Variable Moon Filters

[Ed in UK]

Hi Everyone,

New to Astronomy & Star Gazing and only managed a couple of sessions with my new scope (Skywatcher 130EQ2).

The first object to have my scope pointed at it was, of course, the Moon. But after nearly burning the retina off the back of my eye I realized a filter was in order. Done some digging and decided to go for a Variable Filter (Polariser)  as I understand that some planets may also be quite bright to view sometimes.

My question is, is there any way of judging/setting the filter after it has been put in the eyepiece but before it goes into the scope ?

Can see it could get a bit fiddly if I need to remove the eyepiece several times to adjust the filter/polariser to achieve the required amount of filtration. 

Also is it recommended to have some form of protection on the end of the scope to prevent dust/dirt/moisture from getting in ? 

Many thanks in advance.

Ed

[laowhoo]

Couldn't you just notch it w/ a dab of fingernail polish at the 1/4 turns? Or maybe with a bit of glow in the dark paint. What's that stuff that luminesces on watch faces? I wish we'd gotten the polarizer rather than the 13% b/c I was surprised to find the SkyGlow light pollution filter was much better for my eyes on the moon. Some people just turn on their porch light for lunar.

[Ricochet]

In a telescope with a 2"-1.25" reducer or a diagonal you can put half of a polariser in the reducer/diagonal and the other on the eyepiece and then simply twist the eyepiece to the required brightness. 

For your scope the only way of replicating is if: 

1. You are using a barlow in a which case you can put one part in the eyepiece and the other on the barlow (if the barlow is threaded for filters)

2. Use a short extension tube (which could be a barlow with the barlow lens cell removed) to do the same. This requires that you have enough in travel to compensate for the extra length of the extension tube. 

If neither of these are possible for your set up then the best thing to do is probably to put both halves of the polariser on the eyepiece, then hold the eyepiece up to the moon and adjust it to a reasonable level before putting it into the telescope. This should work quite well for low magnifications but you may find that it dims the image too much at high magnification. 

[Paul73]

Don’t worry. It seems a lot brighter than it is because everything else is dark. And, full moon isn’t a great time for Luna viewing as you loose a huge amount of contrast (fewer shadows except on the limbs).

Try using more magnification until it is comfortable.

Paul

PS. I have a moon a moon filter and a variable polarising filter, but tend not to use them much. If I ever turn my 16” on the moon, I will probably use them! Most of my Moon viewing is done using 120mm or 250mm sans filters.

[Louis D]

Another option is to go the binoviewer route.  The splitter cuts the light in half going to each eye, and both eyes see the same brightness, so fatigue/overstimulation is not an issue.  The moon also takes on a 3D look at full moon.  It's also the only way I can see detail on the face of the moon at full phase.

5 hours ago, Ed in UK said:

Also is it recommended to have some form of protection on the end of the scope to prevent dust/dirt/moisture from getting in ? 

Dust on the mirrors is not much of an issue.  Scratches on the mirror are, so resist the urge to clean them.  Newtonian scopes like yours tend not to dew up because the mirrors are either deep inside a closed tube (which acts like a dew shield) or face away from the sky (reducing radiative cooling), so moisture is not much of an issue for your scope, either.

[Waldemar]

4 hours ago, Ricochet said:

In a telescope with a 2"-1.25" reducer or a diagonal you can put half of a polariser in the reducer/diagonal and the other on the eyepiece and then simply twist the eyepiece to the required brightness. 

For your scope the only way of replicating is if: 

1. You are using a barlow in a which case you can put one part in the eyepiece and the other on the barlow (if the barlow is threaded for filters)

2. Use a short extension tube (which could be a barlow with the barlow lens cell removed) to do the same. This requires that you have enough in travel to compensate for the extra length of the extension tube. 

If neither of these are possible for your set up then the best thing to do is probably to put both halves of the polariser on the eyepiece, then hold the eyepiece up to the moon and adjust it to a reasonable level before putting it into the telescope. This should work quite well for low magnifications but you may find that it dims the image too much at high magnification. 

Hi Ricochet,

You are saying the variable polarising filter comes apart in two threaded pieces...
I can not find anyway to separate them, cause they are not threaded together but are coupled so they can turn but not come loose.

I would like to know how you get them separated without damaging them and use them as single filters.
If that works it would be a top solution for these variable polarisers.

[Ed in UK]

Wow thanks to everyone for your extremely helpful advice.

My thoughts mirrored Ricochet's with attach the filter to the eyepiece then hold up to moon and adjust. I was keen to see if there was another way.

Looking forward to a clear night and getting out again.

[John]

You can't hurt your eye observing the moon without a filter so at least there is no risk. You can safely experiment to find what is comfortable for you. Many observers don't find the need for a moon filter at all even with relatively large aperture scopes.

 

[Ed in UK]

Thanks John.

[Ricochet]

37 minutes ago, Waldemar said:

Hi Ricochet,

You are saying the variable polarising filter comes apart in two threaded pieces...
I can not find anyway to separate them, cause they are not threaded together but are coupled so they can turn but not come loose.

I would like to know how you get them separated without damaging them and use them as single filters.
If that works it would be a top solution for these variable polarisers.

I can't guarantee that every pair of variable polarisers can be separated but on the ones I have seen it is possible. Here are some pictures of my 2" pair. Firstly the two partly together and secondly the special one on its own. Notice on the special filter cell (which is on top in my picture) has the rotating part at the top and then directly under this is a small knurled section. If you grip this section and the second normal filter cell (the bottom one) you can split them into two separate sections as with any other filters. 

