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What's the best size?

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A lot of people use an ED80 as a guide scope (FL=600mm)

Im not sure whether your mount could cope with the extra weight of this however, especially with your 8" scope.

All is not lost though. Do a search on here for Finderguider. A few people, myself included, use their finderscope as a guidescope.

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No, the ED80 is a great imaging scope but is way over the top, and a bit slow, as a guidescope. Also heavy. If all you want to do is guide, the ST80, which FLO do for about 80 pounds, is ideal. At f5 it finds guide stars easily and the body of the Barlow, once the lens has been unscrewed, makes an extender to bring your guide cam to focus. Look no further, it is the perfect guidescope. I have them on both imaging mounts. You even get a finder thrown in, which I align with the imaging scope, and a camera bracket for widefield camera lens imaging. In short, a no brainer!

However, you may find that an 8 inch SCT is not always the instrument of choice (long focal length needs good form from the autoguider and mount, wind is a killer, bad seeing is another...) in which case a reversal of roles has a lot going for it. Guide with the SCT (you will need a reducer to pick up stars easily)

and image with the 'guidescope'. If this appeals to you then spend more on your 'guidescope' but keep it light because you are already pushing your EQ5 pretty hard with the 8 inch. I used to piggyback a WO ZS66 on a Meade LX200. There are a few good light semi apos which will image pretty well. I'd have a look at Ian King's site for the latest cheap fast doublets.


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If you look at the spec range for the synguider... the ST80 is a perfect match...

80mm aperture... 400mm Focal Lenght.... so f5...

I have failed miserably to get the Synguider to work as a finder guider that was with a celestron 50mm finder...

I piggy back a megrez 72 on the CPC800 and a 500mm f8 "M42" telephoto lens as a guide scope...


Edited by Psychobilly
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I had a poor time the other night when I was using my LVI in my TV85. Now that is a fine scope and all of about £2k of kit, but at f7 it was just struggling.

I swapped over to my ZS66 with reducer fitted to bring it down to around f4.5 ish and hey presto i was getting good 5 minute subs!!

Speed is definately a consideration and at f5 I think the ST80 should be ideal.


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Guide rings are quite expensive. I'd try for second hand with a wanted ad because people have been known to stop using their SCTs for imaging. On the other hand, depending on your sky, you might not need to make the guidescope adjust relative to the SCT. None of mine is adjustable. In that case some home made plate running across the top of the OTA and attached by the accessory screws would be fine. Or Baader do a nice upside down Vixen dovetail rail to fit SCTs and a clamp which quickly fits/detaches and to which you would fasten the guidesope. That is how I did it.


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