A Sidereal Pointer

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HOW TO BUILD A

SIDEREAL POINTERGiorgio Carboni, June 1996, updated in February 2004

Translation edited by Robert May

TABLE OF CONTENTS

Introduction

The Astronomical Coordinate System

The first Pointer Design

Construction of the BaseConstruction of the Pointing Device

Orienting the first Pointer

How to use the first Pointer

The Second Pointer Design

Building the Second Pointer

Setting the Second Pointer

Orienting the Second Pointer

Orienting the R.A. Shaft

Orienting the R.A. Scale

Using the Second PointerImprovements

How to make the Scales

Pointing the Telescope

Electronic Controls

Observations

Internet Sites

Bibliography

Figure 1 - The Sidereal Pointer is an instrument that allowsyouto locate heavenly objects by means of their owncoordinates.

INTRODUCTION

In this article, we deal about the construction of a Sidereal Pointer (Indicator?). An instrument that allows you to localizecelestial objects in the nightly sky, just knowing their coordinates. What can be the use of this instrument? Firstly, it canhelp you to learn to know the constellations. It can help you also to locate in the sky the position where point yourtelescope to observe an object invisible at naked eye, like a nebula or one of the heavenly bodies called "MessierObjects".

Since television, newspapers and often astronomy magazines give the positions of heavenly objects only imprecisely andthe position of the celestial objects are precisely defined on the basis of the astronomical coordinate system and sincethere exists astronomy books which contain these values, why not build an instrument which can help us locate everybody in the sky? Thus, let us build a simple tool which help your walking amongst the stars without getting lost. It willguide you in the heavenly observations with the naked eye, with binoculars and with any optical instrument that don't

have an equatorial mount, such as most of the homemade telescopes fail to have. This instrument will also be very usefulfor learning to recognize the constellations and locate the celestial objects not visible to the naked eye, to track comets ofwhich astronomers give you the coordinates, etc. With this instrument, you will also be able to track on a star map thepath of planets and comets. It will also be useful to know in which direction the center of our Galaxy is, and to find outthe other major celestial points. This knowledge will help you to locate where you are and to obtain a new and moreaware relationship with the sky.


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This instrument, which was once known as a Torquetum, was first described by the ancient Greek scientistPtolemy. Subsequently it seems this first model was perfected or reinvented by an Arab astronomer in the XI or XIIcentury. Then, it has been used by several European astronomers since the XIII century. We will describe two modelsadapted to modern and quite simple enough construction.

THE ASTRONOMICAL COORDINATE SYSTEM

In certain respects, the astronomical coordinatesystem (figure 2) is similar to the Earth's coordinatesystem. Like it, it is formed by meridians andparallels. The two systems have in common the polaraxis. In fact, the apparent rotation of the celestialvault is due to the rotation of the Earth itself. TheEarth's axis points approximately toward the PoleStar, hence the celestial vault spins around thatsame star. By the defined virtue of the coincidenceof the terrestrial axis with the celestial one, theequatorial plane of the Earth and sky alsoapproximately coincide. Like the terrestrial ones, thecelestial latitude goes from 0 (at the equator) to+90 (North celestial Pole) and -90 (South celestialPole).

What then are the differences? Mainly, they are asfollows: the earthly meridians are "integral" to theterrestrial surface, while celestial ones are "integral"to the starry vault. In this way, as in the terrestrialsystem a city has always the same coordinates, whilein the heavenly system a star has always the samecoordinates . These coordinates are called longitudeand latitude in the terrestrial system, RightAscension (R.A.) and Declination (D) respectivelyin the astronomical ones that are used byastronomers.




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It is widely known that the coordinates of these celestial objects may change a bit over time. In fact a city, by "floating"with the Earth's crust, due to the convective motions of the mantle that are below, may vary its own position. In a

similar way the stars, due to their own motion, and due to the change of the inclination of Earth's axis, which tracks acircle in the sky in 26,000 years of time, change their location too. For us, who are not professional astronomers, thesechanges are so little in size as to be negligible. Another difference among these two systems of coordinates is that wecount the terrestrial meridians in degrees, while the celestial ones are counted in hours. So we have 24 mainastronomical meridians, each of which divided in minutes and seconds. The Declination is, instead, measured in degrees,like the latitudes of the earth.

By convention, the origin of the astronomical coordinate system has been placed at the intersection between the eclipticplane (the plane of the terrestrial orbit around the Sun) and the plane of the heavenly equator that occurs in the springequinox. This "0" meridian of the astronomical system passes by this intersection at that time.

THE FIRST POINTER DESIGN

We will see how to build two Sidereal Pointers in the following sections. The first is a simple instrument, made of

cardboard, wood and plastic (figure 4), while the second is built of more substantial metal, thus more precise and lasting, but also more difficult to fabricate. Before we start, this article was originally written in Italian so all of themeasurements are done in the Metric system. As such, equivalent inch dimensional stock can be used without anyproblems.




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Figure 4 - The first Sidereal Pointer.

