38 replies to this topic

[Surfer4329]

I have been considering buying the Skywatcher 150i. The main downside it seems to me is the periodic error being so constant that you need 0.5-1.5” exposures for guiding to account for the “reliably” constant periodic error. The reason I say this is a downside is because of the issue of “chasing seeing” on nights of average or below average seeing conditions.

So for those of you with experience with these mounts, do you find that the short exposure times required for proper guiding causes you to “chase seeing”? Or better yet, have you been able to do 2-3” exposure times with good guiding results on these types of mounts?

[klangwolke]

I have three and run two of them at 2 seconds and the smallest at 1.5 seconds. I’ve read comments about running the faster but I get worse results that way.

[atticusbhat]

I usually run my AM3 with 0.5" exposures and regularly see ~0.5" RMS on nights with good seeing, and ~0.7-0.9" on average to mediocre nights. Haven't seen much benefit from >1" exposures, though I will say I usually have at least decent seeing where I am.

[scanner97]

Chasing the seeing with short guide exposures should not be a problem if you use multi-star guiding.  (Which is generally the default these days...)  If you want to buy a good mount, and you want something light, a strainwave like the 150i is an excellent option.

[calypsob]

Without encoders like ioptron or rainbow offer with a strainwave, you cannot improve guiding with longer exposures. With non encoder mounts you have to keep exposures under 2 seconds. Hobym Crux apparently has a PEC program which smooths out PE but I am not aware of anyone who can comment on how it can improve autoguiding. 

[Surfer4329]

I have three and run two of them at 2 seconds and the smallest at 1.5 seconds. I’ve read comments about running the faster but I get worse results that way.

Do you use multi-star guiding? That's the first thing that comes to mind when you say that your guiding gets worse when you do shorter exposures. Approximately, what kind of RMS guiding error are you getting with 2" exposures?

Edited by Surfer4329, 17 June 2025 - 01:19 PM.

[dan_hm]

Multi-star guiding.  No reason not to use it.

[licho52]

Seeing is something that's completely blown out of proportion in discussions (as are tons of other things, I guess people just like to have something to opine on).  Just get the mount, enjoy it and don't look back.

[Brian Carter]

I have never used more than 1sec for my guiding exposures with the AM5 and reliably track around 0.5” RMS. I haven’t had problems “chasing the seeing”.

[Yomamma]

I have been considering buying the Skywatcher 150i. The main downside it seems to me is the periodic error being so constant that you need 0.5-1.5” exposures for guiding to account for the “reliably” constant periodic error. The reason I say this is a downside is because of the issue of “chasing seeing” on nights of average or below average seeing conditions.

So for those of you with experience with these mounts, do you find that the short exposure times required for proper guiding causes you to “chase seeing”? Or better yet, have you been able to do 2-3” exposure times with good guiding results on these types of mounts?

I guide my WD20 with .5 second exposures unless the seeing is bad then I up it to 1 second, which will usually smooth it out some but not a lot.  Both of the rigs I run on it are refractors and are about 24 pounds.

[Tom62e]

Without encoders like ioptron or rainbow offer with a strainwave, you cannot improve guiding with longer exposures. With non encoder mounts you have to keep exposures under 2 seconds. Hobym Crux apparently has a PEC program which smooths out PE but I am not aware of anyone who can comment on how it can improve autoguiding. 

There is 99.99% consensus that longer exposures averages out the movements so you don’t chase the seeing. It almost always improves guiding when the seeing is poorer than average.  You are the only person I’ve ever heard claim this isn’t the case.

 

And to answer the OP’s question: yes, one major disadvantage or a SW mount is the need to guide with .5 to 1 sec exposures, sometimes shorter . . . because it does lead to chasing the seeing. As demonstrated by Yomomma’s post. 

Edited by Tom62e, 05 July 2025 - 03:14 PM.

[Tom62e]

Chasing the seeing with short guide exposures should not be a problem if you use multi-star guiding.  (Which is generally the default these days...)  If you want to buy a good mount, and you want something light, a strainwave like the 150i is an excellent option.

While multi-star guiding may help, it doesn’t by any means eliminate the issue.  Chasing the seeing is a known issue with SW mounts. 

[calypsob]

There is 99.99% consensus that longer exposures averages out the movements so you don’t chase the seeing. It almost always improves guiding when the seeing is poorer than average. You are the only person I’ve ever heard claim this isn’t the case.

And to answer the OP’s question: yes, one major disadvantage or a SW mount is the need to guide with .5 to 1 sec exposures, sometimes shorter . . . because it does lead to chasing the seeing. As demonstrated by Yomomma’s post.

