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Periodic Error in the Real World

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#1 syscore

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Posted 26 December 2016 - 10:47 AM

There has been a lot of discussion about periodic error (PE) and periodic error correction (PEC) recently on a few threads, and after weight capacity, it is often the first spec we look at. Unfortunately, I think many users treat PE as if that were the only tracking error in a mount, and once corrected, the mount will essentially track perfectly. The reality is that PE is only about half of the error in tracking, with the rest of the error being random and uncorrectable, except through guiding or through the use of high resolution encoders. To drive this home better, I used PemPro to measure and correct the PE on my CGE Pro mount. First, I want to show what people often publish and many users see, just the clean PE before and after correction, and then the raw data that shows what is actually going on.

 

A couple points about measuring PE. You want as perfect a polar alignment as possible. PemPro will correct for drift but because of the other errors present in tracking, it is very hard to get what the true tracking of the mount was, after PemPro corrects for drift. In all of these examples I have the drift correction turned off so that we can see exactly what the mount is doing, and I have a very good (< 1 arc min) polar alignment. I find that I get better measurements with longer focal lengths. In these examples I am using an 11" EdgeHD at f/10 (FL = 2800 mm). Also, I like to use my CMOS camera instead of my CCD in order to get faster data points. I am using an ASI178 here and the pixel scale is a ridiculous 0.18 arc sec. You certainly don't need to perform PE measurements at such a high resolution, but this is the equipment I have.

 

The following graphs are what people generally publish and talk about. First, is a measurement of the PE of my mount using PemPro with no PEC correction. This is called the "native PE"...

 

PE_Before.jpg

 

About 9 arc sec peak-to-peak, which is under the 10 arc sec that Celestron claims. So far so good.

 

And this next graph is the PE after applying that curve to the PEC feature built into the mount. This is called the "corrected PE"...

 

PE_After.jpg

 

Now the PE has been reduced to just under 2 arc sec, which is pretty good, fantastic even.

 

Both of these curves look very smooth because they are "best fit" to raw data, but in the next post I am going to show what the raw data actually looks like.


Edited by syscore, 26 December 2016 - 02:21 PM.


#2 syscore

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Posted 26 December 2016 - 10:57 AM

In the previous post I showed what the processed PE data looks like for my CGE Pro. In this post I will show what the raw data behind those curves looks like.

 

First, the raw data before any (PEC) correction is applied...

 

Raw_Before.jpg

 

And then the raw data after (PEC) correction is applied...

 

Raw_After.jpg

 

As you can see, even though the "best fit" algorithm produces a nice smooth curve, the actual tracking of the mount wanders quite a bit away from this curve over each worm cycle. The cause of this comes from gear noise, the bearings holding the shafts, tooth to tooth inconsistency in the worm gear, and a number of other causes. But the data does follow the shape of the smooth worm cycle displayed in the previous post.

 

Beter and more expensive mounts will certainly have tighter raw data, but not perfect.

 

The second graph, after correction, shows the same wandering, but is certainly much flatter, and for some periods of time it is very flat, and then wanders, and then is flat again. Will this support unguided imaging? That depends on the focal length, and at 1960 mm, I can get 2 minute exposures that are "ok" about 40% of the time.

 

In the next post I will show a couple images.


Edited by syscore, 26 December 2016 - 01:01 PM.


#3 syscore

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Posted 26 December 2016 - 11:07 AM

This is a 2 minute unguided exposure of M33 using the 11" EdgeHD at f/7 (fl = 1960 mm). The pixel scale is 0.57"/Pixel, but the image has been reduced to fit the forum. PEC is enabled...

 

M33_Unguided_1960mm_120s.jpg

 

That isn't bad and I think at a focal length of 500 mm or less, I could probably do 3 minutes with most of the exposures keepers. The following is a crop of this image zoomed in 2X...

 

M33_Unguided_1960mm_120s_Crop.jpg

 

Not bad, but at this scale we can see some elongation. Now compare this to a 2 minute guided exposure of the same region...

 

M33_Guided_1960mm_120s_Crop.jpg

 

An obvious improvement.

