Posts Made By: P. Lee Stock

Thought I'd share the finished project.

Here's an assessment of the pros and cons of this design, as I've found so far:

Pros:

- Very powerful, weight/imbalance tolerant drive system
- Strong, backlash free linkage
- Resets instantly by disengaging drive rod from a half-nut

Cons:

- Some periodic error unless leadscrew and connection to motor shaft are perfectly straight
- Tangential error common to all tangent driven platforms

After straightening the threaded rod and making the best rod to motor shaft connector I could, the platform has a small amount of periodic error--which admittedly caught me by surprise. It will move an object around approximately 1/3 of the apparent field of my highest power eyepiece (4mm ortho--305x) in my 8" f/6 newtonian every 45 or so seconds. So, it's not a photography-capable platform as currently driven.

But... I insisted on instant reset and weight/center-of-gravity tolerance (and design simplicity)--these objectives it achieves. Traditional tangent drives are cumbersome to reset and direct drives are sensitive to weight and center-of-gravity issues. In short, I don't like periodic error, but I'd take a small amount over lengthy reset procedures or a slipping table.

I am considering eventually driving the platform using a variation of the linear conversion drive shown via the link below. I imagine instead a timing belt doing a continuous loop while forcefully engaging/disengaging the belt via a clutch where the current pivot (brass thumbscrew) is located. I would also insist that any drive system employed retain fast reset and variable load tolerance.

http://faculty.washington.edu/quarn/stpmtr.html

I posted some photos a few days ago showing how I transfer power and signal across the azimuth axis of a dobsonian without resorting to dragging cables or having to use an electrical connector in the pivot bolt, which you generally want to reserve for your encoder bushing. The one I showed earlier requires a metal lathe and takes a lot of time and effort (http://astromart.com/messages.asp?message_id=127907&page=).

The one shown below, however, is easily made in about a half hour and only requires sheet copper, a couple springs, and some solder. I oversized the copper rings to make things easy, and because my telescope has a large, solid ground board. Also, I was only dealing with transferring two wires, + and -, across azimuth so I could connect my EQ platform directly to an RCA jack on my ground board.

The one linked to above transferred power *and* signal to a stepper motor, which meant 4 wires, plus the telescope design required the hub to be very small so it wouldn't be seen between the spokes of the rocker bottom.

Just thought some of you might eventually run into this concern and wanted to show how I've dealt with it.

I have a set of recent TV plossls and want more magnification. I am considering the 5x Powermate, the 2.5x Powermate, and the 3-6mm TV zoom. Ignoring cost and number element considerations, I'm merely wondering if the 5x Powermate can be used with the all the TV plossls without vignetting. If so, then it appears that I'll be able to add effective FLs of 6.4mm, 5mm, 4mm, 3mm, 2.2mm, and 1.6mm. That would be great, though I'll probably never use 2.2 or 1.6.

Otherwise, the other options are worth considering. What do you recommend?

Thanks.

I posted this in the TV forum, but thought I'd ask here as well. I wouldn't even consider putting this in the eyepiece forum as no one would ever look there ;^) .

I have a set of recent TV plossls and want more magnification. I am considering the 5x Powermate, the 2.5x Powermate, and the 3-6mm TV zoom. Ignoring cost and number element considerations, I'm merely wondering if the 5x Powermate can be used with the all the TV plossls without vignetting. If so, then it appears that I'll be able to add effective FLs of 6.4mm, 5mm, 4mm, 3mm, 2.2mm, and 1.6mm. That would be great, though I'll probably never use 2.2 or 1.6.

Otherwise, the other options are worth considering. What do you recommend?

Thanks.

This is worth checking out, I think:

http://astromart.com/viewad.asp?cid=232816

A lot of Alt-Az telescope concepts floating out there... this one has some unusual features worth logging away in the memory banks.

Thought I'd share some photos of the telescope I am nearing completion of. I'm working on the tube now.

The mount is dark-stained cherry veneer over baltic birch plywood. Six coats clear polyurethane (to keep the grain clear and better see imperfections during the sanding phase), and final coat in satin to better withstand inevitable brushes with shrubbery and vehicle upholstery ;^).

The young man is my enthusiastic woodworker son Joseph, just turned 14 years old.

This has been one of the most enjoyable to read posting streaks in this forum in awhile. It really highlights the benefit of this forum over other ATM forums since the photos are immediately visible as you read the text description.

I asked Herb if he wouldn't mind not purging this forum of it's posts and photos in the future to save storage space as I believe it is developing into a valuable ATM resource and he responded that he had no plans for a future purge. This could become "the" place to hunt down ATM solutions.

Please keep the posts coming--what fun I'm having reading and looking at all your creations! Makes me feel like a rank newbie all over again ;^).

"Lee"

I'm getting ready to attach this 8-inch cell permanently onto the tube and thought I'd post a detailed view. Some features:

- CPU fans to break up the thermal boundary layer on the mirror face
- 3.5" PC case fan in a "down flow" configuration to pull air down the tube and out the back (including the boundary layer as it is blown across and off of the face of the mirror)
- 2" diameter felt support pads according to Bob Royce's recommendations for 8-inch mirrors (this one is a 1.5" thick Zambuto)
- Rubber stoppers instead of springs supporting the cell plate--prevents shifting/rocking of the cell plate as the scope is aimed from zenith to horizon
- Velcro retaining system rather than clips hanging over the mirror face consuming aperture and causing diffraction. It is really nice looking down the tube and seeing the entire mirror face without clips. The mating velcro on the side of the mirror is adhesive backed
- Routered channel in the end ring. It will be slid onto the end of tube and screwed to it. The back plate holding the cell (ala Richard Berry style) screws to the end ring and is quickly released by unscrewing three knobs around the perimeter of the ring
- Rear fan and boundary layer fans are on separate circuits with variable speed control for each, permitting you to dial in the airflow for rapid cooling vs. maintenance during observing (to eliminate the possibility of vibrations at the eyepiece)

Michael,

This looks like a solid design. I'm big into the beauty of wood, but I've enjoyed and appreciated your posts, especially your sharing your methods of work. I really like the ground board design with the three ball bearing assemblies directly over the telescope feet. The scope has a simple, no-nonsense appearance to it--everything where it should be doing what it should do.

Take care,

Lee