Part I: Fast without the furious: Why your SBIG CCD camera doesn’t have USB 4.0Colin Haig
There are lots of marketing claims about fast camera download speeds and ever-faster interfaces. It makes you wonder why your SBIG CCD camera doesn’t require a super-fast interface that races along the data highway into your computer. Why is this?
It’s really two simple reasons:
- Cable Length
- Sensor Readout Rate
In Part I of this blog, we’ll explore Cable Length, and in Part II, we’ll check out the Sensor Readout Rate.
USB 2.0 for Longer Cable Length
When designing an interface between a science camera and the computer, we look at the typical applications for the camera and the available interface technologies. There are always trade-offs the designers and engineers make, and we like to be conservative in our approach. This tends to give greater reliability and longer lifespan to the design.
USB 2.0 cabling has a huge advantage over USB 3.0 and newer versions of the USB specification. That advantage is cable length! With not-so-old-school USB 2.0, you have a generous 5 metres (16.4 feet) of cabling. In a typical small observatory with telescopes under 0.5m in diameter, this is usually plenty enough to get from the camera to the computer, even in the case of German Equatorial Mounts that have to do a pier flip as they cross the meridian. USB 2.0 is plenty fast, running at a peak of 480 megabits/second (about 5x faster than 100BaseT ethernet that was the standard for years). We’ll touch on this more later.
Although slick marketing tries to convince people that a bigger number means something is better, there are some cases where this simply isn’t true. More megapixels in a camera sometimes just means they are smaller and gather fewer photons. In the case of USB 3.0, its theoretical blazing speed comes with a major limitation for our purposes.
Practically speaking, USB 3.0 is limited to 3 metres (9.8 feet) of high-quality cable. For our typical customer’s needs, this is barely enough to get from the camera, down the pier and to the floor for modest sized telescopes. This is one of the reasons our latest USB 3.0-capable cameras such as the AC4040 and STC-7 are also USB 2.0 compatible.
To get around these length limitations, a lot of folks try to make their USB 3.0 camera work with a cable extender device. They sometimes forget that most of these extender devices aren’t designed for cold-weather operation, and when they drift off frequency, reliability goes out the window. Here again, quality engineering costs more, and USB 3.0 performance often degrades when using inexpensive or indoor-only extenders and hubs.
What if your computer rides around on top of your telescope? This trend has become more popular, where people use a nano-PC or embedded computer that is mounted within a couple of metres of the cameras. In this case, USB 2.0 or 3.0 work fine. A faster USB 3.0 connection doesn’t address the other design factor, the sensor readout rate.
In Part II of this blog, we’ll continue with the Sensor Readout Speed Limit and how fast it will go.