Solar Filter Holder

Introduction:

This full-aperture solar filter holder was made to fit a 90mm Vixen refractor, the design can be adapted to suit a sub-aperture filter for a larger telescope. Originally I used ordinary Mylar filter material (offered for sale specifically for the purpose) but later replaced this with the newer Baader Astrasolar film which offers much improved optical performance. The advantages of using this filter holder are:

The film is not stressed in any way, the maximum optical performance of the film is retained.
The holder offers good protection for the filter, and a cap covers it when not in use.
No film is wasted, a circle just a little larger than full aperture is all that is needed.
It's very secure, safety is paramount when visually observing the sun.

Construction:

There is little to say that cannot be gleaned from the pictures. The filter holder consists of three parts, the main body which slips over the end of the telescope, an inner ring to which the filter material is attached, and a retaining ring which holds the filter in place within the main body.

All these parts were turned from a suitable length of alloy tubing obtained as a scrap off-cut (as always!). It was most important that the filter be held securely in place with not the least chance of it being accidentally dislodged in use. For the Vixen refractor this involved lining the dewshield with an additional layer of felt so it was a tight fit on the objective cell, this made sure that there was no chance of it it falling off (little enough chance as it was...). The main body was made long enough so it slid onto the outside of the dewshield and it too was held securely.

The main body was lined with felt to provide a friction fit which would not scratch the painted dewshield. A recess was machined in the front end to hold the filter ring, and also a threaded section to accept the retaining ring.

The filter ring is just that, a plain ring with a 1/4" wall thickness and about 1/8" depth. To attach the film the ring was faced with double-sided sellotape, the film laid on a flat surface and the ring placed gently on top. Excess film was then trimmed off around outside using a very sharp blade. Under no circumstances should the filter material be stretched over the ring like a drum skin in an attempt to get rid of wrinkles, these do no harm (they indicate the film is not under tension). The tape holds the film very securely, when I exchanged the Mylar for the Baader Astrosolar it was the devil's own job to get the Mylar film off! I had to cut it off and then use solvent to remove the old tape. Further, total reliance is not placed upon the tape, the ring and filter film are trapped between the recess in the main body and the threaded retaining ring which holds it in place. There is no chance the film can accidentally fall out.

The front of the filter holder, fully assembled.

The retaining ring unscrewed, the filter brought forwards a little.

Looking into the rear, the black patches are felt.

With the cover in place (Vixen supplied), has a useful central small aperture for CCD use.

All the parts represent a simple turning and threading exercise, quite easy to make if you have a basic thread-cutting lathe. I tried using mine without anodising or painting the holder and could see no evidence of unwanted reflections, so I just left the aluminium in it's natural state. Sticky-back felt or 'flocking paper' is great for producing a good sliding fit where required.

Here the filter is shown being put to good use.

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	The front of the filter holder, fully assembled.
	The retaining ring unscrewed, the filter brought forwards a little.
	Looking into the rear, the black patches are felt.
	With the cover in place (Vixen supplied), has a useful central small aperture for CCD use.