Camera control

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10.2.1 Where to get special camera cables

The following pages describe a number of homemade cable releases and computer interface devices. Please do not build these circuits unless you understand them thoroughly and can use test equipment to verify that they are assembled correctly. It would be a pity to damage an expensive camera while trying to save money on accessories.

Makers of camera control software can tell you where to get the special cables if you can't make them for yourself. One major supplier is Shoestring Astronomy (www.shoestringastronomy.com), which has plenty of up-to-date information as well as products for sale. Another is Hap Griffin (www.hapg.org). By the time you read this, there will probably be others; some Internet searching is in order.

10.2.2 Tripping the shutter remotely

To trip the shutter without shaking the camera, and to hold it open for a long exposure, you need some kind of remote control cable. Unfortunately, DSLRs don't accept the mechanical cable releases that we relied on with our film cameras. Instead, they use electrical cables, or in some cases infrared signals.

It's useful to know which other cameras take the same cable release or infrared controller as yours. For example, Canon's literature says that the Canon Digital Rebel (EOS 300D) takes the Canon RS-60E3 "remote switch," which has a 2.5-mm (3/32-inch) phone plug. So do the EOS 350D and 400D. The same accessories will plug into the cable release socket of any of these.

But the EOS 20D, 20Da, 30D, and higher-end Canons require the RS-80N3, which has a three-pin connector all its own (Figure 10.4, p. 122). Canon also makes the TC-80N3, a cable release for the same cameras that has a digital timer built in.

All current Nikon DSLRs work with the Nikon ML-L3 infrared remote release. The D70s and D80 also take an electrical cable release, the MC-DC1.

Third-party manufacturers make cable releases equivalent to Canon's and Nikon's. Some of them have special advantages, such as extra-long cables, but some are shoddily made and apt to come loose from the camera. Shop carefully.

Making a cable release for the Digital Rebel

It's easy to make your own cable release for the Canon Digital Rebel family (EOS 300D, 350D, 400D, or any camera that takes the RS-60E3). Figure 10.1 shows how. The connector is a 2.5-mm phone plug. That's the smallest of three sizes, used on headsets for mobile telephones; in fact the easiest way to get one may be to take apart a cheap mobile phone headset.

EXPOSE

TIP ^ SLEEVE

Figure 10.1. How to make a cable release for Canons that take the E3 connector (2.5-mm phone plug).

As the illustration shows, the three connectors on the plug are called Tip, Ring, and Sleeve. Connecting Ring to Sleeve triggers autofocus; connecting Tip to Sleeve trips the shutter.

In astronomy, we don't use autofocus, so only one switch is really needed, from Tip to Sleeve. The bottom circuit in Figure 10.1 is for DSLRs. Some film EOS cameras require you to tie Tip and Ring together if you are only using one switch.

What's inside the camera? Figure 10.3 shows an equivalent circuit determined by voltage and current measurements from outside the camera (and suitable application of Thevenin's Theorem). The equivalent circuit shows that you cannot harm the camera by shorting any of the three terminals together, or, indeed, by connecting anything to them that does not contain its own source of power.

In fact, some Canon owners have reported that they can plug a mobile telephone headset into the DSLR and use its microphone switch to control the shutter. Mine didn't work, but there's no harm in trying.

Equivalent Circuit Telephone Cable
Figure 10.2. Cable release built with toggle switch in a small bottle.

Canon EOS 300D, 350D, 400D

Canon EOS 300D, 350D, 400D

Figure 10.3. Internal circuit of Canon camera as revealed by voltage and current measurements from outside.

Customizing a cable release for higher-end Canons

You can't make your own cable release for the Canon 20D, 20Da, 30D, or higher models because the so-called "N3" connector used on it is not available separately.

However, you may want to modify an existing Canon RS-80N3 or equivalent cable release so that it will accept the same phone plug as a Digital Rebel, making it compatible with serial port cables, custom timing devices, and whatever other accessories you may have built.

COMMON (SLEEVE)

EXPOSE

Figure 10.4. Pins on Canon N3 connector (used on 20D and higher models) have the same function as Tip, Ring, and Sleeve on Digital Rebel series. Do not short Tip and Ring together; they have different voltages.

