OAG considerations

[vlaiv]

Just occurred to me that I have not came across this before online and might be worthy advice for anyone considering OAG guiding.

Many people complained that they found OAG difficult and this might be a part of a reason for that.

It has to do with speed of the scope and selection of the guide camera, as well as construction of OAG. There are cases when OAG setup is operating in "stopped down" mode - which makes finding and using guide stars more difficult.

First thing that we need to consider is size of prism. I have not measured it precisely, but as far as I can remember it is something like 7-8mm. Let's say that we are using F/5 scope with this OAG. Optimum distance for guide camera sensor in this case is only something like 35mm away from prism. If you put your guide sensor at larger distance - let's say 50mm away from prism, and your prism is 7mm - you will be effectively running your OAG at F/7. This is equivalent of stopping down your "guide" aperture.

If you mount your guide sensor even further, let's say 70mm, you get even worse results, you will be operating on F/10 in this case.

Some guide cameras have "large" back focus - for example ASI range has back focus more than 10mm, like 12.5mm or even 17.5mm (some cooled models). This means that in above case you need your "OAG stalk" to be only about 2mm long - and that is often not the case.

What should you be looking at when planing for OAG?

How fast is your scope? Slower scopes have less problems with this because light cone is already narrow so no further stopping down is likely.

How long is your "OAG stalk" and can it be adjusted (mine for example has fixed length prism stalk but T2 thread can be moved up and down - used for focusing)?

Choosing a guide cam with small back focus so that it does not "eat up" distance

Mounting OAG closer to imaging sensor - because two need to be in "sync" for focus distance, you need to mount OAG closer to shorten the distance between prism and sensor. This creates a problem if you are using filters - you need to account for that distance as well.

Hope this is useful to someone.

[newbie alert]

Yes, many of us find it very difficult..I've not managed to achieve focus with my guide camera( skyris) with my sct.. a few friends have achieved focus but only using a lodestar and even then having to adapt with a lathe or shortening the stalk ... I'd not even thought of the prism size..

[Astrobug]

I moved to an oag this year. It is indeed a bit complicate but the oag from zwo helps a lot when you use a zwo guide cam. All the spacers are made to match it. The v2 oag is good for the price. Until now i was lucky to get a guide star on each object. It is a f5 scope but i can understand on slower scopes it can be tricky. The main reason to move was the flexure on the guide scope. It was to big on my 200f5 with the 1600mm.

[Davey-T]

I use Atik OAG with Loadstar on SCT and it works OK as long as there are actually stars to focus on 

Dave

[kirkster501]

I run QHY5 on ATIk OAG with my ATIk 460 and EFW2.   Works fantastically and never failed to find a guide star. I was up and running with 20 minutes of setup time.

At long focal lengths you have to use OAG.

[souls33k3r]

Just recently configured my OAG to use with my EdgeHD 8" scope. At F/7 I expect it to be painful but using the Orion TOAG and QHY5L-IIm camera, I was able to achieve focus during the day. Just waiting for the pier to go in and then I'll see how it behaves at night. 

[Allinthehead]

I just ordered one of these.

https://www.firstlightoptics.com/zwo-accessories/zwo-125-helical-focuser.html

Focus can be a pain with the thumb screw.

[Jkulin]

When I bought my Moravian G2-8300 MkII with 7 position 2" FW and built in OAG I did so because it maintained the 55mm back focus required for my Esprit Field Flatteners, however the Moravian OAG really is a pile of rubbish and proved very difficult to use properly with the Loadestar, in addition the locking nut for the focuser fouled the edge of the Esprit Flattener, in addition they used plastic bolts to secure the Guide Camera into position, when I complained to Moravian they claimed that they designed their equipment to work with their guide cameras and no one else's and really it was tough luck!

So I have just been through a series of purchases including the Orion TOAG, ZWO OAG, Starlight Thin OAG and the QHY Medium OAG.

I couldn't use my Celestron OAG as it is 46mm wide, so was fine on my 10" RC as that doesn't use a flattener or reducer but impossible to use with my Esprits or my Newt as that use a CC with a 55mm back focus.

So I ordered at a robbing price from 365 a spare FW front cover for £137.60, I'm no engineer, but worked out that if I made a 3mm aluminium plate to fit in the recess of the current Moravian OAG and tapped it to bolt an alternative OAG from the back and counter sunk the screw, then I should be able to bolt the OAG using the existing OAG holes to the front of the adapter with a further 6 tapped holes with longer counter sunk screws.

