How To...

Build an equa­to­rial plat­form, part two.

Sky at Night Magazine - - CONTENTS - With Mark Par­rish

In part two of our project, we show you how to com­plete and mo­torise the Dob­so­nian equa­to­rial plat­form. We have doc­u­mented our build with down­load­able pho­to­graphs in this month’s Bonus Con­tent so you can repli­cate the parts, but be­cause your choice of drive mo­tor and power sup­ply could dif­fer ac­cord­ing to cir­cum­stances and avail­abil­ity you may have to adapt the de­sign a lit­tle to suit your kit. Our spread­sheet cal­cu­la­tor (also in the Bonus Con­tent) once again comes to the res­cue when it comes to work­ing out gear ra­tios so all you need to do is in­put your val­ues and ex­per­i­ment with the num­bers un­til the ra­tios match your needs.

Com­mer­cial mo­tor drives for tele­scope mounts are avail­able for projects like this, but they can be ex­pen­sive. They use so­phis­ti­cated step­per mo­tors, which move by very small, pre­dictable in­cre­ments. Short pulses are sent to these mo­tors by a spe­cial con­troller to pre­cisely con­trol the mo­tion of the scope. One of these would be ideal but it is pos­si­ble to build a cheaper (al­beit slightly nois­ier) al­ter­na­tive. Our drive is based on a more straight­for­ward DC mo­tor. When the power is on, these mo­tors turn fairly con­stantly, but quickly. We used a home-made gear­box along with the gear­box built into the mo­tor, and an off-the-shelf con­troller to mod­ify the out­put speed to suit our needs.

Aim for side­real

The prin­ci­ple be­hind our mo­tor drive is sim­ple: the tele­scope’s plat­form should ro­tate very slowly at the same rate as the Earth is spin­ning – the side­real rate – but in the op­po­site di­rec­tion. In 24 hours this is only slightly more than one full rev­o­lu­tion (360°), so in one hour the plat­form should turn 360/24 = 15°. This is both a use­ful move­ment for

ob­serv­ing and a sen­si­ble amount for our de­sign to ac­com­mo­date.

Since we know the ra­dius of our north seg­ment (from Part 1) we can work out the length of a 15° sec­tion of the cir­cum­fer­ence. Af­ter mea­sur­ing the di­am­e­ter of our out­put gear, we can work out how many times per minute it must turn to cre­ate the re­quired move­ment. This is the nec­es­sary out­put speed for our gear­box sys­tem.

To pre­vent slip­ping, the out­put gear drives a toothed sec­tion of the north seg­ment. We cre­ated this ‘rack’ by mak­ing a mould from mod­el­ling clay (rolling the out­put gear along a strip of clay to cre­ate

the pro­file) then fill­ing the mould with glue from a hot glue gun. If you find this too fid­dly, you can buy toothed belts with match­ing small gears in­stead. Screw some stops onto the bear­ing strip to pre­vent the plat­form ro­tat­ing too far ei­ther way.

The DC mo­tor you need to buy should come with a built-in gear­box with a pub­lished speed of 10rpm or less. A con­troller can also slow this down, so our home­made gear­box will only need to re­duce the speed a lit­tle. A worm gear with a wheel (ours has 57 teeth) will do the job nicely and these can be bought on­line.

Grab your of­f­cuts

Of­f­cuts of alu­minium an­gle saved from Part 1 are great for build­ing the gear­box, al­though care­ful mea­sur­ing and some fine tun­ing may be re­quired be­fore you get it run­ning per­fectly. Once com­pleted, the gear­box is mounted on a piv­ot­ing arm. A rub­ber band pulls on this so the out­put gear presses against the rack and drives the plat­form. To re­set the plat­form, sim­ply dis­en­gage the piv­ot­ing arm.

The con­troller is con­nected be­tween the mo­tor and power sup­ply (we used a 12V pow­er­tank for our 12V mo­tor). You can fine-tune the speed dur­ing use by ob­serv­ing an ob­ject at the eyepiece and mak­ing ad­just­ments with the knob. This may also be­come nec­es­sary if the bat­tery level drops dur­ing the night.

To set up, align the base­board north­south and get it as level as pos­si­ble. Place the scope on top and switch the board on. You can fine-tune your align­ment us­ing a tech­nique in­cluded in the Bonus Con­tent.

Mark Par­rish is a con­sum­mate crafts­man. See more of his work at but­ton­de­

Prop­erly mo­torised, the fin­ished base will make your Dob­so­nian track at the side­real rate – keep­ing it aligned with the stars

The un­der­side of the base­board; here you can see the bear­ings, drive mo­tor and gear­box

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