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Noob question - Off Axis Guiding


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Hey :)

Complete beginner here!, never used any type of guiding... Not looking to open a can of worms but I’ve a feeling it will!

Off Axis vs Normal guiding...pros and cons

My setup will be a Canon 6d dslr(full frame) with a 70-80mm APO, field flattener and EQ6-pro Mount.

i was intending to buy a generic ‘guiding package’ using an ASI camera and mini scope.

i then read about off axis guiding...
Is this something I should consider vs a normal setup or is there more to it?...just like there seems to be with everything in Astrophotography!ha! 
 

Off topic...

im considering the ASIair Pro and tablet vs using a laptop and something like APT/backyard eos ...if I go with the ASIair I want to future proof/ make sure what I buy works in terms of guiding with it ...if that makes any difference to the off axis guiding question 


Any help would be greatly appreciated as always :)

 

Cheers

Ant 

 

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Leave off axis guiding for another day. For an 80mm scope, use a finder guider or a little guide scope.

No, the light cones into which the OAG dips its prism (so to speak!😁) will probably have different diameters in different scopes at the relevant distance from the objective. Olly

Not necessarily. The pa method used by sharpcap, ekos, and polemaster, needs a field of view of about 1 degree in order for platesolving (near the pole) to work. 

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A "quick" comment, as I don't know too much about it..

But the pro for off axis guiding, as far as I understand, is that since you are guiding from the same light-source (the main mirrors or lense in the telescope), you can get more accurate guiding, as there will be less errors introduced by slack and loose connections. If the connection between the focuser and the telescope slacks a bit, it will move both the camera and the off-axis-guider, basically cancelling that out. Where as if you have a separate guide-scope + camera mounted on the telescope, any slack,"bending" or tugging due to cables, can introduce errors, where the camera will then guide the mount, even though it is not necessary. Giving you eggy stars in your images.
Making sure that all connections are very tight, and that cables aren't dangling, is then a lot more important with a separate guide-scope.

The downside for off-axis, is that the camera will receive a lot less light, than having a dedicated scope. So you will tend to have a harder time, finding bright enough stars for guiding. Having to add another element between the focuser and the main cam, can also introduce problems with reaching focus, or even weight limits.

Anyone feel free to correct me, if I've said something wrong here haha.. :)

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Hi Ant

My impression is that OAG is for obtaining good guiding on long focal lengths, say 1000mm and up.

And it requires a sensitive guide camera too.

Even then I'm sure you will have read that obtaining a suitable guide star can be a trial, involving moving the target off centre and/or rotating the OAG.

OAG is considered the cure for Differential Flexture encountered when using a separate guidescope.

Michael

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You can use a guide scope to polar align with Sharpcap which is a major advantage if you dont have a permanent setup.

OAG is more complicated to setup, and if your scope isn't well corrected across the field then you will be guiding on distored stars. The main advantage is not worrying about differential flexure which becomes more of a problem as focal lengths increase, especially with reflectors.

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On 05/04/2020 at 03:37, AntHart said:

Off Axis

Hi. Most modern guide cameras work fine with oags; you'll always be able to find stars where older less sensitive models struggled. It doesn't matter if the guide star is distorted. Oags give tight stars. Oags weigh less than guide telescopes. Mount the guide camera in a helical focuser and they are just as easy to set up as a guide telescope. Works with all types of telescope, regardless of focal length.

Without the helical focuser they are well nigh impossible which IMHO is from where their bad reputation derives.

Just my oag biased €0.02, but HTH anyway.

Cheers.

Edited by alacant
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  • 4 weeks later...

im going almost the same route as you.  Also choose the asiair.  Also go for oag. The asiair? Maybe because of the easy entry into ap. Maybe because i have to choose between a setup with a laptop or just your phone or tablet. The oag? Probably for having less equipment then the normal way with a guidescope.  
I have followed this hobby for some years without diving into it but for everyone diving into in this era, i think the best way is by automation of many time consuming actions.  As an inexperienced hobbiest i know i have to search for an oag solution that lets me focus the guidecam.  So you probably also have to keep in mind that there’s room for moving your cam or focussing.  And last but not least look for a wide mirror.  To decrease the earlier mentioned issue with not finding a guidestar.  
 

pls let me know what you bought in the end

Edited by Robindonne
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2 hours ago, MattJenko said:

Leave off axis guiding for another day. For an 80mm scope, use a finder guider or a little guide scope.

