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How safe is this filter for solar observing? It screws on 0.965 Zeiss Orthos. Anyone tried this filter before?

I assume that it is safe because it's made by Zeiss.

Thanks for your input.

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Filters that screw onto the eyepiece tend to be frowned upon (regardless of who makes it). It isn't that they can't filter out the light properly (many can). It is just that the light energy concentrated to a small region on the filter by the telescope may eventually cause the filter to crack or shatter without warning. If that happens while you are looking into the eyepiece, you will very probably suffer permanent eye damage. A friend of mine had his vision saved by the UPS man one day. He was viewing the sun in his 2.4 inch refractor using one of those little screw-on SUN filters when the doorbell rang. He had taken the eyepiece out to answer the door and tend to the delivery, but then, he heard a "CRACK" sound. When he got back to the telescope, he could see sunlight streaming out of the star diagonal and onto the ceiling, where, he noted to his dismay, a small piece of that solar filter was embedded into the plaster! It is best to avoid the use of such "screw-on" single unit eyepiece filters and go with a good white-light filter that covers the objective. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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To shorten Stephen's important message... it is NOT safe to use a solar filter at the eyepiece. The sun's light MUST be filtered at the objective end of the telescope.

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I had the exact experience David describes in his post above with my 60 mm refractor telescope and a eyepiece mounted sun filter -- it dangerously cracked -- fourtunately while I was not looking through it...

However, I would sincerely have to disagree with the above in that a Herschel wedge, which is used at the eyepiece end of a refractor, is a safe solar filtering device (when used as directed with the appropriate filters), and is in fact a superior device to any objective mounted filter I have used -- including a 100 mm Zeiss SFO, and the full-aperture Questar filter, which I both own, and are among the highest optical quality available.

Criterion also made a unique eyepiece solar filter in the late sixties and early seveties (with a mirrored barlow lens and internal filter ahead of the eyepiece), which is also quite safe and a bit of a collectors item.

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Bob Yoesle

You are a child of the universe,
no less than the trees and the stars...
And whether or not it is clear to you,
no doubt the universe is unfolding as it should.

Desiderata

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To shorten Stephen's important message... it is NOT safe to use a solar filter at the eyepiece. The sun's light MUST be filtered at the objective end of the telescope.

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I had the exact experience David describes in his post above with my 60 mm refractor telescope and a eyepiece mounted sun filter -- it dangerously cracked -- fourtunately while I was not looking through it...

However, I would sincerely have to disagree with the above in that a Herschel wedge, which is used at the eyepiece end of a refractor, is a safe solar filtering device (when used as directed with the appropriate filters), and is in fact a superior device to any objective mounted filter I have used -- including a 100 mm Zeiss SFO, and the full-aperture Questar filter, which I both own, and are among the highest optical quality available.

Criterion also made a unique eyepiece solar filter in the late sixties and early seveties (with a mirrored barlow lens and internal filter ahead of the eyepiece), which is also quite safe and a bit of a collectors item.

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The above unit shown in the image is a sort of low-power "aluminized barlow". The unit required an additional filter, but the combination worked pretty well. As for the Herschel wedge, I don't put that in the same league as the simple screw-on "SUN" cap filters that some of us old-timers got with our little 2.4 inch refractors. I consider them "time bombs" waiting to steal our vision, so they can be rejected outright. The Herschel wedge is a nice safe and effective way to view the sun in white light, although the cost of the wedge approaches that of some low-end H-alpha systems like the PST. It also is not used in many reflectors due to the internal heating issue, which again is where the front objective-covering solar filter is a very good and economical choice for white light solar viewing. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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