Starting with the idea that there is little glow post about 10 arc seconds. Untrue, from what I have seen. I can see this visually just looking at the field and noticing the background grows darker away from the planet the further one goes. This is true even if I employ an occulting bar, or an eyepiece that allows a sharp cutoff at the field edge with Mars out of the field (e.g. 3-6 TV Zoom on the 3, 4, or 5 settings.) Phobos is still in the steeper part of this gradient, and never really leaves it, even at 20 or 30 arc seconds from the planet's surface.
Maybe we have different optics or different levels of transparency, but aside from some obnoxious reflections and flaring from my Meade #140 barlow, when Mars is outside the field of view, it's very difficult to detect *where* outside the field of view it is, using the gradient around it. The gradient is certainly not strong enough to obscure Phobos even at 12 seconds in.
But what's also interesting is Phobos and Deimos will be optical point sources, meaning their brightness increases by the square of the aperture. The surface brightness of all other light in the field is based on the exit pupil. Maybe there's some optimum between magnification, aperture, and exit pupil as it relates to how easy or hard it is to separate Phobos from whatever glow might be present around Mars.
Averted vision was still stronger for locating Phobos in the first place.
This is my experience as well. Each time I noticed Phobos it was because I was either looking at Mars or still positioning Mars in the view (I actually find that sometimes deliberately inducing motion makes things much easier to see, since that's how human vision is wired), but then I could lock in on it in direct vision.
I knew that Phobos and Deimos were nearing elongation during the time I set aside for Mars observing, on opposite sides of the planet. Upon focusing on the planet with full 20" aperture at 278x I immediately noticed Deimos in peripheral vision, while staring at the planet. Phobos was not so readily seen. I had to hunt for it for a time, waiting for some better seeing/focus to finally give it away in the glare. After that I could find it more readily, although not like Deimos. Neither of the moons were in the spider vane diffraction rays initially.
278x in 508mm aperture is a 1.82mm exit pupil. In contrast, the last time I was easily able to spot Phobos (at about maybe 17-18" separation), it was 246x and 1.54mm exit pupil.
In terms of surface brightness, that's a difference of (1.82/1.54)^2 = 1.4x brighter surface brightness.
In terms of magnitude increase, that's (20/15)^2 = 1.76x point source brightness increase.
So it's interesting that in your 20" at 278x, Phobos would have had proportionally *better* contrast against the surface brightness of any glow, than in my 15" at 246x, and yet you didn't find it as easy to notice.
1. For whatever reason, there is more glow around Mars at the time of your observation (differences in scope contrast, optical quality/cleanliness, transparency, cornea/retinal differences)
2. The extra size of Mars combined with the slightly higher apparent surface brightness in your scope introduced more glare for your eye that overcame the proportionally brighter point source of the Moon. But based on *my* experience, that extra glare would render Deimos just as hard to see (and again, I'm differentiating glare - which is the flooding of the retina with light from Mars - from *scatter* which is a visible glow around Mars that's present even when Mars is not in the field).
I'm still not convinced that seeing conditions can make "scatter" (e.g. glow around Mars) so substantially worse that it would make Phobos (at elongation) harder to see. That doesn't track logically for me and I've never observed that phenomenon to such a degree on any planet. Transparency I get, but seeing conditions would have to be so bad that the sky is basically behaving like frosted glass. And if seeing conditions are truly that bad, then the fainter light of Deimos should have been erased into a blur.
Concerning my seeing conditions, this is a typical night for me: https://www.youtube....h?v=hSGGZMzOAU8
This is one of the best nights I've ever had - better than most of my Mars sessions: https://www.youtube....h?v=kHc3Nkys1XI (which is still poor compared to say, this: https://www.youtube....?v=RavpiN0Axj4)
I have VERY, VERY, VERY bad seeing conditions most of the time. A few nights I get decent result, but they are still poor compared to most other locations.
On the very best nights, I can see about 2-3x the number of stars in M13 than I can on typical nights. The fainter the point source of light, the more susceptible it is to bad seeing.
Edited by CrazyPanda, 20 October 2020 - 11:17 AM.