Filter flatness and its effect on details

[Kunama]

I recently "inherited" a 2" light pollution filter and thought I would give it a test on my TOA-BBHS-Delos optical train.....

It worked ! It cut down the effect of light pollution! BUT while,doing the test I noticed also that the view was distorted so I had a closer a look at reflections on the filter surface to reveal that the filter was not flat and uniform.....

Perhaps the reason they are so cheap is that they are not optically flat. Comparing the filter to my Baader and DGM it became obvious this little Chinese offering was headed for the bin....

I guess it is a bit like photographers buying expensive Canon or Nikon Lenses and then putting a sheet of plastic filter in front for effect....

You get what you pay for...

[MortonH]

Interesting. I guess that's why the premium ones usually state they are optically flat.

How much distortion was there? I wonder if it looks like field curvature to some people or is it worse than that?

[Kunama]

Originally Posted by MortonH

Interesting. I guess that's why the premium ones usually state they are optically flat.

How much distortion was there? I wonder if it looks like field curvature to some people or is it worse than that?

Morton, it was a ripple effect, particularly noticeable when I tried it in daylight to view a straight building line.

[MortonH]

Originally Posted by Kunama

Morton, it was a ripple effect, particularly noticeable when I tried it in daylight to view a straight building line.

Yikes. No good then!