Dobsonian Telescopes

Using the Dobsonian Telescopes

Prepared by Dave Holland and Barry Olson, 2009

Introduction

Our four Dobsonian telescopes are the easiest and by far the fastest to set up and use, so we use them more frequently than all other models. If you are thinking about buying a telescope, give this deign serious consideration. The word "Dobsonian" actually refers to the telescope mount, which was invented by John Dobson in the 1960's. He popularized a scope that was easily made by hand, inexpensive, large, and very stable. The optical part of a Dobsonian scope is of Newtonian design, which is a type of large reflecting telescope. Light enters the front of the tube, strikes a large concave mirror (primary mirror) at the base of the tube, is redirected backup the tube, and then reflected by a small 45 deg flat mirror (secondary mirror) out the side of the tube and into the eyepiece. The flat mirror is held by struts near the front of the tube. The large size of the primary mirror allows visualization of faint objects, and is therefore ideal for searching out deep sky targets.

All telescope mounts have two bearings set up at right angles to each other. All Dobsonian mounts are of alt-azmith design, which means the lower (azimuth) bearing sits like a dinner plate on the ground, rotating in azimuth directions (N-S-E-W). The altitude bearing rotates the scope at right angles to that, perpendicular to the distant horizon (altitude).

Eyepieces

Our telescopes are all stored with the eyepieces removed, and we store eyepieces separately in locked cabinets. Eyepieces come in two common diameters (1 1/4" and 2"), based on the diameter of the eyepiece tube. Most of our telescopes have 2" mounting holes, so if you want to use a 1 1/4" eyepiece you will use an adapter. Check the telescope you are using to see whether an adapter is already in place. Eyepiece adapters are easily removed by turning the small retaining set screws and sliding the adapters out.

Typically inscribed on the side of eyepieces are focal length, design type, and manufacturer. Eyepieces are classified by focal length, with small (8-15mm) eyepieces giving greater magnification than large (50-60) focal lengths. You might be drawn into the notion that greater magnification is better, however there is no free lunch. Smaller eyepieces give a more magnified view, but are harder to point because of the more narrow field of view. Small eyepieces are also subject to more atmospheric distortion, which is the biggest obstacle to terrestrial observing. For average viewing, an eyepiece size of 20-50 mm is most popular. Often we will use larger eyepieces to find an object and then we may switch to a smaller eyepiece to have a closer look. There are also different eyepiece designs, based on the arrangement and number of glass elements, indicated by the name inscribed on the eyepiece. Common designs are Plossl, Nagler, Ramsden, Huygens, Kellner, etc. You can calculate the magnification of your image by dividing the focal length of the telescope by the focal length of the eyepiece. The average focal length of a Dobsonian scope is about 1200mm, so, for example, with a 20mm eyepiece the magnification is (1200/20) = 60X. Therefore magnification is not a useful tool when comparing telescopes.

Getting Started

Your first step is to choose your weapon, pick out one of the Dobs. Remove the vinyl dust tarp, and check for and (if necessary) remove the plastic lid (dust cover) from the top of the scope. Put both the vinyl tarp and the dust cover aside in the equipment room.

Choose an eyepiece and carefully insert it into the barrel of the eyepiece mount. You don't need to slide the eyepiece all the way into the mount, but it needs to be secure as you tighten the small set screws on the mount. Be sure those set screws are tightly holding the eyepiece before you move the scope outside. Otherwise you may later hear a sickening bang as the delicate eyepiece hits the concrete. You may want to check to be sure the Telrad is working now (see below).

Your next step is to move the scope outside. Our Dobsonian telescopes are all resting on round black dollies with a single handle to control the dolly. As you jockey the scope outside, consider how easily you would be able to transport this kind of scope, and observe how much easier it is to move the 8" Dobson compared to the 12" Dobson. Notice how much room these scopes require. You won't get a 12" Dobson into a small car unless you can dismantle it. Open the equipment room door, put one hand on the top lip of the scope and another hand on the dolly handle. Then carefully roll the whole thing across the door threshold and down the ramp. It is more stable to do this by walking backward and dragging the scope over thresholds. Remember that these telescopes are just resting on their bearings, so if you lose balance the whole thing can fall over. Go Slow! Choose an empty area of the patio to set up your scope.

