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Viewing the Sun: How to Build A Solar Filter For Telescopes

How To Build A Solar Filter for your Telescope with AstroSolar Safety Film

by: OZScopes - The Australian Telescope Experts

A Solar Filter on a 10
A Solar Filter on a 10" Dobsonian Telescope.
From "Making a White Light Solar Filter"

A White Light Solar Filter - like BAADER's AstroSolar Safety Film - is a fantastic, inexpensive way to view sunspots and use your telescope gear during the day. It'll fit most telescope (with varying aperture sizes), and once you have some solar film, you'll be able to make the filter yourself.

If you were after purchasing your own BAADER AstroSolar Film, simply give us a ring on 1800 SCOPES (1800 726 737) or drop us an email at support[at]ozscopes.com.au and we'll be able to get that sorted for you.

Solar film typically looks like floppier, smoother version of aluminum foil. With high density coatings on both sides of the foil, you'll see that's its surface is reflective. This'll provide you with security when you're observing the sun head-on.

How Do I Make the Solar Filter Cell?

20-30 minutes

You'll need: BAADER's AstroSolarSafety Film, Some Stiff Cardboard, Cellophane Tape, Double Sided Tape, Some Facial Tissues, Scissors, White A4 Paper

So, you're wondering how to make your own objective solar filter for your binoculars, camera or telescope? Simply follow the instructions below - it shouldn't be too difficult, and it should take about 20-30 minutes. Read on!

Making The Filter Cell

The cardboard frame of your solar filter, shaped and cut
The cardboard frame of your solar filter, shaped and cut.
From "Making a White Light Solar Filter"

1. Measure and note the inner diameter and outer diameter of your cardboard.

2. Get some stiff (make sure it's as sturdy as possible, or it'll be floppy!) cardboard, and cut two equal sized rings out - the inner diameter is to be the same width as the full aperture of your telescope lens/mirror, and the outer diameter to be 10cm (~4:) larger.

3. Stretch out a square piece of facial tissue flat on a hard plane/surface (like your table) and secure all four corners of the tissue with tape - but do not stretch or tear it!

4. Cut a square piece of AstroSolar Film slightly larger than the outer diameter of the 2 cardboard rings - to do this, place the AstroSolar Film between two sheets of clean, white paper. "Sandwich" the film, so that you can easily cut the film without leaving creases or fingerprints on the film.

5. Gently place the AstroSolar Film onto the flat tissue, but do NOT stretch it! The AstroSolarSafety Film cannot be put under stress, or you risk stressing its precision optical property.

6. Hold one cardboard ring with the sticky side DOWN about 10mm above the film, and let it fall down onto the film, so that the ring touches all parts of the film at the same time.

7. Turn around this cardboard ring with the film covered side facing UPWARD, and lay it back onto the tissue. Stick the second cardboard ring against it, carefully.Trim away any overhanging parts of the film.

8. Now, you've created a round ring-holder with AstroSolar Film sandwiched between it, without creases or ripples - and most of all, without Stressing The Film!

The Solar Filter Cell from Underneath
The Solar Filter Cell from Underneath.
From "Making a White Light Solar Filter"

The Solar Filter Cell from on Top
The Solar Filter Cell from on Top.
From "Making a White Light Solar Filter"

Making the "Cylinder"

1. Now,cut several long strips of the cardboard approximately 50mm (2 inch) wide. Try to get at least 3 or more good strips. Place these long, wide strips of strong cardboard around the telescope dew cap (or front end) and tape the ends with double-sided tape (you can also use glue, if you'd like - but be sure not to glue it to the telescope!). Repeat this procedure at least 3x with double-sided tape between each layer, so as to produce a stiff 50mm high cardboard cylinder to precisely fit onto your telescope.

2. The finished cylinder should fit snugly on the telescope, but should still be able to slide up and down easily. The fit is very important - too tight, and you won't be able to get the filter on and off without damaging it; too loose, and an accidental bump or gust of win could knock the filter right off. Hint: For smaller aperture telescopes, you might be able to find the perfect sized cardboard tube to fit over your scope. Simply cut off about 2" worth and use that as your cylinder. If the tube is slightly too large, use and adhesive felt liner or cork pads to make it fit more snugly.

