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Old 21-09-2017, 10:55 AM
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Merlin66 (Ken)
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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


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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Old 21-09-2017, 03:57 PM
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billdan (Bill)
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Very interesting Ken, the old rule of thumb was that the focal length of the guidescope should be around 1/5th of imagescope. But as you have shown, this old rule only applies if the guidecam and imagecam have the same size pixels.

It works against you if the guidecam has larger pixels than the imagecam.

E.g my Lodestar guidecam has 8.2 micron pixels and the QHY12 imagecam has 5.1 pixels.
Bruces' first example
0.2*(8.2/5.1)*1680 = 540mm min focal length for guidescope (about 1/3rd of imaging focal length).

Thanks for clarifying that for us Ken.
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Old 21-09-2017, 04:03 PM
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Merlin66 (Ken)
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It highlights the limitations of the popular "finder guider"
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Old 21-09-2017, 04:50 PM
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When guiding manually in the old days I would ramp up the guide scope with a barlow and hope that there would be a star left bright enough to see against the illuminated red cross hairs... At one point I was going to use my 1200 mm fl refractor as a guide scope.
But now with my auto guide working I cant believe how easy it is..the program even finds a guide star within the field of view...when manual one would have to find the guide star and then adjust the imaging scope from there, a time consuming and most frustrating process.
But I will run the sums but so far my finder guider is way past anything I used in the past.
Thanks Ken.
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Old 21-09-2017, 05:26 PM
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Merlin66 (Ken)
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Yeah, I remember well those manual guiding days.....
Long gone!
The current cameras and software have radically changed things.....
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