#21  
Old 14-06-2014, 08:10 AM
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I don't. A warm CCD is often quite noisy. Even with good electronics. The 8300 chip cleans up fast and -15C is usually very clean. But a decent mirrorless or DSLR has to be clean at room temps.

Also ISO1600 on a D4 is above unity gain I think so again even more impressive from the Nikon as now the Nikon is being amplified compared to an 8300 chip which is not hardly at all.

The new Sony A7s goes up to ISO409600 like a D4 but even cleaner. 4K video that is clean shot in total darkness except for a smallish fire is what it can do.
Its been designed for low light from the ground up though. Its full frame 35mm. Would it match a good cooled mono CCD? I doubt it.

The main issue with DSLRs in what I have seen is they often have no colour in their stars due to overexposure from small wells (typically the megapixel race has meant smaller and smaller pixels and their associated problems).

As Peter pointed out each pixel has surrounding circuitry in a DSLR that is around 40% of the surface area. That's a huge penalty.

Cooled one shot colour CCD and spectrum modified DSLRs/mirrorless (lets not discriminate against the up and coming mirrorless cameras hehehehe) are much closer in performance.

Greg.
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  #22  
Old 14-06-2014, 09:29 AM
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DSLR RAW data is cleaned up significantly by camera firmware. Consequently, images appear better - that is intended. The down side is that DSLR data is not linear - short exposures might come close to linear - this can create calibration problems if not done with care and insight.

For a relatively inexpensive widefield setup, a QHY10 would do just nicely over a cooled DSLR, unless you hate using a laptop and enjoy the relatively less complex DSLR operation. For a number of reasond, I am very tempted to get over the laptop 'complex' and do just that.
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  #23  
Old 14-06-2014, 09:52 AM
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Greg is dead right - the small pixels aren't so much a problem for sensitivity as they are dynamic range. There are physical limits as to how many electrons you can put in a well of a given size.

Rowland - you're right too but that "inexpensive" QHY10 costs 1.5x more than my entire imaging rig.

Simple answer to the original questions: Yes, if you can afford it.
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  #24  
Old 14-06-2014, 10:52 AM
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Quote:
Originally Posted by LightningNZ View Post
Greg is dead right - the small pixels aren't so much a problem for sensitivity as they are dynamic range. There are physical limits as to how many electrons you can put in a well of a given size.

Rowland - you're right too but that "inexpensive" QHY10 costs 1.5x more than my entire imaging rig.

Simple answer to the original questions: Yes, if you can afford it.
I was careful to say relatively... when you consider the options... heaps more than my DSLR setup, that's for sure...
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  #25  
Old 14-06-2014, 10:55 AM
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If only Sony made a low light APS-C version of the A7s...

Why can't DSLR's "bin" or combine pixels to get better low light performance? Too much junk electronics around the pixels? I see no improvement in noise or sensitivity when I use a lower megapixel count.
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  #26  
Old 14-06-2014, 01:29 PM
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Each pixel in a CMOS sensor has its own amplifier, while on a CCD, pixels are read in serial fashion, one at a time through the amplifier (though some have multiple amplifiers, the general idea still holds). Given that there is only one amplifier you can "bin" the signals from adjacent pixels so that the signal is larger relative to the "read noise" in the amplifier (which is fixed). This will result in more dynamic range provided the signal is not too large for any of the "buckets" where the electrons are accumulated.

Of course you get into all sorts of trouble if you are binning from pixels with different colour filters applied to them (as the Bayer mask does above a "one-shot colour" sensor), so people with colour CCDs are less likely to using binning I expect.

Hope that helps answer your question,
Cam

Quote:
Originally Posted by cometcatcher View Post
If only Sony made a low light APS-C version of the A7s...

Why can't DSLR's "bin" or combine pixels to get better low light performance? Too much junk electronics around the pixels? I see no improvement in noise or sensitivity when I use a lower megapixel count.
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  #27  
Old 14-06-2014, 02:00 PM
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Most Osc colour cameras tell you not to bin unless using a faster mode to focus. You def don't bin when taking pics.
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  #28  
Old 14-06-2014, 03:45 PM
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Okay, thanks.

So from what I've read, the best DSLR for astro use will have large pixels and at least a 14 bit AD converter right? Is that why the Canon 1100D works so well in it's class?

I'm just pondering some of the reasons why my Pentax K-x doesn't compete well for astro use, though it could simply come down to heat noise in my case, from living in Hades where it's hot 9 months of the year. Better make that 10 months for this summer.
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  #29  
Old 15-06-2014, 11:53 AM
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Quote:
Originally Posted by cometcatcher View Post
If only Sony made a low light APS-C version of the A7s...

