|
There have been a few topics over the years on CN in regards to image scale, contrast transfer (MTF), pixel size and the like. Although some of the mathematics in the discussions went over my head I could understand enough of it.
In astrophotography there are two types of sources, point (stars) and non-point (everything else!). As apertures increase so does light intensity from stars directly proportionally to the aperture increase. This is aperture flux and is spread over however many pixels your FWHM is.
Non-stellar objects like galaxies and nebulae are a function of pixel size and focal ratio. With this were flux per square arcsec of sky is important.
What this means is that stars saturate slower in smaller refractors than larger reflections BUT both being able to image just as deep into a nebula with the same integration time. This is what makes wide field astro graphs (FSQs, flat field APOs, photographic lens') so good at what they do. They can image bright stars, fainter stars, galaxies and nebulosity in one exposure easier than a larger aperture.
So why don't the 80-130mm refractors or even 8-12" reflectors generally come as sharp as what Peter has posted? Seeing conditions is definitely a big part of that. Mostly though, it's aperture. What aperture allows is for you to image at a higher scale while maintaining SNR and therefore contrast. Contrast by definition is how well defined brightness differences are at small and large scales.
Take an 8" F/4.25 with ASI183 (close to yours Lee) and a 16" F/8 and PL16803.
Both image at 0.58"/pixel but as we're mostly talking about non-stellar objects (contrast in the nebulosity) we only care about pixel size and focal ratio.
2.4^2/9^2 = 14x more light per pixel
8^2/4.25^2 = 3.54x less light concentration
The larger aperture gets 3.95x more flux per square arcsec because the 16" is sucking in 4x the amount of photons as the 8" AND they're imaging at the same image scale.
Replace the ASI183 with a KAF-8300 and its 5.4 micron pixels and the situation changes where the smaller aperture ends up with 27.4% higher SNR with the same integration BUT it's now doing so at 1.31"/pixel. What this achieves is stronger signal on the fainter structures further out from the core of M42 and better contrast on the large scale structures BUT without the resolution to capture those small scale structures anymore.
|