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Old 08-07-2014, 01:31 PM
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Ken,

The A/D like many cameras may be 16 bits but the camera itself is really only capable of producing 11.6 bits of information because of the well depth and read noise.

Log2 (WellDepth e- / ReadNoise e-)

Although that is then artificially increased by the non linear antiblooming capability of the CCD circuit in terms of reproducing the dynamic range of the original target, the camera is still only giving you 11.6 bits of useable data.

Unfortunately some camera makers like to "oversell" to their consumers by quoting the A/D circuit capability ! I am being kind here !!

True 16 bits would be 64,000 e- well depth and 1 e- read noise or 256,000 e- and 4 e- read noise !

Rally

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Originally Posted by Merlin66 View Post
Is this a 16bit camera???
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  #22  
Old 08-07-2014, 06:14 PM
Fizics (John)
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As I understand it is important to have a good match of pixel size, focal length, and seeing conditions to optimize sensitivity and resolution. So it all comes down to arcseconds per pixal. Reference this link http://www.stanmooreastro.com/pixel_size.htm
and conveniently he has a link to calculate arc secs per pixal http://www.stanmooreastro.com/Pixel_size_calc.html

John
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  #23  
Old 09-07-2014, 07:37 AM
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Originally Posted by Shiraz View Post
that's entirely consistent with earlier posts.

Binning occurs in 2 steps.

1. First, the signal from pairs of adjacent pixel is added by putting the charge from 2 pixels (rather than one) into each cell of the shift register - for a saturated 8300, this results in trying to stuff 51,000 electrons in each SR cell. But, the shift register cells cannot hold that much and saturate at 37,000 electrons - so you lose ~1/3 the available full charge in this step.
2. The (37,000 electron) charge packets in the SR cells are then transferred into the output register in pairs, so 74,000 electrons turn up at the output register for each binned pixel. The output register cannot handle more than 55,000 electrons, so some of the available electrons are again lost to saturation and you are again down by about 1/3.

ie, in the two processes, you lose about half of the total number of electrons available from the 4 saturated pixels - you start out with 25,500x4 and end up with ~55,000, simply because you cannot squeeze all of the available electrons from 4 saturated pixels through the transfer electronics.

The final step is that the gain of the output stage is normally set to give 64000 ADU with only one pixel-full of electrons (25,500). With binning, it is presented with a bit more than double that many electrons per pixel, so the gain must be reduced by a roughly a half to keep the maximum reading to 64,000 ADU.
Hi Ray,

Great explanation. How does this effect a non-saturated signal, say one at about 5,000? Would you still get the full 20,000 if you binned?

The reason I ask is that you guys turned me away from taking my RGB at 2x2 binned because you argued (quite convincingly) that I was better off getting RGB at 1x1 on a KAF8300. But if the argument is only for saturated signals, really it doesn't matter to me as these are usually just the stars, which I would tend to take shorter subs for and overlay the saturated ones in PS.

And, yes, cloudy days are great for expanding you knowledge rather than actually doing any astronomy.

Cheers
Stuart
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Old 09-07-2014, 12:00 PM
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Quote:
Originally Posted by rally View Post
Ken,

The A/D like many cameras may be 16 bits but the camera itself is really only capable of producing 11.6 bits of information because of the well depth and read noise.

Log2 (WellDepth e- / ReadNoise e-)

Although that is then artificially increased by the non linear antiblooming capability of the CCD circuit in terms of reproducing the dynamic range of the original target, the camera is still only giving you 11.6 bits of useable data.

Unfortunately some camera makers like to "oversell" to their consumers by quoting the A/D circuit capability ! I am being kind here !!

True 16 bits would be 64,000 e- well depth and 1 e- read noise or 256,000 e- and 4 e- read noise !

Rally
very well summarised Rally - not widely accepted though, which is a pity.

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Originally Posted by rat156 View Post
Hi Ray,

Great explanation. How does this effect a non-saturated signal, say one at about 5,000? Would you still get the full 20,000 if you binned?

The reason I ask is that you guys turned me away from taking my RGB at 2x2 binned because you argued (quite convincingly) that I was better off getting RGB at 1x1 on a KAF8300. But if the argument is only for saturated signals, really it doesn't matter to me as these are usually just the stars, which I would tend to take shorter subs for and overlay the saturated ones in PS.

And, yes, cloudy days are great for expanding you knowledge rather than actually doing any astronomy.

Cheers
Stuart
Hi Stuart. with wells of 5,000 ADU, the binned signal would be 20,000 ADU, but that would then be scaled by about 0.5 (along with the noise). Net result is better SNR from binning. If you use HDR methods, the loss of dynamic range on stars is not a problem.

