Off axis guider or guidescope?

[Nigella Bryant]

Hi all, what's your take on this? Currently I have a 70mm telescope that I could use as a guide scope. Would there be any benefit to getting and using an off axis guider.  I'd be using an altair astro 26c on a C11 @ f6.3 or a 12inch f4 newtonian on the Eq8. 

Thanks for looking. 

[david_taurus83]

Guidescope for refractors, off axis guider for reflectors/mirrors

[fwm891]

If you're thinking of using the same guide camera for each application (not specified) then the 70mm will give you the widest FoV which would be useful in poorer areas of sky in locating a suitable guidstar, plus you can alway align a guide scope to guide on the object being photographed (i.e. a comet head) which you can't with an OAG. An OAG however will be using the same fl as the main image train which should lead to better guiding. Long fl's and OAG's need reasonable star fields to ensure enough guide stars within a FoV.

My2 p'th...

Edited October 7, 2023 by fwm891

[Nigella Bryant]

47 minutes ago, fwm891 said:

If you're thinking of using the same guide camera for each application (not specified) then the 70mm will give you the widest FoV which would be useful in poorer areas of sky in locating a suitable guidstar, plus you can alway align a guide scope to guide on the object being photographed (i.e. a comet head) which you can't with an OAG. An OAG however will be using the same fl as the main image train which should lead to better guiding. Long fl's and OAG's need reasonable star fields to ensure enough guide stars within a FoV.

My2 p'th...

Good point's, thanks for your input. 

[knobby]

Oag will help mitigate any differential flexure and may even deal with mirror movement of you're lucky. Tricky to set up but once done should stay set up.

[ONIKKINEN]

Although i think this is an unpopular opinion, i would recommend an OAG even for a smaller scope or a refractor. Maybe not for a redcat51 or a very small scope like that, but for most scopes i would go OAG and certainly for the 2 scopes you mentioned here an OAG would be my choice.

If you go for an OAG with an included focuser, like the Askar M54 OAG, there is hardly any extra faff or setup required other than figuring out the adapter train for back focus. But after all that is done its no more difficult to use than a guide scope, in fact i think its easier since you wouldn't really have to touch it unless you took the imaging train apart for some reason. No chance of differential flexure ruining an image and you get the best possible guiding performance since the imaging scope will surely outperform any guidescope even if the guide stars are banana shaped (PHD2 guides on bananas and triangles just fine).

So my vote goes for the OAG.

[Nigella Bryant]

2 hours ago, ONIKKINEN said:

Although i think this is an unpopular opinion, i would recommend an OAG even for a smaller scope or a refractor. Maybe not for a redcat51 or a very small scope like that, but for most scopes i would go OAG and certainly for the 2 scopes you mentioned here an OAG would be my choice.

If you go for an OAG with an included focuser, like the Askar M54 OAG, there is hardly any extra faff or setup required other than figuring out the adapter train for back focus. But after all that is done its no more difficult to use than a guide scope, in fact i think its easier since you wouldn't really have to touch it unless you took the imaging train apart for some reason. No chance of differential flexure ruining an image and you get the best possible guiding performance since the imaging scope will surely outperform any guidescope even if the guide stars are banana shaped (PHD2 guides on bananas and triangles just fine).

So my vote goes for the OAG.

Thanks, I was looking at the Askar M54 OAG. 

[Clarkey]

I would agree that theoretically an OAG is the best option. However, I have tried twice on my RC8 and found them to be a real pain. This was using the standard ZWO OAG and I tried the Askar modular one for the larger prism. I live in Bortle 6 which may not help, but I have struggled for guide stars. Also, if you rotate your imaging train to frame a shot you need to recalibrate phd2. For this reason I have stuck with my trusty ST80 for guiding. I don't seem to get too much flex so it's ok.

[AndrewRrrrrr]

i prefer an OAG due to the weight saving and differential flexure thing. the only downside I have with my setup is that to rotate the camera for framing purposes I have to rotate from the rear of the scope itself. this means I need to re-do a phd2 calibration whenever I do this. not too much of a faff but something to remember. 

[tooth_dr]

If using a guidescope then a camera like a 120MM is perfect.  If OAG, then I'd recommended a more sensitive camera.  I use a 290MM for my OAG.  I use a guidescope (just the finderscope with an adapter) with my fast reflector and an OAG with my refractor.  The star shapes with my OAG arent that round but guiding is unaffected.  As said above, once the OAG is set up, there isnt anything else to worry about.  Both work in my experience but with the longer focal length you are suggesting, I'd recommend an OAG and a good sensitive guidecam.

[ONIKKINEN]

1 hour ago, Nigella Bryant said:

Thanks, I was looking at the Askar M54 OAG. 

Its really good for the price. Comes with accessories for both the camera end and telecope end to M42, M48 and M54, and has a sturdy focuser and is sturdy in general. Highly recommended, much better than the other OAG i have, a TS16mm one where if you tally up the price of the accessories will be the same price as the Askar, but the TS one is unstable on the guide camera side.

[AndrewRrrrrr]

and if you're struggling for backfocus, it can be handy to not have to accomodate the OAG. but still prefer the oag for weight minimisation and keeping that weight as near as possible to the centre of rotation - to help with guiding performance. so far so good !!

[Ouroboros]

10 hours ago, Clarkey said:

Also, if you rotate your imaging train to frame a shot you need to recalibrate phd2.

I have only just realised this having just acquired and fitted (though not tested) the Askar M54 OAG.  It hadn’t occurred to me before. I guess you have to make sure you’re happy with framing first, then calibrate. I think I tended to do that anyway. 

