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Hi Les, It really is a matter of the type of astrophotography you wish to persue. In my opinion a refractor makes a lovely imaging scope but the cost can be almost prohibitive. The use of a refractor requires a good quality scope of APO standard. These aren't cheap. If you wish to image galaxies you need a scope with a reasonable Focal length and a reasonable objective size. eg a 102mm objective as a minimum with a focal length of arround the 800 to 900 mm making it an f8 scope. If you want to capture nebulas which are often quite large you can get away with a much smaller refractor in about the 80mm objective class. Say an ED80. This scope will not be of a lot of use with small galaxies.
The fact that a refractor doesn't have a central obstruction does help in the imaging process but again a refractor is not the begining and end of it.
Quality is the next big hurdle in both glass type and general image quality. The ability to collimate a refractor is imperative to good imaging.
Glass lens type is next with FPL 53 lenses are really a minimum to give quality images without aberations and colour shift.
Cost. Quality with a refractor costs money. Takahashi, TEC and AP make some lovely telescopes for imaging but they have a similar lovely price. Cheaper versions are available from the likes of Skywatcher at a much reduced price.
In conclusion your dob will make an excelent imaging scope provided you modify the focal length to suit your camera and use a MPCC or similar. There is no need to spend big dollars on a refractor which gives a dedicated field of view for a big wad of cash.
Sounds a bit Irish I know but experiment with the gear you have and progress to specialised equipment as your experience increases and as you deside which way you wish to go down in the imaging slippery slide.
You may notice most imagers have more than 1 scope to cater for the differing focal lengths required.
The main thing you need to concern yourself with is your mount. A poor mount will never allow you to take long exposure images. A good mount like a good scope cost money, like everything to do with Astrophotography. Some compromises can be made in your scope but the mount is the most important thing to do with long exposure photography.
Keep experimenting with what you have and have a bit of fun. Hope this helps.
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