However, apart from the loss of dynamic range, I understand that binning with the 8300 is problematical for 2 reasons:
1. the read noise increases for binned operation (probably not big issue, but requires longer subs than expected)
2. the saturation in the registers somewhere along the line result in one sided blooming on brighter stars (I have no experience with this chip, but the effect is widely documented). Presumably there are parts of the chip that are not protected by ABG - they would not normally need to be, but are pushed into saturation by the higher net signal.

In summary, imaging at 1x1 with subs that are long enough to bury read noise allows full resolution for targets with small bright features that might benefit from high res. If you decide that you don't have enough colour data at 1x1, you could alternatively use software binning on this data to get the full binning advantages of 4x dynamic range, 2x SNR and no artefacts. So I guess that the basic question is why bother with on-chip binning.

Clouds?*++%. I have a new mount sitting here and an AO to try out - naaah not going to happen any time soon.

Last edited by Shiraz; 10-07-2014 at 11:20 PM.
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  #25  
Old 09-07-2014, 12:53 PM
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So I guess that the basic question is why bother with on-chip binning.
Only if there is a significant read noise advantage? Not the case with many of the current Kodak/TrueSense sensors.
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Old 10-07-2014, 10:29 PM
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Snr

Noise is only read once if you on chip bin whereas software binning results in 4X noise if 2X2 binning - the software reads the noise from all 4 pixels. Better to on chip bin.
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  #27  
Old 10-07-2014, 11:09 PM
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read noise adds in quadrature, so 2x2 binning in software only results in 2x the read noise, not 4x. In addition, the 8300 produces significantly more read noise in hardware binning mode than in 1x1 mode so there is actually not a lot of read noise advantage from on-chip binning.

However, if you take 1x1 subs that are long enough that the read noise is insignificant cf the shot noise, read noise is not an issue at all with software binning and there really is is no advantage to on-chip binning, only disadvantages.

Last edited by Shiraz; 10-07-2014 at 11:30 PM.
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Old 11-07-2014, 10:09 AM
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Originally Posted by Shiraz View Post
However, if you take 1x1 subs that are long enough that the read noise is insignificant cf the shot noise, read noise is not an issue at all with software binning and there really is is no advantage to on-chip binning, only disadvantages.
Good point, Ray. I also like to do my RGB unbinned so I have the option of extracting additional luminance data.

Cheers,
Rick.
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  #29  
Old 11-07-2014, 12:27 PM
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Originally Posted by Shiraz View Post
Software binning is possibly a better option overall - if you expose the 1x1 subs for long enough to bury the read noise you get all the advantages (and disadvantages) of bigger pixels - 4x the sensitivity, 4x the well depth and 1/2 the linear resolution.
Hmmm - the reason to bin in hardware is surely to get a reduction in exposure time or better s/n for the same exposure time (given you have small pixels and can trade away the resolution). Any software binning after the event is not going to give you signal that is not there (ie that is is lost in the background noise).

If we assume our CCD has a readout noise of 10e and out signal is twice that, then in the single pixel example each pixel is readout with a noise of 10e hence we achieve a signal to noise ratio of 2:1 (20e/10e). Even if we subsequently sum the four pixels in a computer after readout the signal to noise ratio becomes 4:1. In adding the four pixels we sum the signal (4 times 20e i.e. 80e) and the noise is added in quadrature i.e. square root of the sum of the noises squared (square root of 4 times 10 squared i.e. 20e). In the binned example there is no noise until the signal is readout by the amplifier so the signal to noise ratio is 8:1(80e/10e) i.e. twice as good as the single pixel readout mode.

Or at least that is my understanding....
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Old 11-07-2014, 03:53 PM
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I don't quite understand what you mean by signal being lost in background noise - apart from the dark current pedestal (which could possibly be considered to be noise), noise is simply variability in the signal - it is not something that in some way can obliterate the signal. Binning is just one way to average numerous independent measures of the signal to reduce the uncertainty (the noise). It doesn't matter if you do it on the chip or in software, the same signal will be there in either case.

Your analysis is correct in that perfect on-chip binning provides 2x the SNR of software binning. However, the 8300 does not do perfect binning - from a few sources, the single read noise in binning is somewhere around 1.3x that in 1x1 mode, so some of the SNR advantage is lost that way.

But the main point is that, if you get enough signal in the subs that shot noise dominates (and that is what you should aim for anyway), then it doesn't matter much what the read noise is in software or hardware binning. In your example let's take a more realistic signal of 1600 in each pixel, for which the total shot noise will be 80 with 2x2 binning. For your read noise of 10, the combined read/shot noise with on-chip binning would be 80.6 and for software binning 82.5 - ie there is only a minor advantage to on-chip binning. That very slight advantage would likely be overwhelmed by the increased single-event read noise in the on-chip binning mode of the CCD (this is certainly so for the 8300). In these circumstances software binning provides better dynamic range, a little bit more SNR and more flexibility.

