Introduction: Airline-portable 8" Dobsonian Telescope
The scope looks funny. It has a square box on semicircular rockers on the bottom, then a single wooden strut leading to a focuser board out of which there sticks out a stalk for the secondary mriror, and a rounded coathanger with a light-shield. The original design used two struts, but one is enough at this size. Telescopes don't really need tubes. Tubes are just there for holding the optical elements in place and keeping out stray light, and both tasks can be accomplished in other ways.
After disassemly, no piece is more than 16.5" long. The second photo shows all the parts reading for packing up. I originally planned for the telescope to assemble on location with no tools, but ended up relaxing that to allow for one Philips screwdriver. Using Philips screws in a few places reduced weight over the big wingnuts I would otherwise have used.
The telescope doesn't look like much, but I've had good views of the Whirlpool Galaxy, the Dumbell Nebula, the North American Nebula, the Ring Nebula and various other deep-space objects. It works on planets, too, though it's best for low-magnification deep-space viewing.
I originally planned to use a 6" F/5 mirror, but I got a very cheap 8" F/4 mirror with a crack near one edge. Turns out the stresses induced by the crack only seriously distorted the mirror in one area, and so I blacked that out. That explains the dark semi-circle near one side of the mirror. There are gory details of how I tested the mirror here.
At the time, my woodworking skills were very limited. The power tools I used were:
- cheap Harbor-Freight jigsaw
- power drill, with a set of hole-saws and a 1.25" spade bit
- friend's table saw (I could have done the cuts with the jigsaw and a good blade instead, like a Bosch Progressor U234X)
- friend's mini-router for one elongated hole, which I could also have done with drill and rattail file (and I could have just had a round hole with better measurement).
The optics were:
- 8" F/4 chipped mirror, bought for $39 on CloudyNights
- 1.83" secondary mirror, bought for $9 on CloudyNights (one end was a bit turned, so I blacked it out)
- Mars's Eye red dot finder (you can also use a Daisy red dot finder from Walmart or Academy, especially with some modifications)
- scrap wood and particle board
- one 14-16" wooden/particleboard/plywood circle, about 1/2"-3/4" thick (see more in step 4 for sources)
- JB Weld
- Titebond II
- silicone glue (the older, smelly kind; the newer non-smelly is supposed to be not as good)
- 1.25" PVC conduit
- 1/4-20 carriage bolts, fender washers and a few nuts
- miscellaneous machine screws and nuts
- 1/16" bondable PTFE (amazon's Industrial and Scientific store)
- 1/4" threaded rod and 3/16" square rod (many ways of doing this)
- one thumbscrew
- some woodscrews
- seven 1/4-20 three-lobe female thru-hole knobs (amazon's Industrial and Scientific store)
- one vinyl record (about a dollar on ebay)
- acrylic flat black paint
- one coat hanger
- some foam board
- four wire ties
- duct tape
- one binder hole reinforcer sticker
- two long extension springs
- three strong compression springs (I used mower engine valve springs).
And if you don't have any telescope yet, you'll need eyepieces. I recommend to start out with a 30/32mm Plossl and a 10mm Plossl.
I also suggest that you read all the steps before doing anything. I am not giving many dimensions as they'll depend on your mirror. I will assume you know how a Newtonian telescope works.
Update: I found my design drawings and after editing them slightly for clarity and to fit with the final version, I posted a couple of them. If you want the SVG files that the images are made from, let me know, but since your dimensions are going to be different from mine, you're better off making your own drawings.
Step 1: Measuring Mirror Focal Length
A crucial step that determines the sizes of many parts is to measure the mirror's focal length (don't trust labeling). There is more than one way to do it. My favorite is to take the mirror outside, put it on a soft chair, and point its optical axis at the moon (a bright star, e.g., Sirius or Vega, should do). The optical axis runs through the center of the mirror, perpendicular to the mirror. Then hold a small piece of wax paper on the optical axis, at around that distance from the mirror surface that your mirror seller told you was the focal length. Adjusting the mirror and the wax paper a bit, you should see the moon on the wax paper, reflected from the mirror. (In a sense, you already have a telescope at this step. If you had a camera sensor in place of the wax paper, you could take a picture of the moon.) Move the wax paper along the optical axis until the moon is as sharp as it can be. The distance from the wax paper to the mirror is the focal length.
