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Histogram, exposure and gain for planetary imaging

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#1 dgreenheck

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Posted 26 July 2018 - 01:26 PM

Can someone please explain how to optimally use the histogram for planetary imaging? I capture using a ASI120MC camera with SharpCap 3.1.

 

I keep reading about people optimizing their exposure and gain settings using the histogram, but it has never been explained to me what features you are looking for in the histogram to indicate that you have those settings dialed in.

 

I've read conflicting information regarding what the gain should be set at. Damian Peach says you should capture at the highest frame rate and not be shy about bumping the gain way up (he suggested up to 70, which results in a lot of noise in my images). Then I read posts from others on these forums saying don't go above 35 since the read noise doesn't improve much past that. Hard to argue with Damian considering his results, but perhaps he has special equipment and his advice doesn't apply to me? Anyways, I'm hoping I can use the histogram as an objective tool to help me answer this question during an imaging session.

 

Also, there is a brightness/gamma feature in SharpCap that appears to shift the histogram to the right. Is there ever any reason to use this or does it just reduce your dynamic range?

 

Sorry, packed a bunch of questions in here, but hoping to spawn some discussion and hear what others have to say.



#2 Tom Glenn

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Posted 26 July 2018 - 04:11 PM

I don't know what the range in gain values is for the ASI120, but every camera has an acceptable range of gain.  What you will find, however, is that individual imagers are all over the map with gain recommendations, and many will obtain excellent images with very different settings.  This is largely dependent on the seeing you have, and how many frames you stack.  If you have excellent seeing, you can take relatively long exposures with lower gain, slower frame rate, and sometimes get a superior result than using high gain and high frame rate.  But in poor or average seeing the reverse is usually true.  You need enough experience to make a judgement call based on the live view of what type of shutter speed you need.  Bright objects like Mars can take incredibly fast shutter speeds (2.5ms and less) while dimmer objects like Saturn require much longer exposures.  This also depends on your scope used.

 

I use Firecapture instead of Sharpcap, but the basic principle of interpreting a histogram is that the number that is listed (and constantly fluctuating somewhat) is telling you what the brightest pixels are registering as, with 255 being the brightest number on an 8 bit camera (pure white).  If anything registers as pure white, the data is lost.  This is called "clipping".  Planets do not have a wide dynamic range, so you do not need to maximize the spread of the histogram like you generally want to in daytime photography.  You also want to leave enough room on the right hand side of the histogram for sharpening, which moves it to the right (makes it brighter).  If you captured at a 90%+ histogram you would almost certainly clip some regions to white after sharpening.  But even more important is that if you capture at too high a histogram, it means you are either using too slow a shutter speed, which results in a low frame rate, or your gain is too high, which yields a noisy image.  You can't go wrong with a 50% histogram on Jupiter, Saturn, or Mars.  Some people go higher, and some go lower.  But 50% is not a bad place to start. 

 

As far as maximizing frame rate, this can make sense, but with modern cameras and small ROIs, you can encounter a situation in which the maximum potential frame rate of the camera is faster than you want.  Take this for example.  On Mars, with a small region of interest, I can attain over 400 frames per second with my ASI224mc.  Because Mars is so bright, I can use a moderate gain value to achieve this result.  However, with Jupiter, I could also theoretically attain the same frame rate by using a 2.5ms exposure, but doing do would require me to use a very high gain and the result is actually worse than if I use a much slower exposure of around 6-8ms and a lower gain value.

 

This brings up the following observation.  Faster frame rates, if achieved by using high gain values, do lead to more frames captured.  However, in order to eliminate the noise, you now have to stack more frames.  A slower frame rate with a longer exposure and lower gain will lead to less frames captured, but you can achieve the same SNR with fewer frames stacked.  So there is a range of parameters in which the results will be identical.  But there is also a point at which the results diverge.  For example, in good seeing, I use a 25ms exposure on Saturn, which limits me to 40 frames per second.  However, the result is superior to what I get with a 10ms exposure, higher gain, and 100 frames per second, despite getting more frames that way.  Results with 15ms and 20ms exposures are largely similar to the 25ms exposure, but use different gain values, to keep the histogram at 50% or slightly below.  In average seeing I use the 15ms exposure, but in good seeing I consistently get better results with a 25ms exposure and lower gain.  It's a good exercise to experiment with different parameters and see which gives you the best result under which conditions.  