The QHY is beautifully designed, the focuser is smooth, it locks the guide camera nicely and is rock solid, I measure the back focus and I need an extra 6mm spacer, so I would rather have had to add more rather than take off, I just need to try it out once I have finished my current object which if we ever get some clear night will hopefully be soon.

[glowingturnip]

i think I was luckier with my Moravian OAG, I can find bright stars to guide on almost every time with my QHY5Liic (might have to rotate it sometimes to find one).  If I recall correctly, I did have the plastic bolts but I switched them out straight away for some metal spares I had in my box of junk.

If the OAG is at exact focus though, I get bi-lobed star shapes which confuses PHD, it jumps between the two lobes, leaving the same pattern in the subs.  However, defocussed slightly I get a nice round blob and good guiding.

I 3D-printed some shims so I can keep it screwed down and at the correct focus.

[Nikolas74]

To be honest , 2 times i have tried using OAG but i was never satisfied with the guiding results comparing to having a guide scope.

Also it seems always to be unstable and difficult to focus on guide stars.

I wanted to have my setup more clean and light , but from my experience i believe the old fashion way of a guide scope and guide cam is a guaranty !!!! 

And i think soon i will go back to my guiding cope.

They say that OAG works best best in longer focal lengths maybe that is the case....my focal length is no more than 500mm. 

[alacant]

Hi vlaiv, hi everyone

Thanks. Nice post

On 15/07/2019 at 12:39, vlaiv said:

Choosing a guide cam with small back focus

I don't think we have a choice. The guide camera sensor has to be at the same distance [1] from the prisim as the imaging camera sensor,

On 15/07/2019 at 12:39, vlaiv said:

because two need to be in "sync" for focus distance,

[1] Agree. Setup with the filter in place. Focus the imaging camera then focus the oag. In that order.

My oag has a mirror rather than a prism (better/worse?). Using a cheap t7 camera on a 254mm f5 has never been a problem even in the most barren stars fields of galaxy season. The improvement over using a guide telescope is remarkable.

Cheers and clear skies.

[vlaiv]

3 minutes ago, alacant said:

I don't think we have a choice.

I believe you do. There are a couple of components that you can adjust in average OAG setup.

For example - my OAG has mirror stalk that can be pushed in or pulled out. We use it to position prism in relation to sensor. Sensors are rectangular (most of them) so you can choose where to position prism - closer to optical axis, further away, etc ...

You can also put certain spacers in optical train, and you can choose where to put them. If you need to provide exact distance between field flattener / coma corrector - opt to put spacers "before" OAG (so that OAG is closest to sensor).

Sometimes you just don't have an option - like if you use integrated systems where filter wheel and OAG are single unit, or even cameras where you have all things prefabricated to fit together. But in this case you can understand before purchasing such items if you are going to stop down your OAG or not depending on scope you will be using it on.

[alacant]

7 hours ago, vlaiv said:

mirror stalk that can be pushed in or pulled out

Hi. OK. I see what you mean. So yes, plenty of adjustment within which only a -very- small subset thereof will work.

Cheers

[kirkster501]

I concur with jkulin that the Moravian OAG is a piece of garbage.  You cannot adjust how far the prism projects into the light cone, it is tricky to focus and the rubber screws to hold onto the camera a joke.

I have repurposed this camera to be a wide field camera with my Samyang lenses and have removed the OAG. 

I plan on buying a CMOS camera to go on my FSQ.

Edited July 23, 2019 by kirkster501

[MarsG76]

Finding a guide star for OAG guiding might not be easy, but when I plan on imaging a object, I check on Starry Night Pro whether there is a potential guide star candidate within the OAG prism circle.

[Jkulin]

3 hours ago, kirkster501 said:

You cannot adjust how far the prism projects into the light cone

With my MKII I could do that, but it was just the general build quality and design, it was rubbish.

[Datalord]

On 23/07/2019 at 18:49, kirkster501 said:

I concur with jkulin that the Moravian OAG is a piece of garbage. 

+1. It gave me almost the same problems until I bought a lodestar. It is horrible, flimsy and I pray every day nothing will accidentally bump it on my setup, because it can only be focused with extreme care and on a clear night. 

Apart from that, I always have guide stars with SNR above 50

[Jkulin]

Yep I've had enough now so hopefully when it cools down a bit I will switch it over to the QHY.