by Matthew Marcus
I got decent views of M3, 2903, eta-Ori (double), Comet Machholtz, the Trapezium (5 stars easily, 6 suspected), etc. However, 4565 showed mostly its core, with the edges appearing just as faint wisps against the bright background. The area around M84 did show a few galaxies, but nothing like I've seen it. After a while of the clouds moving in and out, they moved in in a way which looked pretty permanent. That was when I made the Scope Sacrifice and went home. Before that, I at least got to see the GRS near transit and Io coming out of eclipse. The seeing was good enough to show the Galiliean moons as little disks of differing sizes. With Io close to the planet, its yellowish color stood out more than usual. Using filters really showed the color difference between Io and the other moons.
As is typical on nights of less-than-perfect seeing, I found the GRS obvious at 125x and elusive at 250x. That got me to thinking about why it is that there's an optimum power such that going higher not only doesn't help but it actually hurts. For dim stuff, the reason is obvious - lack of light. For bright stuff, it must be something subtler.
Here's my theory, for what it's worth:
The human eye has some edge-enhancement built in, so that especially when contrast is low, the eye's response peaks at a certain spatial frequency. The response is flat below that peak, and rolls off above. Now, if you have an object seen with an optical system with a limited resolution, there will be information up to some maximum spatial frequency, above which there's not much. This maximum spatial frequency depends on the scope and the seeing. Further, what a scope does is to scale down the spatial frequencies in the object by an amount equal to the magnification. Thus, if there's detail at a spatial frequency of 1/minute and you observe at 60x, it's presented to your eye at a frequency of 1/degree. Most people can't resolve an arcminute but can resolve a degree (yes, I'm playing fast&loose with factors of 2*pi, for simplicity). This is part of why scopes work. The other part, of course, is the light-gathering power.
Anyway, I suspect that the optimum power is that which maps the maximum spatial frequency present in the object, as seen through the scope optics and air, onto the peak of the eye's frequency response. Thus, the eye provides a boost where the object is rolling off. If the magnification is too small, then the eye can't respond to the information present, and detail is lost. This is what happens when you try to see Jupiter naked-eye as anything but a point. If the mag is too big, the peak of the eye's response occurs where the object isn't providing any signal, so the edge-enhancement is wasted. If the seeing is poor, the largest spatial frequency available from the object is reduced, so the magnification must be reduced to keep the match with the eye's response.
This edge-enhancement peak occurs near the top end of the eye's frequency response, i.e. when detail gets to be about as small as you can see. This is why the optimum magnification often seems to be when the detail you want to see appears almost as small as you can see. That's when you're tempted to mag it up, and you get disappointed when not only does new detail not emerge, but detail you saw disappears.
What happened after I left? Did the Scope Sacrifice work?
Posted on sf-bay-tac Apr 03, 2005 15:53:50 PT
Converted by report.pm 1.2 Apr 05, 2005 21:04:52 PT