Sunday, April 22, 2012

Shooting the Sun

I celebrated what is so far the best weather of 2012 by getting my telescope out after lunch and experimenting with imaging the sun. I had purchased an Orion solar filter for my 80mm Orion EON refractor earlier in the year but this is the first time I've been sufficiently motivated to try to use it.

I first very roughly aligned my telescope with the earth's polar axis (I knew about where Polaris is) and then moved the scope so that its shadow was minimized in order to get it pointed at the sun. I then mounted my filter over the dew shade. I had a feeling that the sun would still be bright enough through the filter that my astronomical CCD camera would be overly sensitive and unable to achieve a short enough exposure, so I started out using my Nikon D40. I mounted it using the T-adapter to a 2" barrel with SCT threads. I centered and roughly found focus using my D40 view-finder, but the focusing was a challenge because I needed to utilize the sun-spots to refine the focus and they were very small (low mag prime-focus photography). I resorted to eye-balling it and taking numerous exposures with small variations in the focus. Before this, I did some exposure experiments and found that about 1/800th of a second was somewhere close to optimum. What I found later was that I still had to decrease the brightness somewhat, so next time I'll probably go with a bit shorter exposure somewhere between 1/800th and 1/1250th. My best result in shown below. You can see that there were quite a few large sunspots visible. At this point I'm not sure how much better the result could have been. Tube currents due to solar heating of the scope are a limiter, but I think there was probably room for better focus.

This activity at least gets me somewhat prepared for the annular eclipse next month in Northern California and the Venus transit of the sun on June 5th.