OAG vs guide scope???

I doubt there will be much of a difference in terms of guiding system resolution and number of stars, although that depends on your guiding camera.

The difference will be mainly physical:
-different distribution of weight (might be good or bad),
-smaller weight (OAG wins)
-lack of flexure (OAG wins)
​​​​​​

The OAG will affect spacing  from flattener and like all things between the scope and sensor will reduce incoming light a little bit especially if you have a large sensor (prism might get slightly in the way) and might bring in a tilt, that last bit being far le ss likely than with a filter wheel though.

I would say if it ain't broken don't fix it.

Cheers,
Dimitris

I prefer using OAG mostly due to the total weight of the setup and the ease of use once it is properly set up. With an OAG i.e. from Starlight Xpress, connected directly on the filter wheel, there is very little weight added to the focuser.

Another advantage with an OAG, is that you don't have to worry about focusing the guide camera, as it will stay on focus as long as the main scope does. 

The guiding quality may improve compared to a guide scope with shorter focal length, but having said that, you will probably reach the perfomance limit before this improvement comes into effect due to seeing, stability of the mount, etc. 

On the other hand, some less intuitive drawbacks with an OAG:
- Guiding during an autofocus routine is not possible.
- The pick-off prism is likely to cause vignetting, especielly if you have a large imaging chip.

Martin

Ruediger:

-lack of flexure (OAG wins)

Please consider, you put significant more weight to the small focuser. Since it is a 80mm I would guess it is not the most stable one. So you may introduce new issues there too. But this is guessing since we have no details about the scope.

CS
Rüdiger

Correct. So tilt is a serious issue too, also focuser "flop".

I have always use a guide scope and have had little to no flexure issues.   You need to maintain a 1 to 10 ratio between the arc/sec/pix between the guide camera and the imaging camera.   Some times that is hard to do with long length large scope, since it would demand a large guide scope.  Therefore OAG can be a real advantage there.  

With a guide scope, once the mount is calibrated to the guide scope/camera, you will not need to recalibrate if you rotate the imaging camera for composition.
I calibrate on the meridian at DEC 0, ( recommended by Rollin Christian of Astro Physics) then there is no need to recalibrate until I change the set up.
I prefer the 60mm finder/guider scopes now sold by several manufactures.  They are wide field and I have never needed to adjust viewing angle to fine a guide star.  My problem is just the opposite, too many stars and some time hard to fine single one with no interference from other stars.  I prefer the AP/Baader Verio 60mm guide scope sold by Astro Physics.  Very sturdy, but also expensive ($570).

However, If I am imaging galaxies I use a Celestron C9.25 with a Starizona F7.5 reducer/flattener (FL 1762mm).  That focal length is too long for the 60mm finder/guiders and will  I get about a 1/14 ratio.  So, I use a WO 66 refractor mounted on substantial rings.  It diffidently adds weight.  In that case a OAG would be some advantage, however rotating the camera is often necessary to best frame a galaxy.  And I don't want to be recalibrating the mount for every object.

One you fine good focus in a guide scope and mark the spot, focus should be simply going to that spot each time.  There should be little shrinkage in the guide scope to cause out of focus.  It will be necessary to adjust that seasonally when temps vary from season to season.  

I think one of the most important factors in guiding is the length of the guide camera exposure.  I use 5 sec as a standard.  You do not want to be chasing seeing with short exposures.  Periotic correction and polar alignment can play an important roll here.  The better the correction and alignment the less guide correction will be necessary.  That allows you to use longer guide exposures and combat seeing.

My personal recommendation would be, why seek changes if you have not encountered a problem.  Use the ZWO guide scope and don't give up on it if guiding isn't great right away.  There are many issues that can cause poor guiding.  If after many tries you conclude flexure is causing the problems, then OAG may be warranted.

Lynn K

I have a Stellarvue SVX080T-3SV on Skywatcher HEQ6-R controlled by ASIair pro.  Initial ZWO guidescope was mounted on a plastic shoe mount… 

After acquiring a ZWO OAG and electronic filter wheel and getting the appropriate back focus…..

