The maths of CMOS, read noise, and sub length

[Placidus]

Introduction

We continue to see claims that with the arrival of the CMOS chip, the days of the 1 hour sub are over, and that we can start routinely doing subs of a few seconds only.

There can be compelling reason for really short subs, such as poor polar alignment, dodgy guiding, shallow quantum wells, light pollution, wind buffet, airplanes, etc. Let us assume that we have an arbitrarily dark site, an observatory dome not under a flight path, and permanent polar alignment.

We will derive an easy formula for signal to noise ratio as a function of quantum efficiency, source brightness, readout noise, sub duration, and total number of subs.

We will see that increasing the quantum efficiency alone cannot produce a truly huge improvement over say a 16803 chip.

We will see that for a sufficiently faint target, ten 1-hour subs will yield a 600% better signal to noise ratio than the same exposure time in 1-second subs.

We will also see that it is not a good idea to be testing really short subs on ultra-bright easy targets (say M42) if we are later on hoping to photograph truly faint things.

The results will bear out and put in context comments made by Peter Ward, Rick Stevenson, myself and several others in other forums.

Some easy mathematics

Let the quantum efficiency be Q, the photon flux at a given pixel be P photons per second, the duration of a single sub be T seconds, the number of subs be N.

The signal, or total number of photo-electrons captured in a single sub will be QPT. The higher the quantum efficiency, the brighter the object, and the longer the subs, the more signal we will have.

There are two components to the noise that we are interested in here. The first is shot noise. If we have a very faint source of photo-electrons appearing at random intervals, then during any short interval, we may or may not get a photo-electron, so for some subs our count may be zero, in others it may be 1, in others 2, etc, but not always the same.

The actual number captured in a single sub will follow a Poisson distribution, with an expected value (population mean) for any single sub of QPT as mentioned already, a variance also of QPT, and a standard deviation of root(QPT).

The second source of noise that we are interested in here is read noise. Let the read noise in a single sub have a standard deviation of R electrons.

Over a set of N subs, the expected total signal will be NQPT. That makes sense: the more subs, the higher the quantum efficiency, the brighter the target, and the longer each sub, the more photons we will collect. Dim subjects will have poor signal.

At a single pixel, the two sources of noise under discussion (shot noise and read noise) are uncorrelated with each other and uncorrelated from frame to frame. The variances (the squares of the standard deviations) add linearly.

The total variance is therefore N x (shot noise variance + read noise variance) = N ( QPT + RR ), and the total noise is the square root of that.

(Because we can't type superscripts here, I've written the read noise variance R x R as RR.)

The signal to noise ratio is therefore NQPT / root [ N (QPT + RR)].

We will now examine some important and interesting limiting cases.

The first limiting case is for God's Own Camera, where Q = 1, and R = 0. This does not yet exist. The signal to noise ratio is snr = NPT / root [ NPT] = root(NPT). Since P is a constant set by the Almighty, the signal to noise ratio is proportional to root(NT). With the perfect camera, we can choose to do one ten-hour sub, or ten one-hour subs, and get the same result.

The second is an arbitrarily bright source. Think lunar photography. In the limit as P goes to infinity, QPT + RR approximates QPT as closely as we like. The signal to noise ratio becomes root(NQPT). Since Q is set by the manufacturer and P by the choice of target, the signal to noise ratio is proportional to root(NT). Thus again we can choose to do 1 ten second sub, or ten 1 second subs, and the snr will be the same.


The real world of very faint targets

Our third limiting case is the big one. We will imagine an incredibly faint source, where P (the number of photons per second arriving at a given pixel) is as small as we choose. Recall that the signal to noise ratio is NQPT / root [ N (QPT + RR)]. As we choose ever fainter sources, QPT becomes tiny compared with RR, and in the limit, the signal to noise ratio becomes

snr = NQPT / root [NRR] = QPT root(N) / R.

What does this mean?

(1) Traditional CCD chips have a quantum efficiency around 0.3 to 0.7. Even a perfect chip cannot have Q > 1, so if it takes tens of hours to photograph the outer chevrons of the helix using particular scope with a Q of 0.5, no camera in creation can do it in less than half that time. The advantage of a CMOS chip has little to do with efficiency or sensitivity.

(2) The big advantage of a CMOS chip is the greatly reduced read noise. If, all else being equal, we can reduce the read noise R by a factor of ten, then on a sufficiently faint object, we will improve the signal to noise ratio by a factor of ten. That is astonishing and is a good reason for hoping that Santa will produce a 4000x4000 chip with 100Ke quantum wells.

(3) All else being equal, then for the very faintest targets, the snr is proportional to exposure time T, but proportional only to the square root of the number of subs N. That means that for very faint targets, it will be hugely better to do ten 3600 second subs than 36,000 one-second subs. It will be better by a ratio of 3600 root(10) to 1 root(36000), or a ratio of six to one. Long subs rule.

