There's simply no way that the fans don't cause considerable vibration artifacts for such long exposures. Am I missing something?
1) If your setup is sophisticated enough that fan vibration may cause an issue, you're likely already using a german-equatorial mount and some kind of guiding correction (secondary guide scope, or off-axis guider). These adjust the scope's positioning in real-time to keep on-target. So that removes any large-scale deviation that may be caused.
2a) Small-scale deviation is generally not a problem, since most setups are likely to be limited by the resolution of your entire optical train. For example, my 800mm newtonian combined with a CCD with 7.2mm pixels means I have a resolution of ~2.01 arc-seconds. Assuming perfect guiding, my scope can tolerate any vibration as long as my target stays within ~2 arc-seconds. *
2b) In reality, most locations are limited by seeing conditions (e.g. how turbulent the air is), which averages 1-3 arc-seconds in non-mountain regions. Also the calibration of your mount (periodic errors, guiding errors, etc) are almost certainly a greater concern than the vibration added from the peltier.
That's from an amateur's perspective. Obviously high-end setups will start to worry about things like the fan's vibration, but for someone with a backyard setup (doubly so for someone using a consumer DSLR instead of dedicated CCD), there are bigger causes of inaccuracy :)
* This is a bit of a simplification, since things get a bit more complicated due to FWHM, wavelength, etc.
The main thing I contend with is backlash on my mount - imperceptible amount of play in the worms means I have to overbalance the scope significantly.
As I said elsewhere in this thread, distant quarrying (oh, and moles) cause far more shake than a good fan.
Speaking of... I need to finish that up soon before winter season starts in earnest!
They're typically fans with high quality bearings that also are spinning slowly. There's essentially zero vibrations coming out of that.
It's very hard to get decent results out of any optical stack, especially for photography, without having any sort of fans, heaters, coolers, and other air flow and temperature management devices in various places.
Perhaps they use the fan to initially cool the sensor and then switch it off and let the Peltier cooling maintain the temperature or something?
Seems like some sort of liquid cooling system where the radiator/fan assembly was not mechanically coupled to the camera body (except by flexible plastic tubing) would be a better approach.
[1] For example, I have an antique 400mm focal-length lens that really is about 400mm long. With it attached to the tripod near the middle, and the camera at one end, there is a lot of potential for bouncing around.
It's a high quality fan, and it's hard for a 5g fan to introduce vibrations in a 100kg+ mount+telescope setup.
Such cameras (sold by Nikon [1]) are routinely used on high-end microscopes. They usually have a smaller resolution though - 1024x1024 is pretty common. There are a number of technologies used in scientific microscopy that work well for night-time photography, but most are just too expensive. Electron multiplying CCDs, chilled CMOS sensors, etc. They end up being one of the more expensive parts to a half-million dollar scope though.
[1] https://www.nikoninstruments.com/Products/Cameras/Camera-Hea...
Image stacking can reduce noise afterwards, but there's no substitute for collecting more photons to increase the signal in an image... and the only way to do that is to try and clean up the noise floor (via cooling, better sensors, etc) :)
For $1000+ I imagine the fans actually don't have much impact.
Edit: yes, that's how active systems work: gyro stuff and counteracting by moving the lens or the sensor¹. For astronomy rather a lens, because the sensor would have to move too far for the big shifts the tele causes².
¹ http://photo.stackexchange.com/questions/804/how-does-image-...
² https://photographylife.com/lens-stabilization-vs-in-camera-...