Variable polarizing filter - the “trick”?

[Starslayer]

So I have one of these and understand it but see several references regarding different ways to use it.  Drop in the eye piece shroud for temp use, split in half and attach one bit to the diagonal and one to the ep etc etc.   My issue here is 1. I do not see what benefit can be gained putting the static bit on the diagonal unless you want it more permanent but it is the bit about attaching the adjustable ring bit to the ep and using it like that.  Someone does suggest a rubber o ring dropped in but other than that I cannot get this to work for me. If I lock the ep then it won’t swivel at all. If I leave the ep loose then it does not adjust ( well only a bit and when it feels like it ).  Are those suggesting this “trick” putting the ep in just enough for the lock or compression ring to grab it maybe so that the ep can then be turned at will?  May be just as easy to pop it out to make the adjustment as necessary then lock it back in properly. 

At 6.40 on the video as example.. looks simple but does not work. 

 

Edited August 3, 2021 by Starslayer

[Tiny Clanger]

If you put one filter (probably the rotating one, as it will have less chance of being disturbed  )   on the diagonal inner thread, the other on the eyepiece thread, you can look through the eyepiece while you rotate the eyepiece barrel, which turns the polariser (yes,  the eyepiece needs to be loose in the focus tube) and see the precise effect,  get to the view you want, then tighten the eyepiece in place. That's what  I do, and find it works well, at least, in 'scopes which have a diagonal ...

If you attach both filters to the base of your EP, (as I have to do in my dob)   it's not so easy, you are repeatedly taking the EP out,  turning the rotating filter a little, replacing the eyepiece carefully while trying not to change the filter orientation,  looking through it,deciding the view can be improved,  taking it out, turning the rotating filter a little, replacing the eyepiece carefully while trying not to change the filter orientation, looking through it,deciding the view can be improved,  taking it out, turning the rotating filter a little, replacing the eyepiece carefully while trying not to change the filter orientation ...

I suppose you could place the pair of filters temporarily in the eyecup of the eyepiece , look through as you fiddle with the orientation, then try to hold both in their relative positions with your fingertips as you screw them into the eyepiece base.  I know if I tried that I'd just drop them both, or get fingerprints on 'em. !   I tend to use a completely non-adjustable Moon filter rather than the  polarisers when using my dob.

[Tiny Clanger]

1 hour ago, Starslayer said:

 

At 6.40 on the video as example.. looks simple but does not work. 

 

Works for me ...

[Starslayer]

20 minutes ago, Tiny Clanger said:

Works for me ...

Although he has got the rotating part on the ep not the inner diagonal thread.. He then turns the ep but I see where you are coming from and will give it a try.  Thank you for the rest of your post by the way.

[inFINNity Deck]

59 minutes ago, Tiny Clanger said:

If you put one filter (probably the rotating one, as it will have less chance of being disturbed  )   on the diagonal inner thread, the other on the eyepiece thread, you can look through the eyepiece while you rotate the eyepiece barrel, which turns the polariser (yes,  the eyepiece needs to be loose in the focus tube) and see the precise effect,  get to the view you want, then tighten the eyepiece in place. That's what  I do, and find it works well, at least, in 'scopes which have a diagonal ...

I use the same method with my C11 EdgeHD: the rotating one goes into the diagonal (at the scope end), the fixed one in the eyepiece. Care should be taken to properly orient the one that goes into the diagonal as the diagonal itself causes some polarisation as well, so the orientation of that filter should be checked to maximise the effect when combined with the eyepiece. This can be checked by rotating the filter while looking through the diagonal with the eyepiece and eyepiece-filter attached. Look through the diagonal from the scope end (so in reversed direction) and rotate both the eyepiece and rotating filter. You will find four positions of the rotating filter where the combined filters have no effect at all (so turning the eyepiece alone does not darken the view) and four positions where the combined effect is maximal. Choose an orientation of the rotating filter close to the darkest view, but not fully dark, as that will produce a dark beam through the FOV (but that may be depending on the FOV, I use TeleVue PanOptic and Ethos wide-field eyepieces). Finally insert the diagonal in the OTA and, as TC wrote, leave the eyepiece unlocked and rotate the eyepiece until the resulting view is comfortable, then lock the eyepiece. You will notice that the view can be made yellowish or blueish, depending on which side you are from the darkest setting.

