Advice on using Baader solar safety film to make telescope filters.

[Knarf]

I recently bought some of this film and I am looking forward to making some filters with it. I have an 8" Dob that I want to make a filter for, but in doing my research I'm finding mixed advice about whether I should go full-aperture or not. I am currently leaning towards making a 4" off-axis filter out of foam board. It seems that, with something as bright as the sun, that should be plenty, but I guess I have to consider that relatively little light will make it through the film. The film I bought has 0.00001 transmission. This is not listed as a percentage, so I assume it is out of 1.0. Doing some rough calculation (which is probably incorrect), it should reduce the apparent magnitude of the sun by 13. If the sun is -27, then even with the solar film, it should appear to be -14 in brightness. Therefore, 4" of aperture should be plenty. This is why I'm leaning towards stopping my aperture down. Maybe even 1-2" would be enough?

Btw, the way I arrived at 13 was log(0.0001) / log(2). There might be a flaw in my logic here.

[RickJ]

First off 4" is too big for off axis as you will be shadowed by the secondary. When working off axis you don't want any bite taken out of the image by the secondary as that adds diffraction effects that degrade the image more then the loss of aperture. Also position the off axis stop between spider vanes to again avoid diffraction. 3" is about the maximum for an 8" scope unless the secondary is unusually small.

If you have the material I'd go full aperture. The filter material is designed to be safe when used this way. A brighter image reduces exposure time if taking pictures allowing better freezing of seeing for a sharper image. Daytime seeing however is often rather poor. This is why off axis isn't too bad of a solution. Normally stopping down a scope reduces resolution. A 3" stop has only 3/8ths the resolution of full 8" aperture less a bit for diffraction. When stopped down the diffraction causes a greater loss of quality so it is important to avoid it in this situation. The result is often only a slight gain in visual image quality with full aperture but on those days of good daytime seeing it can make a huge difference. Since this is a one time event I'd be ready for this possibility and go full aperture since you've already spent the money.

Keep in mind that while magnitude 14 is bright by night standards, that is only about 6 times brighter than the moon which is rather low contrast and hard to see in daylight and you'll be looking with a daylight adapted eye. At night it would seem too bright but not by daytime standards!

Also your divisor is wrong. Magnitude is based on the fifth root of 100 (~2.512) so the answer is exactly a 12.5 magnitude drop.

Rick

[Knarf]

Thanks very much for the advice. I wasn't very confident that going full aperture was a good idea, but you've convinced me otherwise. You've also convinced me not to shape it like a batman symbol (I'm guessing this would cause diffraction issues as well? =P).

[Knarf]

Sorry, I just have one more question. On Tuesday, they are predicting windy conditions. 22 mph winds with gusts up to 40 mph. I'm worried that this might be too strong for a thin film. I haven't taken the film out of its packaging because I'm worried about damaging it. Am I being too worried?

[RickJ]

It's very tough. Use a good elastic band to hold it on the scope so it doesn't blow off, that's the bigger danger. Wrinkles aren't an issue, don't try to pull it tight to remove them. Glass filters are more easily harmed in fact.

Rick