The caveats are "very faint targets" and "all else being equal". Thus if your mount can't do a 1-hour sub, or there are intermittent clouds about, or aeroplanes, or wind buffet, or your quantum wells are too small, or you are overwhelmed by sky glow, or you only want to photograph the moon, shorter subs make sense. If you don't have those limitations, longer subs make sense.

(4) Test your new CMOS camera on very faint targets. Under super-bright conditions, the read noise is, as already discussed, almost irrelevant. It is not so informative to be doing your first test shots on easy targets like M42 or the Lagoon, unless that is all you are ever going to photograph. Have a crack at the outer chevrons of the Helix, or a crack at the faint OIII super-bubble in NGC 602.

(5) We will always want to do enough subs to be able to do good data rejection to get rid of satellite trails and cosmic rays, and to dither so that we can also handle hot pixels and bad columns using statistical data rejection. However, ten 1-hour subs is adequate for good data rejection. 36,000 1 second subs will not add any important protection and is kind of unmanageable.

Someone else might like to write an equation for signal to noise ratio that includes sky glow.

[glend]

Mike n Trish, my initial reaction was to just 'let this go through to the keeper', but hidden in your thesis is the attitude we so often hear. While your theory is of course sound, the 'caveats' and 'all things being equal' are real gotcha's in many peoples lives, and i would add another one - budget. In a perfect world of a very dark site in an observatory with a good pier, a high end mount perfectly aligned and corrected, no wind, perfect seeing, etc etc your long sub ccd nirvana camera will out perform on SNR. However, many of us lack the financial ability ( through lack of funds or other priorities), to build the nirvana system. We have to deal with the short comings of our particular situations, and thus compromises are required in order to produce and enjoy imaging. What the new, low entry cost, very low noise, reasonable QE (say 60%) cmos cameras provide to us humble masses, is the ability to achieve more than we possibly could in the past with our obvious constraints. ;)

[Shiraz]

I doubt that anyone has seriously proposed 36,000* 1 second subs. However, let's look at something far more practical to see what is going on.

With a camera having a RN of 10e and using 1 hour subs, the total RN over a 16 hour period will be SQRT(16)*10 = 40. If you have a camera with a RN of 2e and take 1 hour subs you will get a total RN of SQRT(16)*2 = 8. the total number of photons detected will be the same in each case as will the shot noise, so the low RN camera will provide higher SNR.

Now try shorter subs on the low read noise camera. If you take 96* 10 minute subs, the read noise is SQRT(96)*2 = 19.6. ie with subs of 10 minutes, the total read noise is still less than that half that of the camera with 10e RN and 1 hour subs. The camera is still exposed for 16 hours so the number of photons detected and the shot noise will both be the same as before. Even with 10 minute subs, the low RN camera will provide better SNR than the 10e camera with 1 hour subs.

Clearly there will be a crossover point where total RN of the 2 cameras is the same. That will be when you have 25x as many subs with the 2e RN camera as with the 10e RN camera. If you have the same total exposure time, you will still have the same signal and shot noise and the 2e RN camera will produce identical SNR to the 10e RN camera - but with subs that are 1/25 as long.

Low read noise does not allow ridiculous results and it certainly does not reduce the total exposure time by orders of magnitude, but it does allow very much shorter subs to be used than previously. The situation for sky limited performance is even more striking and very short subs can be used in some circumstances. However, there is no gain in SNR available with properly exposed sky-limited subs, just the ability to use very short subs.

edit: in your terms, from the dim target equation and the same total exposure, a camera with 2e read noise and one with 10e read noise will have equivalent SNR when:

QP*(T2)*(sqrt(N2))/(R2) = QP*(T10)*(sqrt(N10))/(R10)

now (N2)*(T2) = (N10)*(T10) for the same total exposure, so

(T2)/(R2*R2) = (T10)/(R10*R10) or

T2 = (T10)*(R2*R2)/(R10*R10) = (T10) /25

ie, for equivalent SNR and the same total exposure, the sub lengths can be 1/25 as long with the 2e RN camera

[lazjen]

So my takeaway from this is:

* I'll need a lot more subs to compare to the deep single subs (ignoring the extreme 1sec comparison being used for the match, but more realistic exposure durations)
* I'll need to be able to process a lot of subs (I am getting used to it already)
* I won't be as concerned about losing an hour sub versus a 10 or 5 min sub
* And I need to pick appropriate targets for my system and location and not expect Hubble deep field imaging from an inexpensive setup (that cost probably much less than 10-20% of the system that can do 1 hour subs).

Seems reasonable to me. :)

Q: Although truly extreme, would it be possible to process those 36000 x 1 sec subs? Could they be grouped into batches of say 100, integrated, then those results integrated, etc until down to the final single integrated result?