The next step is a little awkward. You need to take the scope off the dolly -- this is easiest if you have a helper. The telescope is just resting in the saddle of the vertical bearings, so if one person lifts the scope by the two side handles, it will come right off the lower part of the mount. With two people, one person lifts the scope off the saddle and the other then slides the lower part of the mount off the dolly and onto firm concrete. Notice that most scopes have a front and a back, so when you replace the scope in the saddle, maintain the same relative position. At this point it is really important to store the dolly where nobody can trip. Although this may seem like a small point, an accident on the patio could cause a hip fracture or other serious injury, so you need to be responsible. Those dolly's are very hard to see in the dark.

Now you want to focus on a distant star, any star. Point the scope to an area of the sky with lots of stars. The eyepiece mount has a focusing knob that permits the eyepiece to slide with the shaft of the eyepiece mount, permitting very accurate focusing. When you first look through the eyepiece, stars will look like fuzzy donuts, or you may not see anything at all. If you don't see anything, first check to see if you forgot to remove the telescope dust cover (D-oh!). Your eye position is also a factor, much like with binoculars. Be sure your eye is directly in line with the eyepiece. If you see an upside-down view of two people making out in a distant car, point the scope higher in the sky. To focus, rotate the focusing knob back and forth until you see fuzzy blobs. Focus accurately to get the stars into fine points. If you are having no success at focusing, it could be the eyepiece is being held too far inside the barrel of the eyepiece mount. If you think that might be the problem, loosen the set screw that holds the eyepiece in place, slide it out a little, and re-tighten. Be sure it is still held tight in place. One of our newer eyepieces has a plastic cap covering the internal side of the eyepiece, and until we lose that cap we have to remember to take it off each time. If you forget, you will never be able to focus. I'm hoping somebody will lose that cap soon.

Telrad pointing

Because it is hard to accurately point a large barrel telescope at a tiny point in the sky, most telescopes have devices strapped to the side that permit accurate pointing. The most popular device for pointing is the Telrad, shown in the appended photos. On the right side of the Telrad is a switch --turn the switch clockwise to turn it on. You will hear an audible ‘click’ as the unit is turned on and off. This is a variable switch which allows you to adjust the brightness of the bull’s eye target. You stand on the right side of the scope, put your left ear on the barrel of the scope, and look through the Telrad. If your head is lined up properly you will see the bulls-eye, analogous to the unmagnified sights of a gun.

When you first use a telescope, you want to test the alignment of the Telrad to see how accurately the Telrad Bulls-eye points at the target. The easiest way to do that is to choose a very bright, distinctive star. Line the Telrad up with that star and then look through the telescope to see how accurately the Telrad points at the target. Then readjust the scope so that the target is dead center in the telescope eyepiece. Then go back to the Telrad to see where the true center of the scope is. If the alignment isn't too bad, just remember where on the Telrad the true center is for the evening.

If you feel you need to realign the Telrad, you can adjust the direction it points by adjusting the three screws on the back of the Telrad. The Telrad is battery operated, so if it is not working, replace the battery. Make sure you turn off the Telrad after using the telescope.

Collimation

Newtonian telescopes provide the best views when the mirrors are properly aligned or collimated. Since these are portable telescopes, the mirrors will become slightly misaligned as the telescopes are moved in and out of the storage room. We have a collimating device, called the Antares Laser Collimator, which is used to collimate the mirrors. The collimator is inserted into the barrel of the eyepiece mount, and a laser light is used to align the mirrors. Collimation is easy but not really a beginner technique, and so it will not be covered in detail here. There are good reviews of Collimation technique for Newtonian scopes on the web, for example here. Verifying collimation is optional, but it is good practice to do so regularly. If the image through the eyepiece is not clear, or if stars are teardrop shaped, then you should verify collimation. Have someone show you how to do this the first couple of times.

Driving

A big step forward for beginners is to have solo time with a scope, driving around the heavens. If nobody offers, be assertive and ask for this during the course. Hold the top lip of the scope with your right hand, and stabilize the scope with your other hand. Feel the difference between altitude movement and rotational azimuth movement. Practice pointing at things. Notice how difficult it is to point at something directly overhead. You'll be amazed at how much is out there. But if you don't know where you're going, you will also get tired of seeing endless, patternless fields of stars. If you are lucky, you will stumble across a few globular and open clusters. Watch for satellites zipping across your field of view. Watch for patchy dark areas, often caused by interstellar dust obscuring the view of more distant stars. Be careful not to blind yourself with the brightness of the moon. After an hour or so of aimless traveling, you'll be looking for more direction to find the cool stuff.

