Why didn't I go down the OAG route earlier? - Forums

Topic starter

Apr 15, 2022 · Edited: Apr 16, 2022

Just sharing:

I had my first night using an off-axis guider tonight and just wish I'd done it earlier.

It's a ZWO OAG with a ZWO ASI290MM mini and helical focuser on a 1000mm Newtonian.

Before this I was using an Orion starshooter delux with a 162mm scope.

I've gone from 1 to 1.5 arc secs guiding down to a consistent 0.6 to 0.7 arc secs and now have round stars in just about all my subs rather than just 60% of them

I haven't even had a chance to tweak PHD2 yet. Nice thing is that DEC and RA are both around the 0.5 mark as opposed to RA being twice my DEC value before.

Also, I'm not having any trouble finding guide stars … always half a dozen identified by PHD2's multi-star thingy.

One last benefit: I've shaved another few hundred grams off my payload.

Just thought I'd share as I was always a sceptic of OAGs.

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AirBourn

Apr 16, 2022

I have a similar experience with ZWO OAG and ASI290 on a ED127CF. I'm sold!

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Freneltic

Apr 16, 2022

Where I live I would have to set up a 360° motion detector else some sort of animal could sneak up on me.

Kevin Morefield

IOTD Staff

Apr 16, 2022

Andy,

I'm surprised that for many people the guide scope seems to be the default answer. I see almost nothing but disadvantages to guide scopes:

1) Heavier
2) Less accurate - differential flexure to start with
3) More complicated to use - they need to be focused separately and your main camera refocuses won't also focus the guide scope of course
4) More expensive usually - an OAG should be cheaper than a guide scope + rings to attach

If an OAG won't fit in your backfocus I get it, but otherwise I don't get it. I will say that in the "old days" where you had really small guide cameras, the long focal length scopes did struggle to get stars at times. But today's large, sensitive guide cameras have no such issues.

Kevin

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Paul Muller

Apr 16, 2022

I've used and discarded guide scopes pretty early in my own journey - as you say there's really not much point with modern CMOS being so sensitive and with the large corrected image circles of modern scopes, it's almost impossible to find a chunk of sky that doesn't have a guide star for your OAG - even at 2400mm FL.

Having said that, I am keen to move to an ONAG and all the benefits they bring (total elimination of the risk of no guide stars, Near-IR guiding and focus lock) - the only downside, weight and MONEY!

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Oamaruastro

Apr 16, 2022

When I got started last year, it struck me that an OAG was the way to go since it meant you knew that your guide camera was looking at the same part of the sky as your imaging camera and there was a bit less risk of knocking things out of alignment. Of course there were still teething problems, like making sure the prism wasn't in the way of the imaging camera (that took some figuring out).

But I have a related question: I have been using an ASI120MM for my guide camera and find it a bit disappointing (the imaging is often quite dim and thus my guiding falls off as ASIAIR reports "star lost"), and have been considering replacing it with an ASI290MM. Anyone have advice on that move?

schmaks

Apr 16, 2022

My reasoning is the guide scope was included with my asiair system. Suggestions for a good OAG for a ZenithStar 73 / 183GT setup?

Sean van Drogen

Apr 16, 2022

For me on the short focal length and a more intricate imagetrain lack of backfocus seems the main reason to go for guidescope instead of OAG.
Just not enough space in the 55mm for all my gear camera + filterwheel + rotator already eats up 50mm for me.

Dominik Weinbrenner

Apr 16, 2022 · Edited: Apr 16, 2022

As long as the guide camera sensor is well sampled with the OTA, off-axis guiding is sublime.

I had trouble with guiding when shooting M106 last month using a 200mm f/5 guide scope. No idea why. Once I swapped it for an OAG, all problems went away.

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andrea tasselli

Frequent contributor

Apr 16, 2022

Kevin Morefield:
Andy,

I'm surprised that for many people the guide scope seems to be the default answer. I see almost nothing but disadvantages to guide scopes:

1) Heavier
2) Less accurate - differential flexure to start with
3) More complicated to use - they need to be focused separately and your main camera refocuses won't also focus the guide scope of course
4) More expensive usually - an OAG should be cheaper than a guide scope + rings to attach

If an OAG won't fit in your backfocus I get it, but otherwise I don't get it. I will say that in the "old days" where you had really small guide cameras, the long focal length scopes did struggle to get stars at times. But today's large, sensitive guide cameras have no such issues.