A Sidereal Pointer (figure 4) is made by 2 disks that are held 90 relative to each other. The first one carries the RightAscension scale, and the second the Declination scale. In order for the instrument to work correctly, the R.A. disk has tobe parallel to the equatorial plane. In this way, its axis is parallel to the Earth's axis of rotation and, as such, it's axispoints toward the Pole Star (figure 5). The Declination disk has to be orthogonal to that of the R.A.. In this way, itsdivisions will correspond to the celestial parallels. The R.A. disk is also called the hour diskand the Declination one theangular disk.

You can build Pointers with techniques and materials different from those if you so desire. In any case, the scales of thePointer must be oriented according the astronomical meridians and parallels. If you understand this concept, theconstruction of this instrument will be much easier for you.

The fact that the Pointer is not placed in the center of the Earth, but on its surface, could generate parallax errors withnear objects however, due to the enormous distances of the asters from the Earth, this problem really does not exist.

Now let's see how to build the first Pointer. Figure 4, shows the basic design of the first Pointer. Its scales are in heavy

paper (cardboard or cardstock), the plane of the scale of the Right Ascension is in plastic, the base of the instrument andthe support of the scale of the Declination are in wood. It could be simpler to make the whole Pointer out of cardboard,but in a short time its structure would inevitably be deformed by its own weight. In order to allow the instrument to lastlonger and to be more precise, I make some parts with stiffer materials, like wood and plastic. Let's start theconstruction of the Pointer with it's base.




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CONSTRUCTION OF THE BASE

As the figures 6 and 7-A show, thebase is formed by the pedestal, theplane for the R.A. scale and it'ssupporting leg. For the pedestal, usea board in wood or in chipboard. Thisboard should have a certain weight inorder to give a good stability to theinstrument. Apply 4 felt feet under

the board corners.

A suitable material to make thesupporting plane for the R.A. scale isa 5 mm thick plastic plate. Fasten itat its base by means of two hinges,so its slope can be adjusted. On thelower side, again using a hinge,fasten the leg. The length of this legchanges as a function of the latitudewhere you will tend to use thePointer. Give this leg a suitablelength, then, by means of a littleplastic plate, stop it in such aposition that the corner between the

R.A. plane and the pedestal is equalto 90 minus the latitude of theobservation point (figure 7-A). Witha square check this value. On thisscope, you can also use another copyof the Declination scale to indicatethe angle of the R.A. plate with thescale suitably supported so that youcan read the angle. At this point, theR.A. plane will be close to parallelwith the equatorial plane, and its axiswill be close to parallel to the Earth'saxis.

On the piece on which the R.A. scale

will be put, trace a orthogonal (time)line to the base of the plane andwhich passes on the center ofrotation of the scale as shown infigure 7-B, (red line). I made thisline with red color for clarity in thedrawing. As the background is black,make this line with white color(figure8).

Figure 6 - Base of the Pointer. Notice the hinges, the leg, the stop for theleg and the screw on which the R.A. scale rotates.




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I designed this base for the temperate latitudes ofthe northern (boreal) hemisphere, where most ofinhabitants of our planet live. For our friends inthe south, the absolute angle will be the same andyou will be pointing to the southern pole. In anycase, the length of the leg has to be fitted to thelatitude of the observer. If you desire, you canmake the leg adjustable if you normally changelatitude a lot. For the artic and equatorial regions,more radical adaptations to the base of the Pointerwill be necessary to point the Pointer in the rightdirection. In order to orient the instrument in thesouthern hemisphere with circumpolarconstellations, you will have to refer to differentconstellations than those I indicated to you. The

equatorial constellations will work well for those ofyou in the equatorial regions.

CONSTRUCTION OF THE

POINTING DEVICE

Let's define thepointing device as all the partsthat are placed upon the plane of the R.A.. Byusing little wood boards, build the support of thescale of the Declination as shown by the figure 7-A.

The main components of this instrument are the

scales. Due to the difficulties of drawing them, Isupply them already made for you; I made thesedrawings with a program for mechanical designand I saved them as a ".gif" image. To giveprecision and clarity to the marks, I gave thesedrawings a high definition. For this reason, youwill see them rather big on the monitor, but do not




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worry because they can be printed in the needed size. The same is valid for the indexes drawings.

Downloading the drawings.Click on: "scales of the 1st Pointer design" to open the drawing. Then save it on your hard disk or on a diskette. Make thesame for the drawing of the

Then save it on your hard disk or on a diskette by pressing on the right button of the mouse (a suitable little window willbe opened). Make the same for the drawing of the "sight".

Resizing the scales.It is necessary to adjust the print parameters in order to obtain the scales in the right sizes. With a suitable software forimage editing, open the drawing of the scales and resize it as follows:

- height = 2204 pixels;- height = 19 cm or = 7,471 inches;- width: = it is automatically resized (keep the ratio);- resolution = 116 px/cm or = 295 px/inch.

Do not save the drawing when you have resized it or give it another name if you wish to save the image! You do notwant to overwrite the original file.

