Wednesday, February 18, 2009

Comet C/2007 N3 (Lulin) - 67 Image Stack: f/2.8, ISO3200, 2 seconds

This shot was the product of 67 individual 2-second photos at ISO3200, f/2.8 of Comet C/2007 N3, aka "Comet Lulin."

This comet's green color comes from a type of carbon and cyanogen, a poisonous gas. View this shot to get an idea of how insignificant this comet is in the context of the whole night sky. If you look closely, you can see the faint tail of the comet extending to the lower-left.

Equipment used: Canon 40D , Canon 100mm f/2.8 Macro Lens, Bogen/Manfrotto 190XPROB Tripod Legs, Bogen/Manfrotto 322RC2 Grip Action Ballhead

I shot these in the early morning hours of February 18, 2009, from 12:15AM to 12:25AM EST from my deck, (an hour north of Philadelphia, PA), with a horribly light-polluted sky. Weather-permitting (and it's not looking cooperative over the next few days), I'm planning on finding a darker area to shoot this comet once more, with hopefully even better results.

This wasn't a very well-planned event. I never head about the comet until a couple hours prior from fyngyrz's post. To find out when it would be visible here, I used this interactive sky chart to get my bearings as to where I'm looking, and this page to find out where the comet currently was (look for the downloadable PDFs). 3AM would have been a better time to take this, with the comet higher in the sky and further away from the horizon's light pollution, but this camera gear doesn't pay for itself - I had to wake up at 5:30AM.

Now, from what little I know about astrophotography, it's important to take your shot quickly to avoid star trails - within a second or two. You'd be surprised how quickly those starts move across the sky. To take shots this quickly, you have two options: a very high ISO, and a very wide aperture. Ideally, you go with both. Now, as with anything in photography, there's tradeoffs with both of these options.

With the high ISO, you're introducing a lot of grain. My Canon 40D goes up to ISO1600 with a "pushed" ISO3200. By "pushed", it's really just ISO1600 that's boosted in-camera one stop brighter. Pushing introduces a lot more noise than would be present at the already-high ISO1600.

With the wide aperture, you're more likely to get chromatic aberration (CA). Ideally, I'd like to use my f/1.4 50mm wide open. However, due to the CA that's introduced at that aperture, I get very distinct bands of purple fringing around every star, ruining the shot. I find that this CA drops significantly at f/1.6, almost going away at f/2.0.

So where did I end up with these options? Well, I took to series of shots - one with my 50mm f/1.4, and one with my 100mm f/2.8 macro. With the 50mm, I tried f/2.0 to reduce the CA, and with the 100mm, I used its widest aperture of f/2.8.

Each of the individual shots of this comet are (necessarily) terrible. The grain makes them almost a waste of time to look at. There are several types of noise introduced by the shot including: high ISO, sensor heat, and atmospheric - each of these degrade the image. However, these noises are random. The underlying image that I'm shooting isn't. It's possible to remove the noise with a process called "image stacking."

Basically, image stacking involves layering several photos of the same subject on top of each other, varying the transparency of each photo through the stack, with the most transparent being at the top. You blend the photos together with a subtraction algorithm so that anything that doesn't exist in the other photos -- the random noise -- is ignored. It's a tedious process, but well worth the time, because you're getting what would otherwise be an impossible shot.

To image stack for the comet in this series, I took 67 individual photos, each at ISO3200, f/2.8, and 2 seconds. I then used Apple Aperture to crop them down to the size you see here. I was as careful as I could be to line up the crops with each other - remember, the earth is spinning fast enough that within 20 minutes, the comet will move out of frame! Lining these up took a considerable amount of mind-numbing time. After cropping, I brightened them up a half stop, then exported them all as JPGs.

I then used the image-stacking software called Lynkeos to automatically line up the photos better, then stack them together. I could have done this manually with Photoshop, but was more than happy that Lynkeos did a fairly decent job on its own. I exported from Lynkeos to TIFF format, then post-processed with Photoshop and Aperture to get the most out of the shot that I could.

I plan on stacking the shots from my 50mm in the next couple of days. It'll be interesting to see how that extra stop of light makes a difference. Unfortunately, the comet will be much smaller due to the wider focal length.


Anonymous said...

you need a "scotch mount" or cheap used equatorial mount with a RA tracking motor,plus Telephoto =hours of fun

Blake said...

Dave - You're absolutely right... I'm saving for the 70-200mm f/2.8 IS right now, but after that, I'm looking for a good tracking mount. Those Astrotrac mounts look pretty cool - I have a lot more research to do before I drop that kind of cash.

In the meantime, I might try making a barn door mount and see how far that gets me.