Friday, October 26, 2012

Eclipse 2012 - Nighttime forays

Last week I finally got out under the stars with the solar imaging setup. Having tried it out with good success in the daytime the next level was obviously to do it during the night, using the moon and background stars as test targets.

Set up running at night (click to enlarge).
I got up around 4.30am and went to our garage which is located a couple of hundred meters from our house. I had to go down there because I needed a clear eastern horizon where the low, waning moon would be visible. The position in the sky was similar to where the Sun will be during the eclipse and hence the mechanical arrangement of the setup would be tested under realistic conditions. Polar alignment was done casually by just leveling the mount and aligning roughly towards north. Setting up went well - just twenty minutes after arriving on site I had focused and framed the moon in the camera. Tens minutes later I had executed a test script using Eclipse Orchestrator and was packing up. The entire operation took only 40 minutes! When packing up I discovered that the knob holding the tripod vertical shaft was not fully tightened. Whence the first lesson from this night: remember to check all knobs and make sure that they are tight!

Looking though the resulting images in the following days showed that the camera did respond to changing settings being sent from the PC - great (and unlike the situation I suffered from during the total eclipse in China!!). A typical image is shown below. I quickly zoomed in on the boxed region to investigate the images more closely.

A 2.5 second exposure at ISO200 made through the 100mm f/6.4 refractor.
The image above tells me several things. First the lunar disc size is very close to what I expected and hence the corona will fit according to plan (inner + middle regions are OK, outer regions not fully covered). Second, I should remember to rotate the camera so that east-west appears roughly horizontal (unlike in the image above!) since the corona is widest in this direction. Finally, the earthshine is well sampled; i.e. the choices made automatically by the script wizard in Eclipse Orchestrator seem to be reasonable.

Exposures like the one above also captured several stars. Measuring the diameter of these on several images  gives a FWHM of approximately 3.2 pixels or 7.7 arcseconds. This is quite close to what I expected from this setup. Nice.

However, roughly 20% of all the images revealed some sort of vibration. There was also a 30 second period where the tracking was poor so that images were smeared in right ascension. I do not know why. This really illustrates a main problem for this setup: vibrations and tracking! Hopefully, I will fare better next time by remembering to tighten everything.

Three images. The top is good - showing 3 pixel resolution. The middle suffers
from vibrations while the lower image reveals poor tracking. (click to enlarge)
I'll blog more after I have been out under the stars again. I need more experience with the setup. I also want to find out whether vibrations and tracking can be improved upon. I also need to know more about polar alignment issues and how much declination drift to expect. So much to do and so little time left!

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