Final Position of the Camera

Using SharpCap's Polar Alignment feature
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ademart50
Posts: 13
Joined: Wed May 24, 2017 9:42 pm

Final Position of the Camera

#1

Post by ademart50 »

First, thanks very much for including Polar Alignment in SharpCap. It establishes a great basis for everything we do in each session and has been a real help to my efforts.

I'm having a small problem with the procedure and wonder if you could comment. My setup is an SV102T refractor, an ASI071MC-Cool camera and an AVX mount. My PA process starts with the scope pointed at Polaris from the "parked" position. After the first plate solve, I rotate 90 degrees and wait for the second image/plate solve. Once complete, SC gives me instructions for left-right (azimuth) and up-down (altitude) corrections. So far, so good.

The (mental) problem here is that the camera is now 90 degrees from vertical and the alt/az instructions are resolved with the "wrong" controls. The azimuth bolts actually correct the altitude and vice versa. It seems to me that before making the alignment corrections, I should move the camera back to vertical so the corrections apply to the related controls. I've tried it both ways and haven't detected a polar alignment problem. Plus, the mount is then ready to do a 2+2 alignment.

So, is the camera position critical to the final alignment, or ?

Thanks,

Andy
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Re: Final Position of the Camera

#2

Post by admin »

Hi Andy,

I think it should work OK like this, but I wouldn't want to guarantee it - it's not how I envisaged it working.

You shouldn't have to worry about which direction the scope is rotated to when adjusted - by this point SharpCap knows two things

1) where on the camera image the center of rotation is located (it might be off the image, but that's OK as SharpCap still knows the co-ordinates for it)
2) where in the image the NCP is

Point 1) does not change when you adjust alt/az or when you rotate about the RA axis.
Point 2) does move around on the image for both of these adjustments

SharpCap guides you to put point 2 in the same place as point 1 - at which point you are polar aligned.

You can just follow the 'make the yellow arrow shorter' approach without worrying about suggested directions - that was how the early versions of SharpCap PA worked.

To make things easier, SharpCap now calculates the up/down/left/right movements needed - these apply to movements of the mount and are independent of the position of the camera. For instance up means turn the altitude adjustment to move the poleward pointing part of the mount upwards, while right means adjust azimuth to move the poleward pointing part of the mount right (when viewed looking past the mount towards the pole).

SharpCap can work out these directions regardless of the orientation of your camera - it knows the RA/Dec of both the pole and the current mount center of rotation. It makes a guess at your latitude and longitude based on your time zone and hemisphere, which it uses to convert the RA/Dec values into Alt/Az for those two points in the sky - the difference in their altitude and azimuth gives the direction guidance.

Note that if you really want to end up in the 'home' position then you can start the PA procedure with the first position in the 90 degree rotated position and then rotate back to home for the second position.

cheers,

Robin
robrj
Posts: 62
Joined: Fri Feb 17, 2017 1:03 am

Re: Final Position of the Camera

#3

Post by robrj »

I used to do the polar alignment by returning it to home and it seemed to work okay. I only stopped because Robin said not to do it that way. :)

Also, I found it easiest to adjust using the numbers only. I don't even look at the graphic. I just make a small adjustment and see how it effects the numbers. Plus, they're easier to see from a distance.
ademart50
Posts: 13
Joined: Wed May 24, 2017 9:42 pm

Re: Final Position of the Camera

#4

Post by ademart50 »

Robin,

Thanks, it sounds like you are saying that it may not matter since the PA routines now take the camera rotation into account when displaying az/el corrections-
To make things easier, SharpCap now calculates the up/down/left/right movements needed - these apply to movements of the mount and are independent of the position of the camera. For instance up means turn the altitude adjustment to move the poleward pointing part of the mount upwards, while right means adjust azimuth to move the poleward pointing part of the mount right (when viewed looking past the mount towards the pole).
It's been raining here for a few days. As soon as it clears, I'll try your suggestion. I will leave the camera rotated and see if the up/down is affected by the altitude screw and similarly if the left/right changes with the azimuth setting.

It also sounds promising to start with the camera rotated for the first capture and then return it to vertical for the second capture. There may not be any photography that night, but I'll have a lot to report.

Andy
ademart50
Posts: 13
Joined: Wed May 24, 2017 9:42 pm

Re: Final Position of the Camera

#5

Post by ademart50 »

Well, the weather here has been variable and other "duties" have called even when the night sky was good.

I have had several tries in the last two weeks where I used Polar Alignment with different starting and ending points. There were runs with the camera starting level to the horizon and others vertical to the horizon. In the cases where the camera ended up "vertical", I finished the procedure with the camera as-is and also rotating it back to horizontal.

The only issue I noticed was the original one, specifically that if the camera was vertical, then the left-right / up-down adjustments were made with the "other" alignment bolts. With the camera vertical, the up-down error was corrected with the azimuth bolts.

Other than that dissonance, the polar alignment succeeded in all cases. It's clear to me that SharpCap's math does not care as long as it has a sufficient angle between capture-one and capture-two. The end result was an alignment with the residual error given on screen. For those wondering, I am using a Celestron AVX mount. The alignment bolt pitch is coarse enough that it requires very small/fine adjustments to get the last 30" of error out of the system. More sophisticated mounts may have finer alignment pitch and may be easier to get to zero-zero.

Robin- Thanks again for being a sounding board. Great product, keep it up!
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