What should I expect from guiding at longer focal lengths? - Forums

Most of my astrophotography experience has been with shorter focal lengths—550mm f/5.5 and shorter—where I typically guide with an OAG and an ASI290MM mini camera with PHD2. Generally I’m happy if guiding accuracy is better than an arc sec at those focal lengths. So I just started shooting with a 6” RC that has a FL of 1365 mm. I’ve been using the same OAG (the regular one from ZWO) and ASI290MM mini camera, and the first couple nights were a struggle to find a guide star, let alone get decent guiding. I was probably getting about an arc sec of accuracy, which isn’t nearly as good at 1365 mm than it is at 550 mm. I got images, but they could be better.

Last night I noticed that PHD2’s guiding assistant recommended I bin the camera, so before tonight’s session I set up PHD2 to bin the camera 2×2. So far I’m pleasantly shocked at the improvement. Same target as last night, but tonight I’m not having trouble finding guide stars, and guiding accuracy has generally been 0.5” to 0.6”.

So, two questions:

1) Should I have expected such an improvement in guiding from switching to 2×2 binning?

2) In general, is this good guiding given the focal length I’m shooting at? Welcome inputs from those of you who are more experienced with longer focal lengths.

(Let me add that I’ll soon be upgrading to an OAG with a larger prism and an ASI174 mini, in case you were going to recommend that.)

I have a RC8 and from most forums said OAG was best, I would always struggle to find stars to guide with and when I did guiding was very poor. It was very frustrating, in the end I went an 80×400 refractor piggyback ( I have an EQ6-r ) with an ASI120MM and leave it at 1×1 binning. I have seen some use 50mm to 70mm refractors piggybacked with great guiding, I would choose a cheap refractor over OAG any day. Generally ¼ of your scope FL is sweet. I tried large prism OAG as well it didn’t help, I hope this is helpful

Last night imaging I was 0.46 total rms, I usually get between 0.30 and 0.70 most nights but average around 0.50 to 0.65. 300 sec subs are no issue at all.

Hi, Dave.

I have a Celestron EdgeHD 8, use an OAG (ZWO large) and an ASI174MM mini for guiding (no binning). An EQ6R mount. Nina and PHD2.

On decent nights and at full focal length (2032mm) I regularly average 0.5” to 0.6” over a 5 minute sub, but have come to expect at least as good as 0.7”.

I’ve an AM5N mount too but I haven’t used it enough yet to be able to provide enough information on it’s performance with the C8.

If things are going a bit wonky, I often run the Guiding Assistant after I’ve ensured my calibration run was acceptable, and generally apply whatever recommendations it suggests. I’ve also found that for some reason, I sometimes need to re-run the PHD2 wizard on exactly the same setup to get things working properly. Someone else mentioned the same thing on another topic thread a while back. No idea why, but it does ‘fix’ things sometimes, so might be worth keeping in the back of your mind.

And finally, (and I expect you already know this) difficulty finding guide stars can be sometimes be related to the OAG prism not sitting far enough into the light path. If it’s positioned along the long edge of the sensor, you have more room to play with before it interfers with the image.

I hope that helps a bit, Dave.

All the best.

Paul

Hey!

I am not imaging at that specific focal length but I do image at 1000mm (130PHQ) with a 2600MC Air camera, which is a Duo concept, so esentially works as an OAG. I quickly realized that bin2 is a must have, because at bin 1 you get the stars with so much details that you pick up on even the slightest motion, creating havoc on guiding, as you are mostly chasing seeing. What also helps (depending on mount), is a longer guide exposure, which in my case works best at 3s, for the AM7 mount.

Also, I use the x0.75 sidereal speed, and I use the 160ms guide pulses for both RA and DEC, and the movement ignore setting at 0.2px (cant remember the exact parameter name). In total, this makes my guiding much less sensitive to seeing conditions, doing a decent job at filtering out the atmospheric noise, which I then also fine tune with agressivenes (asiair).

If the seeing is not really bad and if there isnt like wind gusts physically shaking the rig, I am guiding in the 0.30” total rms range, which is more than good enough for my 0.78” pixel scale.

