Q: What are ITAR restrictions on export & transport of night vision devices out of the US?
Q: What is the difference between tube manufacturers?
A: There is a whole lot of discussion about this on various tactical/military sites like AR15.com, but with regards to astronomy applications of night vision, this is a good post about the differences between ITT and L3 for White Phosphor tubes.
Q: It seems that Night Vision is really effective for cutting out light pollution from urban areas. Does this mean that I don't need to go out to dark sites any more? Alternatively, if I live under dark conditions, is NV basically useless for me?
A: The answer to both questions is a resounding "NO!" Night Vision devices amplify the light that is already there. If you have a lot of light pollution, it will also amplify the light pollution. Of course, this can easily be solved by using filters to cut out the unwanted light, and then the NV device can make up for the loss of brightness via artificial amplification. So in a sense, you're getting "free contrast", at the cost of potentially distracting scintillation "sparkling" in your field of view. However, if you are under truly dark skies, with great transparency and naturally great contrast, Night Vision will simply make your experience much more magical. It can amplify just the "good photons" with no decrease in contrast, and no need to block out light-polluted bands of the spectrum. The sky will be "filthy with stars".
Q: Can I use night vision devices to look through a traditional eyepiece?
A: Yes! However, there are optical considerations and depending on the particulars of your telescope, eyepiece, and night vision device, you may experience coma, vignetting, or other optical limitations. See the "Digiscoping" section of the Optical Considerations of Night Vision page for more information.
Q: Isn't Night Vision awfully expensive for such a small, simple device? Is it really worth it?
A: Emphatic yes, followed by a paragraph of eloquent advocacy for NV+astro; coming soon Posts to reference:
Q: What's the difference between Gen2 and Gen3? Why do you recommend Gen3, if Gen2 works and is cheaper?
A: There are some great discussions around this topic on the Cloudy Nights forums. A technical answer can be found in posts like this one or this one. The short answer is that while Gen2 devices use a different type of phosphor that is more sensitive in certain wavelengths, Gen3 devices almost always outperform Gen2 devices in terms of contrast.
It's a nuanced topic because there are several very experienced members of the Night Vision astronomy community who have Gen2 and Gen3 equipment. But for most people just getting started in this area of our hobby, my recommendation is to get the best tube you can afford, and that is oftentimes a Gen3 tube.
A: This is a matter of great discussion on the forums. The following threads contain great information: 'NV afocal observing with TeleVue/TNVC adapter and a 20" f/3', 'Afocal observing'.
In summary, when doing afocal projection, the eyepiece's focal length, divided by the ~26mm focal length of the Night Vision eyepiece, yields a focal reduction or magnification. So, when a 55mm Plossl is afocally projected onto a Mod3 night vision device, it acts as a 0.5x reducer. The downsides of afocal include (1) much header pile of weight hanging off of the focuser; (2) difficult to find robust mounting solutions, except for the Televue-TNVC afocal adatper which only work with Televue eyepieces; (3) great complexity in switching out filters, because they generally need to be screwed into the front of the eyepiece; (4) cost of filters, which now have to be 2" in order to screw into the bottom of eyepieces, as opposed to 1.25" which is all that's needed for the input window of an image intensifier device.
Prime focus has several advantages: (1) fewer optical elements in the light path; (2) much lighter weight and less cumbersone; (3) smaller filters, and the possibility of using cheap 1.25" filter slides.