Why can't DSLR's "bin" or combine pixels to get better low light performance? Too much junk electronics around the pixels? I see no improvement in noise or sensitivity when I use a lower megapixel count.
Why would you want an APSc low light Sony sensor? Full frame is always lower in noise because of the larger collection area. Its about 1-2 stops better due to this (not an exact thing as different chips have different efficiencies). So if say Sony reduced that full frame 12mp A7s sensor down to APSc it would straight away be more noisy.

Greg.
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  #30  
Old 15-06-2014, 01:57 PM
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Originally Posted by gregbradley View Post
Why would you want an APSc low light Sony sensor?
Image scale. When I used to shoot FF film, I had to use some mighty big newts to get enough magnification for anything.

Scope quality. APS-C crops out the blurry bits at the edge and corners that would be present in FF and most of my scopes are of dubious quality.

Money. FF is more expensive.
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  #31  
Old 15-06-2014, 02:35 PM
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Quote:
Originally Posted by uwahl View Post
Where astro CCD cameras really score is if you want to do any science (eg filter photometry)
That's a really good point and one reason to look at a CCD for sure.

Quote:
Originally Posted by alistairsam View Post
DSLR's are usually 12bit or 14 bit, OSC's are 16bit. the difference does allow for a greater range in shades of grey or colour.
I found I could stretch my qhy8 images much more than the dslr images.
just my experience though.
That's interesting. Clearly the difference between 14bit and 16bit let alone 12bit - 16bit is very large. I'd be interested to know the ADC process though. Is it really that the ADC just chops that dynamic range, clipping extremes, or is there a more intelligent compression process that precedes it? If there is, how efficient is that algorithm? It might sound like a significant loss of dynamic range, but it could be less than it sounds. Just a thought.

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QE - although most CCD's are above 55% QE (broadly speaking), I don't know if there would be a difference with QE measurements between the D50 and QHY8 as QE calculations for the dlsr aren't that straightforward.

The D50 is an exception amongst a few others in that it uses a CCD vs CMOS in canons. the QE of most canons are calculated to be between 25 and 40% with the exception of one or two that are quite high.

binning - as someone else mentioned, this is also an advantage with CCD's depending on your sampling and system image scale.

As for your broken CCD, why not get a replacement if its under warranty?
The pixel size on my D5100 is 4.78um, whereas the pixel size in the mono Atik 314e that I bought is 4.65um. Additionally, I found a QE figure for the D5100's sensor, which is 48, vs 45 for the Atik 314e.

As for getting the CCD fixed, well it's already been "fixed" once, and given that I bought it over two months ago, I'm not really inclined to send it back for another long wait while it gets sent through the retailer, back to the manufacturer, which may or may not result in me having a usable camera at the end of it. I'm not convinced that the image quality will be a significant improvement over that of my D5100 now either.

I think probably the best "bang for buck" prospective for me right now is to just use my D5100 and get a decent intervalometer which will enable me to take exposures over 30 seconds.

Quote:
Originally Posted by LightningNZ View Post
CCDs generally have bigger pixels for the same area - that's an advantage for the obvious light gathering potential but it also makes processing much quicker. Mono images are also likely easier to process.

For the same money you tend to lose a lot of area though. Why don't you just get a mono'd D5100 with a cold finger cooler? So you lose a little on fill (which is normally compensated by the micro-lens array) and you definitely lose on QE, but that's still a big, high-quality sensor right there.
-Cam
A mono D5100 with a cold finder would be really attractive; unfortunately cost is prohibitive. I think by the time all of that was done (I wouldn't do it myself as I already fried a brand new dSLR trying to spectrum mod it once before) I'd have been able to get a decent CCD (as opposed to the low-end CCD I've bought previously).
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  #32  
Old 15-06-2014, 04:04 PM
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Quote:
That's interesting. Clearly the difference between 14bit and 16bit let alone 12bit - 16bit is very large. I'd be interested to know the ADC process though. Is it really that the ADC just chops that dynamic range, clipping extremes, or is there a more intelligent compression process that precedes it?
There is no clipping, it's just a difference in resolving an analogue signal to discrete values, or shades for imaging.
The difference between 14bit and 16bit is the number of shades that can be represented.
The 2 bit difference is a factor of 4, or, 2(binary) to the power of two(bits) = 2^2 = 4
So if all else is equal, every shade of grey that a 14 bit system can resolve, a 16 bit system can break that down further into 4 more shades.
To put it another way:
2^14 = 16384 shades (from 0 = pure black to 16383 = pure white = saturated)
2^16 = 65536 shades
65536/16384 = 4

