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Old 21-09-2017, 10:55 AM
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Merlin66 (Ken)
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Join Date: Oct 2005
Location: Junortoun Vic
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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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