Exactly so. There is no need for an OAG with an 80mm scope. They are more or less essential with reflectors but my high resolution refractor imaging is done with simple guidescopes.

Olly

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3 hours ago, MattJenko said:

Leave off axis guiding for another day. For an 80mm scope, use a finder guider or a little guide scope.

Hello.  Would you explain why?   Not that i doubt it but i, and maybe we, really dont know why. Lets say i have one setup with maincam and guidecam on an oag. Is it not easier to just swap the whole combo between scopes?

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OAGs are more fiddly to get working, sometimes require adjustments to the imaging train to get guide stars and overall I have found to be more than awkward to use. For the multiple scope use case you mention, I use the same ST80 guidescope and guidecam and put it on a whole variety of other scopes and it just works. There are loads of stars to choose from, I can choose whatever framerate I need and never have to rotate things just to make it work. Astroimaging is hard, and the simpler things are, the more likely it is to result in usable images, and starting out, its all about minimising things that can go wrong. I am not saying OAGs can't work with small refractors, but they are not required, there is no mirror shift to worry about which is the main reason OAGs are used and you can get good guiding with a less complex setup. Be nice to yourself and get things working to the point where you don't have to worry about them.

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11 hours ago, Robindonne said:

Hello.  Would you explain why?   Not that i doubt it but i, and maybe we, really dont know why. Lets say i have one setup with maincam and guidecam on an oag. Is it not easier to just swap the whole combo between scopes?

No, the light cones into which the OAG dips its prism (so to speak!😁) will probably have different diameters in different scopes at the relevant distance from the objective.

Olly

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On 06/04/2020 at 09:32, SamAndrew said:

You can use a guide scope to polar align with Sharpcap which is a major advantage if you dont have a permanent setup.

A bit OT, but I am quite sure that you can do this with any camera connected to any scope. No need to use a guidescope for that.

CS

Sven

 

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22 hours ago, freiform said:

A bit OT, but I am quite sure that you can do this with any camera connected to any scope. No need to use a guidescope for that.

CS

Sven

 

Not necessarily. The pa method used by sharpcap, ekos, and polemaster, needs a field of view of about 1 degree in order for platesolving (near the pole) to work. 

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46 minutes ago, wimvb said:

Not necessarily. The pa method used by sharpcap, ekos, and polemaster, needs a field of view of about 1 degree in order for platesolving (near the pole) to work. 

I used PHD2 for polar alignment without a view of Polaris, though it probably helped that I had a fairly clear view pretty much all the way to the horizon to the east (west would do equally well) and south.

James

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I agree with @wimvb youcan use the Sharpcap PA tool with your guidescope as it will have the required FOV to enable the platesolve routine to work.

While OAGs  aren’t as tricky to set up and use as they once were, I would go with the separate guide scope on your current set up.

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As a total n00b to OAGs myself, I found the setup somewhat fiddly, as I expected, but in the end it wasn't hard. Basically you want to make two distances approximately the same:

  1. At right angles to the main optical axis, from the pickoff prism of the OAG to the guide camera sensor plane
  2. Along the optical axis, from the pickoff's position back to your imaging camera's sensor.

(Yes yes, the hypotenuse of that right triangle won't be identical to the leg length. I said approximately. (-: )

You may need to drum up some spacers in order to get one or the other in the correct range. Once you've got it within 10-20mm, you can set the rig up in daylight, focus the imaging camera, and then adjust the OAG in and out until it focuses. Helical focuser would be sweet but it's really not a requirement; in my setup, it's not even possible.


The distance to whatever daylit object you're focusing on doesn't matter, it doesn't have to be at infinity. In addition to the above-cited advantages, the OAG gives you a cleaner, lighter rig whose lateral center of gravity won't change as much as the mount tracks in RA.

Edited by rickwayne
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10 hours ago, rickwayne said:

As a total n00b to OAGs myself, I found the setup somewhat fiddly, as I expected, but in the end it wasn't hard. Basically you want to make two distances approximately the same:

  1. At right angles to the main optical axis, from the pickoff prism of the OAG to the guide camera sensor plane
  2. Along the optical axis, from the pickoff's position back to your imaging camera's sensor.

(Yes yes, the hypotenuse of that right triangle won't be identical to the leg length. I said approximately. (-: )

You may need to drum up some spacers in order to get one or the other in the correct range. Once you've got it within 10-20mm, you can set the rig up in daylight, focus the imaging camera, and then adjust the OAG in and out until it focuses. Helical focuser would be sweet but it's really not a requirement; in my setup, it's not even possible.


