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OAG vs Guide Scope

01 Mar, 2018 04:11
I’m staring this topic to keep my questions and discussions separate. I would like the other thread to be just about the camera options and this one to be a friendly Q&A about the OAG vs Scope option. My scope is an ED80 (600mm focal length) and I’m deciding between the two options. Some questions I have are:

 1. At that focal length, is differential flexure really that much of a concern?
2. I currently use the Orion Field Flattener with my Canon DSLR, what’s the feasibility of continuing to use a field flattener with an OAG?
3. I also switch around between using an H-alpha clip filter, a CLS clip filter, and no filter depending on what and where I’m imaging. The filters always change my focus distance a little bit so would that be a big pain when also having to worry about focusing a guide camera through an OAG?
04 Mar, 2018 21:37
Differential flexure is a concern, but can be addressed with a solid mounting.  Finder shoes aren't good.

Depends on the backfocus of the flattener.  Some are only 55mm, they won't work.  Others are 100+mm, they leave enough space foe an OAG.

You'll need to refocus the guide camera.  Depending on the specific OAG, that may or may not be easy,
05 Mar, 2018 02:04
At 600mm you can probably get by with a guide scope. Like a Borg 50mm with a loadstar.  I have used both and have settled on an OAG for my imager.  I find OAG is much more accurate, flexture can be an issue with a guide scope.   But the fov with an OAG is very small, so there are times when a guide star is tough to find. All said, OAG with the right gear is the way to go.  Even better is On Axis Guiding, but expensive and complex.  Check my system and images, I’m doing fine these days.  Good luck!
05 Mar, 2018 08:19
Hi Thomas,
I used a guiding scope (you can see it in my previous setup images) on my refractor but I had a lot of guiding problems. I bought a 16mm oag (but there are 9mm OAG too), so I have enough backfocus for camera. With oag the autoguide not is perfect but the error is about the half than before. I see a lot of people using a guiding scope, with very good results, but in my case it doesn't work well.
05 Mar, 2018 13:03
Alternatively you might consider an ONAG, bit more expensive but conceptually easier
05 Mar, 2018 14:59
Since about 4 years I'm using a finderscope to guide all my telescopes.The scopes are SW 80/600 ED,  SW 150/750 BD Newtonian 2 pieces parallel mounted,  Meade 10" SCT at 1600mm and 2500mm (with and without reducer) Hier you can see the two newtonians with the finderscope.  Hier the 10" SCT with the finderscope  As example hier a 30 minutes single frame  And hier an image with 2500mm focal length (5min, 6min, and 8min shots)

So my conclusion is that a finderscope guiding works VERY WELL, but there are some people who can not operate there gear properly. :-)

Best regards johny
Edited 05 Mar, 2018 15:03
05 Mar, 2018 21:33
in my opinion OAG you choose in 2 cases
A. Your scope is quite long  and bends a bit under own weight.
B. to save weight.
Pros are light weight and it detects bending of main scope and corrects it
Cons = smal FOV for finding guide star, can be fixed with guide camera with good sensor i think smile
Anyway if Your scope is sturdy enough  and weight allows  to have guide scope - buy guides cope.
If You have one of mentioned cases - OAG. focusing OAG is bit tricky in cold weather (below -15C) but in warm weather - no problems, and it is one time setting, u set it and forget it.  smile
Edited 05 Mar, 2018 21:35
05 Mar, 2018 23:53
smal FOV for finding guide star

Finding guide stars is easy with an On axis Guider.  The XY table allows you to scan the whole field of your imaging scope.
06 Mar, 2018 07:05
The backfocus for the Orion Field Flattener for Short Refractors and the Orion 0.8x Focal Reducer for Refractor Telescopes is 55mm from the seat of the male T thread to the film/sensor plane. With a DSLR you will not be able to use either of these devices with an OAG.

You have not specified your mount. Really high quality mounts can do just as well with a guide scope as with an OAG. For lower quality mounts an OAG will decrease the periodic error more than a guide scope.

The lighter the guide scope + guide camera combination the less differential flexure problems you will have. That is why many people are happy with the results from these three Orion products:
Orion StarShoot AutoGuider Pro & 30mm Ultra Mini Guide Scope
Orion Mini Deluxe Pro AutoGuider Package
Orion Magnificent Mini Deluxe AutoGuider Package

Because you have an Orion ED80 I assume you don't have a mount that costs more than US$1000. An ONAG costs $846.00 and is not a good alternative for a low cost setup. I recommend a mount equal to or better than one of the below three mounts plus one of the above guider kits:
Orion Atlas EQ-G Computerized GoTo Telescope Mount
SkyWatcher EQ6 SynScan
Celestron CGEM II GoTo EQ Mount with Tripod

One other thing, the ED80 has a Crayford focuser. These focusers don't work well with heavy camera loads, they wobble and slip, which is another reason not to use an OAG.
07 Mar, 2018 21:53
With a short focal length a guide scope can work very well.  However, do not underestimate the difficulty of mounting the guide scope rigidly enough.  I use a guide scope of 900mm fl on my 4000mm fl reflector, but only for exposures of less than 10minutes.  Longer than that I get visible trailing at my plate scale of 0.47 arc-sec/pixel.  The great virtue of a guide scope is the potentially larger field of view.  It certainly makes finding a suitable star easier, and doesn't constrain the composition.
31 Mar, 2018 16:07
Thomas, the comments above about OAGis right.  With shorter focal length scopes  (under 1000mm) a separate guide scope works fine, sometimes better than an OAG.  After thinking more about this , just yesterday I changed back to a guide scope for my TV NP 127 , 660mm.  I did this because I already had a nice Borg 55 guide scope and on some objects it’s tough to find a guide star OAG @660mm.  I tested last night and I have many star choices, easy to focus, guiding is perfect.  Be aware that flexture is something to watch out for. My guiding is now under 0.2 as long as I want with my AP Mach 1GTO.
31 Mar, 2018 21:00
I started with a guide scope (refractor 80/400) on top of my 8" RC and had massive problems with differential flexure. I switched to an OAG and the problems where gone. Like others wrote it is more challenging to find a guide star, but it is no big deal. Once you have a proper star, the result is way better than it was with a guiding scope. The object you are looking at is at exactly the same position over hours. You can see (and read) my painful lesson here (start with the original picture and the description, then follow the story until you reach G :-) ):
Differential flexure story

My neighbour is absolutely fine with his 8" newton and a guide scope, so i guess the problems start somewhere at a focal length beyond 1000 mm.

Regarding your 3rd question (finding focus with filters): In the beginning i thougt it will be a problem, but in my (little) experience it is no problem at all. Once you have your setup in focus, just attach your filters between the main scope and the OAG, find focus for your main cam and the guide cam should be in focus too.
06 Apr, 2018 14:51
Thanks for all of the thoughts everyone. I ended up getting a 50mm guide scope that is now screwed into the mounting bar on top of my scope. Now just to figure out PHD2 and get to guiding.
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