May we have some color please?

[don]

Selden, thank you for your explanation. By "low temperature units", I assume you are talking about the electronics used to gather the light, since film would freeze and eventually crack at low temps (below freezing). Could CCDs function under low equipment temps and maybe even provide better performance in the saturation deparetment?

Grant, thank you for describing how eta comes into play at relativistic speeds. The shrinking diameter part is a surprise to me. Also, I had not thought of some objects appearing dimmer due to their EM wavelength shifting to higher frequencies. This would obviously make traveling through some wavelengths at relativistic speeds quite dangerous due to them shifting into the x-ray and gamma ray region! Very strange indeed.

-Don G.

[granthutchison]

The shrinking diameter part is a surprise to me.

It's because of something called light aberration - if you're moving quickly, objects appear to be displaced towards your direction of motion, because their light seems to be coming from ahead of you. (Run through vertically falling rain - your front gets wet but your back stays dry. Run through light rays coming at you sideways, and they look like they're slanting in from in front.)
For an object that's directly ahead of you, the light from its edges is displaced towards its centre, because that's your direction of motion - so it appears to shrink in on itself. (And an object dead astern will undergo the reverse effect and appear to expand.)

Grant

[don]

That's amazing.

Sounds like a "light cone" to me -- smaller in front, larger in back.

Thanks Grant.

[selden]

Don,

Refrigeration is often used to extend the life of film. Careful attention does need to be paid to to the rate of cooling and warming. Serious astrophotographers don't always use the types of film available at your corner drug store. They have to buy in bulk to make it worth while for Kodak to make a batch of specialty film, so they also have to store it for a long time.

Refrigeration units are used for film when actually taking pictures, too. See http://members.shaw.ca/jmirtle/coldcam.htm for a description of some of the problems involved with cold-film photograpy.

[don]

Selden,

Using dry ice in a camera is something I had never heard of before. There is quite a noticable difference in photo quality too. Thanks for the link. Simply amazing!

Yes, storing film in the fridge is something we used to do also, before we got a digital camera.

-Don G.

[Buzz]

OK, I agree now with having a faint milky way, even one much more transparant than it is right now. But I would really like to see it more like a single thin cloud with dark bands rather than the current collection of grey blobs. I have to admit that I like the way it looks in Starstrider much better (quote from someone: "ducks and runs") Maybe it is easier to do for them because one cannot travel outside the milky way in StarStrider if I am correct, so they can just use a background picture instead of a 3D cloud.

[mrzee]

G'Day to you too Don and everyone else here!

Was interstate for the last few days and decided that it's about time I got a telescope while I was there. Got myself a cheap ($650 Aus) skywatcher 90mm mak-cas mainly for the portability, it's also dual purpose as I can use it as spotting scope when target shooting.

Even has a camera adaptor, though I doubt very much it would be much use on anything but the moon and sun (with the appropriate filters). Still I'm up for the challenge so might give it a go.

Got a few questions for the "wise ones" about this whole telescope thingy and comparing it to what the computer says the sky will look like. Here goes:

1) I can't get Celestia to match the sky using the mag limiter. Some of the stars seem to be displayed on the screen but not visible and visa-versa. This is only a small discrepency but makes it look confusing for the newbie telescope operator. This effect is present with naked eye observations.

2) The magnitudes for the planets don't seem to be tied in too well with the magnitudes of the stars. Why no mag display for planets. This would make it easy to decide if it's even worth the trouble of looking for it!

3) Looking at Mars I saw the ice cap and it was where Celestia showed it would be. The two moons were shown in Celestia were quite bright but I couldn't find them in the scope. I forgot the left-right reversal in my scope, so corrected my expectations and still no moons??... 10:00pm EST 13/10/03 Melbourne, clear night.

4) A simple explanation why a shorter F number is brighter than a longer F number. Have trouble understanding that two mirrors of equal diameter provide different brightness depending on their focal length??