In really bad seeing I try to mitigate with a 4s guide exposure, which also works for the AM7, helping to average out the atmosphere a bit more.

As for guide stars, the guide exposure length helps with that, and for fine tuning I just shift the guider gain, imaging some targets at 50 or some close to 300 gain on the guide sensor. I noticed in poor seeing it helps to have a higher gain on a bright guide star, clipping it a bit with a flat top peak, mitigating some of the motion. But that only works if you actually have a bright guide star available. I never use a fixed guider gain but always something optimal to find at least one star. In some cases I do have to slightly change the framing to pick up a guide star, there is just no way around that.

Regards

I'm shooting at 1200mm so image scale is 0.65"/px which should be close to yours and I always used a guide-scope with 1/3 of the main scope FL . If it wasn't for mirror shift I'd be doing that even for much longer FL but when I attempted use an AOG the only thing that worked was binning 2x and much longer integrations helped a lot stabilize the guiding to avoid chasing the seeing and the wind buffeting (here always an issue).

My early efforts to use an OAG here in Bortle 8 were pretty frustrating as I had a lot of trouble finding a guide star. I used a guide scope for along time even though I ended up throwing out a quarter of my subs when shooting at 1500mm focal length.

When I decided to bite the bullet and switch back to an OAG, I bought the Celestron OAG and modified it to take an ASI432MM camera with the big sensor and 9mm pixels. I generally get several guide stars even with short exposures and 0.5 arcsec guiding.

Dave Ek · Mar 21, 2026, 04:43 AM
Most of my astrophotography experience has been with shorter focal lengths—550mm f/5.5 and shorter—where I typically guide with an OAG and an ASI290MM mini camera with PHD2. Generally I’m happy if guiding accuracy is better than an arc sec at those focal lengths. So I just started shooting with a 6” RC that has a FL of 1365 mm. I’ve been using the same OAG (the regular one from ZWO) and ASI290MM mini camera, and the first couple nights were a struggle to find a guide star, let alone get decent guiding. I was probably getting about an arc sec of accuracy, which isn’t nearly as good at 1365 mm than it is at 550 mm. I got images, but they could be better.
Last night I noticed that PHD2’s guiding assistant recommended I bin the camera, so before tonight’s session I set up PHD2 to bin the camera 2×2. So far I’m pleasantly shocked at the improvement. Same target as last night, but tonight I’m not having trouble finding guide stars, and guiding accuracy has generally been 0.5” to 0.6”.
So, two questions:
1) Should I have expected such an improvement in guiding from switching to 2×2 binning?
2) In general, is this good guiding given the focal length I’m shooting at? Welcome inputs from those of you who are more experienced with longer focal lengths.
(Let me add that I’ll soon be upgrading to an OAG with a larger prism and an ASI174 mini, in case you were going to recommend that.)

Binning improves SNR which helps your guide program to recognize the stars better. So, yes an improvement is expected. For longer focal length I recommend turning the gain all the way up and use binning if appropriate. You won't care about dynamic range especially with an OAG. You want the maximum numbers of guide stars visible, so your multi star guiding works best.

0.5-0.6” RMS is fine with todays mounts and guide cameras. There might be room for improvement, but I do not know if it will be worth it. If your stars are tight and round, then your guiding is “good enough”.

With my Avalon M Uno mount and 2000mm focal length I guide around 0.25-0.4” total error if seeing permits. But I don't use PHD2.

If my guiding is worse, then I'm 80% sure the images are junk anyway because of bad seeing.

Paul Larkin · Mar 21, 2026, 05:46 AM
Hi, Dave.
I have a Celestron EdgeHD 8, use an OAG (ZWO large) and an ASI174MM mini for guiding (no binning). An EQ6R mount. Nina and PHD2.
On decent nights and at full focal length (2032mm) I regularly average 0.5” to 0.6” over a 5 minute sub, but have come to expect at least as good as 0.7”.
I’ve an AM5N mount too but I haven’t used it enough yet to be able to provide enough information on it’s performance with the C8.
If things are going a bit wonky, I often run the Guiding Assistant after I’ve ensured my calibration run was acceptable, and generally apply whatever recommendations it suggests. I’ve also found that for some reason, I sometimes need to re-run the PHD2 wizard on exactly the same setup to get things working properly. Someone else mentioned the same thing on another topic thread a while back. No idea why, but it does ‘fix’ things sometimes, so might be worth keeping in the back of your mind.
And finally, (and I expect you already know this) difficulty finding guide stars can be sometimes be related to the OAG prism not sitting far enough into the light path. If it’s positioned along the long edge of the sensor, you have more room to play with before it interfers with the image.
I hope that helps a bit, Dave.
All the best.
Paul

