Apodizing Mask

Many people have seen me use a home-built apodizing mask while observing planets and have inquired about its use and design. An apodizing mask (I sometimes call it my “60’s filter”) is used to cut through the seeing much like an aperture stop. Technically, it is supposed to [...]]]>

Apodizing Mask

Many people have seen me use a home-built apodizing mask while observing planets and have inquired about its use and design. An apodizing mask (I sometimes call it my “60’s filter”) is used to cut through the seeing much like an aperture stop. Technically, it is supposed to approximate a gaussian curve for the aperture instead of the sharp edge of the normal scope. In a refractor, it basically removes the first diffraction ring of the airy disk at the expense of fattening the central part of the disk somewhat. In an obstructed design like a Newtonian or SCT, there is some debate about it’s value. Also, the larger the secondary’s obstruction, the less it helps. It isn’t very useful in a scope smaller than 8 inches. For my scope (reflector) and my eyes, it seems to help on those less than perfect nights of seeing. Another “side effect” that it makes is a rainbow pattern around the object–hence “60’s filter” (psychedelic man)! The bottom line is that it helps see detail on planets and split double stars.

Here’s the quick and dirty on making one…
Constructing the screen is very simple. I used three layers of standard fiberglass window screening material. All three screens are cut to the outside diameter of the telescope. Central holes are cut in the following diameters based on the size of the primary mirror: 1st screen: 90 percent, 2nd screen: 78 percent and 3rd screen: 55 percent. The size of the holes in a 10 inch telescope would then be: 9 in.; 7.8 in.; and 5.5 in.

The window screen sections are positioned so that their patterns are rotated by a successive offset of 30 degrees providing a relatively randomized blocking effect of the screen. All three screens are then sandwiched between two pieces of lightweight wood or cardboard and secured into place. The dimensions are not very critical, just adapt to your own scope. To use, simply place your mask into the front of your scope. My apodizing mask rests on top of the spider for my secondary.

Source: Improving Your Reflector Telescope Performance on Planets, Steve Waldee.

Servo Focuser

In the January, 1996 issue of Sky & Telescope, my picture showed up with a “servo focuser” gizmo that a member of my astronomy club (Ken Florentino) created. Since that time, several people have contacted me asking for details on how to get one working. A description follows…

Servo Focuser

In the January, 1996 issue of Sky & Telescope, my picture showed up with a “servo focuser” gizmo that a member of my astronomy club (Ken Florentino) created. Since that time, several people have contacted me asking for details on how to get one working. A description follows…

Description
In case you are not familiar with the servo focuser, it consists of a pair of standard model airplane/car servos connected together such that an observer can control focus of a telescope remotely, but without the common overshoot and binary ON/OFF limitations that most commercial focusing motors provide. Best of all, it works without any batteries! In operation, one servo is mounted to your focuser (the “slave”–I use an “O” ring as a drive belt to go from the output shaft of the servo to the focus knob) and the other is held in your hand. When you turn the control arm of the servo in your hand, the nylon gear train in the servo ends up spinning the small DC motor inside quite fast, so fast that it acts as a small generator. Enough voltage/current is produced to travel down the connecting wires to the other servo (on your focuser) and drives the DC motor in that unit. When the servo output arm turns, you get enough oomph to turn your focuser knob. VIOLA!

Servor Focuser Knob

The “feel” of the thing is quite normal… if you turn the servo arm fast, the focus knob turns fast. If you turn the servo arm slow, the focus knob turns slow. If you reverse direction, the focus knob turns the opposite direction. When you stop, the focus knob STOPS! In other words, it works just like you are turning your focus knob, but you don’t impart any vibration to that 450X view of one of the members of Stephens Quintet! Of course, there are a few limitations:

1) Due to friction and other losses, you don’t get a full 1:1 control; I get about 1.5:1 on mine;
2) If you spin the servo control arm too slow, you won’t generate enough power to turn the “slave” servo;
3) If you are trying to lift a 2 pound Nagler eyepiece straight up, you may have to get a little more creative with your belt drive and external gearing;
4) You have to perform a little minor surgery on the servos (described below).

On the good side, you get:
1) Analog, bi-directional focus control with an intuitive “feel”;
2) Freedom from batteries;
3) It’s cheap! I got my servos from a local hobby shop “junk box” for $2.50 each–any model airplane hobbyist will have a few laying around with missing connectors or bad electronics–this is fine!

