Re: Astro CCDs still dragging their pixel feet
- From: "David Nakamoto" <res07oeg@xxxxxxxxxxx>
- Date: Wed, 13 Apr 2005 21:24:16 GMT
It seems to me that the questioner, no matter what his open or hidden intentions
were, is operating under the paradigm that if the heart of the technology is
similar (CCDs, film, et al), then what applies to one particular use should
apply to another.
Given that technology even in the film era of Astronomy got somewhat complex,
leaving a lot of laymen clueless as to why certain things were done the way they
were, digital imaging should be a bit worse.
Remember film? There were dozens (at least) out there just from one
manufacturer alone, since no one film can handle any and all situations. There
were even emulsions developed by Kodak just for use by astronomers. And then
there was the use of gases to hypersensitize the response, as well as cold
cameras.
There is a similar divide between CCD/CMOS imagers for regular everyday use, and
those for the special conditions for astronomical imaging. As one gets more
into this specific area of interest, the more one gets educated as to why things
are done the way they are, and why it's different from everyday imaging.
There are a lot of people working with CCDs, as part of companies, universities
and colleges, and as individuals, in the astronomy community. I'm sure they've
looked into, and are looking into, whether the newer CCDs and CMOS imagers can
be used. If and when they determine that something is useful, the rest of us
will know about it in a few months time.
--- Dave
--
----------------------------------------------------------------------
Pinprick holes in a colorless sky
Let inspired figures of light pass by
The Mighty Light of ten thousand suns
Challenges infinity, and is soon gone
david.nakamoto@xxxxxxxxxxx
"Roger Hamlett" <rogerspamignored@xxxxxxxxxxxxxxxxxxx> wrote in message
news:dq57e.23970$vv2.15328@xxxxxxxxxxxxxxxxxxxxxxx
>
> "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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