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Guide camera fl/Imaging camera fl

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#1 LuxAstron

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Posted 05 July 2020 - 01:48 PM

OK, I just wrote an entire thing  on this topic, and the page refreshed when hitting f5 accidentally, so lets do a TL:DR

I am stuck on understanding this. 

Oversampling is good. (you just cant see as good as your camera can handle?)

Under sampling is bad.

With guide-scopes and imaging scopes, you want your guide-scope / camera to be 1/3 FL of your imaging camera? After reading further into this, I came across a lot conflicting information I need help clearing up. 

Is the reason for 1/3 ratio being suggested is it is like a generalized "Good area" to be in? I read that the FL ratio between the guide and imaging don't really matter, the most important aspect is the pixel resolution being the same, if not better on the guide scope for more accurate guiding. Yes this will show many more spikes on a graph but is this due to it being tighter guiding and making more corrections? If your pixel size of your guide cam is 5, and your pixel size on the imaging cam is 2.5. This would be bad correct? It would be bad because the same star will have half as many pixels as the imaging, so the guiding will be half as accurate, but would not show as bad a graph because its a more generalized "happy area" for guiding due to it not noticing many of the corrections it should have picked up? Would this not cause the resolution/arc to be much higher because guiding is not as accurate? Everyone is talking about guides being a portion of your imaging scopes FL, but why is it not the other way? Why don't people use a imaging/guide combo if the guide was double the FL of the imaging scope?
Now in my head, I have it as nothing matters but the single star, how many pixels it takes up, how many pixels of that star your camera can see, if your guide camera, can not see more pixels or a higher pixel resolution then your imaging camera, it wont be as "good" guiding(EVEN if the FOV is tiny in comparison, its job is to guide, not show me a good view), am I correct on this aspect?

I am just a bit confused. the situation, If you have a telescope & Imaging Camera @ 2.5microns FL 625mm F/4, and a guide-scope @ 3.5microns  of 625mm(Barlow 3x), I see the guide-scope is slower, but does it matter how much light you are getting if the image is going to be oversampled?) Is oversampling not the issue of having TOO much light/noise coming in?

Does this change the end resolution of pixels of the guidescope to the same if not under that of the Imaging scope. With the imaging scope at .8" and the Guide Scope at .4", I know those are unrealistic conditions, but if you had them would it not be better to over-sample on the guide scope and have twice the pixel resolution vs having less pixel resolution and not as accurate guiding? as its much easier to aim at mthe center of a star thats 10 pixels across on the guide, and only 5 on the imaging. Even though the Imaging is WAY bigger FOV, the guides FOV does not matter right? 

Please someone rock my world, Slay these false notions and ideas, put me on the right path brother. I need to understand this.

I get a lot of people might wonder why even do AP if you don't have photo exp, or purchase expensive equip if you don't know the how to use it. That is why I am asking, only one way to figure this out and that is ask the Amateur/Professional community on here. I love space, I know most if not all of you do. I need to know what is out there, and this is just one step closer to getting me there.



#2 Stelios

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Posted 05 July 2020 - 07:00 PM

Uh... many would categorically disagree with "oversampling is good, undersampling is bad." And sampling considerations apply to your imager, not to your guider. Undersampling leads to squarish stars and a lack of fine detail (drizzle with dithering can fix most of this). Oversampling in effect leads to a larger image with no additional detail, as you are seeing limited (and possibly scope-resolution limited). 

 

None of these affect the guider. The guiding software can find the "centroid' of a largish and even deformed star, and can guide on a square star as well as on a round one. 

 

The ratio between guider and imager is a ratio of *image scale*. The pixel size of the cameras comes into the picture--the real ratio is ps_guide*fl_imager/(ps_imager*fl_guide). The rule-of-thumb is that the guider image scale should be no more than 3-5 times the imager image scale (3 or less being better). There's no lower limit--OAG is approximately 1x ratio and provides the best guiding. 

 

The *reason* for keeping the ratio low, is that the higher it is, the more a star could move in the main camera *without* triggering a motion in the less sensitive guidescope (which could not detect that fine a motion). In other words, the guide star would not move enough to trigger a guider correction, but the star in actuality *has* moved enough to affect the image on the main camera (which has a much finer resolution).

 

So why not guide with even longer F/L than the main scope? Primarily because the guider would issue corrections that would be unnecessary--stars in the main imager would not have shifted enough. If you are using a guidescope, you will also be magnifying the effects of seeing and any flexure inherent. And then there's the issue of a long and heavy guidescope adversely affecting tracking. But people *do* guide with OAG's which have the same F/L as the main scope.

