Masking the aperture down works as you are slowing the F ratio down (decreasing aperture, so you lose resolution and brightness.)
"Decreasing aperture result lose in resolution" is correct
just in a lens that have no CA error (like an apo or masked achro)
in this case each mm of aperture , participate in making resolution
and when you decrease aperture, you lose details
that provided by masked part of lens.
But other wise in fast achro, masking apreture, eliminate TROUBLOUS parts of lens (outer parts) that are responsible of softness in image (details disappear in blurry image)
so it result increasing of resolution IN COMPARE with unmasked achro lens .
If you stop down your ED 80 scope, every millimeter of stopping down , result decrasing in resolution, because you are eliminate good parts of lens that can be used to make extra pixel for you, but in achro, it is better for us
to eliminate outer part as they destroy details.
is it clear ?
For example in my 100mm F5 achro:
I have very blurry image at 166X (6mm eyepiece + 2X barlow) no details are visible on moon.
But when i stop down it to F8 (63mm aperture) i get a very sharp image,
a lot of details that i wouldn't see if i don't mask it.
Of course as you said there is lose in brightness, but as we are speaking about
CA that appear for bright object(like moon), it is not important for us.
I never stop down my scope when I look to ring nebula (or other DSO).
In DSO, you will use lower magnification , and in low power , CA is not a big problem, just there is some rainbow around bright star
(that is not important , a least for visual)
At the end I want to say a subject that I found about CA filters:
there is two effect of CA:
1- in low power:
It make some rainbow around bright object, but it doesn't effect
on details (not noticeable), it doesn't make image blurry(not noticeable), and all rainbows can be eliminate easily with a CA filter (I use orion vblock)
2- in high power:
There is no rainbow, but instead CA make softness in picture and cause to loosing details and sharpness.
In this case , using filter is not useful, it doesn't make picture sharp
I found when i want to see maximum details on moon, a filter doesn't help me
it just eat my voluble pixel and make picture a little soft
(in low power it not important as we have a lot of pixel but in high power we have a little pixels provided by correct part of lens),
in this case I use some additional stop down inside scope!
by using two diaphragm before eyepiece, I can get 66X/Inch extra sharp picture on moon !
but with filter it is impossible to reach this level of magnification
So highest possible magnification in a achro can be
achieved just by masking (in or out) but not by CA filter.
Edited by x935418, 05 March 2015 - 05:51 AM.