Modern digital SLR cameras have made nighttime photography tremendously popular. Star-filled landscape photos and glorious images of the Milky Way and distant galaxies are now within the reach of aspiring astrophotographers. Combining previously unheard-of sensitivity with game-changing image quality, these technological marvels can turn night into day. But what a DSLR can’t do is stop the Earth from turning.
Put your camera on a regular tripod, and in just a few seconds of exposure, the stars begin to transform from intense points of light into elongated streaks. The longer the exposure, or the greater the focal length of your camera’s lens, the more pronounced the effect becomes. Blame it on the turning Earth. The same motion that makes the Sun appear to rise in the east and set in the west also carries the stars across the sky. The only way around it is to move your camera in the opposite direction at the same speed — one rotation per day. In a nutshell, that’s what this nifty tracking platform does.
The Hinge Tracker accomplishes this trick the simplest way possible. A basic regulator circuit powers a DC gear-head motor, which turns a pair of gears that engage a curved length of threaded rod, and makes the hinge open at the correct speed. You can build it in a weekend about $75 or less.
“You can achieve the equivalent of longer exposures by taking several short ones, and adding the resulting frames together with software”
Serious question—why not take many short exposures, no more than a few seconds each, and do the necessary rotation in software as well?
I keep hearing about how longer individual exposures are better, but so far I have yet to see any clear explanation of the reason. It seems like there should be no difference, assuming equal total exposure time.
You want to maximize signal to noise. A longer exposure achieves this. the star field is the “signal”. The ambient light is the “noise”. Doing it via software can acheive the same thing but you will hve to do much more “noise” filtering.
More noise filtering than just averaging the frames? So far as I can tell, that’s all the camera is doing with a longer exposure—averaging the input over time. Software can do that easily, and can even do things like subtracting “dark frames” from each frame before stacking, which is what you would need in order to actually remove ambient light.
I just don’t see any difference between what the camera is doing (especially if it’s digital; at least film is continuously exposed for the entire time, whereas a CCD is periodically reset and the samples accumulated by the camera’s firmware) and what the most basic stacking software would do with many short frames, apart from the fact that the camera can’t rotate and align the individual frames.
Well, it’s just physics… but don’t take it from me… perhaps another reference?
“It should be noted that the most important thing in astrophotography is SNR, or signal-to-noise ratio. The higher your per-frame SNR, the better the results…stacked or otherwise. You could take 120 5-second frames, or 5 120-second frames…the five 120 second frames are always going to produce better results. You could even take 500 5-second frames, and the 5 120-second frames are still going to produce a richer result, since the per-frame SNR is much higher. Each frame contains richer, more complete information that you are unlikely to ever fully replicate by stacking much shorter exposures. ” http://photo.stackexchange.com/questions/40188/longer-exposure-lower-iso-or-shorter-exposure-higher-iso-what-gives-better
Thank you, that reference was very helpful. If I understand it correctly, it’s saying that there are two major types of noise to consider, shot noise and read noise. The shot noise should correlate with the total number of events (photons) and thus with total exposure time, so that part would be no different… but read noise is added to every frame by the camera, more or less independent of exposure time, so more frames equals more noise—and longer exposure times per frame means fewer frames, and less read noise. Does that sound about right?
Has anyone had success finding the parts to make this? I found the motor through scientificsonline.com and on their ebay store but finding the spur gears is not proving very easy. SDP/SI doesn’t have them in stock.
You don’t need the wood block for the ball head mount at all. Just mount the ball head directly onto the hinge with an appropriate screw or bolt. If you align the 0 degree reference mark with the axis of the hinge pin, the degree scale reads declination (or inverse declination) directly. The ball head allows you to rotate the base of the camera about the axis of the lens as desired to compose your photo. For example, you might want the camera base to be in the same plane as the ecliptic. This works easily if the cut in the ball head sleeve is aligned with the 0 degree mark as they are usually made.
Has anyone built this tracker using a Pulse width modulated power supply or a DC to DC buck converter ( LM2596) instead of the linear reg LM317T? As maybe less battery power will be lost as heat. Is either even a feasible replacement for the LM317T?
That said a bit of excess heat may come in handy on cold nights to keep the battery going.
The minimum voltage input for the LM317T is 4.2 volts, (Depending on motor current draw.) would it be possible to power this via a 5 Volt, 2 amp USB charger?
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