Polarising filter - does it really help?

[rusirius]

I was looking to buy a moon or ND filter when I came across a double polarising filter. The blurb claims that as well as being able to reduce the brightness from 40% all the way to 100%, it can also help with splitting doubles as it reduces the glare from the primary so that you can beeter see the fainter secondary.

Is this just advertising hype or does it really help?

Since doubles are one of my interests, I would consider this somewhat more expensive option if it has merits.

[Merlin66]

I have the Orion one, and think it's a much more flexible accessory than the Moon filter. I definately works to reduce brightness ( I sometimes use it behind the white light solar filter just to aid the contrast.)

I understand the concept re double stars, but I'd think it would also reduce the light from the comes and make it even harder to see. You can borrow mine and check it out before you buy??

It should be noted that Couteau in his " Observing visual double stars" never mentions the use of filters at all; he mentions many other things to help and assist the observer but not ND filters......

[Talitha]

Never heard of using a polarizing filter to help detect higher magnitude secondaries. It would definitely tame the glare of the primary, but logically wouldn't it also dim the secondary (perhaps to the point of extinguishing it from view, depending on the magnitude)?

[rusirius]

Talitha, that's what I thought and hence my scepticism.

Here is a link to the description of the item in question:

http://www.rothervalleyoptics.co.uk/celestron-polarizing-filterset-1-25_d1807.html

Interestingly, the description for the Antares and Skywatcher equivalents (which incidentally look identical) make no such claim:

http://www.rothervalleyoptics.co.uk/antares-variable-moon-filter_d953.html

http://www.rothervalleyoptics.co.uk/skywatcher-variable-polarizing-filter-1-25-quot_d2933.html

Merlin, it occurred to me to try it behind a Baader solar filter but I presume it yeilds no additional detail just extra contrast? Your offer of try-before-you-buy is appreciated. I will consider it, but I would hate for it to be lost in the post or something. That looks like an interesting book by Paul Couteau but I'm puzzled by the huge price variance ranging from $5.98 on Amazon US to a whopping £148.01 on Amazon UK!

[Merlin66]

To my eyes, anyway, it gives a bit more contrast; I've been comparing it with a Green filter ( which was the best of all the colours and "specials" I tried.)

One of the best sources for out of print books is Alibris. You're right; sometime the prices are absolutely crazy!!!

[Talitha]

I've heard good things regarding the use of an occulting bar to handle the glare from bright primaries, but not a polarizing filter. (Yes, it's odd that one page mentions double stars and the others don't. )

[rusirius]

I've heard good things regarding the use of an occulting bar to handle the glare from bright primaries, but not a polarizing filter.

An interesting idea.

[Merlin66]

Another great aid to double star observers is a double image micrometer made from a small cleft of Iceland Spar. I built mine almost 15 years ago and it's still going strong!

The basic details are mentioned in Couteau's book and also detailed in Martinez' excellent book "The Observer's Guide to Astronomy", Vol2, page740-743

[Lady Isabella]

Before buying a filter, I would have a go at making an Hexagonal mask.

http://www.carbonar.es/s33/Articles/WhoWasThatMaskedDoubleStar.pdf

[rusirius]

Thank you m'lady. A most interesting article.