Barlow, DSLR, fast triplet

[m37]

Hi everyone

What would be the effect on field curvature if I were to use a decent quality 2" Barlow at around 2.5x on a fast triplet APO flattened with a 2" 0.8x reducer/flattener into a Canon EOS1000D DSLR?

I'm thinking this

http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-ed-deluxe-2x-two-inch-barlow-lens.html

or this

http://www.firstlightoptics.com/celestron-eyepieces/celestron-luminos-25x-barlow.html

It would be my f5.9 GT-81 which has a 0.8x flattener/reducer giving an f of 4.72

My instincts are that f5.9 x 0.8 x 2.5 = f11.8 which should show reduced curvature albeit with longer exposures required. I want to have the option of longer focal length for some targets.

cheers

Chris

[Daniel-K]

Hi chris not sure but if your after a longer FL would it not be an idea to buy a 2nd hand 127 mak? just a thought

[brantuk]

Hmmm... It doesn't really make sense to use a reducer and a barlow in the same imaging train. The reducer drops the f-ratio down and the barlow raises it back up. Plus the extra glass in the train is just going to take away from the quality of the image. I'm afraid I'm not feeling it really lol.

[m37]

It is more the flatness I was curious about. I have to use the reducer as it is also the flattener. Good point bout the mak.

[frugal]

The flattener works mostly at the edges doesn't it? Surely the barlow will mean you no longer have those edges in the field of view. Or do the new edges of the FOV show the curvature just as much because they are larger?