Solar filter aperture question.

[welshrob87]

I'm in the process of planning and making a solar filter for my 200P Dob, I have a sheet of Baader AstroSolar Safety Film so the maximum aperture I could make is about 18-19cm (allowing a centimetre or so to hold the film in place).

My question is should I make it as big as possible or is it more beneficial to have a smaller off centre aperture?

[jimmyjamjoejoe]

Apparently it's better to do a smaller off centre one. With something so bright, having the central obstruction will reduce much needed contrast, some people also notice some central dimming in the image. You also need to consider that the central obstruction will take up more percentage of the objective since you'l be reducing it to 170-180mm probably, yet the secondary stays the same size.

People say that between F25-F30 gives good results, meaning the 50mm off axis hole in the objective cap suits the purpose almost perfectly.

[welshrob87]

Thanks. I did wonder if the secondary mirror mount would cause any problems. Will probably change my plans and make an off centre filter instead.

[jimmyjamjoejoe]

Mine should be with me tomorrow, i'm gonna make an off axis one for the 200p, and a couple for my 12x50 binoculars. I'l either store the rest or donate it to some lucky chap in my astro group.