How deep can you go?

Discussion of using SharpCap for Deep Sky Imaging
Cybermystic
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How deep can you go?

#1

Post by Cybermystic »

I am in the middle of a rather heated debate on Facebook - and I wonder whether anyone on this Forum (Robin especially) can help me. I use a Hyperstar at f#2 on a Celestron C11, and the heated debate is on a Hyperstar forum. I seem to be on my own in believing that you cannot image very faint objects with short sub-exposures. Now having used one for 17 years I am very well-acquainted with the power of the Hyperstar, but I am also well aware of its limitations under non-ideal sky conditions. Now what the argument boils down to is that I say you cannot image very faint stuff, like for instance the Taurus molecular clouds near the Pleiades if your subs are too short, especially with high Bortle skies. Just about everyone else is saying that you can image very faint stuff, even with 30-second subs, provided you take enough subs. Basically I don't get this at all. Surely there is a limit to how deep you can image from skyglow considerations alone? Secondly, if the object is so weak that you don't get a single photon in your 30-second sub, then clearly it won't get imaged at all. Also, if you don't need long exposure times to pick up very faint objects, then why did Hubble spend days on getting the Hubble Deep-Field image?
If anyone can throw any light (sorry) on what this is all about - I would be very grateful.
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Re: How deep can you go?

#2

Post by admin »

Hi,

it depends on your camera...

If your camera was 'perfect' - ie it counted photons perfectly (even if it only detected a fraction of them) and didn't introduce any noise of its own, then you could capture exactly the same detail from 3600 x 1s exposures as from 1 x 3600s exposure. After all, You will be collecting the same number of photons in that 3600s either way, and with no errors coming from the camera, that's all that matters.

Real cameras aren't like that - they inevitably add a bit of extra noise to each image they capture - called the read noise (this is on top of the fundamental noise caused by random fluctuations in the number of photons arriving). Taking long sub exposures is a way to minimize the effect of read noise in the final image. It's particularly important if your camera has a high read noise (for instance typical CCD cameras from 10 or so years back might have a read noise of 8 to 10 electrons). A modern CMOS camera set to a middling gain value might have a read noise of 1.5 to 2 electrons. That means that the sub exposures taken with the CMOS camera can be about 25 times *shorter* than those needed with the CCD camera... That's because the approximate formula for the required sub exposure length is

Read noise squared * 10 / (sky background brightness in electrons per pixel per second)

Now, your very fast f-ratio for the hypercam means that the last bit in that equation (sky brightness) tends to be much larger than for a slower imaging system. For instance, an estimate for Bortle 4 using f/2 optics with a colour camera with 3.75 micron pixels and 50% QE is a sky brightness of about 2.5 e/pixel/second (see https://tools.sharpcap.co.uk for an estimation tool).

Plug that into the sub-exposure length formula with a read noise of 2 electrons and you get

sub length needed = 2 * 2 * 10 / 2.5 = 16 seconds

In those circumstances there are practically no benefits to using sub exposures longer than 16s.

Change to a CCD camera with 10 e read noise, now you need 400s sub exposures...
Back with our CMOS camera, but add a narrowband filter which might cut the background brightness by a factor of 10, you need 160s sub exposures
Of, with no filter and the CMOS camera, but remove the hyperstar and run at f/10, the background brightness will drop right down and you will need the 400s subs again...

There is a lot more about this topic here : viewtopic.php?t=456

You can also see a talk I gave about it here : https://www.youtube.com/watch?v=3RH93UvP358

cheers,

Robin
Cybermystic
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Re: How deep can you go?

#3

Post by Cybermystic »

Hi Robin,

Very many thanks for your really informative reply! And thank you very much for taking the time :D

Regards,
Greg
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Re: How deep can you go?

#4

Post by oopfan »

Hi,

Robin is absolutely correct:
sub length needed = 2 * 2 * 10 / 2.5 = 16 seconds. In those circumstances there are practically no benefits to using sub exposures longer than 16s.
It is the minimum exposure. However, there is nothing preventing you from exposing longer. It is not like your image will be destroyed. I like to expose longer to reduce the total number of frames, and therefore the total time to process.

Of course there are downsides to increasing exposure including:
1. Bright stars become even more saturated.
2. Contamination by passing clouds.
3. Risk of satellite and aircraft artifacts.
4. Guiding/Tracking errors.

My CCD's Read Noise is 5.4 electrons. At Bortle 5, the Brain tells me to expose for approximately 2 minutes. I'll push it to 5 minutes if the star field is relatively devoid of bright stars (i.e. no stars brighter than 10th magnitude.) But that is just me, and I'm happy with the results.

If there are bright stars in the field but I still want to capture long exposures, I'll add a 3D-printed Newtonian Mask that I designed for my refractor's aperture. It gives much more realistic diffraction spikes than adding spikes later in software.

So, to answer your question, and as Robin adroitly answered: The darker your skies, the longer the minimum exposure. Remember that you want dark space to capture at a level significantly above your camera's offset. I have a rule of thumb called the "1000 ADU rule." It says to add 1000 ADU to your offset, and then increase the exposure until dark space registers at or above it. So if my offset is 200 ADU, then I set the exposure until dark space is at 1200 ADU. Now, that 1000 ADU is for CCD cameras. For CMOS cameras, you can redefine it as the "300 ADU rule." I use that rule when I don't want to run The Brain.

