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Old 05-07-2016, 09:12 PM
UBoat
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Trying to understand dark current specifications

I'm still looking for an uncooled imaging camera to use when hiking under dark skies; and trying to assess the range of exposure times that are possible for the SX Ultrastar before making a purchase. I've compared with the Infinity camera and note the latter is limited to exposure times of two minutes because it's optimised for live viewing. The Ultrastar uses the same chip as the Infinity but uses much less electrical power. So I'm not sure that the comparison is fair and I'm wondering if the Ultrastar can mange longer exposures.

Optcorp show a specification that says the Full Well Capacity (FWC) is greater than 23,000 e- unbinned, that the Dark Frame Saturation Time is greater than one hour and the dark current less than 0.1 electrons/ second at +10C ambient. The dark current value implies a Dark Frame Saturation Time of about 60 hours. This is similar to the uncooled performance at 17degC of my ATIK 314L+ that loses about 3% of its dynamic range to dark current with an hours exposure time.

On the basis of that specification the comparison with my Atik314 and the Ultrastar low power consumption I would assess that exposures of an hour rather than two minutes would be feasible with an Ultrastar but I'm cautious because Optcorp are so conservative in simply saying 'greater than an hour to full well capacity' when their specification suggests 60 hours.

My assessment is naive so I'd be glad if anyone can correct or confirm my interpretation and/or say what exposure times the SX Ultrastar is capable of?

Thanks, Peter
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Old 06-07-2016, 01:30 AM
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Shiraz (Ray)
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Hi Peter

I think that the big problem will be the noise component of the dark current, rather than the dark current itself - so signal to noise will be the main issue, rather than dynamic range. On that basis, you only need expose long enough to ensure that the (noise from the dark current + sky noise) will dominate the read noise and you will get as good as possible results with stacking. On that basis, with either camera, 2 minutes sounds about right (maybe a bit longer would be slightly better, but that is probably a reasonable maximum exposure anyway if you are taking along a portable tracking mount of some sort to do astrophotography). You could use longer exposures if the SX supports them, but you would not gain any extra image quality over that which you would get by stacking multiple short exposures (and stacked short exposures would give better dynamic range).

SX are very helpful and would probably clear up any questions re the maximum exposure possible with their software/hardware. the very nifty stacking software suggests that longer exposures are possible if you want to go that way.

Anyway, why not just use your existing Atik without cooling - the older chip doesn't quite match the quantum efficiency of the new one, but it is still pretty good (and it appears to have slightly lower read noise than the new one).

Last edited by Shiraz; 06-07-2016 at 01:47 AM.
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Old 06-07-2016, 07:00 PM
UBoat
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Many thanks Ray for the advice, particularly your outline of how to assess an optimal exposure time. I'll be using a Star Adventurer on an EQ3 Tripod with PHD guiding and was thinking of typically 15 minute exposures though with your advice I could probably drop it to 2 or 5 minutes - I'm doing the mesurements and the math!

I'm a bit puzzled about how to interpret dark current vs. noise. What I see when I run darks with the Atik, the Orion G3 or the ASI120 is a progressive appearance of more and more 'hot pixels' at progressively higher intensities. Is that noise or dark current? I'm using dynamic range to indicate the onset of hot pixels until saturation. So 256 means that there are a large number of hot pixels at an intensity of 8bits and so on until full saturation is all pixels alight at an intensity of 16 bits. I don't know how to express this properly because I've never seen how dark exposure measurements should be quantified and documented? Would appreciate any advice on this you could suggest.

Re your other excellent suggestions I'm following through by phone with Starlight Express as their internet web page doesn't work for me. And have been receiving excellent feedback. Re using the Atik or the Ultrastar, here's my rationale for what its worth.

For power I'm using a minigorilla with a capacity of 6AH at 5v and 3AH at 12V together with a folding solar panel that can completely recharge the minigorilla in a few hours. My estimate is that the Atik will last two to three hours and I'm testing it right now! The Ultrastar should last 30 hours which is a bit more practical for several days camping but it should be feasible to use the Atik for a half night session.

Again many thanks, Peter
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Old 06-07-2016, 07:30 PM
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dark current/dark noise is difficult to measure. As you say, there are two populations - well behaved pixels that generate the basic dark current, plus the lower number of abnormal pixels, including locked-on ones and warm/hot pixels that generate more dark current than normal ones.

the abnormal pixels can almost all be removed with stack rejection when you are putting together an image, so it is probably appropriate to ignore them when measuring dark current. I find that the mean of manually selected small regions (without too many warm/hot pixels) works OK, or you could use a non-linear measure that accounts for outliers, such as Median.