You will want to design your telescope so that the distance along the light path--remember that in a Newtonian telescope the light path bends at right angles at the secondary mirror--from the primary mirror to the end of the focuser tube with the focuser tube fully racked into the scope is about 1/4" less than the focal length. You can achieve this by adjusting the length of the strut, the placement of the primary mirror and the distance from the secondary to the focuser. Basically, the crucial thing is that your eyepiece's focal plane should go where the wax paper was (taking into account bending of the light path by the secondary).
The design in this instructable is good for short focal length. My mirror's focal length is 800mm. For significantly longer focal length, you'd want more struts than one.
Step 2: Primary Mirror Cell
The mirror box and primary cell--the part that the primary mirror attaches to--are made of approximately 3/4" oak. One could also use 1/2" Baltic birch plywood for the box and maybe 1" Baltic birch for the cell.
The idea behind this kind of cell is very simple. The mirror is glued with 1/4" thick pads of silicone glue (thick to isolate the mirror's thermal expansion from the cell's thermal expansion, and thus prevent stress to the mirror due to mismatch) to one piece of wood. This piece of wood has three bolts sticking out of the other side of it, there are springs put on the bolts between the cell and the bottom board of the mirror box, and holes for the bolts are drilled in the bottom board. Finally, wingnuts are put on the bottom board, and adjusting the wingnuts allows the mirror angle to be adjusted for collimation.
Here are some more details. First, I put the mirror dimensions into Graphical Plop to calculate the optimal placement of the silicone glue pads. Normally for a mirror of this size, one would use three pads. However, because I was using a damaged mirror, I wanted more stress relief for the glass, and so I used six. Basically, the pads all get equally spaced around one circle, and Plop calculates how far from the mirror's center the circle goes.
I drew the cell design, with all the cutting locations, with Inkscape. I usually take a drawing like that, print it out the right size, tape it to the wood, and cut and drill right through it--it's easier than measuring.
I acquired three valve springs from a dead mower motor at a mower repair place (I just asked the guy if he could extract them, and gave him $5). These are pretty big springs--about 1" diameter. One could use smaller compression springs from Amazon's Industrial and Scientific store, too, but I like these ones. They keep the mirror very solidly in place.
I think I cut the rectangular base-plate for the mirror box with a table saw, and combined the cutting of the base-plate with cutting the sides of the box (see next step).
I used a jigsaw to cut the cell in the (non-regular) hexagon shape shown in the first photo. Both the cell and the mirror have four holes each for ventilation (one wants the mirror to cool quickly): a large one in the middle (cut with a hole saw), surrounded by three 1.25" holes around the outside (spade bit or hole saw). These holes line up, so cool air can flow through the bottom board's holes and the cell's holes, as well as around the cell (which is smaller than the mirror).
Between the three 1.25" ventilation holes in the cell, after countersinking with the 1.25" spade bit, I drilled 1/4" holes for the carriage bolts that would come out of the cell, all the way through the cell and the mirror box bottom plate.
The countersinking was done in three places: on the top side of the bottom plate and on the bottom side of the hexagon mirror cell in order to keep the springs in place as in the photo, and on the top side of the mirror cell to sink the heads of the carriage bolts in. (Because spade bits are guided by their center, it is a very good idea to countersink before drilling the 1/4" holes.) Countersinking on both sides weakens the cell, but with the really hard oak I was using, I wasn't worried.
I then used JB Weld to glue three carriage bolts into the mirror cell, with the heads being covered with JB Weld (gray filled circles in some of the photos). The carriage bolts need to be sized so that after going through the cell, then having the springs on them and then going through the bottom plate, there is enough sticking out that you can put wingnuts on the bottom end. It's hard to keep the carriage bolts pointing straight--I had to bend them a little after the glue set so they'd fit into the bottom plate well.
Before gluing the mirror to the cell, it might be a good idea to finish the wood of the cell to your liking. The scrap I used was already pre-finished on one side. Normally these days I use a very simple and cheap wood finish: a 1:1 mix of Titebond II and water, in two or three coats (it dries enough to sand in 30-60 minutes or so), sanded lightly after each coat. You might want to leave unfinished the area where the silicone glue will go, so that it sticks to the wood rather than possibly pulling off with the finish.
