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PHD - RMS for good guiding


Andyb90

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Hi Everyone,

I'm interested to get a better understanding of my PHD2 guide graph and if I'm getting good guiding. I read in a post from earlier today the total RMS value should be around half the imaging scale for good guiding.

The 600d camera I use has a pixel size of 4.3 microns

my scope (80mm ED frac) focal length: 500mm

From what I've found the formula for image scale is 

(206 * sensor pixel size) / scope focal length, so for my setup:

(206 * 4.3) / 500 = 1.77 arcsec / pixel.

Is that the correct calculation of my image scale or am I missing something?

If it is that would mean I need to achieve an RMS of around 0.89 or lower for good guiding. Is that correct?

Also is the below format the way PHD2 shows the total RMS value, with the arcsec/pixel value in the brackets?

Tot: pixel ( arcsec/pixel )

Appreciate any help on this.

Andy.

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This is based on RMS being equivalent to one standard deviation. You would expect 95% of readings to be within two standard deviations. So if one standard deviation is half the imaging scale, then 95% of readings are within 1 pixel. Whether that is achievable or not is dependent on numerous factors.

Your image scale calculation looks right. The value in parentheses in PHD2 is the absolute number of arc-seconds RMS value i.e. Tot: pixel (arc-sec ")

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I've just read the following webpage on PHD guiding:

http://www.blackwaterskies.co.uk/2013/07/phd-guiding-basic-use-and.html

Here is a quote from the article where the pixel scale for guiding and imaging are used to calculate an RMS in pixels for the image.

Quote

For example, looking at the RA error in the graph above:

0.2 RMS pixels error x 2.67 arcseconds per pixel = 0.53 arcseconds of error on the guider image

0.53 arcseconds of error / 1.9 arcseconds per pixel imager scale = 0.28 pixel RMS error on final image

I calculated my guiding image scale as follows:

9 x 50 finder focal length = 180mm

qhy5ii pixel size = 5.6 microns

guider image scale =  (206 * 5.6) / 180 = 6.41 arcsec / pixel.

If I do the quoted calculation for my set-up I get:

0.2 RMS x 6.41 = 1.28 arcsecs/pixel

final image RMS error = 1.28 / 1.77 = 0.72 pixels.

For the above example, assuming the RA errors were evenly distributed above and below the central line in PHD the final error would be doubled, giving 1.44 pixels.

This approach seems different to just using the image scale or is it actually similar?

Andy.

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Its basically the same but just converting to imaging pixels rather than arc-seconds. It probably harks back to the days when PHD only reported in pixels.

Edit: So using the guideline in the first post, you'd be looking for an imaging RMS of 0.5 pixels

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