 

The FOV of the guider may be smaller, larger, or different (as with OAG) than the FOV of the imager. As long as it yields a star suitable for guiding (single, sharpish peak) that's all you need from it. The rest of the FOV is for show smile.gif



#3 kathyastro

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Posted 05 July 2020 - 07:10 PM

The 3:1 ratio seems unduly conservative to me.  I guide with a 162mm guide scope on an imaging scope that is operating at 926mm.  That is 5.7:1.  While I wouldn't object to better guiding, I have not had any problems with it.  Suppliers of 162mm guide scopes claim they are good to an imaging focal length of 2000mm (12:1).  And PHD2 claims that the limit is determined by the SNR of the guide camera and that it can guide to about 20:1 if the SNR is good enough, due to its ability to detect sub-pixel movement.

 

I don't think I would want to try pushing the limit just to see what it will do, but a 3:1 ratio is a bit pessimistic, I think.



#4 imtl

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Posted 05 July 2020 - 07:39 PM

Kathy, 

I slightly disagree with you on this. I think it depends on what image scale you have and what are the seeing conditions. Working with 5:1 ratio might be fine but it depends on what image scale is involved. If you have an image scale of 1.5"/pixel and 5.7:1 ratio with your guider then you will be at 8.55"/pixel for your guide scale. Which is simply useless. A star will be swallowed by a single pixel and PHD2, even though it is very good cannot do magic. If someone has a noticeable movement of a star with 8.5"/pixel than they should pack up for the night.

 

So, it is true that 3:1 is not set in stone. But nothing really is. It depends on your system and the local condition. But as a general rule of thumb, especially for beginners, it is good to aim for no more than 3:1 - 5:1. The lower the better.

 

Eyal


Edited by imtl, 05 July 2020 - 07:54 PM.


#5 Stelios

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Posted 05 July 2020 - 09:20 PM

The 3:1 ratio seems unduly conservative to me.  I guide with a 162mm guide scope on an imaging scope that is operating at 926mm.  That is 5.7:1.  While I wouldn't object to better guiding, I have not had any problems with it.  Suppliers of 162mm guide scopes claim they are good to an imaging focal length of 2000mm (12:1).  And PHD2 claims that the limit is determined by the SNR of the guide camera and that it can guide to about 20:1 if the SNR is good enough, due to its ability to detect sub-pixel movement.

 

I don't think I would want to try pushing the limit just to see what it will do, but a 3:1 ratio is a bit pessimistic, I think.

My statement of the "rule-of-thumb" was 3-5 to 1 and I clarified that the lower end is where you want to be. I didn't make up the ROT, just reiterated it. 

 

You are scraping the high side at 5.7 to 1. This doesn't mean you can't guide,but you might do at least somewhat better with a longer F/L guidescope or smaller pixel guide cam, and you would almost certainly do better with an OAG. The "better" might or might not make an actual difference in the final image as so many other determinants exist. 

 

The 162mm guide scope makers used to claim 1500mm top (when I bought my StarShoot Autoguider). Now they claim 2000mm. Next year perhaps they'll guide the Hubble ;) There's rarely a limit to what marketing dictates. 

 

I think most experienced imagers realize that rules-of-thumb (like the guide ratio, or the "50% of mount load weight for imaging" or "swamp noise to between 5*RN**2 and 10*RN**2") are not indicative of cutoff points beyond which failure looms grim--they are just indicators of a range beyond which performance tends to worsen, usually in a gradual fashion. Newbies take these rules very literally--I've seen someone terrified of putting 18lbs of weight on a 30lb-rated mount. 



#6 freestar8n

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Posted 06 July 2020 - 03:42 AM

A lot of advice on autoguiding is simply repeated folklore and if you look for actual studies to show they really apply in the field - there won't be any.  It's just advice handed around that kinda makes sense - but if you think about it, it really doesn't help much.

 

Instead of thinking in terms of focal length, I would look at the target fwhm you are trying to image, and the fwhm of the guidestar - and the size of the guider pixels in arc-seconds.

 

If your guider fwhm is much larger than the imaging fwhm you are aiming for, then the centroid error in the guidestar will likely cause problems.  But the centroid error will depend on how much signal there is in the guidestar and how much sensor noise there is.  There is no simple way to characterize these things.  And if you have a faint guidestar you can always put more signal in it by exposing the guider longer - but then you end up with error contributed by not making the corrections fast enough.