99% consensus? I don’t see a consensus anywhere.
You can’t do long guide exposures on a normal non encoded strain wave and achieve .5 RMS. You absolutely have to have encoders to do 3 to 5s guide exposures if you are right at or over sampling. This is not the case with the standard EQ mount.

[calypsob]

While multi-star guiding may help, it doesn’t by any means eliminate the issue. Chasing the seeing is a known issue with SW mounts.

Right exactly on non-encoded strain wave mounts that have to shoot guide exposures less than one second.

[James Peirce]

Multi-star guiding mainly takes care of that.

 

It’s also a non-issue with guiding at wide to moderate focal lengths.

 

It can become a problem with a narrow field of view and small pixel size. Not just some potential for more limited guiding stars to choose from depending on the setup—meaning you really can end up “chasing the seeing”—but while also combating potentially slower focal ratios for guiding and where the periodic error and secondary harmonics can have a much greater impact on performance. Varying depending on copy variation and the model, of course. My RST-135E performs considerably better than my AM5. There comes a point where a traditional equatorial mount becomes a much more cost-effective solution to good performance.

 

I quite like my strain wave mounts despite the limitations they do have. They’re a lot more convenient in terms of transport, setup, and tear-down for me when imaging on the go.

Edited by James Peirce, 05 July 2025 - 06:19 PM.

[fewayne]

There is 99.99% consensus that longer exposures averages out the movements so you don’t chase the seeing. It almost always improves guiding when the seeing is poorer than average.  You are the only person I’ve ever heard claim this isn’t the case.

 

And to answer the OP’s question: yes, one major disadvantage or a SW mount is the need to guide with .5 to 1 sec exposures, sometimes shorter . . . because it does lead to chasing the seeing. As demonstrated by Yomomma’s post. 

I guess I'm in the 0.01%, then. There was such a consensus, as there used to be for many other cherished shibboleths of AP. "Cool to the max you can manage". "Shoot the longest exposures you can, or read noise will kill your images". "Darks are an absolute requirement for all sensors".

 

I will note that Y merely shared what works for them, not a proof that chasing the seeing is actually a thing. And I'm glad they found something that works. Me, I often find that as seeing degrades, going shorter on the cadence improves performance. I thought that my encoder-equipped HAE mount wouldn't work that way, but it does.

If "chasing the seeing" were as bad as the alleged 99.99% consensus says, Frank and MetaGuide would be hopeless anachronisms. But in fact, video-paced guiding works! Multi-star guiding makes traditional exposures work for the rest of us at short cadences. The secret is that turbulence cells are so much smaller than your FOV that the multiple stars are moving independently, so the seeing-related movement tends to average out.

"Anecdote" != data, but just the other night I was working at 2.5" (130 APO, OAG, CEM70). The seeing started to degrade as the night went on, taking my guiding from ~0.6" to ~0.9" and then higher. I quickened the cadence. The guiding got better. At half a second I figured that ~0.6" again was good enough. Just to check, I went longer for a minute. Ew!

 

On nights when things are crazy, my strainwave mount does just fine at 1" or less, thank you.

[Tom62e]

I guess I'm in the 0.01%, then. There was such a consensus, as there used to be for many other cherished shibboleths of AP. "Cool to the max you can manage". "Shoot the longest exposures you can, or read noise will kill your images". "Darks are an absolute requirement for all sensors".

 

I will note that Y merely shared what works for them, not a proof that chasing the seeing is actually a thing. And I'm glad they found something that works. Me, I often find that as seeing degrades, going shorter on the cadence improves performance. I thought that my encoder-equipped HAE mount wouldn't work that way, but it does.

If "chasing the seeing" were as bad as the alleged 99.99% consensus says, Frank and MetaGuide would be hopeless anachronisms. But in fact, video-paced guiding works! Multi-star guiding makes traditional exposures work for the rest of us at short cadences. The secret is that turbulence cells are so much smaller than your FOV that the multiple stars are moving independently, so the seeing-related movement tends to average out.

"Anecdote" != data, but just the other night I was working at 2.5" (130 APO, OAG, CEM70). The seeing started to degrade as the night went on, taking my guiding from ~0.6" to ~0.9" and then higher. I quickened the cadence. The guiding got better. At half a second I figured that ~0.6" again was good enough. Just to check, I went longer for a minute. Ew!

 

On nights when things are crazy, my strainwave mount does just fine at 1" or less, thank you.

