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Proper Sampling for a Takahashi Mewlon 210 and QHY 5-III-462C

Planetary Camera Acquisition Solar System
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Frank Brandl avatar
Frank Brandl avatar
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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andrea tasselli avatar
andrea tasselli avatar
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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Frank Brandl avatar
Frank Brandl avatar
andrea tasselli:
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.

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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Michael Clive avatar
Michael Clive avatar
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!