Off Axis Vs Guidescope

[Eggmoon]

Hi all,

Looking at options for autoguiding... and all the info out there is quite overwhelming.

Now, first things first... I am not looking for "Worlds best practice" or anything like that. I am starting out taking astrophotos, and enjoying it, but I am not going to be spending a fortune upgrading all my gear to get magazine quality images... I just want to be able to get some passable stuff to show to my astro-illiterate friends.

I do want them to be be decent pics, and don't mind putting in the time acquairing and processing them, but I am a married man with three kids and a dog... so not going to be buying high end cameras with my small amount of spare cash, so the need for perfect autoguiding isnt there, and my scope is an 8" SCT, so not going to be looking at the very faintest and smallest objects.

Basically want something that just does the job well enough for my amatuer / enthusiast / dabbler status. Orion, Rosette, Andromeda, Horsehead, things like that are on my target list.

So... an off axis, or a seperate guidescope? Who wins?

Geoff.

[Poita]

If you have a sensitive guide camera, like an STi or Lodestar, then my advice is to go with the OAG.
With a sensitive camera finding a guidestar isn't an issue, and an OAG gets around all the hassle with misalignment and flex between the guidescope and main scope.
It also means less weight on the mount and less complexity all round really.

I only use a guidescope for hyperstar work, otherwise I always us an OAG.

[bojan]

IMHO, neither is really needed with your scope, with camera in the prime focus, for exposures up to 30 sec or so...
However, you will need to align it properly (EQ, not AltAz as I supposed it is now, judging from what I found on google on this model) to avoid field rotation.

[Eggmoon]

Bojan,

Just started using a wedge and trying out polar alignment... still working on getting it right.

I was kind of hopeful that I might get away without an autoguider like that... but just checking options so far... not comitting to either getting or not getting one yet.


Poita,

As I just said to Bojan... no camera yet... apart from my EOS 60D that I use to get images... just doing some research. I assume that different cameras are more suited to the different methods?

[Poita]

Whatever method you use, the more sensitive the camera, the better it works.
A more sensitive camera means a wider selection of stars to guide on and less messing about.

[Merlin66]

IMHO,
looking at your stated aims...
I'd have a small ED refractor mounted piggy back on the C8 with the DSLR to take some OhhhhAhhhh shots. Then you could use the C8 with a webcam to take shots of the sun/moon/planets - no guiding, just tracking....
You can use the C8 as a guide with a cross-wire eyepiece and the Mk I eyball....or find a s/h QHY guide camers.
Keep it simple....

[LewisM]

I have an OAG for sale on the Trader right now :D :D:D

[Eggmoon]

Ahh Lewis...

That leads to the question... if they are any good, why you selling it?

[TechnoViking]

This is the same decision that is plaguing me! OAG or piggy back scope?
With an OAG, (please correct me if im wrong) you can only use a small percentage of the field of view to find a star to guide with?

Where as a piggyback scope you will have a greater number of usable stars to guide with?

Here are 3 videos i watched, that helped explain a lot!

http://www.youtube.com/watch?v=YK0eLznxGh8 <-Piggy Back

http://www.youtube.com/watch?v=VWXhVyKQk88 <-OAG

http://www.youtube.com/watch?v=8TClzFbxwhM <- Very informative !!

[Merlin66]

It's not just the FOV issues....it's the stability and rigidity of the guide system to maintain accurate guiding FOR THE IMAGING system.

[Peter.M]

You are using a SCT there is absolutely no comparison. There shouldn't even be any discussion, If you want to do any longer exposures that you will require for dso imaging you need to use an OAG. SCTs mirrors move to focus, and thus also move or "flop" as the telescope tracks, a guide scope attached to the tube will not pick up this mirror movement and you will end up with stars that are not round. An OAG should be able to compensate for this movement and you should get round stars.

[TechnoViking]

Sorry to hijack your thread Geoff :abduct:

Any brand OAG better than others? bigger prism's etc?

so many to choose from!