Thanks for sharing your experience, Paul—it’s good to know what numbers are reasonable to expect. While I haven’t rerun the profile wizard in PHD2 (I’ll keep that in mind), I do run Calibration assistant and Guiding assistant at the start of every evening of imaging. And yes, I did make sure my prism is as far down as possible without blocking the main sensor. Good advice. Using the ASI533MM Pro camera helps because of its modest sensor size.

Andrej Flis · Mar 21, 2026, 07:27 AM
Hey!
I am not imaging at that specific focal length but I do image at 1000mm (130PHQ) with a 2600MC Air camera, which is a Duo concept, so esentially works as an OAG. I quickly realized that bin2 is a must have, because at bin 1 you get the stars with so much details that you pick up on even the slightest motion, creating havoc on guiding, as you are mostly chasing seeing. What also helps (depending on mount), is a longer guide exposure, which in my case works best at 3s, for the AM7 mount.
Also, I use the x0.75 sidereal speed, and I use the 160ms guide pulses for both RA and DEC, and the movement ignore setting at 0.2px (cant remember the exact parameter name). In total, this makes my guiding much less sensitive to seeing conditions, doing a decent job at filtering out the atmospheric noise, which I then also fine tune with agressivenes (asiair).
If the seeing is not really bad and if there isnt like wind gusts physically shaking the rig, I am guiding in the 0.30” total rms range, which is more than good enough for my 0.78” pixel scale.
In really bad seeing I try to mitigate with a 4s guide exposure, which also works for the AM7, helping to average out the atmosphere a bit more.
As for guide stars, the guide exposure length helps with that, and for fine tuning I just shift the guider gain, imaging some targets at 50 or some close to 300 gain on the guide sensor. I noticed in poor seeing it helps to have a higher gain on a bright guide star, clipping it a bit with a flat top peak, mitigating some of the motion. But that only works if you actually have a bright guide star available. I never use a fixed guider gain but always something optimal to find at least one star. In some cases I do have to slightly change the framing to pick up a guide star, there is just no way around that.
Regards

Andrej, thanks for all the suggestions. I’ve been hesitant to play with the guide camera gain but I’ll give it a shot, and some of your other tweaks, as well.

John Tucker · Mar 21, 2026, 09:41 AM
My early efforts to use an OAG here in Bortle 8 were pretty frustrating as I had a lot of trouble finding a guide star. I used a guide scope for along time even though I ended up throwing out a quarter of my subs when shooting at 1500mm focal length.
When I decided to bite the bullet and switch back to an OAG, I bought the Celestron OAG and modified it to take an ASI432MM camera with the big sensor and 9mm pixels. I generally get several guide stars even with short exposures and 0.5 arcsec guiding.

Hoping my experience is the same with the larger OAG and guide sensor, John. Thanks.

Tobiasz · Mar 21, 2026, 09:46 AM
Binning improves SNR which helps your guide program to recognize the stars better. So, yes an improvement is expected. For longer focal length I recommend turning the gain all the way up and use binning if appropriate. You won't care about dynamic range especially with an OAG. You want the maximum numbers of guide stars visible, so your multi star guiding works best.
0.5-0.6” RMS is fine with todays mounts and guide cameras. There might be room for improvement, but I do not know if it will be worth it. If your stars are tight and round, then your guiding is “good enough”.
With my Avalon M Uno mount and 2000mm focal length I guide around 0.25-0.4” total error if seeing permits. But I don't use PHD2.
If my guiding is worse, then I'm 80% sure the images are junk anyway because of bad seeing.