 

And this is 5 x 10 minute subs that have been calibrated and stacked with no other processing. PEC is enabled and guiding is enabled...

 

get.jpg

 

In the next post I will summarize.


Edited by syscore, 27 December 2016 - 03:14 AM.


#4 PirateMike

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Posted 26 December 2016 - 11:25 AM

Thanks for posting this information. I am currently working on my mount to get better tracking, and if the weather ever gives me a chance to continue.....   :)



#5 syscore

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Posted 26 December 2016 - 11:25 AM

Periodic error is just the periodic component of the tracking error. It isn't all of the error. Generally, it is only half of the error, maybe even less. As you could see in the raw data, while the periodic error of my CGE Pro is 9", the actual error cycle over cycle is more like 15". But the error within a cycle is usually around 9".

 

The same applies to the data after correction was applied. Even though the PE was reduced to less than 2" after PEC was turned on, the raw data had more like 10" of variance, but within a single worm cycle the variance was much smaller. Less than 3" for some periods of time.

 

Is native PE meaningful? Yes, because it does generally indicate the tightness of the mount's tracking. Mounts with lower native PE are generally tighter in all of their components and track better than mounts with higher native PE. It was very hard to get results approaching the M33 shot with my CGEM, even while guiding, and it had a native PE of around 25". 

 

Is corrected PE meaningful? If we are talking unguided imaging, yes, but keep in mind that it isn't going to eliminate all tracking error, but if the native PE is low enough, and the PEC then reduces that further, it can work for short periods with short focal lengths. In my example above, shooting 2 minutes unguided at 1960 mm, while it was "ok", I would have to throw away too many exposures, which is a lot harder than guiding and keeping all of the exposures.

 

Is corrected PE meaningful when guiding? You will hear different answers from different people on this. This is because if you look at the raw data and the combination of little errors, like seeing, the auto-guider is making corrections every 1 to 5 seconds, and at that time scale, the little errors are much larger than the slowly moving periodic error. Nonetheless, I do think I get a 10% to 20% improvement in my guiding when I enable PEC. And the data certainly shows periods of time that would require less corrections. But keep in mind that I have conquered many of the other subtitles of guiding, like guide camera focus and balancing. Those aspects would cover up a small 10% or 20% improvement if not tended to.


Edited by syscore, 26 December 2016 - 11:42 AM.


#6 prostcj

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Posted 26 December 2016 - 11:59 AM

Thanks for the detailed post.  I have always believed that guiding is much easier than trying to set up a system to go unguided.  I am beginning to wonder however, with CMOS cameras like the ASI1600 becoming prevalent, that many short exposures will be the trend of the future.  If this is the case, I wonder if the premium mount market will dwindle.  



#7 syscore

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Posted 26 December 2016 - 12:18 PM

Thanks for the detailed post.  I have always believed that guiding is much easier than trying to set up a system to go unguided.  I am beginning to wonder however, with CMOS cameras like the ASI1600 becoming prevalent, that many short exposures will be the trend of the future.  If this is the case, I wonder if the premium mount market will dwindle.  

 

Shorter exposures can certainly help a non-premium mount get better results, and that was true even before the ASI1600. The thing is though, the exposures have to be 15 seconds or less (or you start picking up the errors in the mount on many of them) and you end up with thousands of exposures. Sorting through and processing 1000 subs is not a lot of fun. But I have seen users do that with a Dobsonian and get very good results. I doubt that will cause people not to want premium mounts if they can afford them, but it does give users with non-premium mounts a strategy for better images, until they get tired of the number of exposures. :)


Edited by syscore, 26 December 2016 - 02:18 PM.


#8 blueman

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Posted 26 December 2016 - 02:40 PM

Roland Christen recently said, when guiding, PEC is not really necessary. If unguided, well it does matter. The same with the Encoders, if guiding, not really required, but unguided, a necessity.

 

Guiding is actually a very easy thing to do. It requires a little setup to figure out how your system responds, but after that initial work, it just not difficult. A good guider, whether OAG or Guide Scope, requires a good camera and for a Guide Scope, good mounting to eliminate differential flexure.

