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The effects of aperture on guiding

Acquisition Guiding Seeing Conditions Deep Sky
Tony GondolaKyle GoodwinMédéric Hébert
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Did your guiding improve when you switched from a dedicated guide scope to OAG?
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Tony Gondola avatar
Tony Gondola avatar
I'm putting this out there as a topic of discussion as apposed to "what works for you".
It's known that seeing effects vary with aperture size. The basic effect is that if you were to observe the airy disk in a small aperture scope, say 60mm. The disk would tend to stay well defined. The main effect of seeing is xy motion of the well defined disk. This is because apertures under 100mm or so will tend to be looking through one seeing cell at a time. The same star viewed in a large aperture system would tend to be stable in position but with the light broken up into speckles resulting in a bloated ball of light.
Since guiding software works by calculating a centroid for the guide star it seems to me that it would have more of a tendency to "chase the seeing" with a small aperture guide scope system verses a large aperture one. I suspect that this may be the reason that people with larger apertures tend to report improved guiding when going from a small aperture dedicated guide scope to OAG.

Since tiny aperture guide scopes are used my many, it seems, worthwhile to explore this idea a little deeper. What do you think?
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Tony Gondola avatar
Tony Gondola avatar
mmmmm, I'm surprised no one has a comment in this.
AstroShed avatar
AstroShed avatar
Ok, I will start, when using an OAG on a larger aperture scope, it also takes any differential flexure out of the equation, which can help a lot with guiding, this is why most people swap to them. so in this case the guide camera sees the error at the same time as the main imaging scope.
Oskari Nikkinen avatar
Oskari Nikkinen avatar
I did notice an improvement going from a 60mm achromat guidescope to an OAG (8" newt). I dont subscribe to the idea that bigger scopes are more prone to seeing issues, i think its just that a bigger scope is more capable of measuring the effects of that poor seeing.

I have had issues with chasing the seeing even with multi-star guiding, typically with at least 5 stars in use. In these cases the seeing is so bad that good data is not coming through no matter what. In those cases i bump up the exposure from my usual 3s to something like 4 or 5.

Important to note that the guide graph might not be as pretty with an OAG, so a naive quick glance might lead one to believe that guiding got worse. In reality that is just the bigger aperture being able to measure the bad seeing far more accurately than some cheapo achromat suffering from every optical aberration imaginable. At the end of the day its not the guide graph that matters, but the actual data itself. Which should be better with an OAG, if only by a barely measurable amount.

I greatly prefer my OAG to the guidescope for other reasons too, such as ease of use and reliability. No extra dew straps required, always in best focus, more compact to carry and less setup required as it is attached to the imaging train. Just easier and better, hardly any downsides.
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Tony Gondola avatar
Tony Gondola avatar
My largest aperture is 150mm so I really can't do the experiment but it would be interesting to just make a small mask, off axis if it's an obstructed system. Make it say 60mm in diameter. Grab the guiding and FWHM data, Repeat with the full aperture and see if there's a difference. I would guess that you'd have to repeat this a few times because of long term seeing variations but it might be interesting if a trend can be seen in the data.
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David Russell avatar
David Russell avatar
Hi Tony.

my first guidescope was a 30mm ZWO. that got replaced with a 50mm Skywatcher ED Evoguide. that was an improvement.

recently I started using a Stellarvue 80mm Doublet as a guidescope for my 10 inch RC which is 1484mm after reduction.

it works well and I see no need to move to an OAG despite many people saying I would need to do so.

the images have been good too.
Tony Gondola avatar
Tony Gondola avatar
David Russell:
Hi Tony.

my first guidescope was a 30mm ZWO. that got replaced with a 50mm Skywatcher ED Evoguide. that was an improvement.

recently I started using a Stellarvue 80mm Doublet as a guidescope for my 10 inch RC which is 1484mm after reduction.

it works well and I see no need to move to an OAG despite many people saying I would need to do so.

the images have been good too.

