Quote:
Originally Posted by Peter Ward
Stefan has mentioned the elephant in the room IMHO.
Tight & deep sky star profiles need far more than good seeing. A smooth stable and accurate mount being first and foremost. As for the optics I have found that using better than "diffraction limited" helps buffer the effect of atmospheric turbulence.
Larger apertures apart from having higher resolution, also stabilise the image position at the focal plane (i.e. the airy disk might look like rubbish, but it stays still and is not dancing all over the place). Mechanical and thermal stability prevent loss of collimation and focus drift during the exposure (e.g. ZeroDur optics do not expand/contract, hence change focus, with temperature changes).
Taking uber-tight and deep images a bit like Formula one racing, a few percent gained from the tyres, suspension, brakes, engine, aerodynamics etc, often add up to a large overall gain. If the visibility is clear and the track is dry (i.e. good seeing) then all the better. |
Yes, one should be aware that there is a connection between seeing and aperture size.
Also, a good planetary seeing occurrence may not necessarily be a good one for deep sky imaging, or vice versa - at least for a given aperture. The data is captured and processed in different ways. Image distortions are dealt with in software in the case of planetary imaging while that is not the case when measuring FWHM.
It is the average size of convection cells in the atmosphere, that disturb the wavefront, relative to the size of the telescope's aperture that is important. Convection cells that are smaller than the aperture, shift the photons from the Airy disc into the diffraction rings - larger spots - and large convection cells, relative to the aperture, mainly make the whole spot dance around.