 

I have done PEC with my AP900, that got the PE down to under 1". But I don't use it, personally I found that it did not actually improve my guiding, using test subs taken on good nights as basis for this decision.

 

Seeing is your worst enemy, not guiding. Seeing cannot be corrected, well for the most part, there are some systems (adaptive optics) that claim to help. But seeing cannot be overcome if it is poor, no matter what you do. For me, seeing is my main limiting factor for image quality. I think that most imagers deal with seeing more than they realize. It is not uncommon to see someone fiddling with their guiding at our site, when it is the seeing not the guiding that is causing their problem. This causes them to believe that guiding is difficult and full of issues, when most likely the best guiding in the world would not have helped much that night.

 

So, I don't worry too much about PE anymore. I worry more about setting the aggression to fit the night. Normally I will let the guiding run for 15 -25 minutes, while I wait for it to get dark, using the time between sundown and astronomical twilight. During that time I will watch my numbers and try adjusting the aggression to see if I can improve them. Some nights seem to like higher aggression numbers and others lower ones.

 

Guiding does help with seeing though, it can try to keep up with it, with improvements much of the time. Unguided does not attempt to keep up with seeing, so if the seeing is variable, it will not work as well as guided images.

 

So I think it is important to know your imaging site and its conditions, maybe more so than worrying about how good your PE or PEC might be. Sometimes I think we tend to dwell on the technical end of things a bit obsessively. Practical experience in the field is what normally improves your imaging. Hardware certainly makes a big difference, it makes it easier to learn how to deal with your conditions. But in the end, Seeing, is the determining factor for how your night will be.

Blueman


Edited by blueman, 26 December 2016 - 02:43 PM.


#9 syscore

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Posted 26 December 2016 - 02:46 PM

I agree about PEC, but not about PE. Guiding can never make my CGEM with 25" PE produce results like your AP900 with 7" PE.:) Even when you guide, you still must keep in mind the equipment you are working with and your expectations in check with that.



#10 prostcj

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Posted 26 December 2016 - 05:23 PM

Roland Christen recently said, when guiding, PEC is not really necessary. If unguided, well it does matter. The same with the Encoders, if guiding, not really required, but unguided, a necessity.

 

Guiding is actually a very easy thing to do. It requires a little setup to figure out how your system responds, but after that initial work, it just not difficult. A good guider, whether OAG or Guide Scope, requires a good camera and for a Guide Scope, good mounting to eliminate differential flexure.

 

I have done PEC with my AP900, that got the PE down to under 1". But I don't use it, personally I found that it did not actually improve my guiding, using test subs taken on good nights as basis for this decision.

 

Seeing is your worst enemy, not guiding. Seeing cannot be corrected, well for the most part, there are some systems (adaptive optics) that claim to help. But seeing cannot be overcome if it is poor, no matter what you do. For me, seeing is my main limiting factor for image quality. I think that most imagers deal with seeing more than they realize. It is not uncommon to see someone fiddling with their guiding at our site, when it is the seeing not the guiding that is causing their problem. This causes them to believe that guiding is difficult and full of issues, when most likely the best guiding in the world would not have helped much that night.

 

So, I don't worry too much about PE anymore. I worry more about setting the aggression to fit the night. Normally I will let the guiding run for 15 -25 minutes, while I wait for it to get dark, using the time between sundown and astronomical twilight. During that time I will watch my numbers and try adjusting the aggression to see if I can improve them. Some nights seem to like higher aggression numbers and others lower ones.

 

Guiding does help with seeing though, it can try to keep up with it, with improvements much of the time. Unguided does not attempt to keep up with seeing, so if the seeing is variable, it will not work as well as guided images.

 

So I think it is important to know your imaging site and its conditions, maybe more so than worrying about how good your PE or PEC might be. Sometimes I think we tend to dwell on the technical end of things a bit obsessively. Practical experience in the field is what normally improves your imaging. Hardware certainly makes a big difference, it makes it easier to learn how to deal with your conditions. But in the end, Seeing, is the determining factor for how your night will be.