I bought the polarising filters as a set, which both were 2" and added a 1.25" fixed version for the shorter focal length eyepieces I use (those have an inner barrel of 1.25") and find them very comfortable in both lunar and solar observations (the latter in combination with a full aperture Baader ND5.0 foil filter).

Nicolàs

[Tiny Clanger]

1 hour ago, Starslayer said:

Although he has got the rotating part on the ep not the inner diagonal thread.. He then turns the ep but I see where you are coming from and will give it a try.  Thank you for the rest of your post by the way.

You had not edited your post and added the video when I  typed out my reply, so I wouldn't have seen that detail.

It seems sensible to place the freely rotating filter where it is less likely to be accidentally disturbed .

If you can't get on with the Pol.  pair, maybe get some ND filters instead ?

[Starslayer]

1 hour ago, inFINNity Deck said:

I use the same method with my C11 EdgeHD: the rotating one goes into the diagonal (at the scope end), the fixed one in the eyepiece. Care should be taken to properly orient the one that goes into the diagonal as the diagonal itself causes some polarisation as well, so the orientation of that filter should be checked to maximise the effect when combined with the eyepiece. This can be checked by rotating the filter while looking through the diagonal with the eyepiece and eyepiece-filter attached. Look through the diagonal from the scope end (so in reversed direction) and rotate both the eyepiece and rotating filter. You will find four positions of the rotating filter where the combined filters have no effect at all (so turning the eyepiece alone does not darken the view) and four positions where the combined effect is maximal. Choose an orientation of the rotating filter close to the darkest view, but not fully dark, as that will produce a dark beam through the FOV (but that may be depending on the FOV, I use TeleVue PanOptic and Ethos wide-field eyepieces). Finally insert the diagonal in the OTA and, as TC wrote, leave the eyepiece unlocked and rotate the eyepiece until the resulting view is comfortable, then lock the eyepiece. You will notice that the view can be made yellowish or blueish, depending on which side you are from the darkest setting.

I bought the polarising filters as a set, which both were 2" and added a 1.25" fixed version for the shorter focal length eyepieces I use (those have an inner barrel of 1.25") and find them very comfortable in both lunar and solar observations (the latter in combination with a full aperture Baader ND5.0 foil filter).

Nicolàs

Oh dear. This really confused me but after reading several times I think I actually understand. I will give this a go tomorrow.  Thank you.  I think part of the problem is understanding how the filter actually works and until you know that it makes no sense that you actually rotate the part that does not rotate to achieve the goal.  

[Starslayer]

7 minutes ago, Tiny Clanger said:

You had not edited your post and added the video when I  typed out my reply, so I wouldn't have seen that detail.

It seems sensible to place the freely rotating filter where it is less likely to be accidentally disturbed .

If you can't get on with the Pol.  pair, maybe get some ND filters instead ?

Sorry for the over edit.. And no, I will persevere with this.  

[Tiny Clanger]

1 hour ago, Starslayer said:

Oh dear. This really confused me but after reading several times I think I actually understand. I will give this a go tomorrow.  Thank you.  I think part of the problem is understanding how the filter actually works and until you know that it makes no sense that you actually rotate the part that does not rotate to achieve the goal.  

You have two polarising filters . As you rotate the two filters with respect to each other, you get a range of darkening,  from 'not a lot'  (I'd have to look up how much)  steplessly through to 100% opaque. So to achieve the precise darkening effect you require, you need one filter to stay still,  while you rotate the other . When the two filters are joined together to make one unit, obviously the top filter , screwed to the base of the eyepiece,  will stay still, and you turn the other , using the rotating feature on it.

When you separate the two filters to use one on the eyepiece, the other in the 'scope side of the diagonal, , you want to control the relative orientation of the two filters  by turning the eyepiece. So have the non-rotating filter there, giving you an eyepiece/filter unit which moves as one when you turn the eyepiece in  the focuser tube . 

