Definitive guide vs image relationship
A must read from the guys on the PHD group:
Bruce’s approach can be translated into a similar rule of thumb as the historical value with respect to focal length of the guide scope, if we accept a couple assumptions. This reduces the number of calcs needed.
GP = Guide camera pixel size in microns
GF = Guide camera focal length in mm
IP = Image camera pixel size
IF = Image camera focal length
Assumptions:
1. A 1. Centroid accuracy of 0.2 pixels for the guiding software
2. A 2. Movement of less than 1 pixel on the main image is the acceptable upper limit
Guider image scale = GP * 206.265/GF
Imager image scale = IP * 206.265/IF
Acceptable Movement on main image = 1 pixel = GP * 206.265/GF * 0.2
---------------------
IP * 206.265/IF
Rule of thumb
Guider focal length > 0.2*(GP/IP)*IF
Bruce’s first example
Guider focal length > 0.2*(5.3/7.3)*1680 = 229
The 350 f.l. guidescope is OK
Bruce’s second example
Guider focal length > 0.2*(5.3/7.3)*2540 = 354
The 350 f.l. guiderscope is marginal
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