Split ring solar filter

[Ben the Ignorant]

I now have three 80mm scopes (may those who want only one telescope cast the first stone!), an achromat, a semi-apo and an apo. I needed a solar filter that would match them securely, luckily it's easy enough thanks to the dewshields having almost the same diameter. A filter with fixed attachments wouldn't do, and a filter with sliding attachments would be tedious, so I made one that fits several sizes by itself, by means of a spring shaped like a crown.

Since it never leaves the house it doesn't require a shockproof or dewproof case; a very humble cardboard box is it home:

The filter's frame is a sandwich of two mousepad layers with the Baader Astro Solar film in-between. Double-side tape and a screw plus plug assembly keep them together.

 

In order to pinch the dewshield the plugs are slanted toward the inside in a V-shape:

Superglued rubber O-rings make the filter sticky and prevent scratches. The plugs have slots that proved convenient to slide the flat spring inside of them. The extra layers of rubber (electric cable skin) at the rear make the filter adaptable to several diameters. With a rigid frame only the rear or front set of O-rings would touch the dewshield, but the soft mousepad frame yields under the spring's tension, so all rubber pads always make contact. This in turn is allowed by the Astro Solar film's plyability. 

 

The steel spring pushes the plugs against the dewshield, ensuring tight contact:

On the front side screws sold with the plugs keep them in place and improve on the double-side tape assembly. This is my f/7.5 achro with added centering screws on the front lens, the rear one is neatly paralyzed by shims. When I got this scope it was badly decentered; plastic screws recovered from finders do the job and do not pinch the glass like metal would.

 

In order to work the spring has to be split, but out of prudence I didn't want to let metal tips in the proximity of lenses, two pieces of heat-shrink tubing cover them, and make for a more finished look, while I notice on this close-up that some rubber parts were not cut too cleanly:

Will have to rectify that later . What matters is the filter is safe, and the foam can be pressed against the dewshield tight without marring it. Plus, the black foam kills all stray light for maximal contrast.

 

I wasn't sure what length to use, and the steel spring cannot be stretched if it's cut too short, so I made a trial with a steel insert from a windshield wiper, made of regular rust-prone alloy:

Then I used another windshield wiper insert to make the real thing, but this one is rust-proof alloy, looks shinier and will never degrade. I shaped it overall with my fingers, and did the detail work with pliers.

 

The filter locks tight on the semi-apo like it does on the achro, no problem, the dewshields have the same girth; I could swap their caps:

(The white thing is a temporary dovetail bar made from hard maple wood that saved the day in another project. It could be used permanently for very lightweight and cheap telescopes).

 

My triplet's dewshield has this rim making it three or four millimeters wider than the other 80mm scopes, the adaptable filter comes in handy:

The front piles of O-rings are placed so they pinch the rim for extra safety. The flexible quality of the mousepad material and steel spring make it simple to stretch the assembly over the rim when installing or removing the filter, which has an 83mm or 84mm clear aperture to respect the light cone, by the way.

 

This design may not be transferable on all other scopes but I hope you can pick ideas from this.