PS: My experience is that you have more to fear from exposures that are too short, like "raining noise". In the early days, I had plenty of that.

Brian
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Re: How deep can you go?

#5

Post by Cybermystic »

Hi Robin,
I apologise for continuing on with basically the same question, I am o.k. with those calculations saying how long your subs should be - but they don't tell you how deep you can go (in magnitude). What is the last bit of the jigsaw to work that one out? Could you give me a number for the hypothetical case of an f#2 system with a CMOS camera (ASI 2600MC-Pro) and a Bortle 4-5 sky? And could you say in words (cos I'm not too good with numbers) what it is that limits your imaging magnitude? I don't think it's as simple as the object's emission being less than your local skyglow - or is it??
Best,
Greg
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Re: How deep can you go?

#6

Post by Cybermystic »

Brian,

Very many thanks for your confirmatory mail. Any chance you could also look at my follow up question to Robin which is what my lack of understanding is really all about. I want to know what are the limiting factors for you imaging hi mag objects. I know it must be directly linked to your skyglow, but I just can't put this last bit of the jigsaw together.

Best,
Greg
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Re: How deep can you go?

#7

Post by oopfan »

Hi Greg,

How would you define "limit"? I know for me, it is the number of clear nights that I can get the telescope out. The longest I ever went was 11.7 hours of total integration time (see my thumbnail of M81.) Now, if I had Hyperstar like you, I could have accomplished that in one session. I have a 71mm f/5.9 refractor, so it takes me longer. That's the answer to your question: How much time are you willing to invest? The longer you go, the less noise. The more light pollution, even longer.

I've had disagreements with people here on the forum. They say that it doesn't matter if you see the DSO in a single exposure. I say it means a whole lot. It is a reliable predictor of how long it will take to get a good, low-noise final image. In other words, if I can't see the DSO in one frame, then I am assured that it will take multiple nights. This line of reasoning has never failed me.

Finally, what is your noise tolerance? If I had a high tolerance for noise, then I would be proud to show off my single 5-minute exposure of M81. I have relatively low tolerance for noise, so it took me 11.7 hours. Honestly, I wish I could have gone longer, but it was getting late in the season and I don't get that many clear nights.

I know you are looking for "limiting magnitude" but you can't say that in isolation without talking about all the other factors including noise tolerance and number of clear nights, etc.

With my equipment at Bortle 5, I know from experience that I am pushing the "limit" trying to image anything fainter than 23.0 magnitudes per square arc-second. M81 is 21.7 mags/sq-arcsec. I know that if I invest 11.7 hours into a 23.0 DSO, I will see it in the final image, but it will be noisy. Am I willing to invest 11.7 hours for a noisy image? Probably not, unless it is a once-in-lifetime opportunity. With your equipment, Greg, I have no doubt that you could get a fine image, so it depends on many factors.

Brian
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Re: How deep can you go?

#8

Post by Cybermystic »

Hi Brian,

Again thank you very much for your time, and your superb explanation - I pretty much agree with all you say. What do I mean by limiting magnitude? I mean what is the faintest thing I can capture with my rig and my background skyglow given (if you like) an infinite number of subs. The people I am having an argument with on the forum are implying that you can get to any magnitude, given enough subs. I am convinced you cannot image a DSO whose emission is less than your sky background emission, so your sky background directly limits the faintest object you can image.

Greg
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Re: How deep can you go?

#9

Post by oopfan »

Hi Greg,

My experience suggests that there is no limit, it's simply how much time you are willing to invest (i.e. the number of subs.) Remember that your signal-to-noise ratio (SNR) doubles as you quadruple the number of subs. So, if you've put in 30 minutes, then you can double the SNR by capturing a total of 2 hours. To achieve a further doubling, you are now at 8 hours, and another doubling is 32 hours. At some point, you will hit your breaking point, and say 32 hours is enough.

The Hubble Space Telescope may take only one hour, but it takes you 1000 hours. I don't know if any of us have that kind of patience. (I just made those numbers up for dramatic effect.)

PS: You mentioned that you can't image fainter than your light pollution. Not true. My light pollution is 20.02 mags/sq-arcsec, yet M81 is 21.7. Think of light pollution as just another source of noise that you need to overcome with more subs.

Brian
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Re: How deep can you go?

#10

Post by Cybermystic »

Here's the final bit of data from my end that might help with any explanations.

Putting the numbers through Robin's calculator for my Hyperstar with a 2600MC-Pro camera and a Bortle 4.5 sky we get a sub-length of 10-seconds.

A few weeks ago I carried out the experiment to see what I thought should be the optimum sub length. So picking CTB1 as the target I took 3, 6, 9, 12, 15, 18, 21, 24, 27 and 30-minute subs. I got basically called a liar for saying I took 30-minute subs with a Hyperstar, which I must say is charming - however, returning to the results. I saw an increase in brightness of CTB1 from 3 to 6 minutes, and from 6 to 9 minutes, but for all subs greater than 9-minutes CTB1 did not appear any brighter. So I am assuming, just from what I can see on the screen, that my SNR is topping out with a 10-minute sub. I have never believed the calculator numbers as they have always seemed way too low to me. Am I doing wrong by looking at the actual downloaded image?

Greg
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