This is on some updates I made to a 22.4” Dobsonian telescope I originally built in 2002 following the David Kriege and Richard Berry design (K&B) in The Dobsonian Telescope. The updates include a new shorter focal length primary mirror, larger Antares secondary mirror, improvements to the truss, reduced weight, and the addition of tracking and go to.
Some parameters are:
Physical diameter of primary: 22.4", 569mm”, clear aperature: 567mm
Focal length: 80.7", 2050mm (f/3.61)
Secondary mirror minor axis: 127mm, 121mm clear aperture and 23% CO with the Astrosystems holder
Fully illuminated FOV: ~15mm
TFOV with ES 20mm 100 deg ep: 0.97 degree, 0.85 with P2
These were my guidelines for the update:
1) Control cost
2) Use as much of the existing telescope parts as possible.
3) Lower eyepiece height, but primary mirror > f/3.5 to enable use of less expensive eyepieces.
4) Ease of transport and setup.
5) Mirror box to double as storage box.
6) On board drive motors for go to and tracking.
7) No tools required for truss installation.
8) Fans for mirror cooling.
9) For visual use only, no AP.
The 22.4” diameter, 2” thick quartz blank was given to me. I ground out the 0.388” deep curve by hand, mainly using a 10 lb barbell weight (as sopticals mentioned using). I started with #25 grit, then #40 grit, then #60…Then switched to a ¾ size tool for fine grinding. Polishing was started with a full size tool, and completed with a 10” tool which was also used for figuring along with some smaller tools.
I tested the mirror with a Bath interferometer and Dale Eason’s DFTFringe software (https://groups.io/g/Interferometry). The test results were verified by a Foucault double pass null test performed at Ostahowski Optics prior to coating. Terry reported the surface was very smooth and very well corrected. He also said it has a slightly heavy (low) edge, and that is common for large fast mirrors he has tested. He said you usually can’t figure that well without a null test during figuring, which I think speaks well for DFTFringe.
The 127mm (5") supremax 33 secondary mirror was purchased from Antares. PV error: 0.056 wave, rms error: 0.009 wave. It, and coating of the primary were my largest expenditures for the scope.
The original mirror was held by a sling – the old seat belt design. The model at Cruxis (http://www.cruxis.co...ecalculator.htm) indicated that a simple two-point support would work fine for the 2” thick, 22.4” diameter, f/3.6 quartz mirror, but a whiffle tree support was better, so I went with that. I modified the old K&B tailgate design to add an “A frame” with whiffle tree edge support to the mirror cell to make it similar to a JP Astrosystems cell (http://www.jpastrocraft.com/cells.htm). The three whiffle trees of the original 18 point mirror back support were re-mounted on the A frame with aluminum angle and shoulder bolts, and pins were added for the triangles to prevent rotation, mimicking the JP design.
The pads were cut from a sheet of Acetal (trade name Delrin). They were bolted to the triangles then each triangle was placed pads down on a sheet of #400 sandpaper on the steel top of my table saw and sanded flat until I could not see any light between them and the table surface when illuminated from behind, ensuring they were coplanar and flat. I also examined the pad surfaces to ensure they were sanded completely over their surface, with no low spots.
Alt and Az drives
I took inspiration from Andrei H…’s design for the drives, described here: LINK He said that the alt drive worked fine, and cautioned me about the difficulty of making the ground board O.D. precisely fit the belt glued onto it. He recommended I just use the more standard approach of using the ground board as a pulley with a belt around it and a drive pulley on the motor, similar to the photo at the above link under “The azimuth drive”.
I wanted both the alt and az motors in the rocker box for simplicity so I combined aspects of Howard Banich’s (https://www.cloudyni...-a-28-dob-r1089) telescope with Andre’s, using drive belts glued to the alt bearings (Andre) and keeping the mirror box inboard of the alt bearing radius (Banich design) so it passes above the drive shaft and pulleys in the rocker box. This, and the use of a “drive disc” fastened on top of the ground board permits mounting of both alt and az drive components inside the rocker box. The disk is 19.5” diameter, 12mm (~1/2”) thick Baltic birch, screwed to the top of the ground board, as shown in Fig.4. Also shown at center is the ½” bolt screwed into a nut welded to a plate mounted on the back side of the ground board. It inserts through a brass bushing in the rocker box to fasten it to the ground board.
The drive system components are shown in the first photo below. A close up of the altitude drive is shown in the second photo, and the azimuth drive pulley/belt is shown between the rocker box bottom and ground board in the third photo. The Az motor is mounted on a “slide” which permits adjusting belt tension. The fasteners are in slotted holes to enable this movement. The black box houses the drive PC board. Adjacent to the battery are a DC/DC and fuse supplying 12V for the fans from the 20V battery. Also shown in the 2nd photo is a close up view of one of the bearings the altitude bearings ride on. The two power cords laying in the box are for mirror fans.
The blocks holding the Alt drive shaft are Baltic Birch and are adjustable up/down to adjust pulley engagement with the belt on the alt bearings. The shaft runs on ingus bearings seen mounted on the block. (tip of the hat to Andre for those too). The photo below shows a closeup of an Alt drive pulley with the mirror box in place on the rocker box.
The truss is made of 1 ¼” O.D. aluminum tubing from the original scope. The original truss was the folding design described by Guy Walton in the February 2004 Sky and Telescope, vol 107, issue 2: (http://connection.eb...truss-dobsonian)
I was concerned that this was not rigid enough to maintain the alignment necessary for an f/3.6 mirror, so I changed the design. I decided to make the truss brackets to save cost, and to ensure rigid mounting of the truss to help hold optical alignment.
The mirror box and UTA truss brackets are made from 3/16” thick, 1 ½” aluminum angle and are bolted to the mirror box top and UTA bottom ring. The angle for the mirror box brackets was cut with a hack saw at 45 deg, filed, jigged up, and taken to a weld shop to make the corner brackets shown in Fig.9. The truss is clamped with the knobs on the ¼-20 bolts that are threaded into the brackets.
The top truss end brackets, shown in the photo below with the UTA truss brackets, have a pivot bolt with lock nut in the bracket below the pole ends which permits folding each pair of poles together when removed from the scope. The knobs are kept on the studs in the UTA brackets for easy assembly. The truss brackets were made from 2” aluminum angle with a hack saw, bench grinder, and file. The holes visible at the “corner” on each side are for the bolts that fasten the brackets to the truss poles.
The bottom truss brackets were made from 1” aluminum angle as shown in the photo below and fastened to the truss poles the same way as the upper ones. The ¼-20 bolts in the UTA and mirror box brackets fit snuggly into the bottom of the notches in the truss brackets. The intersection of the axes of a pair of truss poles is very close to the knob of the UTA brackets and the lower corner of the mirror box brackets.
Edited by GShaffer, 02 January 2019 - 10:47 AM.