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    Looking through really big telescopes
    From: Frank Reed
    Date: 2010 Feb 1, 16:54 -0800

    Yesterday, under the subject "CN aboard a Cessna 172" I mentioned in my trip highlights that I had visited Palomar Mountain. There's nothing really to see up there except the historic telescope resting quietly in its enormous dome. They don't do public observing because the telescope is really set up as a giant digital camera and digital spectroscope. No human eyes have looked through it probably in decades. Hanno Ix commented that it would be nice if they had public observing sessions to generate some support and money for the research. I said that the view wouldn't be that impressive since the view with the human eye as sensor through a very large telescope is about the same as the view through a 12-inch backyard telescope.

    Jim Wilson then replied:
    "The title of this post is wrong, but the talk about Palomar being the
    perfect machine prompts this reply."

    That's "thread drift", Jim. Nothing to worry about; it happens all the time. I've now switched to a new subject. You should feel free at any point to change the subject in a NavList reply if you think a thread is going to continue in some new direction. People generally leave the subject unchanged because they think the side-discussion will die out very quickly, and they don't want to dilute interest in the main thread topic. But it's nothing to obsess over since all of the messages contain threading information in their headers, and that will soon be visible in a "tree view" of messages on the message boards.

    Jim, you wrote:
    "our travel agent specializes in astronomy vacations, and
    he found that he could rent the Mount Wilson telescopes for a quite
    reasonable price. We first used the sixty inch one, the first ever large
    telescope, to look at Mars and other objects. Later, he rented the 100
    inch one, and we looked at Mars at its closest approach in 60,000 years.
    They had a Mickey Mouse viewing setup, but we could actually look through
    the telescope."

    That's very clever! Just rent it. Your experience brings up one aspect of huge telescopes that used to be quite important. Generally, the useful magnification in observation of planets maxes out at about 400x or maybe a little less because of atmospheric turbulence. Magnify any more and you're just making a blurry image bigger. But for visual observers, there are moments of "perfect seeing" (and yes, for those who don't mess around in astronomy, "seeing" is the technical term for it, at least in English). These are instants when the line of sight to the planet is all smooth air, and much finer details become visible for a fraction of a second. It used to be quite common for astronomers to sit patiently staring at the planets waiting for those moments of perfect seeing. Under those conditions, magnifications much higher than 400x are effective and then a huge telescope has a real advantage. This also applies to modern backyard astronomers who use digital techniques and collect video imagery of planets through their telescopes (rather amazing, they actually use video to collect dozens of frames per second rather than still cameras). Those video frames can then be analyzed and "stacked" to get the best seeing from each frame in the video, a little over here from one frame, a little over there from another. The results with high-end amateur equipment can be as good as professional images of the planets. But if you look with your eyes, you would never see any of that.

    By the way, the center of the Earth was closer to Mars on that night in August, 2003 that everyone marked on their calendars, but the Mount Wilson Observatory was closer the previous night (by an insignificant amount observationally).

    There are two other types of observations that are relevant:
    1) seeing faint field stars
    2) observing extended objects like nebulae and galaxies

    Large telescopes really excel and have all the advantages you would expect from their apertures when it comes to seeing faint field stars. By "field stars", I mean stars that are distinct from each and appear point-like --just plain stars in the field of view in the telescope. So, for example, if you want to see Pluto or its larger cousin, Eris, you need a telescope with an aperture larger than some minimum (about 10" for Pluto, about 60" for Eris since it's further away and darker).

    When observing extended objects like nebulae and galaxies, the real "stars" in the astronomy pageant, there's very little advantage in using a very large telescope with a human observer at the focus. The Orion Nebula looks just about the same in a 60" telescope as it does in a 10" telescope. It's an amazing optical property: the apparent surface brightness (which can be defined in terms of magnitudes per square arcminute, e.g.) cannot be changed by any combination of lenses or mirrors or other standard non-photo-multiplying optical components. It also cannot be changed by distance. So if you lived on a planet one hundred times closer to the Orion Nebula, it would appear in the sky exactly as it does at 100x magnification in a telescope from Earth. That same dusky, "barely there" but beautiful nebulous glow.

    -FER

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