To glue the mirror to the cell, the idea is that you put 1/2" thick gops of silicone glue on the cell so that when they are squished down by the weight of the mirror, they will spread into 1" wide and 1/4" thick discs (see the side view photo). The gobs went at the correct distance from the center (calculated by Plop), and went in between the six previously drilled holes on the cell (the three 1.25" ventilation holes and the three 1/4" countersunk carriage bolt holes). However, for additional safety, I drilled 1/4" holes in the cell where each gob would go, and made sure each gob penetrated through the hole and to the other side of the cell. That would keep each gob from detaching from the cell. There is no similar worry about the glass side because good silicone glue (I used the DAP transparent standard smelly silicone glue; just don't use the less smelly Silicone II as there are bad rumors about it) will stick extremely well to glass.
To keep the mirror even with the cell and to space the gobs at 1/4" thickness, insert spacers. I think I had some scrap wood for that. Pencils would also work. Then leave for about two days for the silicone glue to set well, removing spacers as soon as the silicone has enough rigidity that it won't distort significantly any more.
The last photo shows the bottom of the completed mirror box, with three ventilation holes as well as the nice wingnuts on the carriage bolts sticking out of the box. There are, of course, fender washers between the wingnuts and the wood.
Congratulations: you now have a collimatable primary mirror cell. The three carriage bolts are the collimation bolts. Collimatability is central to amateur Newtonian telescope design--you can make all sorts of things to lax tolerances, and then on the observing field you tighten everything up optically by collimating (see Step 9).
Step 3: Mirror Box
The mirror box is just a box where the mirror cell sits, with the bottom of the box being the baseplate from the previous step. It makes sense to cut the baseplate and the sides of the box at the same time. The inside of the sides of the box should be enough distance away that you can comfortably put carriage bolts in from the inside where you need them to attach the side rockers and the strut. So don't do this step before reviewing the following steps to get a feel for what is coming. (I made my mirror be a little too close to the edges, which means that removing the screws without touching the mirror can be tricky, and I have to adjust the mirror height with the collimation bolts. It's not a big deal.)
You can do fancy joinery, but I just used a butt joint with Titebond II and wood screws. I also drilled a bunch of large ventilation holes with the hole saws, making sure to leave space for attaching the rockers and strut.
I painted the inside of the mirror box, and of the ventilation holes, flat black, using craft store acrylic paint, on top of my Titebond II + water finish (or maybe a Titebond II + water + black paint finish).
As I normally do in such projects, I used Inkscape to make cutting/drilling templates for getting the holes in the right size--I hate measuring, and my laser printer has higher accuracy than I do. I attach the template for one of the sides.
Step 4: Altitude Bearings
If you have a router, you can just cut your 14-16" altitude bearings out of your favorite wood material. I didn't have a router when I made this. There are many sources for pre-cut circles of this size. Here are three:
- Hardware and hobby stores carry wooden circles in various dimensions. Given how light this telescope is, softwood should be fine--just finish it well.
- You can buy a round wooden cutting board online or in Ikea and cut it in half. I recall that Ikea's cutting boards are quite cheap.
- You can see if you can get a cheap or free stool or some other piece of furniture with the right dimensions on craigslist, and cut off the legs.
I eventually cut further holes to match the ventilation holes on the mirror box (using an Inkscape template printout to align them). The altitude bearings attach to the mirror box with wingnuts (and fender washers) and carriage bolts (the heads are on the inside of the mirror box). If working with particle board or plywood, it's really important to use some sort of a wood finish on the raw cuts. I used a mix of Titebond II, water and acrylic black paint.
For now ignore the spring in the picture--that will be discussed in Step 11. And the bottom board is for our next step.
A word about my use of particleboard. It is't ideal, but it's what I had lying around, and it has lasted just fine. Because of how modular this telescope is, you can always replace a part if you acquire better quality materials or your budget increases.
