Tuesday, May 27, 2008

High Dynamic Range Moon

The eye and brain has an amazing ability to be able to perceive things that are intrinsically very bright and also very faint at the same time - and in astronomy this is most evident as one looks at the crescent Moon.

Once the sky darkens when the Moon is just a few days old and sinking fast after the setting sun, one can easily glimpse the phenomena that is Earth Shine. This is where the light from our star, the Sun, is reflected off the still sunlit parts of the Earth's surface, and is reflected back out towards the Moon. It then reflects for a second time back to the Earth whereby we can juts perceive it as a ghostly image of the lunar surface. The effect is most pronounced early and late in the Moons cycle as the bright Sun lit crescent is small compared to the faint Earth lit globe.

Note, you must have your monitor correctly calibrated to see this properly, otherwise it might all look a little bit dark. Try this website to get a rough setup.


Digital cameras are not as good as the eye in this respect. Take a picture of the Moon and you'll find that you cannot correctly expose both the darker Earth lit surface and the bright Sun lit crescent at the same time. Why is this? Well, a digital camera has what we call a fixed dynamic range. That is, it has a finite range of brightnesses that it can capture in one go, and it's generally not as good the the Good 'Ole Eye!

To get around this problem, one can take multiple images at different exposures and use a photo package such as Photoshop to blend them together to create what has been coined a "High Dynamic Range" (or HDR) image. I won't go into detail here about how I did it, just do a Google for HDR blending. In my case, I took exposures from 0.25 to 30 seconds through my ED80 'scope and blended them together. The result was to capture both the brightest part of the Mo
on (magnitude -7.2 according to calsky) to the faintest star just to the top left of the Moon at magnitude 10.1 (according to Sky Charts). Quite some range!

Here's a tighter crop of the Moon showing more of the details of the "shadowed" Maria - note that it looks just like a filtered down full moon as the light source is, for all intents and purposes, coming from the same angle as that for a full moon - albeit with somewhat less illumination. Easily visible are the major Maria, Tycho with its outspread rays, Copernicus, and the "bright" Aristarchus near the far left limb.

Tuesday, May 13, 2008

Saturn

As part of our Mercury evening at Bristol Astronomical Societies observatory last week we took the time to image Saturn whilst the darkness kicked in ready for some deeper sky observing.

We used the prime focus (or Newtonian focus for you purists!) webcam imaging with my ToUCam Pro and IR filter. After initial focusing the firmware was updated to
utilise the Colour-RAW mod using WcRmac for the best results we can get from this webcam.

After experimenting with a 2x, 4x, and no barlow lens, the 2x produced the best overall results given the steady (but rather hazy) conditions. I rather like the result! Depending on which monitor/TFT I view this on, it can look a little dark .... I think I need to calibrate my monitors!


Technical Details: Cyril Swindon is a 12" Newtonian reflector with a newly furbished motor and fine RA controller which worked flawlessly. For such a large scope, it was surprisingly easy to aim, focus, and fine track. 2000 frames were captured, and just under 1000 used to create the final stack. The resulting TIFF was then exported and wavelets, colour balancing, noise reduction, rotation, and cropping was performed with PixInsightLE (which gives, in my opinion, far more controllable results than Registax).

Monday, May 12, 2008

The Innermost Planet

I've only ever seen Mercury once before, and it was a rather special occasion. That evening my fiancé and I watched the International Space Station skim overhead, and spotted all the planets out to Saturn in one sitting.

Mercury was hard to spot initially, but from our high vantage point we finally saw it come out of the murk over Bristol. Venus was easy, beaten only be Earth, and Mars was obvious with its redness in the evening sky. Jupiter and Saturn were obvious to the trained eye that noticed the intruding "stars" in their host constellation, but easily pointed out.

Last week our local astronomical society took the chance the weather gave to us to go out for a Mercury spotting session - possibly the only chance we'd get this year. This was the first image taken after sundown, and the scene was just beautiful. From the low lying orange clouds on the horizon to the deep blue sky overhead and the Moon with its Earth shine reminding us how bright we must seen to our watching companion.

In this image, Mercury can be seen nestling down towards the orange hazr in the bottom right, and the stars Alnath and Hassaleh can be seen just above the moon and to the right of the moon respectively.


This was the first of a number of targets that night ... so more to follow soon ....

Sunday, May 11, 2008

We had some pretty impressive electrical and thunder storms the other night, and not to miss a trick, I set up my 10D on a tripod pointing out the window and set it going, hoping to capture some lightning - confidence was high considering the rate at which it was forking down to Earth.

Out of over 120 frames taken, I captured lightning on about 6 of them. This was the best by far - and what a stonker it was too! It seemed to weave around in circles, in and out of the clouds, before making a very decisive beeline down to Earth. I think I was lucky to get the result I did, but I'm not complaining :)

Whilst watching the storm, there were 2 strikes about 40 seconds apart that forked down pretty close by, but behind the houses over the road. Straight after hitting home there was the most eerie of sights - a large hemisphere of electric blue rose up from behind the house, flickering ever so slightly over a period of about two seconds before quickly shrinking down and being replaced with an auroral-green dome which lasted about a second before fading. This happened twice in a row at the same spot .... I wonder if it stuck some power lines or similar - I'll never know, but it really gave me shivers down my spine when I saw it.


Few technical details now. All my images were taken with a 28mm lens stopped right down to get the longest exposure I could (8 seconds) whilst keeping the sky dark, but not black. This way, when lightning did strike, firstly it would not be washed out by the sky, and secondly with the small aperture it would not bloom too much (the lightning is *very* bright compared to the ambient light levels). It was just down to luck after that - though I could see which area of sky was producing the best strikes so at least I would have a fighting chance!