Making a solar filter smaller than the aperture...

[ryandsimmons]

Hi All,

Having upgraded to a Celestron C8 recently I need to upgrade my solar filter as well, so today I bought some Baader Solar Film to make a new filter for the (probably cloudy) upcoming eclipse.

Problem I have found is that the aperture is 8 inch, but the solar film is coming in at around 7.5 inches width.  Now I am struggling to work out what needs to be done with the gap.

Do I:

1) Simply put 1 inch strips of cover over the area,  Making the aperture not quite circular?

2) Make a full circular blocker, making it a 7 inch filter, not using the outside inch of the OTA?

3) Some other fantastic idea I am missing that is obvious.

4) Use strips of spare film to cover the gap (with LOTS of overlap to ensure safety, but this may cause dark areas?)

I am not quite sure of the best way to approach this, last time I made one was for a 6 inch telescope, so never had to think about this sort of stuff.

Ideally I want to get this finished this weekend, I want to try to do some test DSLR photography on the sun ASAP as a dry run.

Cheers in advance

Ryan

[ronin]

I would go for 2.

You do not need the full 8 inches for the sun, it tends to be fairly bright.

Effectively make a cardboard circle of 8" with a 7" (or 6") hole.

Put the film as best you can over the central hole and fit the cardboard carrier to the scope.

Bit like an old biscuit tin lid with a hole in it. There is nothing like these to describe it these days.

[ryandsimmons]

That is not the answer I was looking for!  It means I need to throw away the rings I made and restart.  :-(

Many thanks!

[ArmyAirForce]

On my 8 inch reflector, I made a filter that is 190mm diameter out of a 200mm wide sheet.

I used to have an off axis solar filter, but positioning the Sun central in the scope gave an off centre bright spot on the solar disc, making it look sunlit itself.

So I'd go for circular as big as you can get away with.

[ArmyAirForce]

Sun lit Sun!

[ryandsimmons]

Cheers chaps!  All I need to do now is see if my shiny new camera can fit the sun in the frame (700d with a .66 focal reducer) or whether I need to borrow the 1d from a mate at work.  (Mysteriously the 700d is missing from the 12dstring FOV calculator.)