Re: Digital reflex vs Webcam

From: Roger Hamlett (rogerspamignored_at_ttelmah.demon.co.uk)
Date: 12/07/04 

Date: Tue, 07 Dec 2004 11:30:47 GMT

"Martin Frey" <martinfrey@snipclara.co.uk> wrote in message
news:o90br0lub3ug0224rqijfce6e2gsc7r9rj@4ax.com...
> rmollise@aol.com (Rod Mollise) wrote:
>
> >If you just want to image the planets, go the webcam route. I'm
convinced
> >nothing will beat 'em (when combined with Registax) for planetary
detail. I've
> >found that you really start getting the details _down_ on Jupiter, for
example,
> >after about 300 stacked frames. You probably will not be able to take
300
> >frames with a DSLR in the 90 seconds or so required to prevent smearing
due to
> >planetary rotation. Also, the small pixels of the webcams are very well
suited
> >to high res imaging.
> >
> >If you also want to shoot the deep sky, and maybe terrestrial images,
get the
> >DSLR.
>
> Expand the thread - where do the dedicated ccds - SBIGs and Starlights
> - fit in with this analysis?
There are a number of 'levels'. The key point about the webcam, is that
they are usually 'one shot colour', and designed to transfer images at
relatively high speed, allowing a _lot_ to be taken. This suits planetary
imaging well.

Webcam - ideal for planetary imaging, and the brightest 'deep sky'
objects.

Modified Webcam - still great for planetary imaging, but can go 'deeper',
though still with limited ADC resolution.

DSLR. Generally a larger chip than the webcam, which limits the frame rate
possible. In most cases can go to longer exposures than the standard
webcam, making them slightly more suitable for deep sky images. Also
usually support an 'uncompressed' mode, getting rid of jpeg artefacts from
the images (but at a further cost in speed). One big advantage, is the
ability to work wihout a computer. Still limited on the maximum exposure
times available, and lack of cooling means that noise really sets an upper
limit here. Already achieving pictures that only ten years ago, would have
been considered 'awe inspiring'.

Dedicated CCD, with an electronic shutter. These can pretty much match the
better webcam for planetary imaging (but at a much higher cost), and
because to the greater bit depth on the ADC, and cooling, can go deeper
still, while retaining detail over a wider dynamic range. Examples of such
cameras, are the Starlight models, and the SBIG ST2000 and similar units.
However generally the download times are slower than a webcam, limiting
the exposure sequences that can be used, because of the greater amount of
data, and longer conversion time needed for the better ADC, are not so
easy to use to grab long imaging sequences. For instance, the 'best' way
on the Starlight cameras, is actually to use them in 'focus' mode, grab a
frame containing the planet, then select just the section around the
planet, and download this at high speed. Like this, frame rates in the
order of 5 to 10fps, can be achieved, against the 'normal use' mode only
allowing perhaps one frame per second (or even less on the larger CCD's).

Dedicated CCD, without an electronic shutter. The problem here is that the
minimum exposure time is limited. This doesn't allow you to snap 'high
speed' frames, and take advantage of the short exposures to grab moments
of good 'seeing'. hence they are not as good for this particular
application.

Planetary imaging, is an area where the 'one shot' cameras (with a built
in filter matrix), are also good, since on the frames where a 'good' image
is recorded, the full colour data is also available. Using cameras with
filter wheels, involves either a long delay to change filters between each
image, or shooting a number of seperate short sequences with each filter,
increasing the time needed, and the risk of movement being objectionable
in the final result.

Most cameras, have their 'ideal' target. Planetary imaging, really does
suit the webcam terribly well, and for the price, this combination cannot
be beaten. Conversely (for instance), 'self guide' cameras (like the
Starlight MX family), are superb for imaging 'moving' objects like comets,
allowing longer exposures to be taken of these, relatively easily, but at
the cost when dealing with deep sky objects, of 'throwing away' some of
the data. The true 'dedicated' astronomical CCD, wins 'hands down', when
it comes to seeing 'deep', and it is very salutory to look at film images
taken a few years ago, from major telescopes, and realise that using such
cameras, many amateurs are now 'seeing deeper', than such professional
equipment, using much smaller apertures. DSLR's, are great for relatively
'wide field' imaging, where the limits on maximum exposure are less of a
problem. However with care, it is amazing just how far 'unsuitable'
equipment can be pushed, and there are some superb deep sky images now
appearing from the better DSLR's, and some very good planetary images as
well.

Best Wishes

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