TheSkySearchers

Twenty years ago I was observing a total solar eclipse in Zambia. During the partial phases, the wind came up while I was looking through my camera, my solar filter blew off and I caught a brief eyeful of unfiltered sun. Determined that would never happen to me again, I devised a better mounting.

There is a small difference between white light sunspot observing and a solar eclipse. During an eclipse, at the start of totality, the solar filter needs to be removed quickly and without applying any force to the optic that may shift it away from the sun, a few minutes later when totality ends, it needs to be replaced also without disturbing the optic. So anything that pushes on the OTA with a lot of friction is not optimal nor is anything that has to be slowly screwed off. I decided to make a flange plate solidly attached to the end of the OTA and then have the filter mounted in a similarly sized plate, and have a quick release clamp to hold them together.

Baader Astrosolar film has four advantages over many other films and glass filters.
1. The Baader film is only 10 microns thick giving it a near perfect optical transmission quoted by
Baader as 96-98 Strehl.
2. The film is coated both sides so slight scratches or missing pinholes of coating won't affect the
eye safety of the coating
3. Being so thin, it doesn't alter the focus the way some glass filters or thicker black polymer filters do.
This means that as the solar crescent is thinning, I can focus on the filtered image and be ready and
focussed to capture the first fleeting phenomena of totality - Baily's Beads and the diamond ring
without wasting time refocussing.
4. It is relatively cheap. I purchased a 20in x 40in sheet 20 years ago and have made a variety of filters
for wide angle and telephoto lenses, small refractors, a 4"maksutov and my 8" cassegrain and I still
have plenty left.

In this article, I give detailed instructions how to securely attach a Baader Astrosolar film holder to your scope, or any other type of thin film to make a secure solar filter mounting that won't blow off but can be removed quickly. The film is recessed so that it doesn't get damaged if placed on a rough surface. I have made a number of flange pieces over the years for different telescopes but I am still using the same Astrosolar film in its recessed holder that I mounted in 2002.

The article describing the construction is here:
https://joe-cali.com/eclipses/EQUIPMENT ... lters.html

cheers
Joe Cali

An excellent write up Joe and the linked article contains great information for people looking to start doing white light viewing as well as some good tips for people imaging.

I have to say, that the square boards look very much as though they would act as sails; so, despite the flange and the optional use of a rubber band, I would not be happy to use this if there was anything more than the slightest movement of the air.

I made mine using two 'cereal box' card strips that fit snugly around the OTA / Dewshield and the design doesn't give anything wider for the wind to catch.

Gfamily wrote: ↑Thu Jun 17, 2021 9:32 am I have to say, that the square boards look very much as though they would act as sails; so, despite the flange and the optional use of a rubber band, I would not be happy to use this if there was anything more than the slightest movement of the air.

I made mine using two 'cereal box' card strips that fit snugly around the OTA / Dewshield and the design doesn't give anything wider for the wind to catch.

Have now used this design at 11 total solar eclipses and one annular solar eclipse, some in strong breeze.

I cut them fractionally small so I basically have to stretch the mounting board over the lens hood. if too tight I trim them open a little or sand them but the foam has to be stretched over the lens hood. I don't use the rubber bands. If I cut a mounting board too big, I make another. It doesn't move once attached. It also acts as a bit of a sun shade when you are looking through the view finder and keeps the sun from heating up the DSLR body.

I can guarantee they work really well.

When travelling, to and from the eclipse, the filter board gets sandwiched between two other flat boards of the same size and this protects them against damage. You will find with yours that if you travel to an eclipse and have a lightweight mounting, pulling it off the tube might move the OTA. If you're just using it for white light and partial eclipses then it doesn't matter.

cheers
Joe

Gfamily wrote: ↑Thu Jun 17, 2021 9:32 am I have to say, that the square boards look very much as though they would act as sails; so, despite the flange and the optional use of a rubber band, I would not be happy to use this if there was anything more than the slightest movement of the air.
...

Turns out not to be a problem for the sizes of material depicted in this thread. Had mine out in a 5-10 mph breeze last week. No noticeable deflection.

I do need to erect an actual sun shade if I expect to do this in mid-summer. I have a heavy duty tripod I'm refurbishing that should allow me to integrate a support for the sun shield so there are no additional forces on the mount.

Have fun and clear skies.

I had a sheet of Baader film laying around and made a lense cap filter. Tried it yesterday and the sun was so bright you had to use other eyepiece filters to dim the light for visual use. After seeing your article I must have the 3.8 photo material. Thanks for the info.

Jones wrote: ↑Sat Dec 25, 2021 5:46 am I had a sheet of Baader film laying around and made a lense cap filter. Tried it yesterday and the sun was so bright you had to use other eyepiece filters to dim the light for visual use. After seeing your article I must have the 3.8 photo material. Thanks for the info.

Hi Jones,
It may be the 3.8 or the V5.0. The V5 visual filter material is bright esp if you used it on an aperture 4 inch or larger. The easiest way to work it out is to take a series of exposures and see which is correct? I give the exposure values for different filter materials early on in the article.

The V5, for my taste, gives too bright an image for comfortable long term visual observing. When I observed the Transit of Venus in 2012, I used an ND6 black polymer filter giving me an image brightness much more comfortable for extended viewing times required for the 7 hour event.

However for a total solar eclipse, the Baader Astrosolar V5 is the best material in my opinion. Coated glass filters are so thick that they can slightly alter the focus meaning you have to refocus the camera in the fleeting last seconds before the eclipse. The Baader film is only 10 microns thick and as such doesn't change the focus. I focus my telescope using live view using the craters on the Moon's limb silhouetted against the solar disk during the latter stages of the partial phase then find it is well-focussed for totality when I remove the filter.