[Placidus]

Excellently put, Glen. My very first astrocamera was a Canon EOS 20Da, which had a cmos chip. I was a very good boy for mentioning the caveats and all-else-being-equals.




Cheers, Ray. We continue to be in total agreement. You've done a real-world comparison between a real CMOS and a real CCD, and shown that you can do shorter subs with the CMOS and get a better result. True !!! But it is still the case that if you can go a bit longer, your result is even better again !

I'm currently looking around for a 16 megapixel CMOS chip with a good software development kit. Until I can find one that I can fully control myself using C sharp, I'm stuck with my existing Apogee 16803, which comes with a really good DLL interface for back-yard developers such as myself.

Best,
Mike

[Placidus]

Hi, Chris,

I think your comments, and Ray's and Glen's, help focus my mind on what it is I'm really trying to say. It's:

(a) When doing your testing, try some faint things. The big advantage of the CMOS chip's low read noise shows up with really faint things.
(b) Once you've got your CMOS chip, it is always better to do the longest subs that are practical given all the real-world issues like wind buffet, clouds, aeroplanes, guiding and tracking, etc.

Huge stacks: Suppose we went for something more reasonable like 600 one minute subs. My own software, GoodLook 64, would have to do that in say 10 batches of say 60 subs (depending on chip size), producing 10 partial 32 bit FITS stacks. (It needs to hold all the subs in memory for good reasons unrelated to the task at hand). But that set of 10 partial subs would be very easy to stack to a final image. I already used that technique for a big mosaic on the Tarantula.

Cheers,
Mike

[lazjen]

Thanks Mike. I think I might try doing an image sometime in the future with a large number of subs per channel ( > 200) to see what the processing challenge is like.

I will try to do faint(er) objects from my site for testing but I need to work out what's the appropriate settings I need to use - exposure time, gain setting, etc.

Even with my slightly haphazard or adhoc testing I've done with the camera so far, I achieved far better results for LRGB from my backyard than I've ever been able to do so in the past with other cameras. In fact, I had basically given up on LRGB and was expecting to really get NB only. For the price, the ASI1600 is a real game changer.

BTW, when you get your desired CMOS system, send your "discarded" 16803 camera my way for, er, "comparative testing purposes". ;)

[Shiraz]

I suppose that the other thing that could be said is that, if you can get the same (or better) result from much shorter subs, there is no need for deep wells.

M&T, the other issue that should be noted is that current CMOS chips have small pixels - they do not match well with long fl scopes, so let's hope that someone eventually brings out a mono CMOS full frame camera with large pixels.

[SimmoW]

Excellent, mature discussion chaps. I hope Ray is right, there has to be an effective middle ground. 1 second subs are extreme, Mike's explanation proves that. 10 min ones are far more real world and attainable.

[Shiraz]

FWIW, suggest that the equation for sky limited imaging is:

SNR = N*Q*T*P / sqrt(N*Q*T*P + N*Q*T*S + N*T*D + N*RN*RN)

where S is the sky signal and D the dark current

the generally agreed setting for sub length is to make N*Q*T*S the dominant term in the denominator, at which point the SNR is independent of RN (ie there is no gain in SNR from using short subs).
Normally, subs will be long enough that N*Q*T*S >~ 10*N*RN*RN - or something similar. ie, N cancels out and the required sub length T scales directly with RN*RN, the same as for narrowband. Since sky-limited exposures with normal cameras come in at 10-20 minutes for a 10e RN camera, an equivalent system with a 2e RN camera could do the same job with 24-48 second subs.

[Atmos]

Here is a camera for you (http://www.andor.com/scientific-cameras/ikon-xl-and-ikon-large-ccd-series/ikon-xl-231) MnT :P
A little over 2e- RN, 350k e- wells and pixel sizes that really suit your long FL :P

It is a great discussion in this thread btw :)

[SimmoW]

Hmm, I bet only God could afford that camera!!

[Slawomir]

Sensor diagonal = 87mm...now you are talking! But it weighs 12 kg...

I nearly pulled a trigger but then I noticed that 2e read noise is only for 100kHz readout...16mp would probably take 2 minutes to download! I think I will have to wait for an improved model...:P

[Placidus]

Thanks everyone for sensible and helpful comments extensions and advice. I'm learning.



That had vaguely occurred to me too. A good point.



Another very good point, not just for CMOS but in general.




Thanks, Ray, that's excellent. You've written the equation in a way that is clear unambiguous and intuitive. It'll take me a while to scrape together the relevant numbers and plug them in.




Wow! That is a big beast. Some of the 60x60 mm real estate would be wasted on us mere mortals with 50 mm filters.

[lazjen]

How do I measure the sky signal for this equation?
I assume the Q figure is different for each channel or narrowband being used?
And for the ASI1600, the gain setting determines the read noise, right?