I'm sorry you are hopelessly lost.  You are just wrong from every perspective.  I hope you eventually see the light.  In the meanwhile, stop giving newbies poor advice.  At least let them know it's your "dissenting" opinion.

Edited by Tom62e, 05 July 2025 - 10:32 PM.

[Yomamma]

I use a WD20 with about 25 pounds on it and when seeing is average or better I run guiding exposures at .5 sec and get really good guiding. I use an ASI174 in an OAG.

 

When the seeing gets worse, I will up the exposures to 1 second and that will tend to smooth out the jig saw a bit but it does not get rid of it.

 

If I run 1.5 to 2 sec guide exposures you can see the PE.

[Tom62e]

I use a WD20 with about 25 pounds on it and when seeing is average or better I run guiding exposures at .5 sec and get really good guiding. I use an ASI174 in an OAG.

 

When the seeing gets worse, I will up the exposures to 1 second and that will tend to smooth out the jig saw a bit but it does not get rid of it.

 

If I run 1.5 to 2 sec guide exposures you can see the PE.

What you have described here is what almost all SW mount users have reported.  It makes perfect sense and is in line with the general consensus.

 

The other downside to a SW mount requiring short guiding exposures is less stars to guide on, which equates to less effective multi-star guiding.  

 

To be clear, I'm not dissing SW mounts.  But there are pros and cons.  They are no better than EQ mounts, just different.  Each one has their pros and cons.  I personally, after using several SW mounts, prefer the EQ mount.

[Tom62e]

Multi-star guiding mainly takes care of that.

 

It’s also a non-issue with guiding at wide to moderate focal lengths.

 

It can become a problem with a narrow field of view and small pixel size. Not just some potential for more limited guiding stars to choose from depending on the setup—meaning you really can end up “chasing the seeing”—but while also combating potentially slower focal ratios for guiding and where the periodic error and secondary harmonics can have a much greater impact on performance. Varying depending on copy variation and the model, of course. My RST-135E performs considerably better than my AM5. There comes a point where a traditional equatorial mount becomes a much more cost-effective solution to good performance.

 

I quite like my strain wave mounts despite the limitations they do have. They’re a lot more convenient in terms of transport, setup, and tear-down for me when imaging on the go.

I agree with everything you said but your first sentence.  Multi-star guiding does not “mainly take care of that”.  It makes a marginal improvement.  That said, multi-star guiding makes a significant improvement over one star guiding in general.  That is the improvement you are seeing.  Everything else equal, multi-star guiding does not correct for “chasing the seeing”.  

 

The seeing affects all stars, not just one.  And again, by having to use faster exposure times when guiding, you have less guide stars to guide on.  So SW mounts minimize the benefit of multi-star guiding to begin with.  And I haven’t even mentioned their susceptibility to mild breezes. 

Edited by Tom62e, 06 July 2025 - 12:02 AM.

[psandelle]

I'm sorry you are hopelessly lost.  You are just wrong from every perspective.  I hope you eventually see the light.  In the meanwhile, stop giving newbies poor advice.  At least let them know it's your "dissenting" opinion.

Out of curiosity, how does Metaguide work again?

 

Also, Rainbow Astro mounts are probably the outlier, then; the non-EC ones really work best for most use cases at .5 seconds (as recommended), and of the three RA mounts I’ve used, that was the case. But that’s only RA I’m talking about here, and I’ve always found they gave me better guiding than other SWG mounts.

 

Paul

Edited by psandelle, 06 July 2025 - 08:15 AM.

[Yomamma]

What you have described here is what almost all SW mount users have reported.  It makes perfect sense and is in line with the general consensus.

 

The other downside to a SW mount requiring short guiding exposures is less stars to guide on, which equates to less effective multi-star guiding.  

 

To be clear, I'm not dissing SW mounts.  But there are pros and cons.  They are no better than EQ mounts, just different.  Each one has their pros and cons.  I personally, after using several SW mounts, prefer the EQ mount.

My WD 20 will not go below .4 RMS, However, my CEM120 will guide in the .3 range all day long, and when seeing is good if will go down into the .2s.  There is no comparison between my WD20 and CEM120, the CEM120 is my choice for good guiding. The WD20 is portable and has no weights and that is why I like it and use it.

[Tom62e]

My WD 20 will not go below .4 RMS, However, my CEM120 will guide in the .3 range all day long, and when seeing is good if will go down into the .2s.  There is no comparison between my WD20 and CEM120, the CEM120 is my choice for good guiding. The WD20 is portable and has no weights and that is why I like it and use it.

