Would i benefit from moving over to an oag?

I follow the 1/4 rule (FL guidescope/ FL telescope ratio greater or equal to 1/4) and never been disappointed with the results (insofar as the guidescope was concerned).

First of all, the focal lengths of guide scope and imaging scope aren't the key metrics here. What maters is relative image resolution. For example, if your sky conditions permit to shoot at 0,2"/pixel but your guide setup resolution is 3" per second, your guiding equipment is most likely no up to the task.

For the question if you need OAG: you should determine if there are any issues. If there are no issues, then I would leave it the way you've got it.

A typical case that makes OAG necessary is that when you have very long exposure times and there's flexure in the system which leads to a changing alignment of the guide scope w.r.t. imaging scope. You'd notice the effect through elongated stars. In my imaging combo, 300 second exposures were fine. 600 seconds started to show elongation and everything above wasn't acceptable. Hence, I switched to an OAG, which is now my standard. It allows me to have 1200 second exposures without issues (unless the guider decides to make crazy stuff).

In addition to the above, the OAG has certainly the advantage that matching the image scales (see example above) is fairly straight forward. However it can have the drawback that the placement of the prism is too far of the optical axis (e.g. if you have a large imaging sensor) so that you cannot gather enough light. This is especially an issue with some reducers (e.g. the reducers on the EdgeHD SCTs). Other issues can be the narrow field of view for long FL scopes (2000mm+) for the guider which can make it difficult finding a proper guide star.

Hope that helps.

Björn

I moved to an OAG for flexture issues a couple of years ago and haven't looked back.  Longest focal length I image with is 1280mm (C8 reduced).  It is an old orange tube C8 and not flat.  The Celestron 0.63x reducer leaves much to be desired and star shapes are terrible for the OAG.  But guiding is fine, last night was about 0.4" to 0.5" RMS.  I have had one target, M13, where I couldn't find any guide stars.  I just switched to another target, didn't try to do anything to "fix" it.  So that is a real problem to consider at longer focal lengths.

andrea tasselli:
So it just differential flexure by another name...

That's how I took it given later reference to "optic differential drift"

I would suggest you to examine your frames very closely, especially those at longer focal length and think about your work process. If you almost always take exposures which are not longer than maybe 120 seconds and you are using focal lengths in the region of about 500-600mm max, then you should be fine with your current system.
If you are heading towards longer focal lengths, you need to be aware that you might most likely need often longer exposure times - like 300sec or even longer. Now - if you guiding delivers perfectly round stars at 300" and longer, then don´t change your winning team. But I expect you to find that your stars are not really perfectly round and then you should consider to move to OAG.
The huge advantage, as stated often before is, that your field of view is coming from the same optical path as your image to be taken with. No differential flexture possible but probably a bit more difficult to find a guiding star in some regions of lower star density. Be careful with OAG when you want to use full frame cameras, for those you need to move the deviating prism far out on the longer image side and that might cause too little light and some distortions of the stars for the OAG - of course all depending on your optics.
Another advantage is - you need one optical system less to be heated in winter to prevent dew to build up on the front lens… a small one, but it is an advantage.
Placing the OAG is a bit tricky sometimes, especially if the BDF is not really convenient - 55mm, which is very often the required BFD can be a pain in the neck if you want to use a mono camera with a filter wheel… here most major manufacturer offer sets which match this rather short distance well - I know them from ZWO, QHY and StarlightExpress. That makes it imo necessary to stick to one brand for imaging/OAG if the BFD is short, their equipment is well matched and meets this requirement, other combinations have the potential to be problematic regarding that short distance.
Anyway, that´s my 2pennies worth….
CS
Georg

I started using an OAG on my RC because of the high FL, and because the main camera, FW, OAG and Guide camera are fixed together so nicely, I now use that setup permanently on my small refractor as well. Also, it works better than the small SVBONY 30mm guide scope I was using before.
Plus, once both cameras are aligned and fixed, autofocussing routines also focus the guide camera in one go.

If you want an OAG, do not take a simple and low cost one. Ask to the people who use one with success.

Well, I use a ZWO OAG with an ONTC 808 and a Lodestar X2. I have also used this set up with a 488mm and 640 mm Astro-Physics refractor and have been doing so for two years now. I would say I have been, in my eyes at least, successful, though of course, others may have a different opinion . Success is subjective, after all!

The Lodestar X2 has a quite small FOV, but I am yet to find an area where the OAG has struggled to pick up stars, virtually always multiple stars for multistar guiding. I am sure if I looked hard enough, such an area exists. If, at some point, I desire to image that area, I'll worry about it then.

OAG performance gains are a no brainer IMO.   I run exclusively OAG in both my scopes.  I currently run two Espirit 100s, but I've also used the same imaging train on an 8" Newtonian at 920mm focal length.   The guiding performance at 920mm with my EQ6 is as low as 0.2 arc second RMS.    Currently with my espirit I hover around 0.4 to 0.5 (they're 550mm)

Another nice benefit is once your OAG is focused to your camera, you can swap scopes all day long and you shouldn't have to refocus it.


Now that doesn't come without it's downsides.   

​​​​​​At longer focal lengths exceeding 1000mm, it will work great, however you may run into issues finding good guide stars.    During galaxy season I've struggled, since star density is much less away from the milky way.  Often times I end up with a single really fat guide star which can hurt performance with a higher HFD. 

With your current setup, I'd run an OAG all day long however.   You'll have the added benefit of better sampling with zero downsides on an Espirit 120.

My most similar setup is my 150/750 Newton that I've used with both a 50/160, 50/200 and ZWO OAG. Main sensor was ASI533MC (3,76um) and guide sensor ASI120MM (3,75um).
i did notice a much better guiding switching from the 160 to 200 guidescope but not as much going from the 200 to to the OAG.

I had some minor issues with not getting enough stars but thats more due to the low level guide sensor.

I did finally go back to the 50/200 on that rig as the weight was placed better. I still use my OAG but only on above 1600mm focal length and with ASI174 guide sensor.

I'm not sure why guiding performance gets quoted when comparing an OAG to a guide scope.  Assuming both adequately sample, they should give the same guide results as this is almost entirely dictated by mount performance (ignoring seeing).  What an OAG eliminates is differential flexure which would only be evident in your actual main camera images, not your guiding RMS.  Plus the OAG may also shed some weight.

 I can guarantee my guiding performance almost doubles with an OAG compared to a guidescope. Guiding at a smaller resolution will give you much more accurate guiding. Compare a star that covers 4 pixels to say one that covers 16 pixels. How much more accurate will your guiding be? Much more. 


As we speak I am imaging with an EQ6 and a 44lb 10" at 2000mm focal length and 44lbs of counterweights. I'm at 35 degrees east. You go show me anyone using a guide scope at 2000mm focal length getting this level of accuracy. 







There's no logical reason not to use OAG in any situation unless you can't because you don't have enough backfocus. It solves differential flexure, better guiding, one less stupid thing sticking off the side causing imbalance. They're cheap, there's tons of options and it's one less thing to blame when you got a ton of other variables in this game.

Matthew Proulx:
There's no logical reason not to use OAG in any situation unless you can't because you don't have enough backfocus. It solves differential flexure, better guiding, one less stupid thing sticking off the side causing imbalance. They're cheap, there's tons of options and it's one less thing to blame when you got a ton of other variables in this game.

Because it is waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay easier/cheaper at least from where I stand. And I have guided with over 2 mt FL. Once the sampling is accurate and the focus sharp and you mechanicals as stiff as needed your guiding is going to be as accurate as you'd need it to be. At least at the small scale where the >90% of us are...