Introduction

Visual sun observation with optical instruments (telescopes, binoculars, etc.) can only be done with proper solar filters. Not using a proper filter, one that filters sun light before it enters the instrument, will result in instant damage to the eyes, apart from equipment degradation.

In this article, I will describe the process I used to assemble a filter cell with the Baader Planetarium solar film for use with one of my telescopes (Meade ETX-90). I used a plastic top cover from a chocolate powdered beverage (Fig. 1) whose size is adequate for 90mm telescopes, but naturally you may find other products with covers suitable for different objective diameters. Meanwhile I have used smaller covers of the same product for creating a pair of filters for my binocular.

Fig. 1

Required Material

• Baader Planetarium solar film
• A plastic or cardboard cover with a diameter slightly larger then the front telescope diameter
• Bristol board or card paper
• Paper glue
• Double-sided tape (such as Scotch 665)
• Adhesive felt or sponge tape
• Scissors
• Compass
• X-Act

Construction Process

All of the following steps must be executed carefully, especially while manipulating the Baader film.

1) Open a circle in the cover, with a diameter slightly larger then the objective's diameter (Fig. 2). I used a knife with reasonable but not very good looking results.

Fig. 2

2) Using the X-act knife, cut two identical rings of bristol board (Fig. 3). Each ring should have an outer diameter sufficiently large to allow it to fit inside the cover almost without backlash, and an inner diameter a bit smaller than the circle opened in the cover. Note that the inner circle of the rings *must* be smaller than the cover hole, so that in the end no imperfections in the cover (due to the cutting) scratch or damage the film. You may use a compass for drawing the circles on the bristol board.

3) In each of the rings, cover one of the sides with double-sided tape (Fig. 4). Avoid crossing the interior of the rings.

Fig. 3	Fig. 4

4) Cut a square piece of Baader film, with a side length larger than the rings' outer diameter (Fig. 4). Put the film on a solid, plain and protected surface (for example, stretch a Kleenex or other soft tissue over a table, fixing the corners using common tape).

Fig. 5

5) Join one of the rings to the film, by carefully settling it with the tape covered side face down. You should align the ring with on of the corners of the film, to facilitate the next step (Fig. 6). After settling the ring, press it against the film with the finger.

6) Turn the film upside-down (Fig. 7), and join the second ring. Align the second ring with the same corner chosen in the previous step, so that it becomes aligned with the ring on the other side (Fig. 8).

Fig. 6	Fig. 7	Fig. 8

7) Trim the excess film and tape tips from the resulting disk (Fig. 9).

Fig. 9

8) Fix the disk to the interior of the cover using some glue. (Fig. 10)

Fig. 10

9) Fix three pieces of felt or sponge to the cover, folding them over the edge as shown on Figure 11. Do not fold the tape pieces too sharply - each tape piece should softly wrap the edge, or else one of the tips will tend to unglue.

Note: These felt or sponge pieces will allow the cover to fit and stay in the telescope without backlash, avoiding the filter to unfasten easily (VERY IMPORTANT!).

Fig. 11

Final Result

Figures 12, 13 and 14 show the filter cell alone, on the ETX-90, and on a binocular (naturally you MUST protect both objectives).

Very important: Before each use, inspect the filter with the naked eye holding it against the sun. You must assure that there are no scratches, holes or other anomalies that may allow unfiltered light to enter the telescope.

Fig. 12	Fig. 13	Fig. 14

After assembling your filter cell, you can use the remaining Baader film to protect the chocolate powder can, that has no cover now:

Fig. 15

I'm joking, of course. What you see on the picture is common aluminium sheet.

Warnings and Important Notes

Very important:

• Make 200% sure that the filter cell does not fall off the telescope easily;

• Check the filter prior each use, as mentioned before;

• Be responsible. Never leave a telescope unattended during daytime;

• Warn those observing with you of the importance of all these safety measures;

• Remember to also protect the telescope's finder, as the risks are the same as observing through the telescope itself; You must cover it completely, or build a smaller filter cell for it. Alternatively, detach it from the scope during solar observations.

• Store your filter safely in a protective container, such as a small (unused!) pizza box.

Tips and Recommendations

• You can use the remaining Baader film to build high quality filtering glasses for observing the sun, useful for occasions such as eclipses or solar transits. For this purpose I have created a PDF document with a template for printing paper glasses (download here). The template fits my head, which means it is probably too big for a children (or too small for others). If needed, you can always try specifying different zoom levels in the printing configuration settings. Finally, you should use the thickest paper supported by your printer.

• As mentioned above, you can also use the remaining Baader film to build a smaller filter for the telescope's finder.

Site via Pumabox.