Hadley 4.5
Latest additions
February 28, 2023 Laser Pointer
29 July 2022
Introduction
This is a homemade telescope project in which the optical elements are all purchased (as opposed to grinding and polishing mirrors) that might reach a wider audience. 4.5 inch spherical mirrors with elliptical diagonal mirrors can be purchased for under $30 (as of this writing) and most of the other parts can be 3D printed for under $25 (filament cost). With a reasonable eyepiece and some miscellaneous mechanical parts (screws, bolts, etc.) the total out-of-pocket cost should be under $100. This telescope will be of much higher quality than most of the department store telescopes that will be purchased this Christmas season.
This is an ideal individual or family project that will show practical STEM lessons and produce a STEM tool that will easily show features on the Moon, Jupiter and it's Galilean moons and the rings of Saturn. It is mostly a 3D printing project using an inexpensive 3D printer (I used an Creality Ender 3 costing about $200 as of this writing) which will also introduce the participants to that experience which may carry over to other aspects of their lives. There are many 'maker spaces' around the country which have 3D printers they will share.
The Hadley Telescope
Fortunately a suitable design already exists and we are free to copy it.
There are quite a few parts to print (some of which are identified above). The following table shows more detail.
Optical Tube Assembly (OTA)
Mirror Alignment Knobs - 25 minutes to print at 0.20 layer depth, 4 grams PLA
The print requires stopping at 6 mm to insert 10-24 nuts (I use the change filament function to get the print head out of the way)
Spider ~3 hours to print. This print requires stopping the print twice to insert 10-24 nuts for embedding. The first pause is at 6 mm (after 30th layer if you are using recommended 0.2 layer height). The second pause is at 11.4 mm (after 57 layers). See image below.
30 grams PLA
Peep Hole - This was added to original design to give more precision to sights. It fits into the lower sight. However, see description of laser sight below for improved performance.
4 minutes to print
.5 grams PLA
OTA Assembly
Assembling the OTA is fairly straight forward. Look at the photo below. You will need three 1/2 inch round rods (aluminum preferred for the weight), each three feet long . The bottom tube, mount interface, sights, baffle and top tube slide over the rods and are held in place with 10-24 screws and nuts. The nuts slide into the rectangular slots to provide threads for the screws which clamp the parts to the rods (don't overly tighten the screws). The relative positions of each part will need to be adjusted as we go along.
The bottom of the focuser is attached to the upper tube body with 10-24 screws and nuts. The top of the focuser simply screws into the bottom part.
The bearings are attached to the mount interface with 10-24 screws and nuts. Once the mirrors and everything else is installed, you must adjust the position of the interface along the tube to achieve a balance. You will set it in the mounting to do this.
Run a nut all the way down a 2 in 10-24 screw and apply a drop of Loctite to keep the screw from turning in the nut. Insert the screw into the diagonal cell and through the central hole in the spider, put a nut on the end of the screw. This may need to be adjusted to make the diagonal centered in the focuser tube. Glue the diagonal into the cell with flexible glue.
Insert three 2 inch 10-24 screw through the three threaded holes in the spider until they stabilize the diagonal cell. We will need to adjust these later to orient the diagonal properly. The spider will only fit one way into the upper tube so the diagonal must be rotated to be visible through the focuser tube.
As with the diagonal cell, run nuts all the way down three 2 inch 10-24 screws and seal with Loctite. Insert into the cell. The mirror will be glued using flexible glue on the three pads. On the back side of the cell, place three compression springs which will go between the cell and the bottom tube to be secured with the three threaded knobs. The picture below shows the knobs holding the springs for the picture.
Mounting
7 August 2022
The Alt-Az mounting is also 3D printed. The various parts are glued together to achieve a Dobsonian mount.
Bottom Half of Rocker Box
20 hours to print (start at Noon, finished next day at 8:00am)
.32 mm layer
400 grams of PLA
Ground Board - 5 hours to print
135 grams PLA
A one inch long 1/2 inch hex bolt is inserted and glued in the ground board to serve as a king pin for the rocker box.
Various materials can be used for bearings. I have used Velcro pads (the loop side), felt spacers used under furniture legs, Teflon strips. In this design a reasonable amount of friction makes the mount more stable while allowing easy movement.
Mounting Assembly
Attach the sides and brace to the bottom half with glue and clamp.
Insert and glue a 2 inch long 1/2 inch hex bolt into the ground board. Attach three strips of Velcro over the feet underneath. (For a heavier mount we would use Teflon strips but the lighter mount needs more friction to be smooth.)
1 September 2022
Finished Telescope
4 September 2022
The stability of the telescope in use is greatly dependent upon the size of the ground board and the related AZ bearing diameter. The Creality Ender 3 used for the example above is limited to an AZ bearing of about 8" diameter. It works reasonably well and allows objects to be found, centered and focused and vibrations tamp out rapidly to allow easy viewing. A larger 3D printed base would require a printer with a bigger print volume (eg, a Creality CR-10 or such).
An alternate choice is to make the mounting out of wood. To that end we designed a different telescope mount that would be more stable. We kept the same design for the sides and simply made a larger rocker bottom and bottom board. The pieces can all be cut from a 1/2" 2x4 ft board (plywood or MDF) costing about $13 at the time of this writing.
These pieces can be cut using a saber saw and hole saws but a more elegant and precise way is to use a CNC router (such as the one available at the Inventors Forge Makers Space).
Turning Tabs
28 February 2023
Some simple tabs added to the base make turning in azimuth much easier. These are simply screwed and glued to the base as shown.
Laser Pointer
28 February 2023
I found the 'gun sight' to be difficult to use if the night is reasonably dark - couldn't see the front sight!. I remade the front sight using glow-in-the dark filament but, because it has to be 'charged' in sunlight, it didn't last long enough until dark. So I decided to use a green laser pointer which can be purchased on Amazon for about $12 and designed and built a mount for it. The mount uses a ball and socket to make alignment easy (just point the telescope at the Moon or a bright star/planet and point the laser to it). Here are the parts needed (plus a few #10-24 machine screws and nuts),
Assembly drawing
Ball
5 grams of PLA or PETG
10 minutes to print
Best to use 0.20 layer height for smooth surface
To align with celestial target, loosen screws in cap to allow easy movement without 'flopping' around. Point laser to target and tighten screws.