I recently purchased my first ever 6" Dobsonian telescope other day and was wondering if it's possible to connect my Canon 400D SLR to it and take photographs through it? Help would be greatly appreciated thankyou.
What sort of photos did you want to take? I'm afraid that without tracking, you'll be limited to very bright objects only - namely the moon. To a lesser extent, you can use it to take images of the bright planets but you'll usually get a better result with a webcam.
The other problem is, that in some newtonians, the DSLR won't come to focus (not enough in focus). This is usually remedied by either:
1) Getting a very low profile focuser and adapters
2) Moving the mirror up the tube.
You might need to trial and error though. Maybe the 6" with it's higher f-ratio might come to focus easier? Try it! Take the eyepiece out, take the lens off the DSLR, and hold the two together, try and achieve focus and take some test shots (on the moon!).
If you do set up the SLR to the focuser don't forget to make some counter weights before trying as it will be top heavy and the scope will dive for the ground. Old speaker magnets will do most times.
What about giving an afocal session a go? All you need to do is take a photo of what appears in your eyepiece of choice by holding in in place by hand. At least this will give you an idea of what type of objects interest you, and what will and may not work if you were to go with a tracking mount.
As the others have mentioned, you almost certainly won't get focus with the dSLR. However, there is one way around this, use a barlow. That will allow you to reach focus (you don't need an eyepiece!). however, the downside is that with a Barlow you won't get the whole moon in your view and pictures of bits of the moon are less exciting on the whole.
You need a t-ring for your camera, available from myAstroshop from $17, and a prime focus adaptor to fit your focuser, again not that much and available from myastroshop and elsewhere.
A dob is very limiting in this respect and the advantages of a dSLR (long single exposures, high resolution) aren't taken advantage of. All in all it's not really worth the effort (and I have tried it). Use the dSLR to take widefield pics of the stars on a tripod instead!
You could try a Lumicon adapter, it clamps onto the eyepiece and has an adjustable platform for lining up the camera with the eyepiece lens. They are a bit heavy and would definitely need counterweighting on a dob.
