Going H-alpha: Building a D-ERF cell.

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WARNING: SOLAR OBSERVATION REQUIRES GREAT CARE AND SAFE FILTRATION.
INSTANT PERMANENT BLINDNESS CAN EASILY RESULT FROM SIMPLE MISTAKES.
NEVER LOOK AT THE SUN THROUGH ANY LENS, MIRROR OR INSTRUMENT UNLESS IT HAS BEEN FULLY TESTED AND APPROVED FOR SUCH USE. YOU FOLLOW MY EXAMPLE ENTIRELY AT YOUR OWN PERIL!

A trip to a charity shop produced this potential filter cell: 

A label, sticky tape and postage stamp, wet sponge holder from yesteryear. I have removed the non-slip rubber cup base and sponge. 100mm Ø x 35mm deep in 1mm thick stainless steel. The front hole is 89 mm Ø.

The water hardness is still clinging from years of use. This will easily rub off to leave a pristine, brushed finish. Now I just need to find a way to get the 90mm Ø, glass, filter disk safely inside it.

The images right show the process from freehand, chain-drilling the base inside the scribed circle. To removing the 'tin lid.' To the completed ring cleanly turned on both sides to 88mm and 93mm Ø holes.

Extreme care must be taken not to have such a flimsy item snatched out of the chuck on the lathe. I filed the rough hole smooth after the chain drilling before risking the piece in the lathe. Then used extremely fine power feed with small depth of cut increments.

The greater depth and larger diameter [compared with the earlier lid] will allow space for cork packing pieces. Plus the deeper cylinder sides will allow radial screws to hold the completed filter holder in the main tube of the 6" refactor. All without risking damage to the filter itself. The gods of creativity do sometimes smile upon the ATMer.

The enlarged hole will allow for later, easy glass insertion. The new and smaller hole will leave a better safety lip to retain the glass disk against small cork pads, glued in place.

The perimeter of the glass disk will sit between thicker cork pads to allow for differential, thermal expansion. It is vital that the filter is fully relaxed in its cell without any strain or distortion. Slight tilting is usually vital to avoid multiple reflections of the solar image.

I must also consider cell assembly and positioning. Without risking the perfect optical surfaces with their multiple optical coatings. The safest way to fit glass in a cell is to lower the cell onto the glass. While the glass itself is resting on a tissue protected object like an inverted drinking glass. This is the safe way to remove and refit objectives in their cells without the risk of tipping and jamming. Providing, of course, patience and care is taken.

Later it occurred to me that the filter glass can be placed at the back of the cell on thin, cork pads. This would allow the radial fixing screws [to the main tube] to easily clear the glass. The screws would be nearer central in the cylindrical walls of the cell for better symmetry. The clear open front of the cell will allow easier insertion into the main tube. With the front rim offering a secure finger-hold during fitting and possible removal.

In the end I had simplicity thrust upon me. No need for radial fixing screws nor spacers. The nearest original baffle of the Celestron 6" was almost the same aperture as the energy rejection filter. So I simply drilled and bolted them together. The whole unit will slide the 50cm down the main tube away from the objective. Then can be turned square to the tube to jam it in place.

Job done apart from the cork pads and lining to protect and support the filter glass. CSK screws would have been more sensible but I had nothing in the small size I needed. I can make up the difference in head clearance with more cork. Or just buy some countersunk screws.

I used a steel rule and scalpel to cut two 30mm wide strips from 2mm cork sheet. Then two more strips only 12mm wide. The two wider bands were sprung into the housing after cutting to exact length. Then removed and glued together with white wood glue while in place in the cell. Thanks to the shoulders on the filter housing there was no need to glue the cork to the metal.

I then glued one of the narrower strips flush with the back of the doubled bands. Now I need to decide how to safely hold the filter in its snug cork bed. I have the cork strip ready but can't glue it if I ever want to remove the filter again. Short, radial screws and nuts holding the second cork strip? This seems a bit crude for my tastes. I'll think about it overnight.

 Now I'm thinking about a narrow but thick strip of vinyl or leather to retain the filter glass. Small Csk. screws at 120° should hold it firmly if the strip is stiff enough. So it's back to the charity/thrift shops looking for a thin belt or strap to act as donor material.

The last two images show the 90mm D-ERF mounted in its cell. The cork offers mechanical protection to the filter glass. With the baffle facing the sun, the arrow on the edge of the filter is pointed correctly. The clear aperture is about 88mm and the lining materials will be blocked from heating effects by the baffle. It will be inserted from the focuser end of the main tube since it will be much nearer that end than the objective end. The low angle reflection from the sun/objective side is purple. More a bluish reflection from the focuser side.

Click on any image for an enlargement. 

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