How to find stuff: Use the Constellations

Dobson telescopes are fundamentally hand directed, so you need to know where you are going. To do that, you need to memorize some basic landmarks in the sky, the constellations. Once you have those landmarks down, you can use them to find cool stuff in the night sky. 88 constellations altogether, but for basic guiding you only need to know about 20. Start with Ursa Major (the big dipper), Cassiopeia, and the twelve zodiac constellations. Learn Orion for winter guiding, and learn to find the great square of Pegasus, and the summer triangle for summer guiding. If you learn 20 stars and 20 constellations you will feel comfortable navigating the night sky. Practice finding landmarks in the sky whenever your are outdoors in the night or morning, not just when you are using your scope, every day. When you stumble out of the bar with your friends, show them Jupiter or Saturn, and impress them when you can name the first star they point to. Notice how the morning view of the sky is so different than the view the evening before. A morning view is what the evening night sky will look like, three months down the road. Didactic lectures don't really help you to learn the night sky, practice does. For an excellent guidebook to help you with this, get a copy of this little gem.

Astrophotography

It is possible to pursue photography with Dobson telescopes, but there are problems. If your scope does not have a motor drive, your shutter speed needs to be fast enough so that stars will not turn into white streaks across the camera sensor (star trails). The required speed to prevent star trails depends a lot on the magnification of the scope, with greater magnification requiring a faster shutter. One way around that is to shoot a series of photographs and then stack the images together digitally. There is also a problem of image rotation when using an alt-az mount. A series of images taken with a camera on an alt-az mount will show the image to rotate with time, which will smear detail in a planet or create another kind of rotational star trail. Therefore even if your Dobson scope is fitted with a motor drive, you will have rotational streaking. To compensate for that, you can shoot a series of very short exposures and stack images after compensating for rotational effects.

To get around those problems, most serious astrophotographers choose a (non Dobson) polar equatorial mount, which orients the mount bearings to be parallel to the planes of right ascension and declination. This confines the artefact created by earth's movement to one plane, the plane of right ascension, and therefore object rotational effects drop out of the equation. Astrophotographers use a motor drive and a camera with the IR sensor removed, to optimize light transfer. So, the complexity and expense is far greater when you want to record images on a camera.

Miscellaneous devices

One of our Dobsons is fitted with an Orion SkyQuest Intelliscope, which offers positional information as to where the scope is pointing. If you want to use the IntelliScope™, you will have to read the manual and understand how it works. That scope works fine as a normal Dobson scope when the IntelliScope™ is turned off. We also have a small table-top 113 mm Orion Starblast telescope on a tiny Dobson mount that is available for use. If you use that tabletop scope you need to be really careful it doesn't fall.

Wrapping up

Shutting down for the night is often the most critical point, as it is easy to forget to do something, and fatigue may disclose vulnerability. A mistake at this point could be a huge error for the club, and it could damage your reputation and your future privileges. It may be days or even weeks before an oversight is discovered. So, give yourself lots of time and do it right.

Carefully replace the telescope back onto the dolly, and gently drag the scope back into the telescope room. Remove the eyepiece and return it to the eyepiece cabinet. After removing the eyepiece, it is good practice to turn the setscrews in the focuser a few turns in. This ensures the setscrews are not holding on by a few threads and prone to falling out from vibration. Lock all eyepiece cabinets. Remember to turn off the Telrad. Replace the dust cover and pull the vinyl tarp over the scope. Go back outside and check to be sure you didn't leave anything. Remember to turn off the red stair lights. Go back into the equipment room and deadbolt the outside equipment room door, turn off all lights and deadbolt the internal equipment room door as you leave. Remember to sign the logbook and describe what you did, and have a look around the clubhouse to be sure the building is secure. Double check the sliding door, the overlying security shutters, and the sliding glass door lock. No member is permitted to remove club property from the clubhouse without prior agreement by the telescope director, and even then you must record what you borrowed in the logbook and the equipment log. Wrap up any garbage and take it to your car. Sign out with your security code, deadbolt the door lock and also lock the door handle lock. Lock the front gate.

Happy Stargazing!

Dave Holland and Barry Olson, Aug 2009

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