Printing the scalesBefore you print the scales, deactivate the function that fits the image to the sheet. Insert in the printer a sheet ofheavy paper or cardstock. Chose the horizontal direction of printing and print. Check that the size of the scales youobtained are right. If there are some little changes in the size, there will be no problem. The important is you processalso the drawing with the sight with the same procedure, so to obtain prints with the same scale. If you do not succeedto obtain prints with the right sizes, change the program you have for image editing.

Figure 8 - R.A. index. You can do it also in metal or in plastic.

Resizing and printing the sightsThe resolution of this drawing is a half of that of the scales, so it is necessary to again resize it as follows:

- height = 1102 pixels;- height = 19 cm or = 7,471 inch;- width = it is automatically resized;- resolution = 58 px/cm or = 148 px/inch.

Put two A4 sheets of heavy paper or cardstock, as you did with the scales, in the printer. Chose the horizontal direction ofprinting. Print two copies of the sights. Check that the size you have obtained is the right one.

Cutting and mounting the scales

- Cut the figures, following the external borders;- with a 4 mm hand punch, make the holes;- glue the small R.A. index to its support (figure 8);- mount the R.A. scale on its plane;- mount the Declination scale and clamp it moderately by its screw;- adjust the orientation of the Declination scale so that the line which goes from -90 to +90 is parallel to the R.A. plane;- glue the Declination scale to its wooden support;- do not glue the R.A. scale to its plane, but let it free to rotate;- glue together two copies of the sight, by keeping its upper part open, as shown in figure 4. The little "V" channel thatemerges will help you to locate heavenly objects and will make the sight more rigid as a pointing device. I called thislittle channel the "Pointer.

ORIENTING THE FIRST POINTER

The Pointer is now basically finished, but before you use it, it is necessary to orient it according to the astronomicalcoordinate system. The operations I will describe in the following have to be made by night and with a good visibility ofthe starry sky. You have to bring with you the Pointer, a clock, a calculator, a flashlight with a red filter, the maps of the




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constellations of this article, the coordinates of the celestial objects that you want to look for. The flashlight is necessaryto read the coordinates on the maps, and to bring some light on the scales. When I will say to observe in the inversedirection, it merely means that you have to see in the opposite direction of the arrow of the sight.

To orient the instrument and the Declination scale:

- put the instrument on a table;- with a level, check the level of the table and if necessary, level it;- align the instrument in the north-south direction, so that R.A. axis points to the Pole Star and the sight looks towardsouth;- set the R.A. Index on the reference line on the R.A. plane (figure 7, red line);- set the Declination index in the -90 position;

- looking in the inverse direction, rotate the base of the instrument so that the sight points to the Pole Star;- if it is necessary, adjust also the slope of the R.A. plane.

At this point, the scale of the Declination should be oriented. From now on, do not move the base any more, but limityourself to rotating the pointing device on the scales. From now on you use the instrument looking in the normaldirection (in the arrow direction).

Orienting the R.A. scale to the heavenly meridians:

- chose a suitable star among the constellations near the equator as I indicated in table 1 and in figure 16;- by rotating the pointing device on the scales (do not move the base of the instrument!), point the sight to the star youhave taken as a reference;- now, by keeping the R.A. Index still, rotate the scale of the R.A. until the value of the Right Ascension of this star is incorrespondence with the index. With a hairpin or a piece of double sided adhesive tape, stop the R.A. scale. Do not movethis scale during the night of observations;

- verify that the instrument points again to that star.

At this point, also the R.A. scale is oriented to the astronomical coordinate system.

HOW TO USE THE FIRST POINTER

The instrument is now ready to be used. It is simple to use: just bring the two indexes to the values of the coordinatesof the of celestial body, and the Pointer will show where it is placed in the sky. While you pass from one star to another,the celestial vault will continue to rotate and your Pointer will quickly lose the reference to the heavenly meridians. Don'tworry! Imagine you oriented the instrument at 22 o'clock, if at 22 hours and 15 minutes, you want to locate an object,subtract from the R.A. of the object the 15 minutes that elapsed from the orientation of the R.A. scale or:

R.A.'= R.A. - td

where tdis the time difference between when you set the R.A. dial and the present time.

Let's do an example. If after 22 minutes of time since you oriented the Pointer, you want locate Cygnus (gamma), a

star of the constellation of the Swan, you have to subtract 22' to its R.A and bring the index of this scale to this newvalue. The Declination isn't modified with time so it's value isn't changed.

At this point, I wish you to have a good time with your Sidereal Pointer! In the paragraph: "Observations", you will findsome information to try with the instrument and to do some first observations. In the bibliography, I indicate anastronomy text that gives the coordinates of thousands of stars and other heavenly objects, as well as a lot of interestinginformation about them. This book can act as your guide during your observations, just as if you would to have anastronomer at your side.

Due to the design of the pointer, you will not be able to point to the stars located between the zenith and some degreesnorthward. The second Pointer has been designed so to avoid this problem and it is for this reason it has to be mountedon a tripod. To also avoid interference between the pointing device and the R.A. plane, the scale of the Declination andthe sight have been brought on the side. However, you can still see a wide portion of sky to be observed with this firstdesign of the Pointer.

