New Dobsonian Telescope

My Biggest Scope!

I enjoyed using a Dobsonian telescope recently in Australia so I decided to get one for myself. The relative design simplicity and easy setup appealed to me. The Dobsonian requires no electric power, no computer, and no tedious initial mount alignment. I can leave it outside an hour or two before dark to let the mirror reach ambient temperature, and later begin observing immediately. Sometimes, I just don't have the energy for the relatively complicated setup of my computer controlled, equatorially mounted 130 mm refractor even though the refractor eventually finds objects automatically and is capable of guided photography. The Dobsonian isn't much good for getting images, but it's fine for visual observing.

I purchased the new scope from Orion Telescope. It has a 12-inch (305mm) diameter mirror with a 1500mm focal length. It came with two Plossl eyepieces, a 2X Barlow lens, and a 9X50 right angle finderscope. The assembled telescope is pictured below.

Dobsonian telescopes use the basic Newtonian design. Looking down the tube you can see the large primary mirror at the base and the smaller secondary mirror suspended near the top of the tube.

Light enters the open telescope tube, reflects from the curved primary mirror, and converges toward focus on its way back up the tube. The converging light then reflects from the smaller 45-degree tilted secondary mirror and heads toward the side into the focusing tube where an eyepiece magnifies the final image for viewing. The next picture shows the secondary mirror supported by four thin vanes.

Light eventually exits the side of the tube and is viewed through the eyepiece there.

To help aim the telescope at desired targets a small finderscope with relatively large field of view is mounted on the tube. The finderscope is aligned parallel to the large scope's optic axis, so, when everything is properly set up, an observer can first place a target in the finderscope's crosshairs and subsequently find the same target centered in the main telescope's eyepiece. Unfortunately, I found the included right angle finderscope completely useless. I'm used to straight-through finders where I look with one eye at the sky while the other eye looks through the finder eyepiece. I found it almost impossible to comfortably sight a star and get that star in the right angle finder eyepiece. Maybe this was caused by my own lack of experience with right angle finders, but the right angle finder was awful! So I dismounted the right angle finder and installed a more powerful 13X80 straight-through finder from my refractor. This didn't eliminate my difficulties because this larger finder's eyepiece position was very uncomfortable. The wide finder body also blocked simultaneous sky viewing using both eyes.

Based on conversations with an expert Dobsonian observer during our Australia trip I decided to purchase a Telrad finder. The Telrad is the rectangular box-like object attached to the tube in the center of the next picture.

The Telrad finder doesn't provide magnification or light gathering power like a traditional finderscope. Instead, the observer views the sky through a tilted piece of glass. The tilted glass is transparent enough to allow starlight through. It simultaneously reflects light from a set of illuminated concentric red circles toward the eye. The observer sees dimly glowing red circles superimposed on a view of the sky. When aligned properly, the observer puts a target object in the center circle and finds the telescope pointing at that same object. The next picture shows a view of the first quarter Moon through the Telrad. My phone camera focused on the nearby Telrad body instead of the distant Moon. This made the Moon and illuminated circles out of focus in the image below. To my eye, however, the first quarter Moon and red circles were clearly in sharp focus.

With the Telrad finder I now have a practical, efficient way of aiming the telescope.

The telescope arrived in several boxes and required assembly. I was surprised to find the big mirror needed to be installed in the tube. Mirror installation made me a little nervous, especially while holding the heavy mirror hovering over the tube almost 6 feet above the floor! It took some time to put everything together, but the included instructions were easy to follow. After constructing the telescope it was time to properly align the mirrors.

I had never performed telescope collimation before, so I consulted several helpful videos and written articles. Two collimation methods were available. The first method involved looking through a small hole centered in the eyepiece holder. The view of multiple mirror reflections seen through the small hole didn't exactly match any of the instructional illustrations available. I was confused and unsure what to do about this. Nevertheless, I carried out the first collimation procedure to the best of my ability. Fortunately, the second collimation method was simpler to understand. The second method involved placing a small laser in the eyepiece position. The secondary mirror was adjusted so the laser beam hit the center of the primary mirror. Then the primary mirror itself was adjusted so the reflected laser beam was centered in the laser "eyepiece". I carried out the laser procedure very carefully and then performed an alignment confirmation check by taking the laser out and looking through the small eyepiece hole from the first collimation procedure. I was puzzled to discover the two collimation procedures produced incompatible results! Alignment by laser caused misalignment through the small viewing hole and vice versa! Consequently, I had no confidence the telescope was properly collimated by either method.

