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Bahtinov mask on planets?


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When it comes to attempting planetary images, one thing I find rather difficult (apart from waiting for good seeing!!!) is focusing properly on planets. I have taken a few images before where the results are just a bit blurry, and some where the image is super sharp, both within the same hour or so, and that is because I played with the focus a little bit. My focuser isn't great anyway, but I struggle to really tell just how good my focus is on a computer screen, especially if the seeing isn't perfect.

Is it possible to focus with a bahtinov mask on a planet to ensure good focus, then go for it? I can't see why it wouldn't work, but thought I would ask just in case!

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Just now, lenscap said:

I tried a B-mask on Mars recently.

Instead of 3 razor-thin intersecting lines you get 3 bands, each about the width of the planetary disc, so it's not very precise and I didn't find it useful.

Ahh. Is that even with the ISO cranked up to make it nice and bright before bringing it back down to capture details? If that doesn't work, any tips on checking focus on planets?

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3 minutes ago, MylesGibson said:

Ahh. Is that even with the ISO cranked up to make it nice and bright before bringing it back down to capture details? If that doesn't work, any tips on checking focus on planets?

I'm a visual observer. If in doubt I focus on a nearby star or planetary moon.

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Will not work on a planet but you can focus on a near by star and then move to the planet without touching focus. It is best to focus by eye directly on the planet but if that doesn't work for you then a star it is.

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12 minutes ago, Freddie said:

Will not work on a planet but you can focus on a near by star and then move to the planet without touching focus. It is best to focus by eye directly on the planet but if that doesn't work for you then a star it is.

I thought stars have slightly different focus to planets? Or is that not the case? I'll give this a try then, thanks!

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2 minutes ago, MylesGibson said:

I thought stars have slightly different focus to planets? Or is that not the case? I'll give this a try then, thanks!

No, suppose you have a 1 m focal length. Even if we focus on the moon at 384,000,000 m (give or take) the difference between that focus position and infinity focus would be 2.6 nm, or 1/200 lambda, which is well beyond the accuracy of my focuser. At 10m focal length this increases to 260 nm, which is half lambda. An object at one astronomical unit away at 10 m focal length we have a figure of 0.67 nm difference between perfect focus and infinity focus.

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On 11/12/2020 at 15:46, michael.h.f.wilkinson said:

No, suppose you have a 1 m focal length. Even if we focus on the moon at 384,000,000 m (give or take) the difference between that focus position and infinity focus would be 2.6 nm, or 1/200 lambda, which is well beyond the accuracy of my focuser. At 10m focal length this increases to 260 nm, which is half lambda. An object at one astronomical unit away at 10 m focal length we have a figure of 0.67 nm difference between perfect focus and infinity focus.

That's a great explanation. I'll focus in a nearby star then! 

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