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- This topic has 6 replies, 4 voices, and was last updated 1 year, 6 months ago by Tim Haymes.
28 October 2020 at 11:07 pm #574778
During the summer I carried out some maintenance on my observatory, including reseating my pier. It supports my Losmandy G11, Gemini GoTo and C11 and I would usually work through the Gemini’s polar alignment process to complete my task.
Simon Kidd (Cottered) recommended using the polar alignment feature in SharpCap Pro. (I had SharpCap Standard version, and the Pro upgrade only costs £10, valid for one year. Renew it as and when required).
I then used my QHY5L-II mono CMOS camera and 32mm F4.3 miniGuideScope to follow the simple steps to polar align my Losmandy G11. I soon had a result from SharpCap Pro that my alignment was within its ‘Excellent’ band and the error was <15″. This is good enough to find my occultation and astrometry targets within the 30′ field of my C11 and video camera.
Alex.29 October 2020 at 9:20 am #583316Callum PotterKeymaster
As Shaprcap only works on Windows, I use PHD2 on my MacBook, with my 50mm EvoGuide and ZWO 290MM camera, and PHD2’s Polar Drift Alignment – it works really well too.
15 arc-seconds is pretty impressive, though!
Callum29 October 2020 at 8:44 pm #583319Nick JamesParticipant
Did it give that number to 6 significant figures too?
I would take that 15 arcsec with a rather large pinch of salt and it would certainly get you into a religious argument with certain people. I remember an endless discussion with a certain Mr. CJRL on this subject many years ago.
At that level of precision people will also argue about whether you should point the polar axis at the refracted pole or the true pole (they are around 50″ apart at 52N) or somewhere between the two. Even tiny amounts of differential thermal expansion can move the polar axis by many arcsec. I suppose at least Leeds doesn’t have many earthquakes to worry about.30 October 2020 at 2:02 pm #583320Tim HaymesParticipant
I hadn’t realised SharpCap Pro had this facility. Thanks for the tip Alex. My EQ6 is drift aligned and pretty good. It will be interesting to check this out more quantitatively, give or take a pinch of sodium chloride.30 October 2020 at 2:15 pm #583321
SharpCap’s pola alignment display was giving error values between 8″ and 15″ as I finished tightening my azimuth and elevation locks. This included a correction for the refracted pole at my latitude. The ambient temperature was around 10 C. Of course, tapping and nudging the mount / ‘scope showed how these values could change.
I fully expect it would give different values if I was to test it on a frequent basis. I don’t do long-exposure astrophotography and I’m not chasing professional observatory alignment standards, just content that it’s reasonably well polar aligned and is doing the job of finding my 30′ star fields.
Alex.30 October 2020 at 2:23 pm #583322
Please let us know how you get on. As you well know, we need good pointing accuracy to locate our asteroidal occultation fields. If it does the job it’s a lot quicker than star-drift and much less expensive than buying a bespoke polar gizmo.
I’ll leave you to discuss how many angels can sit in the gap between the reported and the true pole.
Alex.29 August 2021 at 7:10 pm #584631Tim HaymesParticipant
To update you Alex, i connected my QHY174m-GPS camera to a 50mm F/3.6 finder. This was from Orion Optics originally but suspect it is a standard design. The instrument was in the polar home position. I adjusted the three finder alignment screws to bring the pole star a bit nearer the center using an eyepiece, then plugged in the camera. The scope had been parked from the previous nights observing. I focused the camera and the pole star was at the edge of the CMOS field. Its not necessary to have the pole star detected though.
I went through the routine with ShapCap Pro which is well explained with on-screen popups. The offset was measured from the automatic plate solving as 7′ arc. This was from a drift alignment originally but i knew it was a bit off. Polar alignment plate solve is build into the routine. Then i moved the scope 90 degress in RA as requested by Sharpcap.
After making corrections to the alt/az screws the routine reported 1′ arc. I could have got a bit closer but reckoned I was at the limit of the adjustment screws on the mount. So i settled with this. Now the guiding is very good indeed and gotos are spot on.
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