A decent OAG

[PKay]

After 4 nights of dramas I have thrown my ZWO OAG into the bin.
What a piece of crap.

Can anyone suggest a good quality OAG?

[h0ughy]

https://www.innovationsforesight.com/product-category/on-axis-guider/

[DJT]

Hi Pete
Might be worth enlarging on what issues you found with the ZWO OAG and what setup you were working with.

I have used both ZWO and the Orion TOAG. Both can do the job but I felt that the Orion was pretty crappy with the provided grub screws being made largely out of butter (resolved by changing them out) and the ZWO had flexure issues so wasn’t consistent.

Currently using an Astrodon MMOAG..no issues there but it’s quite thick compared to the other 2 so not sure how that would go with your Newt but if you are looking for quality then compared with the other 2 mentioned, a different league. (And $$$$...)

[glend]

A guidescope works fine usually, except for very long focal lengths.

[billdan]

I use the Orion TOAG (10mm wide) which if you are using a coma corrector with only 55mm backfocus is almost mandatory. That or the TS Optics thin OAG.

h0oughy recommends the ONAG but I don't think they work with Newts, not enough backfocus, they're too big.

The Orion TOAG is a pain to get right and took me nearly 4 hours until I was satisfied. The prism is very small so the FOV is not very big. They also suffer from flex, so I have the Lodestar tied down with a big rubber band to stop it from swaying.

Cheers
Bill

[LewisM]

I didn't have the patience for an OAG - had 4...last one as recent as a month ago - it sat in the box and I sold it new. Personally prefer converted finder guidescopes, and the flexure issue is WAY over rated especially now many of us use short focal length converted finders rather than the days of yore 80mm scopes set in dodgy 3 point rings or long FL achromats in similar set ups, especially now redundant given the sensitivity of newer guide cams.

Using a converted finderscope as a guidescope ensures I get a star each and every time, especially using a Lodestar. Never have I been unable to find a guidestar, even in SEVERE LP where we live.

I have mine in a set of Baader 3 point finder rings - I have had that thing setup for months now, travelled with it, moved it around all set up etc and not once have I had to re-centre the guidescope, and there is NO flexure at all.

Anyway...the Orion TOAG. I called it The TOAD for obvious reasons as you'll find out if you ever buy one. It's a B!TCH of an OAG, and it wobbles and gyrates like Bill mentioned. Really sloppy manufacture. I spent - like Bill - close to 4 hrs getting the bloody thing to work, and even then the stars were astigmatic seagulls. I mean WTF Orion...The prism is it's Achilles heel. I could also notice a flare in my images, so the prism edge must have been slightly chipped or just made poorly (wouldn't surprise me) - I could have blackened the edges but gave up and bought the TS version...which wasn't much better (had to replace the prism). My latest OAG was the Prostar version (a bit thicker and VERY well made - NO slop - and cheaper than the Orion garbage or the ZWO).

Gee, I am Mr. Positive today :lol:

Try the Prostar version from myastroshop.com.au - I didn't set it up, but the quality of manufacture and complete lack of slop was a refreshing change.

Personally, IF - and that is a big IF - I ever go to an OAG, it'll be an Astrodon or Innovations ONAG. Expensive as all get-out. That aside, my guidescope system works, so I am not fixing something that is not broken. 20, 30, 75 minute subs with ZERO trailing seems to indicate that :lol:

[PKay]

Thanks for the feedback chaps.

I like the Astrodon OAG. It really looks the part.

However, for one last time (I swear) I will go back to the guide scope.

And if that doesn't work (again), I swear I will throw the guide scope into the bin as well :)

[04Stefan07]

Let me know which bin, would be happy to empty it for you :P :lol:

[LewisM]

No, you do NOT want a ZWO OAG Stefan. If you think the Orion TOAG is a POS, wait til you see the ZWO :eek:

[Bart]

Starlight Xpress.

