Baader Astro-Solar filter for the 7" f/12 iStar for the Mercury transit.

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Having the 7" refractor available suggests that I use it for the coming Mercury transit on the 9th of May 2016.

I already have a Baader Astro-Solar film filter for the original Celestron 6" f/8 lens stop down cap for a 112mm f/11 approx. Plus the full aperture film filter for the 90mm which really ought to be replaced by  now. So it seems a waste to stop down the 7" when it could and should be used at full aperture for highest resolution. I have plans to do far more solar observing, photography and hopefully some imaging.

Above: Bare solar filter holding alloy rim trial fitted in place over the 180mm 7" iStar objective cell. 

Sorting through my collection of pots and pans from the earlier, 7" straight tube build I found a thin, parallel sided, perforated, inner steamer pan which would allow some clearance over the objective cell. Enough, in fact, to allow a lining of thin, closed cell foam. The 'Funky' foam lining will protect the objective cell's finish during fitting and removal of the filter and ensure a snug fit which won't fall off in a gust or the telescope is accidentally tilted 'nose down.' The alloy filter rim will hopefully provide long service without stressing the special filter foil it is designed to protect.

First I cut the base of the pan away to a depth of 40mm with an electric jigsaw. This measurement ensured the sides of the rim would not strike the collimation screws but would still reach down enough to ensure a safe grip on the objective cell.

In retrospect I should have cut the 185mm hole in the base first to allow saw blade clearance on the workbench. Instead of which I had to work in short arcs over the slot between the workbench jaws. 

The 185mm hole was chosen to give a slight clearance for the light passing through the filter before the objective cell did its usual job of setting the full aperture of 180mm. Any grazing surface will scatter incident light. So it probably best not to allow such surfaces to protrude into the light path.  

The images show the basic, alloy rim after smoothing the saw cuts and giving the alloy a rub over with abrasive paper to tidy things up. Followed by a thorough wiping with a clean rag to collect all the alloy dust and swarf.

The question now is how best to use the alloy filter rim I have made from the steamer pan. The Solarfilm has to be supported without the risk of snagging on sharp edges. The usual ATM construction is a sandwich of two rings of packaging cardboard and double sided tape. The tape is to hold the film smoothly and securely without stretching or wrinkling. I don't think the film wants to be in direct contact with the alloy ring. But, there just isn't enough room for a pair of useful cardboard rings inside the narrow, front rim.

If I made a cardboard strip into a short tube I could stretch the solar filter film over that.Then add a tube of foam made from a similar strip. Both then go inside the alloy rim for security and support. It is a [sort of] a plan but the proof is in the actual making. There isn't very much clearance between the cell and the rim.

Alternatively, I could cut a narrow ring of foam to line the filter rim and cover it with double sided tape. That would protect the film from ever touching the alloy rim. Then I'd attach the solar film, followed by another ring of foam with double sided tape already applied. Hey presto! Protective sandwich completed. Except it will be quite a fiddle working in the bottom of the 40mm deep filter rim. One mistake and the expensive filter foil is wasted.

It really needs a jig which can be built into the foam/foil/foam sandwich and then the rim gently lowed over it once complete. With the strip of cell gripping 'Funky' foam then applied to the inside of the rim in a long strip. Or [rather] several shorter strips with ventilation gaps between them to allow air to escape. If the filter was made too airtight the film will balloon on fitting and suck back in on removal. Best avoided to avoid a repeated chance of damage to the solar film!

I have just ordered some new Baader Astro-Solar foil and may even come up with new fixing ideas by the time it arrives. Some sort of stiffening ring would be really useful to avoid distortion of the foam rings. In fact the stiffening ring could come first followed by the foam rings and foil sandwich.

In the end I made two identical rings out of some 3mm, 1/8" plastic sheeting I had forgotten I owned. The rings should be stiff enough to allow safe handling while applying the double sided tape. The sticky tape can then be trimmed without distorting the rings. It is important that the rings fit easily inside the alloy rim. Or the solar film filter will be pulled out of shape when it is fitted. A relaxed smoothness without tension is desired for best optical performance. This is the advice of Baader themselves, in their printed instructions. Where it is recommend the prepared [sticky] rings are dropped gently onto the film while it is lying on a flat surface. Adding the second ring, to ensure it is concentric with the first, might be best with the first ring already seated in the alloy rim. Fingerprints on the solar filter film must also be avoided. I note that Baader now include thin paper protection for the film. They advise that the film be cut with scissors only while protected by paper on both sides. I must admit to being disappointed by my first sample of Baader film because it had large creases and actual folds caused during manufacturing. This was purchased some years ago. Whether these affected my views of the sun remain unknown.

I used sharp scissors with an angled bias to trim the outside of the double sided tape once firmly applied to the plastic rings. [With the protective top surface tape left in place to allow easier handling.] My careful trimming was to ensure the rings dropped easily to the bottom of the alloy filter rim without any protruding tape foam.

Cutting the inside circle was far more difficult and I used a pair of curved scissors with a final [rather rough] trim with a small bladed, craft knife. The foam backing of the tape does not respond well even to a gentle sawing action with a brand new blade. The foam is soft and sticky but the slightly untidy finish should be be invisible once in place inside the alloy rim. Rubbing around the rings with finger and thumb helped to smooth the tape foam and I carefully removed any straggling pieces which might stick to the active area of the solar foil. The width of the plastic rings is much narrower than the usual filters so even and firm adhesion of the solar film to the double sided tape is vital to the finished solar filter's longevity.

I managed to drop the first ring safely onto the solar foil but it still wrinkled. Adding the second ring to achieve concentricity with the first was an absolute nightmare! I had to carefully peel the 2nd ring off again, trim the foil closely around the 1st ring and then use the alloy rim to get one ring to overlap the other. The result is not remotely pretty but it seems to have worked. I added a few strips of double sided tape to get the rim to fit snugly over the objective cell and to hold the rings safely in place. Of course the weather is not going to cooperate. Gales, overcast and rain forecast for all afternoon! Peering through the filter it was only just possible to see the sun as a dim blob behind the uniform layer of clouds.

Click on any image for an enlargement.

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