Blueman

This would be a good example of the PEC number not telling the whole story.  If your peak to peak PE is under 1", as long as you have a guide alignment, there would be no need to guide since a TEC 140 (.75 reducer) and KAF 8300 will sample at close to 2" / pixel. 



#11 blueman

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Posted 26 December 2016 - 10:09 PM

 

Roland Christen recently said, when guiding, PEC is not really necessary. If unguided, well it does matter. The same with the Encoders, if guiding, not really required, but unguided, a necessity.

 

Guiding is actually a very easy thing to do. It requires a little setup to figure out how your system responds, but after that initial work, it just not difficult. A good guider, whether OAG or Guide Scope, requires a good camera and for a Guide Scope, good mounting to eliminate differential flexure.

 

I have done PEC with my AP900, that got the PE down to under 1". But I don't use it, personally I found that it did not actually improve my guiding, using test subs taken on good nights as basis for this decision.

 

Seeing is your worst enemy, not guiding. Seeing cannot be corrected, well for the most part, there are some systems (adaptive optics) that claim to help. But seeing cannot be overcome if it is poor, no matter what you do. For me, seeing is my main limiting factor for image quality. I think that most imagers deal with seeing more than they realize. It is not uncommon to see someone fiddling with their guiding at our site, when it is the seeing not the guiding that is causing their problem. This causes them to believe that guiding is difficult and full of issues, when most likely the best guiding in the world would not have helped much that night.

 

So, I don't worry too much about PE anymore. I worry more about setting the aggression to fit the night. Normally I will let the guiding run for 15 -25 minutes, while I wait for it to get dark, using the time between sundown and astronomical twilight. During that time I will watch my numbers and try adjusting the aggression to see if I can improve them. Some nights seem to like higher aggression numbers and others lower ones.

 

Guiding does help with seeing though, it can try to keep up with it, with improvements much of the time. Unguided does not attempt to keep up with seeing, so if the seeing is variable, it will not work as well as guided images.

 

So I think it is important to know your imaging site and its conditions, maybe more so than worrying about how good your PE or PEC might be. Sometimes I think we tend to dwell on the technical end of things a bit obsessively. Practical experience in the field is what normally improves your imaging. Hardware certainly makes a big difference, it makes it easier to learn how to deal with your conditions. But in the end, Seeing, is the determining factor for how your night will be.

Blueman

This would be a good example of the PEC number not telling the whole story.  If your peak to peak PE is under 1", as long as you have a guide alignment, there would be no need to guide since a TEC 140 (.75 reducer) and KAF 8300 will sample at close to 2" / pixel. 

 

Yes, there are variables besides PE that everyone has to deal with.

Blueman 



#12 blueman

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Posted 26 December 2016 - 10:11 PM

I agree about PEC, but not about PE. Guiding can never make my CGEM with 25" PE produce results like your AP900 with 7" PE. :) Even when you guide, you still must keep in mind the equipment you are working with and your expectations in check with that.

As I said,  good hardware makes it easier to learn to deal with the other variables. ;)

Blueman 



#13 tolgagumus

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Posted 26 December 2016 - 11:33 PM

Periodic error is just the periodic component of the tracking error. It isn't all of the error. Generally, it is only half of the error, maybe even less. As you could see in the raw data, while the periodic error of my CGE Pro is 9", the actual error cycle over cycle is more like 15". But the error within a cycle is usually around 9".

 

The same applies to the data after correction was applied. Even though the PE was reduced to less than 2" after PEC was turned on, the raw data had more like 10" of variance, but within a single worm cycle the variance was much smaller. Less than 3" for some periods of time.

 

Is native PE meaningful? Yes, because it does generally indicate the tightness of the mount's tracking. Mounts with lower native PE are generally tighter in all of their components and track better than mounts with higher native PE. It was very hard to get results approaching the M33 shot with my CGEM, even while guiding, and it had a native PE of around 25". 

