Fabricate the Spider and Focuser assemblies

Here we are at the final assembly and test stages. The placing of the optical components inside the telescope tube is a very important part of the assembly.

Take a look at all parts now. Are all the wood parts painted? if not then you need to besure to paint all plywood sides and edges so water moisture will not warp the wood later in life.

Take a look at the mirror cell from the front. Be sure there are no glint reflections, no bright metal spots that will cause reflections. Only the mirror should reflect light.

Take a look at the diagonal holder and spider assembly. There should be no sharp edges. If the metal shows sharp edges then file them smooth and bevel all metal edges. Paint all metal parts flat black. Do not paint the diagonal mirror surface!

Check the eyepiece focuser. Crank it all the way down, and look at the bottom that will stick into the telescope tube. Are any of its edges shiny or metalic? If so, then flat black paint again and again tell all parts inside the tube that can cause glints are flat black.

Check the PVC EL rotator cup bolts. The two bolts will stick into the tube where the washer and nuts will be installed. Paint these items flat black, before you install them.

Final Assembly Begins

Mount the whole finished Mirror Cell assembly into the bottom of the telescope tube. Mount the cell assembly so that the 3 bolts do not stick out the bottom of the tube. This will protect the mirror cell from jaring when you set the tube on its end.

Do the two string alignment again to be sure the mirror is pointed up the center of the telescope tube as best as you can see view it form the upper end of the tube.

You will now measure up the telescope tube from the front of the mirror the focal length of the mirror. The 6”f5 mirror has a focal length close to 30”. Therefore any star light coming from infinity stellar objects will focus at the focal point 30” from the front reflecting surface of the mirror.

Prepare the secondary diagonal mirror assembly. The single vane spider will push into the tube. You will place the diagonal mirror into a position where it will reflect the light form the mirror at 90 degrees.

Here is how to determine the distance from the front of the primary mirror, up the telescope tube, to the point where you will want the secondary mirror to sit and reflect the light at 90 degrees through the side of the tube, where the focuser hole will be cut, and through to the magnifing eyepiece.

A Dobsonain telescope is just a Newtonian telescope design.

The Dob is just a simple AZ-EL mount that is simple to create. Most telescopes use equatorial mounts, or fork mounts, or other heavy duty mounts with drives to track the stellar objects.

Here is a picture of a telescope and how it reflects the light from the mirror to the secondary and up to the focuser.

Figure 1: Newtonian Telescope Mirror and Light Path

If you study the drawing above, figure 1, you will see that the stellar light passes into the front of the telescope, down the tube, and hits the concave reflecting mirror. This mirror will then reflect this light back up the tube. As the light converges from the mirror to form the an image it has to hit the secondary mirror and reflect 90 degrees upward through a pre-hole in the side of the tube, and finally up just above the eyepiece focuser device to from the final image.

One needs to determine where to cut this focuser hole in the telescope tube so that the stellar light can pass up to the focuser. This is very simple. Don’t make it harder than it sounds.

The mirror has a focal length of XX inches, in this case 6”f5 or 30” from the mirror front to the final image it creates. If you don’t know the focal length of the mirror then you need to measure it using a very distant light or the moon. Carry the telescope outside with a long ruler. Point the scope at the light or moon and hold a piece of paper infront of the mirror so it make an image on the paper. Measure the distance form the image on the paper to the front of the mirror. This is the infinity focal length, or Fl.

Think about this. The mirror will reflect all light back up the tube, along the center line of the tube. You can't put your head in the front of the tube to see the image, because your head will block all light entering the telescope. Therefore you need some device that will bend the mirror reflected light at 90 degrees and allow the light to exit through a hole in the side of the tube. This is the purpose of the spider and secondary mirror.

To calculate the hole position on the telescope tube take the FL (30”) and subtract the radius of the telescope tube ( half the diameter of the 8" tube is 4”) and subtract the cardboard wall thickness of the tube and any fiberglass you added, say 3/16” for the tube wall, then subtract the height of the focuser when rolled all the way in, say 2.5” for the one supplied, and subtract another ½” for good luck. You want the image to be just above the top of the eyepiece focuser.

So, what was the distance?      30" - 4” - 3/16” - 2.5” – ½” =   22.5” VOILA!!!

Now you need to mark a point on the outside of the tube that is 22.5” from the front surface of the primary mirror.  Do not remove the mirror and cell. Leave them as they would be in final assembly.

The tube should be a bit longer then you need right now. The tube won't be cutoff tell the very end of assembly.

Measure the distance from the primary mirror front to the front edge of the tube. That distance will be YY inches. Subtract the 22.5" from the YY inches. The YY-22.5" is the distance from the front of the tube down the tube to the center of the hole for the focuser.

Mark this point on the outside of the tube.  Mark the hole center point.

Get out the drill and hole cutter and cut a 1.5” hole at this point on the outside of the tube. This is where you will mount the focuser.

