Telescope Reviews: 127mm Mak for astrophotography??

Wrong. Totally.

Popular subject of late. Because I've posted a lot on this in other threads, I'll just do some copy and pasting. In short, there has always been too much misunderstanding regarding f-ratio vs. aperture.

First, I'll refer you to Stan Moore's articles on going deep with your images, particularly since you are the one that stated that this stuff is "quantifiable mathematically"...

http://www.stanmooreastro.com/f_ratio_myth.htm
http://www.stanmooreastro.com/eXtreme.htm

The central point is...f-ratio is a component of two INDEPENDENT aspects, aperture and focal length. When combined with the nature of how CCDs work, you can't just make a blanket statement about f-ratio that's accurate enough to cover all imaging scenarios. There is no rule of thumb.

Read noise is a major player in imaging, but that is easily overcome by imaging longer with each subexposure, or indeed, having a faster f-ratio scope. In other words, as a basic principle, we will use subexposures to overcome any noise attributed to read noise...called "sky limited" exposures. However, each object has a predictable, poisson noise ratio, also known as "shot" noise...which means that you inject noise into the image at the rate of the square root of the number of photons. If done correctly, this is the ONLY noise of real concern.

Practically speaking, this means that as you increase the number of photons of an object (or angular area of the sky), then the ratio of the signal to noise increases. As a simple example, if you have 16 photons for an object, then naturally you will have 4 units of noise (square root of 16). This is a 4:1 S/N. Once you stretch that out in processing, to show the details of the object, this may or may NOT be enough to demonstrate the object without "grain" or "noise." However, if you have a scope that delivers 100 photons for that same object, then you will have 10 units of noise...but that's a 10:1 ratio of signal to noise. Thus, when you stretch that out in processing, you have more signal that you can process...and less noise to hide.

Typically, as you double the aperture, then the S/N of the image doubles. Therefore, an 8" scope will produce twice the S/N of the 4" scope for given object, regardless of the focal ratio. This means that when you stretch the image of the 8" scope, you will have a cleaner image on that object of interest. THUS, we are most concerned with how many photons we can capture of an object, regardless of how many pixels it takes to do it...once we've overcome read noise, it really doesn't matter the number of pixels, as long as we aren't "over-sampled" - which means having too many pixels in an attempt to provide details that the seeing and mount can't provide anyway.

Now, if that object of interest is small, such as a galaxy in a widefield image of Markarian chain, then noise will be hidden by virtue of the fact that there's less of the object to show. However, you'll notice that the backgrounds might be pretty grainy when you blow it up, simply because there is less photons recorded in those areas (lower S/N). But because the field is so wide, incorporating a large FOV of the sky, then noise might not be as apparent in those small areas. It's naturally hidden by the enormous scale of the image. It's why your image looks great when displayed at 640 pixels on your webpage, but looks like cr@p when displayed at 1200 pixels.

In summary, "noise" can be defined as the "uncertainly" of your data. This means that your data has a certain amount of uncertainty that scales in direct proportion to your aperture. The only way to make things more "certain" is to increase aperture, decrease light pollution, improve seeing, get a faster camera, increase your total exposure length...or...just shoot a wide enough field where your final display of the image will never show the noise that's there in the first place. The latter is what we often do, and it deceives us into thinking our fast f-ratio scopes are indeed "faster."

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jay
www.allaboutastro.com