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Off Axis Guiding... Why doesn't everyone do it?

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#1 Naptown Larry

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Posted 21 October 2021 - 01:13 PM

As a beginner... at first examination, Off Axis Guiding seems the the way to go. No need for a separate guide scope, no concerns about misalignment between your main scope and the guide scope, getting a super sharp wide field of view through your main scope. Just plug your guide camera into your Off Axis adapter and you're off to the races!

So what am I missing? What are the downsides of OAG and why do 99% of the people you see guiding, still use a separate guide scope setup?

Edited by Naptown Larry, 21 October 2021 - 02:29 PM.


#2 kathyastro

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Posted 21 October 2021 - 01:29 PM

There are often spacing issues when trying to squeeze an OAG in between flatteners and cameras.

 

When I was starting out, I was using a DSLR on my imaging Newtonian.  The coma corrector, with its T-adapter allowed only 55mm of back focus for the camera.  Since a DSLR with its T-ring is already 55mm deep, there was no room for an OAG.  (As it turns out, there is one OAG that takes the place of a T-ring, so I could have gotten it to work, but by the time I discovered it, I already had my guide scope.)  The back focus issue is common for coma correctors on Newts and for flatteners on refractors and SCTs.

 

A guide scope is simple.  An OAG requires a lot of fiddling with spacers. 

 

There are definite advantages to using an OAG.  I want to use my C-11 for imaging, and an OAG will work better and more cheaply than a guide scope.  But now I have to start fiddling with spacers.


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#3 Mark Lovik

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Posted 21 October 2021 - 01:31 PM

The answer depends on the telescope systems used.

 

Me -

I perform EAA and use short subs with live stacking.  I try to avoid any guiding because it simplifies the complexity of my system.   Less cables, less software to manage, less fuss, lower power consumption, fewer things to break in the night.  I carefully check that guiding errors are not limiting so they do not impact my final views.  If (or when) guiding becomes a problem, then I will add it.

 

Creating a reliable off axis-guiding train can be difficult in many circumstances (limited back focus limited physical space in the optical train, lack of flexibility to make changes to the optical train).  If off-axis guiding is more difficult than traditional guide scopes --> why do it?  

 

Off axis guiding seems to be the solution when there is a compelling reason to do it.  Off axis guiding is also a choice when other approaches fail or become unnecessarily complicated.

 

So the choice of guiding / guidescope guiding / off-axis guiding is not a directive.  Its looking at what works with the least amount of fuss and trouble for the system (telescope and imaging stuff) being used.


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#4 D_talley

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Posted 21 October 2021 - 01:31 PM

Problems with OAG:

No super sharp wide field view through your main scope. You are picking up stars at the edge of your star field, distorted and elongated in some cases. 

Your pick off mirror is small and limited to the number of bright stars it will see when you are on a target object. You may have to move the camera around until you get a bright enough guide star.

Cost.

Getting focus between the guide camera and main camera to work.

 

I use an On Axis Guider with my Meade 14 which allows my guide camera to use any star in the view of the main scope to be a guide star.  However an OnAG will cost more.


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#5 spereira

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Posted 21 October 2021 - 01:58 PM

Moving to Beginning Deep Sky Imaging.

 

smp


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#6 TOMDEY

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Posted 21 October 2021 - 02:14 PM

Back when film astrophotography was the only game in the Universe, and single Deep Sky telescopic exposures often ran for hours (?!), Off Axis Guiding was pretty much essential to keep differential flexure from screwing things up. And one's Polar Alignment and Drive Systems also had to be superlative... far better than what is needed today with the (relatively) nearly-instantaneous "subs" that are both popular and sensible. Back in the ~1980s I collected over a thousand multi-hour DS images at 1950mm focal length ... with a few hundred "keepers".