FOCUS (RING)

NO CONNECTION TO OUTER RING

Figure 10.4. Pins on Canon N3 connector (used on 20D and higher models) have the same function as Tip, Ring, and Sleeve on Digital Rebel series. Do not short Tip and Ring together; they have different voltages.

Figure 10.4 identifies the pins on the camera. You can trace the connections into the RS-80N3 and add a short piece of cable with a female phone jack to match the Digital Rebel, connected to the corresponding wires, so that whatever you plug in will operate in parallel with the existing switches. On the wires entering the RS-80N3, Tip is red, Ring is yellow or white, and Sleeve is the copper shield. These are the same colors used on stereo cables, so if you get your 2.5-mm phone jack by taking apart a stereo extension cable, you can simply match colors - but always verify the connections with an ohmmeter.

Going in the other direction, you can fit a 2.5-mm stereo phone plug onto a Canon TC-80N3 timer unit in order to use it with the Digital Rebel series of DSLRs and time your exposures digitally. The best way to modify a TC-80N3 is to interrupt its cable, inserting a stereo plug and jack. That way, you can plug the TC-80N3 into the rest of its own cable when you want to use it with a higher-end EOS camera.

10.2.3 Controlling a camera by laptop The USB link

Every DSLR has a USB interface through which a computer can change the settings, trip the shutter, and download pictures. The USB connector on the camera is a very small type not used elsewhere, but it is standardized ("MiniUSB-B") and you can buy suitable cables from numerous vendors. In fact, the

10k AW

1N4148, [ 1N914, 1N4001

2N3904, 2N2222

1N4148, [ 1N914, 1N4001

Jkm5 Controller Parallel Port Pinout
Figure 10.5. Two ways to control a Canon shutter from the serial port of a computer. For parallel port cable, use pins 2 and 18 (of 25-pin connector) in place of pins 7 and 5, respectively.

cable that came with your camera is almost certainly too short; you'll need a longer one immediately, or else an extension cable.

For the USB interface to work, you must install the WIA or TWAIN drivers that came on a CD with your camera. These can also be downloaded from the manufacturer. They enable the computer to recognize the camera, communicate with it, and download files from it. Astronomical camera-control software relies on these drivers.

Parallel- and serial-port cables

One crucial function is missing from the USB interface of Canon and Nikon DSLRs, at least so far. There is no "open shutter" or "close shutter" command. Through the USB port, the computer can set an exposure time and take a picture, but only if the exposure time is within the range of selectable shutter speeds (up to 30 seconds). There are no separate operations to begin and end an exposure on "bulb."

Accordingly, you'll need an ersatz cable release, plugged into the cable release socket of the camera (or wired to an infrared controller) and controlled by the parallel or serial port of your computer. In conjunction with software such as DSLR Shutter (freeware from www.stark-labs.com) or any major DSLR astropho-tography package, the computer can then "press the button" for as long as it wants.

Canon 400d Energy Astrophotography
Figure 10.6. The serial cable circuit can be assembled in a large nine-pin connector, then connected directly to the computer or to a USB serial cable.

For details of the cable needed, consult the instructions that accompany your software; there are many variations. Figure 10.5 shows one popular type.

The software interface is very simple. Instead of outputting data to the serial or parallel port, the software simply sets one pin positive instead of negative. On a serial port, this is usually the RTS line (pin 7; pin 5 is ground); on a parallel port, pin 2 (and pin 18 is ground).

The value of the resistor can vary widely; erring on the side of caution, I chose 10 k^, but some published circuits go as high as 47 k^. The second version of the circuit uses an optoisolator to eliminate a ground loop, but if the camera and computer are also tied together by their USB ports, this is a moot point.

If your DSLR isn't one of the Canons with the 2.5-mm phone plug, Figure 10.7 shows what to do. You'll need a cable release or infrared controller for your camera. Find the switch in it that trips the shutter; use a voltmeter to identify the positive and negative sides of it; and wire the transistor across it.

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Reasonable care has been taken to ensure that the information presented in this book isĀ  accurate. However, the reader should understand that the information provided does not constitute legal, medical or professional advice of any kind.

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