Thanks, Tobiasz. Seeing is definitely in play here—it hasn’t been great. One never knows whether to blame bad seeing, bad guiding, bad focus, or bad collimation. :-)

Short answer is yes. My guiding setup is a an 80mm F/5 refractor with a 585MC camera. I shoot at FL’s of 900mm, 1340mm and 1580mm with no issues, round stars. My average RMS with an EQ-6 runs about 0.6”, on nights of really good seeing it can dip down to 0.3”. If the seeing is bad then guiding can degrade to 1.0” RMS. At that point I still get round stars but with an overall loss of resolution.

What I am seeing very little of in this discussion is the mount. Cheaper mounts like most of the Chinese mounts and Celestrons and such are not as well made or as heavy as one would like at longer focal lengths. They also often lack encoders which help a lot.

Regardless of guide chip and OAG size, lighter mounts are going to be more trouble when guiding. This is longstanding wisdom. As Roland at AP has said, the most important thing is “the mount, followed by the mount and then the mount”. A good mount will make life vastly easier (except for paying for it, of course 😉). Quality counts. For example, I have my AP1600 at home to the point where I can go 900 sec at 1000 mm unguided….

A synergistic effect effect of a quality mount is that you can use longer guide times and that translates to more and better guide stars which also improves guiding. I am using a 174 guide camera on a remote system at 2563 mm and because it is on a high end mount the guide time is 7 seconds.

As far as guiders, I know some folks say they get good results with guide scopes with longer FL imaging scopes but 30 + years of doing this have taught me to avoid guide scopes except for very short focal lengths and very rigid setups (so almost never with a reflector). They are just flaky at long focal lengths - there are too many opportunities for differential flex. OAG is the way to go almost 100% of the time at anything more than a few hundred mm.

Just my .02 and I am sure lots of folks will say that they do just fine with other setups but in my experience, the lighter the setup, the greater the frustration.

Bill McLaughlin · Mar 21, 2026, 02:42 PM
What I am seeing very little of in this discussion is the mount. Cheaper mounts like most of the Chinese mounts and Celestrons and such are not as well made or as heavy as one would like at longer focal lengths. They also often lack encoders which help a lot.
Regardless of guide chip and OAG size, lighter mounts are going to be more trouble when guiding. This is longstanding wisdom. As Roland at AP has said, the most important thing is “the mount, followed by the mount and then the mount”. A good mount will make life vastly easier (except for paying for it, of course 😉). Quality counts. For example, I have my AP1600 at home to the point where I can go 900 sec at 1000 mm unguided….
A synergistic effect effect of a quality mount is that you can use longer guide times and that translates to more and better guide stars which also improves guiding. I am using a 174 guide camera on a remote system at 2563 mm and because it is on a high end mount the guide time is 7 seconds.
As far as guiders, I know some folks say they get good results with guide scopes with longer FL imaging scopes but 30 + years of doing this have taught me to avoid guide scopes except for very short focal lengths and very rigid setups (so almost never with a reflector). They are just flaky at long focal lengths - there are too many opportunities for differential flex. OAG is the way to go almost 100% of the time at anything more than a few hundred mm.
Just my .02 and I am sure lots of folks will say that they do just fine with other setups but in my experience, the lighter the setup, the greater the frustration.

Yes, the mount is important. FWIW, mine is an iOptron GEM45.

Bill McLaughlin · Mar 21, 2026, 02:42 PM
What I am seeing very little of in this discussion is the mount. Cheaper mounts like most of the Chinese mounts and Celestrons and such are not as well made or as heavy as one would like at longer focal lengths. They also often lack encoders which help a lot.
Regardless of guide chip and OAG size, lighter mounts are going to be more trouble when guiding. This is longstanding wisdom. As Roland at AP has said, the most important thing is “the mount, followed by the mount and then the mount”. A good mount will make life vastly easier (except for paying for it, of course 😉). Quality counts. For example, I have my AP1600 at home to the point where I can go 900 sec at 1000 mm unguided….
A synergistic effect effect of a quality mount is that you can use longer guide times and that translates to more and better guide stars which also improves guiding. I am using a 174 guide camera on a remote system at 2563 mm and because it is on a high end mount the guide time is 7 seconds.
As far as guiders, I know some folks say they get good results with guide scopes with longer FL imaging scopes but 30 + years of doing this have taught me to avoid guide scopes except for very short focal lengths and very rigid setups (so almost never with a reflector). They are just flaky at long focal lengths - there are too many opportunities for differential flex. OAG is the way to go almost 100% of the time at anything more than a few hundred mm.
Just my .02 and I am sure lots of folks will say that they do just fine with other setups but in my experience, the lighter the setup, the greater the frustration.

