DIY Telescope On A Sliding Scale

Description

Purpose: Based on the plethora of existing resources, I will retell how to make a Dobsonian telescope as cheaply as you can obtain the materials, or as sophisticated as an automated long-exposure telescope. Given that astronomy and astrophotography are cost-constrained activities, this project is more of an activity (and a time sink) than an end product. This is ideal for someone who is interested in astronomy and more willing to invest time than money, as opposed to having a casual interest in the subject, little time to devote to it, and the money to buy one of various automated telescopes that takes impressive pictures with minimal knowledge and effort (e.g., Unistellar, Dwarf, etc.), which is a perfectly rational thing to do if you can afford it. At the end of my DIY telescope journey, I came to appreciate the different types of telescopes, lenses, camera sensors, and sky objects to observe; and finally I knew, from doing this project, what telescope is right for me.

Details

Dobsonian telescope:

Wikipedia - https://en.wikipedia.org/wiki/Dobsonian_telescope
YouTube -

Design - I used a wooden frame rather than a cardboard tube. The telescope is balanced on a mount that sits on a swiveling base, which can be rigged with bike gears and stepper motors to automatically point the telescope. The azimuth can be controlled by a 3D printer threaded rod and stepper motor. Is it worth it to automate that or is it good enough to have an optically functioning telescope and no more? You decide.
Materials list:
1. Primary and secondary mirror (such as 114 spherical mirror)
2. Eyepiece (e.g., 15-25mm Plossl)
1. 3ft ~1x1” fence posts (6 or 7 of them)
2. Craft plywood (~4sqft)
3. Fasteners (nuts, screws, bolts, angle brackets, straight brackets)
4. For spider mount:
  1. Bike spokes (or something similarly thin)
  2. Fasteners (if you want adjuster screws)
  3. Bottle cap or cardboard tube (e.g., aluminum foil roll tube), thumb tacks, duct tape, ect., to mount the secondary mirror.
5. For a motor drive
  1. 2 stepper motors (e.g., Nema 17 Stepper Motor Bipolar 1.5A 42Ncm 42x42x38mm 1.8deg 4 Wires)
  2. RPi stepper motor hat (e.g., Adafruit) and soldering kit if the pins are not soldered. The Adafruit hat was the only driver that worked for me with my stepper motors and Raspberry Pi 4B.
  3. 3D printer threaded rod and nut, 3D printer kit bearings, fasteners (e.g., wood screws)
  4. Bike chainring, bike chain, bike sprocket, fasteners
  5. 12V battery
  6. Breadboard, capacitor for the battery power, lead wires, and ribbon cables for stepper motors
Cost (approximate):
1. Mirrors: $30
2. Eyepiece: $20
3. Wood: $30
4. Fasteners: $30
5. Camera: $32 (I used a Raspberry Pi IMX462 HDR 2MP Arducam camera)
6. Motor drive components:$150
7. Cost summary of your telescope by tiers of functionality:
  1. Basic functioning ~50x magnification telescope: $110
  2. “” with camera: $142
  3. “” with motor drive: $300
Astrophotography
1. Technical constraints: larger pixels have more sensitivity, giving a better dynamic range, but this correlates with a lower resolution for a given sensor area. Less expensive cameras (e.g., webcams, jetson nano / ribbon cable Pi cams) have smaller sensor area than DSLR cameras, and a small sensor area means that it is hard to locate a sky object with a smaller sensor.
2. There is a <$100 pricepoint for Pi cameras and I found that using IMX462 ($32, 6.46mm diagonal) with RPi libcamera to control exposure time was good but low resolution, 2MP. The smaller the sensor (diagonal), the smaller the field of view (FOV). A smaller FOV makes it hard to locate objects and keep them in frame.
3. Higher Pi cam price points are the IMX586 ($200, 8mm diagonal), and IMX283($340-$450, 15.86mm diagonal). See the Arducam website.
4. This brings us to DSLR cameras (Nikon, Cannon, etc.) where a used DSLR is >$300 and the sensor diagonal is >12mm. Use a T-mount or T-ring to connect the DSLR body to a commercial telescope focuser.
5. Image processing (consider using Python OpenCV and NumPy, Qt widgets). Here is my code: https://github.com/acvanp/AstroImage
  1. Take images of your subject (star, planet, whatever), align those images and average them.
  2. Place a blanket over the telescope and take "darks", 4 or so of them, average the darks and subtract the dark average from the image average.
  3. Shine a light into the telescope and take "flats", average the flats and divide the dark-corrected image average by the flats average.
  4. After this you can do touch ups like brightness/contrast/saturation/sharpness. Consider using ImageJ software from NIST.

Build Instructions

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For this particular wooden frame Dobsonian telescope

Measure twice and cut once.

Know the mirror size focal length and design the telescope around that.

The primary mirror mount attaches to the baseboard with 3 adjustable screws. The screws have to push and pull on the mount and thread against the baseboard, so consider various fasteners like friction nuts or pronged tee nuts, and maybe wood glue or epoxy.

With the mirror mount on the base, you can attach the fenceposts to the base and framing to the top of the fenceposts, around about where the spider mount will go.

Use a street lamp or a full moon to find the focal point relative to the body of the telescope and thus determine where the spider mount and lens should go. I carved a rough hole in a small piece of plywood for the lens; used bike spokes for the spider mount and drilled holes in the fence posts to mount the spider mount on the fence posts.

As in the YouTube video of Dobson building his telescope, find the center of gravity for the telescope and build the mount using sections of fenceposts. I wrapped Teflon tape around where the altitude mount pivots in a see-saw motion to make the telescope pivot smoother around the altitude mount. For an altitude motor drive, use the 3D printer screw and a stepper motor. The stepper motor mounts to the stand and the screw pushes and pulls against the body of the telescope (see project images). Make use of the 3D printer bearings that commonly come packaged with the 3D printer screw to have smoother rotating parts.

The azimuth mount can swivel by rotating one layer of the telescope stand sandwiched against another layer of the telescope stand. Use a Teflon gasket and vinyl record like bearings sandwiched between the two layers, as in Dobson's YouTube video. I used a long bolt (such as a toggle bolt) as a spindle. For a motor drive, fix a bike chainring against the underside of the upper layer of the telescope base and fix the stepper motor to the outside edge of the lower layer of the telescope base (see project images). Use shims (I used pieces of the 1sqin fencepost) to elevate the base of the telescope off the ground so that the stepper motor is aligned with the chainring. Use a small cog/sprocket and large chainring to get a good gear ratio for a modest stepper motor to torque the telescope for a functioning drive. I found creative ways to attach the sprocket to the stepper motor, to make the azimuth spindle just tight enough to minimize vibration and just loose enough to allow the stepper motor to move the telescope. Get creative with the mount and the vinyl/teflon bearings. I used a plastic strip from a plastic bottle to keep the bike chain aligned on the gears.

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