How deep can you go?

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

#11

Post by Cybermystic »

Hi Brian,

Just seen your reply - and that's the point where I am completely lost, and you would be in agreement with the guys on the forum - they too are saying you can go as deep as you like if you have enough subs. But I really don't see how that is at all possible. I can see how you can image M81 when it is close to your background, but let's pick an object which is only 1/10th or 1/100th of your background - could you really pull that object out from all the noise? I don't know, but it doesn't make sense to me that you can.

Funny you should mention Hubble :D At f#24 it is 144 times SLOWER than my f#2 Hyperstar, so probably not the best comparison :D But I know what you were implying.

Greg
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oopfan
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Re: How deep can you go?

#12

Post by oopfan »

Hi Greg,

HST's pixel size is MUCH larger than anything we use, so each pixel is capturing MANY more photons per unit time.
could you really pull that object out from all the noise?
Yep, think of light pollution as something that is translucent, not opaque. If it was opaque then you wouldn't be able to see bright stars even.

Brian
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oopfan
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Re: How deep can you go?

#13

Post by oopfan »

Hi Greg,
So picking CTB1 as the target I took 3, 6, 9, 12, 15, 18, 21, 24, 27 and 30-minute subs.
My guess as to why you are seeing maximum brightness around 9 minutes is due to the non-linear response of each pixel. It is not a straight line from 0 to 65535 ADU. When you approach the high-end of the scale, the curve flattens out.

That's another reason not to take excessively long exposures. There is a range of acceptable exposures. The Brain measures the minimum exposure. If you go below it, then you will lose faint nebulosity and risk raining noise. If your exposure is too long, then your pixel response begins to flatten out.

Brian
Cybermystic
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Re: How deep can you go?

#14

Post by Cybermystic »

Hi Brian,

Thank you once again for your excellent replies. And we're beginning to get somewhere (with me), I actually worked out on my own (last night) that the background must actually be "translucent" rather than opaque, so yes, I guess it does then make sense that provided you can spend enough time you can go as deep as you want.

So now we really do come to the last piece of the jigsaw. Why aren't images taken from city centres as good as those taken from dark sky sites? Is it simply because they cannot put in the hours necessary to get the same SNR? Also of course, their sub lengths must also be a lot shorter than a dark sky site. Does that play any part at all, or not??

Greg

P.S. Thank you very much for keeping up with my stupid questions - it is very much appreciated!!
Cybermystic
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Re: How deep can you go?

#15

Post by Cybermystic »

Hi Brian,

I got this reply from a World famous astrophotographer, and it really puts my question much better than I did.

"Stacking more subs at a light polluted site is a process of diminishing returns. No matter how many you stack after a certain point, the inherent limitation imposed by your background noise will limit how faint an object you can measure. Moving to a dark site is the only solution, or decreasing sky flux by using narrowband filters, for instance...."

There is my issue. He is saying basically you CANNOT go as deep as you want with an infinite number of subs as you come up against a limit as defined by your sky background. What is the nature of that limit??

Greg
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oopfan
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Re: How deep can you go?

#16

Post by oopfan »

Hi Greg,
There is my issue. He is saying basically you CANNOT go as deep as you want with an infinite number of subs as you come up against a limit as defined by your sky background. What is the nature of that limit??
You are misinterpreting what he is saying. Yes, he is right about the "law of diminishing returns." Remember in an earlier email, I said "to double the SNR you need to quadruple the subs." Sure, if you've invested 30 minutes, then capturing 2 hours isn't a great hardship. To double SNR again, you need to go 8 hours, and then 32 hours, and then 128 hours, and then 512 hours. Since humans don't have an infinite lifespan, we stop at our pain threshold and say the image is "good enough" or we jump in a car and take a 2 hour drive to a dark site.

So, be careful when someone tells you it can't be done from a city center. It can be done, if you have enough patience. Most people don't have that kind of patience, so they falsely conclude that it can't be done. What they are really admitting is they can't do it. To a lot of people, reality is subjective. It is not.

