OAG Cost for C8

Equipment Reflectors Backyard Astrophotography Beginner
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Paul Campbell avatar
Paul Campbell avatar

Hi, looking for some recommendations for an OAG and C8, using a .63x focal reducer.

I was hoping to get the ZWO OAG L and ASI174MM since it seems like this may be the most robust solution in terms of being able find guide stars. But after looking at cost, it’s close to 800 new for the cam and oag. I’ve looked around for used versions and couldn't find anything atm. And even if bought used, cost is a bit better but still pricey. Anyone use a more affordable setup or just keep looking, or is the 800 worth it? I’m getting pretty decent results with my guide scope (240mm fl on an upgraded finder dovetail, yeah I know its bad), but i’m 100% sure things could be improved and I like the idea of shedding some weight and accounting for any mirror flop. Sorry for the millionth oag discussion.

Tony Gondola avatar
Tony Gondola avatar

People will tend to argue in favor of what works for them. Both approaches have pros and cons. Whenever I contemplate when I spend money in this hobby to first question I’ll ask myself is “will I see a difference in my images”. That’s really the bottom line for me. So, to that thought, are you currently having issues that you feel an OAG will solve? It will certainly help with weight and balance.

Are you seeing poor star roundness despite seeing good HFR or FWHM values?

Does the out of roundness or incidence of star trailing seem to get worse over time and does it suddenly appear at various times in a session. All of that would point to differential flexure taking place which is something an OAG would eliminate. If you don’t see those things and feel like star sizes are are what they should be then you wouldn’t benefit much from the upgrade.

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Paul Campbell avatar
Paul Campbell avatar

Tony Gondola · Jun 12, 2026, 04:39 PM

People will tend to argue in favor of what works for them. Both approaches have pros and cons. Whenever I contemplate when I spend money in this hobby to first question I’ll ask myself is “will I see a difference in my images”. That’s really the bottom line for me. So, to that thought, are you currently having issues that you feel an OAG will solve? It will certainly help with weight and balance.

Are you seeing poor star roundness despite seeing good HFR or FWHM values?

Does the out of roundness or incidence of star trailing seem to get worse over time and does it suddenly appear at various times in a session. All of that would point to differential flexure taking place which is something an OAG would eliminate. If you don’t see those things and feel like star sizes are are what they should be then you wouldn’t benefit much from the upgrade.

I have noticed some subs can have oblong stars in the past but hard to say if that’s due to flexture or maybe just guiding errors, which have now improved a lot. It doesn't happen a ton though and can also be fixed with software. I really just don’t like the way my guide scope sits off center of my ota. I was thinking about getting a secondary rail to put on top of the ota but the lightest I could find was from Farpoint at 1.25 pounds, plus some extra hardware to attach my scope. An extra 1.5 pounds-ish is a big penalty to pay to center the scope. I wish I could get a more robust finder scope type bracket that had the right hole spacing so I could center it dead on the ota. But most seem designed for the finder scope screw spacing. I’m almost considering getting one of those orange the wider format rails then just chopping it down.

Tony Gondola avatar
Tony Gondola avatar

What guiding RMS numbers do you generate on a night of good seeing?

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Paul Campbell avatar
Paul Campbell avatar

Tony Gondola · Jun 12, 2026, 05:39 PM

What guiding RMS numbers do you generate on a night of good seeing?

Around .76 to .85 on good nights. Mount is an eq-g. I’m basically really happy when it stays below 1

Tony Gondola avatar
Tony Gondola avatar

That’s a bit high but not out of the typical range seen for the mount. I’m not sure an OAG would improve that as you really seem to be “mount limited”. If I were to put money anywhere, it would be in that area.

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Paul Campbell avatar
Paul Campbell avatar

Tony Gondola · Jun 12, 2026, 06:24 PM

That’s a bit high but not out of the typical range seen for the mount. I’m not sure an OAG would improve that as you really seem to be “mount limited”. If I were to put money anywhere, it would be in that area.

Makes sense…. Definitely some work to do in that regard. I think it is capable of going lower. I’ve seen it get around 5 rms for some time then it sort of drifts up. Lots of balance and software things to try to improve this. I still haven't given phd2's predictive pec a fair shake.

Its extremely tempting to get a belt kit, or maybe just one of those kits with new gears. I’m fairly mechanically inclined but I just don’t know what “good" feels like with these mounts. Not having a frame of reference makes me very nervous that I could make things substantially worse for the benefit of maybe shaving a few arc seconds off my error. If there was still a service I could send my mount off to and get serviced, I would do it in a heartbeat.

