"Foggy" prism - is this normal?

[bitnick]

I got a Baader T2 prism (the non-Zeiss version), and I'm wondering if there is maybe something wrong with it.

Exactly on axis, it looks clear, but just sligthly off axis, in one direction only, this is how it looks:

And more clearly, more off axis:

Is this something those of you who are owners of prisms recognize? Should I contact Baader for an opinion?

[Stu]

That doesn't look right to me. The prisms in mine are beautifully clear, hard to tell they are there from some angles. I would get in touch with your supplier for an exchange

[bitnick]

Ok. It's really great to be able to ask advice of others with the same equipment! I will contact the supplier. I also sent a question directly to Baader about this; it'll be interesting to read their response.

Thanks!

[bitnick]

I got a reply from Baader yesterday. Apparently, this "fogginess" is normal in prisms. Why? Because they use total internal reflection (TIR), and this only works in a quite limited angle off of perpendicular to the prism surface. Outside this angle, the "reflective" back of the prism starts to get transparent, showing glue or whatever is behind the prism.

The guy who answered even checked two prisms they had laying around, which showed the same effect.

BigSumorian, maybe yours look like that too, in the "right" light? When I took the photos, it was quite light with sun shining in through the windows, indirectly illuminating the inside of the prism. The prism also wasn't mounted to anything (as is obvious in the photos), so light entered in very shallow angles.

Here is a quote of the relevant part of Baader's reply:

Everything is fine with your prism - they work with total internal reflection and not like a normal mirror. You can see this effect with every prism, this is just how the Zeiss- and Baader-prisms work. The "magic" happens inside of the prism, so if you use it on the optical axis, it will work as expected. Only if you tilt it, you will see some strange effects, but they are irrelevant if you look through it as intended. Total internal reflection depends absolutely from the angle; if you are beyond the critical angle, the light will be splitted, and there is transmission as well as reflection, and the view becomes foggy. Depending on the background and illumination, this effect is more or less obvious.

So, everything is fine with your prism, because it is designed to reflect light in the 90°-angle. Only if the view would be foggy when you look at it in the right angle (e.g. through an eyepiece), there would be a reason for worrying.

I then asked about the critical angle of the prism (i.e. the angle where TIR starts/ends - BTW this is directly dependent on the refractive index of the prism material). It would be nice to see if I could correlate this with the effect I'm seeing - just for the fun of it. Maybe shine a laser through it or something. I have not gotten that information yet, though.

[Stu]

That's very interesting! I still don't recall seeing the same effect on mine, obviously you don't see through it when not at 90 degrees but I don't think it looks foggy?? Anyway, I will give it another go tomorrow and perhaps take a picture to compare.

Does it perform ok?

[jetstream]

I'll check mine too.

[Stu]

I've had a look at mine and am afraid I don't see anything like that. It is clear either side of 90 degrees until a certain point when total reflectivity takes over, but at no point do I see any fogginess as shown in your picture.

Did you send them the picture?

[bitnick]

BigSumorian: Does it perform ok?

That is of course the important question. But it is difficult to say, since the prism itself is not in focus when used in the telescope, any fogginess would just increase scatter and dim the image, rather that being directly visible. I'm just starting to use my telescope, and don't even have a mount yet (I'm building it myself), and I don't have the experience to judge the prism OK or not in the telescope under these circumstances.

BigSumorian: Did you send them the picture?

I could not send them the images directly (their contact form did not allow it), but I sent them a link to this forum thread instead, and it seems like they've seen the pictures in it.

Interesting that you do not see this in your prism. Total internal reflection is dependent on the refractive indices of both the medium outside the prism and the material of the prism itself - the critical angle Tc = arcsin(n2/n1), where n2 is the refractive index of the medium outside the prism, and n1 is the refractive index of the prism. If the outside medium is air, then n2 = 1.00, and the TIR angle is pretty much maximized. If there is dirt stuck on the diagonal surface of the prism though, the "outside medium" would be that dirt, and that would have a refractive index higher than air, which would reduce the usable TIR angle and light up that dirt through transmittance. I wonder how the prism is mounted to the back plate of the holder?

Hmm... I really need to know the usable TIR angle of the prism, but Baader has not been forthcoming so far. I've only gotten "total reflection only works 90°, not at other angles, and it is not depending on the material" - which is completely wrong, and hopefully only due to a misunderstanding. I'll continue my dialogue with them until I've gotten this cleared up.

[bitnick]

jetstream: Please do!

[bitnick]

Oh, and it would also be interesting to know the types and dates of purchase of your prisms.

(Sorry for the flood of posts, but I can't edit my posts to add things...)

[YKSE]

I see it in my Zeiss prism diagonal.

It took a good while though, not as dense as in your picture, nor that angle. I can only see it around two specific angles, i.e. looking from diagonal's 90 degree corner, about 45 degree into the prism surface.

[jetstream]

I see it with my Baader/Zeiss 2" too- I left the nosepiece and EP holder on though and this kind of cuts off the view of it a bit. It is definitely there. These prisms offer very low scatter and great color transfer, you will be happy with yours. They quit making mine I think, I do have the # here somewhere.

[Stu]

Still can't see it on mine, had a good look earlier.

[bitnick]

I was kind of hoping on some kind of consensus here, but alas... I guess between Baader's reply and some of you having it in your prisms without apparent drawbacks, I'll leave it at that. Still waiting for Baader's reply on usable TIR angle though.

[jetstream]

What scope are you using the prism with?

[bitnick]

A Maksutov 127 (focal ratio 11.8).

[jetstream]

I have noticed that my prism diag needs its own cooling time and I always place it outside with the scope to cool. Maks might need a fair chance to reach equilibrium to give the best views as does the prism. About 15-20 min does it for me ( diag) and the resulting views are phenomenal, seeing considered.

How are your views with the Mak and prism?

[bitnick]

As I wrote above, I'm building my own mount for this thing (I bought it as an OTA). I've only tried the scope on jupiter, handheld and through window glass - so my views are pretty much non-existent at the moment.

I will of course start a thread describing the mount sooner or later.

Thanks for the cooldown hint!