To large a FOV for PA?

Using SharpCap's Polar Alignment feature
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DavidStevenson
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Joined: Mon Dec 25, 2017 8:24 pm

To large a FOV for PA?

Post by DavidStevenson » Fri Jan 10, 2020 7:26 pm

Hi - I’ve been using PA in Sharpcap for a year or so with mixed success. I’ve stuck with it because I haven’t really been successful with anything else to be honest but I think I have nutted out the problem - too small a FOV. I’m in the Southern Hemisphere, where there are few stars around SCP, and my ZWO 120mm through a 280mm guidescope has a FOV of 1o x 45”. Using some planetarium software this gives me around ~9 stars at mag 10.... :?

So my natural inclination is to go the other way and make a big FOV. I have various solution options at 2x1.5 degrees or 1.5x1.2 etc etc.

So my question is - is there a FOV that is too much? I know that I need to be within 5 degrees but if my FOV strays outside the 5 degree does that impact on the platesolve etc. ie if I solutioned a 5 degree FOV would that work ?

Appreciate your guidance

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admin
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Re: To large a FOV for PA?

Post by admin » Fri Jan 10, 2020 7:39 pm

Hi,

I've had the SharpCap polar alignment working with and iOptron i-polar camera which has a 25 mm lens and gives the field of view of about 11 x 8°. It works, but the results are pretty rough and ready because the short focal length give such a low resolution in terms of calculating star positions. Somewhere around the place I have a 50 mm CS mount lens which obviously gives half the field of view and gives much better results and seems to work quite reliably. Plenty of people use the 130 mm focal length mini guiders with great success.

In general a small amount of the view being outside the 5° limit shouldn't be a problem, although if the area outside the field of view contains most of the bright stars in the frame then that might be an issue as SharpCap would tend to try and choose the brightest stars to plate solve .

Hopefully this answers your question – keep the focal length at about 50 mm or above if you are using a standard guide and you should be fine.

Cheers, Robin

DavidStevenson
Posts: 3
Joined: Mon Dec 25, 2017 8:24 pm

Re: To large a FOV for PA?

Post by DavidStevenson » Fri Jan 10, 2020 9:04 pm

Thanks Robin. Exactly what I needed to know.

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