[Camelopardalis]

Maybe I was lucky, but my ZWO OAG works fine :shrug:

I also have the Celestron OAG, and if you have the back focus to spare, it’s well made with a nice large prism and helical focuser on the guide cam. And doesn’t cost the Earth.

[Merlin66]

+1 for the Celestron OAG. Works well.

I've used many OAG over the years and the best was the old Lumicon Giant Easy Guider. Not suited to every scope, but great on SCT's.

The Newtonian version looked just as good, but I never tried one.

[Paul Haese]

OAG issue might seem over blown if you are imaging above 1.3" per pixel for image scale. Once you start imaging below a second per pixel you need to consider another solution for guiding.

I have used OAGs on refractors, Newtonians and RC's. Every one of them benefitted from using an OAG.

I know that the Astrodon one is probably the best one getting around. It is very strong and serviceable. I also hear that the TS one is a good too.

[gregbradley]

Astrodon MMOAG. I have 2. They are fabulous. Just be a little careful with the little prism as its only glued down and can be knocked off. A new one is something like $130.

There are a range of accessories and they are all strong. It doesn't flex and the helical focuser is very handy for precise focusing.

I have had a couple of Starlight Express OAGs and they are very lightweight. Fine if you are using a small light camera. But start using full frame and above sized sensors and you need the aperture and strength of the Astrodon.

Astrodon MMOAG and SBIG STi guider is my preferred choice and it almost never lets me down.

Reliability and repeatability should be high in the list of requirements. A clear night when you can stay up if needed is not that common and to lose it to poorly performing gear is a bit of a killer of enthusiasm for this pursuit.

Greg.

[doppler]

In regards to the Orion thin OAG, I think the secret is to use thin spacers to adjust the focus and to leave the grub screws and the stalk alone so it's tight against OAG body. I use a Canon 1100d and zwo 120mc guide camera and have found it to be quite stable with the Orion TOAG. At 1200MM focal length my 10" newt is much easier to guide with an OAG and finding guide stars is not a problem with the larger aperture.

[04Stefan07]

How heavy is the Celestron? It looks like it weighs a tonne.

I've noticed some of their products weigh a lot more than their equivalents.

[Camelopardalis]

Not as heavy as it looks :P

Insignificant weight on the size of scope that might benefit from it...

[04Stefan07]

I went for the Orion Thin Off-Axis Guider more because of the weight and the recommendations for an SCT. If the back is too heavy for me then I will need to add weight on the front of the bar.

The Celestron does look good though, looks like it accommodates their focal reducer. I had to get an additional 2 adapters to apply mine on my setup.

[Camelopardalis]

My logic in the smaller prism is that you get less light, and lower likelihood of getting a guide star, especially with a SCT. I think Celestron did a decent job with their OAG, especially for the price.

On a shorter F/L or faster scope the smaller prism might not be such an issue.

[Merlin66]

A couple of points of note
1. The actual available guide fov may be constrained more by the size of the guide chip than the size of the prism.

2. To minimise the "off-axis" coma type problems usually seen in some guide images the prism should be able to be tilted to better bring the optical axis to the centre of the guide camera. This is overlooked in some designs.
The old Meade OAG had a small tapered screw adjuster for the prism.

[Amaranthus]

Rick is totally on the money. Get the prism height right, so that it does not shadow the sensor (mine is quite high) then lock those grub screws down HARD and never adjust them again. No wobble then, it holds its position beautifully. Then, use spacer rings between the guide camera and the OAG mount to find focus, after working out the approximate distance required from the light path to each sensor (imaging camera and guide camera). Obviously, focus is found when the distance is the same.



That is totally expected from an OAG. You are picking off light from the edge of field! The key is, it doesn't matter a jot to guiding. The guide program will find the centroid in the seagull's head, and that will be stable.

[Logieberra]

I’ve got a Astrodon MMOAG. Love it to bits. But the custom adapters! Precise Parts are a stinking rip off...