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Advice: Webcam or dedicated guider?


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Hi Folks

I wonder if anyone had any views about the relative pros and cons of using a webcam over a dedicated guider? For instance, what would be the benefits of buying a dedicated guider such as the QHY5 from Modern Astronomy for £149, or a SP900 for £40. Would rather do the latter to save money, but if anyone can persuade me that the extra investment would be worth it, then fair enough. I realise a dedicated guider would give me an ST4 port, but please explain to this ignorant fool why i can't simply guide with a nice cheap webby and be done with it....

Thanks as always for your help.

PB

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That's about the size of it, sensitivity. There are ways round this though. If your guide 'scope is faster you get more stars to use, so go for a fast guide 'scope and you can use the webcam. Also, if you get oversized guide rings you get more adjustment available for the guide 'scope giving you a bigger area of sky from which to choose your guide star.

Kaptain Klevtsov

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With an unmodded colour webcam you'll not be overburdened by suitable guide stars, that's for sure, if it was that easy everybody would do it.

The QHY5, and other guiders, are far more efficient because they can expose for as long as required instead of being limited to 1/5th second.

The QHY5 and the new SX Lodestar also have chips which are over 3 times as large as the SPC's.

bern

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Also, if you get oversized guide rings you get more adjustment available for the guide 'scope giving you a bigger area of sky from which to choose your guide star.

Just a quick point on this one - if you use a guide star that's some distance from the target, then you open yourself up to the increased possibility of field rotation, unless of course you are a black belt in polar alignment - the further the guide star (and longer the exposure), the more accurate you need to be with you polar alignment. With a webcam, and its reduced sensitivity, you may find you have to repoint the guider by so much that the above happens - I'd personally go for the dedicated guider.

Cheers

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Just a quick point on this one - if you use a guide star that's some distance from the target, then you open yourself up to the increased possibility of field rotation, unless of course you are a black belt in polar alignment - the further the guide star, the more accurate you need to be with you polar alignment.

This I do not understand - why would a guide star a little away from the target cause Field rotation? All the stars move realtive to each other - so I don't get it.

cheers

Ant

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This I do not understand - why would a guide star a little away from the target cause Field rotation? All the stars move realtive to each other - so I don't get it.

It will if you are not perfectly polar aligned. If the star you are on is sufficiently far away from the imaging target, your exposure is long enough and your alignment is off by enough, then the field rotation may be evident as trails (actually arcs), even if your guiding is absolutely perfect in Dec and RA. Of course, you can get away with it most of the time as the drift due to rotation is less than the tracking/seeing error (or even pixel size).

Imagine trying to guide on a single star looking at the zenith in alt-azimuth mode and the guide star is due north of the imaging area - the guide star will remain stationary thanks to the guider, while the target will appear to rotate around the star you are guiding on. If you have a polar mount mis-aligned, then you get the same (but reduced) effect.

It's because the sky and mount effectively have different coordinate systems (as the mount isn't perfectly aligned), and guiding on a point results in an apparent rotation of the "sky's" coordinate system with respect to the "mount" coordinate systems. It varies across the sky too - it's awful at the zenith, but almost zero in the east/west. Obviously, the better your polar alignment, the further away you can guide, and the longer the potential exposure.

I couldn't find a good diagram to explain it (nor draw one myself!) :D - there's a calculator on Michael Covington's site here though:

http://www.covingtoninnovations.com/astro/Polar.html

GC

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