DIY Solar Filter question

[Jammy]

I was thinking about using a 10" dia plastic pipe end cap, cutting a hole in it, and using a sheet of A4 Baader Astro Solar film within the aperture to make a solar filter.

My question is; does it matter if the aperture of the filter is narrower than the diameter of my telescope?

To allow for fixing the A4 sheet, I'd probably have an aperture of 190mm.  My telescope (C9.25) has a diameter of 235mm.

Would this just reduce the light level, or would it create other problems?

I've looked at buying a filter, but this seems a far cheaper alternative.

[michael.h.f.wilkinson]

You would simply be reducing the aperture a bit, no real problem, just a shade less resolution theoretically (limited by seeing anyway, most of the time)

[brantuk]

So long as no light enters the scope at all other than through the badder solar film, then it will be safe to use. Don't forget to use a bit of baader solar film on the front of your finder scope as well - a point often overlooked by most.

[bingevader]

The sun is so bright, that even with the filter, the reduced aperture should be fine.

I'm doing the same with my 8" dob, but making a filter to fit the stopping down hole in the end cap.

I won't be attaching the finder scope at all.

But just for reassurance, I have a filter for my etx90 too, it's only 90mm and has the same 1200mm focal length as the 8" dob, the views of the sun through that are fine and dandy.

[Jammy]

Thanks guys.  I'll make sure that there is no light penetration other than through the filter.  I'll remove the finder too just to be safe.

Now to find the correct size cap....

[LukeSkywatcher]

You really diont need much aperture on a scope to observe the sun. I'm doing it with a 70mm refractor at the moment. In the past ive done it with a 90mm refractor. Once the solar filter you make/buy covers completely the aperture you want to use and no stray light is sneaking in ANYWHERE................you will be fine. You dont even need a finderscope to find the Sun. Just line of sight along the scope tube will do and get you close and then tweak it a bit and the Sun will be there. To focus on the sun, use the edge (any edge) and you are good to go.

[Bizibilder]

Just be aware that the central obstruction of your scope is quite large - 85mm or thereabouts.  You probably need a filter with a minimum of 130-150mm in diameter, 190mm should be fine.  You could go full size but, as stated above, the seeing, when viewing the Sun, is limited by atmospheric turbulence.

[Browny]

Thanks guys.  I'll make sure that there is no light penetration other than through the filter.  I'll remove the finder too just to be safe.

Now to find the correct size cap....

Sounds like a trip to B and Q this weekend ! 

[spaceboy]

I made a similar filter some time ago. I had a 200P and purchased a second dust cap and cut out an opening in that. The limiting factor was the solar film itself as an A4 sheet with enough over hang to safely secure it in place meant that out of the 8" aperture of the scope I was only able to utilize 6" of that as a solar filter which due to the secondary mirror obstruction of the Newtonian this was further reduced to maybe 5".

The filter was good in that I could just put it on as if it was a dust cap. It fit snugly so no concerns of it flying off in the wind and there was no gaps for sunlight to make it in anywhere.

[Jammy]

Here's an update as to how I managed to make a solar filter.

Firstly I bought a 10" plastic pipe end cap from these guys www.thredgards.com/pipend.html 

The cap that I used has an ID is 273mm, and the C9.25 OD is also 273mm.

I cut a hole in the centre of the end cap 180 diameter, and trimmed rim as necessary to fit around the dovetail bar.

I cut two carboard rings to a snug fit within the pastic cap, each with a hole in the centre, again 180 diameter.

I then taped a sheet of Baader Astro Safety filter between the two pieces of card, then taped the two bits of card together and pushed it all into the pipe end cap and taped the card within the cap.

One complete filter!  The way I've made it means that stray light cannot ever enter from the front.

I've tried it out today, and this is a resulting first time photo.

You were all correct about the size of the opening not affecting the image.  Thanks again for the advice guys!

I'll post a photo of the finished filter later if anyone is interested...

[Astronigel]

Where did you buy the baader filter from?

[Jammy]

The Baader AstroSolar Safety Film was from Telescope House.  I bought a single A4 sheet.

[Johnny4365]

Nice pics Jamie. Well done for going down the DIY route. I wimped out and bought a ready made astrozap filter from FLO which I am more than happy with.

I did make some filters for my binoculars. Made them in about an hour on the evening before the eclipse, whence the blue peter look.

Would be interested to see your finished filter.

[LukeSkywatcher]

Nice pics Jamie. Well done for going down the DIY route. I wimped out and bought a ready made astrozap filter from FLO which I am more than happy with.

I did make some filters for my binoculars. Made them in about an hour on the evening before the eclipse, whence the blue peter look.

Would be interested to see your finished filter.

I completely wimped out and bought a Hershel wedge and filters. I was tired of wasting money buying sheets of astrozap filter and making a complete hash of making DIY filters.

[Jammy]

I like the binocular filters Johnny.  Nothing wrong with Blue Peter.  Where's the sticky back plastic? 

Here are a couple of photos of my filter.  Hope you can see what it looks like from these.  The only thing that I hadn't mentioned was that I used some sticky backed rubber window seal on the inside.  This is to make sure that it's a snug fit around the end of the scope when in use, but still easily removed when not required.

[Johnny4365]

That looks like a well made filter Jamie. Good idea to use the window seal for a tight fit.

I like the yellow colour, very sunny.

[Jammy]

They come in a choice of yellow or yellow! haha

[Mav359]

Depending on how DIY you want to get ......

I got these guys to cut me two rings to size out of 3mm plastics, then they were drilled and joined together using counter sunk hex bolts, the solar filter paper is just sandwiched in between. This way if the filter is damaged i just unbolt it and lay a new piece of filter paper between them. This was designed to work with my Dew shield but it might be a way to go if you fancy something a bit more robust, not cheap they charged me £25 a ring but for me worth it