All depends on what you want to do and your resources I guess. Not everyone’s wife is nice enough to understand the importance of owning a mount that can do 15 minute unguided exposures - which is of course why we guide….

When seeing is good I get 0.3 to 0.4 arc sec guiding with my EQ6R and 0.5 to 0 .6 arc sec with my CEM25P (I’m seeing these used on Cloudy Nights for as little as $500 these days). I think I’m going to need better optics and definitely much better processing skills before a better mount would really add value.

Hi,

I am using most a RC10 with reducer for a focal length 1520mm. main camera is a touptek 2600M (imx571) - Mount is a warpastron wd-20P

I am guiding with OAG . The camera I use actually for guiding is the player one xena based on imx585 mono. It is set as bin 2 and setup for high full well hdr. This way I have a sampling near the one of the OTA and also bin permit to catch more ligth . Up to now I always have guiding stars even through clouds (not too thicj) it is working. The 585 as a large sensor to you allways have a stard in the field.

The guidaing precsion depending the seeing goes from 0.28” to 0.7” with ok seeing. (when seeing is bad it can be around 0.8” and when very bad 1.2” ).

in town bortle 7 : guiding an am5n and a RC6 I have guiding between 0.55 and 0.7 as best

in country bortle 4 : 0.28” to 0.7” depending the seeing.

Before using the PO xena 585M for guiding I used

touptek 664C : pixel size 2.9µ bin2 was good no probleme to find star

qhy5iii 200 : pixle 4.5µ bin 1 : most of the time find start for guiding.

Touptek GPM174M : pixel 5.6µm bin 1wery high sensitivity, large sensor, good guiding , always find stars.

I tried som planetary camera that are also ok for guiding with imx678 but sometime they don’t find stars , the sensor is very small.

From my experience choose a guiding camera with a large sensor area if pixel are <3µ use bin2.

In my opinion the player one Xena 585M is one of the best for guiding and it is also very good for solar pictures.

And that’s the bottom line. Spending huge amounts of money is not the world most of use live in. Most of the solutions put forward here reflect the hardware that is commonly used.

Hi Dave.

Let me give you my particular experience. I photograph with a Celestron 14 Edge at prime focus (3910 mm), with an EQ8 mount. From the beginning I had problems locating good stars with an OAG. For this reason I do the control tracking with PHD2, a 90 mm diameter refractor and 900 mm focal length and a ZWO 678MM camera. I have a Bortle 6/7 and with good viewing conditions, I have reached errors of 0.20 ".

I wish you good skies.

Regardless of my setup, I use the same guiding scope, a 50mm Svbony scope around 199mm fl with a ZWO 120 mini. Not sure what the settings are for the 120 mini camera, but I can see stars in the field of view. I have a 82mm refactor fl 480mm, I have a 6” Celestron SCT that I use with a reducer for around 1098mm fl and without around 1528mm fl, I have a 12 inch Meade SCT with reducer 2058mm fl, without reducer 3048mm fl. This is all reported from the ASIAir computer that I use to control everything. I live in Bortle 6 and with good seeing conditions and no breeze, dead calm, I can average between .33 and .6 rms. with poor seeing that number goes up to .9 and can be over 1.0, I achieve round stars on all systems, and guiding is no issue at all as long as I have pretty good seeing and a very light breeze or none at all which can mess with your guiding at higher focal lengths . I truly don’t think I could get any better with an OAG than .33” so I stick with what has worked and what does work. And there is always BXT for correcting those elongated stars from breezy conditions or poor seeing.