Using a barlow with a camera is called "negative projection" and gives the same effect as using a lens. Basically a barlow doubles the focal length of your scope turning a 1000mm scope into a 2000mm there by effectively doubling your magnification of any given eyepiece. Now when you attach an SLR to your telescope it is acting as an eyepiece with an effective focal length of about 50mm. So with your normal scope that will give you a "magnification" of about 20 times. 40 times with the barlow.
Point to note though. If you look for information about image magnification in relation to cameras you are more likely to find information about field of view rather than magnification. FOV is more important information to an imager doing DSOs as it helps to understand how much of an object we can fit on the film/ccd/cmos. Or alternatively what sort of focal length we need to achieve to fit an entire DSO onto our imaging surface.
About the only time magnification is talked about with imaging is when imaging the planets or the moon. And not often then.
For your question on eyepiece/barlow "thing", there are a number of pieces of equipment you can use. The meade tele-extender is only one example. I can be used as a variable extension tube on its own which will increase your "magnification", it can be used with a barlow and as variable extension tube which will double the originanal extension tube, or it can be used with a 20 or 15 mm eyepiece inserted which will give you rediculously large magnifications and focal lenghts (15000mm plus)
http://www.astro-optical.com.au/view...AGING&x=37&y=4
http://www.meade.com/lx200gps/lx_accy.html (about halfway down on the left)
There are other types than the meade that do the same thing.
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