SharpCap Forums

Hello! I'm getting consistent results with Sharpcap, but these differ from PHD2 Guiding Assistant and NINA's 3 point Polar alignment. During a recent trial, Sharpcap gave an error of 10 arc-sec after polar alignment, while PHD2 GA gave 35 arc-min and NINA gave 55 arc-min. PHD2 and NINA also give consistent results between PA runs.
I have an 8" LX200 OTA on a Ioptron HEM44 with a counterbalance. I use an SBIG ST402 camera with a focal reducer on the main scope giving 12x20 arc-min FOV. I use a 100mm f2.8 camera lens with QHY5III678M for the guider. NINA 3PPA was performed with the main scope; PHD2 and Sharpcap with the guide scope (I used a subframe of 1.5 degrees for Sharpcap - no problems with plate solving.)
I'm at a loss to explain the different PA errors with the different methods, and I would appreciate guidance on how to resolve the discrepancy. Thank you and best regards.

Mike

Hi Mike,

the first thing to do is to check your SharpCap results are consistent across repetition and with rotation in both directions. What we are looking for evidence of is any flex between the mount and the camera/scope used for SharpCap polar alignment, as that is the most likely cause of incorrect results. A USB cable's weight pulling on the back of the camera can introduce a noticeable error in some cases, since the direction that the weight pulls on the camera changes as you rotate the mount.

Another way to detect this is to rotate relatively slowly in SharpCap, in stages of about 10 to 15 degrees with pauses. This should give you some additional red crosses (darker red) showing the intermediate calculations of where the image appears to be rotating around. If everything is OK then these extra markers should be very close to the final red cross that shows the rotation point. If they are spread out then that is a sign that things are flexing as you rotate the mount.

Another possibility (harder to diagnose/measure/check) is that the mount may be moving slightly under its own weight as it moves to different parts of the sky. This would be more likely I think if you have a tripod resting on a potentially soft surface, or if you have any imbalance in the two axes.

There is more info on troubleshooting polar alignment issues here : https://docs.sharpcap.co.uk/4.1/#Troubl ... 0Alignment

Hope this helps,

Robin