According to the following article, with only a little change it would be possible to use this instrument with the three setsof astronomical coordinates: horizon (alt-azimuthal), equatorial, and ecliptic. The equatorial one is the one I have chosenfor this project.

http://www.humboldt.edu/~rap1/EarlySciInstSite/Instruments/Torquetum/Turq.html

THE SECOND POINTER DESIGN

The second Sidereal Pointer (figures 9, 10 and 11) is madeup of two shafts which carry the Right Ascension and the

Declination disks. As we saw in the first design, these twodisks must be at 90 apart. On the Declination disk, thereis fixed a small plate with two sharpened screws that serveas the pointer. On each disk is glued a graduated scalethat allows you to orient the Pointer. Then there are 2index pointers, one for each scale, and there are also someclamps to clamp the disks still while operating the




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instrument. The whole instrument is held by a fork, that isdesigned to be mounted on a photographic tripod.

Notice that in this second model, atop the R.A. disk, andcoaxial with it, is a transparent disk which has thereference line for the R.A.. We call it Index diskand on itare attached the supports for the Declination shaft.

Figure 9 - The second Sidereal Pointer Design.




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BUILDING THE SECOND POINTERNow let's see how to build this instrument. As before with the previous design, you are allowed to do all the changes youwish to the design. During this description, we will take into account a part of the design at a time.




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Figure 12 - Second Sidereal Pointer. This image shows some changesto the instrument compared to the figures 9, 10 and 11. In particular,

a Declination brake been added of the on the right support and aDeclination index pointer (on the left support) has been made

with a Plexiglas plate bent to 90 instead of a sheetmetal pointer.

Figure 13 - The Right Ascension disk and fork. Notice the la-teral cuts on the fork to make the bottom clamp more flexible.

Here are the main parts of the Second Pointer Design.

1 - FORK (figure 13)Its main function is to hold the R.A. shaft. The holes through which this shaft passes are made elastic by means ofsawcut slot. The two lateral holes and their cuts have been made so as to make clamp more flexible. The clamp servesto hold the shaft when necessary. The screw in the upper part of the fork serves only to reduce the movement of theshaft in the bushing. You can fabricate the fork by cutting a piece of "C" shaped rolled steel, or bending a steel bar (6 x25 x 250 mm) to the shape. In the back of the fork, you have to tap a 1/4 W (1/4"x20) threaded hole in order to mountthe instrument on a tripod.

2 - R.A. SHAFT. (figure 13)This is made with a round ground steel rod 10 mm in diameter. You can usually find this in good hardware stores. At thebottom of the shaft, you have to attach a little disk or collar to prevent the shaft from slipping off. On the upper part of

the shaft, you will have to make a little flat (this is wise anytime that you have to clamp anything onto a shaft so that theitem can be later removed without worrying about the setscrew scoring holding the part on the shaft) on which the setscrew of the flange will grip the shaft without burring up the shaft. This shaft revolves on two flexible bushings in thefork.

3 - BUSHINGS (figure 13)The two main holes made on the fork have been done with a drill. In order to make the hole smoother for the rotation ofthe shaft, a bushing is inserted in them. To be flexible, these bushings have to be slotted. You need 4 bushings, two ofthem serve for the Declination shaft and two for the R.A. shaft. You can buy the bushings in a bearing shop. You wantOillite bushings and you have to cut them along the length of the bushing. You may also get a longer bushing and cut itin two for two bushings if desired.

4 - HOUR STOP PLATE (figure 13)This little plate is screwed onto the fork and the R.A. disk sets on it. This plate has to be parallel to the R.A. disk, so, if itis necessary, file the part until you obtain the necessary parallelism between the R.A. disk and the fork. On this little

plate, you will mount the R.A. disk brake.

5 - THE RIGHT ASCENSION DISK (figure 13)This disk is made of plastic stock 4 mm thick. Its external diameter is 140 mm and it has a hole of 10 mm on its centerin order to let the shaft pass through it. Notice that this disk has to be free to move on the shaft, and it can only bestopped from rotating by the R.A. brake. On this disk is glued the R.A. scale which can be made with paper. To glue thescale, you may use a clear furniture varnish which you can apply with a brush, or a spray can. You may use it also to




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cover the scale with a thin, protective transparent layer. When gluing the scale to the disk, take a rubber or plastic rollerand push the bubbles out from under the scale. The color of the R.A. and Declination disks should be flat black, so as toreduce the reflection of light from them.

6 - THE R.A. DISK BRAKE (figure 13)This brake is mounted on the plate 4. It is shown in figure 11, in the partial view B, this brake is a clamp which pincheson the edge of the R.A. disk.The threaded rod has to be attached to the clamping knob. The handle is threaded and the threaded rod has to betightened into it. You may also want to upset the threads with a pair of sidecutters so that the threaded rod won't comeout of the knob while in use. You can also use Locktite to hold the threaded rod in place in the knob as an alternative.

