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PHD ver 2.6.5 "Polar Drift Alignment Tool" issues


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After a long break from astronomy, last night I attempted a reasonably accurate polar alignment (PA) of my EQ mount using the simplest of the polar alignment tools in PHD2 - "Polar Alignment Tool".

You simply slew to the proximity of the NCP, selected a star, then loop expose for a few minutes until a stable PA offset value is given. See screenshot.

HOWEVER, and I may be making a noob mistake, but my initial PA offset was ~100 arcmin, with the circle target outside the FOV, so I alt-az adjusted the selected star to the edge of my FOV. Then I repeated the process but the PA error didn't decrease?!

I then slewed the telescope to a new patch of sky, started again, but same problem --> increasing or constant PA error with each iteration..?

All I can think is that I wasn't close enough to the NCP? (A tree obscures the ~ 3 degs of declination around the NCP at my site).

Thanks in advance.

 

PHD drift align.png

Edited by choochoo_baloo
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Yes, sorry should have been clear, I definitely set mine to "Northern" hemisphere. That screenshot is actually scrounged from someone else's youtube guide...

Furthermore, the guide in question states that an advantage of this method is:

(a) calibration not required (b) mount connection not required 

Any other suggestions chaps?

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I maybe totally wrong here but did you set up your mount to a rough polar alignment and do you have a polar scope on your mount as you mentioned just slewing to ncp rather than being in the home position. 

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2 hours ago, spillage said:

I maybe totally wrong here but did you set up your mount to a rough polar alignment and do you have a polar scope on your mount as you mentioned just slewing to ncp rather than being in the home position. 

Again, I should have made myself clearer; I started in what I judged (be eye on Polaris and my latitude) to be a reasonably accurate EQ home position. Any other suggestions?

Edited by choochoo_baloo
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Hi - I'm the author of the tool and the "someone" who made the video tutorial :) 

Did you have sidereal tracking turned on? If the mount is not tracking as normal the RA drift affects the result. If you did have tracking on then I'd suggest to upload your debug log to the PHD2 forum via the help menu option so I can have a look at what was going on. Its a bit too big to attach here.

There's also a slight possibility that RA drift due to periodic error could be affecting the result. 

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18 hours ago, kens said:

Hi - I'm the author of the tool and the "someone" who made the video tutorial :) 

Did you have sidereal tracking turned on? If the mount is not tracking as normal the RA drift affects the result. If you did have tracking on then I'd suggest to upload your debug log to the PHD2 forum via the help menu option so I can have a look at what was going on. Its a bit too big to attach here.

There's also a slight possibility that RA drift due to periodic error could be affecting the result. 

Thanks for the reply Ken. You did a good job on that video, and sold me on using it!

I decided to try PHD2's standard/Meridian Drift align last night instead. I managed to get both altitude & azimuth PA errors < 10 arcmin...which would suggest a fundamental problem, either I was miles off PA in the first place, and/or the aforementioned tree prevented me from being close enough to the NCP.

Now that my PA is pretty good -  the 4 min exposure unguided Lights are streak free - I'm going to re-attempt the "Polar Alignment Tool". Will update this thread with my findings.

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Hi

I still say check into calibration (which is needed for Polar drift and static polar alignment (UNLESS there is a previous calibration for this area stored in the system.:

https://openphdguiding.org/PHD2_User_Guide.pdf

Excerpt: The Polar Drift Alignment tool operates on a similar principle to the regular Drift Alignment tool. The difference is that the drift is measured near the celestial pole to calculate the adjustments needed in both Azimuth and Altitude at the same time. Connect PHD2 to your camera. Be sure that PHD2 has already done a calibration for this set-up. Manually adjust the mount’s RA axis to point within 5 degrees of the apparent pole, then slew the telescope to point to Dec = +90 or -90.. On a German equatorial mount, it’s best to start with the mount in the counterweight-down orientation, pointing at the pole.

Andy

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That's an editorial mistake - I didn't edit out the calibration bit after cutting and pasting instructions from SPA. Calibration tells PHD2 how the camera is oriented but that isn't needed for PDA as the direction to the pole is simply at right angles to the drift. That's why not having tracking turned on is the most likely cause of the problem and I've seen this several times before.  In fact it even happened to me when I made the tutorial video so I made a point of including an instruction to turn it on. Its very easy to overlook because many mounts start with tracking off when in the home position and need it to be switched on manually or start when the mount slews. When tracking is turned on the Dec drift causes the star to rotate around the pole at a speed proportional to the amount of error. With tracking turned off the RA motion adds another element of rotation that affects the calculation quite significantly. One tester found that with tracking off they would have to make only half the correction to get a converging result but the final result was still off when measured by other methods. Once they turned tracking on the tool worked as expected.

Not saying it can't be anything else but the experience to date is that failing to start tracking has so far been the main source of errors.

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