Polarizing Filters

[MapleRidge]

Hi all...

I am looking to install a polarizing filter in the Lunt DS mod that I have created, primarily to reduce the reduce the overall background glare and suppress some reflections between the glass elements. I should be able to mount a 2" eyepiece style filter, but to date the only polarizing filters I have used are the variable version for the WL wedge.

I don't think this is what I want, just a single element if available. When I look at the specs on the Baader filter, it states that it reduces the brightness and I have seen reference here to circular polarizing filters for this purpose.

Can anyone with experience with this help me get my head around the best option and any specific models and sources to order them would be much appreciated.

Thanks,
Brian

[MalVeauX]

Hi,

A basic single polarization filter will do it. Baader's 2" version is fine. It will slightly dim the image (all do), but only a slight be (maybe 2/3s to 1 stop of light from exposure perspective). Slightly tilted it should eliminate any reflections really.

I would avoid the variables (two singles in a series that you rotate to control the effect).

Very best,

[MapleRidge]

Thanks Marty...

The rotating pair is what I have used and knew wasn't what was needed, but the single version looks good.

Brian

[EGRAY_OBSERVATORY]

This is interesting Brian, as I have two of these filters as:-

1 0310255 / 1380 EXPLORE SCIENTIFIC 1•25" Variable Polarizing Filter @ £ 39-90 / 12-09-2017
1 05562 ORION 2" Variable Polarizing Filter G 27087 PO 37719 (1%~40%) @ £ 52-00 / 09-04-2009

Used for both Lunar and Solar purposes, currently up to the last week or so, the 1.25" one was used in front of the Altair GPCAM for WL for three-years without any problems - with the Orion ED-80T-CF scope - using the Lunt 1.25" HW.

That scope and camera have now just been replaced and awaiting 'first-light' as a pair of Sky-Watcher 'ESPRIT' 80ED and just received and fitted yesterday, a new ZWO ASI-120MC-S for full-disk imaging - using the same HW.

Once trialled, I will bear in mind the feasibility of using a single-element filter, but as to what amount/% of light-reduction is required would be a tad difficult to know. So if there is a problem with light-reflection, another option might be required, as this post refers...

Best wishes
Terry

[AndiesHandyHandies]

Hi

I think you need a Circular Polarising Filter, CPL or CIR-PL, which I only found in camera filter sizes.

Tend to have to go up to 49mm and back down to 48mm but too big then. I have one loose in a 2" nosepiece at the moment. Hoya.

Cheers. Andrew.

[MapleRidge]

Terry and Andrew...

Thanks for the info posted.

I will likely be near a higher end camera store later this week and might look for a candidate there. I think I still have a old 2" around but it was in poor condition, but might work well enough to test out and know if it will do the job.

Brian

[EGRAY_OBSERVATORY]

This afternoon Brian, I started the first WL imaging with the new camera and scope and could barely see any surface-detail including two faint spots with the 1.25" polarizing-filter, so took it out and rechecked - still to find little surface detail etc. In any case that filter doesn't now seem to be required with the new camera, as SharpCap easily adjustable without it.

I still have to try out F-stoppers to see what effects that will make for a better contrast in WL.

Cheers
Terry

[marktownley]

Hi Brian,

It needs to be a circular polariser, not linear polariser.

Mark

[MapleRidge]

Thanks Mark...

From what i have gathered so far, the photographic version is what's needed. I see that they are specially labelled as 'circular'.

As far as placement goes, to save a lot of experimenting, where is the first place to start trying to place one the see any reduction in glare/reflections...after the between the two etalons in a double stack or prior to the camera? It will influence the ability to mount it once it is confirmed.

Thanks again,
Brian

[george9]

I did a lot of work with filters for my stock LS80 DSII. I certainly welcome corrections.

To reduce the reflections, you need to put the filter between the two etalons. Anything that dims the image will work because the reflections pass through the filter three times (or more) but the primary image passes through only once. Lunt's high-resolution glass filter I think is just acting as a neutral density filter, and it works. I suspect a linear polarizer will do the same thing, although maybe not as well because much of its attenuation is getting rid of light polarized in one direction, so going through a second time with the same polarization doesn't remove more of that light.

Anyway, the idea behind a circular polarizer is that it cuts out some of the light with it linear polarizer and then the quarter wave plate twists it so that the next time through gets rid of more light. And if you put the circular polarizer side toward the eyepiece, then you don't have to worry about optics affected by polarization (e.g., binoviewers, but not relevant here I guess).