Construction
If you already have two servos in front of you, here’s what you need to do to build a servo focuser:

1) Remove the cover that surrounds the electronics, DC motor, and gear train. Be careful to keep track of the layout of the nylon gears – you will need to restore them to their original orientation when you reassemble the servos.

2) Remove the small electronics PC board and discard it. You’re only interested in the motor and gear train. You may also need to remove a small potentiometer – more on that in step 4). Just cut the wires.

Servo Gears

Servo Motor

3) Solder two wires that will travel from the two electrical connections on one servo motor directly to the two connections on the other servo motor. You may find a small capacitor to control electrical noise already in place between these connectors. I left the capacitor in place on mine, although I doubt it will make much difference. Be sure to keep in mind any rubber grommet or exit hole in the servo case when you route this pair of wires. Also use enough wire to reach from your focuser to wherever you want the control servo to be–I used about 2 feet of wire.

4) This step is the only tricky part… Most servos have a small mechanical tab that is used to limit the travel of the output arm of the servo. This tab is usually located on one of the gears in the gear train. Since you want to be able to rotate the servo 360 degrees in either direction, the tab that restricts the rotation must be removed. Different servo brands use different designs, but generally you will be able to find a small tab that runs into a stop at about +- 30 degrees of rotation. Use a file or X-acto knife to trim this tab off. Be VERY careful here, the nylon gears can slip out of your fingers easily and expose your fingers to that razor sharp blade (ask me how I know!). For this reason, I suggest using a file to grind the tab down.

5) Reassemble the servos (you did all of this to both of them, right?). Be sure to get the gear train back into its original configuration. When you give one servo a spin, the other should respond. To make control easier, I attached a wooden handle to the control servo to allow easy handling with gloves on in cold weather.

6) Connect the “slave” servo to your focuser. You’re on your own on this one. For my telescope, I attached an aluminum plate below my focuser and the servo was attached to the plate with double-sided foam tape. Make sure the output shaft of the servo is parallel with the focus knob for some kind of O-ring drive belt.

Schematics

Key:
A–Wooden control handle attached to control wheel with small screws

B–Control wheel(s) – this is the standard nylon control arm that comes with a servo. The output belt on the slave servo is held captive on the slave servo by stacking two of these wheels

C–Servo output shaft

D–Gear Train section of the servo (this is where the rotation restriction tab will most likely be found)

E–Empty space where the discarded electronics module came from

F–DC motor with shaft entering the gear train section of the servo

G–Two wires connecting the two servo motors

H–Output shaft that is connected to the focus knob via an O-ring

As for a little background on the cassegrain telescope that my servo focuser was mount on, read on…

The telescope is a 8″ f/16 classical cassegrain. The primary mirror is a parabola (commercial SCTs use a spherical primary and a corrector plate) with an f/4 figure. The secondary is a 2 1/4 inch hyperbolic convex giving a 4x image, giving f/16 at the eyepiece. The optics were made by a company named “Cross” sometime in the late 70′s. I am the third owner of these optics. When I acquired it, the tube assembly included a fiberglass tube and the hardware was all manufactured by Parks. The version shown in the magazines (S&T and Amateur Astronomy) has the fiberglass tube replaced with a truss structure. The truss tubes are carbon fiber arrow shafts (an off the shelf archery product). The rest is just the usual micro-Dob design that should be familiar with most ATMs. If you are interested in a how-to article on constructing a cassegrain (especially the hyperbolic secondary), check out the book How to Make a Telescope by Texereau.

by Steve Bygren, steve2822@earthlink.net

In the spring of 1991, I decided to build my own poncet table for my 10 inch dobsonian. I had recently read a number of articles on poncet tables, and I figured I couldn’t live without one. I decided to build [...]]]> or When A Bargain Isn’t Necessarily A Bargain

by Steve Bygren, steve2822@earthlink.net

In the spring of 1991, I decided to build my own poncet table for my 10 inch dobsonian. I had recently read a number of articles on poncet tables, and I figured I couldn’t live without one. I decided to build my own because they are relatively straight forward to construct, and the commercial versions cost nearly three times what my telescope cost me to build.

For those of you not familiar with a poncet table, it is a way to get a large alt az telescope to track the sky in right ascension without having an equally large equatorial mount. Where a ‘normal’ equatorial mount will have a telescope connected to a shaft of some kind pointing toward the celestial pole, a poncet table uses a ‘phantom’ axis to perform the same function. From the drawings below, the advantages of a poncet should be apparent:

1) it is compact and lightweight;
2) any dobsonian style telescope can be placed on one; and most importantly to me
3) it can be built at home with only basic power tools.

