Guiding scopes - required focal length...

[Alveprinsen]

As some of you may allready know, I've been guiding my Evostar 80ED Pro with a 102/500mm Startraveler as guiding scope. Now that is pretty close to 1:1 in focal length between guiding scope and imaging scope.

However, I hear people are using the finder-scopes on their Newtonians.. like.. 50mm focal length scopes - to guide like... 800mm, 1000mm focal length newtonians - and with success..?..!..?..

So my question is: Off axis guiders aside.... what is the "golden ratio" between imaging scope and guiding scope.?.

I just received my Skywatcher Explorer 200P-DS today, and figured I'd piggy-back my 102/500mm guiding scope on it... Thing is, none of the screws either fit, or are long enough. I guess I will have to go shopping for some screws tomorrow.

And on that note, what exactly IS the threading in the Skywatcher dovetail bars and tube rings?

Sincerely, Alveprinsen.

[glappkaeft]

As some of you may allready know, I've been guiding my Evostar 80ED Pro with a 102/500mm Startraveler as guiding scope. Now that is pretty close to 1:1 in focal length between guiding scope and imaging scope.

However, I hear people are using the finder-scopes on their Newtonians.. like.. 50mm focal length scopes - to guide like... 800mm, 1000mm focal length newtonians - and with success..?..!..?..

A 9x50mm finder has a much longer focal length than 50 mm (that's the aparture). I'm eyeballing it at around 200 mm.

So my question is: Off axis guiders aside.... what is the "golden ratio" between imaging scope and guiding scope.?. 

I don't think really think there is one that doesn't depend on your mount, camera pixel scale, guiding software and guide camera (sensitivity, pixel size, etc.). I'm not even sure if the focal length of the imaging telescope is important at all (except to calculate the image pixel scale).

In general to maximize your chance of finding a guide star you want a good limiting magnitude and a wide field of view. The parameters that maximizes this is large aperture, large CCD/CMOS chips and relatively short focal lengths. The obvious thing to do is to save money on the guide system by limiting the size of both the aperture and the guide camera chip.

Back in the day of ST-4 guiders the software needed to resolve the star so using a 9x50mm finder would't have worked but modern guide software with good subpixel algorithms seem to do a good job with just a few pixels.

[LouisJB]

Top tube rings use the camera thread which is 1/4 Whitworth

Bottom threads on rings are different, metric

Skywatcher 50mm finder focal length is 175mm as I recall

I'm no expert but the old rules of focal length to guider focal length ratios don't apply so much anymore, due to improved cameras and sub-pixel guiding. However, it will depend on your guiding camera, low image scale or resolution would give less accurate guiding. An ST80 at 400mm makes a good guider focal length, that is what I'm going to use with a 200mm tube, it it all becomes too heavy I'll try a finder guider, but would rather the longer focal length guider for guiding a relatively long focal length tube.