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Hello everyone, For the next planetary season I'm planning to use a Takahashi Mewlon-210 with a QHY 5-III-462C camera. This camera uses the Sony IMX462 CMOS sensor with a pixel edge length of 2.9 µm. I'm now trying to calculate the optimal focal length / sampling for this setup. As a rule of thumb, I learned, that the telescopes max. resolution should be covered by 3 pixels in order to achieve optimal sampling. Let's dive into the calculation: Max. resolution Mewlon-210 = 138 / D Max. resolution Mewlon-210 = 138 / 210 Max. resolution Mewlon-210 = 0.66" ==> covered by 3 pixels = 0.22'' per pixel. When I calculate the resolution of the telescope without a barlow lens per pixel, I get the following: Alpha = (206.265 x d(pixels)) / F Alpha = (206.265 x 2.9) / 2415 Alpha = 0.25" The resolution is therefore a little bit lower than desired: 0.25" per pixel < 0.22" per pixel. What focal length does the system need to achieve 0.22" per pixels? F = (206.265 x d(pixels) / Alpha F = (206.265 x 2.9) / 0.22 F = 2719mm Conclusion: For my understanding, I would not use a Barlow lens for planetary images with this setup. Using a 2x barlow would lead to some heavy oversampling and therefore a "waste" of integration time. What I don't understand: When I search for planetary images here on AstroBin with this setup, I find a lot of planetary images, which were made with a much longer focal length. My question: Is my calculation above wrong, am I missing something? Thank you, CS, Frank |
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| It is called oversampling and only works (well) with good seeing. I routinely did use f/20 on my 10". Depending on your seeing you might need to use either a barlow and a camera with larger pixels or stick to that configuration and accept that you're slightly undersampling. I'd lean on the oversampling in good to very good seeing. |
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andrea tasselli: Thank you Andrea for your feedback. I know what you mean, a "slight" oversampling is what many planetary imagers prefer. The f/20 was what was often used with pixel edge length around 5 µm - and that was totally OK. I also oversampled in the past. For example I used a C11 and a ASI 120 MM with f/18, while f/16.3 was what the theory says. With the Mewlon, the theory says that an f-ratio of f/13 would be optimal sampling with the QHY and with slight oversampling lets say f/15 would be acceptable. This means that I need a Barlow of 1.3x. I don't know of any barlow with 1.3x (best Barlow in this range that I know is the AP BARADV with 1.8x but that is still too much focal length for this setup). So I think in the end it comes to what you say: Use a barlow with a different cam or accept that I'm slightly undersampling  |
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TMB makes a weak barlow, I think Baader makes a 1.5 I am about to start imaging with the same camera/telescope combo, I say just try it! |
