making off-axis sub-diameter solar filters

[bal...@comcast.net]

Hi SAS:

With the eclipse coming up, I thought I’d shoot out my thoughts on making sub-diameter off-axis solar filters to observe the Sun, either on a regular day or during partial phases of an eclipse.

 

Using either the Baader filter material or the Black Polymer filter material, you want to cut out enough to make a round filter that has a diameter that is [diameter of the mirror minus the diameter of the secondary mirror] divided by two. So for a 10” scope with a 2.5” secondary mirror, you would want [10 – 2.5]/2, or 7.5 / 2 which is 3.75”. You can make it square or round, but you will be taping this filter onto a flat material such as card stock, and the card stock will have a ROUND hole that is the diameter as defined above. Well, almost. You don’t want to be within ¼” from the edge of the mirror, so the hole will be a hair smaller that my formula. What you want is a hole that avoids the edge of the mirror, the edge of the secondary and the spider veins so that the optical path of the hole has no other obstructions. See the picture below. Place the hole on a quadrant that is not on the same side as your focuser in case the focuser tube projects into this pathway. Also try to avoid mirror clips on the primary mirror.

 

 

The filter material will have the shiny side towards the Sun, be totally taped without leaks or any kind of failure, and the card will totally cover the front of the scope. Furthermore, you want to construct a cylindrical wall onto the filter card stock so that it will slip onto the tube of the scope. You want to construct it so that it absolutely will not come off while using it, but it can be removed quickly and easily when totality starts and you are using no filter. You want it secure so that wind won’t blow it off, a kid walking by won’t brush it off, absolutely fail-safe. Yet easily removed and replaced. If you are in a place where totality does not occur, then you can make it so that it is very secure and not easily removed since there will be no point during this even in which no filter is needed.

 

If you are going to do full-aperture rather than sub-diameter, then you can find a way to safely mount this without leak across the whole optical aperture. I do not suggest this with the Bader filter, even though they claim it is safe, it will appear uncomfortably bright. Baader filters make the sun appear bluish, Black Polymer will make the Sun appear orangish. I prefer the Black Polymer for both reasons, it seems safer and I like the color. Neither filter will give you the true color of the Sun. There isn’t any reason to go full aperture, the Sun is bright enough that you don’t need aperture, and the sharpness using 3” hole is excellent, so resolution isn’t an issue either.

 

Good luck, good weather, and BE SAFE!!!!!]

Baldy

 

 

Jeff Baldwin

Lathrop, CA