Wednesday, September 19, 2012

Deep sky objects and stacking

I've seen lots of awesome photos of deep sky objects. I always thought they are so small that you can't see them with your naked eye... But no. They're huge. Sky is full of galaxies and nebulae and many of them are bigger than the Moon. The problem is, they're too dim to see.

A telescope collects light from a bigger area than your eye, so it helps to see some objects. An other way to collect more light is long exposure photos. Ok... no news to people who knows about astrophotography, but I don't know who I'm writing this for. Problem with long exposures is the rotation of Earth, or the apparent movement of the sky. You can compensate it with a tracking mount, but better mounts cost more...

I'm still doing this with budget equipment. My kit:
  • Canon EOS 1100D - 300e
  • 75-300mm lens - 100e
  • EQ 3-2 with one motor - I bought it used for 200e
  • Stuff to put this all together - ~50e
Camera and lens are something one might already have and use for other stuff as well. So if you think of the total price, it would be here about 250e.

Now to the point: stacking pictures. With this budget kit, I can take 15 second exposures. Longer than that and the stars would trail. With better alignment for the mount I could probably go as high as 1 minute, but for now I settle for 15 s. A deep sky object might require several minutes exposure to be seen and perhaps hours to look good. I take a lot of photos with short exposures and then stack them together with specialized software. I've been using Deepskystacker but there are others as well.

To demonstrate the difference between a single 15 second exposure and 15 minutes of them stacked together, I'll show my first "masterpiece": Andromeda galaxy:

A single 15 second exposure


15 minute stack of 15 second exposures
Is there a difference? You can definitely see more of the galaxy. I am going to shoot the galaxy some more the next time it's possible and post an updated version. Perhaps this time I don't fail and adjust the focus in the middle of session so that the rest 30 minutes of exposures are out of focus...