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Which Lunt Herschel wedge with NP101?

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#1 Aleko

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Posted 09 August 2013 - 06:46 PM

Hi folks,

I'd like to hear from actual NP101 owners who use the Lunt wedge for white light solar viewing. I have a wide variety of 1.25 and 2 inch eyepieces, with many of the latter being dual barrel, though I prefer using those as 2 inch. My concern is posts that I read about some eyepieces possibly not coming to focus. Are you using the 1.25 Herschel wedge or the two inch, and do you have trouble coming to focus with some eyepieces? Also, do you use a 1.25 inch polarizer or do you use a 2 inch?

Bottom line, which size wedge and polarizer works best with this scope? Right now I'm all visual, but would like to keep my options open for possible imaging down the road.

Thanks!

Alex

#2 AstroPaolo

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Posted 10 August 2013 - 03:37 AM

the 1.25 should be enough till a 4" refractor, for a 6" better a 2" wedge one.

#3 brianb11213

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Posted 10 August 2013 - 04:23 AM

The 2" wedge has a slightly longer lightpath than a standard 2" diagonal ... this is because the output tube (eyepiece well) is longer than standard, in order to allow for a substantial stack of filters to be used without impacting the internal surface of the wedge. You will have two or more likely three filters in there, including the dense neutral density filter which is required for safety.

If you have more than 1 cm / 0.4" of inward focuser travel available with a standard diagonal, you should be able to reach focus wuth the 2" Lunt without any particular difficulty.

Personally I do not use a polariser. I know some people like them but to me they seem to be throwing away the wedge advantage of high contrast ... if you really want to use a polariser, note that the output beam from the wedge diagonal surface is fairly well polarised so you only need a single polarising filter, but one which is a simple linear polarising type, not the circular polarising type usually seen these days. But do try either the Baader Solar Continuum or a simple green Wratten #58 instead.

It makes far more sense to me to use 2" filters in a 2" wedge ... otherwise you might as well be using the cheaper 1.25" wedge instead ... given that your scope is small enough for the heat loading to be acceptable. A 1.25" wedge will also have less optical length than a 2" wedge, probably less than an ordinary 2" diagonal.

#4 Aleko

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Posted 10 August 2013 - 10:40 AM

Thank you, the above posts contain a wealth of information, and with my being completely unfamiliar with the the solar wedges, are greatly appreciated.

Regarding polarizers, I understand the wedge light being polarized, thus requiring only one other polarizing filter. I thought its use was meant to increase contrast, not worsen it. Supposedly one rotates the filtered eyepiece to adjust to the best view. What is the advantage of the continuum filter? I was totally unaware of the circular type of polarizer (meant for digital cameras?). Are the polarizers sold by astro stores the circular or linear type?

Thanks for the valuable information.

Alex

#5 brianb11213

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Posted 10 August 2013 - 07:44 PM

Regarding polarizers, I understand the wedge light being polarized, thus requiring only one other polarizing filter. I thought its use was meant to increase contrast, not worsen it. Supposedly one rotates the filtered eyepiece to adjust to the best view. What is the advantage of the continuum filter? I was totally unaware of the circular type of polarizer (meant for digital cameras?). Are the polarizers sold by astro stores the circular or linear type?

1. It's the construction of the polarising filter which causes a certain amount of light scatter & it's this that reduces contrast compared with an ordinary dye-in-glass or dichroic (interference) filter.

2. The idea of using an extra filter in the Herschel wedge is to reduce the light to a comfortable level. The wedge itself plus the ND 3.0 filter which comes with it are sufficient fot make the device safe but the image will still be very bright.

3. The continuum filter is like a "narrowband" nebula filter - but at a wavelength where there are few spectral bands, so emission and absorbtion are practically eliminated resulting in maximum contrast. It so happens that the green wavelength used in the SC filter is also at the wavelength where the human eye works most efficiently. The near monochromatic bandpass helps by reducing the effect of seeing degradation due to air currents. Trust me, it works better than any other filter - though the ordinary Wratten #58 green is not far short, a great deal cheaper & if you bought a beginners' filter set you probably have one already. A deep yellow or orange work almost as well.

4. Circular polarisers came in with auto cameras - linear polarisers fool the auto exposure & auto focus mechanisms. The difference between a linear polariser & a circular polariser is that a circular polariser consists of a linear polariser with a "scrambler" mounted on the output side. The issue here is that the scrambler definitely degrades contrast.

I wish I could say whether "polarisers sold by astro stores" are linear or circular. They usually don't say. If you buy a pair of polarisers for use as a variable filter then at least one of them should be a linear polariser, otherwise the device won't work. Unfortunately I have seen examples of pairs of polarisers which do not work owing to both components being of the circular type. This seems to be getting more common as most polarising material is used for making filters for auto cameras, where circular polarising behaviour is required.

By using an ordinary filter instead of a polariser, the issue becomes moot.

#6 Aleko

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Posted 10 August 2013 - 08:47 PM

Brian, not only are you knowledgeable, but you have a gift for explaining things in a clear and understandable way; greatly appreciated! I learned something and also saved a few dollars today. Will try the SC filter as you suggest, and 'til then I do indeed have a #58 that for years has been waiting to be put into a scope!

Thanks,
Alex

#7 lcaldero

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Posted 11 August 2013 - 08:12 AM

The NP101 is a petzval design which is not recommended for use with a Herschel wedge. I understand that the built up heat can damage the lens in this design.

#8 Aleko

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Posted 11 August 2013 - 10:14 AM

The NP101 is a petzval design which is not recommended for use with a Herschel wedge. I understand that the built up heat can damage the lens in this design.


I was worried about this too. A call to Televue cleared my doubts. They said that there is no problem using the NP101 with the solar wedge.

Alex

#9 Bill Cowles

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Posted 11 August 2013 - 04:16 PM

Correct, I have been using a Lunt Herschel wedge on my old Genesis for years, works great.

Bill

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#10 George9

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Posted 20 August 2013 - 04:06 PM

Personally I do not use a polariser. I know some people like them but to me they seem to be throwing away the wedge advantage of high contrast ... if you really want to use a polariser, note that the output beam from the wedge diagonal surface is fairly well polarised so you only need a single polarising filter, but one which is a simple linear polarising type, not the circular polarising type usually seen these days. But do try either the Baader Solar Continuum or a simple green Wratten #58 instead.


I was comparing various options to dim the Lunt 2" wedge. I did not have a Continuum filter, but I had a Lunt 2" polarizing filter, an old Celestron Wratten #58, a Baader "optically polished" 500nm bandpass (similar to Wratten #56), and an Orion Moon filter. This is on a 5" AP apo.

I found that the polarizing filter had very high contrast. I could not detect any loss whatsoever. The 500nm was great, and so was the Moon filter. The #58 did seem to lose something in the granulation although not the sunspots, but I am not sure if it was the optical quality, or that it was darker, or that it was yellower. I would be happy with any of them, so it was close.

George






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