My guiding RMS error has gone from 5 to 10 seconds to 0.4 to 0.9 seconds.  So, for me, the OAG experiment yielded an order of magnitude improvement in RMS error.

I limitation of the experiment is that some of the improvement may be secondary to the new multi-star guiding that the ASIair pro has incorporated. So, the gain may not be all the OAG.

Most likely the improvement has been the use of the Multi-Star guiding function. That can be toggled on and off via the "Brain" button. That way you can isolate your recent addition of OAGing and get a more accurate result of OAG vs GS.  I think your improvement is mostly the result of the updated PHD2 software. I have seen marked improvements on all my mounts with this updated software. 

For the hassles of OAG, I recommend solving your GS mounting issues. if needed put a 3pt ring on the finder draw tube.  Use the Guiding Assistant in PHD2 to dial in the software settings.  With the flexibility of not guiding thru the main tube that a GS offers. Plus I hate to play the herd mentality game but you would be hard pressed to  find a fellow APr that uses OAG on anything other than a movable mirror-long F.L. OTA for that reason. 

Another point is speed of setup. Its getting to the point I can setup within a half hour and another 5min polar alignment with the Polemaster and once the camera cools and the sky is dark. The longest thing I have to wait for to start imaging is for the mount to slew,  platesolving the position, autofocus, the PHD2 does a mount calibration on a guides star and I start my sequence. I did not have to spend a moment planning a guide star position or even wondering if I would even have a guide star to use.  Enough issues pop up in a normal imaging session, a GS will help to minimize any issues that prevents time on target with the main camera.

Cheers

OAGs have a number of issues which GSs don't and vice-versa. I always used GS because they are easier to set-up and keep that way and I have used them on anything from 200mm to 2500mm so focal length isn't an issue. Plus you can use them with DSLRs which is kind of difficult with OAGs. And with lens + DSLR it is just no go. The two kits OAGs I have make it hard to focus the guiding camera and if it is heavy (old style CCDs) there isn't just enough room for that. Plus, with FR, FW and adaptors you might well find yourself out of the recommended BF for the combo, so again you are out of luck there. And with bigger sensor there are issues with flats if you need to move the pick-up prism to find a suitable guide star. GSs have none of that faff but they have their own issues, mostly differential flexure. And additional mass and moment of inertia if the GS is a big fella.

I'm new to AP and only have experience with an OAG (on a C9.25), so don't know what using a guide-scope is like, but I can concur with all of the OAG issues mentioned. Vignetting caused by the prism (at least with my full frame DSLR), very few stars to choose from and sometimes I have to rotate the OAG, and therefore also the imaging camera, to even find one (so, framing is what you get), and potentially challenging backfocus resolution. Calculating what you need for an extender length to accommodate the OAG (assuming you have enough BF length to do so with other components) isn't so difficult, finding parts in the right length, right thread standard, sufficient aperture, and in stock, on the other hand....and not cheap!  For the aperture I needed, all of my options – several stock extenders, spacers, & adaptors, a single Precise Parts piece (but not adjustable), or a custom adjustable piece from a local machine shop – were 3x the cost of the OAG! One other thing I ran into, though particular to DSLR’s, and maybe just my model (Canon 5D/IV), is that the shape of the camera body only allows the (ZWO) OAG to be connected in one position relative to the sensor, which is why rotating the OAG to find a star also means rotating the imaging camera as well.

Cheers,
Scott

With the relatively short focal length of an 80 mm refractor there are plenty of stars to guide off of. Even with the small prism of the ZWO OAG and the tiny ZWO 120MM I have had no episodes of "lack of guide stars".

I "had" to move to a OAG when I got my Edge 8HD. In the beginning I found it very difficult but now I would not go back. Even when imaging with the Esprit 100 I am using the OAG.

In my budget configuration based on a 150pds (f/5) I didn't notice any difference between ZWO OAG and my cheap Orion 50mm guidescope.
The primary F/L is 750 mm and guidescope 160mm.

Used the same ASI120MM-mini with both.

I suspect that with short primary focal lengths (200-1200mm) there will be little guiding improvement with OAG.
But I think that with longer focal length (>2000mm) in combination with long exposure times (>300 sek) a OAG is a better choice.