Polar Alignment Disagrees with PHD2

[poolemarkw]

Hello Everyone,

Last night I used the Polar Alignment tool through my 127mm refractor and .8 focal reducer for a focal length of 762mm, connected to my ASI2600MC Pro camera. I got the alignment to 17 arc-seconds and locked everything down. I then used the Guiding Assistant on PHD2 to analyze my guiding performance and after 5 1/2 minutes it reported that I had poor polar alignment with an error of 5.3 arc-minutes. How can there be such a huge discrepancy?

Thanks,

Mark

[admin]

Hi Mark,

my guess would be that something is shifting slightly in your guiding/optical setup as you rotate the RA axis, which throws off the SharpCap measurement - even a dangling USB cable from the end of the guide camera can cause a noticeable effect. See https://docs.sharpcap.co.uk/4.0/#I%20get%20the%20wrong%20results%20%E2%80%93%20SharpCap%20says%20my%20Polar%20Alignment%20is%20great%20after%20adjustment%2C%20but%20it%20isn%E2%80%99t! for more information on how to tell if this is a problem and what to do about it.

cheers,

Robin

[poolemarkw]

Thanks Robin. I'll do some tests this evening. I wasn't using the guide scope and camera to do the polar alignment, but rather the main imaging scope and camera. Is one preferred over the other?

Mark

[admin]

Hi Mark,

normally I would expect the main scope to be more firmly mounted than the guide scope, but then again, depending on your focuser, the main scope may have more potential for flex there than the guide scope. Anyway, either should work just fine.

cheers,

Robin

[poolemarkw]

Great, thanks. I'm going to do the recommended process of doing the polar alignment in one direction, then do it again going back to see if it remains the same.

Mark