Old 01-09-2015, 09:49 AM
glend (Glen)
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At What FL is OAG Required

Is there some conventional wisdom concerning at what fl should you move from a guidescope to OAG? I've been doing ok with my 60mm ZWO guidescope/Metaguide setup on my 1000mm fl Mak-Newt and the 1250mm 10" imaging newt. With the RC08 at 1625mm fl will I still be able to guide effectively? Mount is a NEQ6 Pro, guide camera is a ZWO130mm (with a pixel size of 5.2), imaging camera is Canon 450D DSLR (with a pixel size of 5.19).

I should add that the RC08 is the lightest of the scopes mentioned above, and has two full length dovetails stiffening it.

Last edited by glend; 01-09-2015 at 10:24 AM.
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Old 01-09-2015, 10:02 AM
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rustigsmed (Russell)
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hi glen,

not sure, although the orion mini autoguider/scope combo reckons it is only suitable to a max of FL1500mm http://www.bintel.com.au/Astrophotog...oductview.aspx
- the guidescope is 50mm vs your 60mm.

an OAG has other benefits such as reducing flexure. so it may be worth considering moving to an OAG anyway. although you will need a good focuser to hold the additional weight.


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Old 01-09-2015, 10:04 AM
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I've always been an OAG man simply because it eliminates a lot of unnecessary cost, fiddling and heartache. I used to use an OAG with my st8300 with my TMB/lomo 80/480 years ago and never ever had problems. Essentially I was guiding at 1 as/p and imaging @ 2 as/p so even a considerably large guide error of 1.5 full pixels only translated to <1 pixel of drift in the image. No flexure, no fiddly adjustable rings, no large difference in scale and it removes the need for extra heaters, rings, extension tubes etc that all cost money and add weight to the mount.

That said Fred vanderhaven guided his 12" SCT at f10 with 6.8 micron pixels using a piggyback ST80 and 7.4 micron pixels. I don't think there are any hard and fast rules. Your results will dictate what you need to do.
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Old 01-09-2015, 10:19 AM
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multiweb (Marc)
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I think the main benefit of an OAG is to avoid flexure issues between the guiding scope and the imaging scope. Usually the heavier the optics the better off you are with an OAG.

If you use an SCT you're best using an OAG. I you use a big refractor as a guider then you're likely to get flexure when weight shifts in various parts of the sky. If you use an small light finder then it's less likely. But as it is separate from your imaging scope, again the primary mirror of your scope might shift, or the tube rings/dovetail flex, so you're better off with an OAG again.

I was getting flexure with my 8" F/4 newt because I had the finder/guider mounted on the tube rings, not on the scope body. Mounting the socket on the tube sorted that out. The scope was moving in the tube rings and that wasn't long FL at all at 800mm but noticeable in the images as a drift that wasn't guided out. An OAG will guide that out.
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Old 01-09-2015, 10:35 AM
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Meru (Michael)
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+1 for Marc's comment. Removes flex and nothing can beat guiding off what the actual imaging telescope sees. But, as a general rule of thumb I've found, up to 1000mm works with a well set up guidescope. Key with guidescoping at longer FLs would be to make sure there isn't much disparity between your imaging scale and guiding scale. This being said once I went with OAG, I have never looked back - it's just better, less weight on the mount, and (in my experience) financially works out to almost the same as buying a guidescope ($600 for the orion guidescope package vs ~$300 for a TSOAG9 + ~$300 for a good but not absurdly expensive guidecam)
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Old 01-09-2015, 12:07 PM
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Shiraz (Ray)
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all the above - it's image scale that matters, not focal length. From my limited experience an OAG will be easier to use than a guide scope at image scales below about 2arcsec/pix.

as a more general comment, an OAG can be difficult to fit into the back focus of a fast scope, so guide-scopes still have a place - not an issue for you though.
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Old 01-09-2015, 01:19 PM
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codemonkey (Lee)
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I started off with an OAG (Orion Deluxe), then got a TS 27mm OAG, then went to a guidescope when I dropped back to 900mm focal length ~1"/px.

I've just recently gone back to an OAG (Celestron) and I can see an improvement already in terms of flex.

I'd recommend an OAG regardless of focal length, and I'd recommend the Celestron; it's better than the other two I mentioned by a huge margin. You can rotate it 360 degrees and it comes with a helical focuser, which are massive improvements on the other two.

The Celestron does need chew up a bit of backfocus though, so it may not be suitable in all circumstances.
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Old 01-09-2015, 05:24 PM
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Slawomir (Suavi)
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Hi Glen,

Echoing others, OAG is the way to go regardless of FL, unless you have limited back focus.

Even better, an integrated OAG such as in QSI cameras or in SX filter wheel.

OAG comes in handy with longer exposures in particular, such as in narrowband imaging.

On a personal note; having used both an OAG and a guide scope (actually, a couple of each), and being eternally enriched by pros and cons of both systems, I am not going back to guiding with a guide scope
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Old 01-09-2015, 06:36 PM
glend (Glen)
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Thanks guys for all the replies. I might give the Lacerta EOS OAG a try, it substitutes for the Canon bayonet and retains the right sensor spacing.
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