Astrophotography with an Off-Axis Guider
by Dennis Allen
In this age of CCD guiders, you may wonder why I bother with an off-axis guilder. Well, a couple of reasons. One, design. I built my telescope to be equally useful for visual or photographic work. Two, cost. For CCD guiding, you need a CCD camera and a laptop to run it. Three, time. The last time I tried CCD guiding, it took me half a night to set up my equipment. Time I could have used actually taking pictures.
I built a 24" truss-tube Dobsonian with a truss-tube kit from AstroSystems (for complete construction details, click here). To do astrophotography, I purchased a third generation dual-axis equatorial platform. This platform gives me an hour of tracking, making photographic work possible and visual use quite enjoyable. To help locate faint objects, I installed a Lumicon NGC Sky Vector. As you will read later, the Sky Vector computer is also useful in photography.
About five years ago I replaced my old Nikon F with a Cannon Rebel XT. What took me fifteen minutes with the Nikon F, I can now accomplish in five minutes with the Canon Rebel. I currently use a Canon Rebel XSi 450D because it has a live preview feature.
You’ll need an external trigger mechanism. On amazon.com I found "Timer Remote Control RS-60E3 For Canon XS XSi T1i XT XTi" for $16.95. Does the same job as the those $150 units.
I also purchased a Newtonian easy guider from Lumicon. This off-axis guider mounts the camera onto the 2" focuser, allowing prime focus photography. Now the off-axis guider comes with an adapter to a t-ring, so I had to purchase a t-ring to Cannon bayonet ring. I removed the t-ring adapters altogether, mounting the off-axis adapter directly to the bayonet ring. In photography, the camera needs to be closer to the main mirror than the eyepiece. The closer, the better.
Setting up for a shot takes a few steps. First, hook up your off-axis guider to your camera. Set your camera to manual mode and plug in your external trigger mechanism. Rewind your platform. Insert your camera/off-axis guider into the focuser. Find a bright star and focus it in the camera view finder. This is where the live preview feature on my Canon XSi is useful.
Now the off-axis guider has a pick-off mirror and a place to insert your illuminated reticle (lighted cross-hair) eyepiece. A double-lined cross-hair box is preferred. You have to slide this eyepiece in and out until the stars focus (don't touch the focuser). Find the object you want and center it. In the cross-hairs find the nearest star. Use the platform's controller to move this star to one of the corner's of the cross-hair box. Now you're ready to take a shot.
In astrophotography, there's a lot of trial and error. Brighter objects like M8 will need less time than dimmer objects like M20. Try 5, 10, or even 15 minute shots. If your external trigger device doesn't have a clock, use an egg timer. Try different ISO speeds: 400, 800 or 1600. Again, try different speeds, and different exposure times.
Suppose you have an object too dim to see in the camera, then what? Well, what you need to do is find and center the object using an eyepiece. Then memorize the star pattern in the eyepiece and/or finder scope. Replace the eyepiece with your camera, focus the camera on a bright star, and return to the object as best you can.
A telescope computer, like the Lumicon Sky Vector, can help. Before you start your run, turn on your computer and align it by shooting two stars (for an equatorial platform, the mode is "ET"). Now find your object and align the computer on it. After you replace the eyepiece with your camera, the computer should be able to take you back to the exact spot.
As I said, there's a lot of trial and error. To increase my productivity, I mounted a camera adapter to the top of my telescope. There I can use my other camera to take piggy-back shots while I'm doing prime focus shots. For milky way mosaics, 5 minute shots with ISO 800 speed and a 50mm lens work fine. Again, try different speeds, different exposure times, and also try different lens sizes.
Anyway, guiding by eyeball is harder than CCD guiding, but you can still get good images. I haven't done too badly, so click here to see the results.
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This web page was last updated 07/27/17