Kevin

1) Not really a concern of mine. To me it matters where the weight is and on a newt it sits in the wrong place, period.
2) Not really. Never had such an issue in over 20 years of imaging
3) Kidding me? I havent' focused one of my guide scope for years!
4) Not really, nor really at all. Guidescope requires a smaller much less expensive camera, even very old ones would do. One of the smaller 30mm guidescopes sells for 30-40 quid over here, with mount in the price.

Michael Ring

Apr 16, 2022

I have also switched from guidescope to oag quite early and never looked back.

There is only one major drawback that I have learned to live with and that‘s re-focusing the oag after switching from uv/ir to narrowband filters because I do not have a complete set of parfocal filters. Perhaps I will some day 3d print something that allows me to autofocus the helical Focuser of my zwo oag´s but so far this issue is not painfull enough to get fixed.

Michael

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Ian Dixon

Apr 16, 2022 · Edited: Apr 16, 2022

Great topic, thanks to the OP for posting. This is something that I haven't thought much about. I recently acquired a 120 mm apo (840 mm focal length) and use a 70 mm apo (410 mm) onboard guiding. The guide scope is mounted using a Losmandy-sized dovetail plate. The small scope weighs about 2 kg, and the large around 10 kg. I know the guidescope is total overkill, but my cost was zero, as the small apo is reapplied to guiding. In this configuration, the mount is an AP900 which stays below .4 RMS in all cases, and can routinely hit .25 - .3 RMS for hours in succession. The rig is coordinated by an ASIair pro, which uses a poor man's version of PHD2 in multistar mode.

https://www.astrobin.com/hi3ax2/C/rawthumb/qhd/?sync

I'm realize that I'm lucky as the mount doesn't seem to care about this magnitude of payload. I guess with OAG I might do better (?), but I guess the real advantage would be to shoot widefield and normal FOV with two parallel rigs, with the OAG doing all the guiding.

I am looking forward to seeing how the 410 mm guide scope does with my C8 Edge @ 2032 mm fl.

https://www.astrobin.com/hi3ax2/0/rawthumb/qhd/?sync

Kind regards,
Ian

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Walter Leonhard Schramböck

Apr 16, 2022

On my 100/500 f5 refractor OAG makes sense and works very well. But on my Celestron C5 with 1250mm focal length and f10 it just did not work. I would have needed a bigger prisma and a more seinsitive guiding-camera, these things are not cheap and would actually cost more than the C5. So on my C5 there still is a small and cheap Svbony guidescope with a cheap ASI120MC-S that was laying around here, and it works good enough for what I am doing with it, no complications - I never had to refocus the guidescope and weight is no concern with a C5.

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Stuart Taylor

Apr 17, 2022 · Edited: Apr 17, 2022

Andy Wray:
Just sharing:

I had my first night using an off-axis guider tonight and just wish I'd done it earlier.

It's a ZWO OAG with a ZWO ASI290MM mini and helical focuser on a 1000mm Newtonian.

Before this I was using an Orion starshooter delux with a 162mm scope.

I've gone from 1 to 1.5 arc secs guiding down to a consistent 0.6 to 0.7 arc secs and now have round stars in just about all my subs rather than just 60% of them I haven't even had a chance to tweak PHD2 yet. Nice thing is that DEC and RA are both around the 0.5 mark as opposed to RA being twice my DEC value before.

Also, I'm not having any trouble finding guide stars ... always half a dozen identified by PHD2's multi-star thingy.

One last benefit: I've shaved another few hundred grams off my payload.

Just thought I'd share as I was always a sceptic of OAGs.

Now this is interesting, encouraging and relevant to me, so thanks for posting! I have only been guiding using a guidescope (as my imaging scope is only 700mm FL) and I generally achieve 0.6 - 0.9" guiding accuracy. But I recently purchased an Edge 9.25 and the FL on that is 2300mm, so I reckoned I'd need to learn OAG. I'll be honest, I'm still very confused about how it works and which equipment to buy, but there seems no alternative to using either OAG or ONAG with such a long FL scope.

