Make a Solar Filter for Your Camera or Binoculars

by wannabemadsci

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Intro: Make a Solar Filter for Your Camera or Binoculars

Make a Solar Filter for Your Camera or Binoculars

The next Total Solar Eclipse over the Continental United States will be April 8, 2024. At that time a partial eclipse will be visible over most of North America. If you want to take some photos of the partial eclipse you will need a special filter for your camera. I made this filter for my camera and similar ones for my binoculars for the August 21st 2017 Solar Eclipse which was amazing and especially spectacular when viewed in the path of totality. Google '2024 Eclipse' for websites to learn more!

To safely view and photograph a partial eclipse you need special eclipse glasses or filters. This Instructable shows how to make a nifty filter for your point-and-shoot camera or for binoculars.

This filter is not only good for viewing partial eclipses, but also for viewing the sun's disk along with sunspots.

I took some photos of the sun and noticed sun spots. I looked at NASA's Solar Dynamics Observatory website and saw the same sun spots!

STEP 1: Start With Certified Eclipse Glasses

The filters used in this project are from inexpensive Solar Eclipse glasses that have been certified as safe for direct solar viewing. Do not use any type of glasses or filters that do not bear a certification for use to directly view the sun. These filters are black on both sides. Not silver. There has been some concern over filters that are silver on one or both sides. Look for certification information on the glasses.

Also note that the filter MUST be installed on the lens closest to the sun. It is is NOT SAFE to have the filter located next to an eyepiece or just in front of your eye. The filter must be installed so that it filters the sunlight as it enters the camera or binocular.

Some cameras have a separate viewfinder for composing your image. This filter arrangement will not provide safe viewing through a viewfinder. Do not look through the viewfinder eyepiece when using this filter.

STEP 2: Measure Your Camera or Binocular

Measure your camera or binocular to determine the size the filter needs to fit onto. You will use this information to design a 3D printed filter holder that will fit snugly onto your camera or binoculars.

STEP 3: Design a Filter Holder

Use Fusion 360 or a similar program to design a filter holder that will fit onto your camera or binoculars.

You need to take into account the diameter of the lens tube that you measured in the previous step and allow a slight gap between the finished part and the camera.

I designed in some tabs with gaps on each side into the perimeter of the filter holder to provide friction fit under tension when the filter holder was slipped over the camera or binocular lens.

On the inside of the filter holder is a rectangular recess that is designed to hold a piece of filter material cut from the eclipse sunglasses.

You will need to 3D print the filter holder. In addition to the filter holder it is helpful to design and 3D print a filter template that matches the recess for the filter in the filter holder. This template will be used when cutting out the filter material from the eclipse glasses.

I have attached example files of the filter holder and filter cutting template.

STEP 4: Cut Out the Filter

Cut out the filter from the eclipse glasses. The filter material that is cut out should fit into the recess in the filter holder.

You can use the template to cut out the filter material if you chose to 3D print one.

STEP 5: Install the Filter Into the Filter Holder

Check fit the filter material into the filter holder. Trim if needed. Note that the filter must fit edge to edge into the filter holder. There cannot be any gaps or blinding sunlight will slip past the filter.

Using a toothpick apply glue to the recess in the filter holder and place the filter material onto the glue to fasten it securely into the filter holder

STEP 6: Install the Filter Onto Your Camera

Install the filter holder, with the filter securely mounted, onto your camera or binoculars.

(Remember to make two filter assemblies for binoculars ;-)

Make sure to only look into your binoculars if the filters are installed.

Enjoy!

11 Comments

arpruss 3 years ago

I wouldn't do this with binoculars! The eclipse filters are rated for naked eye sun viewing. But the binoculars will gather several orders of magnitude more light than the naked eye, so one is apt to be far beyond the safety ratings.
There are proper solar telescope filters one can buy and cut to size.

ElectroFrank 3 years ago

Binoculars are gathering light from a narrower angle, therefore smaller binoculars provide a dimmer image than the naked eye would see. But what you say might apply to larger binoculars.

arpruss 3 years ago

I'm afraid not, at least not for binoculars whose field of view is big enough to contain the whole solar disk. Typical small binoculars have a field of view more than sufficient to contain the sun: the sun takes up 0.5 degrees of sky, and the true field of view of typical small binoculars is at least 7 degrees.

The eye pupil gathers light from at most an 8mm diameter disc, and only a 2mm disc in bright light. Pretty much the smallest diameter of commercial binoculars is 20mm, and 30mm is much more common. Since the light-gathering area is proportional to the square of the diameter, the 20mm binoculars will gather between 6 and 100 times as much light as the naked eye will, as long as we're dealing with an object that fits completely into the field of view--which the sun does.

You can safely test how much brighter the images from binoculars are--even really small binoculars--by looking at the night sky. You will see a lot more stars that are too dim to see with the naked eye. This has nothing to do with the magnification (stars other than the sun are so far away that they are basically a point for any amateur equipment), and everything to do with the fact that the lenses are gathering light from a much larger area.

