Proper exposure for stars, no moon?

[percepts]

"It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) " GPS

Study more, my son, it's the post #...!

You have a talent for being unhelpful. There's always one...

[GPS]

You have a talent for being unhelpful. There's always one...

No, I'm a kind boy, believe me... The post # 47 hints at the reason... Cheers!

[percepts]

No, I'm a kind boy, believe me... The post # 47 hints at the reason... Cheers!

So oh smart one, where in the equation does the fact that on the equator at midnight looking vertically up at a declination 0 star, the star will be moving across the film or sensor at 1 deg of subject view every 4 minutes. 6 hours later, the star will be moving across the sensor at 0 velocity. And in between those two times the star will be moving at maximum speed to start off with and slowing down as you get to 6 hours from midnight. Declination hasn't changed so what parameter has changed and what spatial coordinates are important in calculating that change?

[GPS]

So oh smart one, where in the equation does the fact that on the equator at midnight looking vertically up at a declination 0 star, the star will be moving across the film or sensor at 1 deg of subject view every 4 minutes. 6 hours later, the star will be moving across the sensor at 0 velocity. And in between those two times the star will be moving at maximum speed to start off with and slowing down as you get to 6 hours from midnight. Declination hasn't changed so what parameter has changed and what spatial coordinates are important in calculating that change?

Sorry percepts,
I'm kind but not so much that I'd serve you as a teacher of the basic astronomy that you need to study. Just a detail - looking vertically up in the night (at the zenith) you don't look at a declination 0 star. This time your observing latitude plays roll... Happy studies!

[percepts]

Sorry percepts,
I'm kind but not so much that I'd serve you as a teacher of the basic astronomy that you need to study. Just a detail - looking vertically up in the night (at the zenith) you don't look at a declination 0 star. This time your observing latitude plays roll... Happy studies!

Well I did say equator which is close enough to celestial sphere for this discussion. So what declination is zenith from celestial sphere equator.

You haven't answered previous question.

[GPS]

If a star that's moving on the celestial sphere after 6 hrs stops to move it's the time to take pictures of it - no star trails, what bliss! Or maybe it's a time to take sleep, who knows what could happen in the next 6 hrs.

[percepts]

If a star that's moving on the celestial sphere after 6 hrs stops to move it's the time to take pictures of it - no star trails, what bliss! Or maybe it's a time to take sleep, who knows what could happen in the next 6 hrs.

I din't think you got it. Very noisy discussion.

[GPS]

Don't care about that, it's not important. Cheers!

[Kirk Keyes]

And further more how about you explain:
"It's useless to give him advice about f2.8 or f4... Stars being point light sources depend on the actual aperture opening dimension in their effects on the film. (a f4 on a 300mm lens is not the same as f4 on a 65mm lens, for a point light source.) "

Well, I'll bite...

300mm/4 = 75mm
65mm/4 = 16.5mm

The 300 mm f/4 will gather much more light as it has a greater aperature than the 65mm f/4. As was said earlier, recording point sources like stars is dependant on the absolute aperature/diameter, and not the f/number of a lens. The 65mm lens will perform much worse at reacording star trails than the 300mm.

[GPS]

...
Having said all that, since your camera is digital, you should have zoom software in the back to see the images in fine detail. Therefore why can't you just take some shots and check the trails or not at the time, so you can be 100% sure you have no star trails. Thereby negating the need for any of this astronomical stuff.

Absolutely right, percepts!