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Old 14-04-2010, 05:45 PM
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telecasterguru (Frank)
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OAG decision

Am thinking of going down the OAG path and doing away with the piggy back scope system.

Was going to get an Orion Deluxe OAG. Any advice or alternatives? I have looked at this a while ago but now I think is the time to give it a go.

As well, using PHD for guiding, wouldn't shorter exposure times make for better guiding? Wouldn't it be better to use half second exposures rather than 2 second exposures? I would have thought that smaller increments would keep better tracking.

Thanks

Frank
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Old 14-04-2010, 06:14 PM
Doomsayer
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Oag

I'd recommend the Astrodon MMOAG for its thin profile, load capacity and reliable connections. I use one of these with my RC and a large camera load. If backfocus is not an issue the lesser MOAG is a good option. As is the Van Slyke Engineering OAG - which has excellent adjustability for its pick off mirror. All of these are quite expensive and may require special adapters to fit securely and reach focus with the guide camera. None of these are cheap items however.

Being able to place the prism so that it doesn't pick off aberrated stars needs to be considered as well.

guy
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Old 14-04-2010, 10:29 PM
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Matty P (Matt)
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Hi Frank,

I was in the same situation a couple of months ago. I really wasn't happy with the results I was getting with my guiding so I decided to give an OAG guider a go.

I bought myself a Orion Deluxe OAG and had my first light with it a few nights ago. The results really spoke for themselves. I was shooting 10 minute subs with perfectly round stars on my EQ6.

I am really happy that I went down the path of the OAG. I think for the price the Orion is great value for money. I don't really think you can go wrong with it however if you are interested in spending more definitely have a look at the more expensive OAG's Guy suggested.

Matt
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Old 15-04-2010, 06:49 AM
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telecasterguru (Frank)
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Guy and Matt,

Thanks for the advice. Will go shopping today and see what I can find.

Any thoughts on the shorter subs for guiding?

Frank
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Old 15-04-2010, 05:51 PM
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Frank you will only find out the best guider exposure times through trial and error. Some people use 2-3 seconds others use 5 sec and so on. It really depends on a lot of things including the accuracy of your mount, guide star brightness as well as the seeing conditions (easy to set the correction time too short and chase the seeing for a horrible result). My times change to suit the conditions and what the mount is doing. A good mount will need few corrections, a crappy mount will need a lot of correction. Have a play and see what works best for you. It will be interesting to see how you get on using an OAG on the RC...you have plenty of backfocus to use up in any case.
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Old 15-04-2010, 06:07 PM
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Frank, as Mark says 2-3 secs is about optimum for the average mount (some say 1, but I dont believe it), any faster and you are chasing seeing and guiding will get worse. The longer you go, the more averaged out seeing wobble will be, but also the more stable the mount needs to be.

I have an MOAG and its very stable. As with all things, the more you spend, the better they are and less need to upgrade later with heavier cams and bigger chips.
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Old 15-04-2010, 06:35 PM
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multiweb (Marc)
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Quote:
Originally Posted by Bassnut View Post
Frank, as Mark says 2-3 secs is about optimum for the average mount (some say 1, but I dont believe it)
That's because you lucky so & so have a setup on a pier in an obs so it's practical. If I don't guide a least every 1s on the field depending how well I've set-up, my guiding will "go south" between wind gusts. I need PHD to keep on the ball.
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Old 15-04-2010, 07:19 PM
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Bassnut (Fred)
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Marc

I guided 2secs with my G11 at 2m FL plus, the mount is hard pressed to handle faster corrections let alone chasing seeing. If you use 1 sec to good effect, then perhaps the seeing at Wiruna is better than sydney (likely). Anyway, if it works, why not ;-).

With an OAG though, 1 sec exposures maybe harder to achieve, given the reduced chance of a suitable bright guide star.
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Old 15-04-2010, 08:17 PM
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telecasterguru (Frank)
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I have always tried to guide at around 1.5 seconds.

The reason I ask about this is that I have been reading about adaptive optics and although I still don't really understand how it works, they seem to move the mount a very little bit many times. Is this right?

Frank
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Old 15-04-2010, 08:41 PM
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multiweb (Marc)
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Quote:
Originally Posted by Bassnut View Post
Marc

I guided 2secs with my G11 at 2m FL plus, the mount is hard pressed to handle faster corrections let alone chasing seeing. If you use 1 sec to good effect, then perhaps the seeing at Wiruna is better than sydney (likely). Anyway, if it works, why not ;-).

With an OAG though, 1 sec exposures maybe harder to achieve, given the reduced chance of a suitable bright guide star.
Got a point. You're braver than I am. That's why I'm getting an SXV-AO in a week or too.
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Old 15-04-2010, 08:54 PM
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No, adaptive optics move a motorised mirror (or lens) in the image train to shift the image. Mirrors or lenses are much lighter and can be moved faster due to thier lower mass. When adjustment exceeds the mirror/lens travel, a mount "bump" is issued to allow resuming mirror/lens guiding within the travel they have. Good polar alignment means less bumps.

AO is fast enough to actually chase seeing, unlike with mount guiding, there is no (reasonable) correcting speed limit, although a very bright guide star is required (short exposures) to operate at the fastest speeds.

I often get 0.5 to 0.2 sec exposures with planning, 0.1 if im lucky, with a min of some 0.05 on rare occassions (20hz).

AO at speed virtually eliminates PE, which is a nice bonus, converts an average mount into a PME PE wise with proper tuning.
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