Re: Astro CCDs still dragging their pixel feet

On Thu, 14 Apr 2005 06:00:09 GMT, Chris L Peterson
<clp@xxxxxxxxxxxxxxxxxx> wrote:

>On Thu, 14 Apr 2005 00:55:04 -0400, RichA <none@xxxxxxxx> wrote:
>
>>GO ask professionals WHY they use
>>hundred-megapixel cameras composed of banks
>>of large CCDs. I think the CFH scope has a 340 MEG CCD
>>array they used.
>
>The main reason is because they use telescopes with very long focal
>lengths, which means they need large sensors. In many cases they don't
>need lots of pixels, but there are practical limits to the size you can
>make a pixel. Most professional telescopes are oversampled. Outside of
>surveys, most professionals are working with fairly small images- a few
>hundred pixels square is very common (even if this was isolated from a
>larger sensor).
>
>
>> They are TRYING to duplicate the
>>resolution of film plates while gaining the photon
>>efficiency of CCDs.
>
>Virtually all CCDs already exceed the spatial resolution of film. Large
>amateur CCD cameras (over about 5MP) exceed 35mm film both in resolution
>and in potential image scale.
>
>The size of detector you need is dictated by the FOV you are after. Most
>wide field imagers want sensors with a physical size approaching 35mm
>film and a pixel count of 5-15MP. Such cameras are available- not cheap,
>but not out of range of many amateur imagers. DSLRs (especially Canons)
>also do a very good job here, although with some penalties do to noise
>and the color sensor.

True, and the entry level for DSLRs is now 8 megapixels. It's likely
that it will be 12 before the year is out and there is every
indication that their use in amateur astronomy will continue to grow.
Especially since they now seem to be able to control high noise levels
previously associated with CCDs and long exposures(the Canon's are
CMOS) pretty well.
The high end of DSLRs is now 16 megapixels (Canon) and in one case,
22 megapixels (Mamiya), but at a very high price, around $7000-$20000.

>People (like myself) who image individual objects rarely require more
>than 1-2 MP- any more is simply wasted area.

I guess it depends on the size of the object and the f.l. of your
telescope as well, and that determines the size of the sensor you'd
need. A small sensor limits you to a certain object class, unless you
happen to be using a very small, widefield telescope. I guess the
alternative is doing mosaics?
-Rich
.

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