I guess, we all use averted vison AV, where required. Of course, it's a very helpful technique.
It needs a bit of getting used to. You have to step back from the normal bright light perception mode: you try to perceive sth, but still you look aside. Then by focusing your mental attention to the 'side image' you get a deeper detection level. Simply because the retina is more sensitive to low light outside the fovea centralis (aka optical axis receptors). That's pretty easy so far, also pretty straightforward to understand/explain (except maybe the mental attention thing).
Now field sweeping is similar, in as far as it requires some special 'attention mode', too.
Is it also so straightforward to explain - in terms of mechanisms ? I'm not sure.
Most of us might have employed the technique of field sweeping FS as well. I.e. moving the telescope over some faint fuzzy in order to increase the level of perception somehow. This seems to work because evolution has trained our eye/brain system to recognize moving things very easy, because of suspected urgency. The leopard is after you, you better have a special operation mode for that... There is no doubt an evolutionary benefit from having such a mechanism built in.
Some observational examples:
I've used FS many times at faint stuff and low contrast situations. Hunting the horsehead B33 in a smaller telescope you better first find the edge of IC434 running N-S. Moving the scope perpendicular to it (E-W) will help a lot detecting the edge. Then you go for B33 and try the same maybe... The visual system VS (eye + brain) performs something like an edge detection. In image processing there are specific filters for such tasks. In some way or the other the VS achieves the same with similar methods, I guess.
Same for e.g. the pillars of creation in Eagle Nebula IC4703/M16. The smallest pillar is a bit of a stretch. Try to use field sweeping and hey it pops up (well hopefully).
There are more examples, also some interesting ones (I might add some below lateron :-).
FINALLY a somewhat more complex finding:
I noticed that - when moving - the resolution of the image (as) I perceive (it) suddenly jumps up. At rest, with low contrast, I first see some foggy smeared out image, say resolution seems like 10arcsec (because of the low light level). I know that at low light levels the retina cells are coupled by the VS to increase the S/N ratio (I guess pretty much like a CCD would be binned to reduce the read noise, if not sky-limited). Now, when I move the image resolution suddenly seems to jump to say 3 or 1 arcsec (i.e. by a pretty large margin). At least - to my brain - this seems the subjective impression I have. Of course, I have a hard time then CONSCIOUSLY recognizing things while they move - so fast, and so many details simultaneously ! But still it looks/feels like increased resolution.
I really also notice this sometimes when I look e.g. at Jupiter with good seeing (I hardly do, it spoils my darkness adaptation, of course). As soon as I move my scope the image looks much sharper, all of a sudden. I see cloud details that were hidden when at rest.
I would really be curious if you could comment (especially) on this latter finding / impression / subjective observation.
Any scientific facts on this (from VS physiology etc) ?
Is your experience/feeling about FS similar or totally different ?
Any situations where it seemed especially evident / useful ?
For those in the sketching business: what about the importance of panning to you while you observe + sketch ?
Thx for comments.
Last edited by petersurma; May 17th, 2023 at 03:41 PM.
I actively/consciously use field sweeping when observing large extended deep-sky objects. It helps me to compare different regions of sky when trying to verify whether or not something detected is repeatable. And it helps me to move it through the averted vision sweet spot again and again.
For small or stellar objects, part of the time I try to fixate my averted vision on them and part of the time I use field sweeping (maybe not actively, really hard to tell).
At dark sites and with small exit pupils (dark sky background in the eyepiece) I regularly detect what I call "visual noise" in my vision - flickering here and there on the retina. So any signal coming from actual celestial objects obviously must overcome this noise. It makes me wonder what the best method is: fixating (similar to stacking in astrophotography) or field sweeping and trying to repeat detection to tell real signal from noise? Well, I use both methods, but maybe I could use observation time more efficiently by optimizing visual detection strategy... Who knows, maybe a combination of both methods yields the best visual results?
I wonder what you folks think about that?
I'm also interested in professional literature about this.
In his article "Observing secrets of deep sky objects revealed" on Sky&Telescope's website, Alan MacRobert states: "The peripheral vision is highly sensitive to motion under certain conditions, wiggling the telescope makes a big, dim ghost of a galaxy or nebula almost pop into view. When the wiggling stops, the object disappears again into the vague uncertainty of the sky background."
But he also cites Roger N. Clark's book "Visual Astronomy of the Deep Sky", saying that for faint/tiny objects the eye can build up an image like photographic film with an integration time of 6 seconds, thus recommending fixation on such objects. I took a look at his book, where he indeed stated this, but I didn't find a citation of other literature there.
I already wrote an email to you, but my emailer has a sec problem w/Google currently (your ping reply has arrived thx). Will be solved in 1-2d, I hope.
I agree with everything you're saying. Although my personal feeling is not that I move things thru the av sweet spot, but the increased 'sensitivity' is basically everywhere, once I move. Still it's interesting to hear that you seem to use it a bit differntly - ok everyone might have his own technique, sure.
Wrt literature I got a bit of disencouraged recently since I tried to read a review on the use of fMRT for the investigation of the visual cortex. This one: https://www.annualreviews.org/doi/ab....070203.144220
Hard to understand (that's what I expected, sure) but also: not so terribly much known about these (fs) effects neither, which amazes me a bit. Well definitely there are specific areas for movement recognition in the visual cortex and you can see this in MRT scans ! (so the 'true + hard science' says it's there, finally :-)
For us it is worth noting that FS does come pretty naturally with use of Dobsonian scopes. We need to push anyway, so we see no problem in pushing for improved vision, again. That might be in contrast to people running equatorial mounts, who love 'steadiness' once they found the object (I guess that the regular case). Another argument for the dobsonian way of life. On the other hand FS perhaps seems so natural that it's not worth talking about !?
Thx for the S+T article. Interesting overall, however there is not really very much more than what we already knew about FS. However concerning retina-based 'integration' I never felt real positive about it. More on the contrary, my eyes are (sort of) tiring once I stay focused on a single point for a longer time. I guess that biological mechanisms tend to classify constancy into 'being normal' and thus rather suppress it in order to detect any changes on it with higher sensitivity. (You may remember the image of 3 circle areas each coloured differently - after some time looking at it, you then switch to a white sheet - result: you gonna see 3 circles in complementary colours - explanation: firing green receptors trying to normalize red into grey, then on white background still firing green etc.).
So far so good (hope to get email working soon again) - we'll see what others might say wrt their 'way of FS'...
Last edited by petersurma; June 4th, 2023 at 08:17 PM.
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