Diagonal/polarising filter - odd discovery

[Paz]

I was moon watching on sunday afternoon in daylight and testing out the benefits if using a single polarising filter to tune out some of the atmospheric scatter. I have used double polarising filters before to dim images, but I've never used one in this way before.

I had the polarising filter screwed into an eyepiece, and was using a tak prism diagonal at first. Rotating the eyepiece and therefore filter back and forth through 90° I could see how the filter tuned out some of the atmospheric scatter and in effect darkened the darks to improve contrast.

I then swapped to a mirror diagonal just with intention of comparing diagonal performance but something odd happened.

The view I was presented with was striking blue to the point that I thought maybe I had moved my filter wheel on to the Blue filter in my filter wheel as by chance I happened to have one in there. That wasn't the case and twiddling with the polarising filter angle resulted in a view that went from black and white to blue depending on the angle!!!

When the tak prism diagonal was in the view went from brighter and more bleached to darker and crisper depending on angle but at all times I think it was always grey/black/white.

Why would a mirror diagonal show blue at some angles but the tak prism appears no to?

[MarsG76]

Very interesting... especially when a mirror reflects all wavelengths equally.. I'd expect the prism to act like that....

I'll be interested to read the responses.

 

[vlaiv]

For prism I'm not surprised, some prisms polarize light in certain way - for example Herschel wedge / prism polarizes the light, so you can use single polarizing filter to tune down amount of light by turning eyepiece with polarizing filter attached.

Mirror diagonal is probably dielectric kind (not regular silvered or aluminum) - that could explain strange behavior of light. Dielectric coatings ensure maximum reflection by employing interference between different layers of dielectric material. It is quite possible that some wavelengths that are reflected due to interference get polarized, and some don't which produces distinct "color filter" kind of effect when combined with polarizing filter in certain position.

Other explanation would be that you hit an odd angle to the sun. I know that daylight is somewhat strangely polarized - you can see that by holding your polarizing filter (or polarized sun glasses) in different angles and looking at the different parts of the sky - you will notice that there are areas where sky gets darker and some areas don't seem to be effected. If Sun was at such an angle to produce polarized light in the part of the sky where Moon was, and if for some reason blue, due to atmospheric scatter was not polarized (not coming from sun directly but bouncing around so polarization has been randomized - regular day skyglow) - prism might not be affected because it also (re)polarizes the light before reaching polarizing filter and mirror just passes the light as it is.

Now that I think about it - second explanation seems far less likely, but who knows ....

[Paz]

6 hours ago, vlaiv said:

For prism I'm not surprised, some prisms polarize light in certain way - for example Herschel wedge / prism polarizes the light, so you can use single polarizing filter to tune down amount of light by turning eyepiece with polarizing filter attached.

Mirror diagonal is probably dielectric kind (not regular silvered or aluminum) - that could explain strange behavior of light. Dielectric coatings ensure maximum reflection by employing interference between different layers of dielectric material. It is quite possible that some wavelengths that are reflected due to interference get polarized, and some don't which produces distinct "color filter" kind of effect when combined with polarizing filter in certain position.

Other explanation would be that you hit an odd angle to the sun. I know that daylight is somewhat strangely polarized - you can see that by holding your polarizing filter (or polarized sun glasses) in different angles and looking at the different parts of the sky - you will notice that there are areas where sky gets darker and some areas don't seem to be effected. If Sun was at such an angle to produce polarized light in the part of the sky where Moon was, and if for some reason blue, due to atmospheric scatter was not polarized (not coming from sun directly but bouncing around so polarization has been randomized - regular day skyglow) - prism might not be affected because it also (re)polarizes the light before reaching polarizing filter and mirror just passes the light as it is.

Now that I think about it - second explanation seems far less likely, but who knows ....

I can confirm the mirror diagonal was a dielectric one. If I get a chance later this week I'm going to try reproducing the same effect and see if any other clues present themselves.

[Louis D]

@Paz Do you have a cheap mirror diagonal with an ordinary aluminum coating to A-B test against?

[Paz]

3 hours ago, Louis D said:

@Paz Do you have a cheap mirror diagonal with an ordinary aluminum coating to A-B test against?

Oh yes, I've got plenty of those, I'll dig one out.