Eh, not going to get into another deep debate with you about minutia that really don't matter. Skies of 20.8 are still more than dark enough to get excellent astro images. My parents home is up near the Indian Peaks wilderness here in Colorado, deep yellow zone, often measures 20.8mag/sq". It's around 8700 feet or so. I know quite well what 20.8mag/sq" skies are like. That's the only statistic I care about here. That's right at the beginning of where you want to be...yellow/green zone border. I consider yellow zone the real start of the artificially light polluted zones. Start there, and compared to any imaging in the city, your results with the same gear will be significantly better.
The difference between a green/yellow zone border and a black zone is small, maybe 2-2.5x. So 18 minutes of integration would become 36 minutes. You can easily get 60 minutes in either location, so the difference is moot.
The difference between a green zone and a red zone is quite large 12, 15, 20x. So 18 minutes of integration becomes 216-360 minutes. Well, getting hours of data is not impossible...one could technically acquire 4, 5, 6 maybe even 10 hours of data on a single night if they had night for long enough and a wide enough view of the sky (I've acquired about 12 hours of data across multiple targets on a single winter night on a couple of occasions). However, getting 6 hours of data is certainly more difficult than getting one.
The difference between a green zone and a white zone at the heart of a metropolitan city can be huge, 40x or more. So 18 minutes of integration becomes 720 minutes. Now we are just getting into the realm if insanity here. What you might need 12, 14, 20, 30, 50 hours of data to do in a white zone, you could do in a couple of hours in a green zone. I personally don't know why anyone even bothers to image with skies like this unless they are doing narrow band imaging. Generally an exercise in futility...and anyone trying should find even a park in an orange zone and try imaging there!
The results would be significantly better in much less time.
A green/yellow border zone at 20.8 is
great compared to imaging in the city.
Black body particulates--bzzzt. Black body is irrelevant at visible wavelengths unless the particulates are hundreds of degrees Centigrade. The problem was high humidity causing low transparency, not particulates. And with the high humidity, clouds were rolling through. A few of the frames I threw out were due to clouds.
You misinterpreted. I never said anything about black body
emitting light. I said that inversion layers trap moisture and black body particulates (i.e. dust),
which scatters and absorbs light.
I need to point out, the integration time for my data above is incorrect. I originally missed about half the data when I integrated (don't ask me how, but when I checked the FITS headers for the integration, it listed less than half my total subs were used
)...so the proper integration time is 22x420 seconds. Instead of 336 minutes, as Roger stated, it is 154 minutes. The examples he has used above were processed nearly 15 months ago.
OK, so you had 14x the exposure. Still, the images are pretty close, though yours should be much deeper. I maintain it is 1) mainly processing methodology, and 2) your older camera.
Well, I still don't believe I've ever seen a true native scale, 100% crop of your data. Only downsampled crops. So, who can really say what the actual differences are.
Downsampling hides all flaws.
My 5D III is definitely a noisy camera. I've never denied that.
I proclaim it quite often, actually. Because DESPITE the fact that it's one hell of a noisy camera...moving from my back yard, which usually ranges from 18.7-19.3mag/sq" (spring/fall it tends to be more of an orange zone, summer and winter it shifts back to red zone, barring moon or particularly bad transparency which can push me brighter than 18mag/sq"), to my dark site, which ranges as wide as 20.8 to 21.6, and averages around 21.3, is a completely game-changing move.
The key here is getting out of the city to dark skies. Green zone, blue zone, gray/black zone. Outside of a couple of red zone images, I don't think I've ever seen an image from you that was made from anything worse than the border of a yellow/green zone. That is a CRITICAL factor in your ability to get good images with short integrations. All the low noise capabilities of the 7D II mean less and less and eventually become moot once you start moving into the city. Imaging from a red or white zone (at least 90% of the astrophotographers I know live and image in a red or white zone) is going to make light pollution the most significant source of noise. At that point, LP is swamping any other source of noise (except perhaps dark current on a hot night)...read noise and object signal noise just won't matter at that point:
SNRgreen = 50/SQRT(50 + 10 + 0.1*30 + 2.4^2) = 6:1
SNRorangered = 50/SQRT(50 + 150 + 0.1*30 + 2.4^2) = 3.4:1
Even if you completely eliminated dark current and read noise, your city SNR is still limited primarily by city LP:
SNRnodcrn = 50/SQRT(50 + 150) = 3.5:1
And having a city LP flux of only ~3x your object flux is not all that bad...it can be significantly worse! A white zone can be pure hell in comparison. Using the 7D II in the city, with all of it's low noise advancements, dark current suppression and all, represents an improvement in SNR of maybe 2-4% over having an ideal camera with zero electronic noise. However, ditching the city for dark skies represents an improvement of 76%!!! The best SNR you could get in this case, with a perfect noiseless camera and zero LP of any kind, would be about 7.07:1, which is twice as good as imaging in the city. The green zone gets pretty close.