Dark current within a pixel can be averaged over many dark subs and that will give you the underlying dark current in each pixel. However, when you take lights, you also get dark current and the value you get in a pixel in one light will be different from that in the next light due to random fluctuations in the dark current. You can subtract the average dark value for that pixel to get rid of it in any sub, but you will still be left with fluctuating values in successive light subs due to the random fluctuation. This is dark noise and it will generally have shot noise characteristics. It can be reduced (relative to the signal) by longer total integration and you can increase the total integration time either by stacking a few long subs or lots of short subs. Provided the short subs are not so short that read noise becomes a problem, you will get identical signal to noise from either approach. The cutoff point for how short the subs can be is fairly flexible and determined by numerous factors (including pixel size, fno, read noise, sky brightness, dark current), but for the low read noise cameras you are considering, you should be able to use subs in the 2-5 minute range for typical optics - at least that would be a good place to start experimenting.

Last edited by Shiraz; 07-07-2016 at 07:44 AM.
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Old 07-07-2016, 09:14 AM
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If you want a good uncooled camera for hiking in dark skies a modified DSLR would be your best choice. It does not require anything else to run, no computer, power supplies etc. Just an intervalometer (plenty of them on Ebay). A Vixen Polarie is smaller than most cameras and that is your tracking mount.

You can't beat a modified DSLR unless you go to more expensive astro cameras and they are no longer hiking portable.

An astro CCD needs a power supply, computer to run. A lot to carry if hiking.

It also needs a mount.

Greg.
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Old 08-07-2016, 09:29 PM
UBoat
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Responses to Ray and Greg

Hello Ray

Thanks for those useful tips for taking and quantifying noise in darks and flats. Distinguishing the locked in hot pixels is straightforward, but as for the remainder I'll try taking and comparing a series of darks at different exposures and watch for pixels that reproducibly show more dark current than most. In the ASI120 these are clearly linked to the electronics surrounding each pixel but in the Atik such pixels seem randomly distributed. Which all makes sense in light of your explanation.

Do you know if there has been much work done on the 'spatial noise' that might arise from stacking many 1 or 2 minute subs? IE noise introduced by the rotational alignment and stacking of successive images in which pixels may not quite line up, particularly square pixels. Does it degrade resolution more than stacking a few long exposures?

BTW I took your suggestion and have been testing the Atik using a brand new Minigorilla rechargeable battery pack. The operating time is 9 hours drawing on a capacity of 2.8AH at 12V which suggests a current of 300mA for the Atik without cooling. So its capable for a night's viewing. The cooling would add another 500mA and the Atik would then only last about an hour.

No word from SX on example images and no examples from any of the three blogs I've asked, so I'm thinking its very rare for people to use the Ultrastar to record images, which doesn't mean that it can't be done just that people don't.

Many thanks for all your advice, Peter

Hello Greg,

Interesting suggestion to use a DSLR and I can see that a portable system based on it would be very attractive for large FOVs. For my purposes I think the DSLR size and battery consumption would be an issue. If the Ultrastar produces images as well as the Atik 314 in uncooled mode then it would be almost impossible to beat in terms of size, weight and power consumption. Another possible candidate is the ASI1600 CMOS which is less costly and producing some impressive deep space images.

Your point about the computer is very valid but I'm after better load capacity and tracking than a Polarie could provide and will be using the Star Adventurer autoguide and improved polar alignment capability. I also expect to use live stacking; and possibly plate solving for AAVSO work so will be taking a tiny slow antiquated HB netbook that has proven effective and runs all night and can be recharged with the solar panel. Looking forward to tablets becoming fully astroimaging capable!

Many thanks to you also for your advice. Peter
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Old 09-07-2016, 03:36 AM
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Quote:
Originally Posted by UBoat View Post
Hello Ray

Do you know if there has been much work done on the 'spatial noise' that might arise from stacking many 1 or 2 minute subs? IE noise introduced by the rotational alignment and stacking of successive images in which pixels may not quite line up, particularly square pixels. Does it degrade resolution more than stacking a few long exposures?

Peter
you can use that form of variability to increase the resolution by using drizzle when you stack - it only helps if the image is undersampled, which will depend on the focal length, pixel size and seeing. probably worth doing in any case with your fairly small pixel count, since it is an excellent way to upscale images. Otherwise, there is no deleterious effect from many vs few subs that I know of and you would probably gain some resolution and definitely some dynamic range from multiple short subs.

edit: just had another thought - the Atik might not need to be cooled by a huge amount. Even if you can stabilise it at 10C say, you will get much more effective dark subtraction than at a varying ambient. You might not need much power at all to stabilise it at a moderate temperature rather than get it really cold - depends on whether they have used switch-mode control for the Peltier. Might be worth checking how much power it uses if you set a high temperature.

Last edited by Shiraz; 09-07-2016 at 09:24 AM.
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Old 12-07-2016, 08:20 PM
UBoat
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Have to wait til' next week to try that out - but interesting thought.

I'm expecting to be working in a freezer suit with low ambient temperatures so could be the fan and cold ambient air will be a very effective cooler anyway, uncontrolled though!
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