 

Also, you can adjust gamma to help you focus in live view....BUT return gamma to "50" before capture.  Otherwise it distorts your data in undesirable ways.  Gamma can (and should) easily be adjusted in post-processing, but not before.  


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#3 dgreenheck

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Posted 26 July 2018 - 06:02 PM

I don't know what the range in gain values is for the ASI120, but every camera has an acceptable range of gain.  What you will find, however, is that individual imagers are all over the map with gain recommendations, and many will obtain excellent images with very different settings.  This is largely dependent on the seeing you have, and how many frames you stack.  If you have excellent seeing, you can take relatively long exposures with lower gain, slower frame rate, and sometimes get a superior result than using high gain and high frame rate.  But in poor or average seeing the reverse is usually true.  You need enough experience to make a judgement call based on the live view of what type of shutter speed you need.  Bright objects like Mars can take incredibly fast shutter speeds (2.5ms and less) while dimmer objects like Saturn require much longer exposures.  This also depends on your scope used.

 

I use Firecapture instead of Sharpcap, but the basic principle of interpreting a histogram is that the number that is listed (and constantly fluctuating somewhat) is telling you what the brightest pixels are registering as, with 255 being the brightest number on an 8 bit camera (pure white).  If anything registers as pure white, the data is lost.  This is called "clipping".  Planets do not have a wide dynamic range, so you do not need to maximize the spread of the histogram like you generally want to in daytime photography.  You also want to leave enough room on the right hand side of the histogram for sharpening, which moves it to the right (makes it brighter).  If you captured at a 90%+ histogram you would almost certainly clip some regions to white after sharpening.  But even more important is that if you capture at too high a histogram, it means you are either using too slow a shutter speed, which results in a low frame rate, or your gain is too high, which yields a noisy image.  You can't go wrong with a 50% histogram on Jupiter, Saturn, or Mars.  Some people go higher, and some go lower.  But 50% is not a bad place to start. 

 

As far as maximizing frame rate, this can make sense, but with modern cameras and small ROIs, you can encounter a situation in which the maximum potential frame rate of the camera is faster than you want.  Take this for example.  On Mars, with a small region of interest, I can attain over 400 frames per second with my ASI224mc.  Because Mars is so bright, I can use a moderate gain value to achieve this result.  However, with Jupiter, I could also theoretically attain the same frame rate by using a 2.5ms exposure, but doing do would require me to use a very high gain and the result is actually worse than if I use a much slower exposure of around 6-8ms and a lower gain value.

 

This brings up the following observation.  Faster frame rates, if achieved by using high gain values, do lead to more frames captured.  However, in order to eliminate the noise, you now have to stack more frames.  A slower frame rate with a longer exposure and lower gain will lead to less frames captured, but you can achieve the same SNR with fewer frames stacked.  So there is a range of parameters in which the results will be identical.  But there is also a point at which the results diverge.  For example, in good seeing, I use a 25ms exposure on Saturn, which limits me to 40 frames per second.  However, the result is superior to what I get with a 10ms exposure, higher gain, and 100 frames per second, despite getting more frames that way.  Results with 15ms and 20ms exposures are largely similar to the 25ms exposure, but use different gain values, to keep the histogram at 50% or slightly below.  In average seeing I use the 15ms exposure, but in good seeing I consistently get better results with a 25ms exposure and lower gain.  It's a good exercise to experiment with different parameters and see which gives you the best result under which conditions.  

 

Also, you can adjust gamma to help you focus in live view....BUT return gamma to "50" before capture.  Otherwise it distorts your data in undesirable ways.  Gamma can (and should) easily be adjusted in post-processing, but not before.  

Thanks for taking the time to write up that detailed reply, Tom. Incredibly useful info there.

 

Sounds like it's just experimentation and experience with a dose of knowledge (as all things in astrophotography seem to be lol.gif).