Proper telescope-rated solar filter material is not expensive: https://agenaastro.com/baader-astrosolar-visual-filter-film-nd-5-eco-sheet-2459286.html
And binoculars are just a pair of telescopes.

I am sorry for going on and on about this, but this is a very important safety point. DO NOT USE SOLAR FILTERS NOT RATED FOR A TELESCOPE OR BINOCULARS WITH A TELESCOPE OR BINOCULARS.

Of course, even with telescope-rated solar filter material, the material needs to go in front of the objective like in this Instructable (rather than behind the eyepiece, where it will be burned away).

DrRadium 2 years ago

Thanks for the reference for the Baader film. Ironically following this link leads fairly directly to answers about solar film standards and safety. The Baader AstroSolar Visual Solar Filter Film states that it is "visual optical density (OD) or neutral density (ND) of 5.0 which reduces the intensity of incident sunlight by a factor of 100,000 (approx. 16-2/3 stops)." and goes on to state "This is suitable to make your own front-mounted solar filters for telescopes, binoculars, spotting scopes, or large camera lenses." but it then states it NOT for use in direct viewing of the sun:
"Effective mid-2017, the eye safety norm EN ISO 12312-2:2015 for naked eye solar viewers does not apply to front aperture filters covering long-range optics, such as this item. AstroSolar Safety Film 5.0 should not be used for naked eye solar observation or production of solar viewers that provide an unmagnified view of the Sun."

If it filters out more light, why wouldn't it be suitable? It says to get the solar viewer eclipse glasses. Following a link on the same page to the referenced Baader Solar Viewer AstroSolar Silver/Gold Eclipse Glasses / Shades reveals why. It states: "The optical density of the AstroSolar Silver/Gold Solar Viewer is OD 5.0." It filters out the SAME amount of light as the telescope filter, not less.

Continuing on: "Baader Planetarium's solar glasses are CE-certified and comply to the Norm EN ISO 12312-2:2015 – thus they are certified for safe and direct viewing of the Sun. AstroSolar Silver/Gold has no added treatments to attain fine-optical properties necessary for high magnification work. It is not suited to being used in combination with telescope, binoculars, spotting scopes or camera lenses."

So the glasses don't have the fine optical properties to be used with magnification. Nothing about being unsafe for telescopes at all. So what is the difference?

They anticipate this question:
"4. Why does Baader not use this film for telescopes?
When using precision-grade AstroSolar Safety Film for telescopes, perfect optical quality is most important. However, this film must be treated with great care since the metal and protective coatings applied onto both sides of the film are very similar in their technical properties to the reflective surface applied onto astronomical mirrors. Since a Solar Viewer is used visually at zero magnification, the optical grade of the film is not of prime consideration. Instead it is most desirable to apply protective coatings of highest durability grade, to end up with a protective film of much higher resilience against accidental rough handling.
This is being achieved for AstroSolar Silver/Gold Film with its metal hard coated layers applied onto both sides of the film, making it scratch resistant without sacrificing sharpness and without creating a double image of the Sun due to adding up several layers of dark and/or reflective film onto each other. A single layer of AstroSolar Film provides a perfectly sharp and energy balanced solar image – including the ability of identifying large sunspots and without transmitting irresponsible amounts of infrared (heat) radiation as with simpler sooth-filled energy absorbent films which do not offer added IR-blocking by metal reflective coatings.
Due to the dual side metallization process designed for AstroSolar Silver/Gold Film, the film remains safe for a straight look into the Sun, even in case the front side of the viewer would suffer a scratch or mild kind of abuse. Still every solar viewer must always get a thorough inspection by the user to detect possible damages due to wrong handling and must be discarded in case a defect would be visible."

It appears that the difference between direct viewing and telescope viewing solar filters is not that telescope filters block more light, or that direct viewing film is unsafe on telescopes but in the optical quality of the filter (higher for telescope) and robustness/safety after damage (higher for direct viewing).

At least for Baader glasses, they are perfectly safe to use the way the author does in this Instructable. They might even be safer as the they are harder to damage. But the quality of the images will be lower with more distortions. And they will have a fixed size smaller than the aperture for most binocular lenses, requiring more adaptation and thus more potential for an error letting unfiltered light past. The telescope film comes in larger sizes making it easier to fit to lenses. But they will be easier to damage in such a way as to let too much light in.

