Neutral Density or Variable Polarising lunar filter?

[Adz]

Hey guys,

Well the title says it all really. I need a lunar filter for my scope which is an 8" Dob. These two types seem to be quite different.

Even using the lid with 2" hole on my scope lets in too much light for my eyes.

I understand that one is fixed and one is adjustable, adjustable at the expense of quality?

What does neutral density and what does polarising mean?

Which will give me the better views?

Thanks

[Capricorn]

An ND simply cuts out a given percentage of the incident light.and is independant of the wavelength. So a 20% filter allows through 80% and blocks 20%. Think cheap sunglasses.

Polarising filter operates by blocking the light that is not oscillating in the same plane as the filter is orientated. Reflections off of water is polarised which is why polarising glasses help see through. The glare is blocked but enough of the light from below the surface gets through so you see fish. Thats the theory at least but no-one has told the fish.

A variable polarising filter is simply 2 sheets of polarising material back to back that you can rotate in relation to each other, so the blocked percentage can be varied.

Light is polarised by reflection off of a non metallic surface. Guess what the moon is and how we see it. So I would guess that the moon exhibits polarised light in some degree. A single sheet if rotated would probably show changes.

As the angle of incidence of the light is a factor the light striking the moon may be too oblique in the majority to show much polarisation.

ND's come in just about any value, 1% - 100%.

A polarising filter will always block a fair percentage so I would guess the lowest block is at least 40%. Think 50% for a variable as you will not get them perfectly aligned.

To vary a polarising filter you need to rotate one sheet relative to the other. As I assume the filter is in the eyepiece not sure how this will work, will say I have never looked at how one is constructed. Also to change an ND filter means taking the eyepiece out so altering the filter level is probably much the same work in either case. Remove eyepiece, fiddle with filter and replace eyepiece.

I must remember to go view the moon through a polariser one night and see if it exhibits polarisation of light and to what extent.

The difference is that through the ND the image will be as you see by eye but dimmer. with the polarising filter it will also be dimmer and it will be polarised also.

[Merlin66]

For your purpose; the ND filter reduces the light by a fixed amount - the polarised filter can be "tuned" to give variable reduction in light.

[Adz]

Thanks for the help guys.

So the 0.6 ND filter lets more light in that the 0.9?

I was looking at these two and don't know which one to get:

U.K. Astronomy Buy & Sell

or this

U.K. Astronomy Buy & Sell

It is for use with an 8" dob.

[Moonshane]

personally when a filter is needed, I prefer a Baader Neodymium to any ND filter.

it's also generally useful for light pollution combat and some planetary targets.

[Adz]

Thanks for that Shane, what does the moon look like through it?

[Moonshane]

really nice and natural, grey and contrasty.

the brightness is well controlled but I find it OK/preferable on 50% or less without the filter.

I'd recommend the 2" as you can add it to your adapter and it prevents constant changing to a new eyepiece as you'd do with a 1.25"

[Adz]

So it will take away that overpowering glare on my eyes?

I use an orion precision centering adapter, I only have 1.25" eyepieces. Would I still be able to put the 2" BN filter some how and not have to screw it into each eyepiece each time?

Thanks

[Moonshane]

hi there

the orion unit is threaded for 2" filters so would be ok as long as you check the length of the nosepiece of your eyepieces. I extended my arrangement with a baader fine tuning ring screwed on first. http://stargazerslounge.com/diy-astronomer/122780-novel-use-baader-fine-tuning-rings.html

it takes the glare away for me just as much as a ND filter did.

I am sure you'll be happy with it.

[Adz]

Thanks I have just checked and annoyingly my Nagler does poke out the bottom on the adapter, it is my only eyepiece to

But the orion adapter has to go into a 2" adapter before it can go into the focuser so perhaps it could slot in the bottom screw there?

[todd8137]

i have the 12"dob and have both thesea would not bother with any just get a dark blue the variable twists and it gets lighter or darker i use a dark blue but when the moon is real bright they do not work much at all the dark blue is so so,every one as there own idea what to use a lot of it is suck it and see it might work for you it might not

cheers let us no how you get can you not lend some filters ? befor you buy?

cheers

[Moonshane]

Thanks I have just checked and annoyingly my Nagler does poke out the bottom on the adapter, it is my only eyepiece to

But the orion adapter has to go into a 2" adapter before it can go into the focuser so perhaps it could slot in the bottom screw there?

no reason why not if it's threaded for filters. I'd personally let it be though and get the fine tuning ring as they work very well.

[acey]

I have a 2" variable polarising filter and a 1.25" neutral density filter. Either works fine on the Moon with any of my scopes (though since I rarely look at the Moon, neither filter gets much use). The variable filter consists of two polarisers in a cell, and by rotation you can vary the transmission from nearly 50% to virtually zero.

Both filters screw into the barrel of the eyepiece, so you can't adjust the variable polariser unless you remove the eyepiece from the focuser. If you want to be able to see the effect of varying the transmission you need to hold the filter between eye and eyepiece.

Incidentally, contrary to wide belief, you can't damage your eyesight by looking at the Moon through a telescope, no matter how large the aperture. The Moon seen through a telescope has slightly less surface brightness than when seen with the naked eye. But because you're looking at night, the Moon appears particularly dazzling. A filter makes it more comfortable to view and may help bring out some detail.

An alternative would be to expose your eye to bright light (comparable to daylight) before looking at the Moon through the telescope. Obviously you wouldn't want to be looking at anything faint during the same session - but if you're looking at the Moon then DSOs are going to be off the menu anyway.

[Adz]

Thanks for that input Acey, it is very interesting to hear about how the moon is actually slightly less bright when viewed through a telescope than the naked eye.

I went with a 2" Baader neo thingy from FLO. I shall get back to this thread when I can give a first light