I don't think I would go with OAG unless I had issues that going that way will solve. Like you, I've found 80mmx400mm to be a good guiding combination up to 2000mm. It just works and never has a problem with finding guide stars after a flip or if i change targets in the middle of the night.
Tony Gondola avatar
Tony Gondola avatar
Since I first posted this question I've had opportunities to compare guiding accuracy with 3 OTAs, 80mm F/5, 90mm F/10, 150mm F/6 and 200mm F/10. Bottom line is that with PHD2 at least, there wasn't a noticeable difference. It's clear that the seeing and performance of the mount really swamps any differences that come from the way different apertures respond to seeing effects.
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Médéric Hébert avatar
Médéric Hébert avatar
Good to know I'm not missing out using a guide scope instead of an OAG since I can't fit one in my imaging train.
Darryl Ackerman avatar
Darryl Ackerman avatar
We shouldn't be focusing on the whether a guide scope vs OAG offers an improvement in guiding numbers. We should focus on the quality of the image when making the comparison. I can guide my Mach2 with a 50mm guide scope or an OAG on my SVX130T and get .25" RMS with average seeing from both.

Using the OAG, we eliminate differential flexure for one and guiding is close to or at the same scale as the imaging camera. Using an OAG, the guide camera can correct for the same star movement that the imaging camera sees because it's as close to or at the same scale, and most, if not all differential flexure is eliminated.

The benefit is sharper/clearer, better overall quality of your images. 

To say it another way...

If the imaging scale is far off, say a 200mm guide scope on a 2000mm imaging scope, the scale is 1:10, sure the guiding may be great, from the guide cameras point of view... but that's not the same point of view as the imaging camera. 

You would want to test the theory on a night with average seeing, use the guide scope for a while, then use the OAG. Compare the eccentricity and FWHM and unless the conditions changed drastically, and the guiding numbers are similar, you'll most likely see the images taken while using the OAG yield better results. I'd use 300" or more subs to really get good enough data to compare.

remember the 1:5 rule. The imaging scale for guiding should be no greater than 1:5.  This site does the math https://astronomy.tools/calculators/guidescope_suitability

The reality of it all is... Seeing is the limiting factor for guiding. You can still get good results using a guide scope, I'm not saying you can't, but when people want to really up their game and get the best quality images they can, they go OAG.

Darryl
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Médéric Hébert avatar
Médéric Hébert avatar
The image scale between my guider and my camera is currently 1:2.39, which probably helps a lot. And so far I haven't seen flexture on my setup.
With an average HFR in the 1.8 - 2.1 range, I think I'll be fine without an OAG.
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Tony Gondola avatar
Tony Gondola avatar
Darryl Ackerman:
We shouldn't be focusing on the whether a guide scope vs OAG offers an improvement in guiding numbers. We should focus on the quality of the image when making the comparison. I can guide my Mach2 with a 50mm guide scope or an OAG on my SVX130T and get .25" RMS with average seeing from both.

Using the OAG, we eliminate differential flexure for one and guiding is close to or at the same scale as the imaging camera. Using an OAG, the guide camera can correct for the same star movement that the imaging camera sees because it's as close to or at the same scale, and most, if not all differential flexure is eliminated.

The benefit is sharper/clearer, better overall quality of your images. 

To say it another way...

If the imaging scale is far off, say a 200mm guide scope on a 2000mm imaging scope, the scale is 1:10, sure the guiding may be great, from the guide cameras point of view... but that's not the same point of view as the imaging camera. 

You would want to test the theory on a night with average seeing, use the guide scope for a while, then use the OAG. Compare the eccentricity and FWHM and unless the conditions changed drastically, and the guiding numbers are similar, you'll most likely see the images taken while using the OAG yield better results. I'd use 300" or more subs to really get good enough data to compare.

remember the 1:5 rule. The imaging scale for guiding should be no greater than 1:5.  This site does the math https://astronomy.tools/calculators/guidescope_suitability

The reality of it all is... Seeing is the limiting factor for guiding. You can still get good results using a guide scope, I'm not saying you can't, but when people want to really up their game and get the best quality images they can, they go OAG.

