SOME FACTS ABOUT Equatorial tracking Platforms.
When you plan to build your E.P. on the Equator. You are on latitude 0 that means that both ends of the E.P. will have to be alike and the North and the South tracking arches are equal.
When you plan to build your E.P. on the North or South pole you are on latitude 90 that means that on both locations your platform would only have to turn around. Counter to the Earth's turning.
So for every degree in latitude there is the special requirement to build to that latitude. First they are often made with 2 bearing wheel on each arch.
The farther away we go from the Equator the larger the Polar side bearing-tracking arch will be compared to the Equatorial side arch, and yet we want to build the platform level.
In latitudes near the Equator we keep both arches below the platform, and use 4 bearing wheels.
Towards the pole,with 3 wheels.When we come to about 40 degree Latitude some people build the Equatorial-side pivot above the platform and farther North or South that is the most common bearing.
The arched polar-side bearings are often also different past 45 degree,where the vertical bearings VNS become more practical, and have a slightly different design since they are part of the vertical ellipse or hyperbola of the latitude cone instead of the altitude square line and are part of the narrowest sections of the vertical ellipse/hyperbola.
Closer to the Poles than 75 degree, few Equatorial Platforms are build.
Building an Equatorial Tracking Platform
1 Build it to the latitude where you will mainly use it,when traveling you can wedge it to the new latitude up to 5 degrees North or South (1 degree = 300 mile +/-)
2 Build it to minimum 15 degree swing, good for one hour observing,the total arch length should be min. 50 degree
3 With three adjustable vibration damping legs(important for photography)two legs on the Polar facing side, one on Equator facing side.
4 The Center Of Gravity (C.o.G) of the whole telescope is the center around which the E.P. should turn. C.o.G. is always lower than the altitude pivot point of the Dob. (OTA)
5 Keep the tracking wheels inside the base (ground) legs and min. 30 degree apart.
6 you can use an Equatorial-side pivot above the swinging table when latitude is higher than 45 degree. Or on a single wheel (or 2 wheels)
below the platform (-45Degree lat.)
7 Use a powered tracking wheel or two coaster wheels and a separate driving screw
8 There should be no free space (slop) in the Dob bearings, (Alt & Decl.)or in the driving causing movement when passing mid point of travel especial with photography.
9 Providing a speed adjustment possibility in tracking is necessary for
photography, (deep Space- planetary speed)
10 The drive system should have a emergency slip capacity, a (neutral for resetting) and a positive cut-off switch at the drive end.
11 Mount a level on the platform ground-board, three adjustable (screw)
legs, long enough for leveling and for adjustment of latitude, a sight
for aiming at Polaris and a magnetic compass
When finished make it all dew resistant
DESIGN FOR YOUR OWN EQUATORIAL PLATFORM
The first thing you have to do is, find out where the Center of Gravity (CoG) of the whole telescope is, that is the OTA, complete, the rockerbox and the ground board,(if you are planning to place the existing ground-board on the platform) and the tracking platform board.
The simple way is: find long enough 2 x 12 or ladder, lay it flat on the ground on an angle iron, a pipe or…. as balance-point, next lay the complete scope on it so it stays in balance (without spreading the scope parts apart or moving the balanced board)
Mark on your 4 ft long straight-edge the distance from the bottom of the tracking platform board to the balance point and you have the CoG named "A" See schetch(no need to remember numbers)
Next make a full size drawing like the schetch on a 4x4 panel (plywood or cardboard)Draw a horizontal pencil-line about1/3 from the near edge across this represents the bottom of the platform board, about 1/3 from one side draw a vertical line from the first line to the distance of CoG."A" above it, draw a line "C" reprsenting your latitude from point "A" to the base platform line, another line "D" Altitude from point "A" at 90 degree to the other direction on the platform line.
Draw a circle (the size of the Ebony Black bearing on the bottom of the
rockerbox)= "B" centered on COG line and horizontal line.
Mark one bearing point at the Equator-side of the bearing circle and two more 120 deg. apart, inside the "B" circle for the future bearing-points, mark out the form of the board around the circle (it should be minimum as wide as the circle and 1/3 longer from North to South).
Measure the distance from the CoG to the point where the polar line "D" intersects with the platform line. This is the minimum Polar edge of the new platform board.
Mark the length of line "D" along the platform line from the intersection
point, from that mark make a compass curve line named "E" beginning at one
polar corner of the board to the other. This is the curve line for the Polar bearing.
Knowing the size of the arch, Cut the top-side first to the “D” altitude angle with
the face side the narrowest.
Afterwards cut the arch very accurate with a router.
The curve can be banded with a metal or laminate strip.
On the Equatorial end you can mount a "D" angled support post (with pivot
bolt in line with latitude angle-line “C”) from top of the tracking-board
to intersect square with the polar line "C" which will form the equatorial
pivot for the 3point support.