[dan_adi]

On 07/10/2023 at 17:49, Nigella Bryant said:

Hi all, what's your take on this? Currently I have a 70mm telescope that I could use as a guide scope. Would there be any benefit to getting and using an off axis guider.  I'd be using an altair astro 26c on a C11 @ f6.3 or a 12inch f4 newtonian on the Eq8. 

Thanks for looking. 

OAG for everything.

[ollypenrice]

I'd only use an OAG if I had to, and that would be with a reflector and the likelihood of mirror flop. (This is a rather extreme term for small amounts of mirror movement.) Given that many very high-end setups at long FL run on direct drive mounts without autoguiding, flexure can't be that much of an issue.

With a C11 I'd err towards an OAG.

On small refractors, I'd consider an OAG to be bonkers, quite honestly. You disturb it every time you do anything to the imaging scope. On my refractor rigs I would say that I never touched my guidescope-guide cam once in ten years, other than to scrape spider webs off the lens - when I remembered. 

Olly

Edited October 11, 2023 by ollypenrice

[Ouroboros]

Well, having just acquired an OAG with my little refractor I’ll let you know whether I find it “bonkers” or not @ollypenrice.  So far so good. It works. It makes a nice compact system I can keep as a unit (without a ‘sticky out’ guidescope) that I can easily pack for travel and carry out to the mount.

I see certain advantages of a guide scope.  They’re easily swapped between different telescope, if you use the finder shoe that is.  They’re useful for visual too.  Teamed with a computer, ASIair or similar they can be used in GOTO mode with plate solving to put you smack bang on the visual target through the main scope. Assuming that is you have the two scopes lined up. I found that a real asset and it seemed to have re-enthused my visual astronomy. You can even acquire some images for  your records.   A sort of visual/EAA hybrid mode. 

[Elp]

Although my Z61 (360mm FL) guides much better with an OAG (achieved 0.6 RMS at one point with an azgti when the GS (200mm FL) RMS is usually 0.8-1.2 when the mount and weather is behaving). Trying to guide the C6 at 1000mm FL with the same GS, is a lesson in pain, but works well with the OAG.

[Nigella Bryant]

I'm reading these posts and I read one, I'm persuaded on an OAG then I read another post and persuaded the guidescope is the way,  I'm like a cork tossed to a fro in a sea of opinions, lol. 

[Elp]

Once you've tried one it's like marmite.

[tomato]

I use an OAG on my Esprit 150 rig (1050mm FL) and a 60mm guide scope with the RASA8/SY135, so I take a horses for courses approach. I use an ASI 120 mini with both, never have a problem with finding guide stars with either method.

I guess this reply doesn’t really help you choose.

[carastro]

I have only ever used a guide scope or finder guider.  I have read too many nightmare threads of problems people have had with OAGs that would put me off that option.  
 

However l do gather that they are necessary for scopes that suffer with mirror flop.   Though l had a 150p and a 130pds reflector for a while using my finderguider and had no guiding problems but it could be more difficult with larger mirrors.   

 

[Padraic M]

I'd agree with most of the posters here that an OAG is the best bet for both of the scopes you mention - both with mirrors, and especially the long f/l.

HOWEVER, I'm writing this with gritted teeth, as in my own personal experience (I'm sure everyone is different!) an OAG is the single piece of astro kit that I have most repeatedly wanted to fling against the wall as I found it to be an absolute pig/beast/dog to set up. I'm trying really hard to pick words to describe the experience that won't get me blocked from the forum!

I was using a C8 XLT which I finally gave up on as it's not really suitable for DSO AP, and the star shapes at the edge were horrible. Guiding was not great at all, but this is likely to be because of the poor optics out where the OAG was trying to operate.

On the plus side, once you do eventually get the back focus correct and the focus correct and the prism depth correct and the sensor angle alignment correct, you can lock everything up and leave it indefinitely. As @ollypenrice said though, you can do that with a guide scope too!

If I ever got to upgrade to an Edge HD I would bite the bullet and strap on an OAG.

[blinky]

OAG would be my choice - I swapped from guide scope ages ago and once focussed (which can be tricky as you may not get round stars, so finding focus is difficult) its lighter, easier to balance, and in theory since its the same focal length as your imaging scope, should give better results

[ollypenrice]

7 hours ago, blinky said:

OAG would be my choice - I swapped from guide scope ages ago and once focussed (which can be tricky as you may not get round stars, so finding focus is difficult) its lighter, easier to balance, and in theory since its the same focal length as your imaging scope, should give better results

Why easier to balance? I think it's the opposite. You can put a guidescope on a sliding dovetail to move it fore and aft for fine tuning in Dec without having to struggle with the main scope.

You can also position the guidescope off-centre as a way of getting dynamic balance right. The usual instructions on how to balance assume a system which is symmetrical in balance side-to-side but, with focus motors etc these days, it won't be. I like being able to use guidescope position as a way of reaching balance.

It can also offer be a way of moving your OTA up or down in the clamshell, in cases where hitting the tripod/ground or hitting the observatory roof is the issue.

Olly

[Nigella Bryant]

4 hours ago, ollypenrice said:

Why easier to balance? I think it's the opposite. You can put a guidescope on a sliding dovetail to move it fore and aft for fine tuning in Dec without having to struggle with the main scope.

You can also position the guidescope off-centre as a way of getting dynamic balance right. The usual instructions on how to balance assume a system which is symmetrical in balance side-to-side but, with focus motors etc these days, it won't be. I like being able to use guidescope position as a way of reaching balance.

It can also offer be a way of moving your OTA up or down in the clamshell, in cases where hitting the tripod/ground or hitting the observatory roof is the issue.

Olly

Thanks Olly, much appreciated.