Sunday 13 January 2013

Sunday Sun and Circumzenithal Arcs - 13th January

I was optimistic sunday would offer something in the way of clear skies following a night of freezing temperatures, however unsurprisingly the clouds played a game of cat and mouse all day; it would often be clear blue skies overhead, but around the horizon a mixture of haze, cloud and intermittent fog often conspired to hide the sun.  It wasn't until late afternoon that all this thinned down enough to warrant me getting the double stacked PST.

Even then the sky was far from clear, with the usual array of contrails and high cloud, however there was an optical effect that caught my eye - a circumzenithal arc.  This is the rainbow like smile shape at the centre of the frame opposite.  Circumzenithal Arcs (CZAs)  it arises from refraction of sunlight through horizontally-oriented ice crystals, generally in cirrus clouds, rather than from raindrops.  More details on them can be found here.  This picture was taken with the HD Hero2 GoPro camera whose near fisheye lens is going to prove very useful for capturing optical effects like this.

The shot with the DS PST was by no means straightforward and took some getting too.  I had decided as i'm still shooting on the photo tripod that I was going to use a focal reducer to get the whole disk on the chip of the DMK31.  I rapidly realised the PST (as usual!) did not have enough back focus and so decided to remove the section of the eyepeice holder that holds the Induced Transmission Filter (ITF) - this had the effect of flooding the DMK with infra red radiation however would reach  focus.  I decided to resolve this by using a peice of heat absorbing KG5 glass in the optical train and using a Baader 7nm Ha filter on the front of the external etalon as an ERF.  This sorted things out.  The extra field of view the focal reducer gave me, along with keeping the exposure shorter than a 1/30 second meant I was recording at the full 30fps the DMK is capable of and as a result managed to record avis of nearly 2000 frames.  Despite as can be seen from the sky shot above there was passing clouds, the image of the sun came out very well for the conditions.  The other positive was that it was fairly obvious looking today the sun is slowly but surely getting higher and higher in the sky which is only a good thing for solar observing!