I need help understanding something. I often hear experienced imagers say they want long focal length scopes to do images of DSOs. They say it "increases resolution of extended objects". I am looking at adding a focal reducer to my CDK 14 ZWO 2600 combo. Using the same sensor for both configurations. Shooting DSOs. CDK 14 REDUCED: 1717mm at f/4.8, 0.45 arc sec/pixel image scale . Compare that to the CDK at native f/7.3 at 2563 focal length, 0.3 arc sec/pixel image scale. Now, assume that you're at Sierra Remote Observatories with both configurations, and your average seeing in your images is 1.5 arc sec (i know it gets .9 arc sec or less there but that's at zenith and not all the time). So, you'd want to try to target 1/3 the seeing for your Max resolution= 0.5 arc sec/ pixel. So here are the questions: Although the f 7.2 configuration IN THEORY gives you more resolution with the same sensor, wouldn't it be true to say that the seeing , even though excellent, cannot support that additional resolution since your seeing should have a limit at 0.5 arc sec per pixel? Therefore, in the real world, not in theory, both configurations are limited by seeing at their max resolution, so the f/4.8 scope will have the same apparent resolution (to our eyes, not mathematically) in the resulting images (in reality, not theoretically) as the f7.2 scope? Therefore, the f 4.8 configuration in the field brings the same resolution as the f7.2 AND adds the additional benefit of being much faster. Obviously, this all assumes the SAME APERTURE for both scopes, just different focal lengths and f ratios. Is this correct? Lastly, while we're here… it says the dawe's limit is 0.33 arc seconds for my system on astronomy.tools. doesn't that mean that i can't achieve any higher resolution than 0.33 arc seconds, anyways?
Please help: comparing two fl, f ratios, pixel scale, etc
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Your assumptions would be, in the main, correct. As minimum sampling with a 1.5" seeing a sampling of 0.5"/px would meet the requirements. Optimum sampling, however, accounts for a somewhat larger ratio, between 3.5 to 4 but in the main your image scale of 0.45"/px would be pretty much close to it. For imaging purposes what achieves the best resolution at a given locale with a given optical system (including detector noise) is a complicated affair but if background noise isn't a factor than probably your CDK sampling would do quite well.
As for resolution, nothing satisfactory can be said unless one brings in the MTF of the optical system, so saying the Dawes' limit is 0.33" (which is just one metric that can be used, other exists) doesn't say at what contrast this can be achieved. On the flip side, it is well known that there are subjects (specifically, thin very long lines pair) which shouldn't be resolved by some given system but have been routinely shown not be true (both visually and photometrically).
As for resolution, nothing satisfactory can be said unless one brings in the MTF of the optical system, so saying the Dawes' limit is 0.33" (which is just one metric that can be used, other exists) doesn't say at what contrast this can be achieved. On the flip side, it is well known that there are subjects (specifically, thin very long lines pair) which shouldn't be resolved by some given system but have been routinely shown not be true (both visually and photometrically).
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Yeah from what I can tell, the dawes limit is for visual and doesn’t directly apply to imaging. I’ve been looking at jupiter shots lately, and people routinely seem to out-resolve what the “math” says they should.
I’m a noob so take my babbling with a grain of salt, but I would go for native if the fov works for you, and mosaic for larger targets. I’m assuming you can process that amount of data because you can afford to host a cdk14.
I’m a noob so take my babbling with a grain of salt, but I would go for native if the fov works for you, and mosaic for larger targets. I’m assuming you can process that amount of data because you can afford to host a cdk14.