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PST and polarising filters


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After the ITF failed on my pst mod, I am now using an additional 2" Baader Ha 35nm filter on the objective end of the etalon, i.e. the light goes objective > DERF > Ha 35nm > etalon > BF

There are some internal reflections from the etalon and also the image is a little bright at lower powers so I am looking to increase contrast. Apparently both of these issues can be addressed (possibly) by using a polarising filter and I ordered a variable polariser which comes in two parts - a polarising filter and a variable moon filter. Used together these rotate one against the other and create variable brightness.

I was planning to put the polarising filter (I think this is what is referred to as a 'top pol' filter?) here in the chain  objective > DERF > Ha 35nm > etalon > top pol > BF

my logic being that the already polarised light through the etalon / DERF would be varied with a turn of the etalon relative to the top pol.

having tried it against a ceiling light I cannot see any dimming but when both filters are placed in the diagonal with just the BF, the brightness is variable by turning one filter against the other.

Am I correct that the light through the etalon/DERF is indeed polarised and I only need the top pol?

Maybe I need the sun and a field test to establish the results but just thought I'd check my theory first. Not worried about safety just increasing contrast at lower powers

Cheers

Shane

 

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Hi Shane

Have you tried tilting the Baader to reduce the reflections?

Can't help on the other points. Fortunately my ITF is clear despite being as old as the hills (in PST terms).

Paul

 

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Hi mate, it's definitely coming off the back of the etalon as it jiggles when I tune it and also it was there before I took out the ITF and replaced it with the Baader filter. The DERF is slightly tilted as it needs to be though.

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Have you tried a single polarising filter on the eyepiece? A mirrored surface in the light path (which I believe the PST has) will produce a polarised beam so a filter at the eyepiece can be rotated to vary the brightness ...

Worth a try as it's completely non-invasive.

AndyG 

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I use binoviewers so need to rotate the etalon. I'll upload see pics in a bit. I suspect this is something that I'll just have to test but at least thanks to your post I understand what causes the light to be polarised.

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3 hours ago, Moonshane said:

it's definitely coming off the back of the etalon

Not clear what you mean by this - is the light coming out of the back of the etalon bouncing back onto the etalon? That would suggest a shiny surface aft of the etalon that needs blackening.

If indeed the light is polarised then rotating your top pol on its own should attenuate, as I think you were suggesting.

Michael

 

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Have you not considered just getting an ITF Shane? £70 is a kick in the falls but it's hardly breaking the bank and it arrives quicker than you might think. You could sell the 2" Baader Ha 35nm filter to help fund it as you no longer need it for the 60mm Mod. Just a though as it would resolve the problem in a way it was originally intended.

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The problem is still there when the old it was in place. I think the reflection is now between the gold side of the bf and the etalon where previously it was between the itf and the etalon.  I'll know more when I try it in some sun :0)

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I wonder if this logic works:

one mirrored surface creates polarisation so do odd multiples create polarised light and even multiples effectively cancel each other out so you then need to polarise the light? I am thinking that I need both bits of a variable polariser as I have the DERF, the additional Baader 35nm Ha, the etalon and the front end of the BF so four which creates unpolarised light?

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12 hours ago, Moonshane said:

The problem is still there when the old it was in place. I think the reflection is now between the gold side of the bf and the etalon where previously it was between the itf and the etalon.  I'll know more when I try it in some sun :0)

Sorry...you did say that in your post doh! I got hung up on the fact you mentioned your views are too bright at lower magnification and figured a like for like itf would resolve this better than the baader filter. Then again the replacement ITF also seems to offer a brighter than stock view (not surprising after the rusty itf it replaced) so you may still find them a little brighter than desired ? This sadly is all I have to offer on resolving the brightness having little experience with polarizing filters in HA scopes. FWIW I didn't like 2 part polarizers for use on the moon but do find them of benefit in WL observing.

In reply to the reflections issues. For what its worth I also have the same internal reflections with mine but put it down to the stupid prism being the culprit. Obviously not the case with your stage 2. With recently changing my ITF I did notice the The filter holder does have ever so slight a tilt on it, needless to say for the same reason internal erf's do. Depending where I focus my attention on the solar disc the reflections become apparent so figure this is when the ITF or even erf tilt is not as favorable. Maybe if you could introduce some tilt to your Baader filter? May help???

I mention the tilt on the internal erf also due to your recent mention of how well Stu's PST mod performs. It became apparent to me Stu has an objective derf as apposed to an internal one. This both reduces risk of internal reflections and the inside of the ota becoming an energy trap. Given we both experience reflections using internal erf have you asked if Stu is suffering similar reflection issues??

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Stu has an internal sub aperture ERF so I think it's Peter's 'magic' that's created such a great instrument. In fairness when I use higher magnification the reflections issue improves and vice versa. I am hoping the polarising filter will be a double whammy (brightness and reflection reduction).

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My mistake :icon_redface: I seen this IMG_0244.JPG

And assumed it had an objective filter.

I'm all out of ideas then mate and maybe your idea of a polarizer will sort your problems. Hopefully someone more knowledgeable can better advise you on them than I can. What I would consider though is each piece of glass you introduce in to the mix effectively has two surfaces for dust to settle. If you do indeed have to add two filters to polarize this is a lot of surface for potential dust bunnies in the views. This was the reason why I gave up on a polarizer for lunar observing. It didnt matter how often i blasted them off with my rocket blower, there was always a speck of dust grabbing my attention every time i twisted the polarizer. I also found the further the filters were apart the harder it was to obtain a sharp image.

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Ahh I see now. That internal Derf is some way down the OTA. I thought the whole idea was to have it as close to the lens as possible so that the energy went back out through the objective rather than gathering inside the OTA??? I still don't get whats on the objective though :icon_scratch: Is it a lens cell so that the OTA didn't have to be shortened ?? I know on my PST mod Peter also did a blanking cap so the hole for the derf can be capped and the scope used as a regular achromat with the use of extensions of course.

Sorry for going slightly off topic here.

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20 minutes ago, spaceboy said:

I still don't get whats on the objective though :icon_scratch: Is it a lens cell so that the OTA didn't have to be shortened ??

That is the lens cell Nick, it doesn't have a dew shield fitted. It looks like there is a D-ERF fitted to the dewshield but infact this is the objective fitted to a larger section of OTA and it then steps down to a smaller section which helps keep the scope nice and compact.

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Something I might try if one comes up cheap enough is a contrast booster. I was surprised to see this improves the views for WL and on some features I felt it's better than the continuum filter. Probably wont make a blind bit of difference to Ha views but it's fun to experiment now and again.

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