People have come up with different ways of mounting the circular polarizer. I got a B+W Kaesemann circular polarizing filter, took it out of its cell, and put it in the cell that the Lunt high-resolution glass filter comes in (I think the 49mm filter worked). Someone else used a rubber lens cap.

George

[MapleRidge]

Hi George...

I have always figured between the two etalons, was the way to go. I have the Lunt filter that fits ahead of the DSII module, but in my mod unit won't fit in place. I might be able to use the filter out of it and make a new holder. A project for another slow day.

Brian

[DavidP]

I’m having a huge glare problem that I assume is coming from between my two stacked Lunt 40 mods.
I’m wondering, what a good option is for a circular polarizer and adapting it between the two T2 fittings. I’m of course wanting to pass as much light as possible, but also keep the path short.

[Bob Yoesle]

I've use B+W Kasemann CP filters in my designs. Depending on your blocking filter components, there are alternatives that can help compensate for the 50% transmission reduction the CP will introduce - mainly replacing the ITF or BG38 with a UV/IR block on KG3. If your system otherwise lack a RG630, the BelOptic ITF replacement filter adds the RG to the UV/IR on KG3 element.

[EdAstle]

You could put a T2 filter drawer between them. I have this one:
https://www.altairastro.com/altair-2-in ... 2379-p.asp

I've the same issue with light ping-pong between two rear stacked Lunt 40 front ends (inside RAFCamera adapters).
I've not got a polarizer yet - our paths crossed tonight during my research.

I unscrewed the white nose off the Lunt 40 red base to have a look. The tilt etalon is mirror-like both sides.
The red ERF in the white nose is also slightly tilted.
It's a hall of mirrors. All works fine when on the front where it's supposed to be!

I was looking at EO polarizers:
https://www.edmundoptics.co.uk/f/circul ... her/14781/
"Greater than 42% Transmission". Sounds about right. If it's 100% they ain't working.

I've decided not to bother. The ghosts are off disk (just) and apart from the exceptional prom I'm more interested in surface detail.
I'm messing with a Vixen M80 f/11.
Altair 150 DERF -> Vixen M80 -> KG3 -> (UVIR* + EO 0.5nm Ha) -> TZ4S -> Lunt40 -> Lunt40

*The EO Ha is 25mm diameter and falls through the hole of the 1.25" snout unless rested upon 25.4 glass I borrowed from an Altair UVIR.

[Oak]

There is a space under where the etalon sits in the Raf adapter that will fit a photographic circular polarizer. So no need for additional light path. I have tried a Hoya brand in mine and it did remove the reflections, but was too dim for me. I need to replace the bg in my blocker and try again. Photo below. Filter wedged in place with some Velcro tape.

[EdAstle]

Top tip Derek. Sometimes the answer is right in front of you.

I looked at the latest Hoya CPs today. ND2.2 so I can imagine the light loss.
https://hoyafilter.com/product/hd_mk_ii ... ifications
"ND Number (Filter Factor) 2.2"
"Transmittance EV correction 1-stop"

Bob has championed the B+W Kasemann several times but they are rare as hens with Bugs Bunny teeth here in UK.

5 hours later browsing intensely (insanely?) I found "B+W Master High Transmission Circular Polarising Filter 49mm" for £122.

https://schneiderkreuznach.com/en/photo ... /polarizer
"The multiplying factor with HTC is 2 to 3, which is equivalent to 1 to 1.5 f-stops"

The BW sounds slightly "worse" than the Hoya, but due to Bobs mighty knowledgeable gravitas + BW industry respect I've just parted with £122 for the BW.

If all your upstream energy rejection is "suited and booted" could you not remove the red ERF from the Lunt nose instead of messing with your blocker? Might give you a small degree of latitude?

[Oak]

Thought about removing the erf, but I am using the etalon by itself a fair bit so I need to sort a good way to quickly put it back on.

[EdAstle]

I've bought a B+W "Master" CPL.
There's just 50 seconds between these two images.
FireCaptured + AS4 stacked. Nothing else. The tiny hole in the cloud blanket afforded no more fun.


If that link doesn't work then maybe this wont either
https://www.astrobin.com/full/22vyr2/0/

[marktownley]

The polariser is a clear improvement Ed