5) Is it possible to show the current cursor position as celestial coordinates? This would make it so much easier to find objects using a scope. Compass heading would be nice too, so would a zenith line on the celestial grid when given a latitude on the surface, rotate when on surface keeping the horizon level.... only suggestions, maybe one is worthwhile???


6) How a open source program can generate enough interest for me to spend over a weeks wages to buy a telescope???? Not to mention the many hours looking at the sky and possibly more money for a non dual purpose scope.....



Many thanks for all the info guy's and for such a great program.

[Buzz]

Hi MrZee,

I have answers to some of your questions:
- the moons of mars are really very small. The magnitude in Celestia is not realistic.
- a larger focal length results in higher magnification, so the same amount of light is spread over a larger area and the image will be less bright.

[granthutchison]

3) Looking at Mars I saw the ice cap and it was where Celestia showed it would be. The two moons were shown in Celestia were quite bright but I couldn't find them in the scope. I forgot the left-right reversal in my scope, so corrected my expectations and still no moons.

Two things:
1) Although the Celestia magnitude of the Martian moons is probably not too far off (the brightness calculation is a simple one), they're very close to the very much brighter Mars, which tends to wash them out - your computer screen is incapable of generating the sort of brightness contrast that exists in the real world.
2) The orbits for the Martian moons are currently defined as simple ellipses, rather than highly accurate CustomOrbits: so they're correct for 1 Jan 2000, but are likely to be slightly adrift by now, thousands of orbits later.

Grant

[mrzee]

Thanks for the info guys.
I keep forgeting the dynamic range of a monitor is nowhere near the capability of real life vision. Didn't realise the moons orbits were based on a simple model either.

With the F ratio: I'm aware the larger focal length gives more magnification but from what I've been reading says the faster (smaller F ratios) are meant to give a brighter image with the same magnification than compared to a slower (larger F ratio). They claim it's only noticable with film as our eyes won't see any differrence. This makes no sense to me....

Thanks again.

[Buzz]

Maybe they assume it will be compensated by your eyes, because eyes can handle a large range too...

[AdamOfTheTable]

I totally forgot that computer monitors are such a limitation. I guess some new kind of display technology will have to be able to display realism in the future, as phosphorus is limited (as this thread made me remember). I guess the problem with monitor range is partially the fact that phosphorus glow will bleed over if too bright. It can't be confined realistically. Sometimes bright things in Celestia leave ghostly trails as I move them. Then again, my monitor is huge, but I think slightly damaged. There's a little discoloration visible in certain backgrounds, like it has burned pixels or someone put it too close to magnetic stuff. It was made in 1996 I think (Cornerstone Color 20/77). That's a real bummer that graphics are limited when it comes to realistic contrast.
Oh and yeah, the Milky Way being grey did make me think. I like those cool 3ds nebula models up close, but they look annoying and unrealistic from far away (not to mention making the program jerky when they load into view in the distance). They are annoying in solar system skies. I'm taking them out for now.

[mrzee]

Just thinking out loud.
Though a monitor hasn't the same range as direct visual observation, in the case of the martian moons it seems to be working for the monitor rather than against it. So shouldn't it be possible to introduce a fudge factor to display the appearance as would be seen with the eye or through a telescope? I suppose to do it correctly would take a large amount of proccessing power as brightness of objects and their proximity would need to be taken into account. Maybe seperating the magnitude limiter so independant adjustment is possible depending on the type of object would work? I have found similar issues with the galaxies and addons. No big deal but it's hard for me to relate the screen view to the real view for some parts of the sky. Though that also has a bit to do with the fact I'm still in-experienced looking up at the sky and I cant get the correct rotation about the horizon so I need to visualise the sky rotated as well as watch for magnitude variances.

Got some time to practice again as my telescope has a quality control problem and has to be sent back to be replaced:(

Maybe the sky's above Melbourne will clear up by the time I get it back....