Figure 14 - The R.A. index as seen from below. A flange and the twoDeclination axis elastic bearing supports are attached to this disk.

7 - R.A. INDEX DISK (figure 14)The Index disk has to be made with a 4 mm thick transparent plastic plate (Plexiglas), and with an external diameter of136 mm (or 4 mm less than the R.A. disk so that the R.A. clamp can hold onto the R.A. disk). This disk also has a centerhole and bushing through which passes the R.A. shaft. The Index disk has to be attached to the R.A. shaft (figure 11). Ablack plastic flange links the disk to the shaft. On the bottom side of the disk, it is necessary to scribe a thin radialgroove. This scribed line needs to be orthogonal to the Declination shaft and filled with black Indian Ink or paint. Inaddition, the distance around the disk in either direction needs to be exactly the same or the pointer that this line is willnot repeat when the R.A. shaft is turned 180 around. In this way, you will have a reference line that will work as an

indexfor the R.A. scale. The Index disk also has the important function of supporting the Declination shaft. To do this,you have to use two elastic supports to support the shaft and provide the necessary resistance to easy movement, one ofwhich should have an adjustment knob on it.

8 - THE R.A. FLANGE (figure 14)The flange is in black plastic and its external diameter is 30 mm. It has the function of linking the R.A. shaft to theindex disk. Three flat headed screws hold the flange on the Index disk and a set screw attaches it onto the R.A.




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shaft. To insure that this set screw holds without scoring the shaft there needs to be a flat upon which the setscrew setsfiled onto the shaft.

9 - DECLINATION SHAFT SUPPORTS (figure 14)The main function of these supports is to hold the Declination shaft. You can make them from an aluminum bar with asection of 12 x 25 mm. The hole through which the shaft passes is made elastic with a saw cut. To make the rotation ofthe shaft smoother a bushing is inserted in each hole as you did with the R.A. shaft support. The supports are fastenedto the disk of the Index by means of two flathead screws (figure 11, partial section A). They have to be located in such aposition that the shaft is at 90 to the index line of the R.A. (it may be best to carve this line after having fastened thesupports onto the disk). One of these supports also holds the Declination index pointer, the other has a clamp which canrestrain the shaft.

10 - DECLINATION SHAFT (figure 14)The Declination shaft is made with a ground steel bar 10 mm in diameter. At the left end, it connects with theDeclination disk by means of a flange. At the right end, there is a handle which also has the function of a counterweight.

11 - DECLINATION FLANGE (figure 14)It is like that of the R.A. flange and has similar functions. It is connected to the Declination disk and to its shaft bymeans of screws.

12 - DECLINATION SHAFT HANDLEThis is made up of a steel tube with a thin bar through it. Its function is to help in moving the Declination disk. Itsweight also used to balance the instrument, keeping the Declination side from falling downward.

13 - DECLINATION BRAKE (figure 12)A knob has been mounted on the right support. With it you can tighten the elastic Declination shaft support to slow downor stop the rotation of the Declination disk.

14 - SPACER RINGS (figures 10 and 11)These rings are used in several places to stop the axial movement of the two shafts. They have the shape of bushings.You also should put flats on the shaft where the setscrews are to keep from galling the shaft itself.

15 - DECLINATION DISK (figures 11 and 12)This disk is in plastic 4 mm thick. It has adiameter of 120 mm and a hole in its center inorder to center it on the shaft. The color of thedisks should be black. A scale is glued on thisdisk as we described for the R.A. disk.

16 - DECLINATION INDEX (figures 12 and14)This index serves as a reference for theDeclination scale. It can be made from analuminum strip 2 mm thick and suitably bent(figure 9), or better yet, from a Plexiglas platehot-bent by an angle of 90. On this plate, youhave to mark a reference line.

17 - SIGHTING PLATE (figures 10, 11 and 14)You can make this from a plate of black plastic 4mm thick, 30 mm wide and 200 mm long. Mounttwo screws on the extremities of this plate. Thetop of these screws should be sharpened. The linewhich connect these two screws has to be parallelto the 0 line of the Declination scale.

18 - SCALESTo get the drawings of the scales, click here:Scales for the 2nd Pointer, then save the imageon your HD. To save and print the scales, followthe same procedures we described for the firstPointer.

19 - A TABLE SUPPORTThe Pointer has been designed to be mounted ona tripod although you can also build a support toput the instrument on a table. Keep in mind thatthe table will prevent you from pointing tocelestial objects near to the Zenith.

Most of the mechanical parts can be made with

ordinary tools. However, some of them will bebest made with a lathe. To get them made, ask amachinist friend to make them. The pieces whichhave to be made with the lathe are theDeclination, R.A. and Index disks, the rings andthe counterweight. The internal and the externaldiameters of these pieces have to be machined so




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the amount of work is minimal so the cost ofthem should be low. Even then, accurate workwith hand tools, a clever approach to the workand appropriate stock, you can do the work byhand.

SETTING THE SECOND POINTER

When you are finished with building the Pointerand before you use it, you have to align it. Figure15 explains how to do this operation.

