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25 Jul 2018
Glad to announce of release of INDI Library v1.7.4 on 2018-07-25. A few drivers were added in this release as we continue to improve & stabilize existing drivers.
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TOPIC: subpixel accuracy - autoguiding

subpixel accuracy - autoguiding 1 week 6 days ago #29497

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Would someone happen to know what "subpixel accuracy" in the context "guiding" means"?

is this a parameter to be set (eg in guide module) or a ratio related to the primary / guding scope or pixel size ccd / guidecam?

I came across the topic as I was wondering about the ideal guide scope (length/aperture) for my 500mm refractor, with ASI120 & ASI071 cams.

many thanks, Helge

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subpixel accuracy - autoguiding 1 week 6 days ago #29498

You can't set it. It's a result. The guide camera images a star. The guiding software then calculates a star profile (centroid) from the pixel data. The software then calculates where to move the mount, such that the peak of the calculated profile always stays at the same position. In theory, the mount movements needed to keep the star at the same position may be smaller than one pixel. If the software can keep the guiding error below one pixel of the imaging camera, you have subpixel accuracy.
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Wim

Opticron Oregon 15*70, Skywatcher 150PDS + ST80, AZ-EQ6 GT + EQ3-2 Goto, ZWO ASI174MM-Cool + Pentax K20D (retired) + ZWO ASI120MM, homemade ditherbox, PixInsight, Raspberry Pi with INDI & PHD2, cloud cover

www.astrobin.com/users/wimvb/
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subpixel accuracy - autoguiding 1 week 6 days ago #29499

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very clear, many thanks, Wim

...then, would anyone know KStars/Ekos subpixel-accuracy? For phd2 I have read somewhere that the accuracy is somewhere around or even less than 0.3.

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subpixel accuracy - autoguiding 1 week 6 days ago #29501

That depends on your guide cam, guide telescope and RMS. If, for example, the guide cam plus scope give 1” pixels and the RMS is less than 1” you get sub-pixel accuracy.

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Modded Canon EOS 700D with a WO ZS 71 ED on a SW HEQ5-Pro /
QHY9 CCD and filter wheel with an 8" RC on a SW EQ6-R

subpixel accuracy - autoguiding 1 week 6 days ago #29508

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Hi Wim,

Is this correct for ASI120 with 3,75my:

= 206 x 3,75 / 170mm focal lenght
= 4,54”

Means: with an RMS equaling 1, I will have on average a shift of 4,54” ? Somehow sounds a lot, or am I wrong?

So I would be better off with a scope with a longer focal length?

In case you can recommend some sort of literature, would be great if you can share a link.

Thanks once again, Helge

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subpixel accuracy - autoguiding 1 week 6 days ago #29509

The unit of RMS is arcsec so with 4,54” pixels and 1” RMS you get sub-pixel accuracy.

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Modded Canon EOS 700D with a WO ZS 71 ED on a SW HEQ5-Pro /
QHY9 CCD and filter wheel with an 8" RC on a SW EQ6-R

subpixel accuracy - autoguiding 1 week 6 days ago #29514

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Think, now I got it: if rms = 1” arc sec, it is very roughly a quarter of my arc sec per pix!

B)

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subpixel accuracy - autoguiding 1 week 6 days ago #29515

If you set the correct FL and pixel size in your guider software, guiding error will be displayed in arcsevonds. Otherwise it will be displayed as pixels.
I'm not aware of any books, specific on guiding. Two general books on astrophotography are
Making Every Photon Count, by Steve Richards, and
Long Exposure Astrophotography, by Allan Hall.
A good online resource is Craig Stark's website:
www.stark-labs.com/craig/articles/articles.html
Stark is the original developer of PHD guiding software.
As for best guiding fl, a general rule of thumb is that the guiding pixel scale shouldn't be more than about 2 times the imaging pixel scale. In practice, if your guiding is too coarse, you can't get good pictures, while if it's too fine, you'll be guiding the noise.
Hope this helps.

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Wim

Opticron Oregon 15*70, Skywatcher 150PDS + ST80, AZ-EQ6 GT + EQ3-2 Goto, ZWO ASI174MM-Cool + Pentax K20D (retired) + ZWO ASI120MM, homemade ditherbox, PixInsight, Raspberry Pi with INDI & PHD2, cloud cover

www.astrobin.com/users/wimvb/
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