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PHD2 Static Polar Alignment Question

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#1 Kaos

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Posted 24 January 2019 - 12:37 AM

I recently changed my EFW from a 5 position to an 8 position EFW. My scope is in an observatory and I rarely have to polar align it. However, I bumped the mount when re-balancing and had to do a polar alignment. I used the Static Polar Alignment (SPA) tool in PHD to do it. After some prior emails with Ken, I realized that I had to change the Hour Angle setting to get what I think is a good star match for the stars surrounding Polaris. My Hour Angle setting was 5.9 to get the pattern to match as close as I could (eyeballing to star pattern on the right) to the imaging scope camera after I did the plate solve to center Polaris. I have a profile set up in PHD for my imaging scope and camera for PA purposes. The entire process took maybe 10 mins (I spent an hour drift aligning the other night for comparison sake).

 

Once I ran the Rotate routine, I got the recommended adjustments to the Alt. and AZ. I made the recommended adjustments and reran the SPA. and made a small additional adjustment. The reported PA error was 0.3 Arc Min. I was not sure if this was accurate as I thought I had it pretty close the other night when drift aligning. I had to make a greater adjustment initially along the altitude axis than I would have guessed. Since I was unsure whether SPA gave me an accurate reading I decided to use the Guiding Assistant to check it. I let the GA run for about 6+ mins and the reported error on several runs was 0.0 - 0.4 Arc. Min.

 

Now I know the PA is really close, but the one question I did have was whether I have to adjust the Hour Angle after the first rotation because the rotation moves the stars (per the image) slightly off pattern of the SPA tool of the initial Hour Angle setting. If you have to adjust the HA after each rotation, then the PA would still be off. I am trying to figure out if I can trust the SPA (assuming I get the initial HA close). I did not adjust the HA after the initial Rotation, but was not sure if I should have (I was determined to get the PA to 0.0 in the SPA since I could not image due to the clouds).

 

Note for Ken if he reads this, an overlay pattern on the actual image would be most helpful when trying to set the Hour Angle or determine if you need check the Camera Flip. The overlay would be turned on BEFORE you start the Rotate routine and turned off once you start the Rotate routine.

 

I do love this tool for PA purposes and I think I am getting close to getting this tool figured out. Once I understand the Hour Angle thing a little better, I am sure this will eliminate the future need to spend 40 mins to an hour drift aligning!

 

Thanks

 

Kaos



#2 KenS

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Posted 24 January 2019 - 02:08 AM

For best accuracy it is normal to fine tune the hour angle after rotating. You want to get the visible references stars to be the same direction and distance away from their target positions - as much as possible.

You actually can adjust the hour angle anytime. Its just more convenient at the start to make sure you are looking at the right stars. If the mount can report its ra/dec then the overlay is adjusted for the mount movement

But the only time it matters is just before you make your adjustments.

Just want to be sure you have the overlay turned on using the Show Orbits checkbox. It shows the target positions for the reference stars and their orbits around the centre of rotation.

There's also some video tutorials if you haven't seen them yet. https://www.youtube....Gwbutr2z_yabD8 

By the way, it is not possible to get a perfect PA due to refraction. Anything under 5' is good enough for most purposes. You can get a better idea by calculating how much field rotation you will get http://celestialwond...xErrorCalc.html



#3 Kaos

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Posted 24 January 2019 - 02:28 AM

For best accuracy it is normal to fine tune the hour angle after rotating. You want to get the visible references stars to be the same direction and distance away from their target positions - as much as possible.

You actually can adjust the hour angle anytime. Its just more convenient at the start to make sure you are looking at the right stars. If the mount can report its ra/dec then the overlay is adjusted for the mount movement

But the only time it matters is just before you make your adjustments.

Just want to be sure you have the overlay turned on using the Show Orbits checkbox. It shows the target positions for the reference stars and their orbits around the centre of rotation.

There's also some video tutorials if you haven't seen them yet. https://www.youtube....Gwbutr2z_yabD8 

By the way, it is not possible to get a perfect PA due to refraction. Anything under 5' is good enough for most purposes. You can get a better idea by calculating how much field rotation you will get http://celestialwond...xErrorCalc.html

Thanks Ken...That is helpful to know that I should fine tune the Hour Angle after the Rotation and adjustments have been made before re-Rotating again after the adjustment were made. The Youtube link you provided does not work.

 

Is it possible to create an overlay "template" when adjusting the Hour Angle initially?

 

Kaos



#4 KenS

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Posted 24 January 2019 - 03:07 AM

Oops - something went wrong with my cut and paste: https://www.youtube....IGwbutr2z_yabD8

Doing the overlay before rotation is difficult as I'd have to make an assumption about where the centre of rotation is. I'll have to think about how to make it work.



#5 Kaos

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Posted 24 January 2019 - 01:42 PM

Oops - something went wrong with my cut and paste: https://www.youtube....IGwbutr2z_yabD8

Doing the overlay before rotation is difficult as I'd have to make an assumption about where the centre of rotation is. I'll have to think about how to make it work.

Thanks Ken...Video #4 was what I needed to see! 

 

Kaos




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