The imprecise collimation procedures were disappointing. For example, the laser beam changed direction with the slightest tightening or loosening of the eyepiece holder. The laser beam also changed direction when the telescope was moved from horizontal to vertical. In spite of confusing collimation results my first view of stars through the telescope was fine. Stars came to sharp focus and the Moon looked great. Apparently, if the optics are close to perfect alignment the telescope works very well.

I took a chance ordering the 12-inch telescope because it might have been too heavy to easily move. The tube weighs about 50 pounds. I can lift the tube, but find it hard to handle. It's awkward to grip, lift, or carry for any distance by hand. I use a hand truck to move the tube to my observing position. Through experience I'm getting better at lifting and placing the tube on the mount. The mount itself is easier to carry. It weighs about 34 pounds, has a handle, and includes additional places to grip.

I did discover one issue with the mount, however. The bottom triangular base which rests on the ground is attached to the circular upper base by only one central shaft. While carrying the mount the two base pieces wobble around quite a bit causing their connection to loosen. This ends up affecting the operation of the azimuth encoder. (More about this shortly.) I fixed this problem by using three c-clamps to hold the two base parts firmly together during transportation. Once the mount is in place I remove the clamps so it will be free to turn in azimuth. You can see two of the c-clamps on the base in the picture below.

The telescope comes with encoders and an electronic pointing system called IntelliScope. IntelliScope should be able to accurately point the telescope at dim objects not visible in the Telrad finder. The system requires a relatively simple three step initialization. First, the tube is pointed vertically to the zenith. Second, the telescope is pointed to a bright star. Finally, the telescope is pointed at a second bright star more than 60 degrees away from the first star. After all three pieces of information are entered into the handheld Computerized Object Locator, the telescope should, in theory, point to any object selected on the generous hand controller menu.

How well does IntelliScope work in practice? The first time I used it the system worked pretty well, but I didn't have time to test the pointing accuracy on more than a couple objects. Targeted objects were not exactly centered in the eyepiece, but they were close enough to be found and easily centered in low power eyepieces. The second and third time I used IntelliScope the system malfunctioned and didn't work at all. During these malfunctions the altitude encoder seemed to be working properly but the azimuth encoder at the bottom of the base wasn't recording telescope movement. I suspected the azimuth encoder was knocked out of proper alignment by loose, wobbling bottom base parts while moving the mount. So I carefully reinstalled the connection between the base parts and added the c-clamps mentioned previously to keep the base from wobbling during transport. The c-clamps seem to have solved the problem because Intelliscope worked reasonably well during my last observing session. For example, after performing initial alignment I entered the Andromeda Galaxy, M31, into the hand controller as a target. When I moved the telescope to the position indicated by the hand controller the galaxy was not in the field of view but slightly off to the side. I was able to find it not far from the field of view with small searching movements. Once again, I didn't have time to look for numerous target objects. In the future I'd like to explore the pointing accuracy more thoroughly.

So how good is the view through the eyepiece? The Orion Nebula was, of course, spectacular! Open star clusters look very nice. The Ring Nebula was good at 150X. The sky from my suburban back yard is not truly dark, but I could see the Crab Nebula and M78. During my last observing session on February 2nd I could see comet c/2017 T2 PANSTARRS near the Double Cluster in Perseus even with a first quarter Moon brightening the sky.

At my observing site it's a constant battle against light from neighboring houses. Neighbors turn on back yard flood lights to let their dogs out. Each one shines like a searchlight right on my telescope. When one light goes off, another comes on. It's very discouraging. Occasionally, I get lucky and all the lights are off. 

Just for fun on February 2nd I decided to attach my phone camera in front of the eyepiece with a Celestron NexYZ universal smartphone adaptor. The following pictures were all taken with a Samsung Galaxy 8 Plus phone through a 25mm eyepiece. Exposures needed to be brief since Earth is turning and the mount is not tracking this rotation. The next cropped image shows the Moon in a 1/200 second exposure. Focus quality varies from place to place, and the limb has a hazy glow along its length. Focus was best near center of the camera's field of view near the middle of the terminator. The image isn't so bad considering all the layers of glass and possibilities for misalignment.

A tracking system is really needed to take images of stars and nebulae. Once again, just for the fun, I tried imaging the Orion Nebula. A 1/4 second exposure  revealed some nebulosity and the four bright stars of the Trapezium in the center of the nebula. Stars didn't trail too badly during the 1/4 second. (Click on the images for a larger view.)

Increasing exposure time to one second showed more nebulosity, but, unfortunately, star images are now elongated due to Earth's rotation.

A two second exposure shows even more nebulosity and hints of color, but the stars and nebula are trailing badly.

A lower power eyepiece and target closer to the celestial pole would minimize trailing and allow longer exposures, but, really, an unguided Dobsonian telescope is not made for imaging. I look forward to more visual observing on mild spring nights in the future.