The disadvantages of a poncet are
1) you can only track for short periods of time before ‘resetting’ the mechanism; and
2) precise photographic tracking is nearly impossible.

Since I was interested in tracking objects at high power with my ‘dob, but not astrophotography, a poncet seemed like a good project to take on.

See the following sketches to make sense of the formulas below.

Schematics

Refer to the following photo of the finished poncet platform.

Finished Poncet

My first obstacle was to come up with a way to produce smooth round bearings for the cone shaped rotating surface. It occurred to me that most lumber yards carry precut round disks in ‘standard’ diameters. All I needed to do was to come up with the proper geometry to take advantage of these sizes. After lots of sketching, CRC references, and head scratching, I arrived at the following formulas:

let:
R = radius of bearing surfaces (R(1)…large disk, R(2)…small disk)
a = angle to the celestial pole
phi = angle of disk cutouts (phi(1)Large disk, phi(2)Small disk)
L = distance between the large and small disk
C = chord of the disk cutouts (C(1)Large disk, C(2)Small disk)
h = height of the disk cutouts
S = circumference of an arc

initializing:
R(1) = 15 inches (the sizes available to me from a local lumber yard)
R(2) = 6 inches
a = 39 degrees (approx 0.68 radians – we are at approximately 39 deg north latitude)
C(1) = 14 inches (the base of my DOB is approx 14 X 14 inches)

find:
L = (R(1) – R(2)) / sin(a) = 14.3 inches (this value needs to be at least as long as C(1))
phi(1) = 2 * arcsin(C(1) / 2 * R(1)) = 0.97 radians (approx 55 degrees)
h = R(1) – (R(1) * cos(phi(1) / 2)) = 1.73 inches
phi(2) = 2 * arccos((R(2) – h) / R(2)) = 1.56 radians (approx 89 degrees)
C(2) = 2 * R(2) * sin(phi(2) / 2) = 8.43 inches
S(1) = R(1) * phi(1) = 14.55 inches

The goal of these formulas is to determine the depth and separation of the conical sections needed to give me nearly one hour of tracking for my DOB These formulas give me a platform that is about 14 X 14.3 inches, and using C and phi, I can now cut out sections of the precut disks that I purchased at the lumber yard. Since a 30 inch disk will need to rotate 15 degrees per hour, or 3.9 inches, to approximately track the motion of the stars, a bearing separation on the larger disk of about 10.6 inches (14.55 – 3.9) will give the desired hour of tracking. Try your own dimensions and see if it works out for you. For more information on this design, see the “Gleanings for ATMs” article in the September 1988 issue of Sky and Telescope.

With a little ingenuity, I was able to hand cut all the wood pieces and bolt/glue everything together to form the rotating platform of my poncet. The next step was to build a baseboard with the 5 bearing points attached at the correct distances and angles. Because this design uses a ‘virtual’ point of rotation instead of physically rotating about the point of a cone, a 5 point bearing support system was used. Since I expect to do most of my observing at or near 39 degrees of latitude, my bearing supports were attached at this angle. One of the benefits of the 5 point bearing system is that on the small disk, two bearings support from the edge of the disk, and the third supports against the side of the disk. This geometry allowed me to use a piece of 90 degree angle aluminum to attach the bearings. Once the bearings were completed, placing the rotation platform on the base resulted in a very smooth and free moving action. In order to protect the wood disks when the 30-50 pound telescope is riding on top, I glued a strip of 3/4 inch wide aluminum on the wood disk edges, making the motion even smoother. This completed the basic construction of the poncet table.  It also ends the genuinely”useful” information in this article.

What follows is the meandering path that lead to a fully functional platform…

The next step in this project was to come up with some kind of drive mechanism. This task led me through the most frustrating part of this project. The dilemma I was faced with was running my poncet on either a low DC voltage, or 110v AC. My initial efforts at driving my table were in an attempt to run on DC. I chose DC because you can easily get from 5 to 12 volts from either a car battery or cells, both of which are easily available whenever I go to Badger Flats. The choice of running DC introduces difficulties in locating a motor and gearing system that will produce the low rotational rates needed to drive the poncet. In a box of electrical odds and ends, I had a couple of DC motors that seemed likely to perform the job adequately and off I went. My first DC motor selection ran at about 8 rpm, and by locating some sophisticated drive train parts (O rings), I had the parts necessary to drive the rotating platform. Once I determined the final output speed of my drive system, I attached the whole mechanism between the base and rotating platform and gave it a try. It became obvious that the mechanism I constructed allowed far too much ‘play’ to drive the platform.