 

I did have one question: you say gamma of 50. In SharpCap, the brightness ranges between 0 and 20. Are you saying that I should be capturing with the brightness at the midpoint? Right now I have it set to 0.



#4 Tom Glenn

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Posted 26 July 2018 - 06:15 PM

 

I did have one question: you say gamma of 50. In SharpCap, the brightness ranges between 0 and 20. Are you saying that I should be capturing with the brightness at the midpoint? Right now I have it set to 0.

Brightness is different than gamma, so make sure you are identifying the correct slider.  I'm not sure about Sharpcap, but in Firecapture, brightness affects the black point.  If you increase the brightness you are moving the left hand side of the histogram off the left edge, essentially setting pure black to a few points above 0 on the 0-255 scale.  This is generally advisable to avoid clipping to black, although this is less important on the planets versus the moon, which has much more subtle shadow detail.  Gamma relates to the midtones of the histogram, in between black and white.  Generally, increasing gamma brightens up the midtones without affecting the black point or white point, while lowering gamma does the opposite.  Although in the current version of Firecapture, moving the gamma slider has the opposite effect, which is very confusing.  Lowering gamma in post-processing is one way to add contrast to an image.  But you want the midtones to be set at a neutral level and not skewed between either black or white when capturing.  This can equate to a gamma setting of "50", or it might be listed as a "neutral" setting, or potentially as "1".  Hard to say without seeing the software, but you want gamma to remain at default (unless the default setting is wrong, which would be unfortunate).  


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#5 APshooter

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Posted 26 July 2018 - 08:25 PM

I read somewhere that a gamma of 50 in Firecapture is essentially unity gain for most of the smaller ZWO cameras like the 224MC.  Turning the gamma to 0 would be bad, as you suggested above, Tom.  Turning it up increases contrast and helps with focusing...but yes, definitely return it to 50 before capturing.



#6 ToxMan

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Posted 26 July 2018 - 09:17 PM

I haven't been using the ZWO ASI120MC. But, have noticed noise is a problem for many users.

 

Filling the histogram is a balancing act. It is valuable to look at and understand what it is telling you. Too high video gain setting is where the noise comes from. Too low, and the subject will be dim. And, those features closer to the black point value begin to fade into each other You want to find a happy medium...Not too dim, not too high of a gain setting. Also, with slower frame rates and congruent shutter speeds, you can get a brighter subject, lower gain, and less noise. Fill at least half the histogram. I try to get 60% with my camera.

 

I think the problem is that we shoot ourselves in the foot. Sure, a larger primary mirror gathers more light. That's why many go "big." Then we add barlows, increase focal length for a given aperture, and this dims the subject. So, magnification maneuvers with barlows have to be taken into consideration. If you are getting too much noise, you can not only turn down the gain or frame rate, but also consider if you need to turn down the magnification...shorten the focal length to brighten the subject making it easier to fill the histogram. High-res planetary imaging makes us want to push the envelope. Let's face it, some equipment combinations are not going to give us good images at f25 or f30, and probably should be at f20...or less. And, then there is the problem of over-sampling for the seeing conditions. Or, under-sampling...This is about dialing in our rigs for optimum performance.

 

For the camera I'm currently using, the default gamma setting works. I don't mess with brightness slider. It's default is zero. Not sure if this is true with your ZWO.  So, open histogram...pay attention where noise creeps in at various gain settings. And if your gain setting is not filling the histogram at least 50% and controlling noise, then decrease frame rate/shutter speed, or your magnification to a lower f ratio, shorter focal length for your aperture.



#7 RedLionNJ

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Posted 27 July 2018 - 11:11 AM

I read somewhere that a gamma of 50 in Firecapture is essentially unity gain for most of the smaller ZWO cameras like the 224MC.  Turning the gamma to 0 would be bad, as you suggested above, Tom.  Turning it up increases contrast and helps with focusing...but yes, definitely return it to 50 before capturing.

Unity gain for the planetary cams (at least the ZWO ones) is a whole different subject.  The gain slider in FC has a specific level identified for each cam which represents unity gain.  This is totally independent of the gamma setting.