This is recommended when your location is Polarly past the 45 degree
latitude. If less than 45 degree I recommend to build a 3 point support
below the EQ platform board (a curved board and 1 Equator side bearing)
The arched bearing board should be laid out in the same manner as the Polar-
side arch but will be much shorter since the latitude line "D" will be
shorter and with only one bearing moving only 1-3 inch. +/-.
When using tilted support arches without the rail place the support
bearing wheels in line with the arch and 8 degree from the end of the arch, the "back-up" wheels lined up with the curve of the arch and placed 1 inch
closer to the center, then mount travel stops on outer ends of arch.
An alternative tracking which I use on the polar-arch is to mount a “RAIL” of 6mm
(1/4”) round aluminum on the lower curved edge and clamped on each end and the center, the bearing wheels are Sliding glass door metal rail wheels with ball bearings, since they run on a rail you need only one wheel on each side, the equatorial side can be either a single bearing wheel on the arch with a backup wheel for side pressure or a pivot post.
Vertical mounting becomes more desirable past 45 Degree latitude but is not needed even when using an Equatorial pivot post
The wheels which I used before where from inline skates (8 wheels from 2 skates from Salvation- Army) you can use only the ball- bearings, this lowers the platform 0.75 to 1”)
To build the VNS (Vertical North Section)
Lay out the curved Polar bearing arch "D" as per plan, with the altitude angle, to change that angle to the vertical we must lengthen the radius of arch “E”.
From the pivot point of Altitude “D” line, go up vertical to meet with line “C” the latitude Angle; the length of this vertical line “VC” is the radius of the VNS.
Then divide it in three equal sections, the center Section is not used, the top-sides of these sections have to be square.
Locate the two sections on the standard locations and center-mark each one, twist each section so that they are square to the radius line from the pivot point on the equatorial-side.
Mount a bolt in line with the latitude line and a BAL bearing on top.
The two bearing wheels on the ground board need to line up with the arch sections center, and since the wheels roll around on a curved pad, the afore mentioned “rail” can work only if the “rail” is curved to the horizontal radius as wel as the arch curve, which I found very hard to line both sections in the proper curve. And since you will likely have the Equatorial pivot installed the rail is not needed. The curved paraboloidal radius of VNS is longer than the circular radius, laid out
as described above depending on latitude and CoG
Following is a step-by-step direction layout for a VNS1 Use a 4 by 4 ft card-board
2 Mark on the right lower side the outline of your planned E.P, min 24” long and 20” wide mark the center of the future EP. 20” from the right side of the board edge and 12” from bottom edge.
3 In center of EP place a horizontal line from board edge to board edge.
4 From EP center of the line make a vertical line the height of the CoG.
5 A line of your local latitude angle from CoG to the left to the centerline.
6 Another line square from the latitude line at CoG down to the centerline at right
7 The pivot point at the intersection of the centerline and the CoG line is the center of the Ebony Black circle on the bottom of the
rocker-box or the center of your telescope.
8 Extend line 5 to the extended left EP board edge line.
9 Make an arch from point 8 intersection pivoting on left EP board edge center-line down to the centerline extension on the right (the pivot point of the Hyper-bolic arch section.
10 Draw an arch curve from one left EP board corner to the other; this is the tracking arch curve.
11 You will need the two outside parts of this curve each minimum 20 degree
12 Mount the sections on the left edge of the EP board with center 10 degree from corner and perpendicular to the radius point on the centerline.
13 Equatorial pivot point is where line 6 meets the centerline.
14 The ground-board leg on the Equatorial side is outside of the pivot point.
15 Polar-side legs close to the polar side EP board corners
16 The rail sections can be mounted on the VNS following the tracking curve.
I Whish you a good time building the VNS tracking platform
Peter De Baan
PS. I find the best way to cut the curved tracking-board is with an
extended (lexan) ¼” router base long enough 4 ft for the longest radius and
cutting in layers each about ¼ “deep, using a powerfull router with a 1/4" updraft carbide tipped bit
You do not need: a computer, a calculator or even a ruler when following
these directions.
You will need a pencil, a 4 ft. straight edge (drill holes in it for pencil-
circle scribing), eraser, full size layout board, saw, drill, screw-driver,
screws, glue, hammer, plane, sander, router, eye & ear protection and clamps.
Have a clear sky and warm feet. Peter De Baan
You can use Digital Setting Circles with the platform using the tracking-board as the DSC ground board. Line up your Equatorial Platform to Polaris while the platform is on the starting end of the arch as done without the EP. Then when you have your aim set, start tracking and when you want to change the aim to another location return to the starting point and you do not have to realign.
alignment.pdf
