jeff_zylland

I have looked through some night vision scopes. The near IR illuminated ones work great, but the intensifiers that run off ambient light can work very well, too--but you definately pay for quality. Good genII or genIII night vision scopes cost in the thousands to tens of thousands of dollars. These scopes usually have an optional illuminator and you get the best of both worlds. Then there are far IR scopes that can see infrared from ambient heat, but these are generally not available to civilians, except in a few luxury cars. Many night vision scopes can be made to adapt to camcorders.

I don't think there is a lens that passively "gathers" so much light as to make the night brighter. I believe this breaks the 2nd law of thermodynamics--making a diffuse energy source (ambient light) more concentrated without doing work. It sounds like an urban legend. You need an active image intensifier tube to achieve this.

I have done quite a bit of astrophotography (and have since quit for the present because it takes too much time, but still have some old images posted). For the most part, this is correct that exposing a point source depends mostly on aperture, whereas exposing a finite surface depends on focal ratio, as Tom pointed out.

There are a few factors which can change this in the real world. At a fixed aperture, star trails of Orion will be smaller at a smaller focal length (note this is not the case for photos around polaris because the angular movement is the same at all magnifications), perhaps allowing dimmer stars to be photographed. Also, the density of exposed stars will be higher in the shorter lens--giving it a "brighter" appearance. Compare a standard Schmidt camera photo (f/1.5-2) to a Schmidt-Cassegrain photo (f/6-10) and you will more stars in the S-camera images by orders of magnitude.

As mentioned, diffraction plays a large role in photography--especially of stars. What you are seeing when you see a star through perfect optics is its diffraction image--the airy disc surrounded by concentric diffraction rings. At a fixed aperture, a longer focal length has the same amount of light travelling through a larger airy disc--thus the light density is, in fact, dimmer--just as if the star was not a point source, but a very small object. Reciprocity failure in film emulsion compounds this dimming effect. Essentially, light response of film is no longer linear at very low light levels and there is a cutoff level at which light can no longer be recorded. The dimmest photographable star in a certain setup is just at the limit of reciprocity failure. Increase the focal length, and you increase the airy disc size and dim the image below the detectable limit, independent of exposure time. Thus, with perfect equipment and conditions, you should be able to photograph dimmer stars with faster optics. This effect is only slight or nonexistant at "normal" photographic apertures (f/2-8), where the blurring due to imperfect optics (typically worse in faster optics) may have a greater effect. This is very noticable if you are doing high-powered telescopic photography: "eyepiece projection" photography typically results in f/20 to f/30 effective focal ratios.

But:

Sky fogging is related to focal ratio, as another (Tom?) mentioned. You can pick a focal ratio at which skyglow is below the reciprocity limit--at this focal ratio you can take an all-night exposure. Compare this to a smaller focal ratio (same aperture) that may fog up after a few minutes. It seems that the larger focal length (larger focal ratio for same aperture) might be able to take much longer exposures and perhaps capture dimmer stars.

By the way, with a short lens (<50mm) pointed at the ecliptic, you can take shots for about 20 seconds without much detectable star movement (ie, the diffraction and "bleeding" of the image is comperable or larger than the movement). You don't need a research grade drive system with that kind of allowable error. A "barn door" tracker works fine. You do need accurate polar alignment, though, if you want to take long (>5-10 minutes) exposures. Without accurate alignment, field rotation creeps in, which, because it is rotation, affects images independent of focal length.