Step 5: Rocker "box"
Dobsonian telescopes ride on a "rocker box". Except that this very low-profile design has more a rocker board than a box. Here's the design. At the bottom there is a 3/4" thick particleboard circle (black, so it's hard to see under the vinyl record) a little larger than a vinyl record (i.e., a little over 12"). A thinner circle, say 1/2", would also be fine. The circle has a drilled 1/4" hole in the middle, and a carriage bolt going up through it (head on bottom). The circle stands on three feet (I used scrap wooden circles produced by the hole saw when cutting other things). On top of the circle there is a vinyl record, with the carriage bolt coming up through it. On top of the vinyl record, there is the top assembly.
See the previous step for sources of circles for the bottom circle. Since getting the roundness exact doesn't matter much for this--it's only aesthetic--I cut this circle by attaching a jigsaw to a stick coming out of a pivot, attaching a piece of particleboard to the pivot, and then rotating the particleboard against the jigsaw. The blade I was using was too cheap and the result isn't that straight, but it's good enough.
The top assembly is a rectangular piece of particle board (plywood would be better, but I didn't have any in the budget), with two pieces of 3/4" thick softwood attached to the sides. Between the top assembly and the vinyl record, there are pads of bondable PTFE, glued with JB Weld to the underside of the top assembly and riding on the vinyl record. This is the "azimuth bearing."
The pine boards are angled inward and bondable PTFE is glued in for the altitude bearings to ride on (white squares on photo). There are wooden stops to keep the altitude bearings from jumping out, and in the middle there is a wingnut on a home-made PTFE washer to adjust azimuth tension with. Ignore the springs for now.
Many variations on this design are possible. A wider board, with taller side-pieces, would allow the rockers to be further apart, making balance easier. However, it would decrease portability.
At some point, I used a hole saw to make three holes in the bottom of the bottom circle to decrease weight. I spaced the holes so they wouldn't affect the strength of the board.
Step 6: Strut
My original design called for two struts. The struts would start out far apart, at two opposite edges of the mirror box, and then move closer together as they went up, with the focuser board being mounted to both of them, producing a nice trapezoidal effect. But it turned out that one strut was enough for stability, and so I tossed the other one in the scrap pile, and reduced the size of the focuser plate.
The strut doesn't go straight up, perpendicularly to the bottom of the base plate. It is flush to one side of the mirror box, and angled inward from the other wall. The first photo shows the original design, with two struts and not yet cut down to size. I am also enclosing a design drawing, from Inkscape, which I modified for the Instructable to be closer to the final version of the design. The drawing wouldn't fit on an 8.5x11 page, so I printed it out in two full-size sections, one containing the mirror box area and one containing the focuser board area (step 8), and used each section as a drilling, cutting, angle and placement template. (Because initially I was using two struts, it was actually slightly easier to get all the placement nice and symmetric.)
The strut is 1" square poplar. Oak would be better, but I was working with a limited budget, and it was supposed to be a stop-gap, but I stuck with it as it worked well enough.
The strut attaches on the inside of the mirror box, its bottom touching the baseplate (with the bottom trimmed a little at an angle). It has two carriage bolts that come in from the inside, through the strut, then through the mirror box, and are held in place on the back of the scope with fender bolts and wingnuts.
I did find that eventually the square hole made by the carriage bolt on the inside of the strut got rounded, and so I resquared it by building it up with JB Weld. This probably wouldn't be an issue with a harder wood.
At the very end of the telescope-making process, and this is definitely optional, I cut the strut in half, with a long diagonal cut, and made the two halves assemble together with four machine screws. The nuts for the machine screws are permanently JB Welded in place. Since the strut is about thirty inches long, cutting it in half makes it possible to fit it into a smaller suitcase and decreases the chance of its breaking in transit. As far as I can tell, when the two halves are assembled, it's about as solid as if it were never cut.
The side of the strut that faces the mirror is painted flat black.
Step 7: Focuser Board and Focuser
This focuser board was made out of 3/8" (or maybe 5/16") particle board. It has a big hole drilled in the middle with a hole saw, a push-pull focuser attached to it, and an elongated hole for attaching the secondary stalk (this was cut with a mini router, but one could just drill round holes for the ends and just file one's way between them). The telescope's strut from the previous step screws to the board, and a red-dot finder also attaches to it.
