Plate solve questions

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turchinc
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Plate solve questions

Post by turchinc » Fri May 24, 2019 12:12 am

Hi,
I have a couple questions regarding plate solving and syncing the mount.
1) the blind solve from the menu does not sync the mount, but can I (e.g. via scripting) manually sync these coordinates to the mount? I tried but RA seems to be read-only. I do this sometimes when my mount gets "lost" by slewing incorrectly or when I cancel mit slew.
2) the plate solver (correctly) finds a local astrometry (ansvr) on my machine. If I wanted to use another, i can manually select but the solve-field.exe only seems to be in the ansvr folder. Is that intentional?
3) In general, polar align works great but I have much more trouble with the plate solving function. I assumed they polar align was also plate solving, how come one is so much better than the other? This was also the reason I wanted to try using another plate solver.

Cheers
-chris

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Re: Plate solve questions

Post by admin » Fri May 24, 2019 3:12 pm

Hi Chris,

You can (or should be able to) sync the mount via scripting – the correct way to do this is to call the SyncTo(ra, dec) method on the mount object rather than to set the RA and DEC properties directly.

SharpCap can only use external plate solvers based on the astrometry.net engine using the solve-field.exe tool. It doesn't understand how to communicate with any other external plate solving programs. If you have installed a different variety of astrometry such as AstroTortilla then you can point SharpCap to the solve-field.exe application for that installation.

The plates solving for polar alignment uses an internal plate solving tool that I wrote myself which is focused on just the area around the poles and is designed to be both fast and reliable. Unfortunately the techniques I use would not scale out to doing a blind plate solve over the whole of the sky.

I find that there are two tweaks to getting the astrometry.net plate solving engine to work nicely. One is to make sure that the option to downsample the image is selected in the SharpCap options. Astrometry.net seems to work quite poorly with very high-resolution input images, and reducing the size of the image by a factor of two or three seems to help a lot. The second trick is to adjust the Sigma parameter in the same options page until you get about 50 to 100 stars detected by the astrometry.net plate solving engine (watch the notifications or the log to see how many stars it detects). Too few stars tend not to work at all while too many stars tend to take a very long time.

Of course the other thing you need to check is to make sure that you have installed the plate solving databases for the correct scales when installing astrometry.net.

Hope this helps, Robin

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Re: Plate solve questions

Post by poolemarkw » Thu May 30, 2019 1:29 pm

I'll piggy-back on this thread. I just tried to use the Plate Solving tool last night and kept getting the error:
"Plate solve failed - only - 1 stars found. Maybe too few? Try reducing the 'sigma' value in the SharpCap settings....". It was initially set to a value of 50 and I tried lowering the value by 10 each time, all the way down to zero and still received the same error. Any ideas for what I should try next? I'm using Astrotortilla as the plate solver engine.

Thanks,

Mark

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Re: Plate solve questions

Post by admin » Thu May 30, 2019 8:02 pm

Hi,

It would be worth trying more exposure and/or more gain to see if that helps. If you are using the display stretch function in SharpCap to brighten up faint images then that can make it look like there are plenty of stars in your view on screen, but turning the stretch off suddenly reveals that the images almost entirely dark which could be the reason that the plate solver cannot find any stars. If all of that checks out then save an image to disk of the field of view and try uploading to astrometry.net to see if that solves it. Also share the image here if you can.

Cheers, Robin

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