve and re-synchronise option. In that case the plate solving tool always reports the results in J 2000 coordinates and SharpCap will then convert those to JNOW if the Ascom driver reports that it is using JNOW as its coordinate system.

Aside from the values reading out slightly differently, is the display in SharpCap in JNOW actually causing anything to go wrong?

Cheers, Robin

curtismacc
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Joined: Fri Nov 15, 2019 4:19 pm

Re: CPWI and Sharpcap RA/Dec Do Not Match

Post by curtismacc » Sun Nov 17, 2019 6:11 pm

Robin,

Thank you for the clarification. On the CN forum someone reported that Celestron's SW developers indicated that the CPWI Ascom driver uses JNow coordinates. So based on that and your reply, the Celestron mount requires coordinates in JNow, which SC is providing and displaying, but the CPWI SW displays the coordinates in J2000 even though the mount uses JNow. That would not have been the way I would have written the CPWI SW, but they must have had a reason that makes sense to them.

I have not been able to test whether anything bad happens as a result of this as I just had shoulder surgery and am limited in what I can do now. Based upon what appears to be the case, I would doubt that there is a problem, but I will reply back here for the record once I have had a chance to thoroughly verify the behavior. But that will have to wait until I get the use of my right arm back.

Best Regards,
Curtis

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Re: CPWI and Sharpcap RA/Dec Do Not Match

Post by admin » Mon Nov 18, 2019 7:25 pm

Hi Curtis

Yes, I'd agree with you that they've made an odd decision there, especially since they could have made the ASCOM driver work in J 2000 and then configured it so that it reported that it was using J 2000 and everything would work fine and been consistent…

Hope that your shoulder recovers quickly so that you can get back to astronomy when the weather permits!

Cheers, Robin

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