Polarizing Filters

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Polarizing Filters

Post by 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


Brian Colville

Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
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Re: Polarizing Filters

Post by 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,


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Re: Polarizing Filters

Post by 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


Brian Colville

Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
ASI1600MM, ASI432MM, ASI294MM Pro, ASI174MM, ASI462MM
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Re: Polarizing Filters

Post by 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


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Re: Polarizing Filters

Post by 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.
Last edited by AndiesHandyHandies on Sat Aug 14, 2021 9:13 pm, edited 1 time in total.


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Re: Polarizing Filters

Post by 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


Brian Colville

Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
ASI1600MM, ASI432MM, ASI294MM Pro, ASI174MM, ASI462MM
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Re: Polarizing Filters

Post by 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


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Re: Polarizing Filters

Post by marktownley »

Hi Brian,

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

Mark


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Re: Polarizing Filters

Post by 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


Brian Colville

Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
ASI1600MM, ASI432MM, ASI294MM Pro, ASI174MM, ASI462MM
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Re: Polarizing Filters

Post by 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


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Re: Polarizing Filters

Post by 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


Brian Colville

Maple Ridge Observatory
Cambray, ON Canada

Photos: https://www.flickr.com/photos/185395281@N08/albums

10'x15 Roll-off Roof Observatory
Takahashi EM400 Mount carrying:
C14 + Lunt 80ED
Deep Sky Work - ASI294MM Pro+EFW 7x36/Canon 60D (Ha mod), ONAG
Planetary Work - SBIG CFW10, ASI462MM

2.2m Diameter Dome
iOptron CEM70G Mount carrying:
Orion EON 130ED, f7 OTA for Day & Night Use
Ha Setup: Lunt LS80PT/LS75FHa/B1200Ha + Home Brew Lunt Double Stack/B1800Ha on the Orion OTA + Daystar Quantum
WL, G-Band & CaK Setup: Lunt Wedge & Lunt B1800CaK, Baader K-Line and Altair 2nm G-Band filter
ASI1600MM, ASI432MM, ASI294MM Pro, ASI174MM, ASI462MM
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