First off: If this is your first total solar eclipse, don't bother with photographing C2. WATCH IT! You don't want to be boring behind the camera when the magic happens. You need to see it with your own eyes. Then use binoculars to take a look at the corona. You will regret not looking directly if you spend the entire eclipse watching through a view finder or video feed.
So let's say the remainder of this entry is for veterans. You know what C2 looks like and you'd like to get an image. Just when it is "safe" to remove the filter. Under a clear sky (no clouds in the way) I would say no more than 15 seconds before C2. And yet here is the rub - some predictions of eclipse timings can be off by that amount!
Why the error? A number of factors with most being your own. Starting with an accurate timer - got one? Using a short wave radio to get the time synch signals works best. It will be off slightly depending on propagation delay times. Internet time could be off slightly, same with the time on your cell phone. We are talking about getting the time to within 5 or fewer seconds for this to work best. About the best timing that is easy to get is from a GPS. But even then, make sure it is really accurate. A cheap GPS will not do the trick. A good GPS also helps in getting your location locked down.
So given a good location and good time, you can plan to remove the filter about 15-30 seconds before C2 is scheduled to appear.
Note that you may need to focus slightly. Some solar filters refract more than others. For this reason it may be advised to use a higher focal ratio as the depth of focus is greater. Note this is DEPTH of FOCUS, a technical term for astrophotographers. Click here for a great write up can be found about Depth of Focus and other related topics.
The hardest part is making sure the image is in the field. Using a DSLR with a video view finder (NOT a direct optical path), you can accomplish this quickly. If using a direct optical path you might be able to see the image projected onto the glass from the side. DO NOT LOOK DIRECTLY!
Automation is best, when it works right. I have tried a number of methods with varying success and will explore that in a later blog or two. Getting the automation to start at the right time is best solved manually. So if you have an exposure sequence programmed, you can start it up about 5-10 seconds before C2 and get great results. Fast exposures are needed at C2 just like C3. Click here for an eclipse photography exposure calculator.
I've had some success with C2 images but find that watching it is still the real fun.
|2008 TSE C2 from jet - double image caused by window of aircraft|