#8 Tom Glenn

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Posted 27 July 2018 - 12:57 PM

Unity gain is the gain setting at which 1 electron (1e-) corresponds to one Analog to Digital Unit (ADU) in the sensor.  So on a graph of gain settings, unity gain occurs at the gain in which e-/ADU=1, such as this taken from the ZWO website for the ASI224 (note this chart applies to the sensor when capturing 12 bits--in 16 bit capture mode).  Unity gain here is 135.

 

ZWO_gain_asi224.jpg

source: https://astronomy-im...roduct/asi224mc

 

It's best not to worry much at all about this though, because it can quickly get confusing and has more to do with DSO imaging in which photons are very limited in supply.  But in theory, there is a benefit to sample at or above the unity gain settings, because you will be sampling at a sub electron level.  This really only applies to DSO imaging, because in this case if you set the gain too low, you get a banding artifact, or posterization of the final image in the darkest regions, because the sensor was unable to differentiate between different levels of very small numbers of electrons, so they got bundled together (leading to a bucketing effect, or posterization).  Increasing the gain setting to arrive at 0.5e-/ADU sampling is often recommended.  But in planetary imaging, photons are not limited in the same way they are in DSO imaging, and following this type of advice will often lead to suboptimal gain settings.  Also worth noting that in the charts provided by ZWO, for example, they are all given for a camera operating in full bit depth mode (essentially in 16 bit capture mode). Planetary imaging is done in 8 bit mode, so the unity gain settings are completely different.  For the ASI224mc, unity gain in 16 bit mode (only 12 bits captured though) is 135, whereas in 8 bit mode I believe unity gain is 360.  For many targets, it turns out that a gain of 360 is a good option with this camera, but in some circumstances, this will be too high.  If you were to sample at 0.5e-/ADU in 8 bit mode, you would be using a gain of well over 400, approaching 500, which is useful in some circumstances with this camera (Neptune, Uranus) but in general you don't want to set gain quite that high for most planets or the moon.  


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#9 kevinbreen

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Posted 29 August 2018 - 06:25 PM

Hi Tom

I just tried to capture the moon but clouds rolled in.
Quick question, what brightness setting would you recommend for Lunar? You mentioned raising it slightly to avoid clipping into black ...
And another quicky - is 60% histogram about “right” in your opinion for Lunar.
I was going to ask what your preferred gain setting would be but I see you from above that you don’t use the 224 that I do, and optimal gain is sensor-specific.
Your lunar images above are beautiful by the way

Kev

#10 Tom Glenn

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Posted 29 August 2018 - 08:32 PM

Hi Tom

I just tried to capture the moon but clouds rolled in.
Quick question, what brightness setting would you recommend for Lunar? You mentioned raising it slightly to avoid clipping into black ...
And another quicky - is 60% histogram about “right” in your opinion for Lunar.
I was going to ask what your preferred gain setting would be but I see you from above that you don’t use the 224 that I do, and optimal gain is sensor-specific.
Your lunar images above are beautiful by the way

Kev

Kevin, there are no absolute answers here, but I would say to use a 70% histogram on the moon.  This is based on recent experience doing a lot of lunar imaging and processing.  You definitely don't want the histogram to bee too low, but 70% gives you enough cushion for sharpening.  The times I have captured at above 70%, it still works out OK, but you run into issues with whites getting clipped pretty quickly with sharpening, so then it becomes a PITA because you have to use multiple versions of the image and layer masking in PS to save it.  

 

As for brightness, I didn't even touch the slider in the past and the default was 0 and this was fine.  Now I use a setting of "8".  I don't think it matters much, but a slight increase in brightness means you have no black in the image.  In principle this is better, and allows you to control the black point later, but looking back at older images I don't see much of a problem with a 0 setting (although I still do 8).

 

As for cameras, I used to use the ASI224mc on the moon, and in fact, some of my best resolution images come with this camera and a 2x barlow with my C9.25 Edge.  The reason I now use the ASI183mm for lunar imaging is that I quickly became dissatisfied with the tiny field of view of the ASI224mc.  It's great for high resolution work, but I found myself wanting more field coverage.  Interestingly, when using a larger sensor you solve one problem but create another.....namely the spread in dynamic range becomes extreme, and so the histogram settings matter even more, and processing a single image can become much more difficult.  When using smaller sensors, you usually have less of a dynamic range in the field of view, and in some ways this makes processing easier because you don't have to stretch the histogram as much.  

