Be Ready For The Next Celestial Event
Don't get caught unprepared when then next great comet comes by.
You can get ready for some piggyback astrophotography by making the simple piggyback camera mount described on this web page. With it you can get some great astro images of the next comet, or many of your favorite wide-field stellar objects like galaxies and star clusters. An example is this photo below of the Comet 17P Holmes, which graced our skies in 2007.
If the concept of
astrophotography is new to you, check out the introductory book
Getting Started: Budget Astrophotography
While commercial piggyback mounts are available, they tend to be specific to some of the popular telescopes. But the DIY piggyback mount on this page will work with any 35mm camera (SLR's are best) or any digital camera capable of taking time exposures. The mount described on this page clamps around the telescope's tube, and so can likely be tailored (by selecting the right sized hose clamps) to fit almost any telescope. However, check out the tailored mounts if you have a popular telescope design. It'll save you some work and maybe mount easier.
To take the photographs, you'll need an astrophotography camera. As mentioned, a great camera for the job is a 35mm camera, something like my trusty old 35mm SLR. The camera you choose doesn't even have to have a removable lens as does my old SLR, as some great pictures can be obtained with the camera's stock lens. But the camera must be able to take time exposures, or allow the shutter to be locked open. Most likely, you won't be able to take exposures of over a few minutes unless you live on a remote mountain top somewhere, but a few minutes will be sufficient for many targets. Some DSLR's (digital cameras) can even deliver results with a few seconds of exposure.
You'll need a telescope or at least a sight of some kind for guiding. For the long exposures, you'll need a clock driven telescope or sight. It doesn't need to be a giant telescope. One whose drive could handle the additional weight of a camera is all. I currently have 3 telescope than can do the job admirably, my old MeadeETX 90, my Discovery f/5 Newtonian, and my Celestron NexStar5SE. Each has a clock drive, allowing me to use the telescope to keep on a target as the camera gathers the image.
Start with a 2 by 4 (No Kidding)
The piggyback camera mount shown above is nothing more than a specially cut piece of 2 by 4. Cut off about a 3 1/2 inch length of 2 by 4, giving a square block.
Using a table saw, radial arm saw, or a circle saw, and cut out the center section to a depth suitable for your sized telescope.
It's best to make these cuts with the grain of the wood.
The idea is to have a block that will sit on its side rails with the middle area not touching your telescope.
Add a Camera Mounting Screw
Using the above illustration as an example, drill a 1/4 inch diameter hole through the center of the block.
Run a 1/4 inch screw through the block and tighten on a nut to hold it securely. This screw will be used to hold your 35mm camera or digital camera.
Be sure the screw is long enough to go through the block, a wing nut, and still have a 1/2 inch or so of threads left to mount to your camera.
Thread on a wing-nut in an upside-down configuration. This wing-nut will be tightened against the bottom of the camera to hold it in position.
Finish Off with a Hose Clamp
Go to a hardware store and purchase a hose clamp, shown above, that is long enough to go around your particular telescope. You can likely use a single clamp to go around a small Maksutov Cassegrain or refractor.
If you can't find one long enough because you use a bigger telescope, such as an SCT or Newtonian, you can buy two or three clamps and fasten the ends together. Just thread the tongue of one clamp into the screw of another.
Cut the clamp and drill a hole through the resulting ends (or one end of each of your numerous clamps).
Mount the non-tongue ends of the clamp(s) to the wood block with screws. Now you can run the clamp around your telescope and tighten the hose-clamp screw to secure the mount.
The Piggyback Camera Mount Fastened to Telescope
Above is an image of my piggyback mount on my 6 inch f/5 Newtonian telescope.
You can see the hose clamp band extending around the telescope tube, securely holding the specially cut 2 by 4.
The 35mm camera camera is turned several times onto the protruding screw, and then the inverted wing-nut is tightened against the bottom of the camera, allowing me to align the camera lens with the telescope. The camera shown is taking advantage of the SLR's ability to exchange lenses, as the original lens is replace with a 135mm telephoto lens.
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This particular telescope doesn't have electronic controls in the RA and Declination axes. Rather it uses manual screw controls. To get the best pictures, I use a reticle eyepiece and defocus on a moderate to bright star in the field near my object of choice (or the center of a comet). If a guide star is bright enough, defocusing it slightly lets it better illuminate the reticle lines of my eyepiece. If I need to make a tracking adjustment with the manual controls, I cap the lens of the shutter-locked camera while I make a pointing adjustment, then uncap the lens when finished. This ensures that any vibrations I cause or mistakes I make in adjusting do not affect my photograph. |
And Finally, the Proof
Above you see an image of Hale Bopp that was taken with the camera and 135mm telephoto lens shown in the previous picture. I had a different telescope then, but the setup was the same.