 

OAG is still something to seriously consider now-a-days --- but not essential, when you can alternatively merge piles of subs after the fact, and the software/firmware is doing most of the work. If I ever get back into it... I'd undoubtedly buy a complete turn-key system and just pick up the data the nest day... which might explain why I just have lost interest... and returned to my roots --- visual and enhanced visual!    Tom

 

PS: Here's one of those old film images... glad also that I no longer subject myself to that kinda torture!    Tom

Attached Thumbnails

  • 49 Toms Cocoon onto TP 2415 circa 1980 Astrola.jpg

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#7 rgsalinger

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Posted 21 October 2021 - 02:16 PM

I think that the main impediment with OAG's is that they don't come with setup instructions. Even if you buy the OAG/wheel/main camera and guide camera from the same vendor it's not obvious what you have to do. Without detailed instructions people have no idea how to properly position one, how to handle flattener/reducer constraints, and how to test focus one. I was fortunate in that the OAG I bought 11 years ago did, in fact, come with some instructions about how to set it up. 

 

The other big thing is that the difference between in focus with stars and out of focus with no stars is very small. That leads to frustration. The final nail in the coffin is that the combination of no instructions and tight focus (which get worse with fast scopes) means that it's easy to try to set one up that will never work without additional space between the OAG and the main camera. The tendency is to just bolt/screw it all togther and this rarely works.

 

Every one of my refractors required a spacer between the main camera and the guide camera to get good focus, IF I wanted to position the OAG prism just above the long side of the main camera chip. Every one. 

 

Rgrds-Ross


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#8 unimatrix0

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Posted 21 October 2021 - 02:37 PM

It can be a challenge already to have the required backspacing , people with filter wheels, filter sliders, rotators, and then stick spacers between to get that 55 mm or whatever the flattener wants. Adding an OAG maybe not even possible unless something else need to come off  or adding an OAG starts the guessing and measuring game all over to get back to the spacing. 

 

Nowadays a guide scope can be as small as a small flashlight, almost the size of a black Sharpie marker.   Both a guide scope or an OAG will need another camera and wire and some people also like to swap their telescopes around or swap their cameras on the telescope. It's easier just to stick a guide scope on a telescope than mess with with an off axis guider. 

 

Also, there are no concerns about misalignment between the main scope and the guide scope. They don't have to be aligned, not sure where you got that from. 


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#9 t-ara-fan

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Posted 21 October 2021 - 02:39 PM

I think that the main impediment with OAG's is that they don't come with setup instructions. Even if you buy the OAG/wheel/main camera and guide camera from the same vendor it's not obvious what you have to do. 

The ZWO website has some pretty detailed instructions about how to put together a main camera, optional filter wheel, OAG, and guide camera.  Using a ZWO mini guide camera pretty much ensures you can get focus.  When I put all the stuff together it worked the first time. All I did was twist the helical focuser on the OAG a quarter turn and I was done. Really easy. But I imagine mixing parts from different vendors ==> nightmare. 

 

Achieving focus is a little tricky at night. If you do it in the daytime looking at a distant object it is easy. And probably also with the moon. I was concerned that changing focus for different filters would change the focus of the guide camera on the OAG. However it must be by such a small amount that I have had no problems.

 

I had heard about problems finding a guide star, with people actually having to rotate the OAG and main camera to find one.  I have not had that problem.  See the pic below. This is with my refractor at 1,000mm FL, ZWO OAG-L, ASI174MM mini guide camera, 5 second exposures, Bortle 2-ish skies. LOTS of guide stars, I can do multi-star guiding no problem.

 

2021-09-13 OAG has lots of stars.jpg


Edited by t-ara-fan, 21 October 2021 - 02:46 PM.

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#10 T~Stew

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Posted 21 October 2021 - 02:41 PM

As a beginner... at first examination, Off Axis Guiding seems the the way to go. No need for a separate guide scope, no concerns about misalignment between your main scope and the guide scope, getting a super sharp wide field of view through your main scope. Just plug your guide camera into your Off Axis adapter and you're off to the races!

