What is an acceptable centering error?

Quick question, I had to reformat my imaging laptop recently and finally got a chance to try it out last night after re-installing SGP (and everything else). No backups… sigh.

Everything seemed to be working fine except somewhere during the sequence it failed to center a target. M14. Thing is the image left on the screen showed M14 pretty well centered!

I had forgot to tweak the centering settings and had left it with 1 attempt at less than 30 pixels so I guess it missed that parameter.

I was just wondering what would be considered an acceptable ‘error’ for centering?

The focal length is around 1,460mm (with focal reducer) and the pixel scale is .76

Also I was wondering about the rotator error, I don’t have a rotator, is this just ignored?

Thanks in advance

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I use 10 px

I would think that … at least with my setup 7as or 10pixels would be a bit ambitious.

I could be wrong. That’s why I am asking. :slight_smile:

I use 9 px

@UlteriorModem

Clearly the results that can be achieved depend on the quality of the mount and its alignment. You want it to be as accurate as possible without causing centering failures. All image processing software can register frames to sub pixel accuracy, so that is not really an issue. It will be necessary to simply try some value and see if it works reliably. A permanent setup that is well aligned can probably achieve 5 pixel centering; a portable setup might need 10 to 15 pixels. I have seen some SGP users saying they have trouble with anything less than 50 pixel tolerance.

Charlie

Thank you Charlie, the numbers are a lot lower than I had expected. The manual states “anything less than 100”!

I guess I will start with like 20 and see how that goes. Thanks.

I set my centering error to 50 pixels. it think it depends on on your image scale and how good of a mount you have. For me I have both a CGEM and CGE Pro mounts as well as two SCTs, HD11 and a C8. For these I set my error to 50 pixels because of the long focal lengths. I also have a ES102 with a 714 FL and I set the centering error to 25 pixels. Seems to work well for me. It typically takes 3 iterations and usually end up with a centering error of between 20 and 35 pixels for the SCTs and between 15 and 25 for the ES102. I don’t see the need to shot for anything better. it will just take more time or fail because if trying to push for a lower centering error.

Hope this helps,
Mark

Thanks for the input Mark, more helpful than “I use X” :slight_smile:

I actually got a chance to mess with it last night and coincidently I ended up at 50 as well. At a focal length of 2,000 a pixel scale of .76 on a CGX-L I tried 20.

It failed not buy much but repeated attempts could not get closer. In fact strangely after the 3rd attempt the error started to become worse but usually around 25-30 pixels :man_shrugging: Although the conditions were not ideal for testing with passing clouds.

I set it for 50 with a max of 6 attempts and it usually resolved on the second try.

But the passing clouds reminded me I did not have recovery enabled :laughing:

@UlteriorModem - You actually hit upon the issue of centering convergence that’s been talked about many times on this forum where the centering error actually increases. I believe @freestar8n, (Frank), and others have talked about this centering issue. It appears there’s some debate on how the centering routine/math is working that is not allowing the centering to converge but actually may get worse. That’s why/how I landed on 50 pixels as well. After about 3 attempts the error seemed to not improve or gets worse.

Mark

Thanks, yes I have seen those conversations but really did not think much of it.

But when I saw it last night I was like “Oh so that’s what they were talking about”

I think it’s a mistake to use centring tolerance as pixels because the tolerance that the system can achieve is as an angle. With pixels you need to convert to an angle using your plate scale to see what you are trying to achieve.

People using a long focal length telescope and a small pixel camera will have problems because they are trying to achieve accuracies that their hardware can’t achieve. They need to set a hgher value for the centring error than people who are using a short focal lenght scope and a larger pxel size camera.

As for the tolerance I think that asking for a centring accuracy better than an arc minute is asking a lot of a medium priced mount. A high end mount should be better, more like 10 arc seconds.

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I understand your description.

Is there any way to ‘estimate’ how many arc seconds 50 pixels at a pixel scale of .76 is?

Would there be a practical way to use arcseconds as a unit of measure?

50 * 0.76 = 38 arc secs.

As the pixel scale is something that the plate solve determines it would be totally practical.