Astrophotography with a camera lens and a fixed tripod - Part 3
I've pulled from my archives a couple of other examples of astrophotography using a camera lens and a fixed tripod that some readers may find interesting.
Comets
The first example is from a sequence of images I took of comet Holmes (17P/Holmes) back in 2007. Comet Holmes wasn't a spectacularly bright comet, but it was quite visible to the naked eye from darker locations and I believe it was about half angular size of the moon when at its largest (though that wasn't when it was its brightest). The first image is a crop from a shot made using a Canon EF70-300/4-5.6 zoom at a 120mm focal length setting with an exposure of 1 second at ISO 3200
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Some level adjustments have been made to this exposure to bring out the fainter stars
The second image is a crop from a shot made using a Canon EF 500/4.5 at f4.5 and with a 1.6 second exposure at ISO3200. Looked at closely, the star images are slight trails.
Crop from EOS 40D image using an EF500/4.5L lens, 1.6s at f4.5, ISO3200
Most bright comets show a more direction tail than comet Holmes did when these images were taken, so this may not look like what you might traditionally expect a comet to look like, but it is how comet Holmes looked on October 27th 2007.
Naked eye brightness comets aren't very frequent, but one usually comes along every couple of years. Very bright comets are much more rare and you might only see one each decade. The orbits of thousands of comets are well known and their position in the sky can be calculated quite accurately. Their brightness can also be estimated. However the interesting thing though is that you never know when a new comet will appear and how bright it will be. So although no major bright comets are predicted to appear this year, that doesn't mean one couldn't be discovered tomorrow.
The brightest predicted comet for 2012 goes by the name of C/2009 P1 (Garradd) and might just be visible to the naked eye from very dark locations through March 2012 after which it will get dimmer. Although it may be difficult to see without a telescope it should be fairly easy to see with binoculars and to photograph as long as you know where to point the camera. A web page at http://www.aerith.net/comet/catalog/2009P1/2009P1.html shows it's position in the sky throughout 2012
Comet Holmes will return in 2014, but normally it's a faint comet visible only in large telescopes. The brightening (by a factor of around 500,000x) in 2007 was quite unusual, though it could happen again. It was also bright in 1892 when it was discovered.
Star Fields (Constellations) - Orion
Finally here's an example of an image of the constellation of Orion I shot a few nights ago. I'm in suburban NJ with fairly bright skies at night. This doesn't make star photography easy. For this image the basic equipment was a Canon EOS 7D and a Canon EF 50/1.8 lens. I focused on one of the brighter stars using manual focus and Live View. ISO was set to 1600 and the exposure was 3.2 seconds. First, here's the shot as it come out of the camera as a JPEG using auto white balance.
Camera JPEG, EOS 7D, EF 50/1.8, 3.2s @f1.8 ISO 1600
As you can see, the sky isn't black as it should ideally be. The night sky in this area is bright enough to cause an overall background "fogging" of the image. It's a little darker at the top than the bottom and this is because (a) there's some vignetting since the lens was wide open and (b) the sky is brighter closer to the horizon, because that's where all the artificial light is coming from.
US East coast from space. You can see why my pictures have bright skies!
However we don't have to accept the JPEG that the camera puts out. I shot this in RAW and JPEG and since the RAW file gives you more latitude for corrections, I first opened the image in DPP and edited it. The first step was white balance correction. I corrected the background to a neutral grey rather than the red/magenta in the original image. I'm not saying that a neutral grey is the correct color of the background sky from my NJ location, but it looks better that way! Most of the sky glow is from artificial light with a warm color temperature around 3500K, but I just used the "eye droper" to select the background sky and set it to a neutral grey. The image then looked like the image below on the left. On the right is the same image, but with -0.5 brightness adjustment, -5 shadow adjustment and color blur correction turned on:
Left - White balance corrected in DPP; Right - White balance and density corrected in DPP
Finally the image was cropped and a reduced size version is shown below. In this image stars down to magnitude 9 are visible in the uncropped full size original. The faintest stars visible to the naked eye under dark skies are magnitude 6 and in an urban location the faintest stars you can see are magnitude 3. The difference between the image and the naked eye view in this case is 6 magnitudes. Each magnitude represents a relative brightness of 2.512 (actually the 5th root of 100), so if we can see stars that are 6 magnitudes fainter on the image than by eye, that means we are seeing stars that are 251 times fainter. Not bad for a 3.2 second exposure with an inexpensive lens! Note that at 3.2 seconds, the stars are just starting to show evidence of trailing when the image is viewed at 100%.
The diffuse bright area about 1/3 of the way up the frame is the Orion Nebula (M42) a large cloud of gas illuminated by stars embedded in it. It's about 1340 light years from earth and is a location of very active star formation.
Tracking mounts
If you want to get a little more serious about astrophotography you will need a tracking mount. That's a mount that moves the telescope in such as way as to keep a star or planet stationary in the photographic field. The tracking mount can also be used with a normal camera mounted on it if you want to take wide angle shots of the sky or use your telephoto lens.
Here's an example for $599.99.That's no more expensive than a mid range consumer camera lens. Add on around $80 for the electronics and motor for tracking.
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