So what am I missing? What are the downsides of OAG and why do 99% of the people you see guiding, still use a separate guide scope setup?

There are pros and cons to both, neither will be ideal for everyone in all situations. A deal breaker for me, is that an OAG won't work for any of my imaging setups. I have enjoyed imaging and guiding for almost a year now, with 5 lenses from a Rokinon 135mm to the big 500mm f/4 Canon, and 4 different cameras. None can be easily adapted to OAG, but my 30mm miniguidescope mounted to an arca quick release clamp easily clamps onto any of my setups with a single knob.


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#11 duck

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Posted 21 October 2021 - 02:42 PM

I have a SBIG STV that I've used > 15 years in conjunction with a 5" f/15 D&G refractor, guiding an 18 1/2" f/4.25 Newtonian.  Differential flexure limits my subs to 4 minutes in a lot of orientations.  Can go longer if the scope is near vertical.  Started out guiding manually.  It's tedious, for sure.  An OAG won't fit in the short space between the tube and the focal point.  Finding a guide star 8 feet off the ground (fork mounted puts the Cass position feet flat on the floor) would be impossible.  The separate guide scope is easily offset to a bright guide star located with a 32mm Plossl.


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#12 rgsalinger

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Posted 21 October 2021 - 02:54 PM

The ZWO OAG manual that I have (is there a new one) doesn't mention the need to properly position the OAG just above the long end of the main camera chip. That OAG allows the user to rotate it to achieve that, but it's not mentioned. Second, the idea of moving the prism up and down is a last resort, not the right way to approach getting an OAG working as the manual suggests. 

 

It fails to mention the limited depth of field or make a suggest about how much to move the prism (still a bad idea) each time. (I can't have the right manual but that's the one that I got with my old and still in use ZWO OAG. 

 

Finally, bear in mind that field of view area is cut 1/4th by a doubling of focal length. So, with a lot of people using 2 meter or longer SCT's, it's imperative to get a big chip and a big prism. Again, not mentioned in any OAG instruction book I've ever read.


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#13 TelescopeGreg

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Posted 21 October 2021 - 03:31 PM

One item not mentioned yet is that adding an OAG to a scope adds a substantial weight to the imaging train.  The weight itself isn't so much the issue as the position, which is typically out at the tail.  That will negatively affect Dec guiding, and depending on your mount, might be enough to be a problem.  A guide scope, on the other hand can weigh a bit less, but more importantly can be positioned in the center of the OTA (above the rotation axis), minimizing the effect on guiding.  A guide scope mounted there will affect RA guiding, but that can be minimized by mounting it close to the OTA (i.e., not up on stilts).


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#14 scadvice

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Posted 21 October 2021 - 03:58 PM

I agree with rgsalinger's comments. A fixed prism design eliminates a huge number of problems attempting to align both the cameras to focus.

 

Two fixed rules that people seem to miss when setting up a OAG is that the back focus is a fixed reference number and you must first focus the imaging camera before the guide camera... lock the focuser down...recheck that  the imager is still in focus after the lock down...then focus the guide camera. About half the time they cannot get the guide camera to focus because they don't have enough in or out travel then start moving things around forgetting the rules. The imager must always be in the focus position first.

 

Also, I've had a number of people when calculating the back focus not realize that light traveling through the glass of the prism adds length to the back focus, about 33% of the glasses thickness. Example: A 8mm tall prism adds 2.4mm to the guider back focus. Why is this important? If your reference back focus is 55mm then the guide camera's is 57.4mm. Another reason you only focus with the imager first.

 

Later, back focus fine adjustments of the reference back focus length should only be done between the flattener and the prism. once the final imager focus is secured then readjustment of the guider focus needs to be done.

 

Understanding these points when using a OAG system makes planning and using one much simpler as your less inclined to make basic mistakes.


