I decided that I wanted to try a Bahtinov mask for focus tests on the 3 inch refractor, after having tried one on a telephoto lens with some success. The Moon was about so I thought it could be a good test. This image is what I managed purely by using live view to focus. The live view was monitored on a tablet, by using the HDMI output from the camera, and a video capture dongle.
It doesn’t look too bad; poor at the edges, as before when using only one ring of extension tubes with the eyepiece for projection. It’s a better quality if I use more rings, and a smaller field of view with the eyepiece.
However I tried this setup with a Bahtinov mask, made by printing the grating pattern on transparent film.
This was then mounted on the end of an old cardboard container:
The tube is big enough to pop loosely over the end of the refractor.
The mask works by generating diffraction spikes perpendicular to the edges; a brighter spike (larger area), plus two slightly fainter spikes at 30 degrees offset each way. Because the central spike is on one half of the aperture, it works like the famous Hartmann focus test; as you move through the focus position, the central spike shifts sideways between the other two which are crossed. Here’s the image of a star in focus (pretty well):
… and here’s an image that’s not in focus:
It works pretty well.
Unfortunately, in my excitement I then forgot to take the mask off before taking some trial Moon exposures – I really couldn’t understand why they were darker, and looked out of focus! All the sharp endes of craters etc were being diffracted into a mess. That’s why you have to use a star to set up the focus. Another thing learned – my learning curve is a bit slow in this astrophotography lark!