Quote: I believe some of the reason for the descrepencies lie in these to explinations. The energy distribution is growing larger and larger so the tails are growing dimmer at the tips while they are growing ever larger, and the off axis illumination of the scope itself may be further reduceing the brigtness of that blur at low powers so that we never really see the full extension of the comatic/astigmatic blur.
Quote:But my original post concerned why, in eyepieces of equal apparent field, people notice less coma at higher powers.
Joseph Cannavo FrontCollimation 16" F5 Teeter Dob Driver with novel azimuth friction clutch, and axial (rotating) electrical connection. Tom Osypowski equatorial platform 10" F5 Lightbridge Mid 70's RV-6 4" Orion 100mm ED Mr Keeyoots (My Cat).
Quote: Jon,When illumination falls off with increasing field angle, it is not the case that ever smaller *central* portions of the primary are utilized.Rather, one side of the primary is clipped, while the opposite side is completely seen, with room to spare.
Quote:And the eye's huge dynamic range makes brightness *much* less important than contrast. A diminution of 50% by vignetting, if not excessively abrubt, is rather difficult to discern. A decrease in contrast of 10%, however, is probably easier to detect.
17.5" Dob "Beta Version"
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Quote:My take, doubling the mag decreases the brightness of any arbitrary star by a factor of 4. Also reducing the flare that's visible, especially the outer area of it. The dimmer the star or starfield, the less visible the flare will appear as well.F/4.5 viewing starfields- ok. But without some kind of corrector or accurate mount/drive on planets, forget it.M.
Quote:Stars are point sources, but comatic flares due to optical abberations are -not-. They subtend a finite area.M.
Part of the explanation could be the fact that the human eye itself is an optical system, with its own aberrations. When the magnification decreases, the exit pupil diameter increases, and it is well known that optical aberrations of the eye also increase with pupil size.
For example, look at the pictures posted here by Photonovore.
The original document (large !) can be downloaded with this link :
By considering that the receptor is not the eye but the retina, the optical aberrations of the instrument, for example coma, will add to the aberrations of the eye to degrade the quality of the image. So, even if the size of the coma blur due to the telescope is constant, on the retina the resulting image is certainly worse at low magnifications.
Quote:Jean-Charles,Yes, your point should be added to those I mentioned in my first post.But it is not necessarily the case, as many people do not have any significant astigmatism that would operate at large exit pupils and not at small ones.So whereas yours is a valid point and may very well hold sway in a certain percentage of case, I doubt it does in most.
Quote:All the posts so far have been looking for an explanation why many people DON'T see the same coma at high powers as at low. I'd like to know what's different about the observers who DO see coma at all magnifications.
Quote: All the posts so far have been looking for an explanation why many people DON'T see the same coma at high powers as at low.I'd like to know what's different about the observers who DO see coma at all magnifications.
Quote:But, this effect - if significant - would actually work in reverse. Just as the keener ear more easily discerns poor fidelity in an audio system, noticing off-axis coma is an exercise in visual acuity. Thus with sharper vision, the off axis coma of the primary mirror would stand out more - not less - relative to a sharp on-axis image. Similarly, with decreasing visual acuity, the same amount of off-axis coma would present a less apparent deviation from the consequently softened on axis image.
Quote:I should try the same experiment with my telescope, and verify if coma appears differently in my two eyes at low powers.Jean-Charles