 

Is corrected PE meaningful? If we are talking unguided imaging, yes, but keep in mind that it isn't going to eliminate all tracking error, but if the native PE is low enough, and the PEC then reduces that further, it can work for short periods with short focal lengths. In my example above, shooting 2 minutes unguided at 1960 mm, while it was "ok", I would have to throw away too many exposures, which is a lot harder than guiding and keeping all of the exposures.

 

Is corrected PE meaningful when guiding? You will hear different answers from different people on this. This is because if you look at the raw data and the combination of little errors, like seeing, the auto-guider is making corrections every 1 to 5 seconds, and at that time scale, the little errors are much larger than the slowly moving periodic error. Nonetheless, I do think I get a 10% to 20% improvement in my guiding when I enable PEC. And the data certainly shows periods of time that would require less corrections. But keep in mind that I have conquered many of the other subtitles of guiding, like guide camera focus and balancing. Those aspects would cover up a small 10% or 20% improvement if not tended to.

I am not sure what you mean by it's not all the error? Do you mean it's not all the error in the mount or considering PA error, cone error and stuff like that all together to do unguided imaging?



#14 prostcj

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Posted 26 December 2016 - 11:47 PM

A good mount will definitely help with poor seeing conditions because you need longer guide exposures so that you are not chasing seeing with your guider. The mount should not only be able to go 10 seconds without a correction, but the error should be small so little correction is required.
I think another factor overlooked is backlash. Every mount design that requires lubrication has backlash, some mounts just have a lot more than others. A guider loses its effectiveness with backlash.
I'm hoping my Mesu 200 will allow very effective guiding due to its friction drive which has no backlash and a very gradual PE. At least in theory, won't know until I get it. Fingers crossed.

Sent from my SAMSUNG-SM-G925A using Tapatalk

#15 blueman

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Posted 27 December 2016 - 12:51 AM

It is funny to me that people talk so much about chasing seeing. If a mount doesn't try to keep the star centered during turbulent seeing, the the star image will still suffer. The star will continue producing light and that will show up as smear or multiple areas of multiple exposure. So, if the mount could keep up with seeing, that could actually help IMO.

 

Roland told me once, that he was getting better results with 1 second guide exposures and high aggression settings, kind of counter to what is considered the case by many.

Blueman 



#16 prostcj

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Posted 27 December 2016 - 01:39 AM

A short guide exposure during bad seeing means the mount is trying to move based on a guide star location that doesn't exist. A longer guide exposure means the mount guiding will use more of an average which keeps the erroneous mount movements down.
The turbulence is what's causing the guide star to jump around, unless you are using adaptive optics, you have no chance of "keeping up" with fluctuating stars. Even with adaptive optics I don't see how a system with backlash would react fast enough unless it was intentually placed off of PA so it was always guiding in one direction which would then make the AO useless. 

More information on how seeing can affect guiding http://openphdguidin...ed_settings.htm
I agree seeing will ultimately limit data quality though.
I didn't realize there was any dispute around allowing for 5-10 sec guide exposures if seeing was poor. I guess I learned something new.
It would be nice if a mount could keep up with seeing conditions, I guess that would mean all data would be truely diffraction limited.
My impression was that the only way to get to diffraction limited data collection was through "lucky imaging" techniques such as that used in planetary imaging.

Sent from my SAMSUNG-SM-G925A using Tapatalk


Edited by prostcj, 27 December 2016 - 02:01 AM.


#17 syscore

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Posted 27 December 2016 - 02:36 AM

"I am not sure what you mean by it's not all the error? Do you mean it's not all the error in the mount or considering PA error, cone error and stuff like that all together to do unguided imaging?"

 

I was only showing that periodic error is the error that is repeatable, predictable and can be negated with periodic error correction, but that it is only part of the "mechanical error" in a mount. There are other causes such as bearings, flexure and imprecise gearing that is not predictable and periodic error is generally only half of the mechanical error. The terms you mention are also causes of tracking error, but are longer term and can be predicted and modeled to some extent.