If you have the money, you can skip much of this step and install a light-weight focuser, such as the Kinematics 1.25" helical Crayford.
The shape of the focuser board changed in construction. The initial larger shape (first photo) was for the two-strut design, and I eventually cut the board as small as I could make it to save weight at the top of the telescope after switching to a one-strut design.
Let's start with the focuser. The idea is simple: there is a draw tube that slides between three PTFE pads, with one of the pads having adjustable tension. The tube is pulled gently forward and back for focusing, with a bit of rotation making things even easier.
First, cut three wooden posts that will attach to the focuser board around a hole large enough for the draw tube. Glue bondable PTFE (with JB Weld or a superglue) to two of the posts. The third is slightly more complicated--see the second photo. Take a thin square of metal (I think I cut it from a spare PC expansion card slot cover) about half of the size of the face of the post. Glue it to one half of a rectangular bondable PTFE pad on the bondable side (again, JB Weld). Then glue the other half of the bondable PTFE pad to the bottom half of the post, leaving the top half of the PTFE with the metal backing not glued to the post. Then put a thumbscrew through the post (if the wood is hard enough, you can just put thread in the wood with the thumbscrew; otherwise, use or make a threaded insert). The thumbscrew pushes the metal, which pushes the attached PTFE against the draw tube, and the tension is thereby adjusted.
Of course the location of the three posts needs to be fine-tuned with the thickness of the draw-tube.
Attaching the posts to the focuser board proved hard. They were very hard to keep in place long enough to glue. And glue wasn't enough--glue would just pull away with the top layer of particleboard. So, I temporarily glued them with a weak glue (it may have been a white school glue), then drilled in and attached woodscrews from the bottom, and then finally removed the weak glue and JB Welded them in place, with a very generous bead around them. Unfortunately, they didn't end up as square to the hole as I wish, and I have some wobble.
In the third photo, you see the whole assembly.
The draw tube is PVC. The eyepiece size this is meant to work with is 1.25". Unfortunately, I couldn't find any PVC tubing with an inner diameter close enough to 1.25". I finally got some electrical conduit with inner diameter 1.35" in the hardware store. Really cheap stuff--I bought about 12 feet for about three dollars, and had enough left over over the couple of inches I used here to make a tripod for a binocular mount. So something had to be done to bridge the distance.
Now, I still had the vast majority of the 1/16" bondable PTFE--way more than the small squares I needed for the altazimuth mount and focuser pads. The inner diameter of the PVC was about 0.10" wider than the outer diameter of the eyepieces. I took the PTFE and cut some 1/4" wide strips, about two inches in length. I glued them inside my prospective draw tube (JB Weld, need you ask?), spaced at around 120 degrees. You should be able to see them in the third photo.
It was still too snug for my eyepieces. So I just used a flat file to file the PTFE down.
I added a thumb-screw for good measure, but it's not really needed. It's still a bit too snug. It's hard to remove an eyepiece without shifting the scope (that's also a function of the altitude movement being too easy).
Eventually, as in the third photo, a red dot finder got attached. And the focuser board screws onto the strut (Philips screws, with nuts permanently JB Welded to the other side of the strut).
I keep on thinking of upgrading to a super-light helical Crayford focuser. Too bad that I can't make one myself. (I've made a helical Crayford, but it was quite heavy.)
The design drawing is for the original full-size one. I eventually cut off all unnecessary parts. Note the overlap between one of the focuser posts and the strut--that's because the strut is on the inside of the board and the focuser post on the outside. The angle of the strut in the drawing is carefully set out based on the full drawing of mirror box, struts and focuser box (see step 6).
Step 8: Secondary Mount and Stalk
The secondary mirror attaches with a stalk to the focuser board. I cut a mount out of softwood. It's jointed like in the picture to allow for one axis of adjustment. The other axis is rotation around the stalk. The first picture is before I drilled a 1/4" hole for the stalk. I also eventually sanded/cut the mount down to a smaller size.
Obviously, one should paint/finish the secondary mount before attaching the mirror. It might not be a good idea to paint or finish all of the diagonal part where the mirror will go, so the glue doesn't pull off with the paint/finish. Glus the secondary mirror with silicone glue, keeping toothpicks between the mount and the mirror for spacing (toothpicks are removed once glue is set), with the same reasoning as one uses thick gobs to glue the primary. The second photo on this page is a photo of my gluing process from another scope.
