I don't have the info in front of me, but I remember reading in a tech
magazine that Canon was coming out with a tweeked model of one of the
digital dSLR that was tweaked for astronomy.  It was supposed to
operate better at night, handle low light better, etc.  You may want to
check Cannon's site or check with a major mail order house.  I have not
seen it advertised other than the one magazine.  That might be your
best bet.

The difference is significant, though may largely be due to the inherent
much higher quality that the D70 will be giving you over a P&S camera.
I can look at my long-exposure night shots from a P&S, and then from after
I got the D70, and the difference is immediately obvious even at reduced
screen-display size.  I actually went out and tried to re-do some of my
shots after getting the D70, because the difference was so great.

Note that the noise reduction (dark-frame subtraction) reduces fixed pattern
noise and hot pixels, but does not reduce random noise (and in fact might
even increase it a bit).  Other kinds of in-camera noise reduction are
best avoided.

A great method of minimizing noise that is particularly suitable for this
kind of shot is:  take three of the same shot in a row (full exposure,
whatever you've determined is correct) without moving the camera at all.
Load all three into Photoshop, make them into layers in the same file,
adjust the top two layers to 50% opacity, and flatten.  This "averages"
the three images and (using three shots) will double the signal-to-noise
ratio.  The S/N increase goes by square roots, so you would need to
average nine shots to triple it; three shots is enough for most purposes.
Obviously if the camera moves at all between shots, the whole thing goes
down in flames; using the remote control is a good idea for this.  And
switch to manual focus so the camera doesn't re-focus for each shot.
They need to be identical.

If you use the above technique you will have no need for additional noise
reduction of any kind.  (Keep the dark-frame subtraction NR on in-camera,
though.)

Scott,

The main difference is in the sensor:

- the pixels are much bigger, collecting more photons for a given flux

- they don't have the live preview facility, and are optimised for just
single-shot images, and not a continuous video feed.

The comparison is that whereas P&S will go up to ISO 400/800, DSLRs will
go up to ISO 3200 - for approximately the same noise level in the image.
Compare the sensitive areas of the "1/1.8 inch" chip and the chips used in
DSLRs.

 http://www.dpreview.com/learn/?/Glossary/Camera_System/Sensor_Sizes_01.htm

Not only will the DSLR have a lower noise at a given ISO, you may be able
to afford faster fixed focal length lenses as well.

David

The Nikon D70 with NR, works very well for night photography.Vey clean,
colourful images, with little if any burnout.  I have used mine often for
this, and achieved some sales from the photo library they are lodged with.

The difference is profound. The D70 is typical; there is no need for a
'tweaked' Canon or any other DSLR. They are very good; better than
film. If you start doing exposures of 20 minutes or so you will get
purple noise around the corners of the D70. This disappears with NR
turned off. The trouble with NR is that it takes as long as does the
original shot -- an exposure of 2 minutes means NR of two minutes. This
is because NR works by taking a shot of a black mask with an exposure
time equal to the photo and then it compares the two. For that reason a
lot of photographers, including myself, just turn the NR off and remove
the noise in Photoshop.

Some of the astrophotographers here have said that DSLRs work better
for that than film cameras, but I do not do astrophotography at the
moment.

I just re-read you posting.  The focusing issue is interesting and I
don't have a quick answer to that.  I would suggest you shoot something
with the lense wide open and then stop it down and reshoot it and
compare the results.  Even at f2 or so, a lense hits "infinity" well
before hitting "distant" scenery (unless you are using some sort of
monster lense like a telescope attached to the camera).  So focus
shouldn't be too much of a problem.  By setting it to a higher f-stop,
you'll increase your depth of field and possibly cure the problem.  If
that cures it, then it is a focus problem.  But it might not fix it,
too.  It is possible with a P/S camera that you're really taking it
past it's limits and what you are seeing is not a focus problem but is
a blur.  That could be caused by a couple of things.  One could be even
the slightest of movements.  You should be using a good quality tripod
and probably a rf shutter release to get rid of that.  On a windy day,
you might even want to sand-bag your tripod.  On a dSLR, locking your
mirror up would also help.  The other possibility is that the lense
isn't performing well and you are getting some difraction.  I think
most good-quality SLR lenses probably have better coatings on them than
P/S camera.  If the small light is dead-center, you might also get some
sort of internal lense reflection that is right on top of the image.

I am sure that you've already ruled out dust and grime on the lense.  I
am a big fan of filter to protect lenses in most cases, but at night I
remove them.  Also, keep on your sun shade to help block out any
ambient light that might be sneaking in.

Finally, where are you shooting this?  If it's hot out, you might be
getting more noise than if you are on an Antartic adventure.  If it is
just after dark, you might also be getting some heat shimmers, too.

Good luck.

Both the Canon 20D and the Nikon D200 will provide excellent night
results.  You may want to consider a separate power supply or battery
pack, though, since it takes quite a bit of battery power to hold long
exposures.

The "Noise Reduction" for long time exposure is actually referred to
the Dark Frame Substraction technique. The tricks is to take another
picture with the same shutter speed but with shutter closed. This would
let those hot pixels visilble by the camera and let it rule out these
hot pixels by simple substraction.

The noise reduction referred for high ISO pics are something totally
different. It is a noise filter afterall and will smooth out the noise
and soften the picture image. In contract, the Dark Frame Substraction
technique will increase noise as noise can be doubled in extreme case
of substraction where the brightest and darkest pixels can add to the
most difference.

To eliminate hot pixel, the best way to shoot at a higher shutter
speed. Here is an example of mine. The picture was taken with my
ancient DC Olympus C-3000 which will have many hot pixel and fading of
colors for longer exposure time:-

http://www.geocities.com/ricehigh/Gallery/pano3456small.jpg

Best Regards,
RiceHigh
http://www.geocities.com/ricehigh

I do terrestrial night photography with exposures of 30 seconds quite
frequently with great results. I'm using a Canon 20d. I used to have a
Canon G series and the 20d (and the D70 that I had between the two)
beats the pants off of the G. That said, I think the 20d beats the D70
in this application, but that's me and my bad experiences with noise on
the D70.

The problem that you get into is that raw exposures (you'd be shooting
raw, right?) do better when slightly overexposed, but noise increases
pretty dramatically as exposure time increases...there's a point of
diminishing returns that I have yet to isolate even for my own work. I
still shoot a number of exposures at different settings and sort it out
later.