Last edited by Shiraz; 11-07-2014 at 04:12 PM.
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Old 11-07-2014, 04:34 PM
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Originally Posted by Shiraz View Post
I don't quite understand what you mean by signal being lost in background noise
I must have expressed it badly for a given temp/ccd/gain/exposure noise will be fixed whereas signal will vary with the brightness of the object, thus for a short exposure faint details are lost as they cannot be distinguished from the noise. We use longer exposures or multiple exposures to deal with this.

I take your point re s'ware binning but it has been my experience that s/n is better with hardware binning - I guess it depends on the chip.
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Old 11-07-2014, 05:51 PM
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I must have expressed it badly for a given temp/ccd/gain/exposure noise will be fixed whereas signal will vary with the brightness of the object, thus for a short exposure faint details are lost as they cannot be distinguished from the noise. We use longer exposures or multiple exposures to deal with this.

I take your point re s'ware binning but it has been my experience that s/n is better with hardware binning - I guess it depends on the chip.
This has been a very interesting thread.
The 8300 seems to not be the best chip to demonstrate the advantages of hardware binning, and good at showing when its NOT usefull!.

So, yes, it does seem to depend on the sensor and I agree its all about read noise (when the sensor allows a binning advantage).

My 6303 (NABG) sensor has high read out noise but large well depth. 100000 per pixel cell and 330000 for register cells.

Anecdotally anyway, I get better S/N with hardware bin 2*2 because the 6303 has high read noise and its difficult to overcome read noise in dark skies. It requires very long exposures.

Binning with urban imaging, even with short exposures with an 8300 sensor does appear to be fruitless.

Last edited by Bassnut; 11-07-2014 at 06:05 PM.
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Old 11-07-2014, 06:28 PM
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Binning is a bit like megadata. Ideally you alway use 1x1 but sometimes its time efficient to use 2x2.

2x2 is fine for non critical detail in images like O111, S11, Ha if its not full of detail or the main item in the image.

If you want the best possible image and have tons of time, clear skies, no bad weather and reliable gear then sure go for 1x1 and add up the signal to noise ratio.

Greg.
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  #34  
Old 11-07-2014, 06:43 PM
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Keep them coming gents, I am learning tonnes just by reading through the posts.
Bo
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Old 11-07-2014, 06:50 PM
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Quote:
Originally Posted by gregbradley View Post
Binning is a bit like megadata. Ideally you alway use 1x1 but sometimes its time efficient to use 2x2.

2x2 is fine for non critical detail in images like O111, S11, Ha if its not full of detail or the main item in the image.

If you want the best possible image and have tons of time, clear skies, no bad weather and reliable gear then sure go for 1x1 and add up the signal to noise ratio.

Greg.
Well, thats a bit sweeping, dunno about that.

For colour non res critical data your still better off, depending on sensor (as discussed) , and if your not using RGB to add to bin1 Lum, as in NB, and you are imaging in dark skies, even huge NB colour data is better in bin 2 unless you also have long, as in or more than 1hr subs.
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Old 11-07-2014, 08:53 PM
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Binning and read noise

I have been at this for a very short time so limited experience but my impressions from listening and reading are as follows:-

1 The major reasons to bin or not are driven by seeing, focal length, pixel size and the need to avoid under or over sampling.

2 When a CCD is read the charge in each pixel or group of pixels if binned is shifted to the output registers and then read. It is at this point that read noise is introduced. Therefore whether you read 1x1, 2x2 or 3x3 pixels you only introduce read noise once. This means that your signal to noise ratio increases if you on chip bin and that's good.

3 Binning however sacrifices resolution and that may be not so good.

4 There are other sources of noise (eg dark current) but I am under the impression these are not affected by binning

Can anyone tell me if I have that approximately right and if not where my thinking is off track.

There have been suggestions that the 8300 chip suffers from extra read noise above that experienced when not binning if on chip binning occurs. I would be grateful if anyone can explain how this happens and better still put some sort of value on it. I am afraid I am one of those irritating nerdy types who just HAS to know how things work
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Old 11-07-2014, 09:07 PM
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Another reason to bin is the faster read time and smaller files. I do time series photometry and will usually use 2x2 binning unless my target is saturating with the binned image and a reasonable length exposure (10 secs).
The binned data has less time between exposures and are much faster to process.
Even binned I have 1.3 arcsec/ pixel so don't under sample.
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  #38  
Old 11-07-2014, 10:11 PM
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some of the posts in this might be worth a read as well: http://www.cloudynights.com/ubbthrea...6/Main/5494372
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