 

None of this is directly tied to focal lengths or their ratios.  If your imaging fwhm is very large then you can tolerate error in your guide corrections - but if you are striving for very small imaging fwhm, you had better have a very small and strong guidestar image - in arc-seconds.

 

With OAG I aim for sub 2" fwhm in my images, and my guidestar is small and round and also sub 2" in most cases.  I could go ahead and guide with a 50mm guidescope with perhaps 1/10th the focal length of the sct and the fwhm might be 3" or so - as long as there is no flexure.  It won't be as good as with oag but it can be acceptable.

 

And keep in mind, with a high end mount you don't need to guide at all and you can get fairly small and round stars.  So you could do pretty well with no guidescope at all.

 

For beginners, my main advice would be not to feel you need a big and heavy guidescope with an sct - because it will probably be limited by flexure.  If you want to guide with a guidescope, just get a 50mm one with good optics and a sensitive guide camera with very small pixels and low read noise - like a 290.  Focus the guidescope carefully so the guide star is small and round - and keep exposures short to avoid flexure.  A 50mm guidescope with good optics can make a much smaller guidestar, in arc-sec, than a much larger and longer guidescope.  And it's the fwhm, in arc-sec, and signal strength that will limit the accuracy of the guide error.

 

Frank



#7 LuxAstron

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Posted 06 July 2020 - 02:05 PM

Thank all of you so much, I really appreciate all of this information. I have come to quite a bit more of an understanding as to what you want to avoid and what you are hoping for. 

 

 

The *reason* for keeping the ratio low, is that the higher it is, the more a star could move in the main camera *without* triggering a motion in the less sensitive guidescope (which could not detect that fine a motion). In other words, the guide star would not move enough to trigger a guider correction, but the star in actuality *has* moved enough to affect the image on the main camera (which has a much finer resolution).

I agree, this makes 100% sense and this is part of where my brain is having conflicting issues. So in my head, I am seeing the FOV from my main imaging camera, which is 183, 2.4 Pixel Size, 21MP, 1 binning. My Guide Camera is a 1.25MP 3.4 Pixel Size and 1 binning. 

In my head, I am looking at the pixel sizes, of 2.4 and 3.4, The goal is to bring them as close to each other as possible in order to best match the resolution for good guiding if I have been reading correctly, so wouldnt a barlow drop the effective pixel size to that of the imaging camera or lower, and this would be "Better as the guide would see closer and in more detail the same star the imager is seeing, even though this would be like seeing a super zoomed in piece of what the imager sees. 

If I can overlay in my head, the pixels from the guiide camera, perfectly over the pixels from the imaging camera but zoomed in, now if I could double the imaging cameras pixels, so every square on the imaging pixel, is 2 on the guide cam. Would this not mean better guiding due to higher resolutions on the guide cam.

I am using a OAG. I am trying to get the best guiding possible, and I want to make sure I can make my setup as tuned as possible. 
I have the 6" astrograph, a f/4, 610MM FL, with the 20MP and the 1.4MP guide. 

The guide camera image does NOT matter correct? just as long as it can keep accurately on the center of a star? my idea is that the bigger the star, the more pixels its taking up, the easier it is to find a perfect center, then I guess that is what you are saying comes into play, is the software can already determine the easily. 

I was going to do a guide scope, until I looked into it and saw that they were more expensive the longer the FL, so i opted for a OAG instead. Now for the OAG, I just want to make sure I am utilizing this correctly, as I CANT find a lot of information on using it properly, tuning it, making sure it even works with my setup.

 
I see these images others are taking and its insane, the amount of detail, and from what I see, its because they have SUPER tight guiding. At this point I am starting to believe imaging is based not off your camera, but the amount of time you put into pictures. Which 100% is all controlled by the accuracy of your mount, which is in turn tuned by your guidecamera. 

I don't want to be doing the guiding half-butt, I want it to be on par with the camera, so I am not being held back on the quality of the images and not utilizing the main camera because of something as simple as a miss configured guider. Again, so sorry for asking the same questions if I have, I just really want to make sure I am understanding the entire concept. Thank you, you all have been wonderful and a huge help.

 

 

Instead of thinking in terms of focal length, I would look at the target fwhm you are trying to image, and the fwhm of the guidestar - and the size of the guider pixels in arc-seconds.