ORIENTING THE SECOND POINTER

It is night and the starry sky is well visible. You are in a dark place, without lights around you. You have brought withyou the Pointer, the tripod, the flashlight with a red filter, the constellation maps and the coordinates of the celestial

objects that you want to find. The flashlight is necessary to read the coordinates on the maps and to bring some lightonto the scales and screwtips of the pointing system. The reflection of the flashlight onto the screwtips will help you topoint the instrument toward the sky.

ORIENTING THE R.A. SHAFT




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In order that your Pointer is able to point properly, it is necessary to reference it to the astronomical coordinatesystem. The first thing you have to do is to align the R.A. shaft towards the Pole Star. After setting the Pointer, if youadjust the Declination at 90, the line passing through the pointing screws is parallel to the main shaft. Then you point ittowards the Pole Star, you have to:

- put the Declination disk at 90 and to stop it from moving;- by moving the tripod head, point to the Pole Star;- lock the head of the tripod;- verify that the Declination is still 90;- release the Declination disk.

After you have done this, don't move the tripod or the main axis of the Pointer. Now, at this time, the pointer is pointing

to Polaris and the plane of the R.A. disk is parallel to the terrestrial and celestial equatorial planes and the Declination isoriented properly. You just have to refer the R.A. scale to the celestial meridians. At this moment you are already ableto observe where the heavenly equator lies in the sky. To do this, bring the Declination scale to 0, and rotate thepointing system around the main shaft.

ORIENTING THE RIGHT ASCENSION SCALE

Now you must align the Right Ascension scale to the celestial vault. To do this, choose a star you can recognize and youknow the coordinates, then point the instrument toward it and rotate the disk of the R.A. until you bring the value of theRight Ascension of that star under the Index of the instrument. In the following table and in the figure 16, I indicatedsome constellations you may recognize with ease. They are placed close the celestial equator, so that the error you willmake while orienting the R.A. scale will be minimal.

Table 1 - A set of ReferenceConstellations

to orient the R.A scale.

Constellation Period

Orion November-March

Lion January-June

Eagle June-October

Pegasus August-December

To orient the R.A., scale you have to:

- according to the season, choose the suitable constellation in table 1;- look for it in the sky;- chose a known star of that constellation;- move the pointer of the instrument to point the instrument to the star you have chosen;- tighten the clamp of the Declination disk;- with the clamp on the bottom of the fork, stop the Index disk;- rotate the scale of the Right Ascension scale until the value of R.A. of the star is under the Index;- with the R.A. clamp, stop the R.A. disk (during your observations, this scale must always be keep this position);- read the hour on your watch, or better start a stopwatch;- unlock the Index disk;- unlock the Declination disk and use the instrument.




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At this point, both scales are oriented according to the astronomical coordinate system. If you don't succeed in findingthe constellations I indicated to you, you can also align to Ursa Major and the Cassiopeia which are very easy to recognize(figure 17) although you must point to those stars more accurately than stars near the equatorial region. Adjust the R.A.scale on the basis of one star of these constellations. As these stars have a Declination high enough that this adjustmentwill be afflicted by a certain amount of error. The orientation you obtain will be precise enough to use the Pointer tolocate one of the star of the equatorial constellations I indicated in the figure 16 and then you will be able to perfect theorientation of the R.A.. scale.

When they are low on the horizon, the circumpolar constellations can be hidden by haze. As Ursa Major and theCassiopeia are on the opposite of each other to the Pole Star, if one is low, the other is high, so at least one of theseconstellations should always be visible in a decent evening for astronomical observations in the boreal hemisphere.




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USING THE SECOND POINTER

Now the instrument is ready to indicate to you any celestial body of which you know the coordinates. Its use isextremely easy: just rotate the scales to the coordinates of the celestial body, and the Pointer will show where it islocated in the sky. To see the screws in the dark, you may use an flashlight with a red filter, holding it about 1 meteraway. The reflection of this light on the tips of the screws will appear like stars, so you will have two shining points thatwill guide you in the sky.

This instrument has an error of some tenths of degree. It is interesting to note that Hipparchus of Nicea, astronomer ofthe Hellenistic age, about 2.100 years ago was the first man to determine the position of the stars in the sky, and madea catalogue with the coordinates of 850 stars. This job let him discover important things, such as the precession of theequinoxes. Once you have found your target, you can look at the celestial body with the naked eyes, or a binoculars ortelescope.

While you are doing all these maneuvers, the celestial vault keeps rotating at the speed of about one degree every 4minutes. So the Pointer quickly loses its reference to the celestial meridians. Never mind! Here how you cancompensate foe this difference. If you set the instrument at 10 p.m. and at 10.15 you would like point a star, you justhave to subtract from the Right Ascension of the star the 15 minutes elapsed since the orientation of the Pointer:

R.A.' = R.A. - et (where et = elapsed time since the orientation of the pointer)

and don't tell me that it is difficult!There is another little error to consider: the sidereal day is 3' 56" shorter than the solar one, and so if you use the Pointerall night long, at dawn you will have an error of one minute and a half in the Right Ascension. If you multiply this 3' and56" for the 365 days of the year, you obtain another day which is the one that the Earth "loses" with an entire rotationaround the Sun. In other words: relative to the Sun, the Earth make about 365,25 rotations in a year, whereas to thestars the Earth make one rotation more. So, after 6 hours, we have made an error of one minute in the R.A. (this also isto be subtracted in the formula above).