If you have the focuser and install bolts, center the focuser on the hole cut in the tube wall, and drill for the focuser mount bolts. Put the bolts through from the out side of the tube and attach the washer and nut on the inside. When all done paint the washers and bolts on all sides and do a final assembly of the focuser.

Check the focuser by looking at it from the inside of the tube and be sure it has no highly reflective glints or bright shiny spots. If it does then use the flat black paint and spray those spots. Do not spray the mirror or diagonal mirrors.

Figure 2: Determining the Focuser and Spider Assembly position along the tube

The next step after the focuser is all mounted square to the tube, is to assemble the single vane spider and secondary mirror holder.

The spider assembly will be manually pushed into the tube until the diagonal mirror, or secondary mirror, is aligned with the center line of the focuser and reflects the telescope mirror light up into your eye.

The mounting of the spider is a bit tricky. You want the spider to be across the center line of the telescope tube.

Then angle that the spider assembly sits in the tube is not too important. The most important part is that you want the diagonal mirror center exactly in the center of the tube so it will align with the center line of the primary mirror.

Figure 3: Mounting the Spider Assembly in the tube

Figure 4: Align the Secondary in the center of the tube

If you look at figure 4 you see that the spider can be placed on any diameter of the tube. The spider does not have to be square to the focuser, it can be at any diameter across the tube. The important function of the spider is to hold the secondary mirror so the center of the diagonal mirror is on the center line of the tube, and on the center line of the mirror, so the image gets reflected up to the focuser.

The focuser holds eyepieces that you use to magnify the telescope image of the object you are viewing.

Spray paint the spider and any other glints flat back and install the spider.

To install the spider you have to hold the whole spider assembly inside the tube with your hand or with a C-clamp.

Place your head above the focuser in the center line of the focuser 1.25" hole. Set the spider so that the secondary mirror is centered on the focuser hole. Now tip the spider and rotate the secondary mirror so that you can see down the tube.

Align the secondary mirror and spider so that when you look through the focuser on its center line you can see the center of the primary mirror in the center of the secondary mirror.

Drill two screw holes to hold the spider in place. Then do a final secondary mirror centering. Once the secondary mirror and spider are working you can tweek the 3 mirror cell screws so that you can see your eye reflected right back on top of itself from the center of the mirrors.

Cut the end of the telescope tube off now so that the spider is within the front of the tube.

Paint the end of the tube with the flat black paint, being careful not to get paint on the mirror or the diagonal mirror.

Well we are almost done. But not quite.

The telescope tube now has to be balanced. Obtain a standard eyepiece that you will use in viewing. Place this in the focuser. Go out side and see if you can focus a star or moon image.

If you cannot get a focus by running the rack and pinon or eyepiece up and down as far as it will go then you have a problem.

The place where the focuser is located may be too far form the mirror. Maybe you over estimated the calculated distance.

The first method to use in adjusting the focus is to loosen the 3 mirror cell screws each about 1/4". If the springs are not tight enough then replace the springs so you can maintain a tightness between the mirror cell and back plate.

If you moved the mirror up a bit then go check the focus. Move the mirror up more to find the focus. If you find you have to move the mirror more than about 1/2" then you need to close up the old focuser hole and drill a new hole some distance down the tube.

Determine a new distance and move the focuser and spider closer to the mirror.

You can close up the old tube hole by using the cardboard piece you cut out and epoxy resin. Put tape on the inside of the tube and push the card board hole cut back into the hole. Pour some epoxy resin around the hole and let it set up. Then sand the old hole down and paint.

Now go recalculate the focuser distance. Drill a new hole and do it all over.

If the telescope will not focus do not cut off the end of the tube yet.

Once you have a successful focus image with your eyepiece then you are probably safe to cut the excess tube length off.

Installing the Elevaton PVC rotator plastic pipe cups

This last section will cover the balance of the telescope and the mounting of the elevation pvc rotator cups.

Install all parts of the telescope. Install a typical eyepiece of the weight you will use. Using a ruler standing on edge place the telescope tube about halfway from front to end on the edge of the ruler. Move the telescope up and down the edge of the ruler until you find the distance where the front end of the telescope and the back end are balanced. Mark this point on the tube.

Generally the eyepiece focuser is straight up when you view. So place the telescope tube flat on the table so the eyepiece focuser is straight up. Find the balance point of the whole telescope tube and mark this point on the tube.

Prepare the PVC EL rotator plastic pipe caps as follows. The flange part that will bolt to the outside of the telescope tube has holes for bolts already. Choose two opposite holes, 180 degrees apart, and using a drill with counter sink bit, drill counter sink 45 degree impressions into the top of each hole. Remember the bolts that go through the pvc holes must be flush because the pvc caps have to rotate in the elevation direction when pressed into the Dob elevation cutouts of the wood support box. The bolts can not stick out or you can not rotate properly.

