Plate Solving and resync - too many stars

[Riiskyyy]

Hello,
i just got into deep-sky astrophotography (or astrophotography at all) a few weeks ago and already tried some things to learn more and maybe soon be able to finally take some pics. I already had some other problems with sharpcap which I could fix by myself, but the only thing missing and which I can’t figure out at all is the Plate Solving and Resync. First of all some context: When Polar Aligning with SharpCap and ASTAP there are no problems, i can use the Noise Reduction and the Plate Solver works just fine and can calculate the image for me. The problem I have is, that my mount (iExos-100 pmc-eight) isn’t correctly synced so slewing to an object after polar aligning just isn’t right, that’s why I want to Plate Solve and Resync via SharpCap.
Now my Problem is, that whatever i tried, the Plate Solver always complains about too many stars in the sky, mostly around 500-650 stars. It doesn’t matter if i change my exposure or gain or activate/deactivate the 8-bit thing. I searched everywhere for something like the Polar Alignment Noise Reduction, but couldn’t find anything and that’s why i’m asking in this forum if that’s the problem or if it could be something else since normal plate solving for polar alignment with Noise Reduction works just fine, but there isn’t any setting for that when i’m doing plate solving and resync.
If there’s anything unclear in my question just ask and I will try to give as much information as possible and thanks in advance.

[admin]

Hi,

which plate solving program are you using? Is it Astap?

If there are lots of stars being detected, it's possible that some are being detected from noise in the image, which as you have guessed can ruin the results. If you are using Astrotortilla/Ansvr/ASPS then you can increase the 'sigma' setting for those plate solving tools to try to fix this problem, but Astap doesn't have that option.

Things you can try are

* Make sure you can see some stars in the image (ideally 20 or more) without needing to stretch the image
* Decrease gain and increase exposure to reduce the amount of noise in the image without changing the brightness
* Use the option to always send 8 bit images to the plate solving application.

You could also try switching to SharpCap's built in plate solving engine (new in the most recent updates to SharpCap 4.1).

If nothing above helps, then please share one or two of the images that are failing to solve, ideally in FITS format - they will most likely be too large to upload here, but can be shared via dropbox/google drive/etc.

cheers,

Robin

[Riiskyyy]

Hello,
yes im using ASTAP. What does the sigma value do specifically to help with the noise? I already adjusted gain and exposure to both sides like up to 30 second exposure or only 200 gain but the problem was still there. The 8 bit thing also didn’t change anything, neither while on or off. Next time I have a cloudless night, i’ll try to use the other Plate Solvers. And i’ll send an example in a few hours of some examples where it didn’t work.
Do you need any other info?

[admin]

Hi,

yes, a sample of an image that fails to work (saved using the 'Snapshot' button) would be great.

For Astrotortilla and related solvers based on the astrometry.net engine, the 'sigma' parameter defines how significant the brightness of the star needs to be relative to the general noise in the image to be considered worth detecting. Higher sigma values cause the program to ignore faint stars and only detect those that are brighter.

cheers,

Robin

[Riiskyyy]

Hey,

I'm sorry for the long wait, the christmas stress
Anyways, heres a dropbox link with some example pictures: https://www.dropbox.com/scl/fo/t5ttxwph ... lrchc&dl=0
Those werent the exact pictures used for the Plate Solve and Resync, but the settings were some I tried while doing it so the picture should be around the same.
Another problem, which I didnt mention but maybe has something to do with the first problem is, that I manually adjusted the mount, which caused the RA and DEC Data to be out of sync, which ive already read in another forum about. Since the Plate Solve and Resync only works in 15 degree off maybe the faulty RA and DEC Data is at fault too? Anyways, i dont know how I can resync the mount otherwise, is there a way y'all could help with that too?

[Riiskyyy]

I also just looked at the pictures I gave and I've noticed a lot of noise, but I also took some pictures with much lower gain and exposure, but I sadly dont have any examples for that currently, but I'll add them next time I'm able to do some photos.
And if I understand correctly, the Plate Solve and Resync solves the picture it takes and updates the RA and DEC Data to the ones it gets from calculating the position?

[admin]

Hi,

I just picked one of those files (IMG_30s_200iso_6C_2023-12-18--00-43-28.CR2) and converted to PNG. I then loaded the PNG into SharpCap's test deep sky camera and tried to plate solve. With the 'Plate Solve Only' tool (which searches the whole sky), the image solved fine using both AstroTortilla and SharpCap's build in SharpSolve plate solver.

I suspect that you have correctly identified the issue as being to do with the mount co-ordinates being offset. When you use 'Plate solve and resync', SharpCap tells the plate solving engine to search within 15 degrees of the current location, which is a big enough area to deal with ordinary pointing offsets, but not if a large offset has been introduced.

You can set the search radius in the plate solve settings (recent versions of 4.1) if you need to get around this issue.

cheers,

Robin

[Riiskyyy]

Hello Robin,
does that mean that I can set the radius to any amount I want and it should work?
And did I understand the usage of plate solve and resync correctly?

[admin]

Hi,

you can set the radius in SharpCap as large as you want, but there are two things to be aware of

1) Bigger radius means searching a larger area of sky, which means that the search will take longer

2) Some ASCOM mounts will reject a SYNC command if the offset is large (more than maybe 5 or 10 degrees) - this is a limitation imposed by the mount ASCOM driver, not by SharpCap. This means that setting a 90 degree radius in SharpCap may not work to sync the mount co-ordinates.

Plate solve and resync isn't quite as you are thinking - what it is designed to do is as follows

* read the co-ordinates from the mount - these are taken as the co-ordinates that you want to end up looking at
* capture an image and plate solve it
* set the mount co-ordinates to the plate solve co-ordinates using the 'sync' command of the mount (at this point the mount co-ordinates will be correct)
* Move the mount back to the co-ordinates read in the first step - after this the mount will be pointing at the thing it should have been pointing at to begin with.

Note that this describes the default behaviour of 'plate solve and resync' - it can be adjusted in the settings (see https://docs.sharpcap.co.uk/4.1/#Config ... %20Solving). The second option of 'Sync the mount only' is pretty much what you were thinking.

cheers,

Robin

[Riiskyyy]

Is there some list for the ASCOM mounts it works on or does it work with the iexos-100 pmc right? But i guess there’s nothing like a list. Is there an error message for the case that the mount rejects it when the radius is too high?