Digital SLR vs. dedicated Astro CCD Camera

"Gregory" wrote in message
ink.net...
Hello all,

I am thinking about getting into astro imaging, and
am wondering something...

These one-shot colour DSLRs are pretty popular, and

Why should a newbie such as myself pay $6000 for a
dedicated 6-MegaPixel astro CCD, or $1300 for a
low-end 0.4-MegaPixel astro CCD, when $1400 can buy a
less sensitive, less specific, but perfectly adequate
large-format dual-purpose CMOS camera such as Hutech-
converted Digital Rebel?

For a newbie, no reason really.

I sold my dedicated entry level CCD camera, bought a Modified Rebel with a
TC-80N3 controller modified for the Rebel, and a Stilleto IV focuser for the
EOS cameras.

I sold my fork mounted SCT and purchased a Losmandy G-11 with a Hardin
Optical (Guan Sheng) 200mm F4 reflector with Vixen coma corrector and Direct
Wide adapter for the EOS camera. I added an Orion 80ED piggy-back guide
scope/finder using Vixen's 232mm (R200SS) tube rings and a set of guide
scope rings from ScopeStuff. I then put the entire setup on a ScopeBuggy,
which I level using a couple short length 2x4's and Celestron anti-vibration
pads under the ScopeBuggy leveling screws.

Where before I would get psychologically overwhelmed by the prospect of
setting everything up, including tripod w/wedge, fork mounted SCT, table,
cables, power and laptop, I now only have to concern myself with polar
alignment, and focus. The TC80-N3 is a programmable controller with exposure
time, sleep between frames, and number of frames. The modified Rebel has
mirror lockup which I set to 3 seconds, and in general with the 200mm F4, a
2 minute exposure at ISO800 pulls in a lot of light. Set to take 15x2
minutes, you can integrate 30 minutes without needing to guide.

There are those who are truly "into" it, like Michael Downing (and I've
personally seen his observatory, which is awesome), and there are guys like
me, who just like to roll the scope out to the backyard once a month or so,
and see what they can do with a simple DSLR for a lot less money.

The only person you have to satisfy, is yourself. I wasn't happy, until I
got automated without cables. The G-11 mount runs on a Celestron 17AH Power
Tank, which will keep it going for a couple of days, and the camera runs on
its own internal battery, of which I have two, so I never run out of power.

I pull the card from the camera when it's full, move the images to the PC in
the den, plug the card back into the camera and select my next target. Whie
the camera is accumulating more data, I go in and fool around with the
previous images. In a 4 hour session, I can image 4 or 5 objects, and spend
as little as an hour outside fighting the bugs, or the cold. The next day I
go to work on processing, which is where the lion's share of "the work" is
done anyhow.

But, that's just me, right now. One day I might move back into a CCD with
serious capabilities, but then again, maybe not. I seem pretty happy with my
results for the time being.

Gregory wrote:
Hello all,

I am thinking about getting into astro imaging, and
am wondering something...


Why should a newbie such as myself pay $6000 for a
dedicated 6-MegaPixel astro CCD, or $1300 for a
low-end 0.4-MegaPixel astro CCD, when $1400 can buy a
less sensitive, less specific, but perfectly adequate
large-format dual-purpose CMOS camera such as Hutech-
converted Digital Rebel?

Gregory

Speaking for myself I have been stuggling with hand guided film SLR's
for a few years and I also eyeing dedicated CCD's for some time and
didn't consider the cost was worth it. DSLR's I would not condider even
worth looking at.

When I started to see results from efforts with the Nikon D70 I changed
my mind about DSLR's. I bit the bullet about 6 months ago and bought one
and on the first try I exceeded anything I had ever acheived on film
with nothing but rough guiding. Also I had little trouble convincing the
rest of the family it was a good buy.

See my results at www.bdas.net

A couple things to think about.

1. What do you want to image ( is it big a dim or small and bright)?
Consider the field of view you want. In my case the D70 on a 250mm/F4.7
has a usable FOV of about 1 degree. It works for DSO's but tricky for
planets. In fact the webcam works better.

