"Safe" use of eyepiece sun filter

[Barnacle]

Hi All,

I have a 0.965 eyepiece sun filter.

As these sun filters are unsafe when deployed at the eyepiece end (ie, they heat up, crack and we go blind), I've decided to test and fit it (securely) at the objective end of the 5x24 finderscope instead of destroying it with a hammer.

Tested it on the sun, it works great, clear solar disk (the camera fails to focus clearly and overexposed the sun).

Photos attached.

Now I can use this "solar" findescope (with cross hairs) to aim my telescope (fitted with a glass solar filter for observing the sun), instead of capping the un-usable finderscope (or burn a circle in the forehead if uncapped) when observing the sun.:)

Clear skies,

Bill

[astro744]

It will work fine until it cracks at which point it's too late if you happen to be looking through at the time. I had one crack on a 60mm f11 achromatic a very long time ago (pre Internet era and pre warnings on such filters) and thank God I wasn't looking through at the time. Smash it now.

To align the Sun cover any unprotected optics and then look for the smallest shadow your equipment casts when pointing at the Sun.

[Barnacle]

Thanks for sharing your experience.

But will the filter still crack if it is placed at the objective end?

I can understand these eyepiece filters crack when used at the eyepiece end, as per link below:
https://www.youtube.com/watch?v=G5xb3b-vRd4

But find it hard to contemplate these filters cracking when placed in front of the objective lens just like other solar filters, eg, glass or film solar filters? They will not heat up and expand and should not crack when placed in front of the objective lens.

Clear skies,

Bill

[OICURMT]

If it is made of glass then no, you won't have any problems. The way you've set it up is safe as there is no concentration of solar rays.

[astro744]

Sorry, completely misread your question as your title set off alarm bells in my head. The words "safe" and "eyepiece Sun filter" do not normally go together.

I guess it would be no different to welding glasses but emphasis on guess. How safe are welding glasses anyway for solar viewing? Even if safe you have to ensure that the secure mount stays that way and the filter doesn't fall out. You may get some heat buildup between the filter and the objective, not sure unless tested.

Just prior to the last eclipse I found out that even Baader AstroSolar film was not safe unaided and needs glass in to optical path to eliminate UV according to the latest standards that came into play after a previous eclipse. A newer unaided version is now available.

The solar finders are not that expensive ($29 for Kendrick) or you can make one similar and it's a far safer option.

[ZeroID]

I made up a front filter for my finder from a bit of leftover Baader N5 Solar film. No problems with it so far.

[Barnacle]

Hi All,

The camera has over-exposed the image, the sun is not bright looking thru it, similar colour to my glass sun filter fitted to the objective lens end of my main scope.

This eyepiece sun filter is only good for the 5x24 finderscope or smaller aperture, not for larger scopes.

This eyepiece sun filter is made in Japan, it is made of thick glass with an aperture of 11mm, very dark brown in colour.

Putting at the objective end, it will not heat up.

Thanks for comments.

Bill

[Wavytone]

Bill it isn’t safe and should be put in the bin.

Being dark brown implies the filter material is a dye - which absorbs light in the visible spectrum but almost certainly allows infrared and UV to pass through. You won’t feel a thing but exposure to the sun through this is going to damage your retina.

The only safe solar filters are metallic film types that look like a mirror - these refkect the uvan infrared as well as the visual light. Even 2 layers of aluminised Mylar do ok.