Very good Arun, I have never heard that before. So if you move from an STT 8300 with the Kodak Chip 5.6 Pixels to a Starlight Express 814 with 3.6 pixels, on an 2000 mm f/9 scope what effect would you expect to see?
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Aubrey
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Sent from for Windows 10
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RMS stands for Root Mean Square. It is a measure of, on average, how large the deviations are from the baseline. An RMS value of zero means a perfectly flat graph.
In the context of autoguiding, we are concerned with how far the centroid of the guide star deviates from the point where it should be (or we want it to be). These deviations are measured in two directions which are at right angles to each other - Right Ascension and Declination (RA and DEC),
Now think of your star as a circle with two diameter lines at right angles to each other.
If the deviation in the direction of RA is greater that in DEC, the diameter of the circle is greater in the RA direction than in DEC. The circle is "stretched", it is no longer a circle, but an oval or ellipse, longer in the RA direction than DEC.?
If the RA and DEC RMS are close to each other, then the circle remains a circle.
Now you can also think about the case where the deviations in RA and DEC are both close to each other, but both very large. That's still a circle, but a rather bloated one. Your images will have round stars, but large ones. Fine features of your image will be smeared.
The ideal case is when both deviations are much smaller than one pixel in your image. At that point, all the deviations are "contained" within one pixel, and your guiding no longer limits the size and roundness of your stars, nor feature resolution. The finer your image scale, the more demanding your guiding becomes, and the more you'll be able to see faults in your guiding. Many people consider that a value of lower than 0.8 " RMS to be a good number to aim for. Because at point, under most conditions, deviations caused by things like atmospheric seeing become more critical. With proper polar alignment, balancing, appropriate clutch tightening, etc., you should be able to guide consistently under this value with good mounts. That has certainly been the case for me (with some work) with the GM811G and a small (80mm) refractor.
