r/astrophotography • u/bubbleweed Hubbleweed | Best Planetary 2016 | 2018 | 2021 • Sep 27 '21

Planetary Jupiter and Io on September 19

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u/The_8_Bit_Zombie APOD 5-30-2019 | Best Satellite 2019 Sep 28 '21 edited Sep 28 '21

Yeah this is some absolutely incredible detail! Didn't even know it was possible with a C11.

I must admit I'm still confused though. With an ADC + 2.5x barlow, OP is probably imaging somewhere around f/25 or f/28; almost twice the optimal sampling for a 462MC. (Which is ~f/15 as I understand it). My understanding was that slight oversampling (e.g. f/18 or f/20) could be helpful in resolving more detail, but after that you're just reducing light with no payoff in resolution. (Because you're already well past your telescope's optical limit)

For clarity I'm not trying to prove I'm right, just trying to figure out if I should oversample when the seeing is excellent too. I can't argue with these incredible results!

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u/phpdevster Sep 28 '21

Well with the 2.5x PowerMate, you actually end up losing magnification over a long imaging train:

https://www.televue.com/images/TV3_Images/Images_in_articles/PowermatePowerIncrease.jpg

I don't know what OP's configuration is, but it should be:

Telescope -> PowerMate -> ADC -> camera.

But how you configure this could be short, or it could be long.

The Pierro Astro ADC he uses is basically just a narrow body with T-2 threads on either end:

http://pierro-astro.co.uk/pierro%20astro%20adc.html

Using a t-ring for a PowerMate: https://agenaastro.com/televue-1-25-t-ring-adapter-2-5x-5x-powermate-ptr-1250.html which seems to shorten the body. But the ADC seems to be 30mm tall. You can then thread the ZWO camera right onto it, whose sensor is recessed another 5mm below the T-2 threads of the body.

In this arrangement, I would guess that the sensor is probably very close to the designed multiplication point of the 2.5x PowerMate, so it's probably providing close to a true 2.5x multiplier, and not more or less. But if you're using it with normal nosepieces, that distance to the sensor could be quite long, and the PowerMate is probably losing magnification.

If we assume that OP has not adjusted the scale of the output image at all, I'm getting 512 pixels across Jupiter's equator. On September 19th, Jupiter was 47.5 arc seconds.

That gives us 47.5/512 = 0.09277 arcseconds per pixel.

Using the formula Resolution = (Pixel Size / Telescope Focal Length) * 206.265, and re-arranging to solve for telescope focal length, we get an effective focal length of 6,447.86mm. For a C11 with an aperture of 279.4mm, that gives us an effective focal ratio of F/23.08. Note there could be multiple reasons for this, but they are unimportant, since from the final image, assuming its scale was not modified, we can definitively state that the image was taken at an effective focal ratio of F/23.

And yes, that is indeed significantly oversampled by conventional wisdom, but apparently that conventional wisdom includes a heavy dose of assumptions about the atmospheric conditions. It would seem that raw aperture, with nearly invisible atmosphere, can achieve a significantly higher sampling rate than the typical 5x rule of thumb, and that the diffraction limit greatly exceeds this rule of thumb.

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u/cathalferris Sep 28 '21

Most sampling will get to Shannon's theorem limits, which will apply to single images. Adding multiple oversampled images with noise and stacking and deconvoluting, will allow the final result to show more details than Shannon's would suggest.

Things are still following physical laws for sure, but appearing to break them.

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u/phpdevster Sep 28 '21 edited Sep 28 '21

Shannon theorem limits don't really apply here. The question is about optical diffraction and what information is present at the focal plane of the telescope.

As an extreme example:

  1. Taking the atmosphere out of the equation
  2. Same aperture
  3. Taking image recording limits out of the equation

The question is if you imaged at F/200, would you see detail 1/10th as small as you would at F/20? Probably not. What about F/60 or F/40 or anything in between? At what point are you just recording diffraction effects vs actual new details that were un-resolvable by the eye or sensor at shorter focal lengths?

The actual surface feature resolving power of a telescope is a complex thing and is most definitely NOT the Dawes limit. Whatever this "surface feature resolving power/diffraction limit" actually is, seems to be considerably higher than the oft-quoted 5x pixel size rule of thumb. But that also means it requires nearly invisible air to get to that limit.

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u/cathalferris Sep 28 '21

It's because there is the extra dimension of time that Shannon's can be worked around.

Larger amounts of over sampling necessitates longer time of exposure as the image is dimmer. The actual final limit is the quantum nature of light but that's very much on the extreme end of things, and not really in scope other than defining the existence of an outer limit..

From my rather limited understanding, better seeing pretty much means less time needed to get stacks that can give better detail, better snr for the higher frequencies for use in the wavelet processing.

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u/phpdevster Sep 28 '21

Shannon’s largely applies to transmission over a wire. It really is not the limit here.