Quote:
Originally Posted by billdan
Thanks for the further update Gaston, I'm now wondering if the Edmund Specs of 6 lambda is a misprint.
I had a look at Thor Labs Cold Mirror specs and they claim one tenth lambda surface error.
Unfortunately their biggest size is 25mm at this spec in fused silica. Their Soda-Lime mirrors are larger but no mention of surface quality.
Sorry for hijacking your thread Charles but if you intend to still go ahead with building one, have a look at the Thor specs before committing to the Edmund product.
http://www.thorlabs.hk/newgrouppage9...tgroup_id=6108
Regards
Bill |
The Edmund Optics's specification is correct and provided in peak to valley (PTV) unit. Those mirrors are often used to manage heat in light sources, not for imaging applications, for which the surface flatness is not much an issue.
The Thorlabs cold mirrors are small since it is much easier to manage flatness with diameters at or below one inch. My guess, for the price, the flatness is a 10th of a wave RMS, the PTV error is more in the range of a half wave or more, seldom enough.
With one inch diameter, when set at 45 degree, the useful working surface becomes an ellipsoid with a minor axis below 18mm, not much left for most imaging cameras.
Also they do not have an AR coating on the back which may create significant out of focus ghost images.
Larger multi-coated dichroic mirrors with a good enough flatness are much more expensive, they are usually custom designs.
Soda lime float glass is a common, inexpensive, substrate used in windows, bottles and jars. I would expect a PTV flatness in the 5 to 10 waves, or more depending of the size. For such glass flatness is usually not provided since it is understood it will be large.