How long an exposure is necessary? For comets, not that much. 3 to 5 minutes
will do wonders. In fact, in my backyard, there's enough light pollution that exposures longer than that will tend to saturate my exposures.
For some of your favorite star objects you may need to go up to 10 or 15 minutes, if your site is dark enough to allow it. If not, you can still get some nice photos of most objects with just a few minutes.
Above you see another example of what can be captured with just a piggyback setup and a small telephoto (135mm) lens. It's an image of the North America nebula in Cygnus. The bright star in the image is Deneb. You can see that my inexpensive little telephoto has a wee bit of coma near the edge of the field of view. The North America shape is not well resolved at this magnification, but it is the nebulosity at the lower left of the image.
Contrast this with what you see when looking at Cygnus with either naked
eye or telescope. The nebula just doesn't show then. But with just a
regular camera and a minute or two exposure, it registers clearly. A
book I recommend, even though it is a very old one, is
Skyshooting: Photography for Amateur Astronomers
Some Astophotography Tips
Obviously, to make use of the simple piggyback mount you will need a clock driven telescope on an equatorial mount. Almost any will do, though if you use 35mm camera equipment as I do, you may not be able to use a small telescope because of the weight of the camera. As the photo below illustrates, if I use my NexStar 5SE for piggyback photography, I need to use it with its built in equatorial wedge.
Any telescope of 5 inch diameter or bigger will likely have a beefy enough
mount to do the job, and some smaller ones may also. You need the equatorial
mount for several minute time exposures because such exposures on a
computerized altazimuth mount will lead to a rotating field of view. A
telescope much like the one I use is the
Orion Astroview 6 Inch Telescope
If you want to go digital, be sure you choose a camera than can do time exposures.
Once the camera is attached to the telescope, the procedure I use is as follows:
This procedure, simple though it is, can give very good results in that you are generally seeing a magnified image in the telescope versus what the camera sees, and tracking errors will be easily detected and corrected before becoming big enough for the camera to record.
To make a point about developing, notice the image above of the Pleiades. It was taken with the apparatus described in this web page, using a camera and a 135mm telephoto lens. It shares an issue with my Holmes image shown previously, that of enlarged star images.
You may notice that the star images aren't pin-point images, but disks. I've wondered about this for some time. Usually my star images are point like, or if guiding was a problem, small trails. This disk phenomenon is new to me. I suspect it is a developing anomaly, probably caused by an automated system trying to make a good print. I got my images for the website from the photo store developed prints.
It reminds me that there is a step you should take with your film camera as a standard procedure. Make the first 2 or 3 frames some general, simple, day-time photograph. An everyday type of photograph, say of your dog or something. Leave an instruction with those doing the development that the film has star pictures, but they can use the first couple of frames to set their equipment. They should not let an automated system try to determine best exposure times for prints on each individual photograph. I believe it was my failure (or their failure to note my instructions) that lead to these enlarged star images.
As to star trails from poor guiding, note that guiding is most important in the first minute or even the first 30 seconds, if you are using a film camera. Film suffers what is called reciprocity failure. That is, film loses its sensitivity to light as it is exposed. So it is very sensitive to light in the first few seconds, and becomes less so over time. So any guiding error in the first few seconds will cause a star trail. Small guiding errors that average out aren't as critical after the first 1/2 minute or so.
This isn't so true if you are using a digital camera. Digital cameras, while having their own problems, don't suffer from reciprocity failure.
Barn Door Mount
Even if you don't have a telescope with a clock driven mount to use as a base for a piggyback mount, you can consider building a barn door mount, sometimes called a hinge mount. These simple mounts use a few common things you likely have in your garage or workshop plus maybe a couple more. There are motorized versions and manual versions.
The idea of such a mount is to have a simple device that can sit on a regular camera tripod and track stars well enough for a camera to collect great star photographs. You can't use a telephoto on such a camera with much luck, but you can get wide field star pictures that are impressive.
Check out the Build a Hinge Tracker site for how to construct a manually driven model, and the Build a Motorized Barn Door Tracker for instructions on how to build a simple motorized tracker.
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