Sorry for the late response. I got side-tracked.
Deep Sky is all about collecting LOTS of photons. The type of objects you are photographing like nebulas and galaxies are already kinda soft and fuzzy by nature so atmospheric distortion is not so much of an issue. Here you are basically wanting long exposures with low noise.
In the past this was the domain of expensive, dedicated astronomical CCD cameras. These are still top dog, but they are also top dollar. However, like the amateur webcam revolution for planetary photography changed the equation, amateurs have found they can get results that rival the astronomical CCD’s with consumer level digital SLR’s. Camera like the Canon 10d, 20d, Digital Rebel can produce amazing results. As good as the best astronomical CCD? No. But pretty darn good. (Examples:
http://ca.geocities.com/bradydjohnson@rogers.com/M42andFriends.htm http://ca.geocities.com/bradydjohnson@rogers.com/M31_76ED_300D.htm ) They cost less than the dedicated CCD’s, and they can be used for normal daytime photography (unlike astronomical CCD’s).
There are other tools like the Meade DSI and others. They are a kind of lower end option but can be a good place to start.
But if you already have a digital SLR or were thinking of getting one for regular daytime photography, THAT would be the way I’d go. (SLR being the key here. The front lens needs to be detachable.) Something like the Canon Digital Rebel would be a great choice.
A couple of things to consider. You will need an equatorial mount. A wedge mount or German Equatorial (I prefer wedge). You will need to learn to pretty accurately polar align. Not trivial, but you can learn how. The tracking criteria are much higher for deep sky photography as compared to planetary. As planetary photography’s main challenge is beating the atmospheric turbulence. For deep sky, the main challenges are tracking error, image noise, and gathering enough exposure time. (Both types share the problems of focusing but that’s a WHOLE ‘nother discussion thread!)
Back in the old film days, there was a useful limit to how long you could expose film. This was due to the background radiation of the sky starting to “fog” the film. This limited how much light you could gather on a target before the fogging effect outweighed the benefit of additional exposure. This was called reciprocal failure. Digital photography has the beautiful property that it is not subject to reciprocal failure like film. There is no real limit to how long you can gather light on a digital image. The image just keeps building information.
However, long exposures do face the problem of tracking errors. Polar alignments are seldom perfect. Worm gears have minute flaws that cause the drive rate to vary slightly over time. Machined parts are imperfect so that the axis of the mount and scope are not perfectly orthogonal. All these things will contribute to causing the image of a star to slightly trail and smear across a frame of a long exposure. You can attach a second scope to the main one and target a nearby star at high magnification and correct for the errors real-time, but you would still have to deal with the issue of field rotation and it’s tedious.
But digital imaging has another beautiful property. The image responds in a linear fashion to exposure time unlike a film that responds logarithmically. What this means is that with film if you exposed for 10 minutes to get a certain level of detail, you might have to expose for another 10 minutes just to get 20% more information. When combined with the problem of reciprocal failure, it presented a real problem. Digital imaging on the other hand, responds in a linear manner. 20 minutes gathers twice the information as 10 minutes, etc. AND it doesn’t have to worry about reciprocal failure so there is no limit other that tracking. However, the image can be broken out into separate exposures and stacked later with software. 10 exposures at 1 minute each can be stacked to produce an image that is essentially the same as if it had been a single 10 minute exposure. This is important because the amount of tracking error that will accumulate during 1 minute exposure is a lot less than what will accumulate during a 10 minute exposure. Not only that, but there are a LOT of things that can go wrong during a 10 minute exposure. Image carefully guiding during a 10 minute exposure only to have an airplane fly across the view in the last 30 seconds. Now you have a nice ugly light streak right across you image! If I had been taking 10 exposures at 1 minute each, I would just keep and stack the other 9 and toss the last one. Of course you also need software for all this. I use ImagesPlus, some people use IRIS, or even Photoshop. There are others I’m sure.
So to wrap up, if it were me, I would get a good digital SLR that I could use for both astrophotography and normal daytime photography. I would get at least an 8” SCT with wedge mount (10” is better. 12” is upper end of what would be portable). Either just electric drive or computerized depending on your budget.
If you didn’t want to drop that much, then you might start out with something like the Meade DSI as an entry level camera. I don’t own one, but I don’t remember seeing any horror stories (You’ll need to research that one. A good source might be CloudyNights.Com).
Of course, if you are rich and money is no object, there are some dedicated astronomical CCD’s that are truly phenomenal. (See SBIG, but you can easily drop 10 large)
One side note. You might notice I keep mentioning the 8” SCT. There are a couple of reasons I think that is a great choice. Anything smaller and you would find it too limiting very soon IMHO. Larger scope increase in weight and cost exponentially. The 8” SCT is SOOO popular that it is manufactured in truly prolific numbers. Because of economies of scale, that makes the 8” SCT far and away the best performance/cost ratio out there. That’s my $0.02.
Clear Skies,
Wab