On to plan ‘B’: use a threaded rod/tangent drive common to many tracking platforms. This is the portion of my project that ended up costing the most cash. After hearing how well the machined threaded rod worked on other tables, I headed into the Yellow Pages to find a machinist that could produce the threaded rod sections that I needed to be able to run my table with a 1 rpm motor. Between of the high quality rod that the machinist recommended, and the labor fees he charged to do the work, I spent nearly $50 just for two pieces of 3/8 inch 24 tpi rod. (ouch!) Suddenly, I realized why the commercial operations selling poncet tables were charging so much for them. With that experience behind me, I scavenged through my other odds and ends boxes to produce the bearings, brackets, and springs needed to complete the tangent drive. Now I had to restart my search for a suitable DC motor to run this thing. It was about this time that it occurred to me to use a stepper motor with either a dedicated stepper controller or use my PC to control it. Since I had an article describing the construction of a stepper motor interface that used a PC printer port and a BASIC program to control a motor, I figured that it was only a matter of time before I would have a low voltage DC motor running at whatever speed I setup in the BASIC program. Well, to make a long story short, the PC, program, interface card, and stepper motor all worked as predicted, BUT, the torque was too low to reliably drive my tangent arm setup, not to mention that the batteries in my computer were being drained unusually fast by all the I/O to the printer port.

On to plan ‘C’: build or buy a power inverter and run a 110 VAC synchronous motor to drive the poncet. A 110 VAC 1rpm motor was easily found with a trip to a local surplus/scrap dealer (OEM Parts in Colorado Springs). Incidentally, nearly all of my projects are made up of parts from this unusual store. Back to my story…

I discovered a VERY old power inverter that used a noisy, vibrating mechanical relay and capacitor to generate 110 VAC from a car battery. It could only produce 10 watts, but after gingerly connecting it to my poncet and turning it on, to my surprise, IT WORKED! I put my DOB on the table, and it still worked! I could finally see light at the end of the tunnel. I even took the whole assembly out to out local observing site, Badger Flats, to give it a real test run. For nearly 2 hours, it seemed that I finally had a stable, reasonably accurate drive to allow me to crank up the power without having to constantly tug the scope around to keep an object within my field of view. I was eager to tell someone of my success, and strolled over to some of the other observers present to brag about my belated success. When I returned with some willing guinea pigs to test it out, I noticed the lack of noise near my setup. Silence…that OLD power inverter I was using fried itself and was now producing an odor that was only slightly less offensive than a skunk! Back to the drawing board.

Lets see, this must be plan ‘D’… I had a working system that only needed a reliable source of 110 VAC power. The decision to be made now was whether to buy a commercial inverter to generate the 110 VAC from the 12 VDC available from my car battery, or to build one based on yet another article I had been saving from an appendix of Michael Covington’s astrophotography book. Thinking that I could build one much cheaper than the $75 that a new one would cost, off to Radio Shack I went. After spending about half what a new inverter would cost, I had all the fixin’s to produce a variable frequency inverter that ought to do the job. I spent the next week soldering and drilling and taping everything and I finally had what I hope will be my last boondoggle on this project.

As of this writing, I have a functional poncet table that does a pretty good job of pulling my DOB around the sky, and although I spent a bit more that I expected to, and worked a lot more then I planned to, I have the poncet that I yearned for nearly 18 months ago. Would I do it over again if I knew then what I knew now? SURE I would. The drive system and motor were a bit of a hassle, but the use of precut wood disks made it all much easier than if I had tried to cut them by hand. Considering that a commercially build poncet platform can cost between $800 and $2400, my project was a bargain even with the dead ends that I followed.

Leonid Meteor Shower

What?…listening for meteors? Why not!

Got a short-wave radio? Or an AM/FM with a short-wave band you ignore most of the time?

Good. Tune the short-wave to a weak and distant station. Try to [...]]]>

Leonid Meteor Shower

What?…listening for meteors? Why not!

Got a short-wave radio? Or an AM/FM with a short-wave band you ignore most of the time?

Good. Tune the short-wave to a weak and distant station. Try to stay above 15 megahertz (MHz). Keep the volume real low. Now, when a meteor comes barreling into our atmosphere and becomes visible, usually between 80 and down to 25 miles or so, it will ionize the air along its flight path.

This very temporary “reflector” of radio waves should cause the volume of the station’s signal to rise sharply with even the possibility of Doppler change in pitch to occur. Give it a try and let me know of your success or ongoing trials.