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#15 dx_ron

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Posted 21 October 2021 - 07:45 PM

Good points. I just picked up a used TOAG and figured out that with my flattener (55mm backfocus requirement) I needed the shortest M48-M42 I had (~6.5mm), then the TOAG then spacers to the main camera. Even at just 6.5mm behind the flattener the ASI120mm mini was just barely hanging on to the guider platform at its focus. That's when I went back to the box for the 120mm that I bought used last Fall and realized it had a 1/4" extension ring in it waytogo.gif


Edited by dx_ron, 21 October 2021 - 07:45 PM.

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#16 DirtyRod

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Posted 21 October 2021 - 08:06 PM

I started off with an OAG because of the long focal length of my scope. Had issues with the ASI120mm pick8ng up enough stars and focus so I went to a 60mm guide scope. 
 

The guide scope was dirt simple, provided good guiding, and was always in focus 

 

i went back to the OAG but it required an upgraded guide camera with a larger sensor. It’s lighter than the guide scope setup by 2 lbs and lowers the center of gravity but it has a much smaller FOV. Guiding is good but it does require rechecking focus every time as well as adding to the complexity of the imaging train.


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#17 ChrisWhite

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Posted 21 October 2021 - 08:34 PM

It's a shame, as using an oag would eliminate some other issues that beginners get stuck on, and they are really not that complicated. Just not a lot of "how to" from tge manufacturers on how to use. Zwo does a decent job with their connection diagrams though. I recommend you jump into it. I did and it wasn't so bad.
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#18 OldManSky

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Posted 21 October 2021 - 08:35 PM

As a beginner... at first examination, Off Axis Guiding seems the the way to go. No need for a separate guide scope, no concerns about misalignment between your main scope and the guide scope, getting a super sharp wide field of view through your main scope. Just plug your guide camera into your Off Axis adapter and you're off to the races!

So what am I missing? What are the downsides of OAG and why do 99% of the people you see guiding, still use a separate guide scope setup?

I already have the separate guidescope. I have no concerns about misalignment between the main scope and guide scope — they’re not exactly aligned and I don’t care. I also get super sharp wide fields of view in my main scopes. I just plug my guide camera into my guide scope and go. I don’t have to worry about spacing to match focus, or how much focus changes in the main scope with filters, because that won’t affect my guide scope. I have a nice wide, bright fov in my guide scope, and don’t have to hunt for guide stars or worry about the pickoff prism position. I can move my guide scope to any of my scopes in 5 seconds without disturbing any imaging train.

:)


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#19 bobzeq25

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Posted 21 October 2021 - 11:23 PM

As a beginner... at first examination, Off Axis Guiding seems the the way to go. No need for a separate guide scope, no concerns about misalignment between your main scope and the guide scope, getting a super sharp wide field of view through your main scope. Just plug your guide camera into your Off Axis adapter and you're off to the races!

So what am I missing? What are the downsides of OAG and why do 99% of the people you see guiding, still use a separate guide scope setup?

Guidescope is simpler/cheaper.  You do not need to have the guidescope aligned with the main scope, you just need to have it solidly mounted, so the alignment/misalignment does not change.

 

It works, except for:  long mainscope focal lengths.  Mainscopes with floppy mirrors, like SCTs.

 

That's why most people use a guidescope.  When you see the large majority of imagers doing something, the default position is that they know what they're doing.  Not that they're ignorant or dumb or lazy or cheap.

 

I use both, the OAG with my 130/910.  At that focal length, OAG is (slightly) better.

 

One downside of the OAG is avoidable, but has tripped some people up.  There are combinations of scope/reducer/OAG/camera that simply won't work, you can't get the backfocus of the reducer right.  Take care to avoid that.

 

The main downside of the guidescope is flexure, which you avoid by solidly mounting the guidescope.  A finder shoe is bad.  Two spaced rings is good.  Triangulates the load.  What keeps houses up.  <smile>


Edited by bobzeq25, 21 October 2021 - 11:29 PM.

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