[allan gould]

I know I'm going to be swimming against the tide here, but here goes.
I use a 5" refractor, 10" SCT and a 10" GSO RC on a Losmandy mount. All scopes have another Losmandy rail on the top to which I securely add my guidescope fitted with an x/y shifter and DSI II Pro guide camera.
I have a set of rings around the guidescope that CLAMP the OTA of the guidescope securely and tightly. THERE IS NO FLEX IN THIS SYSTEM. If I use rings with three screws to hold the guide OTA there WILL be flexture.
My SCT does not suffer from mirror flop and I have successfully guided it at 2950mm focal length for 40 min subs with round stars. That is not to say that these subs are not full of plane and satellite trails.
I've tried three sets of OAG and I do have a lodestar and found for me they just give unparalleled frustration. Ive never failed to get a GUidestar with the DSI and my 72mm guidescope, NEVER. The x/y shifter helps here and is invaluable and I guess that's why they are not made any more.
I should add that all scopes are fitted with moonlight focusers with stepper motors for autofocus using Sequence Generator Pro.
So there you go

[Tandum]

My experience has been the opposite of allan's. I use a tak fs60 with a QHY5 in solid rings as a guide scope but when I had a GSO RC8 I could not get round stars without an OAG. I initially borrowed an SX camera/wheel/oag combo to prove it was flex in the guiide scope and then bought an orion OAG. It was cheapish and the guide camera bit rotates around to aid you find a star. I had to modify it to remove slop but it worked fine.

I have lent that oag to a couple of very experienced people and both have tossed it out of the system as being too hard to setup and use. I'd recommend you try a guide scope first and hope it does what you want.

[allan gould]

Good one Robin and so I guess that it comes down to what ever works for you is what you should stick to. I have a qsi583 wsg with the built in OAG and never was happy with it either and so I use a guide scope.
It's extremely hard and difficult to get all of the flex out of a guidescope setup, many try and fail. It's extremely hard to find a nice star with an OAG to guide on. But then there are those that have no problems...... The cheapest option is an OAG but get a good one with thin dimensions but a large prism.
Best of luck. Robin is perfectly correct in his assessment as am I.
Allan

[Tandum]

I guess I should add that I've just bought one of these OAGs (http://www.teleskop-express.de/shop/product_info.php/language/en/info/p2751_TS-Giant-Off-Axis-Guider---50mm-clear-aperture---length-10mm---very-stable.html) for a big newt as I didn't want to add the weight of a guide scope. That thing was very close to $500 delivered. It better work ok :)

[g__day]

Large, long focal length SCTs - Off Axis Guiders OAG (or the more up market) or On Axis Guiders (ONAG - beam splitter using a cold mirror - so IR goes to the guide camera and visible light to the main camera) which means the gude camera sees exactly what the man imager sees - so easier to find stars and less coma).

With a separate scope - mirror flex or droop in the SCT becomes impossible to isolate out - so its like a case of horrible differential flexure - running most of your shots.

[allan gould]

Mirror flex in an SCT is really not a problem due to the advent of mirror locks and using a good Crayford focuser such as moonlight etc. I really don't know why people obsess about this as its fairly simple to overcome at least for Meade scope of 10" and above as a DIY fix that costs 50 cents.

[Meru]

Hey Geoff :) sorry for the essay but when I started I always found it better to have more info than less, and this is a very common Q found everywhere so here it goes:

I've gone through quite a few scopes and have tried both. First and foremost that I wish I did properly is was having really good Polar Alignment. Regardless of OAG or Guidescope, a poor PA will give you really bad images and you will end up blaming the equipment (I should know; I did that!). Not all of us have the time or money or 'guts' to leave expensive equipment outside and in permanent observatories (I get scared just thinking about leaving my stuff in my backyard while imaging!). A really simple way of making sure you put your mount in the same spot everytime is to glue washers to the ground. Alistairsam showed me this and it works wonders. Even though my PA is still a bit off, every night I put my mount out and I can easily get 30sec unguided images, which is considered bare minimum to be able to do guided photography. It allows repeatability and once you get really good PA, you dont need to worry about it anymore.