The secondary mirror's center hangs basically over the center of the primary mirror and in front of the center of the focuser tube, though you can move it a few millimeters away from the focuser, using the calculator here, to optimize light gathering.
I initially made the stalk out of threaded rod. In the second picture, the threaded rod passes left-to-right through: 1/4" wingnut, fender washer, focuser board, fender washer, nut pressed against fender washer from inside, then a couple of inches are exposed, then there is another nut, then the secondary mount, and then one more square nut (more on that below).
It turned out that the stalk had enough flex due to gravity to make collimation questionable. So I JB Welded in parallel with it a section of square rod, gluing it with a lot of JB Weld to the two nuts, the one abutting the focuser board fender washer and the one abutting the secondary mount. Two parallel rods will have a lot less flex (at least in the direction that matters).
To adjust the tilt of the secondary by hand, I needed hand-adjustable tension on the two axes that the secondary mount can move. One axis, the one perpendicular to the stalk, was a machine screw, around #10. I then superglued a larger nut (maybe a 5/16") to the smaller nut to make it easily movable by hand. The bond has held up well over the over one year since I've made the scope, though a little wingnut would be safer (one doesn't want a part falling on the primary mirror). The other axis has a gray square nut. I actually cast that nut out of JB Weld, making a little squarish mold out of painter's tape and putting a 1/4-20 nut in the middle. Again, I could have bought something, but I wanted a very low-profile hand-adjustable nut and that would be hard to find commercially. It had to be low profile, since I didn't want it sticking out past the shadow of the secondary mirror on the primary mirror.
Attaching the stalk to the right place in the elongated hole of the focuser board requires collimating the scope.
Step 9: Collimating
There are instructions for collimating in various places online, some of which use simpler equipment than a laser, and if you're an experienced amateur astronomer, you'll already have your favorite procedure. I myself used a home-made crosshairs laser collimator, but one can do this with any standard 1.25" laser collimator. I strongly recommend the deluxe versions with viewing windows, and I think a laser collimator, while perhaps less precise than some other methods, is the easiest method. With practice, collimation of this telescope takes about two minutes. I do it every session, as well as when I carry the scope to a new location (e.g., to get around trees that are blocking the view).
Here is basically the kind of procedure that I follow. Start by assembling the whole scope, except for attaching the secondary stalk to the focuser board. Note that typically if the scope is fully assembled, before observing you only need to do steps 6 and 7.
1. Put a donut-shaped ring reinforcer right in the center of the primary mirror. It's tricky getting it exactly centered. One way to do it is to draw a circle of the same size as your primary mirror (the outermost diameter, if it's beveled), and mark its center. (If you drew it with a compass, the hole is the center. If you printed it out with a computer, make sure you include cross-hairs or a dot at the center in your drawing.) Make a hole in the center just big enough for a fine-tipped marker. Cut out the circle. Align it carefully with the surface of the mirror (do not rub it against the surface). Using a fine-tipped marker, make a small dot in the center. Then glue a ring reinforcer exactly around the dot. (You may need to pick through the ring reinforcers to find one that is not lopsided.) This center part of the primary mirror is not used for astronomical observations as it is in the shadow of the secondary mirror, so you're not losing any aperture.
2. Put the secondary stalk in the middle position in the elongated hole. Tension the springs over the primary mirror's collimation bolts by about 1/4".
3. Draw (e.g., with Inkscape) and cut out an elliptical paper target marking the center of the secondary mirror. (Some people put a dot on the center of the secondary mirror. I don't like marring its surface.) I recommend cross-hairs marking the two axes of the mirror. The target in the photo is more complicated than it needs to be.
3. Insert the laser in the focuser tube, and if it has a viewing window, point the window downward so you will see it from the bottom of the telescope. Turn on the laser, being careful never to shine it in anyone's eyes.
4. Adjust the tilt of the secondary mirror (you may need to slightly loosen the nuts on its mount and then tighten them) until the laser is centered in the center of the primary mirror.
