variable polarising filter question

[domstar]

Hi everyone,

I like my Lunt Herschel wedge although it hasn't had much use for a couple of months. As a stop gap, I've been using it with a moon filter but of course I need a polarising filter. I know I only need a single polarising filter as the wedge has one built in.

My question is - if I get a variable polarising moon filter would I be able to use the two parts in two different eyepieces? It drives me mad unscrewing and screwing the filter every time I change eyepieces. I'm a total butterfingers too so it would be a benefit if I could screw two filters into two different eyepieces at the beginning of the session. Would this be a sneaky way to get two for the price of one with a possible bonus of use on the moon too or is there something I haven't thought of?

Thanks

[Dinglem]

I have a variable polarising filter like this one https://www.firstlightoptics.com/moon-neutral-density-filters/variable-polarizing-moon-filter.html

You are able to seperate the two filters so can be used independantly.

[domstar]

@Dinglem Thanks. That's the sort of thing I'm interested in if it works as two single polarizing filters with a Herschel wedge,

[johninderby]

Opticstar does one that works just fine and is cheaper.

http://www.opticstar.com/Run/Astronomy/Astro-Accessories-Imagers-Opticstar.asp?p=0_10_5_0_3_133

[domstar]

@johninderby Blimey, that is cheap. Thanks.

[LukeSkywatcher]

A variable polarizing filter can be separated and each part used in different positions in the optical train. This makes it easier to use. The top part that you turn, obviously needs to be closer to your eye so it turns freely. 

[domstar]

@LukeSkywatcher Hi Paul. I wanted to ask you because I know you have one and a Herschel wedge too. I want to use one part on my 18mm eyepiece and one part on my 10mm eyepiece so I can swap eyepieces without all the juggling, unscrewing and rescrewing. My idea is that the Herschel wedge already has a polarising filter so I could use each part of the variable polarising filter on its own. I hate mucking about with filters and I have pretty clumsy fingers. Would it work as I have guessed?

[Paz]

On 25/11/2018 at 16:23, domstar said:

@LukeSkywatcher Hi Paul. I wanted to ask you because I know you have one and a Herschel wedge too. I want to use one part on my 18mm eyepiece and one part on my 10mm eyepiece so I can swap eyepieces without all the juggling, unscrewing and rescrewing. My idea is that the Herschel wedge already has a polarising filter so I could use each part of the variable polarising filter on its own. I hate mucking about with filters and I have pretty clumsy fingers. Would it work as I have guessed?

You can put one polarising filter in each eyepiece as you describe. The Herschel wedge doesn't need  a polarising filter itself, the wedge by its nature polarises the light and then a single polarising filter works with that just as if you had 2 polarising filters working together.

[domstar]

@Paz  Thanks very much. That's what I wanted to know. 

[LukeSkywatcher]

On 29/11/2018 at 19:11, Paz said:

You can put one polarising filter in each eyepiece as you describe. The Herschel wedge doesn't need  a polarising filter itself, the wedge by its nature polarises the light and then a single polarising filter works with that just as if you had 2 polarising filters working together.

True. 

"I want to use one part on my 18mm eyepiece and one part on my 10mm eyepiece so I can swap eyepieces without all the juggling, unscrewing and rescrewing"

I put the bottom part on the wedge (insert into scope tube), and the top part on an EP. Im not sure if you totally separate both parts and put each on different EP's that it will work. A variable polarizing filter only works (AFAIK) when both parts are connected somewhere in the same optical train. 

I may be wrong, but i think they need to be attached somewhere along the train of a single EP.  So putting one half on an 18mm and the other half on a 10mm, in my mind wont work. Ive never tried it. As Paz said, the Hershel wedge is already polarized, so really doesnt need a filter.

Hope this makes sense. I havent done much solar observing in the last 18 months.

[johninderby]

Or simply use an eyepiece extension tube and leave the filter on the end. Usefull if you add a continuum fitter as well.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4213_TS-Optics-1-25--extension-tube--35mm-length-w--brass-compression-ring.html

[LukeSkywatcher]

Just now, johninderby said:

Or simply use an eyepiece extension tube and leave the filter on the end. Usefull if you add a continuum fitter as well.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p4213_TS-Optics-1-25--extension-tube--35mm-length-w--brass-compression-ring.html

Polarizing filters are (IIRC) double threaded so you can add a a continuum filter to the polarizing filter. 

 

[johninderby]

Using an extension to hold the filters just means simply being able to change eyepieces without swapping the filters over.

[Paz]

The optical train would look like this...

Objective lense...Herschel wedge...ND3.0 filter...single polarising filter...eyepiece.

In this train the Herschel wedge is one of the polarising agents and the single polarising filter is the other.

The Herschel wedge takes out about 95.5% of the brightness and the ND3.0 drops it by another 99.9%, then the polarising filter tunes it down further from there.

4 hours ago, LukeSkywatcher said:

True. 

"I want to use one part on my 18mm eyepiece and one part on my 10mm eyepiece so I can swap eyepieces without all the juggling, unscrewing and rescrewing"

I put the bottom part on the wedge (insert into scope tube), and the top part on an EP. Im not sure if you totally separate both parts and put each on different EP's that it will work. A variable polarizing filter only works (AFAIK) when both parts are connected somewhere in the same optical train. 

I may be wrong, but i think they need to be attached somewhere along the train of a single EP.  So putting one half on an 18mm and the other half on a 10mm, in my mind wont work. Ive never tried it. As Paz said, the Hershel wedge is already polarized, so really doesnt need a filter.

Hope this makes sense. I havent done much solar observing in the last 18 months.

It is ok to split the variable polarising filter into its two single polarising components, the optical train would look like this...

Objective lense...Herschel wedge...ND3.0 filter...single polarising filter (I.e half of a pair)...eyepiece.

In this train the Herschel wedge is one of the polarising agents and the single polarising filter is the other.

The Herschel wedge takes out about 95.5% of the brightness and the ND3.0 drops it by another 99.9%, then the polarising filter tunes it down further from there.

[domstar]

12 hours ago, johninderby said:

Using an extension to hold the filters just means simply being able to change eyepieces without swapping the filters over.

That's a great Idea. I never knew that. 

[domstar]

@Paz Yes that's what I thought. It's good to have someone confirm it.

@johninderby I've just ordered the opticstar. It's a ridiculously low price and free shipping.