If yours do not come apart then ones that do are available on eBay directly from China. 

[Louis D]

51 minutes ago, Ricochet said:

I can't guarantee that every pair of variable polarisers can be separated but on the ones I have seen it is possible. Here are some pictures of my 2" pair. Firstly the two partly together and secondly the special one on its own. Notice on the special filter cell (which is on top in my picture) has the rotating part at the top and then directly under this is a small knurled section. If you grip this section and the second normal filter cell (the bottom one) you can split them into two separate sections as with any other filters. 

If yours do not come apart then ones that do are available on eBay directly from China.

Looking even on ebay, it's clear both kinds are sold there.  I never knew they came in two distinct versions.  I'll have to look out for the separable type if I ever get one.

[Cosmic Geoff]

3 hours ago, John said:

You can't hurt your eye observing the moon without a filter so at least there is no risk. You can safely experiment to find what is comfortable for you. Many observers don't find the need for a moon filter at all even with relatively large aperture scopes.

 

I second that. I have never used a Moon filter. But if you feel the need to spend your money, astro dealers will be happy and so will you.

[JOC]

My variable polariser splits in half and I think was a 2nd hand skywatcher one.  I have tried all options above, my delgx skywatcher x2 barlow also splits apart to take the sections.  Put one half at the bottom of the barlow and the other half on the eyepiece then just loosen the screws to twist one bit or the other.  Puting half on top of the EP works if you aren't viewing too vertically.  

The other thing you try is reducing the size of the telescope apperture.  My Dob has a cover which has a smaller cover over a hole about 2" across.  This cuts out a lot of the light.

Another alternative is to wear a pair of sunglasses, even a polarising pair!.  

You could even do sunglasses and half the filter on the bottom of the EP which you could then twist,  Depending on the polarising grid pattern of the sunglasses it may still work to lighten and darken things.

FWIW having tried everything I just rule 5 it and view the moon with no filters and no sunglasses even when it is full.

[Luna-tic]

19 hours ago, John said:

You can't hurt your eye observing the moon without a filter so at least there is no risk. You can safely experiment to find what is comfortable for you. Many observers don't find the need for a moon filter at all even with relatively large aperture scopes.

 

It won't damage your eye looking at FM unfiltered, but it ruins your dark adaptation for viewing anything else for quite a while. If you go out to observe the Moon, look at everything else that's visible first.

18 hours ago, Louis D said:

Looking even on ebay, it's clear both kinds are sold there.  I never knew they came in two distinct versions.  I'll have to look out for the separable type if I ever get one.

Orion's dual polarizer comes in two pieces which screw together, and have a sleeve which allows you to rotate them to add or cut light passage. I've been attaching the coupled pair to my EP, holding it just over the diagonal and adjusting the light, then inserting them. I think I'll try the trick with attaching one to the diagonal or Barlow, and the other to the EP, sounds much easier if you need to adjust the light.

[Louis D]

26 minutes ago, Luna-tic said:

It won't damage your eye looking at FM unfiltered, but it ruins your dark adaptation for viewing anything else for quite a while. If you go out to observe the Moon, look at everything else that's visible first.

Except for this one trick I learned by accident.  When the full moon and the Orion nebula are both up, take a good long look at the moon to get your cones fully activated, then quickly swing your scope over to the Orion nebula.  In my 8" Dob, it looks slightly green for a moment!  It's a really cool effect.  You can also try looking at a white card lit by a flashlight while your scope is aimed at the nebula and then looking into the eyepiece.  It's not an artifact.  The Orion nebula really does appear slightly green to the naked eye, it's just that your rods normally dominate your cones at night, so you see it as basically black and white.

[John]

36 minutes ago, Luna-tic said:

It won't damage your eye looking at FM unfiltered, but it ruins your dark adaptation for viewing anything else for quite a while. If you go out to observe the Moon, look at everything else that's visible first..

 

Actually looking at the Moon first can help with the resolution of planetary features. The white card trick mentioned my Louis above is also used by planetary observers for a similar reason.

Fainter DSO's are not really on the menu when there is a bright moon in the sky anyway, filter or no filter.

Still, moon filters don't cost much so if folks want to use them, why not ?

[Luna-tic]

7 minutes ago, Louis D said:

Except for this one trick I learned by accident.  When the full moon and the Orion nebula are both up, take a good long look at the moon to get your cones fully activated, then quickly swing your scope over to the Orion nebula.  In my 8" Dob, it looks slightly green for a moment!  It's a really cool effect.  You can also try looking at a white card lit by a flashlight while your scope is aimed at the nebula and then looking into the eyepiece.  It's not an artifact.  The Orion nebula really does appear slightly green to the naked eye, it's just that your rods normally dominate your cones at night, so you see it as basically black and white.

I'll give that a try Tomorrow night. Supposed to be clear, we have a gibbous waning moon, and Orion will be up early. Going to try for first light with my new WO GT81.

My statement was based on losing much of my night vision in the eye I was using to view with, for some minutes after ceasing. If I closed the non-viewing eye, I was almost completely night blind, and trying to view things like doubles or open clusters was a waste of time until I regained the night adaptation with my viewing eye.

[JOC]

There was a fab. moon here last night, I went out at around 8pm and it was rising very low on the horizon and looked truly huge with the lensing effect of the atmosphere!