2. Data storage - A 6Mpix camera spews out 500 MBytes of raw images an
hour. Think about how to process it.

3. Processing - I use IRIS and it is magic for post processing the data,
but it needs a huge amount of disk. Don't start with less than 10GB if
you have a set of 100-200 images. Each 48bit image needs 36MB for each
stage of the processing.

Finally as with anything that uses semi-conductor technology you can be
sure that both DSLR's and dedicated Astrocams will fall in price and
improve in performance.

I am finding that the camera is not the limiting factor already. Things
like the mount, the skyglow and autoguiding are all things I could spend
money improving.

Mike L.
www.bdas.net

On Tue, 05 Jul 2005 21:04:53 GMT, John C wrote:

Hi, Chris.

I'd disagree there; I switched to a DSLR because the small chips in the
usual dedicated CCD imager are far too small for typical DSO work at
focal lengths above 2000mm. In a 12" SCT even the DSLR can barely
accomodate globulars and smaller galaxies. The dedicated cameras are
fine with shorter scopes or very small objects like planetary nebulae.

That makes no sense to me. You match your pixel size to your optimal
resolution. With a long focal length instrument you generally need a
large sensor, but you don't need lots of pixels. If your sensor has
small pixels (say 7um in a 300D) , the longest focal length that makes
sense to use is around 1500mm (beyond that, you are probably
oversampling). What you should be doing is using a focal reducer to
increase your FOV; you probably won't be losing any resolution.

With a short focal length instrument, your pixel scale is large. If you
want a large FOV (which is a common goal with short focal length scopes)
you need lots of small pixels to avoid severe undersampling. That's why
megapixel cameras are useful here.

I use an ST8i for imaging. The sensor is 1K x 1.5K pixels, 9um. That
sensor covers a 14x20 arcsecond patch of sky when I use my 12" SCT at
2280mm focal length. That is plenty large enough for the vast majority
of DSOs. I'm oversampled at 0.8"/pixel, so I almost always bin the
sensor 2x2 and produce 512x768 pixel images. That's the same as saying
that I could be using a little ST7 on the same scope at an even shorter
focal length and still capturing all the available resolution for most
DSOs.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

In article ,
Chris L Peterson wrote:
On Tue, 05 Jul 2005 21:04:53 GMT, John C wrote:

Hi, Chris.

I'd disagree there; I switched to a DSLR because the small chips in the
usual dedicated CCD imager are far too small for typical DSO work at
focal lengths above 2000mm. In a 12" SCT even the DSLR can barely
accomodate globulars and smaller galaxies. The dedicated cameras are
fine with shorter scopes or very small objects like planetary nebulae.

That makes no sense to me. You match your pixel size to your optimal
resolution. With a long focal length instrument you generally need a
large sensor, but you don't need lots of pixels. If your sensor has
small pixels (say 7um in a 300D) , the longest focal length that makes
sense to use is around 1500mm (beyond that, you are probably
oversampling). What you should be doing is using a focal reducer to
increase your FOV; you probably won't be losing any resolution.

With a short focal length instrument, your pixel scale is large. If you
want a large FOV (which is a common goal with short focal length scopes)
you need lots of small pixels to avoid severe undersampling. That's why
megapixel cameras are useful here.

I use an ST8i for imaging. The sensor is 1K x 1.5K pixels, 9um. That
sensor covers a 14x20 arcsecond patch of sky when I use my 12" SCT at
2280mm focal length. That is plenty large enough for the vast majority
of DSOs. I'm oversampled at 0.8"/pixel, so I almost always bin the
sensor 2x2 and produce 512x768 pixel images. That's the same as saying
that I could be using a little ST7 on the same scope at an even shorter
focal length and still capturing all the available resolution for most
DSOs.

