Re: Astro CCDs still dragging their pixel feet
- From: "Roger Hamlett" <rogerspamignored@xxxxxxxxxxxxxxxxxxx>
- Date: Wed, 13 Apr 2005 09:14:49 GMT
"RichA" <none@xxxxxxxx> wrote in message
news:5tmo51t0qcbekeenp9e3sskr3igefj7u3f@xxxxxxxxxx
> Compared to consumer digital cameras, astro CCD cams
> are still WAY behind when it comes to the number of
> pixels. The latest one is the Meade DSI Pro, an "improvement"
> on the still new DSI, that boasts "4x" the resolution
> of the DSI. Here is the spec;
> Specifications
> CCD Sensor: High Sensitvity Sony® ExView HADT Monochrome CCD Sensor
> Pixels: 510 x 492 pixels (250,000 pixels)
>
> That's about 1/160th the count of the average 4 megapixel entry-level
> consumer digicam. In fact, the colour HAD sensor is used in consumer
> video cams because they can't process 30fps (video) if the pixel count
> is much over 1 million.
>
> Meade mentions that it's capable of exposures of over 1 hour, but
> they intend it to be used to make very short exposures then combine
> them for the finished result because an hour on a non-cooled digital
> camera would be a disaster. They do offer an RGB filter set with it
> for true colour reproduction.
>
> But I can't help wondering when a company will offer a 4 megapixel or
> larger CCD or CMOS-based astro cam with cooling for a price far below
> that of the SBIG and other high priced cameras now on the market.
> When they can sell a consumer camera with 4 megapixels for $200, how
> expensive can the CCD/CMOS element possibly be, even in small
> quantities??
> -Rich
Smaller pixels = less photons gathered per pixel...
This is not a 'good thing' for deep sky imaging. Ideally, you want pixels
that are as large as possible, within what is sensible to get good spatial
'sampling' of the incoming image.
The big problem though in cost, is the difference in technology. The '4
megapixels for $200' cameras, are using CMOS sensors (noisier than CCD's),
with the ADC built into the chip, and usually lmited to perhaps 10bit
resolution, with only about 7 bits really useable, given the noise levels
on the chip. A CCD sensor, is less noisy, but more expensive. Add a 16bit
ADC, and price again rises (though the Meade DSI, has a 16bit ADC, the
actual circuit noise, limits the useable range to only perhaps 11 to
12bits - still an improvement over the webcams. Unfortunately, each extra
useable bit involves not only improvements in the ADC itself, but
reduction in the noise from every source (thermal, electrical in the
camera etc..), and bigger 'wells' for the electrons too.
For a small pixel camera, if you look at the Starlight M8C, this is a
typical consumer CCD, improved with Peltier cooling, and a better ADC. The
useable 'real' ADC resolution, is limited to perhaps between perhaps about
12, and 13bits, and this from a chip that still costs nearly 3* your
'$200' price. This will be a perfect 'planetary' camera, where the small
pixels will still gather plenty of light, and will also suit short focal
length camera lenses, for relatively widefield imaging.
If you want the same resolution, as you can get from cheap cameras, then
simply attach one to the scope. However if you want better signal to noise
performance, then this comes at a cost, and the cost rises significantly
for each extra 'bit' of useable ADC range.
With 'mono' cameras, small pixels are less of a problem, since the pixels
can be 'binned' in the camera, to behave like larger pixels.
Unfortunately, this cannot be done with the 'one shot' colour cameras, so
you are stuck with them...
Best Wishes
.
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