From my experience I found that using a guidescope will only really work up to a Focal Length of 1000mm or so; from there the best bet is OAG. Up to a 1000mm, even a cheapie 400mm guidescope (Like the Orion 80ST) will do a good job with a cheapie QHY5, provided you have good PA. This is how I used to guide my 80ED, 120ED, and 8" Reflector (Infact I bought a $30 webcam, stuck it onto a 1.25 barrel, and that worked just fine to guide, and I never touched the QHY5 again. See the first three images below). I got round stars on most if not all my subs and I loved the simplicity of it. You have access to a much larger variety of stars since your field of view is significantly larger. Plus I'm not sure how much you do photography, but the f/ratio also plays a big role in how well a guidecamera will do. For example, with a guidescope at f/4 or f/5 a QHY5 will be just fine. At f/8, the image will be substantially darker and you will need a more sensitive camera. Alternatively you can increase the number of seconds a guidecamera captures before using that image to guide - but this relies on a good PA once again. If it's poor then your stars will be all smudged and your guidecamera will be unable to produce anything useful. I also never ever had a problem with flexture; maybe I just got lucky lol.

When I got my RC8, I tried using a guidescope with it - and it frustrated me to the point where I almost sold the scope off. I found it impossible to get nice round stars at 10/15min, and when your imaging at f/8 and on top the secondary blocks 50% of light, you can imagine 15min subs really are need on the faint stuff. Agreed that my PA wasn't very good then but I found the whole ordeal a nightmare with. Plus for a focal length of 1.6m, a little 80ST mounted on its back at 0.4m just couldnt cut it. And the idea of adding a 80ED or bigger meant my mount would be working ALOT harder and it wouldnt be very stable. That's when I got lucky enough to borrow a friend's lodestar & OAG unit. It was a bit of a pain to set up, getting all the right distances and adaptors all right. But once I got it working, with the same PA, I started getting perfect round-stars 20min subs (See the fourth B&W pic of NGC253). Even at 1.6m and f/8, I always found a star to guide on, using 1 sec or less intervals on the lodestar. It all just worked, and didnt get in my way of imaging. My webcam and the QHY5 really struggled to get anything however, and illustrates the point about having a sensitive guidecam. Alot of people shy away from buying these $600+ guidecams and then fail at OAG because they can never find stars so it is expensive.

Since then I've returned the lodestar to my friend and am completely convinced that OAG is the way to go. I think it is sage to say that the majority of users will have a similar opinion; low FL, use guidescope. High FL, use OAG. However there are always exceptions and I have seen amazing images taken with the RC8 using just their finderscope as the guidescope, and others using OAG at FLs of 600mm. Since you have a 8" SCT, I would say that OAG is the way to go. Though I recommend that you don't even use the 8" SCT. At those FLs, every single component has to work flawlessly and your PA spot on before any real images will come. Also the equipment will be quite dearer since they have to be more sensitive and of better quality to avoid hassles down the track. I take it from your post count that you are a beginner in astrophotography :) and like me, the learning curve is quite large and insanely steeper when dealing with FLs that SCT bring. You'd be FAR better off learning with a smaller scope; everything is 10^23 times easier :P and there are always great bargins to be found here. And if you don't like it or are struggling, this forum is amazingly helpful and the classifieds section really help us experiment with our equipment.

Anyways sorry for the essay! Hope all this helps. Just choose a path and stick it out. It may work, and if it doesn't, try Plan B :) Clear skies!

[Eggmoon]

Dont apologise.... a great set of answers there.

And thanks everyone.... been a very lively discussion, very enlightening. I'll mull all this over for a while... but yes.... a very good discussion. Very glad I asked the question.

:D

Geoff.

[Poita]

As Meru said, with your scope you would want an OAG and a really sensitive camera and it really just works. So many more variables when using a guidescope and you can chase your tail for months.

But I would also agree that starting out imaging on something short and light like an ED80 is easier.

[Poita]

Just shows how it comes down to what works for you. I found guidescopes on my SCT to be nothing but a hair tearing exercise, but haven't once had trouble with the SX OAG and Lodestar. It always just works first go.
To me the OAG is a lighter more straightforward setup with less variables, but getting 40 minute subs with a guidescope is a great setup.

The moral is, try before you buy if possible and see what makes sense for you, it is obvious from the replies that both systems can work, it comes down to what *you* can get to work :D