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Samir Kharusi | all galleries >> Sizing Planetary Images To Compare vs The Theoretical Limits For Your OTA > Saturn_C8_C14_Nyquist_Sizes
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Samir Kharusi


Azaiba, Muscat, Oman

Saturn is always an interesting example when we talk of diffraction limits. For reference note the 2 simulated stars separated at the Rayleigh criterion for a C14 in the bottom frame. It's immediately obvious that we can see features, resolved or not, that are finer than the Rayleigh criterion in the Nyquist-display. Check out the Encke Gap in the JPL-NASA simulation for the C14. This Gap is very much smaller than either the Rayleigh Criterion or the FWHM for a C14, yet there it is, glaringly obvious in this Nyquist display of the simulated view. So care needs to be exercised when we talk of "diffraction limits". It's also looking for disappointment if one is striving to detect the Encke Gap in an 8" scope, again see the C8 simulation. When using smaller scopes people often mistake the so-called Encke Minimum (diminuation of brightness within the A-Ring) for the Encke Gap, a very, very much finer feature. IMHO the described Nyquist-display method gives an excellent feel as to what is possible under perfect conditions. I do not claim that it is impossible to detect finer detail. Progress is always ongoing, and unfettered imagination knows no bounds, but the Nyquist-display level of detail seems a very appropriate benchmark to use for one's own imaging. It's a benchmark that can only be achieved when everything is perfect, so it will always be just beyond reach (for most of us anyway). By the way, the C8 images displayed at the top were acquired using a mix of either a 2x Barlow or a 5x Powermate, part of an exercise to gauge whether massive over-sampling at f50 (at acquisition) is substantially better than at f20, study available at my website, here . Please do not confuse deliberate over-sampling at the acquisition stage (a desirable practice) with the Nyquist-reverse-sampling for display that is advocated here. One can also do an exercise using Nyquist-display to check out the detectability of the Cassini Division in ever smaller OTAs. When I played with this, I came to the conclusion that excellent imaging practices with a 2-inch aperture scope should be able to detect the Cassini Division, but not with a 1-inch aperture. Care to prove me wrong? Do drop me a line if you can show it up with a 1-inch aperture. Seeing conditions are a very minimal issue when using such a small aperture.

By the way, in case you are wondering what magnification at the eyepiece of your scope will deliver a planetary diameter, visually, that subtends angular widths similar to the ones you are viewing here, I guesstimate it at between 30x and 40x per inch of aperture. Of course this will depend on your monitor's pixel pitch (mine is at about 90dpi but it's a large desktop monitor) and how far your eyes are from it (mine are at 31 inches, 80cm). Smaller laptop monitors may have finer pixel pitches, but then you tend to view them from closer up. So, somewhere between 400x and 550x sounds about right for a C14 to get the view above. Of course, beyond 300x seeing interferes acutely on most nights. Consequently it's rare that people claim to have seen the Encke Gap visually, even with C14s. Do not confuse that with the Encke Minimum, a much broader band.

Also interesting to sort out what magnification is "needed" to view all the detail that your diffraction-limited OTA is capable of delivering to the eye. Assuming you have 20/20 eyesight, and the planet you are viewing is bright enough that your vision does not get impaired from its daytime capabilities, all you need is 12x per inch of aperture, a surprisingly low figure. But that's all you need for a planet to appear "razor-sharp". This also corresponds to photographs being printed at 240 to 260dpi. Use a coarser dpi in a small print and all edges will appear ever so slightly soft. Ditto with a telescope when you use more than 12x per inch. But of course, you'd like to relax (similar to employing a magnifying glass when examining a print for tiny details), maybe the view is dim and your vision is no longer as perfect as during daytime, and so more comfortable viewing for people with normal vision is at 20x to 30x per inch of aperture. The higher the magnification the more you will notice that the view has gone soft. In larger scopes it also goes soft from seeing. People who claim to see more at 50x per inch, no matter how perfect their OTA is, and no matter how good the seeing is, simply have rather tired eyes ;-) This hobby is full of vision-impaired retirees! If you are one of these characters, and you still claim that your eyesight is fine, just look up with your naked eyes near Vega and see if you can split the wide pair in the Double-Double. In my younger days I used to be able to do that with a glance. Now, I have creeping astigmatism, so 12x per inch is nowhere near enough :-( I also need reading glasses :-( And no matter how large the OTA, seeing is very rerely good enough to use more than 300x at the eyepiece. So if your vision has detereorated with age, go right ahead and use 500x or 1000x or whatever makes you comfortable, but please do not attribute it to the supremely superb quality of your OTA (an affliction that tends to affect owners of premium APOs). There is no way that I am aware of to get around the physics.

Philips ToUcam Pro

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