If I know these 3 things, I can get a good value for T? Then effectively after that, increasing N should increase SNR at nearly the best possible "rate" I can theoretically achieve (for the sky, camera, etc)?

Is this right?

[Camelopardalis]

Can't you just point it a patch of sky near your target and measure the background values in between stars?

[Placidus]

:lol::)

Wow! Your image is highly relevant and much appreciated! We had to do 2x2 on-chip binning to get anything like that result.





I just had a crack at measuring the sky signal. I looked at a single 1 hour dark and flat corrected frame near the south galactic pole (in Sculptor) taken around midnight. The brightness of the starless background was 1650 ADU.

That very much surprised me. It is a high number, much higher than our dark current or the read noise, expressed in consistent units.

With a gain of 1.5 e-/ADU, 1650 ADU is 2,475 electrons. Taking Q = 0.5, that's a photon flux of 4950 photons per pixel per hour, or 1.375 photon per pixel per second.

Is that typical for rural sky? To tell, we need to divide by the unobstructed aperture, and also divide by the angular view of the sky per pixel.

1.375 photons per pixel per second / 0.14 sq meters / 0.3 square arcsec view per pixel = 32.7 photons per square meter of unobstructed aperture per square arcsec of sky.

From that, one can work out the sky brightness in mag/square arcsec, but the calculations are horrific (http://coursewiki.astro.cornell.edu/Astro4410/EstimatingPhotons), requiring knowledge of sky absorption, reflectance of primary and secondary mirrors, bandpass of filters, coatings on correctors and ccd chamber, etc, etc, etc. I end up with a value somewhere between 21 and 22 mag/sq arcsec, which is pretty pleasing, but there are so many assumptions that all I can say is yes, 1650 ADU per hour is actually pretty much to be expected :).

The really good news is that for working out signal to noise ratio, you don't actually need to do any of that. You just need your actual raw measurement of photons per pixel per hour, and plug it into the formula for snr.

[RickS]

I'd like to play but I'm sitting in a hotel room in Singapore preparing slides for a conference presentation. See if you can keep the discussion going for a couple more days please :lol:

[lazjen]

Should be easy Rick - it'll take me a while to understand things enough... :)

Ok, we're aiming to solve T given we know or choose other parts of the equation. The main, difficult variable is S. Also, for practical purposes (darks), we're going to limit T to various values (60 sec, 600 sec, etc).

The sky signal will vary based on the time of the night, phase of the moon, etc. Is it best for T to be such that you're under sky-limited exposure length, or is going over ok (but "wasting" time)?

[glend]

Chris, Ray has produced a very useful chart that shows optimal broadband sub times for differing Gain settings , for three different f ratios and two different levels of darkness (SQM), it is published in the ASI1600 thread here.

I would suggest eliminating variables such as moon effect, and time of night light effects ( like nearby sports fields, neighbors, etc).
Of course narrowband is not as suscepable to 'external influences' imho.

[lazjen]

Thanks Glen, I have seen that chart (and will use it for now), but I wanted to understand more, plus be able to do rough calculations quickly for other cameras and situations.



So, you're suggesting finding a time to measure the best conditions then? New moon (or no moon around) night, neighbours asleep with lights off, etc.

This will then lead to finding the time to being sky limited under the best conditions, which will be longer than in most times of the night/month (suboptimal conditions). If you're taking exposures over a number of hours or nights, you're likely to keep T consistent (for darks, stacking, etc). Therefore, if I understand correctly, a proportion of those exposures are going to be longer than the current sky limited value at the time of being taken. And this in turn should mean that it's ok to go above this time, but you're not going to gain anything extra (you're just wasting a bit of time).

Is this right?

[Merlin66]

It's interesting to see this discussion on theoretical SNR....
I doubt many AP imagers actually measure the SNR in their images.

In spectroscopy we must measure the actual SNR in our data to comply to the ProAm campaign requirements.
We usually have to target SNR>200

The shot noise contribution is not IMHO affected by the use and summing of subs. The total signal determines the shot noise.
Data of 100,000 photons will have a shot SNR of 316 whether it came from 1 or ten subs.
Also remember that ANY manipulation of the data will add noise. This goes for darks, flats, colour balance corrections etc. etc. etc.

In the SimSpec spreadsheet we use to analyse the performance of the spectrograph Christian Buil presented a method of calculating the anticipated SNR which includes all the factors discussed.

(See also: Howell's "Handbook of CCD Astronomy", Sect. 4.4, p73 gives the "CCD Equation", Budding & Demircan's "Introduction to Astronomical Photometry, Sect 5.2.6 - Noise in photometry, p188, Appenzeller's "Introduction to Astronomical Spectroscopy", Sect. 3.4.2, p78)


(The SNR comments above are based on the original post:
3) All else being equal, then for the very faintest targets, the snr is proportional to exposure time T, but proportional only to the square root of the number of subs N. That means that for very faint targets, it will be hugely better to do ten 3600 second subs than 36,000 one-second subs. It will be better by a ratio of 3600 root(10) to 1 root(36000), or a ratio of six to one. Long subs rule.)

