Sunday, December 26, 2010

The Earth moved (under my feet)

First, Season's Greetings, everyone!

I know, not your typical greeting card, but I was think about riding my motorcycle in the snow, then decided it was too cold and had a virtual ride instead!
There was a Lunar Eclipse.
Which you already know, unless you're an ostrich. I’m going to go through the thought process and technique for creating the image I did. Perhaps you will find it instructive and give you some idea of things to try. If you’re not a photographer and not interested in the details, just scroll down to the photo.

The eclipse started at around 1:30 am and ended several hours later. Well, it was cold! -15°C. Since the earth moves (yeah, yeah. I already did those jokes), I couldn’t just set the camera up, turn on the interval timer and go to bed. Furthermore, the exposure changes.

My first idea was to do a composite multiple exposure image in camera. I did a little arithmetic (360°/day = 15°/hour, so a 180mm lens on a 35mm camera (120mm on a DSLR) which the internet says shows about 13° along the diagonal so if you get the movement perfect, you’ve got less than an hour). I tested it with 4 shots at 5 minute intervals and found it to be relatively accurate. So I have a composite picture of 4 small moons about a lunar diameter and a half apart. Not great. and I’d have to be right on, making sure the camera pointed directly along the path the moon would follow.

Then there’s the exposure issue. The bright full moon was properly exposed at 1/200 sec at f/11, ISO 100. Clearly, I would need a longer exposure at totality. Anyone care to guess how much? Well the difference between the full moon shot at 8:30pm and a totality image at 3:30 am was... wait for it... OVER 10 STOPS! The best totality shot was 1.6 seconds at f/5.6, ISO 400.

So clearly, I wasn’t going to be able to automate this process. I decided to shoot individual shots at maximum magnification (which for me is 400mm – my 70-200 lens and 2x teleconverter. Experience tells me that to get a sharp image with that combination, at anything under 1/100 second even on the tripod, I would have to do everything right. The results show that I was correct. More later.

I did a test around 8:30pm, as I said 1/200 sec was the best exposure. I used the self-timer to stabilize the camera. An alternative method was to use the mirror-up setting and a cable release. I locked the mirror up with one push of the release, waited several seconds for the camera to stabilize, then released the shutter. That worked all right down to about ¼ second. Slower than that, things got a little fuzzy because, well, the earth moves. With the angle of view of the 400mm lens combination (less than 5° along the diagonal, you can see definite motion trails in exposures as short as 1 second. I didn’t realize that at the time so it was lucky that I bracketed my shots.

The Nikon D300 can be set to bracket up to 9 exposures – I chose to do 5 – at 1 stop apart. By the way, you can set the interval timer to do bursts too: I tried it – I did four 5-shot bursts 5 seconds apart, all automatically! Not good for this eclipse thing, but something to file away for future use.

I figured out what the moon’s path was going to be with the help of the Star Walk app on my iPad. In hindsight, pretty obvious: at the winter solstice, full moon directly opposite the sun, due East to due West. I found a spot for the camera where ambient light wouldn’t interfere and where I had a clear view to the West.

So I sat at the camera, out in the cold for 2 hours doin’ ma thang. Brrr. A couple of times I went inside, but the hassle of de-fogging my glasses, getting dressed and undressed was too much and besides as the eclipse started, it was fascinating to watch. So I stayed out there for the most part.

By the way, you go through batteries like mad in the cold! Especially if you use LiveView to help focus and frame. I used up 3 batteries.

I was not happy with my tripod/head. I have a good one – a Gitzo heavy duty tripod with a Manfrotto ball head. But at 400mm, when you centre the moon in the viewfinder and lock down the head, it moves so much that it’s actually touching the bottom of the frame. And I had put the tripod mount on the lens collar so it was balanced, too. Somewhere on my wish list is a Wimberley mount...

At 3:30 am I called it a night, since the moon had been in totality for the better part of an hour and I was too tired to calculate how long before it would start to come out of the penumbra. I took the CF card out of the camera, put the camera and lens in an airtight drybag to warm up in dry air, and went inside. I immediately uploaded the images to the computer and was a little disappointed with the sharpness of the totality images which had been at really long exposures.

My goal was to put multiple exposures on one image. So I opened up a new image. I decided I wanted about 1000 pixels per lunar image and I had pre-selected 13 frames. A little air on either side meant that I needed 16000 pixels. A HUGE file.

I wanted the moon to follow a curved path. So I used the elliptical selection tool and scaled it up much bigger than the image itself so I could see the path I wanted. Then I filled everything with black and stroked the path with white and put it on its own layer, which I would use but make invisible before printing. Now I brought in the selected images and placed them along the white curve. They overlapped so I changed the blend mode for each layer to “lighten” – remember I was working on a solid black background.

The last two images, by the way, were shot without the teleconverter so I had to scale them up by 200% to make them match. I tweaked the exposure values for each layer as I went along so that everything matched, more or less.

Finally, I added some text layers, a keyline to define the edge of the image and that 8:30pm really sharp moon photo to balance the page. Here’s the result. Was it worth it? You tell me..

You can see a larger image by clicking on the picture, and an even larger one by going to my Smugmug site here. Not the full 16,000 pixel one, though -- that's saved for printing and hopefully for sale.
I haven’t printed it yet. I got the order process wrong at Costco for a poster and will have to go back again next week. I’ll let you know how it turns out. They can do posters up to 24x36 for $20. We’ll see...

I have more images to show you of other stuff but I’ve written too much here. Next time!