Sunday, October 28, 2012

Eclipse 2012 - Testing, testing, testing...

The total solar eclipse in Australia will take place just 16 days from now. There is only four days until the journey begins!!!

Since the last blog post I have managed to get two more nights with testing the solar imaging setup on stars. Besides just gaining experience handling the setup I have investigated several issues:

Some drift measurements (click to enlarge)
  1. Polar alignment and tracking. Imperfect polar alignment leads to a systematic tracking error that makes the stars appear to drift across the camera detector. Sometimes I set up the mount roughly using a compass and other times I did a careful polar alignment using the Astrotrac polarscope. By shooting several images over a known time period the drift in pixels/second can be measured. Some results are shown in the figure at right - typical drift values are 0.05-0.5 pixels/second (0.1-1 "/second). Conclusion: accurate polar alignment is nice to have but not essential. Drift of the FOV should not be a problem if I tweak the framing 5-10 minutes before totality. Exposure times up to 5-10 seconds at f=640mm should not be significantly smeared due to such drift.
  2. Vibrations: On something like 10-40% of  these star test images do I see effects of vibration. This is the largest threat to obtaining sharp images! I am loading the tripod/astrotrac system to the limit of their specifications and when you do that even slight perturbations can induce oscillations. Conclusion: I expect that wind will be the main threat on eclipse day and that 10-40% of my images will show effects of vibration. There is not much I can do about it at this time.
  3. Refractor collimation: I checked the collimation of the Borg 100ED refractor used for this project using an artificial star. This method is easy to use and highly accurate. Collimation was nearly perfect, but I do not want to suffer from poorly aligned optics on eclipse day. Hence, the collimation gear is coming along. Conclusion: scope was very well collimated and I do have the required equipment (incl. a 7mm eyepiece and 5x powermate) packed.
  4. Resolution: the stellar FWHM in the central region of my images is around 3.3pixels=8". These values are fairly reproducible from focus run to focus run and from night to night. Conclusion: my  focusing hardware/method works. My magnification is not large enough to really sample the variations in seeing.
  5. Field flatness: halfway into these investigations I remembered that the large chips in DSLRs coupled with fast refractors typically do require using a field flattener. Fortunately, I had a TS 2" universal field flattener lying around and could easily pop it in with just the right spacing (120mm) to the DSLR chip. See the image below - the effect is quite substantial. Conclusion: use a flattener when using a DSLR camera!
Effect of using a field flattener
(click to enlarge)
I now feel fairly confident about the functionality of my setup, its limitations and my basic ability to handle it. I'm almost ready to pack!

In addition to photographing the total eclipse I also want to capture some telephoto views of the Southern night sky. In the next blog posting you can read more about how I hope to accomplish that!

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