Figure 5: Elevation telescope tube rotator cups

Take one of the PVC EL rotator plastic pipe end caps and place it so the center of the piece is aligned with the balance point of the telescope. The eyepiece focuser point sky ward, straight up. Place the pvc cap against the side of the 8” tube, on center line with the tube.

Figure 6: Mount the pvc EL cups on each side of the tube at the center of balance

Drill two holes to install the bolts along the length of the tube. Align the pvc end cap so the mount holes are in line with the telescope center line along the tube length.

Drill the two holes. Paint the bolts and nuts and washers flat black.

Push a bolt through the pvc pipe cap prepared counter sunk hole, through the tube wall, and install a washer and nut on the inside.

Note that the bolts that hold the pvc cap to the tube must be couter sunk. You will see that two of the mount holes in the pvc cap have been counter sunk to allow the bolt heads to be flush. If they are not flush you will find you cannot rotate elevation of the telescope in the Dob mount.

Do this for both pvc pipe rotator caps. One on each side of the tube, 180 degrees apart.

Figure 6: Attache the plywood covers that will hold the pvc cups from fallling out of the Dob mount

There are two pieces of plywood cut to fit the top of the Dob support box. These are cut to fit over the pvc rotator cups.

Install the two wood supports by using hindges on one end, and a locking hindge on the other end.

Final Optical Alignment of the Telescope

Your are now ready to align the final mirror to diagonal to center of the focuser.

This alignment must be as accurate as you can perform. This type of final alignment is done each and every time you take the telescope out to view. If any of the optical parts are loose or out of alignment the image quality can be very poor.

The images will be blury, egg shaped or warped or flairy, or even multiple images.

Also note that cheaper eyepieces may not be antireflection coated and when a bright star, planet or the moon is viewed you will get mulitiple ghost images. The only correction is to buy more expensive and bettrer eyepieces.

Here is the general method to algin your Dob scope.

Obtain an old 35 mm film canister. The empty plastic bottle that 35 mm film comes in when you buy it at the store. Keep the top and the bottle.

Carefully drill a small 1/8" hole in the top and the bottom of

the film canister. You must drill these holes exactly in the center line of the film canister top and bottom. If the holes are

tilted or off center then you telescope alignment will be crooked also. So, make a good alignment canister. Film canister fit in the 1.25" focuser easily.

There are other alignment methods, one uses a laser alignment fixture. Another method is to look at a star and when slightly out of focus you see the shadow of the spider. Move the mirror until the shadow is centered on the shadow. The remove the eypeiece and align the spider diagonal so you see the whole mirror when looking throught the center of the focuser.

So, the method you will use will be to put the plastic film canister in the eyepiece focuser. Now look through the holes and see your eyeball looking back at yourself. If you can't see the mirror then adjust the diagonal holder. If you can see your eyeball then adjust the mirror 3 bolts until your eye is centered on your eye and the film canister holes. Then remove the canister and view from the center of the focuser to adjust the spider and diagonal tell you can see the hole mirror and usually some of the inside of the telescope tube.

If the diagonal is sized properly you will see the whole mirror and only a little bit of the inside tube wall. If the diagonal is too large you will too much tube wall and that is just a waist of diagonal.

To adjust the diagonal you loosen the nut on the slider bolt of the spider. Loosen the nut so it is just tight enough to allow you to slide the bolt along the single vane spider so the holder is centered on the spider and then rotate the diagonal holder so the reflection off the diagonal mirror is centered. Centered means you can see your eyeball right back on top of your eyeball image.

Your done!!!

Put the tube in the Dobsonian box support and go out and view.

Put in a 25mm eyepiece and test if an image of a star will focus. Once you find focus try another eyepiece like a 12mm and refocus. If that all works then the focuser has been placed properly on the tube.

If you find you can only focus the longer focal length eyepieces, and not the shorter focal length eyepieces, then you will need to move the mirror forward in the tube. You don't have to move the back plate of the mirror cell at this time. Use the spring action of the 3 mirror cell bolts and let the mirror move forward up the tube by say about 1/4". So, loosen each of the 3 mirror cell bolts about 1/4". Then realign the mirror to the eyepiece focuser. Now try and focus the longer focal length eyepieces again. If they work then great. If not then you will run out of mirror cell adjustment and you will have to then

move the mirror cell back plate up the tube by say another 1/2".

You will have to use your judgement on how far you need to tweek the mirror cell inorder to focus all types of eyepieces.

For now go view the moon, the planets and the brighter stars to get a feel for moveing the Az-El Dobsonian mount.

Tracking the stellar object with the Dobsonian mount is a lesson with inself. It takes practice, practice and more.

Good viewing.

Figure Last: Figure your done with this telescope and ready to start another.

The final conclusion is that the 2005 fall semester Astronomy 101 Lab Dobsonain 6"f5 telescope is ready for student use.

Final assmebly is a set of steps. You have to do it to learn it. The learning is in the doing of the assembly and figuring out what is needed to mount all the parts together into one working assembly.

Trail and error is the name of the game.

Good Luck. Think ahead.