Guidescope/Camera guidelines
acb
Are there any guidelines or formulas to determine the required focal length of the guidescope?
I suppose it depends on the pixel size and sensor size of your guide camera and also on the focal length of the imaging scope and the pixel size and sensor size of the imaging camera.
Does anybody have any idea?
bw_msgboard
Hi, this is Bruce. I posted my response on the Stark Labs forum last night. As I said there, what probably matters more is the relationship between the two image scales rather than two focal lengths – I think you already knew that. The goal is for the tracking/guiding to be seeing-limited, you want to keep any centroid error well below that. Beyond the simple stuff I described, it becomes a matter of your seeing conditions, the sensitivity of the guide camera and how faint the guide star is, the size of the star disks on your main system, etc. I don’t think there’s any cookbook answer for this. And if you’re imaging at long focal lengths (fine image scales), the issues of differential flexure will usually take over and most of this other stuff becomes irrelevant.
Cheers,
Bruce
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acb
Bryan
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. Centroid accuracy of 0.2 pixels for the guiding software
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
Bryan
bw_msgboard
Hi Bryan, thanks for doing this.
Bruce
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Bryan
Sent: Tuesday, September 19, 2017
8:59 PM
To: Open PHD Guiding
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