It was whilst I was making a solar filter for my telescope that I seriously started to think about how I was to find the object of study in the eyepiece. I knew that one recommended method was to move the scope such that the shadow cast by it onto the ground was minimal i.e. a round telescope tube's silhouette would be circular when directly aligned with the sun. If the 'scope was not aligned then the shadow cast would become elongated.
I realised that for next to zero cost, I could fabricate a finder that could fit in the existing finderscope mount thus solving a secondary problem, that of the original finderscope if left without lens caps could train a focused image of the sun onto the top of my head!
32mm (1¼") diameter offcut of PVC water pipe (you could use a cardboard tube)
1 piece 50mm (2") wide masking tape
1 square cut from a silvered wine bag liner or similar opaque flexible material such as cooking foil...
2 elastic bands
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Step 1: Construction
I based the finderscope on the principle of the camera obscura, that is a pinhole pierced in an opaque film at the front of a tube would allow the sun's image to be projected onto a translucent screen at the other end (also a little like a pinhole camera with the film being replaced by the translucent screen).
I had a short length of 32mm (1¼") diameter PVC pipe and I covered one end with a piece of opaque, silvered plastic from a 'bag in box' wine dispenser. I used an elastic band to attach the square of film. The 'screen' at the other end of the tube was simply a piece of 50mm (2") wide masking tape stretched over the end and adhered around the wall of the tube. I pierced a tiny hole in the approximate centre of the silvered plastic film.
I removed the original finderscope from the telescope and slid the plastic tube in place and attached an elastic band to the outside of the tube (see photo above)), this fitted into the finderscope mount. The finderscope on my Celestron C8 is a 6 x 30 and this meant that the 32mm outside diameter PVC tube fitted perfectly and still allowed for adjustment using the screws on the finderscope mount.
Step 2: First Light & Modifications
Once the finderscope was in place and the filter attached to the telescope, I was then able to align the finderscope.
Firstly I had to find the sun in the main telescope (with the solar filter in place) for this I used the technique described in the introduction. Once I was able to get the image of the sun central in the eyepiece of the telescope, it was then possible to adjust the finderscope so that the bright image of the sun was visible on the masking tape 'screen' (see image above).
Final adjustments were made on the finderscope to centralise the image.
The offcut tube I used was 350mm (14") long. This was a quick job as I needed the scope for immediate use. A shorter length tube could be used and it would give a much wider field of view in which to find the sun and this image would be smaller but brighter. This finderscope would also work with a larger diameter PVC tube. However, I used the scrap I had to hand and this fitted the existing finderscope mount.
Here's another example of its use for the transit of Mercury:
You can find more optical and astronomy articles on my blog, including how to make a locking solar filter for your 'scope here and more of my films here
Wishing you Clear Skies, especially for the 21st of August!
Participated in the
Unusual Uses Challenge 2017