This reference also provides the OD to match, 5, for any solar filters to be used on binoculars, though one should be very cautious of buying cheap solar filters or eclipse glasses from unknown sources as the claimed OD may not be real. This I suspect may be the more pressing concern for safety. At the very least, look for the claim to to CE certified and follow the EN ISO 12312-2:2015 standard. If they met this standard, they would say so. Any that don't, do not meet it. arpruss is right to harp on this point as it is better to be over-concerned and under-blinded. And the link provided did a great job of providing technical data on solar viewing filter standards. I feel a lot more comfortable making this project knowing what I found out from the agenaastro.com links.

arpruss 2 years ago

Good points. I should probably note that even if one brand's naked-eye-rated filter has the same filtering as their telescope filter, this might not be true of all brands. Generally, when one is so close to serious danger, I think it's best to stick to the explicit purposes for which the stuff is made. (And if it fails, you can probably get more money by suing if you were using it for the intended purposes, though money won't replace the eye.)

DrRadium 2 years ago

You also raise good points, especially in regards to brand differences (more on that below). I did a little more digging about standards for the two types of filters and found out more. Most telescope filters do not meet the ISO 12312-2 international safety standard for solar filters because they let too much UV through. This isn't considered a problem due to most optics having UV absorbing coatings. However because they do not meet any specific 3rd party safety criteria you have to rely entirely on brand reputation. For glasses that meet ISO 12312-2 you have an 3rd party safety standard with defined criteria. I couldn't see the entire standards because I didn't want to shell out $57 for them, but there were a preliminary set of 'these are not included' statements none of which excluded use for telescopes. I wanted to find out what OD was required,I was not able to get the data directly, but I did find scientific papers that made reference to the ISO 12312-2 standard. One of the better and more interesting ones was at https://iopscience.iop.org/article/10.3847/1538-38...
It tested ISO 12312-2 shades for compliance with the standards. It noted "The current ISO 12312-2 upper limit of luminous transmittance, 0.0032%, provides maximum exposure durations of over 25,000 s and 33,000 s, respectively." That is slightly under 5OD. It also noted that the target luminous transmittance is to reduce it to a comfortable level (not minimum safety), thus the very long durations for viewing safety of 25,000 sec, being 416 minutes or almost 7 hours of solar noon viewing. A true 5OD filter provides 82,000 sec, or a little under 23hrs of continuous solar noon viewing. It is hard to imagine a 5OD filter not being safe on binoculars for a few minutes viewing at a time.

This paper looked at actual ODs from shades claiming to meet ISO 12312-2. 3 of 22 failed dimentional standards (too small I think), They all did well in IR and UV but many failed or were borderline for meeting luminous transmittance standards! Fortunately, all but one lens on one set of shades that fell near or outside standards were too dark not too light. They did not let enough light through. Only one borderline case of letting too much light through was found. This should be confidence inspiring that a brief look through binoculars using these filters is extremely unlikely to damage vision. If it does, it probably was via letting light around the filters not that the filters let too much through.

Even so I appreciate the point about the telescope filters being built for the purpose. These data suggest that barring disreputable filters, the biggest source of risk is probably not the filters letting into too much light, but light getting in around the filters. Good quality telescope filters are available in sizes more appropriate to binoculars and telescopes. Making it far less likely to screw up the build and have light get around the filters. You can have the filter wrap over the entire aperture leaving no chance for light leaks. As leaks would seem to be the worst risk provided reputable telescope filters or at least ISO 12312-2 filters are used, I think ease of a clearly safe build should be give serious weight in filter selection. AAS has a list of reputable vendors for both ISO 12312-2 certified glasses/shades as well as telescope filters at https://eclipse.aas.org/resources/solar-filters
Personally for binoculars and telescopes, I'd consider a telescope filter from one of the listed companies as most desirable, then a listed company's ISO 12312-2 direct viewing shades, then an unlisted company's ISO 12312-2 direct viewing shades, in that order of desirability. I would only make the filters using one of these.

I would not regard any unlisted company's telescope filter nor any shades that do not state they meet ISO 12312-2 as even worth considering for someone who would like to keep their vision. Despite a telescope filter being made for the purpose, one is relying 100% on reputation for safety with telescope filters, unlike the situation with direct viewers. No reputation should equal no trust. While I'd prefer a telescope filter for fit issues, I'd go with filters from shades with ISO 12312-2 certification over any brand X telescope filter. Regardless, I wouldn't trust any set of filters until all possible light leaks had been confirmed covered, using a nice bright light that isn't the sun.

ElectroFrank 2 years ago

Have you grasped that through a telescope, camera or binoculars of even moderate size, the size of the objective lens makes the image brighter ?

More light from the sun is gathered by the large front lens and focused into a small area (your retina), so that the output brightness is greater than when you are simply viewing through a plain filter.

Khovet1 3 years ago

Do you have a source for the glasses?

wannabemadsci 3 years ago

Just Google "eclipse glasses" or go to Amazon.com and search for "eclipse glasses" and you will find plenty of inexpensive cardboard and filter eclipse glasses. As I mentioned, only use ones that are certified for direct view of the sun.

msameer39 3 years ago

Very nice idea.
I have used the solar filter which was made up of some dark glass material (3mm thick) on my telescope and it got cracked by the Sunlight. I will strongly suggest everyone to be cautious with converged sun rays.