A 20 inch f/3.6 'Celano' motorized Dobsonian telescope

After reading some good stories about fast telescopes, I decided to get rid of the ladder I needed for my 20 inch f/5 Dobsonian and ordered an f/3.6 mirror from Mike Lockwood (Lockwood Custom Optics). The mirror plus the 4.5" secondary, tested by Mike before coating, arrived on May 21st (2009).

I did't build a completely new telescope for this mirror, but rebuild my trilateral computerized 20" f/5 telescope (link in the menu below).Eyepiece heigth is now 170 centimeters, so even I (171 centimeters) can look in the eyepiece standing with both feet on the ground. Assembling and disassembling the scope takes about ten 10 minutes.

The mirror cell

The mirror back rests on an 18 points cell. I use roller bearings for the edge support. Pictures of the mirror cell are here and here is explained how I made the roller bearing supports.

The secondary cage and wire spider

The secondary cage consists of two 15 mm thick baltic birch plywood rings, with 20 mm diameter aluminum tubes between them. I didn't use only one ring this time, because of the weight of the secondary (745 grams) and the weight of the focuser plus paracorr and eyepiece. And I find a cage much easier to transport to a dark site, because I don't need an extra box for the ring assembly: the cage is already a box.

The spider is a wire spider, made of 0.48 mm diameter metal cable named 'diamant litze' in German, a very strong, non stretching cable often used in model airplanes. The distance between higher and lower attachement points on the hub and cage is 140 millimeters, giving a 60 angle where the wires cross. The hub is made of 15 mm square and 12 mm round tubing and is very lightweight. The secondary mount is a piece of 110 mm diameter, 2 mm thick aluminium tubing. The secondary is glued to the mount with three RTV blobs. To make the mount as lightweight as possible I have drilled (not visible on the photograph) lots of 12.5 mm wide holes in it. The wire spider supports the 1.1 kilo (2.4 lbs) secondary/holder assembly very well. The collimation is stable from zenit to horizon.

Connecting the trusses to the mirror box and secondary ring

To connect the trusses to the secondary cage and the morror box I used the same method I used for the five 12 incher I build some time ago (link in the menu below), as can be seen on the picture above. The trusses stay connected. They are made of 20 mm diameter 1.5 mm wall thickness aluminum.

Go-to and tracking: Celano stepper drive system

The first 8 years I used to drive the scope with Mel Bartels' stepper system like the former, f/5 version. This is shown here. August 2018 I decided to get rid of the laptop and the 64:1 primary timing belt/pully reduction. I bought a Celano controller and two steppers with a built in 71:1 reduction from the Dutch shop Deepskyparts. I had already motorized a 30 cm for a friend with this system, shown here.

Total reduction for my new system is 1264:1 for the azimuth and 1291:1 for the altitude drive. Taking microstepping (60 ms per full step) into account, I arrive at a microstepsize of ~ 0.085 arcseconds.The controller contains over 15000 objects (NGC, IC, Messier, Abell PN, Abell GC, Arp, Barnard, Hickson, Palomar, Holmberg, DSP catalogs) and it is possible to enter coordinates for objects not present in these catalogs. The new drive system is shown on the pictures below.

Total view, with both motors and timing belts

The altitude drive

The azimuth drive

both drives plus electronics case and handpad

Performance

As written above, the f/3.6 mirror is made by Mike Lockwood. The mirror delivers excellent views! With the Televue visual paracorr and Naglers, the stars don't show coma from center to edge. I often use UO ortho's on small faint objects and compared the view in a 13 mm T6 Nagler with a 12 mm UO ortho, on M13. The FOV in the Nagler was much wider of course but besides that I hardly saw any difference. In both eyepieces the stars were pinpoints to the edge of the cluster. And the startest shows a very smooth surface, which is very important for deepsky observing. I tested the mirror with my Bath interferometer which also showed a good mirror, as can be seen in this report.


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Go to: A 20 inch f/3.6 computerized Dobsonian Go to: Building a trilateral computerized 20 inch f/5 Dobsonian
Go to: Project: five 12 inch lightweight Dobsonians Go to: Motorizing a 12 inch lightweight Dobsonian
Go to: 20 inch telescope Go to: Equatorial platform Go to: Scotch mount Go to: Binocular mount Go to: Dobsonian tips
Go to: Bending aluminium Go to: Collimating Go to: Making a Krupa collimator Go to: Dotting the primary
Go to: A ballhead type telrad/finder mount Go to: Mirror making log of 300 mm mirror Go to: Astronomy
Go to: Using digital finder charts at the eyepiece Go to: Astronomy links Go to: Building a spherometer
Go to: Building a Bath interferometer Go to: A Foucault-Ronchi-Lyot tester Go to: Building a mirror making machine
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Email to: Jan van Gastel