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Piggyback VS OAG

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#1 hongxu chen

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Posted 22 August 2019 - 07:06 AM

Hi all,
I am losing my gear’s weight now. Has anyone compared OAG with piggyback for guiding? Will it cause any problem that piggyback won’t?

Edited by hongxu chen, 22 August 2019 - 08:19 PM.


#2 TOMDEY

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Posted 22 August 2019 - 07:25 AM

Yes... Off-Axis-Guiding is far less susceptible to differential flexure problems... so, generally recommended.

 

But, there are other ways to address differential flexure... e.g., this patent, which works fine. We incorporated it into a bunch of imagers for stuff at work. >>> I used it on scopes at home, too, with great results!    Tom

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#3 carolinaskies

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Posted 22 August 2019 - 12:35 PM

Hi all,
I am losing weight for my gear now. Has anyone compared OAG with piggyback for guiding? Will it cause any problem piggyback won’t?

OAG has a couple of limitations most forget to mention. 

1..... The OAG takes up part of the backfocus distance available to an image train.  Depending on your imaging configuration this can make for a difficult time trying to fit in filter wheels, external focusers (SCT/MAK), focal reducers, field flatners, etc.  

2.... an OAG picks off a portion of the field of view.  Non-adjustable OAG prisms and even adjustable OAG prisms DO have difficulty with fields where dim or non-existent guide stars are located.  Further, due to the diminished light reaching the OAG a quality aka 'expensive' guidescope will suck up some of your budget.


While there is this idea of 'differential flexture' being an issue with external guidescopes, the reality is this isn't the main issue most face, it's actually mismatched guidescopes with main OTAs especially with long focal length systems.  Relativistic movement of a guidestar needs to be identified faster than the probable error of the main OTA (optical/imaging syste)/mount accuracy.   If the guidescope/guide camera has too slow a delta change vs motion recorded on the main imaging sensor then data is worthless, stars bloat or trail, and generally the time was wasted.  A well paired external guidescope which records delta change fast enough and stays faster than any change in the main imaging train will be just fine.  

In the old days there were few guidescopes and relatively small guide cameras making the issue of flexure more pronounced with long focal lengths (and ratios).  Also, given the modern techniques of stacking, even with changes in temperature, a well monitored system can be adjusted if either guidescope or mainscope sufferes from temperature differences. 


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#4 TOMDEY

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Posted 22 August 2019 - 01:27 PM

OAG has a couple of limitations most forget to mention. 
1..... The OAG takes up part of the backfocus distance available to an image train.  Depending on your imaging configuration this can make for a difficult time trying to fit in filter wheels, external focusers (SCT/MAK), focal reducers, field flatners, etc.  
2.... an OAG picks off a portion of the field of view.  Non-adjustable OAG prisms and even adjustable OAG prisms DO have difficulty with fields where dim or non-existent guide stars are located.  Further, due to the diminished light reaching the OAG a quality aka 'expensive' guidescope will suck up some of your budget.
While there is this idea of 'differential flexture' being an issue with external guidescopes, the reality is this isn't the main issue most face, it's actually mismatched guidescopes with main OTAs especially with long focal length systems.  Relativistic movement of a guidestar needs to be identified faster than the probable error of the main OTA (optical/imaging syste)/mount accuracy.   If the guidescope/guide camera has too slow a delta change vs motion recorded on the main imaging sensor then data is worthless, stars bloat or trail, and generally the time was wasted.  A well paired external guidescope which records delta change fast enough and stays faster than any change in the main imaging train will be just fine.  
In the old days there were few guidescopes and relatively small guide cameras making the issue of flexure more pronounced with long focal lengths (and ratios).  Also, given the modern techniques of stacking, even with changes in temperature, a well monitored system can be adjusted if either guidescope or mainscope sufferes from temperature differences. 

Good points!

 

Your #2 is (most-ideally) addressed with an OAG that entirely evades the designed/used FOV. Such exist... but are very hard to come by! We designed and built those for stuff at work. Properly done, you wind up with the best of both worlds. Or, conversely, on-axis guiding that beamsplits off the main field... off-band.

 

And, indeed... with the short focal lengths and short exposures that avocationals use now... the flexure doesn't matter --- as much.

 

Nevertheless... it's sure nice to just fire up and use a system that simply doesn't suffer these issues! Then you have the luxury of doing arbitrarily long exposures, when instrumentation and conditions call for that!    Tom



#5 hongxu chen

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Posted 25 August 2019 - 01:00 PM

Thanks for all you guys’ advice!

Edited by hongxu chen, 25 August 2019 - 01:00 PM.


#6 Akwilliams

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Posted 25 August 2019 - 02:26 PM

Just from my own experience, when i first tried OAG it was a pretty bad experience!  I went back to a 60mm guide scope!   I then revisited the OAG and with a bit of effort and time i got it working a treat.  I'd never go back to a guidescope!

 

I think most who switch will agree that from a setup point of view, its the best way to go.   But, its not going to transform your guiding.. In my experinece,  both deliver much the same results.



#7 t-ara-fan

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Posted 26 August 2019 - 11:38 AM

The OAG looks at a star at the edge of the optical path.  Depending on the optics, that "star" might look more like a seagull or an oval.  Which makes finding the true center of the star difficult.

 

With my ED80TCF and Orion flattener, the stars on the edges are not good.  I tested my OAG on that system, not that the short FL needs and OAG, and it wasn't that great.

 

Next up I will try my OAG with my EdgeHD 8 and see if the stars on the edge are in better shape.



#8 MHamburg

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Posted 29 August 2019 - 03:51 PM

So I am using the unfortunately-no-longer-made Taurus Tracker III OAG which incorporates a second port which acts as a view port (flip mirror). With my Edge 11 HD, Lodestar autoguider, and Moonlite SCT focuser I am getting good results. I actually like to leave the guide star slightly out of focus (a bit bloated) as it seems to aid tracking.

Michael

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