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OAG vs Guide Scope

zakkhogan

By zakkhogan
in Discussions - Cameras

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Luke

Luke

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I prefer an off axis guider for SCT because it helps keep weight down and can help rule out potential problems like flex, mirror shift and difference in focal length, because the imaging camera and guide camera both look through the same scope.

The one main pain of using the off axis guider for me is that you have a smaller field of view to pick your guide stars from. A lot of the time I was okay using a QHY5 guide camera, but sometimes I had trouble finding stars. My QHY5 broke so I've replaced it with a Lodestar and hope its extra sensitivity will mean I can nearly always get a guide star quickly. I've not had a chance to test it yet with the SCT.

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zakkhogan

zakkhogan

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I prefer an off axis guider for SCT because it helps keep weight down and can help rule out potential problems like flex, mirror shift and difference in focal length, because the imaging camera and guide camera both look through the same scope.

The one main pain of using the off axis guider for me is that you have a smaller field of view to pick your guide stars from. A lot of the time I was okay using a QHY5 guide camera, but sometimes I had trouble finding stars. My QHY5 broke so I've replaced it with a Lodestar and hope its extra sensitivity will mean I can nearly always get a guide star quickly. I've not had a chance to test it yet with the SCT.

thanks luke,

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r3i

r3i

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Off Axis Guiding comes into its own with long focal lengths such as what you'll have with your scope. They take a little bit of setting up to get main camera and the guide camera into focus at the same time but this can be done during daylight by pointing your scope at a distant object.

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zakkhogan

zakkhogan

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Off Axis Guiding comes into its own with long focal lengths such as what you'll have with your scope. They take a little bit of setting up to get main camera and the guide camera into focus at the same time but this can be done during daylight by pointing your scope at a distant object.

Presumably this is the same as the par focal method?

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r3i

r3i

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Presumably this is the same as the par focal method?

Yes I guess it is - Usually when I think of the term par focal, it's about setting up your observing 'devices' (eyepieces, cameras) to come into focus at the same place when using a single insertion point (e.g. a diagonal etc.). With an OAG setup, both guiding and imaging cameras are connected at the same time, but yes they need to be par focal.

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ollypenrice

ollypenrice

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I fought long and hard against using an OAG but love'm now especially in scenarios where there is a risk of mirror flop.

Sent from my iPhone from somewhere dark .....

Heh heh, yes, I'm coming round to them as well, though only on setups which never change. They are a bit of a pain to focus and adjust for depth but when it's done it's done for good.

Olly

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Gasman

Gasman

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One consideration you might have which I've just found out myself going down the guide scope route is if you are observing through a slit such as found in a dome! My C8 has plenty of space looking through my Pulsar dome slit on its own but now I've put an 80mm guide scope on top of it its very tight to get a clear view with both scopes .

Best

Steve

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ollypenrice

ollypenrice

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One consideration you might have which I've just found out myself going down the guide scope route is if you are observing through a slit such as found in a dome! My C8 has plenty of space looking through my Pulsar dome slit on its own but now I've put an 80mm guide scope on top of it its very tight to get a clear view with both scopes .

Best

Steve

Very good point.

Olly

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  • 3 weeks later...
kirkster501

kirkster501

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Rebooting this thread. I am going to buy a C9.25 primarily for visual but also to use in closer scale imaging on globulars (my favorite DSO subject [although I am a M81 fan as well, its beautiful IMO :) ])

So I need a OAG. I already have a QHY5 from my ED80/ST80 setup. What mechanism focuses the QHY5 though? There is no focuser on the OAG devices is there?

Was looking at the brightstar OAG. Anyone got any advice please?

Rgds, Steve

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ArmyAirForce

ArmyAirForce

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On my 9mm TS OAG, the guide camera mount, slides on the prism shaft to adjust the focus. The main imaging camera is focussed first, then the guide cam adjusted into focus and locked in place. After that, adjusting the main focus changes the guide cam focus to match.

On the image below, you can see two knurled screws between the guide camera mount and the 2 inch tube to mount in the scope focusser. One sets the depth of the prisim into the field of view, the other allows the guide camera to slide up and down the prism shaft.

scope10.jpg

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Luke

Luke

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With my DSLR and Celestron radial guider, I got the DSLR in focus first, then moved the guide camera up or down the barrel until it was in focus (the barrel allowed for a bit of movement). It was a bit fiddly, but I then drew a pencil line on the QHY5 where it meets the barrel, which made it simple to get it in future. I don't think perfect focus is needed anyway on the stars (I think a slight blur is recommened?).

It might seem a pain to have no focuser, but I think once you get the distance, you've got it nailed :)

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Coco

Coco

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I sold mine because I found some areas of interest were baron of suitable guide stars, the adjustment to find a good one frustrated me.. I now use a 135mm f/3.5 M42 threaded camera lens, now I'm spoiled for choice, although imaging at 2,000mm would be problematic.. maybe the my QHY5 isn't sensitive enough for OAG?

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r3i

r3i

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I sold mine because I found some areas of interest were baron of suitable guide stars, the adjustment to find a good one frustrated me.. I now use a 135mm f/3.5 M42 threaded camera lens, now I'm spoiled for choice, although imaging at 2,000mm would be problematic.. maybe the my QHY5 isn't sensitive enough for OAG?

If you're imaging away from the galactic plain then suitable guide stars are going to be fewer, but I've always managed to find one (then again the number of times I've imaged recently has been few and far between). It does help to have an OAG that allows you to rotate the guide camera around the axis of the scope, which I have had to a few times.

I would have thought the QHY5 should be fine as a guide camera, I understand quite a few folks on here use them.

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Coco

Coco

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If you're imaging away from the galactic plain then suitable guide stars are going to be fewer, but I've always managed to find one (then again the number of times I've imaged recently has been few and far between). It does help to have an OAG that allows you to rotate the guide camera around the axis of the scope, which I have had to a few times.

I would have thought the QHY5 should be fine as a guide camera, I understand quite a few folks on here use them.

indeed Ive been using the same one for 5 years ;)

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Luke

Luke

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I did struggle to find a guide star sometimes with the QHY5, C11 and OAG, but I would have carried on using it if I hadn't broken the QHY5 :eek:

I've gone for a Lodestar now hoping it will make it quicker to find a guide star, but not had a chance to test it yet. I need a clear night, not much wind, and not much moon. I'm thinking 2015 at this rate?!

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Tim

Tim

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Fwiw I have never failed to find a guide star with my oag, even at 2800mm and f10 and under bad UK skies. The celestron oag has great adjustability. I use the lodestar, qhy5? Not nearly sensitive enough to provide the right kind of star to guide on at longer focal lengths.

There was a used oag in classifieds the other day, dunno if its still there?

Charts

Tim

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