[Peter Ward]

I have nothing useful to add to this other than: amen :)

[Placidus]

Totally agree. Never said anything to the contrary. Completely obvious.



Merlin, are you agreeing or disagreeing with my statement 3?

You seem to be disagreeing on the grounds that the partitioning of shot noise into many short subs does not affect the signal to noise ratio. That is like saying that since tables have four legs (true), they cannot be used for serving food. Your true statement does not disprove my true statement.

Partitioning the same total exposure into a large number of subs increases the snr by adding many doses of read noise instead of just a few doses. The equation that I gave (for the case of no dark current noise and no sky noise) demonstrates that. It is still the case with the more complex equation that adds in the role of dark current noise and sky noise, although the effect is less or much less in sky limited shots.

[lazjen]

However, 288000 one second subs would be equivalent, right? If you can't take long subs for whatever reason, just do more. It will just take longer overall and what's wrong with that? Really, long subs only rule if you count elapsed exposure time (and also assume zero failures in the capture process).

I'd prefer to get back to the practical issues for each person. How does one determine the best for their own setup. And if possible, I'd really like some of my questions from earlier posts explicitly answered. :p

[Merlin66]

Guys,
I was referring to shot noise and shot noise only....
The quoted original para 3 doesn't say that the difference in SNR is due to the varying shot noise....later quote: "by adding many doses of read noise"
Theory is great, but what about the noise you add to your images through processing??????
Lazjen,
I'd love to help if you can explain how you currently measure the SNR in your images....what "improvements" are you looking for????

[Shiraz]

Ken, our measure of "SNR" is the perceived graininess in an image that may be arbitrarily stretched and possibly smoothed or sharpened. There is no maths for "graininess" in this environment so we use the well developed physics of signal and noise in a linear imaging system as a surrogate - on the simple basis that, if we can get better true SNR in the underlying linear image, we will have better perceptual graininess in the pretty picture.

Using SNR theory enables us to optimise our equipment in a systematic way and relate problems in the "pretty picture" domain to the underlying physics. We do not need to measure true SNR to do this (although some of us do when pushing the limits or testing our understanding of the theory), but that doesn't invalidate the approach.

[lazjen]

I'm looking for the starting point. As in, I make a decision to go for target X from location L using gear G (camera with specific settings, mount capable of handling M minute exposures consistently).

I need to answer questions like this:

* I can do this image over multiple nights, what's the best T value I can use to get the maximum theoretical SNR in the result?

* The weather is looking like rubbish and this night is likely it, or I'm at a dark site for just a night. Given I liked 20+ images per channel to process I want to take (and therefore constrain T), what camera settings do I use to get maximum theoretical SNR? This can also help determine if target X is actually feasible or I should pick something different.

In both cases, T has a max value of either M or the Sky Limited time at L (SL), whichever is smaller (actually I suppose there's also a limit based on the camera's capacity). Although I assume if SL < M, going to T = M won't hurt, but is just a waste of the extra time? This is one of the questions I'd like to know, because if I work out that for a number of common situations, that I should use a value of say 30 - 75 seconds, I'll just pick 60 sec to cover those cases, etc. If it's 200 to 280, I'll pick 300 sec just for the simplicity.

Is this all obvious to everyone else and I'm missing something basic here?

[Merlin66]

Ray,
I understand the "pretty pictures"......
The graininess of your image can actually be measured to give a good estimate of SNR using the processing software or Image J.
It could be of interest to compare perceived graininess it's the measured SRN.

[Shiraz]

try this maybe? http://www.iceinspace.com.au/forum/showthread.php?t=117010 Measure one parameter for your camera and quickly choose appropriate T for any situation thereafter?

[lazjen]

I'll (re)read it. I think it will help. I think I need to go collect some data and check the results to make these equations and numbers more real to me.

[Shiraz]

if you do decide to use it with a 1600 camera, be wary of the actual gain though http://www.iceinspace.com.au/forum/showthread.php?p=1263049#post126304 9

With the 1600, a sky background ADU of around 400 (above bias) will be in the right ballpark.

[lazjen]

Yes, the ASI1600 will be the camera I test with and want to use the most, so that post is also useful.

[Placidus]

We thought it was time to do some actual calculations with a real galaxy, (a) using a luminance filter, and (b) using an H-alpha filter.

The finished images (not the raw data) can be seen on Ice In Space here (http://www.iceinspace.com.au/forum/showthread.php?t=149053).

The results are extremely illuminating.

