Polaralignment jumps during plate solving

Using SharpCap's Polar Alignment feature
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PCase3
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Joined: Mon Jan 11, 2021 3:47 pm

Polaralignment jumps during plate solving

Post by PCase3 »

Hi everybody,

sorry for my bad english. I am trying to polar align my mount. I´ve a NEQ6 with an Canon EOS 200D + 135mm Photolense.
Yes, get a DSLR working to polaralign with sharpcap, isnt easy, but i managed it. With an exposure of 6sec and an ISO of 6400 i get more than enough stars (45 stars). With the transfer delay between my camera and my laptop i need about 10-15sec. per pic.

My problem is now, that my platesolving jumps +-5arcmin during the alignment. That means, without any adjustments i can´t get closer to zero. Sure, i get values near zero, but they are jumping between the solvings.

E.g. sharpcap says iam inaccurate and have to move 5arcmin to left. So if iam doing nothing, this value jumps in a range of 10arcmin.
The best option i have and how i am doing it now, is to adjust as long as i get in a range of +-5arcmin. I have to wait about 10 minutes and watch the values before i could do another move, just to see where my middle point of 10arcmin is. The values are jumping randomly. All is very stable, nothing is moving. This costs me more than 90min for my polar alignment and at the end i dont know how accurate i am, until i take a pic with 10min exposure.

What iam doing wrong?

Yes, i will buy a guidingscope in the next few weeks, but what if this scope doesnt fix my problem?
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admin
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Re: Polaralignment jumps during plate solving

Post by admin »

Hi,

the field of view that you have with the combination of the 135 mm lens and the DSLR is really too wide for the polar alignment to work nicely – the recommended fields of view are between 1° and 2.5°. Fields between 0.5 and 5 degrees usually work well, but yours works out at 9.5x6.3, which may be part of the problem. This is going to be because some of the bright stars in the image are going to be outside SharpCap's star database area (which goes out to about 7 degrees from the pole).

If the results are jumping from frame to frame, it's probably because different stars are being detected on each frame – look at the star highlight boxes that are drawn on each frame and see if they are moving around. You may be able to adjust the star detection settings to reduce this effect - my first suggestion would be to turn up the noise reduction option in the star detection settings.

Once you switch to using a guiding camera and guide scope, everything should run much more smoothly, so there may be limited gain in trying to solve this issue if you are planning to switch fairly soon.

Cheers, Robin
PCase3
Posts: 2
Joined: Mon Jan 11, 2021 3:47 pm

Re: Polaralignment jumps during plate solving

Post by PCase3 »

Hi Robin,

and thanks for your help.

Because of the bad weather conditions i wasn´t able to test anything.

I think your considerations about the bright stars outside the FOV of sharpcarp are right, but what i didn´t understand is the process of hopping.
With hopping i mean that my mount, my scope and cam are stable and untouched. At this point i would suspect, that every star on my taken pictures doesn´t moving during 10 frames and are on the same pixel (sure they move, but just a bit). So the plate solving should place the star mask on the same pixels everytime, regardless of used stars. And this is what is not happening. The star mask is hopping over my taken pictures.

I tested this with my refractor 70/900mm, which reduces my FOV extrem. The polar alignment is more precise now, but the hopping is not gone.
The hopping is now between 20s.

I tried to change the noise reduction from 1.5 to 2.5, changed the min. pixel for a star from 5 pixel, to 3 pixel, to 10 pixel.
Nothing happend.

best greetings
Patrick
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Re: Polaralignment jumps during plate solving

Post by admin »

Hi Patrick,

the best thing is probably to either record a video showing the problem for me to look at, or alternatively capture 5-10 frames with the 'snapshot' button while the problem is happening - I can use those frames to try to recreate what is going on.

thanks,

Robin
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