Dark skies. That's the key. Beginners really should understand that, as that is why they can't get the same quality as you with only 18 minutes when they try. The camera tech really doesn't matter UNTIL your imaging under dark skies. Then, once you are...sure, technologies such as the dark current suppression in the 7D II might start to matter. But for a beginner...just getting out to the dark site and learning to acquire high quality data will do far, far more for them than worrying about which camera to buy or which processing techniques to use, or even how big an aperture they need. For a beginner it just doesn't matter. Hell, a DSLR and a wide field lens slapped onto a basic camera tracker will allow them to make awesome images...but only from dark enough skies. Try to image the milky way from the city? Pointless. Truly. No point in even trying, it's a waste of time. (And no one gives a crap if the stars go blue as you move away from the milky way, and many people prefer it from an artistic standpoint. Most beginners would be ecstatic just to have the milky way image at all, damn how the colors come out!)
Cameras come and go, and you can learn techniques over time regardless of which camera you have. Data quality, however...well, a 60-70% or greater improvement in sub quality...now that is something that will last forever.
Sir Canon, the only thing you are missing is the dark skies.
Well, again your are jumping to conclusions without data, and *bold* conclusions at that. Try waiting for the additional information I asked Sir Canon for. I say processing is also a factor. All we need to do is look at this:
http://www.clarkvision.com/articles/astrophotography.image.processing2/
See Figure 9 and 8 different people came up with significantly different results.
Roger
You missed the point there. No amount of processing will overcome any limitations
baked into the data at a low level. Your prior Andromeda images demonstrated that quite well, I think. The blotchy, banded color noise issues of the red zone version were apparent even despite the considerable amount of downscaling. However, if you get high quality data from a dark site...then your free to process, and reprocess, and reprocess the same data over and over as your processing skills improve, and you can get better and better results.
Sir Canon is no different than anyone else asking the same questions he is. He is only missing the data quality, and the data quality comes from eliminating artificial light and improving SNR. I.e. he is only really missing the dark skies. His processing skills will improve with time as he processes more and more data, experiments with more techniques, a wider range of software, etc. Over time as one's processing skills improve, THEN the quality of the camera might start to matter, if it was an issue in the first place.
I don't believe that one particular processing technique is radically superior to another. I see people make excellent images with PS, PI, IP, MDL, StarTools and even Gimp (it's got 16-bit now, apparently). Processing skill is something you can work on forever, and you will always find ways to improve (especially if improvements in processing tools and algorithms continue to be made...both StarTools and PI have some very advanced algorithms that can make the most of your data.) However even with exceptional skills...poor SNR, pattern noise, correlated noise, high dark current, etc. Those issues are baked into the data. You can only process within the bounds of those limitations. Some of your own images suffer from correlated noise, and you have mentioned in each of those that your ambient temperatures were quite warm. So even the 7D II suffers from such limitations. Sometimes with the right processing tools, you can push noisy, banded, blotchy, streaky, low SNR data to the extreme limits, but the limits will always be there. To break out of those bounds, you have to improve the underlying data.
There are other ways to get better data other than going to a dark site. Some are better than others. At the bottom of the list would be a bigger scope. However, getting a larger aperture is going to gather more light pollution right along with more light from deep space. It isn't really going to solve the problem...all it
will do is allow you to overcome some of the limitations imposed by LP
more quickly, by getting more exposure in less time. Instead of say 10 hours to get an integration from a red zone that is half as good as 60 minutes from a dark site, you might need only 6 with a big, fast aperture.
However no one should be lead to believe that dropping a large, heavy, 10" fast newtonian with a bigass momnt arm on their piddly little $1500 mount is suddenly going to allow them to make awesome images with 18 minutes of data. It's just not that easy. And that big, long moment arm of the newt? That will make acquiring good tracking and maintaining good guiding that much more difficult (and usually requires a higher end mount with a much greater capacity and more reliable tracking), throwing another curve ball at the beginner who is already overwhelmed and frustrated that he can't get awesome results with only 18 minutes of light polluted data from his back yard.
Adding an LP filter won't do anything magical that either. It can help, like the newt, but it's not any more of a magic bullet.
One option for nebula imagers is to move to narrow band. I'm not really one to recommend this to beginners most of the time. It, like the Newtonian, adds complexity and can add more frustrations (although it is getting easier thanks to companies like ZWO and QHY). This is something I recommend to people who have already moved a bit past the beginner phase and have already conquered the basics. Blocking out all light except the few bands your really interested in can be as game-changing as driving out to a dark site. It has an up-front cost, however it also has the benefit of being extremely convenient...you can just set up in the back yard, fire everything up, and let a few hours of NB exposures rip for the night for a few nights. With three channels of NB data, you have a HUGE advantage in high quality, high contrast, highly detailed subs for creative coloring options at you fingertips, and you can loose your artistic side on it with gusto.
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To all you beginners out there asking the same questions as Sir Canon!
DARK SKIES. If there was ever a key to astrophotography, that's it. Dark skies rule. Light pollution stinks. Ditch it.
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Anyway, I'm done here. I don't want to have another long, drawn out debate, as it will get way off topic and into completely pointless territory and help no one. Beginners: If your asking the same questions as Sir Canon...just try it! Dark skies. You'll see the differences for yourselves, you'll enjoy the brilliant view of the sky...and you won't have to keep listening us two debate the same old crap over and over.
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Catching Ancient Photons