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IMPROVEMENTS

HOW TO MAKE THE SCALES

The scales are the most important part of this instrument, but they are also the most demanding part to be made. Inthe following paragraphs, we will see some techniques which can be used.

Printing on cardboardThis technique is suited for the first Pointer. You have simply to print the scales on a cardboard 0.2 or 0.3 mm thick. Thisoperation will be done with a computer and a printer. I already described this technique in the chapter on the

construction of the first Pointer. To protect the scale of the R.A., you can put it on a disk cut from a transparent plasticsheet. This disk, made integral with the support of the Declination and provided with a black line, can also do thefunction of the Index disk.

Printing on paperThis technique is suited for the second Pointer. You have to follow the same procedure of printing on cardboard, with thedifference that, lacking of stiffness, the scale on paper has to be glued on a rigid support. For the support, it is better toavoid using wood because it has a tendency to warp. Plastic or metal are more suitable. To glue the scales, you can useglues, transparent varnish or such. There are also double faced adhesive sheets which can be cold-applied and also oneswhich can be hot-applied. These latter have the advantage you can position them without problems, then pass a hot ironon them to cause the scale adhere to the support. The scales on paper should be covered with a protective varnish suchas a transparent furniture varnish or transparent varnish for cars. It is also possible for these materials to tend todissolve the marks of the scale. To avoid it, look for another varnish or, before you apply it, fix the drawing with asuitable spray for pencil drawings. These varnishes can be used also as a glue.

Printing on white adhesive plastic sheetIn a stationery store, you can find special plastic sheets used to decorate T-shirts. With a ink-jet printer, you can printthe scale on one of these plastic sheets. Cut the drawing and apply it on a metal or plastic disk by means of a hot iron.

Printing on a transparent plastic sheetWith a computer and a laser printer, you can print on plastic sheets used for transparencies (those which are used inexecutive meetings). Use the transparent sheets for laser printers as they are special sheets which are able to resist tothe high temperature of the oven which fuses the toner to the paper, without becoming deformed. Once you have cut it,the scale can be glued on a rigid support, for example in plastic or in metal. The color of the support has to be white toallow you to see the divisions of the scale. To allow the division to last longer, you can print the sheet in a reversedirection and glue the printing in contact with the support. This technique is simple and effective. If you use thismethod to obtain the R.A. scale of the first Pointer, do not glue it to the supporting plane.

Mechanical ScribingThe laboratories which produce plates use special machines to make scales. Some are basically mechanical dividers,

other are pantographs, or use lasers and are controlled through computers. As support, special plastics for scribing(formica, phenolic resins, brass, aluminum, stainless steel and other metals) are used. Some metals can also beanodized. The cost of these tools are out of reach of an amateur so, to use them, ask a laboratory which works on platesand bring with you a diskette with the drawings of the scales.

Transfer of the toner of laser printers.There is a method to make printed circuit boards which avoids using ultraviolet light and acids and which could be alsoused to obtain scales. This system is based on the transfer the toner obtained with a laser printer and for this reason it iscalled: "laser printer toner transfer". It consists of doing a print by means of a laser printer and, with a hot iron (withoutusing steam), transfer the toner (and so the drawing) on another part, for example a plate of aluminum or stainlesssteel. To make the transfer easier, the receiving surface has to be well cleaned. The different parameters of thisprocedure, such as the temperature of the iron, the pressure of the iron on the paper, the time of the pressing, etc. haveto be experimented with to obtain the best process. There are also transfer papers that are suitable to this process andwhich easily release the toner, once you put them in water. I tried this method without obtaining satisfactory results,anyway, with the right paper and experimenting with the parameters, it is possible to make good scales. The method is

cheap, so you can try it if you desire.

http://www.pcbpaper.com/detail.htmlhttp://www.5bears.com/pcb.htmhttp://www.qsl.net/k5lxp/projects/PCBFab/PCBFab.htmlInternet Keywords: laser printer toner transfer pcb iron

POINTING THE TELESCOPE

The pointing devices of the Pointers I described are quite somewhat uncomfortable and inaccurate. This is not only dueto the systems I suggested, but also to the Light Pollution which, raising the luminosity of the background of the nightsky, which reduces the visibility of the stars. A little telescope with a magnification of 1 - 2 X would be a goodsolution. This telescope should have a wide field of view and an illuminated reticle with adjustable brightness to assist inviewing the sky.

ELECTRONIC CONTROL

If you have enough knowledge in electronics, you might provide your instrument with a remote control with which, bytyping in the coordinates of a heavenly body, the Pointer would indicate it by itself.




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By providing the R.A. shaft with a suitable constant rotation, the Pointer would keep itself oriented to the celestialmeridians all during the night.