The polariser with the ring that turns could potentially catch on the eyepiece tube, and not therefore rotate as a unit with the eyepiece. So it is better to have that one  in the front of  the  diagonal, where there is no movement to disturb its setting ( a setting helpfully explained by Nicholas.)

I think that's the best I  can explain it !

Edited August 3, 2021 by Tiny Clanger
clarity !

[Starslayer]

5 minutes ago, Tiny Clanger said:

You have two polarising filters . As you rotate the two filters with respect to each other, you get a range of darkening, ranging from 'not a lot'  (I'd have to look up how much)  steplessly through to 100% opaque. So to achieve the darkening effect you require, you need one filter to stay still,  while you rotate the other to achieve the effect you want.. When the two filters are joined together to make one unit, obviously the top filter , screwed to the base of the eyepiece,  will stay still, and you turn the other , using the rotating feature on it.

When you separate the two filters to use one on the eyepiece, the other in the 'scope side of the diagonal, , you want to control the relative orientation of the two filters  by turning the eyepiece. So have the non-rotating filter there, giving you an eyepiece/filter unit which moves as one when you turn the eyepiece in  the focuser tube . 

The polariser with the ring that turns could potentially catch on the eyepiece tube, and not therefore rotate as a unit with the eyepiece. So it is better to have that one  in the front of  the  diagonal, where there is no movement to disturb its setting ( a setting helpfully explained by Nicholas.)

I think that's the best I  can explain it !

An eloquent and sensitively tactful explanation much appreciated by an ageing yet not totally over the  hill novice. As said, will try this tomorrow but it makes sense to me now.  Maybe you will understand the confusion caused by those saying to put the rotating part on the ep.  

[Starslayer]

So the plot thickens.  Tried this again today with the rotating part on the diagonal and the fixed part on the bottom of the ep.  I can now adjust it but.... and this is a big but 🤔..... I can either have it all light or darken just one side of the view.  Twist the ep and it goes all light of then darkens just the other side of the view.   Hazard a guess, this is something to do with me using / having stock an erect prism diagonal.  I have nothing else to try it with other than my 2" xlt diagonal and of course I cannot screw onto the back of the diagonal with that.  Sorry rubbish pic but you get the idea.   

[Louis D]

Yes, there is sharp edge down the center of erect image diagonals to effect the left for right swap.  Reflections off glass tend to be strongly polarized.  That's why you can see through other people's windshields with polarizing sunglasses on sunny days.

There's really no reason to use one at night.  That sharp edge adds a diffraction line to bright stars.  Try using a plain mirror diagonal and you should see the entire image darken evenly.  Metallized mirrors don't tend to polarize light.

[Tiny Clanger]

3 hours ago, Starslayer said:

 Hazard a guess, this is something to do with me using / having stock an erect prism diagonal.  I have nothing else to try it with other than my 2" xlt diagonal and of course I cannot screw onto the back of the diagonal with that.  Sorry rubbish pic but you get the idea.  

That's the problem, a prism diagonal ... I should have checked you were using a mirror diagonal, it never crossed my mind .

As far  as I can see, your choices are , buy a 2" pair of polarisers for your 2" mirror diagonal, or go for a few 1.25" ND filters , or buy a 1.25 mirror diagonal ...

Looking at  FLO , the cost of basic (astro essentials) 1.25" ND filters is £9 each, so £30 ish for the set of 3 different strengths. (or the Baader option for £37 each ) The 2" variable pol is about the same cost as the 3 astro essentials NDs 

If you want a very cheap solution, there was a fairly recent thread on here about a £10 ish mirror diagonal folk were buying from Amazon , you could get one and reserve it just for use with the polarisers, and leave one filter screwed in place ....

At least if you decide to buy something different a 1.25" polarising pair will be easy to post, so (unless you can return them to the seller for a refund, which would be ideal ) you can recoup some of their cost by selling them on here !

Heather

[inFINNity Deck]

I use 2" TeleVue mirror diagonals, indeed no sharp edge here. I have no prism diagonals at hand to test the filters with.

Nicolàs

[Starslayer]

Thanks to all.  I understand the options and I think the best one is revert to plan A and simply attach the whole kit to the ep and adjust as needs must.  Quite simple to do any way. Good learning curve though.