I would think that you would want to match the Airy disk to the pixel
size. The size of the Airy Disk at the focal plane is based on the
focal ratio of the optical system, not the focal length. Using a
wavelength of 550nm, the diameter of the Airy Disk is 1342nm times the
focal ratio. For the 7.38 micron pixel in the 300D, I get an optimal
focal ratio of about 5.5. This seems to work with my 102mm f/5 and
80mm f/6, both seem to do well with my Rebel. The 80mm being a semi-
apo does a bit better; even though the theoretical Airy disk is smaller
in the 102, the blue wavelengths are focused better in the 80.

Rob Johnson
take out the trash before replying

On Wed, 06 Jul 2005 02:21:00 GMT, (Rob Johnson)
wrote:

I would think that you would want to match the Airy disk to the pixel
size. The size of the Airy Disk at the focal plane is based on the
focal ratio of the optical system, not the focal length. Using a
wavelength of 550nm, the diameter of the Airy Disk is 1342nm times the
focal ratio. For the 7.38 micron pixel in the 300D, I get an optimal
focal ratio of about 5.5. This seems to work with my 102mm f/5 and
80mm f/6, both seem to do well with my Rebel. The 80mm being a semi-
apo does a bit better; even though the theoretical Airy disk is smaller
in the 102, the blue wavelengths are focused better in the 80.

Maybe if you are in space, but on the Earth it is rare indeed for your
performance to be limited by diffraction. Seeing is almost always a much
more significant effect, and that is what you should normally base your
image scale on. Basically, you make an image of stars, measure the FWHM,
and then calculate your optimal scale from that. In most cases, the FWHM
of a star will be considerably larger than the Airy disk.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Chris L Peterson wrote in message
...
On Wed, 06 Jul 2005 02:21:00 GMT, (Rob Johnson)
wrote:

I would think that you would want to match the Airy disk to the pixel
size. The size of the Airy Disk at the focal plane is based on the
focal ratio of the optical system, not the focal length. Using a
wavelength of 550nm, the diameter of the Airy Disk is 1342nm times the
focal ratio. For the 7.38 micron pixel in the 300D, I get an optimal
focal ratio of about 5.5. This seems to work with my 102mm f/5 and
80mm f/6, both seem to do well with my Rebel. The 80mm being a semi-
apo does a bit better; even though the theoretical Airy disk is smaller
in the 102, the blue wavelengths are focused better in the 80.

Maybe if you are in space, but on the Earth it is rare indeed for your
performance to be limited by diffraction. Seeing is almost always a much
more significant effect, and that is what you should normally base your
image scale on. Basically, you make an image of stars, measure the FWHM,
and then calculate your optimal scale from that. In most cases, the FWHM
of a star will be considerably larger than the Airy disk.

_______________________________________________ __

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

not only in space, diffraction limit is reached on Earth too sometimes .
Small apertures like the ones the OP is mentioning, 80mm and 102mm are
diffraction limited in typical seeing conditions of a couple of arcseconds .

best regards,
matt tudor

Chris L Peterson wrote:
On Tue, 05 Jul 2005 21:04:53 GMT, John C wrote:


.


That makes no sense to me. You match your pixel size to your optimal
resolution. With a long focal length instrument you generally need a
large sensor, but you don't need lots of pixels.
Chris, if I purchased a camera for my C11 for DSOs to use without a
focal reducer, then would my best option be the ST9 by SBIG ? Thanks.

On Thu, 07 Jul 2005 17:18:13 GMT, "David G. Fitzgerald"
wrote:

Chris, if I purchased a camera for my C11 for DSOs to use without a
focal reducer, then would my best option be the ST9 by SBIG ? Thanks.

The ST9 is a good match to those optics. But I'd probably look instead
at an ST8, since it has a somewhat larger sensor (so larger FOV). With
the ST8 you can bin 2x2 to get effective 18um pixels, about the same as
the ST9's 20um pixels, but you still have the ability to use the smaller
pixels if you run into a session with extremely good seeing, or if you
want to use the same camera on another scope of shorter focal length.
The ST8 is 30% more expensive than the ST9, but gives you a lot more
versatility.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com