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Old 27-09-2016, 04:42 PM
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DJScotty (Scott)
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Gain settings with the ASI 1600 mm

Hi there all.

Going to open a can of worms here and hopefully get some discussion going.

As you know the ZWO camera is really starting to gain some traction and producing some amazing photos.

However, I am finding that it does have one limitation, which I am hoping playing with the gain settings will fix.

I tried doing 3 minute subs on unity gain the other night through the red filter and just got a grey sheet with some white dots.

Now, my proposition is this. If the gain is set to highest dynamic range (ie lowest sensitivity), that would enable you (force you) to take longer subs, which, if I understand the rules of this game correctly, is the end goal? Are you not limiting the collected photons by have lots of short subs? Isn't having longer subs better in the long run?

Because, if I understand it correctly, photons don't come in different strengths (they just vibrate at different rates), and all that increasing the gain (ISO in a DSLR) does is increase the "impact" the collected photon has, not actually collects more photons... and in the process, increasing the noise?

So, if you are not actually collecting more photons with higher gain, and actually having a detrimental effect on the end image, wouldn't it be better all round to aim for longer subs and lower gain, and thus increase the chances of collecting more photons and producing cleaner images?

This is what I will be aiming for anyway. But judging by the sensitivity of this sensor, I think 3 minutes in broadband and 5 minutes in NB is the limit anyway, so they're not really "long" in the grand and traditional way of doing things.

Interested in everyone's thoughts on this.

Not wanting to start a fight, but opening up a healthy discussion!

Cheers,

Scott
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Old 27-09-2016, 05:26 PM
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Atmos (Colin)
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The end game is exposing long enough to have the faintest regions of the frame above read noise OR not saturating your subject, whichever comes first. The total number of photons that are captured is determined by overall exposure time, not the number of subs nor the length of single subs.

For instance, my QHY 22 collects the same number of photons with 4x150s as my QHY9 did a single 600s; both are sky limited by this point. As read noise drops the sky limited exposure time drops as well. My QHY22 has a read noise of 4.7e- whereas the QHY9 had 8.5e-. At unity gain you're closer to 2.2e- which means that you probably only need to shoot closer to 60s to get above the read noise in broadband.
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Old 27-09-2016, 06:01 PM
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Quote:
Originally Posted by Atmos View Post
The total number of photons that are captured is determined by overall exposure time, not the number of subs nor the length of single subs.
Hi Colin. Thanks for your quick response. So what i am reading there is that you should get the same results from 2 x 30 minute subs as with 60 x 1 minute subs. Is that correct?
If so why do we aim to do 30 minute (or longer) subs especially in narrowband .

This is the one bit I don't get.
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Old 27-09-2016, 06:22 PM
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As long as you are over the read noise then 60x1 will be the same as 2x30. If you had a camera with higher read noise (KAF-8300) then 60x60s would not be equal to 6x600s in LRGB but 2x1800s would be overkill.
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Old 27-09-2016, 06:27 PM
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We don't aim for 30 minute narrowband sub with the ASI1600, they are not necessary. With the ASI1600, you get high sensitivity and very low noise. Narrowband does not require very long subs with this camera, 300 seconds is enough at Unity Gain settings. There are plenty of examples in the Astrobin ASI1600 group it is worth having a look through those images. Shiraz here on IIS has produced a suggested optimal broadband chart for this camera, and Beta testers on Cloudy Nights did a lot of work on settings and exposures.
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Old 27-09-2016, 07:49 PM
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Hi Scott. Hope some of this is helpful.

As Colin points out, if you image for an hour you will collect the same number of photons regardless of gain settings or sub length - you don't get more photons by using higher gain and with the 1600 you actually get less noise at higher gain.

If you use short subs/high gain, you will collect relatively few photons in each sub. When you stack the short subs, you will get back all of the signal, but will have read noise from the larger number of subs (each one contributes read noise). To keep the total read noise manageable, you will need a low-read-noise chip if you want to use short subs - the 1600 is suitable.

If you use high gain with the 1600, you will have limited well depth. However, you will have a lot of full wells to add up in stacking (one for each sub), so you get back the dynamic range over most of the gain region, particularly since the read noise drops with higher gain. With broadband, you could use any gain from 0 to about 150 with appropriate sub lengths and get essentially the same results in the same total time. The big advantage is that you can use very short subs at high gain - makes guiding etc easier. suggested gain/sub table attached, along with graph of total well depth.

With narrowband, the basic rule is always use the longest practical sub length to get the lowest read noise. However, balance that against the need to keep from saturating bright stars, so choose the longest sub that will give you acceptable stars. FWIW, I have found that 5 minutes under moon is OK with an f4 system at gain 100 - under dark sky, I am currently testing 10 minutes at gain 200.

edit: also, the "unity" gain setting is actually very high gain cf other chips - the 1600 behaves much like a normal CCD when it is used at gain 0, so using it at gain 139 is really pushing it into a region that is not reachable by conventional CCDs.
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Old 28-09-2016, 07:33 AM
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Originally Posted by Shiraz View Post
Hi Scott. Hope some of this is helpful.