Tony Gondola avatar
Tony Gondola avatar

My EQ6-Rpro has belts and it generally runs between 0.6 and 0.4 RMS. Based on that it might be worth doing the conversion. That’s with a 6” f/6 newt and an 80mm guide scope.

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Paul Campbell avatar
Paul Campbell avatar

Definitely tempting. I may just start saving up for something else. Once I get another mount i’ll be a little more comfortable experimenting with my non-daily driver. Couldnt hurt to have two big mounts. Would you recommend the pro? Sounds like the guiding numbers are good.

For my focal length, of around 1337 (after .63x reduction), my pixel scale is at .58. From what i’m reading, I should be guiding from 1/2 to 1/3 that level. But I know most mounts don’t hit that .2 or .3 rms range, and mine sure as heck never will. The demands for precision are astonishing, really.

I did listen to a very interesting talk from the creator of Metaguide and he had some thoughts on “chasing the seeing”. If I understood correctly, he was advocating for setting your mount to be as responsive as possible. Where a lot of resources suggest to use longer exposure lengths to minimize noisey star images, he seemed advocate to make your mount as responsive to change as possible. And eluded that other guiding software wasn’t built for that concept in mind. I’m curious to try it out, but I doubt my mount is tuned enough to react accurately or quickly enough to test that concept.

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Tony Gondola avatar
Tony Gondola avatar

You could try MG, some people use it so it’s worth a try.

The EQ6-Rpro is pretty much a standard for non-encoder mounts. A lot of people use it with good results so I have no reservations about recommending it. Paired with Green Swamp Server, it just works. I’ve gotten good results down to a pixel scale of 0.33”

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Paul Campbell avatar
Paul Campbell avatar

Tony Gondola · Jun 12, 2026, 09:32 PM

You could try MG, some people use it so it’s worth a try.

The EQ6-Rpro is pretty much a standard for non-encoder mounts. A lot of people use it with good results so I have no reservations about recommending it. Paired with Green Swamp Server, it just works. I’ve gotten good results down to a pixel scale of 0.33”

I had never heard of green swamp server! Sounds like it has some interesting features.

Sounds like the pro version is the next step up.

Dave Stirling avatar
Dave Stirling avatar

Paul Campbell · Jun 12, 2026, 10:11 PM

Tony Gondola · Jun 12, 2026, 09:32 PM

You could try MG, some people use it so it’s worth a try.

The EQ6-Rpro is pretty much a standard for non-encoder mounts. A lot of people use it with good results so I have no reservations about recommending it. Paired with Green Swamp Server, it just works. I’ve gotten good results down to a pixel scale of 0.33”

I had never heard of green swamp server! Sounds like it has some interesting features.

Sounds like the pro version is the next step up.

+1 for using GSS with the EQ6-R Pro. Great package.

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Paul Larkin avatar
Paul Larkin avatar

Hi, Paul.

Thinking about your original question. Here is my own experience, which may help (or not).

I have the ZWO OAG-L and ASI174mm (mini), about which you specifically asked, which I use on my C8 (Edge HD 8) as well as my SkyWatcher 120ED refractor.

I keep the OAG>Filter Wheel>Camera train as one assembled unit and just screw the whole lot as a single unit via the front of the OAG to the scope as required. Add spacers as required to achieve overall back focus with/without the reducer.

I originally had the Celestron OAG with my DSLR, but it was way too thick to achieve the correct back focus with the filter wheel and camera setup once I moved to a mono astro camera.

Depending on seeing (and I suspect declination), I can get from 0.3 to 0.7 with the C8 on my EQ6-R mount. I also have an AM5N mount which is so much lighter and easier to set up but I cannot get the same guiding results consistently (at longer focal lengths). I read recently that this is due to the constant ‘periodic error’ induced by the meshing of the teeth in the drive mechanism, so I stick with the EQ6-R for the C8.

So, cost aside this works well for me (the clouds the past few months where I live do not!). As some say, buy once, cry once. Never had a problem finding guide stars with the OAG-L. I don’t expect I will ever need anything else (famous last words).

And to follow a couple of other comments, I moved to the Green Swamp Server (which I discovered by accident) and have so far found it much nicer than EQMOD which I had been using previously.

Hope that helps, given your proposed setup is similar to what I am using, even if only a bit.

Cheers.

Paul

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