 

Many users tend to believe that periodic error makes up more of the mechanical error than it actually does. No doubt, the industry uses the term like they use to use 600X magnification. The reality is that you have mechanical error and only part of it is periodic and the split is often 50/50, even with premium mounts. Nonetheless, premium mounts are made more precisely, have much lower native PE, and much lower mechanical error. It isn't their low native PE that makes them better, they are simply made better, and as a result of that, have lower native PE as well as lower mechanical error overall.

 

Also, I was showing that while using PEC to negate the periodic component can improve unguided imaging, it can only go so far, and it offers very little improvement for guided imaging. This is mainly due to the short window of time (< 5 sec) in which guiding operates and in that time frame, the main errors are seeing and random mechanical errors. The only way to improve that is to make a better mount with lower mechanical error and image on better nights of seeing. Periodic error correction doesn't greatly affect what is happening in the short window of time an auto-guider operates. This is why schemes to make premium mounts out of regular mounts by correcting periodic error using encoders do not actually result in premium performance.

 

While native PE can be a useful measure of a mount's performance, it is not a guarantee. You can have low PE and yet have large non periodic mechanical error. Just because a manufacturer claims PE numbers that are as low as what a premium mount achieves, that doesn't mean the manufacturer has addressed all of the other mechanical errors similarly. The only real proof is how well the mount images. And keep in mind the focal length (pixel scale) you intend to use it at.


Edited by syscore, 27 December 2016 - 02:44 AM.


#18 blueman

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Posted 27 December 2016 - 02:36 AM

Personally, I use 3 second exposures, then there is a delay between them due to download and software interpretation, so that it is more like 3.5-4 seconds between corrections. This is usually good enough for my seeing.

 

However, I have tried 1 second with high aggression as Roland mentioned, it worked about the same, not enough difference to be able to say one was better.

 

Longer exposures cause me problems due to over exposure, you don't want your star to get saturated. So if I were to want to extend the time between corrections, I would add a delay in the guiding setup. So, for instance, 3 second exposure and a 3 second delay would give 6.5 to 7 seconds between them.

 

But for me, when I tried this technique, I found that guiding was not as good. But as I said before, I tend to adjust my guiding for the night before the imaging session starts. But mostly I only change aggression not exposure length.

 

Still, what ever works best for your system, with your seeing, well that is what you should use. ;)

Blueman



#19 syscore

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Posted 27 December 2016 - 02:43 AM

It is funny to me that people talk so much about chasing seeing. If a mount doesn't try to keep the star centered during turbulent seeing, the the star image will still suffer. The star will continue producing light and that will show up as smear or multiple areas of multiple exposure. So, if the mount could keep up with seeing, that could actually help IMO.

 

Roland told me once, that he was getting better results with 1 second guide exposures and high aggression settings, kind of counter to what is considered the case by many.

Blueman 

My M33 image was with 1 second guiding. My mount simply does better that way. I have tried longer guiding exposures, and I wish I could use longer guiding exposures, but I always end up back at 1 second. And the results speak for themselves. It really depends on your setup and your seeing and the only rule to follow is the "what works best" rule.



#20 orlyandico

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Posted 27 December 2016 - 04:13 AM

I've seen the Paramounts at GRAS/iTelescopes with multi-second guide exposures. Of course they use MaximDL which also tends to be far less aggressive than PhD.

 

IMHO a longer guide exposure does do a lot to average out the seeing, but the problem is if the star gets saturated, you don't get an accurate centroid anymore.  I have often wondered about putting an ND filter on the guide scope so that I can use 5-10 second exposures and still have non-saturated stars..



#21 HoriaC

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Posted 27 December 2016 - 02:12 PM

Hi Orly,

 

 

IMHO a longer guide exposure does do a lot to average out the seeing, but the problem is if the star gets saturated, you don't get an accurate centroid anymore.

 

If the programmers went by the book (see also https://en.wikipedia.org/wiki/Centroid ), there should be nothing special or inaccurate when computing the centroid for a saturated star.

