Corrections keep changing

Using SharpCap's Polar Alignment feature
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Mbuna
Posts: 3
Joined: Wed Aug 18, 2021 3:04 pm

Corrections keep changing

#1

Post by Mbuna »

I love the PA function in Sharpcap, it was the reason I purchased the pro version!

Everything works ok, until the part where I have to make the corrections. It picks a star, I begin adjusting and suddenly it picks another star and I have to make other adjustments. Sometimes it first says I have to adjust ‘up’ and without touching the moot, I have to adjust ‘down’ … the whole process sometimes takes 20 minutes or more because it keeps shifting from one star to another, and giving me opposite instructions. And when I adjust ‘up’ the ‘left’ or ‘right’ value also changes. I a not sure if that’s normal, I assume I have to make the adjustments in the rotated state (not rotate it back to the hone position)

I’m using a ZWO ASI 120mm mini cam through a 181mm focal length guidescope.
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admin
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Re: Corrections keep changing

#2

Post by admin »

Hi,

a little variation is usual (particularly if the seeing is poor, or you are using a high gain which gives noisy images). If you are trying to get the error down to 00:00:00 then you will definitely see that sort of problem with suggestions of up, down, left, right because the variation will be large compared to the tiny remaining error.

However, if you are seeing significant fluctuations, say changing from up a couple of arc minutes to down a couple then that is odd. Part of it might be related to the other thing you mention - that the selected star keeps changing. That could be a sign that the star detection settings are not quite right and that SharpCap is not consistently picking up the same stars in every image. If that is happening (different stars have the red/yellow boxes round them in each image), try increasing the exposure a bit or turning up the 'Noise reduction' option in the Polar Alignment star detection settings.

cheers,

Robin
Mbuna
Posts: 3
Joined: Wed Aug 18, 2021 3:04 pm

Re: Corrections keep changing

#3

Post by Mbuna »

The last thing you mentioned is happening indeed. I am following the directions and suddenly the yellow box is around another star, pointing to a way different position than before.

A short arrow suddenly becomes a longer arrow in another direction. Normally I get around 14 to 17 selected stars. I’ll try adjusting the settings as you mentioned, thank you!
Mbuna
Posts: 3
Joined: Wed Aug 18, 2021 3:04 pm

Re: Corrections keep changing

#4

Post by Mbuna »

I tried increasing the exposure and noise reduction, but no effect.
- It sees a lot of stars (20, uses 15 for plate solving)
- solves the image, instructs me to rotate the mount and solves again
- the correction window appears, with enough yellow boxes around stars

And when I apply the directions and get closer, it switches to another star. For example It says ‘up 00.13.00’, I tilt my mount up, ‘up 00.08.00’ and suddenly ‘down 00.12.00’. And I can see another star is selected, while the original selected star is still surrounded by a yellow box
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admin
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Re: Corrections keep changing

#5

Post by admin »

Hi,

ok, thanks for checking that. Can I also check that you are using the most recent version of SharpCap (4.0.8060 or newer). Older versions (particularly 3.2 and before) may have issues that have been fixed in 4.0.

I think maybe the best thing to suggest would be for you to try to take a screen recording of the problem happening and to share that - it seems like a fairly dynamic problem, so screenshots or log files will probably not help explain why it is happening. There are lots of ways to record your screen - my favourite is a free program called 'OBS Studio'.

thanks,

Robin
chongo228
Posts: 204
Joined: Fri Nov 24, 2017 3:34 am

Re: Corrections keep changing

#6

Post by chongo228 »

I see you're using a 120mini and guide scope. If I remember correctly I used about 6-8 second exposures and low gain when I PA with my 120 and guide scope. The hard part is getting SC to see the stars and not the noise that a non-cooled camera brings to the table. My cooled camera can get away with very short exposures for PA. With a non cooled camera don't be afraid to make the exposures longer. Once you get on track you can try to back them down and increase gain to find the fastest exposures possible.

One thing to try is to not mess with adjusting the mount at all until your RA and DEC numbers stabilize between images. Play with the black level and all the sliders until the readings are consistent. If you get too far gone just reset to default and start again. I want to say black level and exposures are the two main settings to mess with. Take a night that might get clouded out or with bad seeing and plan to do nothing else but learn PA and all the sliders. I did this with PA and auto-focus a long time again. Taking your time and learning what everything does will save you so much time down the road. Once you can watch the frames roll in and the readings don't change much then go to adjusting your mount. This takes any variables of flexing or cable snags out of the equation.

Another is some guide scopes can be very hard to get into focus. If you're out of focus then SC will have a harder time seeing the stars. My ZWO guide scope was very hard to get into focus because the rotation lock would change focus when I tried to lock it in place. I put a helical focuser on it and never had to touch it again. You don't have to be dead on but it helps.....especially if you have noise from a non-cooled camera.
SonnyE
Posts: 9
Joined: Sat Apr 18, 2020 5:37 am

Re: Corrections keep changing

#7

Post by SonnyE »

I love chasing the all zeros with Sharpcap.
It is a personal challenge for me. :roll:
It takes the most minutest tweaks to do. Barely perceptible movements, more like pressure changes.
I can't say it makes a difference. But I find it fun to do.
But just the slightest changes.

I'm using an ASI 290 Mini with a 50mm guide scope.
I wonder if the Guide Scope camera you have is causing the difficulty? Just curious... :?
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