3. Lastly, glue the AstroSolar film holder onto the 50mm cylinder while the cylinder is still mounted onto the front end of your telescope.

4. Congratulations, your homemade solar filter is ready! Lift the filter up to the sky and inspect the cell for light leaks before you use them for your telescope or binoculars. You MUST be sure that absolutely no unfiltered light gets through.

5. You can use a black felt-tip marker on the inside of the filter (for aesthetic appearance) to cover tiny pinholes in the filter's material. Be sure to store your completed Solar Filter properly in a clean, dry area.

Safely Storing Your Filter

It's very, very important to store your filter safely so that it's not damaged. If your filter is small enough, you might want to keep it in a plastic food-storage container or Tupperware. It'll protect the filter material, and also keep the cardboard material from becoming wet or soft.

How To Clean Your Solar Filter

Now that you've made your very own solar filter, you'll need to know how to clean it. Find out how to clean your BAADER Planetarium AstroSolar Material.


Failure to do so can result in permanent blindness, or complete and irreversible damage to your eyes!

1. Before using your solar filter, check the filter's fit and integrity - if it's damaged in ANY way, it MUST be replaced. If your AstroSolar safety film has arrived damaged, we WILL replace it free of charge - simply contact us. DO NOT use if the AstroSolar material has any holes or the cell is weakened and doesn't stay on the telescope.

- Slide the filter on and off your telescope to be sure it won't be knocked off by accident or get blown off by a gust of wind
- You can try taping the filter onto the telescope for added safety
- Make sure the filter cell and cylinder are consistently and safely glued together
- Hold the filter up to the sky and look for pinholes in the filter cell itself, or light leakage where the filter cell and cylinder connect
- Tiny pinholes in the filter material can be covered on the INSIDE with a black felt-tip marker.

2. NEVER use the filter at the eyepiece, or any other part of your telescope's optical path - the concentrated ray of light will burn the filter up instantaneously - it's much like burning dried leaves with a magnifying glass. The filter will ONLY work when attached to the FRONT aperture of a refractor objective, in FRONT of a Schmidt plate, or in FRONT of the tube of a Newtonian telescope.

3. If you use binoculars, protect BOTH objective lenses with a filter.

4. If you've punctured the AstroSolar filter, REPLACE IT - the material itself is about as fragile as glass. Once the integrity of the filter has been compromised, destroy it completely to avoid accidental use.

5. EMPHASIZE the importance of safety to those who observe with you, especially children - intentionally removing or damaging the filter can endanger eyesight - there's is no room for jokes. Never leave the telescope outside unattended during daytime!

6. If your telescope has a finder, you should also put a filter on it or use a dust cover and secure it with tape. Unprotected viewing through the finder will lead to the same kind of optical damage as viewing through the telescope. In addition, an uncovered finder directed at the sun can lead to exceedingly unpleasant scalp burns while you're viewing.

7. In all cases, it is advisable to interrupt solar observation occasionally and look at other objects. If you have any doubts at all, especially in cases of known excessive eye sensitivity, consult your optician or ophthalmologist.


WARNING: Improper use of the AstroSolar Safety Film may lead to serious and irreparable injury if proper precautions are not taken during solar viewing .

Use This Information At Your Own Risk: There is no warranty expressed nor implied and we are not liable for any of your past, present, nor future actions. Should you perform these modifications to the letter you may still damage your vision and eyesight. Even if it appears to function properly your actions may cause its integrity to be compromised, causing you harm. By utilizing these ideas and instructions in an attempting to undertake solar viewing, you do so at your own risk & peril.

Derived from:
1. "Making a White Light Solar Filter" by Mike Salway
2. Solar viewing with BAADER AstroSolarSafety Film
3. Making an Inexpensive Filter Cell for BAADER AstroSolar Material (Click to view PDF)