In a telescope, can a close-by object be sharper than the stars? Is it possible?

Question

Because there's less intervening atmosphere to see through.

Does seeing work that way?
Can it be, say, 1" for a celestial body and 0.5" for a tree a few hundred yards away?

Assuming your telescope is large enough that Dawes limit doesn't push in of course.

Is Airy disk size directly proportional to to airmass? something like if 1 airmass = 2" then 10 airmasses= 20"?

Answer 1

Stars aren't sharp. In their best earth-seeing conditions, they have discs of about 0.5 arc seconds. They really are dimensionless points, but they appear to be 0.5 arc seconds. 0.5 arc seconds corresponds to 1/4 mm at 100 meters.

I will pass on Airy disk discussion. That has to do with the physics of light and occurs even on telescopes in outer space. So I'm unclear on your air masses and what they're doing there. Airy disk size is a function of telescope aperture. The bigger the scope the smaller the Airy. In a 4" scope you can see the Airy disk almost any night by using a 3.5 mm eyepiece on a bright star. In a 14" scope you can go for years without seeing an Airy. They become tiny and extremely sensitive to seeing (they breakup). If my 14" were in outer space (and I could use it) I could get an Airy every time.

I've done a fair amount of terrestrial observing using astronomical telescopes. I've done it with 50mm, 80mm, 102mm, 108mm, 114mm, 8 inch, 9.25 inch, and 14 inch astronomical telescopes. I've done it over forests, over grass, over lakes, over rivers, and over asphalt.

The main problem is that you use telescopes for terrestrial observing during the day. During the day you have maximum thermal effects everywhere. So things get pretty messed up. The messed-up-ness from thermal effects during the day is a huge deal and interferes with solar observing too. It also makes it hard to observe stars in the daytime (which can be done). So it's not just "looking up" vs. "looking across." You look up at night when things are calming down. You look "across" by day when things are very agitated.

The other thing that messes up your daytime views is that the telescope heats up and you get wicked things going on inside that telescope.

My own daytime tests under "rigorous" conditions indicated that the telescopes were not performing as well as they should. This was over a mix of grass and asphalt. But I didn't estimate whether they were doing worse than "at night." Seeing is extremely variable after all--by day and by night.

I can also say that some of the most extraordinary views can be had. I've seen the color of a heron's eye at a distance of one mile and I've seen veins in the wings of flies buzzing around pine cones on trees that were far away. I have read texts from magazines that I posted on street signs.

What is mind blowing about using an astronomical telescope by day is that you have a *reference* for what you are seeing. You *know* it's impossible to read a magazine at the far end of the street and there you are doing it. Separating a double star may be harder but we don't relate to it in ordinary life.

You will also discover that some eyepieces, especially the Televue Pan Optic series, are "unacceptable" for terrestrial viewing due to their distortions. A straight wall or pole will look bent. Other eyepieces may add a lot of false color that can be annoying.

Well, if you want to get your best resolution using a telescope by day here is my advice. Observe on a cloud covered day near dusk so that temperatures are uniformly cool and have been cool all day long. Keep your tube out of the sunshine. You don't want night because then all you can see is electric lights or whatever the moon lights up. So "dusk but still light," and cool. Then it is likely that you will be able to push your resolution limits higher on nearby objects than you do most nights.

hope that helps,

GN

Answer 2

Theoretically, the less atmosphere you are looking through, the less distortion you would have. That said, looking at a tree several hundred yards away, you are also contending with a lot of turbulent air just above the ground. Ergo, I would guess that any objects near or at ground level would be more distorted than objects overhead.

A better comparison would be an object that is several hundred yards in the air directly overhead...compared to celestial bodies overhead. In that case, then I would think the closer object would have greater clarity.

Answer 3

Hi. Its a physical property of light due to diffraction. So I do not think so. However, since the tree is so much brighter, your pupils are constricted and may make the image APPEAR sharper.

Answer 4

Since you consider the intervening atmosphere,
of course.

Answer 5

No matter how good your telescope is stars remain nothing more than a pinpoint of light when viewed through it. Use your brain, man.