Brian
Cybermystic
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Re: How deep can you go?

#17

Post by Cybermystic »

Hi Brian,

And there my friend, you have completed the last piece of the jigsaw :D :D :D Thank you so much. So the answer is you can do as well from a city centre as a Bortle 1 zone - IF you are willing to put in all the hours necessary.

By the same argument that would mean you can also image objects as dim as Bortle 1 zones in a city centre too??

Greg
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oopfan
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Re: How deep can you go?

#18

Post by oopfan »

Greg,

Many people focus on one thing. A person who is educated to become a lawyer is a lawyer for the entirety of their career. Medical doctors are medical doctors, although they may switch specialties from pulmonary to gastrointestinal. I am a software developer, always been, always will be, but over the past 40 years I used my skills in the medical field, then finance, and then back to medical where I am now.

My first exposure to the medical field was very exciting. Interestingly there is a strong relation between what I did then and astrophotography today. We designed and developed a very sensitive 16-bit A/D and amplifier to listen to brain waves using electrodes placed on the scalp. Then we developed various devices for stimulating the person: LED goggles for the eyes, headphones for the ears, and a means of applying an electrical shock to the extremities. Brain waves (i.e. EEG) are high amplitude. The brain's response to stimuli (i.e. eyes, ears, extremities) are low amplitude. The brain's response is synchronized to the stimulus, but the EEG is random. It sounds familiar, right?

A typical patient will have a tumor on the optical nerve or the auditory nerve or the spinal column. As the tumor grows in size, it puts pressure on the nerve, ultimately leading to loss of vision or hearing or feeling, respectively. When I was active in this field, the primary treatment was surgery to remove the tumor. Prior to surgery, the technician needed to establish a baseline. Now, depending on how severe the loss of sensation was, the technician would have to stimulate and then capture sometimes thousands of samples. We would Live Stack the samples! After some time the EEG (i.e. the "noise") would diminish so that the brain's response to the stimulus could be clearly seen. The technician wanted to achieve a target signal-to-noise ratio so that the baseline (i.e. "fully obstructed") state of the patent could be accurately measured. Depending on the severity, it could take 30,000 or more samples. Often it took a long time to capture. Once they established the baseline, then the surgery began. As the tumor was removed, the "signal" grew and it took fewer and fewer samples (i.e. "subs") to get the same signal-to-noise ratio. In the old days, before the invention of this equipment, the only way that the surgeon knew he was successful, was to wait until the patient came out of the operation. Basically, he'd take a pin and prick the patient's toe, and ask "can you feel that?" With the equipment, the surgeon has real-time feedback, just like you do when you Live Stack!

Brian
Cybermystic
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Re: How deep can you go?

#19

Post by Cybermystic »

Hi Brian,

That is extremely interesting and links between totally different fields fascinating.

One more thing that came to me since the last question. I am not happy with the short sub-exposure times given by the calculator, especially for dim objects.

Here's my problem. Suppose the calculator gives me an optimum 30-seconds for the exposure time for my camera with my skyglow. Now suppose the arrival rate of photons from my very dim object is only 1 photon every 10 seconds (or less). Then aren't I going to have to image for a damn sight longer than 30-seconds to get enough signal photons to work with?

Greg
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oopfan
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Re: How deep can you go?

#20

Post by oopfan »

Greg,

Given your aperture and Hyperstar, I would be shocked if you are only pulling in a couple photons in a 30-second exposure. Also, remember that the photon rate varies from one moment to the next. So over the course of a stack you can talk about "fractional photons". The Brain is giving you the "minimum advisable exposure". In my opinion, feel free to double it if you have any doubts. You will enjoy a slightly higher SNR but be careful that there are risks that I discussed before. But overall, doubling the exposure is perfectly fine.

It is always good to experiment. Choose a DSO that is kind of noisy in a 30-minute stack. First, capture 30 minutes using 30-second subs. Then, capture 30 minutes using 60-second subs. Compare them. If you don't see a heck of a lot of improvement, then stick with the Brain.

Brian
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