Polar Align Reversed Directions

Using SharpCap's Polar Alignment feature
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rmollise
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Joined: Mon Mar 06, 2017 8:00 pm

Polar Align Reversed Directions

Post by rmollise » Mon Mar 06, 2017 8:05 pm

Hi Robin:

Saw your comment on the blog, and also one of your comments on CN on the subject of some of us having up/down - left/right reversed during the procedure, as in moving the polar axis up/down changes the left/right numbers.

That's what happened to me, and once I understood the problem, it didn't hurt the anything.

My setup is a simple one...QHY5L-II directly into a 50mm guide scope.

Again, once I knew what to do it was easy to get the star where it needed to be despite the reversal. :)

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admin
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Re: Polar Align Reversed Directions

Post by admin » Mon Mar 06, 2017 8:27 pm

Thanks for getting in touch - the way it is supposed to work is like this

* SharpCap knows where the pole is (that's easy - the RA/Dec for a date come straight out of lookup tables)
* SharpCap knows the point that the sky appears to rotate around - that's where the RA axis is pointing
* It can also transform any point in the image to RA/Dec co-ordinates (because it has plate solved the image)

so... SharpCap can work out the RA/Dec that the mount axis is pointing at. Once it has that it can work out the Alt/Az of both the pole and the center of rotation point. Knowing both the the points in Alt/Az co-ordinates should make it easy to work out the up/down/left/right adjustments needed, but it isn't working out that way it seems :(

The directions are supposed to be interpreted in terms of movements to be made if you are standing on the equator side of the mount looking towards the pole. They aren't supposed to be in terms of the movements on the screen.

Just looking through the code it seems possible that I have left/right switched round, but I will have to wait for a clear night to check. Up/down seem to be correct.

The other problem is that SharpCap needs to know your latitude and longitude to be able to convert RA/Dec into Alt/Az. It estimates the latitude as 45 and the longitude from your time zome offset from GMT. This isn't precise but it should be good enough to get the directions generally OK.

If anyone does get a clear night sooner than I do and can confirm or deny that left/right are inverted (and whether up/down are the correct way round) it would be very helpful.

cheers,

Robin

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Re: Polar Align Reversed Directions

Post by admin » Tue Mar 14, 2017 10:24 am

Just an update that I finally got a bit of clear sky the other evening and had a chance to test out whether the direction instructions indicate the right direction.

Here (about 60 miles west of the Greenwich meridian, so basically at Longitude=0). I had instructions to move mostly up and right and when I adjusted to move the mount so that the objective end of the scope moved in those directions the numbers got smaller and the alignment got better.

This was using the current 2.10 beta build. It may be worth trying this build if you are having trouble with the direction guidance as it can use your latitude and longitude from your ASCOM mount (if you have enabled SharpCap to talk to your mount in the Hardware settings window). Having the correct Lat/Long should improve the accuracy of the direction/distance text.

For anyone still having issues with this, I'm still interested in reports of cases where the directions seem wrong - To make sense of the bug I would need

* A frame captured in each of the two orientiations of the RA axis used for polar alignment
* A screenshot of the final adjustment phase showing the arrow and direction/distance text before making any adjustments
* Another screenshot showing what happens after you have made an adjustment (please note down what adjustment you made to alt/az)

thanks,

Robin

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