What is the correct size for a guide scope?

[dazzystar]

Hi All,

Can someone offer some advice on the minimum size of guide scope to suit a Startravel 102 refractor (f4.9) please? It'll be used with a camera for guiding only.

Thanks
Daz

[ONIKKINEN]

I'm pretty sure you could get away with using one of the 30mm F4 scopes for your setup: https://www.firstlightoptics.com/guide-cameras/astro-essentials-32mm-f4-mini-guide-scope.html

 

[dazzystar]

Thanks.

[rl]

There may be a couple of points to make here...you don't tell us what cameras you are using which may invalidate some of my comments. I'm not sure what experience you have but your question possibly implies you are just starting out with astrophotography. 

A guidescope only has to work....by which I mean that all you need is one star in the field with a good enough signal-to-noise ratio on which to guide. Once you're guiding properly there is not much point in fitting a bigger guide scope. There are a number of parameters to trade here; aperture  can be traded for exposure time to some extent (improving signal-to-noise) , which comes at the cost of update rate on the mount. If you're imaging in the milky way then a very small aperture (25mm) may well be enough. If you are pointing at the more barren parts of the sky then the bigger the better but 50mm aperture covers most situations with reasonably short exposure times (0.5 to 2 seconds).

As regards focal length, a longer focal length will allow better detection of guiding error for a given camera pixel size, at the cost of smaller field of view and hence choice of guide stars. Software like PHD2 can calculate the errors down to a small fraction of a pixel so the guidescope focal length can be a small fraction of the main scope.

To cut the waffle and get to practical options; Your main scope has a f/l of 500mm; I would expect either a 30mm aperture 120mm f/l or a 50mm 200mm f/l to give good results with normal guide cameras assuming the mount is aligned on the pole reasonably well. Half the secret is minimising the amount of work the guiding system has to do in the first place. Mechanical stability to eliminate flexure will be more important than the exact aperture. 

A word might be in order to manage expectations. Assuming you are starting out in astrophotography, a ST102 ain't the ideal scope for several reasons. The Chromatic Aberration will be severe and worse than seen by the  eye since a camera will be sensitive over a greater range of wavelengths. The field curvature will also be severe which will show up if you're using a DSLR. But if it's what you've got then have a go...we all have to start somewhere and it will still let you see stuff in greater detail then the naked eye will ever show, even with a few purple haloes. Just don't spend too much cash on kit that can't be used on a better setup in the future.

Edited February 3, 2022 by rl

[dazzystar]

18 minutes ago, rl said:

There may be a couple of points to make here...you don't tell us what cameras you are using which may invalidate some of my comments. I'm not sure what experience you have but your question possibly implies you are just starting out with astrophotography. 

A guidescope only has to work....by which I mean that all you need is one star in the field with a good enough signal-to-noise ratio on which to guide. Once you're guiding properly there is not much point in fitting a bigger guide scope. There are a number of parameters to trade here; aperture  can be traded for exposure time to some extent (improving signal-to-noise) , which comes at the cost of update rate on the mount. If you're imaging in the milky way then a very small aperture (25mm) may well be enough. If you are pointing at the more barren parts of the sky then the bigger the better but 50mm aperture covers most situations with reasonably short exposure times (0.5 to 2 seconds).

As regards focal length, a longer focal length will allow better detection of guiding error for a given camera pixel size, at the cost of smaller field of view and hence choice of guide stars. Software like PHD2 can calculate the errors down to a small fraction of a pixel so the guidescope focal length can be a small fraction of the main scope.

To cut the waffle and get to practical options; Your main scope has a f/l of 500mm; I would expect either a 30mm aperture 120mm f/l or a 50mm 200mm f/l to give good results with normal guide cameras assuming the mount is aligned on the pole reasonably well. Half the secret is minimising the amount of work the guiding system has to do in the first place. Mechanical stability to eliminate flexure will be more important than the exact aperture. 

A word might be in order to manage expectations. Assuming you are starting out in astrophotography, a ST102 ain't the ideal scope for several reasons. The Chromatic Aberration will be severe and worse than seen by the  eye since a camera will be sensitive over a greater range of wavelengths. The field curvature will also be severe which will show up if you're using a DSLR. But if it's what you've got then have a go...we all have to start somewhere and it will still let you see stuff in greater detail then the naked eye will ever show, even with a few purple haloes. Just don't spend too much cash on kit that can't be used on a better setup in the future.

Thanks very much for the reply. The camera is an ASI183MC (not cooled).