 

With the ASI224 on the moon, gain settings of 200-300 are pretty typical in my experience.  The real strength of this camera is the frame rate, which allows you to get a ton of frames in 30-60s captures, and you can reduce noise by stacking lots of frames.  By way of example, here are two images from last year that I took with the ASI224 (click for full size).  I haven't posted these before since these were taken before I joined CN.  The exposure in both images was 5ms, and the gain was 240 for the first, and 290 for the second.  Frame rate was 150 fps.  These captures were 10,000 frames and then 2500 stacked.  That's another advantage to a small FOV, you can get tons of frames without the file sizes becoming too huge.  With the ASI183mm, I never capture more than 4000 frames when using the full sensor, because even that amounts to an 80 GB file.  

 

20170910-040705_TG.jpg

 

20170910-043249_TG.jpg


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#11 DMach

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Posted 31 August 2018 - 12:04 AM

Interesting discussion, always enjoy reading these and picking up new info.

 

I did a lot of reading when I started imaging, trying to get a jump start on the "best" settings. But in hindsight, the most accurate advice I read back then was "you'll need to experiment and find the best settings for your telescope, depending on the aperture, target, seeing etc".

 

So keeping in mind that my setup is as below as YMMV:

 

Celestron 6SE

ASI290MC

TeleVue PowerMate 2.5x

 

I have experimented a fair bit. I started by trying to maximise frame rate, but in the end have found I get better results when I prioritise signal-to-noise per frame over frame rate. I found that anything above a gain of 300 gives me too much grain. As long as the seeing is decent (and you kinda need it to be for sharp images anyway) anything above 50fps (20ms exposure or less) works fine for me.

 

Also, I find the stacking works best when I aim for a histogram in the 60-70% range (leaving buffer for sharpening without clipping). This may be a consequence of my smaller aperture, which inherently gives less contrast/detail and may make it hard for the stacking to be accurate at lower histogram levels (pure speculation here).

 

For dimmer targets (e.g Saturn) I find I need to accept a lower histogram (maybe 50%) and longer exposures. So I rely heavily on good seeing and transparency here.

 

Last but not least, collimation and focus are obviously critical ... but it took a bit of time and practice for me to get these spot on (reproducibly).

 

Good luck, and have fun!


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#12 FlankerOneTwo

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Posted 01 September 2018 - 03:14 PM

Good discussion! One aspect that I didn't see discussed is the effect of atmospheric seeing. Good seeing implies relatively large, slow moving atmospheric cells of homogeneous density - therefore longer exposures can be used. Poorer seeing implies smaller and/or faster moving air cells and shorter exposures are needed in order to minimize blurring caused by transition between cells. The size of the cells also dictates the maximum aperture that can be used to image without covering seeing cell transitions boundaries. The combination of air cell size and wind speed dictated exposure time.

 

Long story short, in average-ish seeing (1-2" FWHM) and average wind speeds, max effective aperture maxed out at 4" and required a 3 ms exposure to negate the effects. Real life seems to suggest you can go above these a bit both in aperture and exposure length so the model probably isn't perfect and/or probably you don't need to be perfect to get good results.

 

For myself (and I don't profess to be an expert), I select exposure time based on visual estimate of seeing and then adjust gain to yield a peak histogram around 70-80% (other use less). I'm typically stacking at least 2000 frames. If noise is objectionable after stacking at these settings - well, you can't beat the seeing.

 

https://www.innovati...FullVersion.pdf


Edited by FlankerOneTwo, 01 September 2018 - 03:14 PM.


