With the later nights coming in, my thoughts inevitably turn to the long, observation-free horror that is the English Summer. So I decided to make a solar filter for my 5″ Mak. You can buy these pre-made for a variety of sizes of telescope, but I prefer the Blue Peter Challenge of making things myself from household objects.
So I hotfooted it down to Charles at MC2 Telescope and spent a few quid on a sheet of Baader Solar Film - free cup of coffee thrown in, thanks Charles. The film comes as an A4 (297 x 210mm) sheet which can be cut to size. For larger scopes it is best to make an off-axis mask, and indeed your scope may well come with one built into the front cap, but with my Mak having a central obstruction a full aperture filter is best.
The next job was to work out how to securely attach the filter to the telescope. Baader recommend making a holder from cardboard, but that struck me as being a little flimsy and I wanted something that wouldn’t degrade over time in my garage.
So as I wandered around the garage my gaze alighted on our paint cupboard – you know the one, just like yours it’s full of 3/4 empty cans of paint. It so happens that the new plastic type 2.5 litre cans are 150mm inside diameter, just right to slip over the 145mm OTA with a bit of room to spare.
I picked the one that was closest to empty – dusky pink, daughter’s room, since you ask – and checked with my other half that we wouldn’t be needing it any more.
Now I know what you’re thinking – how’s he ever going to get it clean? Well, large amounts of hot soapy water, a good handful of wire wool, and about 40 minutes of elbow grease, and voila! Clean as a whistle. Make sure you use an indoor matt or silk emulsion for this – it won’t work with gloss which is oil-based and a total nightmare to get off.
I then cut the bottom of the can off, since I wanted to use the top – with the lid attached it gives the can much more ridigity and provides a good base for attaching the filter.
I also carefully cut around the inside of the lid, just leaving the structural part which ensures a tight fit to the can body.
Then I lined the paint can with a couple of layers of what the craft shop woman described as “funky foam” – it is about 1.5mm thick and comes in sheets for kids to cut out shapes and generally make a gluey mess. This gave a snug fit over the OTA – this is one filter you do NOT want to fall off! For this reason I also added an eye fixing to the outside of the can – using the sawn-off top of a water bottle stopper (£1 from poundland). This allows the filter to be tethered to the scope for extra security.
For the filter itself I followed the Baader recommended approach of sandwiching the filter between two rings with double-sided sticky tape, but instead of cardboard I used foamboard. This is a fantastic material to work with, since it cuts like card, has the rigidity of thick plastic and is a fraction of the weight. It is available at your local craft shop in A4 and upward sheets, and only costs a couple of quid. I chose white since it is less likely to get hot in use. Then I araldited the filter to the top of the can, and let the whole thing dry solid.
The completed item feels solid and is a snug, but not tight, fit over the OTA. I then turned my attention to making a glare shield, again out of foamboard. This fits over the rear telescope fitting and the focuser spindle, and increases the contrast of the view by blocking sunlight from around the eyepiece.
Finding the sun can be tricky, as normal optical methods are not safe to use. I was planning to make a solar finder using an RDF base as the starting point, but in the course of testing the kit I noticed that the copper pipe I had used to make a carry handle for the mak was projecting a perfect crosshairs target onto the glare shield. Bonus!
Once I had aligned the telescope the view through my Baader Hyperion was lovely. This is only a white light filter, so no PST-style prominences or granularity here, but a large group of sunspots were visible, with a surprising amount of structure available. Adding a no. 23 orange filter improves the view even further. The photo from SOHO below gives a reasonable simulation of the sunspot view and resolution as I saw it (although you will not see the granulation shown in the image).
This is a great cheap way of doing daytime astronomy, and will keep me busy until such time as I can afford/justify a PST or Lunt scope.