And I think that's okay so long as consumers understand this before they purchase (manufacturers are way too generous with their performance claims—they focus on all the pros and none of the cons).  There is so much hype that harmonic SW mounts are better than EQ mounts and it's just not true, generally speaking.  Of course, there may be some high-end SW mounts that perform better than some EQ mounts, but generally speaking they are not better - pros and cons. 

 

You named one of the big pros - the SW mounts are much more portable.  For me that isn't a factor I much care about right now.  Currently I only image from my backyard observatory.  Therefore, I actually prefer a much heavier mount, so it performs better in the wind, which I get a lot of in Fountain Hills, AZ.  SW mounts don't handle even mild wind well—one of the cons.  The poor RA guiding in another con.

 

Like you, I can say that neither my WD20 nor my Pegasus NYX-101 ever performed better than my EQ6-R and not even close to the performance of my EQ8-R.  That said, will I ever buy another SW mount?  Probably yes, I would like to have a second mount to put on a HD Tripod I have permanently set up in my backyard (but not in my observatory—so the mount would need to come in during the hot summer days). 

 

The SW mount (perhaps the 150i) would be my secondary rig for wide-field imaging only, while the rig in my observatory will handle the mid and long focal length imaging. I have learned that most SW mounts do not perform well at long focal lengths.  But I understand this going into the purchase, so I won't be disappointed.  I just want the community to be aware that SW mounts do not live up to all the hype . . . but despite this, they still have a place in the hobby. 

[Tom62e]

Out of curiosity, how does Metaguide work again?

 

Also, Rainbow Astro mounts are probably the outlier, then; the non-EC ones really work best for most use cases at .5 seconds (as recommended), and of the three RA mounts I’ve used, that was the case. But that’s only RA I’m talking about here, and I’ve always found they gave me better guiding than other SWG mounts.

 

Paul

I'm not saying they won't perform better at the recommended .5 sec exposures.  In fact, I am saying they will perform better at the recommended guide exposure.  But therein lies the problem.  Their need to guide at short exposures lengths makes them susceptible to poor guiding when the seeing isn't very good.  

 

The outlier would be Klangwolkethe, the gentleman who said his SW mount guides at 2 seconds without an issue. That I have never heard before. I would be interested in testing that SW mount. 

 

Klangwolke - which SW mount do you use that guides better at 2 seconds? I think the community would be interested to know this.

Edited by Tom62e, 06 July 2025 - 11:27 AM.

[James Peirce]

I agree with everything you said but your first sentence.  Multi-star guiding does not “mainly take care of that”.  It makes a marginal improvement.  That said, multi-star guiding makes a significant improvement over one star guiding in general.  That is the improvement you are seeing.  Everything else equal, multi-star guiding does not correct for “chasing the seeing”.  

 

The seeing affects all stars, not just one.  And again, by having to use faster exposure times when guiding, you have less guide stars to guide on.  So SW mounts minimize the benefit of multi-star guiding to begin with.  And I haven’t even mentioned their susceptibility to mild breezes. 

I may not have worded that clearly enough, because there is a reason why I went into there being issues with guiding at a particular scale. Until you start hitting relatively more demanding focal lengths (relative to pixel size)—say, 1300 mm at 3.75 µm, while in practice this is going to vary considerably with copy variation and model of mount—multi-star guiding does take care of it in the sense that you can comfortably produce guiding which is accurate enough to exceed the scale at which you are capable of resolving with the optical system. It only starts to really fall apart when the setup comes closer to reaching the limits of what the atmosphere and optics allow to be resolved and there’s a struggle to find an adequate sampling of stars. It can easily account for additional degradation when using an SCT (at least with my AM5 in this case—my RST-135E keeps error low enough).

 

Yes, seeing affects all stars, but not equally, and if you can sample enough stars to guide on, you can still calculate an improved correction be extrapolating the star centers across all stars being sampled—meaning you can correct out a significant error that would have been accounted for when guiding on one star under bad conditions. So as long as you have enough stars to sample, it can count for something. You can also afford yourself some latitude in how corrections are managed. Under those conditions you wouldn’t want to be sending aggressive corrections anyway, otherwise you’d be introducing error due to over-correction. You can usually account for the peaks in periodic error and secondary harmonics by sending a couple corrections rather than needing to correct it out in 1s.

 

In any case… I made that distinction for a reason. “Chasing the seeing” with adequate multi-star guiding, with a setup rather as I outlined above, just isn’t an issue. And there comes a point where it is, and that’s one of the reasons why, in my previous post, I ought right said that there comes a point where a telescope deserves an equatorial mount instead.