5. Now you need to adjust the height of the secondary mirror stalk. The simplest way is just to stick the paper target from step 3 on the front of the secondary mirror with some paper tape (not putting any tape on the reflective part, and being careful not to rub the target against the mirror) and then to adjust the height of the stalk until the laser hits the center of the target. However, if you off-set your secondary from the center (in Step 8 of this Instructable), you should do so here: Go to the offset calculator website, enter your data, and it will tell you how far off-center to put a dot (or, better, cross) on your target. The dot goes up from the geometrical center of the target, where up is measured with the telescope's primary mirror horizontal and pointing upwards. Then adjust the height of the stalk (and if your hole for the stalk bolt is wide enough, the horizontal position, too) until the laser hits the center of the paper target or the offset dot. Then, remove the paper target, and adjust tilt a little again to recenter the laser light on the primary mirror. You can try putting the target on again, and do some fine adjustments again.
6. Now you need to align the tilt of the secondary mirror. Put the scope at about 60 degrees up from horizontal. You'll notice a small shift in where the light from the laser hits the primary. If the shift is too large, your strut and/or stalk are not rigid enough and you may need to change materials. (I get a small shift that doesn't seem to significantly affect views.) Readjust the secondary until once again the center is hit by the laser, and be as precise as you can. (You should be able to get about half a millimeter precision by eye, as it's easy to judge the center of the donut by eye.) See the third picture (which shows a cross rather than dot laser collimator, as that's all I have).
7. Adjust the wingnuts on the three collimation bolts until the laser beam goes back on itself. The theory is this: the laser light comes out of the laser, hits the secondary mirror, bounces off the center of the primary and if everything is aligned it goes back on itself, hits the secondary, and comes back to the collimator. If the collimator has a viewing window, at perfect alignment the laser light will go through the hole in the center of the viewing window.
The first time you're collimating, the laser beam coming back off the primary mirror may be so far off that it isn't even hitting the secondary. You will then see the beam hitting your ceiling or a wall if you're working indoors. Adjust it first until it hits the secondary mirror. Finer adjustment now depends on the kind of laser you have. With a non-deluxe laser that emits a dot, you look at the inside of the focuser (being careful not to get the laser light in your eye) where the laser light comes back, and you try to find the red dot marking where the returning beam hits. You then adjust the tilt of the primary with the three bolts until the dot hits where the beam comes out. With a deluxe laser that emits a dot and has a viewing window, you first try to get the red dot close enough to show up on the viewing window (you may initially see the dot on the inside of the focuser board, say), and then you adjust to put the dot in the center of the hole in the viewing window. With a cross-laser, you have multiple options. You can align the returning cross in the viewing window (if you have a viewing window). Or you can align the arms of the returning cross on the lines of the outgoing cross on the inside edge of the focuser tube. Or if you look at the secondary, you might see both the incoming cross and the outgoing cross, or parts of them, and can align them into a single cross (photos four and five).
There are more sophisticated methods, such as using a Barlowed laser or an autocollimator. You can search the web for Newtonian telescope collimation.
Note: You might find that some eyepiece you have doesn't come to a focus no matter how far in or how far out you move the focuser. You can often use the primary mirror collimation bolts to fix this. If the focuser needs to come further in than it can, turn the three collimation bolt wingnuts so that the primary mirror moves up, towards the focuser, by a little. If the focuser needs to come further out than it can, turn the wingnuts so that the primary mirror moves down, away from the focuser, by a little. Of course, after that you should re-do steps 6 and 7.
Step 10: Light Shield
In a telescope without a tube, there should be a light shield behind the secondary mirror, opposite the focuser, which is basically like a portion of the missing tube. I thought long and hard about various complex designs. And then I went for one that doesn't look great, but is very cheap, light and transportable. I deformed a coat-hanger so it still had a hook but the main part became a circle. I hung some folded (and duct-taped together at the center) 1/4" foam board from it with wire ties. I had an extra hole near the top of the strut from a design decision I changed my mind about. The hook of the coat-hanger goes in the hole, the top of the coat-hanger sits on the top of the focuser board, and then the foam board hangs down.
It doesn't work well in wind. But fortunately at a dark site you don't really need it at all.