I have looked into FWHM and I am pretty good at understanding most things, but having trouble grasping the concept and how it applies to stars. Is it the amount of light? You don't have to explain as I understand its a very complex subject. AS far as you are talking in terms of the guider pixel in arc-seconds, could this be the same as Resolution  Pixel / Arc Seconds? I have seen calculators for this and I have used them extensively, this is where a lot of my confusion has actually stemmed. Every calculator I read says everything I have is going to be great, but I noticed my Guide camera pixel size is larger then my imager, my imaging / guiding ratio is 1:1.56. .8"/sec to the guider which is 1.25"/sec

If I were to put a barlow, 1.6x on the guide, it would bring the pixels to .8", and .79" which is the closest to 1:1 I can get, would this be IDEAL for imaging and guiding? As any changes will be changes made in both,or is this where the scale starts to see like "Diminishing Returns" and your sacrificing the speed of the scope for the same pixel size?

I don't know if I amndering about something completely irrelavant and wont have any effect on me, or if this is a very fine line which would be considered too aggressive guiding, and I would be doing harm.

I just want to get the tightest guiding I can, I guess the goal is low RMS deviations?

Thank you. 
 


 



#8 freestar8n

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Posted 06 July 2020 - 10:47 PM

 

I have looked into FWHM and I am pretty good at understanding most things, but having trouble grasping the concept and how it applies to stars. Is it the amount of light? You don't have to explain as I understand its a very complex subject. AS far as you are talking in terms of the guider pixel in arc-seconds, could this be the same as Resolution  Pixel / Arc Seconds? I have seen calculators for this and I have used them extensively, this is where a lot of my confusion has actually stemmed. Every calculator I read says everything I have is going to be great, but I noticed my Guide camera pixel size is larger then my imager, my imaging / guiding ratio is 1:1.56. .8"/sec to the guider which is 1.25"/sec

If I were to put a barlow, 1.6x on the guide, it would bring the pixels to .8", and .79" which is the closest to 1:1 I can get, would this be IDEAL for imaging and guiding? As any changes will be changes made in both,or is this where the scale starts to see like "Diminishing Returns" and your sacrificing the speed of the scope for the same pixel size?

I don't know if I amndering about something completely irrelavant and wont have any effect on me, or if this is a very fine line which would be considered too aggressive guiding, and I would be doing harm.

I just want to get the tightest guiding I can, I guess the goal is low RMS deviations?

Thank you. 
 


 

This stuff is not simple and it doesn't lend itself to simple guidelines.  fwhm is the full-width-half-max of the star - and corresponds to its width in the image.  For a decent image all stars will have nearly the same fwhm regardless of total signal in them.  fwhm in the star images captures how good your seeing, focus and guiding is.

 

Meanwhile what matters about the guide star is its centroid - and how well it represents the error the guider needs to move to make a correction.

 

Neither is dependent simply on focal length or pixel size.

 

If you have a guidescope with 10um pixels and a guidestar that is 2um in diameter - if it is placed right at the vertex of 4 pixels it will provide a signal into each pixel that will capture any deviation of the star from that point.  And even if the star isn't right on the spot there will likely be signal in adjacent pixels to provide an error signal.

 

So - this stuff just doesn't lend itself to simple guidelines based on pixel size and focal length.  But you know you want to have a strong guidestar signal that is well sampled by the pixels for a good centroid to use for an error signal and correction.

 

Frank


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#9 Ken Sturrock

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Posted 07 July 2020 - 12:45 AM

Meanwhile what matters about the guide star is its centroid - and how well it represents the error the guider needs to move to make a correction.


Bingo.

Yet once again. There is no magic ratio. What matters is that the guide camera is well-sampled enough to calculate a reliable centroid. Once you have a reliable centroid, which is measured in fractions of a guiding pixel, the guiding routine will do the best that it can. The imaging train is just along for the ride - regardless of what it is.

 

Having said that, many of the popular ratios do result in reasonable image scales for guiding but that's more of a happy accident.



#10 LuxAstron

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Posted 07 July 2020 - 06:39 PM

 

 

So - this stuff just doesn't lend itself to simple guidelines based on pixel size and focal length.  But you know you want to have a strong guidestar signal that is well sampled by the pixels for a good centroid to use for an error signal and correction.

 

Frank

Thank you Frank, I appreciate the information. It makes more sense then it did before. I see how its not only based off the pixels and fl, as much as it is off seeing. I have learned quite a bit, Thank you everyone for your responses, and being patient. I will let you know how it works out with my guiding and all. I know I am going to make sure I keep a strong guidestar. I will try the best and I am sure there programs have some readouts I will start trying to really utilize, thank you. I will check out a few guides for anything else I might have a question about. You guys are awesome.
 


Edited by LuxAstron, 07 July 2020 - 06:45 PM.



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