Between 12 and 16bit is 2^4, or 16 shades.
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Old 15-06-2014, 04:19 PM
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Quote:
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Between 12 and 16bit is 2^4, or 16 shades.
Err...no

Try 61440 shades ;
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  #34  
Old 15-06-2014, 04:29 PM
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Yes, full scale.
Per 12bit shade, there are 16 off 16bit shades, all else being equal.
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  #35  
Old 15-06-2014, 05:03 PM
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Yes, full scale.
Per 12bit shade, there are 16 off 16bit shades, all else being equal.
Not sure what you are trying to say here....

The maximum number of shades you can get from a 16 bit system is 65536
A 12 bit system can only deliver 4096

a big difference when you start stretching small parts of the data....
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  #36  
Old 15-06-2014, 05:16 PM
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Yes, but Simon is simply showing why you go from 12-bit = 4096 to 16-bit = 65536, i.e. 4096 x 2^4 = 65536
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Old 15-06-2014, 05:27 PM
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A few comments

Most cameras only have about 12 bits of effective resolution. They may manipulate the data so that it covers 16 bits, but there is normally no more than 12 bits of information above the noise. A 12 bit DSLR that provides 12 bits of real data will produce just as many usable shades of grey as a 16 bit system that provides 12 bits of real data. The only chips that can get much better than 12 bits of real data will cost as much as 10 DSLRs.

The dynamic range of chips with small wells is pretty much the same as the dynamic range of chips with big wells, since the big chips have higher read noise. Run the small well chips with more numerous short subs and you get the same dynamic range as large well chips with longer subs - you can't use short subs with big wells because of the high read noise. Images from chips with small wells generally have saturated stars because their users have been convinced that they should use long subs - if they use short subs, the problem goes away.

small pixels do not have more noise than big pixels - in fact they have less noise due to the reduced amount of silicon. Images from full frame DSLRs look less noisy because the sensors are more sensitive due to the bigger pixels intercepting more photons - thus the gain (and noise) can be lower. However, this extra sensitivity comes at the cost of much reduced resolution due to the bigger pixels.

The extra hardware around a CMOS pixel is not a problem - microlenses are used to focus the light from the full pixel size down to the physical pixel, so there is no light lost by collision with non-sensitive hardware. Modern CMOS is more than competitive with CCD technology in all respects.

Last edited by Shiraz; 15-06-2014 at 06:46 PM.
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  #38  
Old 15-06-2014, 05:45 PM
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Quote:
Originally Posted by Peter Ward View Post
Not sure what you are trying to say here....
Perhaps if I quote myself....
Quote:
Originally Posted by MrB
...every shade of grey that a 14 bit system can resolve, a 16 bit system can break that down further into 4 more shades.
And change 14bit to 12bit?

So ...every shade of grey that a 12 bit system can resolve, a 16 bit system can break that down further into 16 more shades.
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  #39  
Old 15-06-2014, 05:52 PM
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Quote:
Originally Posted by Shiraz View Post
A few comments

Most cameras only have about 12 bits of effective resolution. They stretch the data to 16 bits, but the lower bits contain junk. A 12 bit DSLR that provides 12 bits of real data will produce just as many usable shades of grey as a 16 bit system that provides 12 bits of real data. The only chips that can get much better than 12 bits of real data will cost as much as 10 DSLRs.

snip

.
Interesting idea but not quite true. It does depend on the well depth and the gain used in the CCD. My ST10XME is 16 bit and I use it to do photometry. If the number of levels of grey was less I would not be able to measure the difference on brightness of 2 stars anywhere near as accurately. It is not just stretched data. Big CCD pixels have greater well depth so can be broken into many more individual levels.
I don't know about the noise level at very low signal but this is not where I measure the stars.
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Old 15-06-2014, 05:53 PM
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Quote:
Originally Posted by Shiraz View Post
The extra hardware around a CMOS pixel is not a problem - microlenses are used to focus the light from the full pixel size down to the physical pixel, so there is no light lost by collision with non-sensitive hardware. Modern CMOS is more than competitive with CCD technology in all respects.
Normal CMOS is catching up to CCD but in it's current form I don't think will surpass it (CCD have evolved too). However sCMOS is a different story, for example take a look at

http://www.andor.com/scientific-came...-scmos-cameras

Andor is the company that bought up Apogee, which was then bought by an even bigger fish, Oxford Instruments...

When you have to email them to find out the price of the camera, you have to be pretty serious about your purchase.

EB
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