For a luminance shot, the sky glow is the overwhelming contributor to the noise, far exceeding dark current or in particular read noise. Consequently, 130 six-minute subs with our gear would be almost as good as 13 one-hour subs.

For a 3nM h-alpha filter on this faint target, the results were completely different. The snr for 130 six-minute shots was 1.2, and leaped to 2.6 with 13 one-hour subs. That vindicates our practice of using long subs with faint narrowband objects.

We examined theoretically what would happen with a zero readout noise camera. It would produce almost no improvement with a luminance filter, because of the overwhelming effect of sky noise. Conversely, with an h-alpha filter, it would produce about a 30% improvement for a 1-hour sub, (not enough to justify our throwing out our 16803) and a 280% improvement for a 6 minute sub, pretty enticing for those who need short subs for practical reasons.

Our grand conclusion is that one has to actually make the measurements and do the arithmetic for the specific case (equipment, location, subject) in mind.

A first draft of the detailed methods, results, and discussion paper is attached. There will undoubtedly be mistakes, even howlers. The most recent version can be seen at DropBox here (https://www.dropbox.com/s/pt7qmra72rtzg72/NGC%207793%20Read%20noise.doc?dl=0) .

[multiweb]

Very concise paper straight to the point. :thumbsup:

[Slawomir]

From the recent fascinating discussions and explanations, I have finally arrived to a well-known fact, that the latest CMOS-based cameras allow many more people interested in astrophotography to actually dip their feet into this fascinating hobby due to a significantly lower cost of the camera and a much lesser need for accurate guiding. This is wonderful. It appears that CCD-based cameras will continue, at least for a few more years, to provide a greater capability to produce superior results in DSO imaging, but at a higher cost and by putting significantly higher demands for accurate guiding.

[Peter Ward]

I'm not sure how having a crook mount helps regardless of the camera being used...in fact...I'd preach just the opposite. Your results will be even better if the incoming flux accurately stays in the one spot on a sensor.

I'd suggest newbies buy the best mount they can reasonably afford....and only then think about the camera/scope.

[Shiraz]

Thanks folks, lovely to see some real data on what sky-limited imaging means in practice:thumbsup:.

your conclusion that 6 minute subs do as well as 1 hour subs on your system flies completely in the face of much accepted wisdom, but it agrees with the theory - nice to see. If you attempt to scale your results to incorporate a 2e RN camera, you will see why the users of the new CMOS chips only need 30-60 second subs - there is just no point in going any longer.

If you have not already seen them, the following provide a sound analysis of sky-limited imaging concepts http://www.hiddenloft.com/notes/SubExposures.pdf
http://www.stanmooreastro.com/eXtreme.htm

nice work - more please.

[lazjen]

I think you've taken Suavi's statement and twisted it a bit. I can't see that he's proposing anything against the common wisdom of buying the best mount you can afford.

What it means though is if someone comes to the hobby with a budget of X, that because this camera is very capable and relatively inexpensive to the traditional CCD offerings, a higher percentage of X can be spent on the mount than might have done otherwise.

[Peter Ward]

Well, yes, that works too :thumbsup:

[Camelopardalis]

But it also means that, say, if said mount could only track well for 30-60 seconds carrying a (R)C8, rather than the 1 hour as seems to be expected, then the owner might get subs out of it that would, after processing, give the owner pleasing results...that has to be a good thing.

Or to be more blunt, (within the margins of diminishing returns) it's no longer a rich man's game :thumbsup: :lol:

[Slawomir]

That's exactly what I attempted to communicate, but apparently ineffectively.

Hopefully more people will be able to get into this hobby earlier on in their lives, when funds are often quite limited, and that IMO would be the most significant outcome of implementing CMOS in astro-cameras.

[Placidus]

My first astro-camera was a Box Brownie. My mount was half a house-brick. Very steady. A roll of film took eight whopper images. I tried doing really long star trails, and learned about sky glow. Short star trails were better. But when they came back from the chemist, there was no charge, because the laboratory thought they hadn't come out. There was no picture of Snoopy chasing a beach ball. So very economical. :) My next mount I made myself using a motor from a gramophone, some Meccano pulleys and rubber bands, and a piece of half inch threaded rod for the final drive. The counterweight was a pineapple tin filled with the guts from an old car battery. Probably got lead poisoning.

Mike

[rustigsmed]

wasn't it mentioned a while back that CCD production was going to really drop off and we were going to be stuck with cmos "in the future"? if they get implemented across the board other astro camera makers will no doubt use the chips and potentially bring pricing down across the board? it would be nice to have access to clean larger sensors at cheaper prices and/or to be able to utilize other top notch solutions other camera makers have implemented.

[Atmos]

I personally don't see CCDs ever dieing, especially for the scientific community. At least not until a CMOS chip with 15 micron pixels, 250,000+ e- well depth and 95%+ QE is made.

