Quote:
Originally Posted by Xeteth
This is the kind of stuff I guess I'm still trying to learn and wrap my head around. For reference my refractor is a triplet apo FCD100 series. Could you elaborate on what a few of these mean (perhaps point me in the direction of where I can learn this stuff), QE, resolution and stressing the optics, etc. I think I should wrap my head around these things before making a purchase. Why do you say the QHY9 could give better LRGB images?
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A has been mentioned, QE is quantum efficiency. This refers to the theoretical amount of photos that make it to the sensor that are actually detected. The KAF8300 is around 56% peak while the ASI1600 peaks at ~65% from memory. The ASI1600 also has a higher peak into the UV (~2x) than the QHY9. This means that the less corrected your refractor is into the UV then more bloated stars will become in the blue channel.
This then moves into pixel sizes and how the ASI1600 with its smaller pixels stress' optics more. Let's just say that your refractor creates star sizes of 7 microns in red, 6 in green and 10 in blue. With the QHY9 (5.4 micron pixels) all of the stars will be contained within less than 2 pixels. As you move into the deeper blue (towards UV, where the spot sizes are more likely to reach 15 microns), the QHY9 becomes very insensitive so there is not much detectable bloat; stars are their true colours.
With the ASI1600 (3.8 micron pixels) the red and green are both below 2 pixels BUT the blue is at 2.6 pixels. It has a considerably higher QE towards the deep blue so there may even be a fainter blue halo reaching out to 3-3.5 pixels.
This means that you could have the centre 2 pixels being correct star colour but then a blue halo surrounding any non-dim star.
For narrowband none of this matters as you're only imaging a VERY thin slice of the spectrum. I've also just been picking random numbers purely for illustrative purposes. You're refractor, although not entirely cheap, is on the lower end of the triplet imaging refractors so it is built to a price point. Excellent value but it can have its limitations. Most of the halos and what I've been discussing can be processed out but my point in all of this is that if you're planning on LRGB images you will near certainly get better star colours out of the QHY9 than ASI1600. If you plan on doing narrowband then the ASI1600 is a definite winner over the QHY9 due to the higher QE and MUCH lower read noise.