Darryl

Another part of that is how you image. Mainly I'm thinking in terms of differential flexure. I think it exists to one degree or another in rigs with separate guide scopes, no argument there. The thing is, unless you have something really loose it's going to be a very gradual change in pointing between the scopes. Back in the day when a single exposure was hours long DF would absolutely destroy you if the rig wasn't built like a tank. Fast forward to CCD imaging it could still bite you but and things were better. These days, integration times can be so short that I doubt that DF is really the problem that it used to be.

Anyway, the original question was if the way seeing effects the image in different apertures has an effect on guiding accuracy, we've not really talked much about OAG which is really another subject entirely. I do think you hit the nail on the head about seeing, it really rules over just about everything.
Tony Gondola avatar
Tony Gondola avatar
Médéric Hébert:
The image scale between my guider and my camera is currently 1:2.39, which probably helps a lot. And so far I haven't seen flexture on my setup.
With an average HFR in the 1.8 - 2.1 range, I think I'll be fine without an OAG.

I'm at 1:2.25 and that's worked well but then I tend to do very short subs, usually 15" to 60". The only time I have had a problem was when imaging too close to a full moon and it wiped out the S/N ratio in the guider.
Médéric Hébert avatar
Médéric Hébert avatar
Tony Gondola:
Médéric Hébert:
The image scale between my guider and my camera is currently 1:2.39, which probably helps a lot. And so far I haven't seen flexture on my setup.
With an average HFR in the 1.8 - 2.1 range, I think I'll be fine without an OAG.

I'm at 1:2.25 and that's worked well but then I tend to do very short subs, usually 15" to 60". The only time I have had a problem was when imaging too close to a full moon and it wiped out the S/N ratio in the guider.

Have you tried longer sub? I'm often doing 180'' to 300'' (I've managed to do 900'' when seeing was great). So unless you saturate at 60'', I think you could do longer
Jung avatar
Jung avatar
With a small 71mm and 91mm refactor upgrading to a OAG did not make a noticeable difference.  Upgrading my guide camera from a ZWO ASI120MM-MINI to a ToupTek G3M678M was an improvement and adding an IR-pass filter to it was yet another improvement.
David Russell avatar
David Russell avatar
I shoot 120 second subs, and guide exposures are generally 4.5 seconds. the imaging RC is 1484mm focal and the guidescope is 560mm focal, so about 1 to 3 ratio.

I have had no major issues shooting 180 second subs, but the loss ratio is higher due to sate-lights so I keep my exposures shorter to reduce the loses.
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Tim Hawkes avatar
Tim Hawkes avatar
Tony Gondola:
Since I first posted this question I've had opportunities to compare guiding accuracy with 3 OTAs, 80mm F/5, 90mm F/10, 150mm F/6 and 200mm F/10. Bottom line is that with PHD2 at least, there wasn't a noticeable difference. It's clear that the seeing and performance of the mount really swamps any differences that come from the way different apertures respond to seeing effects.

I wonder also whether guiding using the multistar option in PHD2 averages things out?
Jim Raskett avatar
Jim Raskett avatar
I was having increasing issues guiding with my guide scope and seeing. One problem was that I was using too high of an imaging scale (>6) and even more troublesome was my atrocious seeing and lp which had been getting much worse the past few years.
I posted my guide logs to the PHD guiding group forum and Bruce responded that I need to half my guider image scale, get a better guide camera (I was using an Orion SSAG) or for the best scenario, go with an oag/better camera.

I use low focal length imaging scopes, a 61mm and 102mm, and never thought that an oag would be the way to go. I really didn't feel like messing around with an oag, but I took the jump and went with it. I also upped my guide camera to an ASI220 mm mini (great camera). 
Not at all troublesome to set up.

My guiding definately improved. I strugged some nights to get under 1 arcsec/px total rms error. with the guide scopes. Now, I routinely run in the 0.6-0.7 range with often as low as 0.5 average over the entire imaging session.

After installing the oag, which is a standard ZWO model, I really like how slick the setup is and not having the guide scope/camera/cable hanging off of the scope.
When I swap scopes, I remove the whole assembly (camera, filter drawer, oag) as a unit and have had no issues swapping.

