[gregbradley]

Here are some simple guidelines to help you make a decision.

1. The mount is the most important part of your astroimaging setup.
2. Start with shorter focal length scopes and medium sized cameras like DSLRs. Using a short focal length scope and a really small chip like in a video camera is much the same as long focal length in the first place. It will show up all the tracking errors, noise etc. DSLRs are hard to beat as an entry point into imaging. You can also use the camera during the day. DSLRs to perform best need to be "modified" where the filter in front of the chip is replaced with a Baader one or clear optical glass. It makes them a LOT more sensitive. You can pick up modified Canon 350Ds for probably $400 these days. Incredible value. You'd probably even get them to throw in the adapters needed to insert the camera into the focuser of your scope.
3. Refractors are less fussy than mirrored scopes usually. They don't need collimation, they are lighter, smaller, less wind affected, they don't have spikes in the stars and they cool off easily and cut through the seeing better. There are many upgrade focusers if the focuser is not good enough.
4. Triplets are better than doublets. Apochromats are better than achromats. APO means apochromat which means the main colours all focus at the same point after going through the glass lens. Achromats do not and will show bright blue halos around bright stars.
5. Autoguiding is essential and a decent autoguider is vital. The Orion one appears to be very good (no reviews on it that I have heard though). The main thing is no flex. You want everything tight. If the autoguider setup bends even very slighty from something then it will cause eggy stars. So make sure everything is tight and strong.
6. Undermount your scopes don't put the heaviest possible scope on the lightest mount. See rule 1.

So the advice you were given in the first reply is very good.
100mm for a refractor with ED glass in the lens even though it is only a doublet it will still perform very well. ED means extra low dispersion. Dispersion here means the amount the colours split apart when going through the glass (and therefore the harder it becomes to make them come back to focus in the one spot later on).

$4000 is quite a bit and you should get something very good. You will also need a bit of software to process the images. Several here use Deep Sky Stacker. I am not 100% sure but I think it may be free. Images Plus is the Rolls Royce for DSLRs and it costs about $300 or so. Photoshop is also long term a really needed piece of software and very expensive.

Another possibility is to buy 2nd hand and your $4000 becomes $6500- $7000 new price. astromart.com is a good mart as is the marketplace on this site. Often people sell whole setups with images already taken with that exact setup so you know what it is capable of. Astro hobby people are generally nice and trustworthy types.

Greg.