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Solar observing, and Herschel Wedge questions

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#1 Couder

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Posted 04 August 2020 - 09:47 AM

I have several refractors. My 4" and 6" Clarks came with Herschel wedges, which I use frequently. I made an adapter to use them on my 3" refractor and another 4" refractor, no problem. I also have a 1.25" Bausch & Lomb Herschel Wedge, which I use on all the above scopes. But my other refractors are 8" F13. Looking around, I see the available Herschel Wedges are for use up to 2500MM Focal length. The 8" refractors are 2641MM. I realize it is only 141MM longer (5.6") but I figure the manufacturers put the max there for a reason. I am hesitant to try my current Herschel wedges on my longer Focal length scopes, obviously I don't want to crack the prisms. Would they be safe? What about the new ones that say 2500?

By the way, I have been using my Clark wedges for many years, for hours at a time with no problems. I have taken the scopes to public observing places and they have been trained on the sun all day with no problems, so I know they hold up under long term use.

I do have a couple other solar accessories for the 8" scopes, but would like to have wedges on both at the same time.

The first picture is one of the Clark Herschel wedges, the second a Dawes type solar eyepiece (two rotating rings, the bottom with graduated size holes, the top with graduated density filters), the last a polarizing solar eyepiece.

http://articles.adsa...000005.000.html

 

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#2 dhkaiser

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Posted 04 August 2020 - 09:57 AM

Can't answer your questions, but find your equipment fascinating.


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#3 Doug Culbertson

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Posted 04 August 2020 - 09:58 AM

Can't answer your questions, but find your equipment fascinating.

+1



#4 Spectral Joe

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Posted 04 August 2020 - 06:21 PM

If the manufacturers are specifying a maximum focal length, it's to ensure you don't have a vignetting problem. For your 8 inch F/13 the image size would be almost an inch, a bit big for a 1.25 inch wedge. A focal length limit has nothing to do with cracking the wedge.

 

Speaking of cracking wedges and limits (this comes up a lot), think about this: People are always worried about the "heat at the focus", the heat at the focus is due to light being absorbed by something. Sure, put your hand there and it will get burned, your hand absorbs quite a bit of the light. But put a piece of BK7 there (the wedge) and what is there to be absorbed that wasn't already absorbed by the objective? The majority of spectral energy from the Sun is in the visible, what long wave IR there is is absorbed by the objective, leaving the visible to get through. BK7 has very, very little absorption in the visible, and so it doesn't get hot when exposed this way. I know, the explanation is that at the focus "the heat is concentrated". If you take the time to calculate the actual effect of this exposure you see that there is almost no effect, the glass is unaffected. The aperture limits manufacturers put on wedges are likely to keep the absorber and heat sink on the end from getting too hot and burning someone. In the old days there was no heat sink, the unused light went out the open end and could be a hazard. Wedges have been used on big refractors for many years.

 

As an illustration of the lack of heating of optical glass at the focus of a refractor, here is a photo of the scanning prism of a spectroheliograph. The spot on the prism is 5mm in diameter, it's the relayed image of the objective, which is 100mm in diameter. The temperature of the prism does not rise at all, the prism, and other optics in the imaging train, have been exposed like this for hundreds of hours with no effect.

 

Illuminated scan prism (Medium).JPG

 

If you're still worried, follow the manufacturers aperture limits to keep safe and the focal length limits to avoid vignetting. The best thing to do, as always with science, is to calculate the effects and go from there.


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#5 Couder

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Posted 04 August 2020 - 07:35 PM

Thanks, so I'm guessing a 2 inch would be ok?

#6 sunnyday

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Posted 04 August 2020 - 07:59 PM

+1

+2



#7 Spectral Joe

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Posted 06 August 2020 - 10:08 AM

Thanks, so I'm guessing a 2 inch would be ok?

If you want a full disk view then the field stop in the eyepiece has to be larger than the image, 2 inch eyepieces probably meet this requirement. Smaller field stops won't hurt anything (they're after the wedge) but they do limit the field of view.



#8 Couder

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Posted 06 August 2020 - 11:25 AM

A 2" wedge would work OK. I have been observing the sun/planets for years too, my eyes just don't want to pick out those "faint fuzzies" anyhow. I already have a scanning adapter (you may have a more scientific name for it) I use to scan the perimeter of the sun.

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