I don't ever plan to go back to a guide scope with my current scopes. I do have a Samyang 135F2 that I will set up for imaging some day and I will use my ZWO 30 mm guide scope and 120mm mini with that.
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Tony Gondola avatar
Tony Gondola avatar
Médéric Hébert:
Tony Gondola:
Médéric Hébert:
The image scale between my guider and my camera is currently 1:2.39, which probably helps a lot. And so far I haven't seen flexture on my setup.
With an average HFR in the 1.8 - 2.1 range, I think I'll be fine without an OAG.

I'm at 1:2.25 and that's worked well but then I tend to do very short subs, usually 15" to 60". The only time I have had a problem was when imaging too close to a full moon and it wiped out the S/N ratio in the guider.

Have you tried longer sub? I'm often doing 180'' to 300'' (I've managed to do 900'' when seeing was great). So unless you saturate at 60'', I think you could do longer

I've been getting good results with short subs, tighter stars, less clipping and the ability to take advantage of seeing changes on short time scales. The small 585 chip I use is very unforgiving so it pays to be very picky about sub quality. The upside of that is it's no problem to calibrate and stack large numbers of subs. A typical session will result in anywhere between 1200 to 1600 subs. That stacks in siril in about 30 min. so processing time is a non-issue. Lastly, I'm in Bortel-8 and if I'm shooting broadband targets the shot noise kills me if I'm not keeping the exposure time short.
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Tony Gondola avatar
Tony Gondola avatar
Jung:
With a small 71mm and 91mm refactor upgrading to a OAG did not make a noticeable difference.  Upgrading my guide camera from a ZWO ASI120MM-MINI to a ToupTek G3M678M was an improvement and adding an IR-pass filter to it was yet another improvement.

I've found the same thing with the use of IR pass or 25A red for guiding.
Tony Gondola avatar
Tony Gondola avatar
Tim Hawkes:
Tony Gondola:
Since I first posted this question I've had opportunities to compare guiding accuracy with 3 OTAs, 80mm F/5, 90mm F/10, 150mm F/6 and 200mm F/10. Bottom line is that with PHD2 at least, there wasn't a noticeable difference. It's clear that the seeing and performance of the mount really swamps any differences that come from the way different apertures respond to seeing effects.

I wonder also whether guiding using the multistar option in PHD2 averages things out?

That might very well be, all of my testing has been with using multi-star.
Tony Gondola avatar
Tony Gondola avatar
Jim Raskett:
I was having increasing issues guiding with my guide scope and seeing. One problem was that I was using too high of an imaging scale (>6) and even more troublesome was my atrocious seeing and lp which had been getting much worse the past few years.
I posted my guide logs to the PHD guiding group forum and Bruce responded that I need to half my guider image scale, get a better guide camera (I was using an Orion SSAG) or for the best scenario, go with an oag/better camera.

I use low focal length imaging scopes, a 61mm and 102mm, and never thought that an oag would be the way to go. I really didn't feel like messing around with an oag, but I took the jump and went with it. I also upped my guide camera to an ASI220 mm mini (great camera). 
Not at all troublesome to set up.

My guiding definately improved. I strugged some nights to get under 1 arcsec/px total rms error. with the guide scopes. Now, I routinely run in the 0.6-0.7 range with often as low as 0.5 average over the entire imaging session.

After installing the oag, which is a standard ZWO model, I really like how slick the setup is and not having the guide scope/camera/cable hanging off of the scope.
When I swap scopes, I remove the whole assembly (camera, filter drawer, oag) as a unit and have had no issues swapping.

I don't ever plan to go back to a guide scope with my current scopes. I do have a Samyang 135F2 that I will set up for imaging some day and I will use my ZWO 30 mm guide scope and 120mm mini with that.

That's interesting, I'm sure the smaller pixels helped a lot in your case.
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Jim Raskett avatar
Jim Raskett avatar
Tony Gondola:
Jim Raskett:
I was having increasing issues guiding with my guide scope and seeing. One problem was that I was using too high of an imaging scale (>6) and even more troublesome was my atrocious seeing and lp which had been getting much worse the past few years.
I posted my guide logs to the PHD guiding group forum and Bruce responded that I need to half my guider image scale, get a better guide camera (I was using an Orion SSAG) or for the best scenario, go with an oag/better camera.

