Solar Filter for C9.25 - Home made

[Dennis]

Hello,

Here are some photos of the Baader Solar Film Astrophotography Solar Filter that I have just completed. The design is made up of an OTA mounting ring and a Baader film retaining ring as follows:

OTA mounting ring:
9mm plywood ring; OD=11.5ins, ID=10.5ins.
6mm plywood ring; OD=11.5ins, ID=10.5ins.

These were glued and screwed together to form a 15mm deep ring that fits over the corrector plate cell on the C9.25 OTA.

Baader film retaining ring:
4mm plywood ring; OD=11.5ins, ID=9.25ins (top ring)
4mm plywood ring; OD=11.5ins, ID=9.25ins (bottom ring)

The bottom ring is glued to the assembled 15mm OTA ring. Then, the Baader solar film was then placed on the bottom ring whilst the top ring was then screwed down in 6 places.

On the inside of the OTA mounting ring, I cut x 3 shallow recesses at 120 deg spacing in which I glued thin foam strips to snugly secure the filter on the OTA. I also drilled and tapped x 3 holes for nylon locking bolts so that once fitted, the filter can be firmly locked in place.

Cheers

Dennis

[[1ponders]]

Jeez Dennis, I was going to post my cardboard and gaffa tape ones but how can I after seeing your work of art :lol:


Nice workmanship mate. Now I know you definately have too much time on your hands.

[acropolite]

Excellent idea Dennis, particularly the locking bolts. I was going down the cardboard and stickytape route, but your idea looks better.:thumbsup:

[netwolf]

Great work Dennis, I like the nylon threads concept. Did you make the plywood rings yourself? Are the available of the shelf?

Regards

[Dennis]

Hello,

The plywood rings were cut from left over’s from other projects and I used a router to cut the rings. It was quite fiddly and time consuming, clamping the work and cutting the circles, so unless you have the equipment, time and skills, I really wouldn’t bother with a wooden frame.

The cardboard models, if well made, certainly look strong enough and are also more lightweight if balance may be an issue? All I have to do is slide the C9.25 in the dovetail mount to re-balance the ‘scope. With a dob, this may not be so easy?

Solar filters tend to get used infrequently so if handled and stored correctly, there is really no advantage in making a wooden model, unless you have too much time on your hands…

If you build up the reinforcing around three points on the circumference wall of the cardboard model, I'm sure it would be possible to insert some nylon locking screws and provided you don’t tighten them too much, they should provide a good enough safety grip.

Cheers

Dennis

[netwolf]

I think the Nylon thread or some method to keep it tightly on is a must. Becuase the last thing you want is a strong breaze or child to knock it off while viewing the sun.

Regards

[Dennis]

Other options for securing a cardboard ring are:

Using a hole punch, punch a couple of holes in the cardboard ring at 180 degrees and thread some string through the holes and fasten the string to the top and bottom mounting rails (if you have them), or around the OTA.

You could also add (glue/staple) 3 tabs or ears that stick out from the cardboard mounting ring and then tie some string around the OTA that passes over the top of these tabs or ears, thus securing the ring to the OTA.

Cheers

Dennis

[[1ponders]]

Or adhesive velcro tabs.

[netwolf]

What about modifying dust caps to be the cell?
Are extra dust caps available for purchase?
My Dob has a dust cap that plugs in tightly.

Regards

[JimmyH155]

I'm planning on using thick rubber bands over the tabs.:D

[originaltrilogy]

Why does the wrinkliness of the film not make for bad images when looking?
This is a very professional looking job.

[Dennis]

From memory, the information sheet that came with the Baader Solar Film recommended that the film should not be fitted taut - a looser fitting (not stretched) was preferred and did not affect the optical performance.

Cheers

Dennis

More details here:
http://www.baader-planetarium.com/sofifolie/bauanleitung_e.htm

[alistairsam]

Hi
I wanted to make a Mylar solar filter for my 8" F4 reflector ota and wanted to know if it's necessary to have a full aperture filter
If not how do you work out what the size of the aperture should be and why are some of the solar filter apertures off centre?
Does it depend on focal length of the telescope and eyepiece?

[brian nordstrom]

:thumbsup: Nice job Dennis , 1 question , where did you get the nylon bolts ?

:shrug: I have never seen these for sale .
I bet the sun looks nice in this .
Awsome mate .
Brian.

[brian nordstrom]

:eyepop: Petr , the wrinkles are there to stop the film tearing under expansion as the film heats up , and mate thats the LAST :eyepop:thing you want to happen .

I would not know how ( or why ) the view is so clear with all those wrinkles in it , but mate it works very well .
Brian.

[Dennis]

Hi Brian

I obtained the (nylon) screws from:

http://www.hobbyparts.com.au/store/partslist/screwshexheadplastic/fasteners/wide/61/

Cheers

Dennis

[originaltrilogy]

I understand it works, but it amazes me. We worry very much about optics and then wrinkly film does not affect image, I don't understand even though is true!:shrug:

[Merlin66]

The thickness and surface accuracy of the Baader Sheet is sufficient that the wrinkled sheet still performs 100%!!
Dennis,
What sized Baader sheet did you use for the 9.25?? I thought the A4 size was too small????

[Dennis]

There were 2 sizes available when I purchased mine. The one I ordered was:

AstroSolar ™ Photo Film D=3.8 (100x50cm/39x20 inch)

More details here (http://www.baader-planetarium.com/sofifolie/details_e.htm)– scroll to the bottom for the sizes.

Cheers

Dennis

[Merlin66]

OK thanks for that!
(BTW the ND3.8 is the photographic version - ND5 for Visual)

[Dennis]

I’m not advocating “building in” any wrinkles when making these cells and my C9.25 example is probably at the worst end of the scale.:)

I used contact adhesive when I assembled the cell and would have liked an extra second or two to iron out the undulations but the adhesive beat me, setting too quickly.;)

Cheers

Dennis