 

Kind Regards,
Horia



#22 freestar8n

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Posted 27 December 2016 - 06:19 PM

Well this thread has many different ideas and recommendations - but mainly I applaud syscore for breaking it down and giving some examples.  It used to be that people on CN were constantly comparing PE curves and doing all kinds of things to make the value smaller - but in all those years there were hardly any examples where reducing or smoothing out the PE actually resulted in better *guided* images.  It was a classic case of confused cause and effect: High end mounts have low PE and produce tight stars - therefore if I reduce the PE of a mid-range mount - it will be more like a high end mount - and will produce tight stars also.  But there have been almost no examples where systematic improvements of the mount or the PEC actually produced better guided results.

 

In my case I also have a cge-pro and I just record about 8 turns of the worm with the PECTool and load up the average.  This has a clear benefit of reducing the PE and I do in fact use PEC while guiding - but I get very round stars either way and I haven't actually measured an improvement with PEC - but I can believe it is in the sort of "10-20%" range syscore mentions.  It should show as very slight elongation in RA - if it is there at all.

 

Now this thread has the usual concerns about guide exposure and "chasing the seeing" - but happily blueman cites someone who says that even for premium AP mounts - guiding at 1 second intervals shows benefit.  I agree completely - and that has been my focus for many years.

 

Now HoriaC mentions centroid algorithms and how - nowadays - they are straightforward.  Actually I don't agree with that.

 

There has been a slow enlightement in CN about guiding - from PE to OAG - but there are still some myths that remain - and I think the best thing to do is just forget everything you ever thought about PE and guiding - and start from first principles.

 

If the mount has some undesirable motion - like PE or gearbox noise - it will drift over time and then you want the autoguider to measure the drift and correct it.  There are two ways to reduce the drift - either reduce the overall amplitude - or make corrections more rapidly.

 

Let's say we need to correct 0.5" drift every second with a guide pulse at 0.5x sidereal.  That is 7.5"/second - so it would take only 67 milliseconds after one second.  If it drifts that far in one second and you only correct it every 5 seconds - you will have an error 5x as big.  Obviously if there is slow and steady drift - why on earth are people focused on reducing the overall *amplitude* of the drift over a worm cycle - rather than just correcting it more often??

 

So there is clear benefit in correcting more like every second - where the typical errors are perhaps 0.2" - in which case you are making fast but very small corrections.  If you find those corrections are not helping - then you can turn down the aggressiveness.  But as long as you are constantly measuring and correcting with low latency - there is no downside.

 

Most importantly - this will let you address what is really causing the stars to swell - and that is fast gearbox noise.  In order to remove that - you need to chase the mount itself - and forget about chasing the seeing.

 

I have more to add but will stop there.

 

Frank



#23 orlyandico

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Posted 27 December 2016 - 07:42 PM

Hi Frank,

 

Not sure how this would work, and I understand that Metaguide has a novel centroiding algorithm.

 

But 1-second guide exposures would be very much dominated by seeing effects, wouldn't they?

 

Also, the assumption is that the gearbox noise is worth chasing. This is true for low- to midrange mounts, but not high-end ones. For example, the Tak NJP (which uses a stock Vexta gearbox reduction) has around 4" p-p of gearbox noise. My guess, based on Vexta motor specs, is that the gearbox noise would have a period of around 30 seconds. That's only 0.1" / second which is well below the seeing threshold.

 

Manufacturers like AP make their own gearboxes which presumably would have even lower gearbox noise than the Vexta's in the Tak NJP.



#24 prostcj

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Posted 27 December 2016 - 07:42 PM

Guiding is reactionary, not pre-emptive.  I guess if you are guiding inside of your image scale by a good margin and the system has enough of a drift it could make sense to perform frequent reactionary corrections.  This technique will not work as well for longer focal lengths and smaller image scales. 

 

Not sure why members feel the need to "brow beat" on these forums based on personal experience. 



#25 tolgagumus

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Posted 27 December 2016 - 08:05 PM

Saturated guide star is never a problem. There is always a dimmer star. With my mount permanently mounted I sometimes do 30 40 second guide exposures. I discovered this works when I tried to use an older SBIG camera with the guide chip behind filters. I said why not, and it works as good. The only draw back was dithering. It took too long to settle. 




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