I use low focal length imaging scopes, a 61mm and 102mm, and never thought that an oag would be the way to go. I really didn't feel like messing around with an oag, but I took the jump and went with it. I also upped my guide camera to an ASI220 mm mini (great camera). 
Not at all troublesome to set up.

My guiding definately improved. I strugged some nights to get under 1 arcsec/px total rms error. with the guide scopes. Now, I routinely run in the 0.6-0.7 range with often as low as 0.5 average over the entire imaging session.

After installing the oag, which is a standard ZWO model, I really like how slick the setup is and not having the guide scope/camera/cable hanging off of the scope.
When I swap scopes, I remove the whole assembly (camera, filter drawer, oag) as a unit and have had no issues swapping.

I don't ever plan to go back to a guide scope with my current scopes. I do have a Samyang 135F2 that I will set up for imaging some day and I will use my ZWO 30 mm guide scope and 120mm mini with that.

Absolutely!
Also, the much lower difference in image scale helps PHD2 react more accurately.
The oag is just a more slick setup with a bit less weight and bulk.
Kyle Goodwin avatar
Kyle Goodwin avatar
Yes, an OAG will usually help guiding, but not because you're guiding with a larger aperture.  It helps because it eliminates sources of differential flexure between the main imaging scope and the guide scope.  If your setup is very rigid, like a refractor, then you may not gain much from going to an OAG.
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Andrew Lambourne avatar
Andrew Lambourne avatar
This is a really interesting thread. So far it's focused on the effects of seeing, flexure and aperture on guiding, but I wanted to think out loud based on observations of the guide graph from AISair over different nights. Just for background  I use 60mm f4 guidescopes: one on an 8" Newtonian and one on a 140mm refractor. Both are fixed with two rings to a metal bar bolted between the tops of the scope rings.

Watched the guiding graphs, I've noticed a few patterns:

a) guiding in Dec invariably wanders less than the guiding in RA, and the magnified view of the guidestar jumps left and right more than up and down (tracking errors?)

b) the guidestar image can go from being fairly sharp to being very fuzzy from exposure to exposure: I usually guide with 1 sec or 2 sec exposures (seeing?)

c) the graph limits usually stay within +/-4", often going to +/-2" for a few minutes or more on a still night, and every now and then jumping to +/-8" in response to a bigger jump in RA offset (wind effect or 'sand in the gears'?)

The probable causes would likely affect both an OAG and a guidescope - in all cases, the guidance is based on a feedback loop which reacts to an error, rather than being able to predict it. In the case of (eg) 1sec guiding exposures, the reaction can't start until on average half a second after the issue. So for transient effects such as 'seeing', I imagine it's possible to end up permanently chasing them if the exposures are short. This suggests maybe increasing the exposure times on the guidecam and reducing the aggressiveness of the feedback loop - my subjective impression is that 2sec exposures and 70% feedback gives better results than 1sec 90% feedback. Next time the skies clear I'm going to try 5sec to see what happens.

Wind impulses are likely to create a short oscillation on one or both axes, which will die down itself rather than benefiting from being chased, so again longer exposures not a problem.

Potentially larger but more occasional effects due to tracking errors are essentially step-changes or longer term periodic errors, which will be detected and corrected more quickly with shorter guiding exposures and a more aggressive feedback loop - so you can't win them all, hence tracking accuracy is still important.

Unless something is loose, I imagine flexure will probably manifest fairly slowly over a session, and from exposure to exposure may not be as large an effect as the others?

And as for the cheap and cheerful guidescopes, one might try a green filter on the guidecam to reduce chromatic aberration at least.

So in conclusion, I suspect there's a wider range of factors (ie tracking accuracy, wind impulse) to take into account overall, and perhaps some experimentation on OAG-based and on guidecam-based systems might shed light on the relative effects of all of these factors on overall accuracy?
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