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CCD imaging with a Refractor - Size of Focuser


Andyb90

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Hi Everyone,

Just wondering how important focuser size is when it comes to CCD imaging with a refractor.

If you have a 2" focuser with reducer, filter wheel and CCD hanging off the end is there a risk of overdoing it weight wise?

Generally speaking is it better to look at a bigger focuser, say 2.5" and upwards?

Andy.

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I'd say it was more to do with having a sufficiently unobstructed light path. The bigger your chip the more this matters. The other factor is the proportion of the light cone. A fast F ratio means a steeper light cone so the need for more clear aperture in the light path.

What scope and camera are we talking about here?

Olly

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I was looking at the below scopes, possibly with a ZWO ASI1600mm cool. Can't seem to find the chip size though, only the diagonal of 21.9mm.

I noticed the Espirit and new Star 71 both have bigger focusers than the GT-81.

Skywatcher Esprit 80

Diameter : 80mm
Focal Length : 400mm
F-ratio : f/5
Linear Power Focuser Diameter : 2.7" 


William Optics GT-81

Aperture - 81mm
Focal Ratio - F/5.9
Focal Length - 478mm
Focuser - 2"


WILLIAM OPTICS STAR 71 II

TELESCOPE APERTURE - 71mm
TELESCOPE FOCAL LENGTH - 350mm
TELESCOPE FOCAL RATIO - F/4.9
FOCUSER SIZE - 2.5"

Andy.

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When considering weight, it depends on the manufacturer and type of focuser, which varies a lot. A decent focuser can cope with well over 4/5 kg of imaging equipment. If you can get adaptors which screw in rather than compression ring connections then the more rigid it will be and the better the end results too, although specialist items like the Baader click lock adaptor are worth looking into as well. Also, the less the focuser has to be racked out to get focus the better, so an advantage of a reducer can be that the focus point is brought inwards, meaning the weight is hanging off a shorter tube of metal rather than a longer one.

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The ASI1600 isn't actually that heavy at less than 500g. Add a kilo for filter wheel and filters, then with the odd adaptor you are looking at just under 2kg for an imaging train, which is well within tolerance for most decent focusers, bearing in mind the other considerations I mentioned above. I recently got one of these cameras and have been testing on a 2" TS RnP focuser and it handles it easily when screwed in properly.

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