Step 11: Balance Issues
Remember those springs in some of the photos? Well, here's how they got to be there.
Partly because of the super-low-profile rocker, the scope has some balance issues. Specifically, with anything but a very light eyepiece, it wants to tilt backwards when it is pointing close to zenith (the eyepiece sticks out backwards and tips it) and it wants to tip forwards when it is pointing closer to the horizon.
A good deal of the problem can be handled by adding an extension spring on each side. One end of is screwed to the rocker box. The other end goes over one of the two carriage bolts that are used to attach the rocker box. This helps a lot, especially with the near-zenith issue. Experimenting with spring placement and choice may help.
There is still a problem with heavier eyepieces closer to the horizon. To fix this, when observing things close to the horizon with a heavier eyepiece, I hang a small drawstring baggie with a weight inside from one of the wingnuts attaching the strut to the mirror box (I can often leave the baggy in place for higher elevations--it then sits on the ground and doesn't affect things). I don't need to pack any counterweight with me when traveling by air, just the baggie, since one can always find a piece of scrap metal, or a rock, or, if all else fails, another eyepiece to put in the bag at the destination.
Step 12: Packing for Air Travel
I made a thick carboard cover for the sides of the mirror box (covering up the side ventilation holes, and cut a piece of scrap see-through plastic to cover the top of the mirror box for travel, and another piece to cover up the ventilation holes on the bottom). I usually put some paper towel on top of the mirror and then some bubble wrap, all under the plastic cover. I realize the paper towel may scratch the mirror, and if I had a more rigid plastic top cover, I wouldn't worry about it. But I do worry about something denting my thin plastic top cover and hitting the mirror.
As a final step, I drilled an extra hole in the baseplate of the mirror box near a corner, so I can bolt the secondary mount, with secondary, inside the mirror box (with a wingnut on the bottom side) for travel. In the second photo, the secondary is the bubble wrapped object in the corner of the box.
I put the mirror box in my backpack. It puzzles airline security most of the time, but I haven't had trouble--I just need to allow a few more minutes for inspection. Dressing nicely and being generally non-suspicious-looking may help. The TSA Blog recommends when traveling with home-made electronics that you tell the staff what you are carrying ahead of time. Last time I went through security, they didn't want to listen before they inspected.
Everything else--strut halves, screws, screwdriver, bolts, focuser board, focuser tube, secondary stalk, altitude bearings, rocker box, light shield go in a suitcase, in zippable bags as needed. I protect the pointy ends of the strut halves (remember they're cut diagonally) with a bubble envelope. I pack eyepieces and laser collimator either in bolt cases or boxes with bubble wrap or bubble envelopes. I've taken to labeling everything (e.g., "fragile - telescope optics - eyepiece" or "laser collimator for telescope") so airline security isn't too badly puzzled when they open the suitcase for inspection. Last time I traveled, I put most of the equipment in a duffle bag, which I then put in the suitcase among clothes. In the duffle bag, I included my nametag from our local astronomy club and two brochures for our astronomy club, thereby giving it all a bit more legitimacy for when TSA opened the bag.
Everything survived transport well, except one of my eyepiece had a broken plastic eye-shield (which I superglued back to functionality).
Step 13: What Have I Seen With This Scope?
This is a telescope I like for wide views (a 2" focuser would improve it!), especially when I travel. I successfully used it for a star party on one of the Gulf Islands in British Columbia, and I think people enjoyed the views.
Here's what I've seen through the telescope, based on memory and observing logs:
Terrestrial: Boats and ships, distant killer whales. Works pretty well for terrestrial targets with a 30mm eyepiece. Of course you have to face backwards or the image is upside down.
Non-terrestrial solar system objects: Moon, Jupiter and some of its moons, a ringed Saturn. Planets look good enough to impress people who aren't experienced amateur astronomers, but it's really not a scope optimized for high-magnification planetary work.
Deep space: M8, M13, M17, M20, M22, M24, M27, M31, M32, M39, M42, M44, M51, M57, M71, M81, M82, M103, Veil Nebula, North American Nebula, Double Cluster, ET Cluster. (For a few of these, I used an O III filter.)
Deep space objects are where the scope really comes into its own.
Runner Up in the
Celestron Space Challenge