OBSERVATIONS

This little instrument can be very useful for many astronomic observations, even with the naked eye. Figures 16, 17 and18 give you the coordinates of some important celestial points and interesting objects to observe. Among them, there isthe famous Andromeda Galaxy, a nebula of such a bright magnitude that you can observe it with the naked eye orbinoculars, while you can observe the other ones only with instruments that have a larger aperture. However you canscarcely perceive Andromeda because of its low surface brightness. In any case, its light travelled for 2 million years

before getting to your eyes! Other objects that can be observed with low aperture instruments are clusters of stars, suchas the Pleiades. We give you the coordinates of central star of the elegant constellation of Cygnus as an example of howyou can use this instrument for recognizing constellations.

You may also want to obtain an astrolabe. It is a map of the constellations with a rotating elliptical window on it. Withan astrolabe, you can tell which part of the sky is visible at the time you are doing your observations. With the Pointer,you can locate single heavenly objects and also constellations, but with the astrolabe you will be able to better see theposition of each constellation to the others. Together, these two instruments will be a great help in your discoveries ofthe night sky.

An interesting point to find is the center of our Galaxy: R.A. = 17h 42' 30" D = -28 59' 18

Another interesting point is the famousAries Point, also called Gamma Point. It is the zero point of the astronomicalcoordinate system and so it has these coordinates: R.A. = 0 and D = 0. This point is placed at the vernal intersection ofthe equatorial plane with the ecliptic one (the plane on which lies the orbit of the Earth around the Sun). It also marks

the moment of the spring equinox.

By pointing the main shaft of the instrument towards the point with coordinates R.A. = 18h 00' D = 66 34', for someminutes the ascension plane will be parallel to the ecliptic one. On this plane are placed most of the planets, and theconstellations of the Zodiac one after the other in a majestic ring-a-ring-a-roses. As they are placed at 30 one from theother, after localizing the first one, all the others are quite easy to find with the pointer. Furthermore, on the eclipticplane also lies the apparent motion of the Sun on the celestial sphere. When the Moon is moving on the ecliptic, itsshadow may hit the Earth, and so who ends up under it sees the Sun darken in an eclipse. You could try to use thePointer to forecast eclipses.

The coordinates of the constellations of the figures 16 and 17 and the coordinates you can find in figure 18, are enough




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to set the pointer and do some first observations. In astronomy text books, like the one indicated in bibliography whichis cheap and very well done, you can find maps of constellations and the coordinates of a lot of interesting celestialobjects. With this book, the astrolabe and the Sidereal Pointer, you will be able to find all the constellations you want,and at least learn to recognize them. In addition, this book gives you a lot of interesting information that will satisfyyour curiosity and your desire to learn something more about astronomy.

As the planets move relative to the stars, they do not have fixed coordinates that can be referred to but rather paths thatare marked with special tables. To find the planets, the so-called ephemeris almanacs, published yearly, are useful andyou can find them in any book-shop. Those almanacs also report the position of little planets and some closecomets. Remember that, if the instrument is pointing towards the Earth this does not mean that there is somethingwrong but rather it is right, and the object is in that direction and not in the night sky!

In its simplicity, the Pointer is really useful: it is a valuable guide to astronomy, and by constructing it you have done aninteresting exercise of mechanics and physics in building an equatorial mount. Furthermore, you have learned how thecelestial coordinates system is organized and you have found in which direction the center of our Galaxy is. One of thebest qualities of astronomy it is not so much to show far objects, as is to drive us to reflect about ourselves, ourcondition, the sense of life and the whole.

INTERNET SITES

On the Internet there are a lot of websites which can supply you the coordinates of heavenly objects and many otheruseful information on astronomy.

http://www.astronomical.org/constellations/obs.html Maps of the constellations and, for each of them, the coordinatesof the stars and other celestial objects

http://www.absoluteastronomy.com/ Coordinates and other information on heavenly objects.http://www.fourmilab.to/yoursky/ Your Sky, an interactive planetarium.http://www.geocities.com/m_s_pettersen/index.html Build an astrolabe.http://my.execpc.com/~tgrunewa/astro/astro_links.html Links on astronomy.http://www.seds.org/billa/psc/hist1.html Important Astronomers, their Instruments and Discoverieshttp://www.humboldt.edu/~rap1/EarlySciInstSite/Instruments/Torquetum/Turq.html The Torquetumhttp://www.21stcenturysciencetech.com/articles/fall01/Tanawa/tanawa.html Building a Torquetum

Internet Keywords: torquetum, celestial objects, heavenly objects, astronomical coordinate system, star atlas, astronomylinks.

BIBLIOGRAPHY

Patrick Moore, The Guinness Book of Astronomy, Guinness Publishing,1988

This book supplies the maps of the constellations, the coordinates of a lot of stars, variable stars, double stars, starclusters, nebulae, galaxies, etc. You will find also a lot of other information on heavenly objects and on the constellations,which will be able to satisfy your curiosity of knowledge in astronomy.

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A Sidereal Pointer

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