As Colin points out, if you image for an hour you will collect the same number of photons regardless of gain settings or sub length - you don't get more photons by using higher gain and with the 1600 you actually get less noise at higher gain.

If you use short subs/high gain, you will collect relatively few photons in each sub. When you stack the short subs, you will get back all of the signal, but will have read noise from the larger number of subs (each one contributes read noise). To keep the total read noise manageable, you will need a low-read-noise chip if you want to use short subs - the 1600 is suitable.

If you use high gain with the 1600, you will have limited well depth. However, you will have a lot of full wells to add up in stacking (one for each sub), so you get back the dynamic range over most of the gain region, particularly since the read noise drops with higher gain. With broadband, you could use any gain from 0 to about 150 with appropriate sub lengths and get essentially the same results in the same total time. The big advantage is that you can use very short subs at high gain - makes guiding etc easier. suggested gain/sub table attached, along with graph of total well depth.

With narrowband, the basic rule is always use the longest practical sub length to get the lowest read noise. However, balance that against the need to keep from saturating bright stars, so choose the longest sub that will give you acceptable stars. FWIW, I have found that 5 minutes under moon is OK with an f4 system at gain 100 - under dark sky, I am currently testing 10 minutes at gain 200.

edit: also, the "unity" gain setting is actually very high gain cf other chips - the 1600 behaves much like a normal CCD when it is used at gain 0, so using it at gain 139 is really pushing it into a region that is not reachable by conventional CCDs.
Quote:
Originally Posted by glend View Post
We don't aim for 30 minute narrowband sub with the ASI1600, they are not necessary. With the ASI1600, you get high sensitivity and very low noise. Narrowband does not require very long subs with this camera, 300 seconds is enough at Unity Gain settings. There are plenty of examples in the Astrobin ASI1600 group it is worth having a look through those images. Shiraz here on IIS has produced a suggested optimal broadband chart for this camera, and Beta testers on Cloudy Nights did a lot of work on settings and exposures.
Quote:
Originally Posted by Atmos View Post
As long as you are over the read noise then 60x1 will be the same as 2x30. If you had a camera with higher read noise (KAF-8300) then 60x60s would not be equal to 6x600s in LRGB but 2x1800s would be overkill.
Alright. I think I am getting it. Thanks everyone.

SO because this 1600 sensor has such low read noise, the signal is appearing sooner than with an 11000 sensor for example. And because of that you can get away with shorter subs. Is that correct?

My next quandary is about the "collection" of photons. I recall, somewhere (and I believe it was Mike and Trish, and Colin I think you were in this discussion) that the other reason you do longer subs is that you increase the probability of a photon striking the sensor for the reeeeeally dim stuff. The discussion, if I remember correctly was around hour long 2x2 binning of the helix and having areas where only one photon per hour (statistically speaking of course) would strike.

Once again, I probably misunderstood this, but what I took away from it was that by taking multiple shorter subs you run the risk of missing that elusive photon in the download and dithering gaps between subs. Am I really off the track there?
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Old 28-09-2016, 11:17 AM
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I would have put it that the signal is appearing above the read noise sooner.

the total number of photons striking the sensor for the really dim stuff is independent of the sub length - it depends only on the total viewing time. It is quite OK to have some of the subs with no target photon in them - in floating point average stacking, you can end up with a signal equivalent to a fractional photon - this is the probability that a photon would be detected in any given sub and it is a perfectly valid signal, even though there is no such thing as a fractional photon. The only advantage of long subs is that the combined read noise from the multiple subs will be reduced if there are fewer of them - you don't somehow magically increase the detectability of faint signals by using long subs and you certainly do not need to have at least one target photon in each sub for the signal to be valid.

Take the mentioned example, where you have on average 1 detectable photon per hour and image for (say) 10 hours. With a a traditional camera using 10x subs 60 minutes long, you would detect ~10 photons in total - maybe a sub or two would have no photons, many would have 1 photon and some maybe more than one. With a CMOS camera using 100x 6 minute subs, you would still detect ~10 photons - most subs would have no photons, but ~10 of them would have 1 photon and occasional ones might have more. After stacking 10 hours of data, the signal result is the same for each camera (~10 photons detected). The read noise adds in quadrature, so the total read noise increases with the square root of the number of subs. For the chosen examples, let's assume that the 10x60 minute subs are taken with a traditional camera having 10e read noise - after stacking, the total read noise for these 10 subs will be 10xSQRT(10) = 33e. If the 100x6 minute subs are taken using a camera with 2e read noise, the total read noise will be 2xSQRT(100) = 20e. ie for this example dim target, the lower read noise camera produces the same signal and roughly half the noise, even though it is using very much shorter subs.

There is always a slight chance that a photon will come along in the downtime between subs - just image for a bit longer.

Last edited by Shiraz; 28-09-2016 at 03:15 PM.
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Old 28-09-2016, 05:45 PM
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Ok. That makes sense. Thanks very much for your information and taking the time to write a response.

Definitely some food for thought.

Cheers
Scott
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