[glend]

SCMOS is designed to meet the needs of the scientific community and there are some caneras in that market now but they are pretty expensive, like the Andor Zyla, Hamamatsu Orca, and others with QE at min 82% now with ultra low read noise. Interestingly the ASI1600 is on par with the best of the sCMOS cameras in low noise performance.
Not sure extreme well depth is that important for 'relative' short sub imaging, as has been pointed out in posts below this one, ultra low noise, and great QE yes for sure. Just shoot and stack more shallower subs, thus processing capability becomes more important, not to forget storage and raw speed, but these are cheap to acquire.

[Atmos]

Combining all of the information that has been discussed thus far basically says that for the absolute best SNR is having a low RN camera with large wells coupled with the longest exposures possible on a target. Take the iKon 231, 2.1e- read noise and 350k e- well depth, easy to take 1 hour subs with very low read noise, 10 hours with this camera and you'll have some of the deepest data possible. You can do 10x1h subs and not even come close to saturating on something like the Helix.

10x 1 hour subs at 2.1 e- leaves a 6.6e- RMS where as a 400x90 with the ASI at 1.3 e- has a 26e- RMS.

[Peter Ward]

Nice. :thumbsup: I shudder to think what that little puppy would set you back...

[Atmos]

I haven't even been game to do some digging :P
It is a specialised instrument though, just buying ONE 3nm filter for that is probably out of my price range :lol:

[glend]

But doesn't that point out the obvious issue for all of us? Relative cost of performance, sure 10 x 1 hrs with that camera is going to give you great data, but at what cost? I suggest that some sort of unit cost for performance would help. Is that SNR? And factor in the cost of a mount that can deliver great 1 hour subs.

If low cost CMOS cameras, used in a different way, can get you 80% of the expensive solution, then most likely the market will go in that direction. I would suggest that far more ASI1600s are being sold to amatuer imagers than iKon 231s.

[Atmos]

I think you're missing my point a bit Glen, the iKon 231 is a scientific instrument, it is not for the amateur market as such. My point was simply that CCDs are not the way of the past and that CMOS sensors, although great for the amateur community, just are not up to the standards of the scientific community. The SCMOS sensors are a step in the right direction and probably fine for narrow spectroscopy but not for the larger professional astronomical community.

99.99% of the amateur community does not have a large enough imaging circle for the camera. They don't have a focuser capable of carrying what will amount to a near 20kg payload (once a filterwheel/adapters are added). If you have a telescope that can handle the camera then you by definition have a mount capable of doing 1+ hour subs.

[gregbradley]

Sony A7s full frame mirrorless camera has something like 10 micron pixels.

Greg

[glend]

Colin you chose to compare the budget 1600 with the iKon. i did not bring the iKon into the discussion as a comparison or reference to Astro cameras, so yes i am missing a point of their relevance to our use, other than possible flow down effects into our products. My reference to sCMOS cameras was in relation to emerging benchmarks for next gen CMOS for astro use.

[Shiraz]

But I think that you need to consider sky noise Colin - the conclusions from M&T's broadband analysis apply in NB if the read noise is low enough.

at Ha, a fully dark sky produces about 0.5 p/s/m2/arcsec2/nm.
lets make up an appropriate scope for the ikon231 camera - with aperture of 1m2, sampling of 0.1arcsec2, 3nm bandwidth and assume 50% optical efficiency (including QE). if you evaluate the photon flux per pixel using those values, you end up with 0.75 photoelectrons/s/pixel for the sky noise.

Now the camera read noise is 2e. The sky noise will overwhelm the read noise when it is about 3x as large, ie to be dominant, the sky noise needs to be 6e rms. You get that noise from 36 photoelectrons. And at 0.75 photoelectrons/s, you get 36 photoelectrons in less than a minute = you would then be sky-limited on any longer subs.

ie, you would not be able to get any advantage from NB subs longer than about 1 minute with the camera in question on a reasonable telescope. (for interest it would be 20minutes at 10eRN)

An ASI1600 reaches the Ha sky limit with a 10 inch scope and 0.75 arcsec sampling in about 10 minutes at high gain under average dark sky. When it is sky-limited, it will go just as deep as any other sky-limited system (including one based on the ikon231). It just takes less total time on a big scope. Very low read noise CMOS chips really are a game changer and we are just starting to scratch the surface on what they can do - even the ASI1600 is way more than a budget entry-level camera, even though it doesn't cost very much.

ref:https://www.gemini.edu/sciops/telescopes-and-sites/observing-condition-constraints/optical-sky-background

[Peter Ward]

Couldn't help myself.....circa $US230,000 apparently....ouch!

[Camelopardalis]

I'll have two please...one in red and one in yellow :lol:

[glend]

That would buy 115 x ASI1600MM-Cs assuming that AUD! Imagine that mosaic.

