Monday, July 23, 2007

A discussion of frame rate and shutter speed

I've received a couple of emails to ask about how to get good planet images. Frankly, I ain't very skillful but if you care to know how I think, here it is.

Let's forget what you hear about frame rate, shutter speed, or whatever in your mind first. Now, let's start from scratch about what contributes to a good planet image:

1. High quality raw frames: it simply means clean and correctly exposed image. We do stacking for this reason, to reduce noise.

2. More high quality raw frames: hey, please don't trick me, no I ain't. Some people said higher fps is always good, because we have more frames to choose from, to freeze poor seeing, etc. However, I would say it's only part of the story. What we want is not millions of frames, but instead, we want hundreds of good frames!

Under pooor seeing, one would say, we need to get more frames to choose from, to freeze seeing. But this usually mean that we will need to use faster shutter. If you want to use 30 fps, you will need 1/30s or faster. That means underexposure or higher gain. Underexposure means less signal, which we won't want. Higher gain means more noise, which we won't want too.

So, how to combat seeing? Let's watch the following video:

Very bad, right? One can say we can get more frames so that we can pick the best from! So, grabbing more frames means a higher chance of good frames. That's right, but consider that, we are collecting a lot of rubbish frames too. And the worst thing is, those best frames are compromised at the same time, for they're either underexposed or noiser than what we can get with slower shutter and lower gain!

The net effect is, we generates a lot of rubbish, and we compromise also our good frames!

One would say that we have no choice, but it's the best compromise under poor seeing condition. I would say, no, that's not. If you spend time in the night sky, you will know poor seeing is not a night long event usually, you can get good moments sometimes. Patience is the key. My friend Mr. Eric Ng said that, he ever waited for four hours before first clicking the record button, for what? to wait for the best seeing.

So if you want to get image, the key is patience. If you just want to get the best image without patience, go for the compromised route, but you won't get the best possible image.

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