Why is a field flattener used w/ refractor?

I've been looking for a 2" barlow to aid use for both visual and dslr photography. I found an AP barlow that should work or I'll get a TV Big Barlow.

AP's website also shows the 2.7" and 4" field flatteners. I'm just wondering why the use? I guess it seems odd a flattener is used with a refractor. I'm just looking for some basic info. I've purchased a new dslr and soon will be ready for 3 or 4 minute long exposures. I'm just using a meade 102ed with a 2" focuser so it's not really anything I can use but I'm curious.

What barlow is preferred for photo use - AP or TV?

Thanks for your help.

Don

AP refractors are designed for widefield imaging. To get the sharp edges, especially with a larger format camera, a field flattner is a big help.

As far as a barlow, that will double the effective focal ratio of your scope and so exposure time will be quadrupled. with all those pixels, I am not sure a barlow will be a help, especially if the mount does not track perfectly, you will probably loose more in the long exposure than you gain with by the higher nominal magnification.

Jon

If you do someday try wide-field imaging with the dslr,
you can get the TV 0.8x reducer/flattener designed for the
TV 102; its got 2" on the front end and T-thread on the
back end. Most refractors have field curvature with
radius about 1/3 the focal length, so any 102 F/7 to
F/10 should work fine with it. Won't need it of course
for lunar and planetary; there you probably want a Barlow
or PowerMate to make the image larger. As mentioned by Jon,
you don't need a 2" Barlow; 1.25" will work fine.