#13 Tom Glenn

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Posted 02 September 2018 - 07:01 AM

Haven't read that entire document yet but just glanced at it.  Seems to provide interesting information and formulas, but I feel like those formulas may provide misleading information for most people form a practical standpoint.  You cite an example of requiring a 3ms exposure to freeze the turbulence, and that may be true from a mathematical standpoint according to the formulas provided and the conditions, but from a practical standpoint, using super short exposures can be detrimental.  I can tell you that I rarely use exposures of less than 8ms, and often use exposures of up to 25ms with excellent results.  Individual results will vary depending on conditions of course, but most people will not be using exposures of less than 3ms except on something super bright like Mars.  Even the moon, which is bright enough to support sub-ms exposures, always gives better results in my experience with longer exposures and lower gain.  The benefits of completely freezing the turbulence with a super short exposure can be quickly offset by the reduced light and necessary increase in gain.  I think that slightly longer exposures, while allowing some movement from turbulence (at least according to those formulas), can be overcome with stacking and deconvolution.  But again, I don't bother imaging in conditions of heavy turbulence, so your experience may vary.  But I can't think of many objects, other than Mars, Venus, and the Moon, in which it would even be possible to use an exposure of 3ms.  Certainly not for Saturn, at least not on my scope (my results on Saturn have always been best using above 15ms exposures, but this is in good seeing).  Interesting article though for sure.  


Edited by Tom Glenn, 02 September 2018 - 07:04 AM.

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#14 RedLionNJ

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Posted 03 September 2018 - 09:24 AM

With all due respect, I attended Gaston's talk where he delivered this content. It was clearly aimed at classic astrophotography - very, very minimally-related to the techniques used in modern planetary imaging.

 

With amateur-sized instrumentation (up to, say, a 16" scope), steady skies ("excellent seeing") is by far and away the #1 factor in getting good data. When the skies permit, it doesn't matter if you have a 320x240 pixel, 30fps webcam or a [insert latest planetary cam here]. If you're imaging Mars right now, you would get equally good results with both devices.

 

As far as really short frame lengths go, 3ms sounds to me like it's asking for trouble. Each night out (sometimes each HOUR, or each TARGET), I experiment a little to try to find the sweet spot where as many crucial criteria as possible are satisfied. These include an exposure/gain combination which swamps "shot noise", as well as an exposure which may not entirely "freeze" the waving/distortion/de-focusing, but will at least minimize it and also hope to include sufficient "just-distorted" frames to build up a nice Gaussian stack which will be amenable to wavelet-processing. Then there's also the practicality of USB bandwidth, histogram requirements, etc. Kind of like a hexagon made of a taut string and if you adjust one side, there's a knock-on effect on all the other sides. With anything less than close-to-perfect seeing, it's tough, very tough.



#15 Achernar

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Posted 03 September 2018 - 12:30 PM

Interesting discussion, always enjoy reading these and picking up new info.

 

I did a lot of reading when I started imaging, trying to get a jump start on the "best" settings. But in hindsight, the most accurate advice I read back then was "you'll need to experiment and find the best settings for your telescope, depending on the aperture, target, seeing etc".

 

So keeping in mind that my setup is as below as YMMV:

 

Celestron 6SE

ASI290MC

TeleVue PowerMate 2.5x

 

I have experimented a fair bit. I started by trying to maximise frame rate, but in the end have found I get better results when I prioritise signal-to-noise per frame over frame rate. I found that anything above a gain of 300 gives me too much grain. As long as the seeing is decent (and you kinda need it to be for sharp images anyway) anything above 50fps (20ms exposure or less) works fine for me.

 

Also, I find the stacking works best when I aim for a histogram in the 60-70% range (leaving buffer for sharpening without clipping). This may be a consequence of my smaller aperture, which inherently gives less contrast/detail and may make it hard for the stacking to be accurate at lower histogram levels (pure speculation here).

 

For dimmer targets (e.g Saturn) I find I need to accept a lower histogram (maybe 50%) and longer exposures. So I rely heavily on good seeing and transparency here.

 

Last but not least, collimation and focus are obviously critical ... but it took a bit of time and practice for me to get these spot on (reproducibly).

 

Good luck, and have fun!

I use the same camera you do, and I agree that when the gain is much above 300 using Sharp Cap, the video is too noisy. I get better results with gain settings of 300 or less.

 

Taras




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