Expanded polar alignment data base

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jhart
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Joined: Wed Aug 26, 2020 10:06 am

Expanded polar alignment data base

#1

Post by jhart »

Hi Robin,

Would it be possible to expand the SharpCap polar alignment data base to allow polar alignment pointing locations more that 5 degrees from the NCP?

Thanks,
Jeff
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admin
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Re: Expanded polar alignment data base

#2

Post by admin »

Hi,

in theory, yes, but it requires a whole lot of extra complicated spherical co-ordinate maths (you can no longer assume that the area of the sky you are working with is well approximated by a flat plane). There are also questions about how accurate the whole thing would be if the distance between the rotation center point and the areas being imaged become large. I did try an alternative approach that used plate solving at lower declinations taking measurements at various RA values, but the results seemed unreliable.

cheers,

Robin
jhart
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Joined: Wed Aug 26, 2020 10:06 am

Re: Expanded polar alignment data base

#3

Post by jhart »

Hi Robin,

Thanks for the explanation. Those details sound very complicated for reliable results. Could I ask another question? Would the flat-sky assumption work for reliable results if it was used for a seasonal "wedge" of sky extending from the NCP? That is, if there is a house or wall blocking polaris but the sky is clear 20 or 30 degrees to the east or if, as in my case, there is a tree blocking polaris but 10-15 degrees above it there is clear sky, could a wedge of sky directly east or above (or other directions from) the NCP be selectively available (depending on the time of year) to download to use without having to do all of the spherical math? That still may involve the same spherical problems and a ton of work for not very good results but I thought I would ask.

Thanks again,
Jeff
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Re: Expanded polar alignment data base

#4

Post by admin »

Hi Jeff,

in theory you seem to be able to get away without the spherical maths for any region of sky up to about 10-12 degrees across - that's why the SharpCap star database goes out to about 6-7 degrees from the pole. What you have to do though is transform the sky co-ordinates to make the center of your region the 'virtual pole' and use a polar projection from that point outwards. So, the spherical geometry maths doesn't really go away, you just encounter it in a different part of the problem :(

cheers,

Robin
jhart
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Joined: Wed Aug 26, 2020 10:06 am

Re: Expanded polar alignment data base

#5

Post by jhart »

Hi Robin,

Thanks again for the tutorial on the spherical sky. Would the complicated math to solve for that reality be the reason that some of the new polar alignment algorithms using three plate solve points at most any location in the sky, have results with varying degrees of accuracy?

Jeff
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Re: Expanded polar alignment data base

#6

Post by admin »

Hi Jeff,

my theory on that sort of polar alignment algorithm is that they are based on measuring the error the mount has when it does a GOTO a star in that region of the sky - for instance, one star might be Regulus...

The mount does a GOTO to Regulus, then you center Regulus and the important info is how far did the mount need to be moved to do the centereing. But... That assumes that the only error in the GOTO is due to polar alignment (or possibly polar alignment and cone error, if the code is smart enough to handle both). What happens if your mount just has a bit of backlash and tends to not land quite perfectly for GOTO movements due to mechanical issues? That error is measured as part of the centering offset and goes into the calculations of polar alignment. Those calculations are then going to be incorrect to some degree because they are receiving incorrect input data.

cheers,

Robin
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