The distance to whatever daylit object you're focusing on doesn't matter, it doesn't have to be at infinity. In addition to the above-cited advantages, the OAG gives you a cleaner, lighter rig whose lateral center of gravity won't change as much as the mount tracks in RA.

Ok. So basically what you are saying is: when you found a perfect sturdy combination of maincam, oag and guidecam, you are able to swap this to other lightcones😉 without having to do much adjustment?

Edited by Robindonne
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As you have a good mount you might find it easier, quicker and cheaper to by a second hand st80. As long as it is attached to your main scope securely you will be good to go.

You can even stick a cheap SW autofocuser on the thing. They are heavier but your mount will cope as I have had my quattro 8 and a st80 strapped to it for guiding.

I keep looking a oag's but cannot fault my st80's and have been using then for a couple of years.

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On 06/05/2020 at 11:20, Robindonne said:

other lightcones😉 without having to do much adjustment

Hi

Yes. Just switch the whole oag-camera assembly from one telescope to the next.

We use 250 f5 : 200  f4 : 150 f8 : 150 f5 : 130 f5  [1] all with the same prism settings with a dslr with no problems. We also get visitors with refractors. Still no issues. You do not need to change the oag focus or prism position. No calculations. No theory. Just practical in the field imaging.

Our zwo120 has its sensor fully illuminated with all the telescopes. Simply change the whole camera-oag assembly from one telescope to the next, focus the main camera and done.

This is based upon in-the-field-hands-on experience.

Cheers and HTH

 

[1] **EDIT: add to that a new hands on test with the same oag/dslr assembly on a 200 f5

Edited by alacant
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1 hour ago, alacant said:

We use 250 f5 : 200  f4 : 150 f8 : 150 f5 : 130 f5 all with the same prism settings with a dslr with no problems.

Sounds good.  So theoretically ,when both cameras are focused on one scope, the combination stays in focus on different types of scopes? What is the thing mentioned earlier in this thread where users say that fl can influence the assembly and therefore needs adjustment? Is it safer to start the calibration of both cameras on a particular  focallength or aperture? Like calibration on an 14” is more likely to be spot on focused on an 8” compared to vice versa?

 

Oh and to ask for some buyersadvice, im thinking of buying the attached item from FLO.  Do you think i will get it working when the asi120 will be directly mounted on the oag when using a dslr as maincam? Or do i need a small helical focuser for the guidecam?   Cams used are asi120mc-s and a eos60d.   If it can be answered then thanks a lot.  

336DBB8C-9EF8-4968-956A-DE4BA355EFE8.png

Edited by Robindonne
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2 hours ago, Robindonne said:

fl can influence the assembly and therefore needs adjustment?

It would be good to know which of their oag-telescope combinations need adjustment. So we can avoid them.

2 hours ago, Robindonne said:

helical focuser

You can focus by moving the guide camera platform on the oag stalk, but a non rotating focuser makes certain you're dead centre of focus. Even if you use it only once it's worth it. Try the former and you'll soon see what I mean;)

Cheers and stay safe.

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3 hours ago, Robindonne said:

asi120mc

You need a 1.25 eyepiece size guide camera for a helical focuser.

The large diameter 120s will need to be focused by trial and error:(

HTH

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4 hours ago, alacant said:

Hi

Yes. Just switch the whole oag-camera assembly from one telescope to the next.

We use 250 f5 : 200  f4 : 150 f8 : 150 f5 : 130 f5 all with the same prism settings with a dslr with no problems. We also get visitors with refractors. Still no issues. You do not need to change the oag focus or prism position.

Our zwo120 has its sensor fully illuminated with all the telescopes. Simply change the whole camera-oag assembly from one telescope to the next, focus the main camera and done.

This is based upon in-the-field-hands-on experience.

Cheers and HTH

That's good. I'm surprised because I didn't find this on the occasion I tried it but maybe that was just a specific case. 

Olly

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3 hours ago, ollypenrice said:

That's good. I'm surprised because I didn't find this on the occasion I tried it but maybe that was just a specific case. 

Olly

I really hope your bad experience was an unfortunate moment.  Because if it works then i’m gonna spend my money on a very good assembly and save on a guidescope guiderings tuberings mountingbars etc etc.
Im still in the need of buying most of probably not needing items. so all beginners including me have to decide which direction to go for the guiding.  
Oag seems to be a good (old)new option.

 

 

 

Edited by Robindonne
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