[Camelopardalis]

:lol: you can tell I don't have that kind of toy money when my first thought was the power drive at all :D

[lazjen]

Imagine the disk space. :)

[SimmoW]

I can report some favourable results with going from 5 min subs to 10 min @ minimum gain 75 with the ASI, using 3nm OIII FILTERS. Noticeable but not massive difference, but I only compared an individual sub, the difference would be significant when stacked. And this was in bright moonlit conditions, although the target was well away from the moon.

[Camelopardalis]

What kind of increase in pixel values are you seeing in the nebulosity?

[SimmoW]

This is where I need to learn much more Dunk! I don't yet have a grasp of how to measure ADU or pixel values. We are away right now, will post comparison pics on return and try and get some stats using pi. Normally I cannot see much difference in comparison shots but in this case I could definitely see an advantage with only minor sensor glow. Will try 15 next time

[Camelopardalis]

:D open the fits file up in fits liberator and just hover the pointer around an area and you'll see values in the upper right pane...besides x and y you've got the pixel values, or ADU.

You can do the same in PI, but I think by default it uses normalised values just to confuse the issue.

If you register the images on top of each other you should be able to make some pretty accurate comparisons. Pick a favourite region of nebulosity, and some stars, and see what sort of values you get. That's it!

[Shiraz]

provided the sky didn't change much, the ADU values above bias will have doubled if you doubled the exposure time :).

in PI, you can also put a small preview block in a smooth region (keep away from hot pixels and stars) and then select it the "statistics" process - mean will give the average signal and MAD is normally a good estimate of the noise.

[SimmoW]

Thanks chaps, will give it a go. Not sure what the record length exposure is for the 1600, will try to break it's record soon!

[SimmoW]

Right, finally got onto the PC and did some visual comparisons, yowzers! Definitely more detail and contrast with the 10 min subs versus 5, both at 75 gain (default gain is meant to be 139 but I wanted to try minimal, which apparently have more noise than 139, but just wondered what the results would be).

Links to original 300 (https://drive.google.com/open?id=0B761Sh7cppxSdlVGa0NIby1vVF U) sec and 600 (https://drive.google.com/open?id=0B761Sh7cppxSZ3lNQ29CWjhSZ2 M) files.

[Shiraz]

Maybe want to be a bit careful just looking at subs Simon. The subs will continue to get better as you increase the sub exposure - without limit. But what you are actually after is the best result in a stack and there is a point where the stack quality does not improve with longer subs.

if you want to compare 300 with 600 seconds, you should stack two of the 300sec subs to get the same overall exposure.

[Shiraz]

Ha measurements for the 1600 at gain200:

with a 6nm Ha filter and f4 optics, sampling at about 0.75arcsec.
measured dark sky in 10 minute sub = 11.7 photoelectrons
measured noise = 3.87 photelectrons rms
expected shot noise (from 11.7 photoelectrons) = 3.41 photoelectrons rms
read noise <1e rms

ie, even with subs as short as 10 minutes the shot noise of 3.41e is completely overwhelming the read noise of <1e under what was fairly dark sky - very much shorter subs could have been used without penalty. It would have been close to sky-noise limited even at gain 100, where the dynamic range is much better. For comparison, my H694 would not have been anywhere near sky-noise limited at 10 minutes, since it has read noise that is significantly higher than the measured shot noise

Also, the measured noise is a fraction of an electron higher than the combined shot noise (due to the signal) and the read noise - ie there is a smidgin of fixed pattern noise in there, but it is vanishingly small. This camera seems to work predictably right down to the theoretical limits.

The maximum useful sub length needed for Ha imaging with my system is around 10 minutes at gains of 100+ and shorter subs will do the job if the sky is bright.

[Slawomir]

That's very useful, thank you Ray. I will try your method to determine optimal sub length for my gear, as I relied on SGP or PI for doing that for me, with varied success. Also, I can't wait to actually take my telescope out - it has been ages...

[Camelopardalis]

Great stuff thanks Ray, I was thinking of experimenting with some narrowband from the city so that gives me something to grapple with. I expect I'll be sky limited pretty quickly by the looks of it :eyepop:

[Shiraz]

it isn't actually my idea:) - M&T did the same sort of thing earlier in this thread and it seems to go back as far as these papers at least: http://www.hiddenloft.com/notes/SubExposures.pdf
http://www.stanmooreastro.com/eXtreme.htm

it is a good way to work out how your system is performing and theory behind it is the basis for the various optimum sub-length calculators and also the "Really Easy" method.

yeah Dunk, if you have read noise of say 2e, you only need about 6e shot noise to overwhelm it - and you get that from just 36 photoelectrons, which is really miniscule - it won't take long under bright sky. In fact there may be a case for going for longer than optimum just to keep the processing load down - the only thing you will lose is dynamic range.