Interested that you are using the ZWO because that has a small prism, and from what I have heard, you need as large a prism as you can get (to maximise the choice of guide stars). So I was going to go with the Celestron OAG (which has a 12x12 prism)

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andrea tasselli

Frequent contributor

Apr 17, 2022

Stuart Taylor:
but there seems no alternative to using either OAG or ONAG with such a long FL scope.

I don't think so. But in your circumstances (SCT) you'll probably better served with an OAG.

Stuart Taylor

Apr 17, 2022

andrea tasselli:
Stuart Taylor:
but there seems no alternative to using either OAG or ONAG with such a long FL scope.

I don't think so. But in your circumstances (SCT) you'll probably better served with an OAG.

Thank you. This is the conclusion I am reaching
Why not an ONAG (out of interest)? What scope would an ONAG be suitable for

andrea tasselli

Frequent contributor

Apr 17, 2022

Any scope, really. But it is pretty expensive (the kit is). Sturdy focuser is a must.

Stuart Taylor

Apr 17, 2022

andrea tasselli:
Any scope, really. But it is pretty expensive (the kit is). Sturdy focuser is a must.

So why are you saying that in my circumstances (SCT) an OAG would be better? (if an ONAG works with any scope)

andrea tasselli

Frequent contributor

Apr 17, 2022

If you are willing to pay the rather steep price for really not a lot of benefit to you compared to a traditional OAG then by all means.

Andy Wray

Topic starter

Apr 17, 2022 · Edited: Apr 17, 2022

Stuart Taylor:
Interested that you are using the ZWO because that has a small prism, and from what I have heard, you need as large a prism as you can get (to maximise the choice of guide stars). So I was going to go with the Celestron OAG (which has a 12x12 prism)

My scope is only 1000mm FL (actually 906mm after my coma corrector). That, combined with the relatively wide sensor (for a guide camera) gives me enough stars. I think you would need to go for an OAG with a larger prism and a guide camera with a larger sensor than mine. There is a ZWO OAG-L which you could combine with the ASI174MM Mini. The guide camera sensor size is what is critical for you to get the field of view and then you need a big enough prism to match it. My guide camera field of view is 0.36x0.20 degrees; the 174MM on your scope would be 0.28x0.17 degrees, so broadly similar.

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andrea tasselli

Frequent contributor

Apr 17, 2022

The cost of both would probably be in the same league of the scope to be guided, if bought used (a standard C9.25 I mean).

Stuart Taylor

Apr 17, 2022

andrea tasselli:
If you are willing to pay the rather steep price for really not a lot of benefit to you compared to a traditional OAG then by all means.

Thanks, this is very helpful advice!

Stuart Taylor

Apr 17, 2022 · Edited: Apr 17, 2022

andrea tasselli:
The cost of both would probably be in the same league of the scope to be guided, if bought used (a standard C9.25 I mean).

Assuming you are referring to Andy Wray's post (immediately before yours) - No. My Edge 9.25" cost me £3k. The combined cost of the ZWO OAG (£106) and the ASI174MM Pro (£500) is nowhere near that

Andy Wray

Topic starter

Apr 17, 2022

andrea tasselli:
The cost of both would probably be in the same league of the scope to be guided, if bought used (a standard C9.25 I mean).

The OAG-L and ASI174MM Mini would cost about £700. I couldn't afford that, but not sure what else you would use to guide a 2350mm scope. My OAG and guide camera cost me £444 which is also more than my Skywatcher 200PDS OTA which cost £379.

Andy Wray

Topic starter

Apr 17, 2022

Stuart Taylor:
andrea tasselli:
The cost of both would probably be in the same league of the scope to be guided, if bought used (a standard C9.25 I mean).

No. My Edge 9.25" cost me £3k. The combined cost of the ZWO OAG (£106) and the ASI174MM Pro (£500) is nowhere near that

FYI: you would need to go for the OAG-L which has the 12mm prism.