Equatorial Platform for my 10-inch GSO Dobsonian
Having recently purchased my dob (end October 2018). While waiting for it to arrive I decided amongst other things to build an EQ Platform for it to save some nudging at the eyepiece whether just me observing or at some observing event etc.
After searching the internet for a while and watching various videos of other people’s platforms I decided to build mine based around this web site:
http://www.reinervogel.net/index_e.html?/Plattform/plattform_VNS_e.html the vertical north sector (VNS) version in particular, as it formed a more compact unit at my 52° north latitude.
At this stage I hadn’t got a telescope to take physical measurements from so I relied on the details given on the TS website concerning sizes and weights, with a couple of emails to TS gave me other details not listed.
https://www.teleskop-express.de/shop/product_info.php/info/p1192_GSO-10-inch-f-5-Dobsonian-Telescope-Deluxe-Version.html
First. I needed to find a close approximation for the scopes centre of gravity. The only way I could do this was to take the physical sizes and weights for the base (rocker box) and the tube (OTA). These were given as 12.2 Kg and 15.5 Kg respectively. TS gave me 442mm as the spacing between the three feet under the ground-board.
The only way I could do this was to make some close approximations for the rocker box panel sizes, the circular ground-board and azimuth base were given as 540mm OD, the panel height at 640mm. Front panel I guessed at based on the size in the photo in the TS advert relative to the 305mm OTA diameter (having the actual scope to measure is so much easier!).
Armed with my known panel and base board sizes I calculated the surface area of the rocker box and given its assembled weight I found the weight per square cm (cm²) and multiplied this back to the individual components to give me their weights.
I used Excel to calculate a string of weight x distance sums, coarsely at first and then refining the distance steps until I found a balance between the rocker box and OTA distance/weights numbers and used that for my centre of gravity height.
This then allowed me to start using the information in the web site (above) to shape the vertical north segments for my 52° north latitude. The website gives templates for different latitudes around 50° north/south latitudes and a run time of about 1 hour. I was going for a run time of circa 45 mins to keep the segments shorter, so they didn’t stick outside the baseboard which in the dark I thought might be dangerous at ankle level. The segments are set at an angle (perpendicular to a line back to the pivot point).
My reading of the website calculations:
· From the intersection of the CoG line and latitude lines find the length of the perpendicular side of the triangle.
· Multiply this length by Cos (latitude + small angle below top plate) Total 65° in my case.
· This gives the half height of the ellipse
· The 1/Cos (angle formed by the roller supports) gives a modification to the ellipse shape to compensate for the angled segments
I used Photoshop to create a full-sized ellipse and the segments. I then printed off full scale, one of the segments on an A4 sheet and transferred that to the plywood.
I purchased some 18mm birch plywood (1 piece: 1220 x 610mm (4ft x 2ft)) which let me layout and cut the ground board and top plate plus the two north segments with lots left over to cut bearing mounting blocks, various supports for the VNS’s and the pivot block assembly.
After drawing out the components on the plywood I’ve used a jigsaw to cut out the main parts and glued the north segments together at a 25.2° angle so they form a single unit.
I found a small 12 volt DC geared motor and further 40:1 reduction gear on ebay to power the platform. The motor is 2 rpm giving me an output with the reduction worm gear of 1 rev in 20 minutes. Given the circumference of the elliptical north segments I had a drive roller of 9.91 mm diameter to give me 1 rev in 24 hrs. The motor speed is controlled via PWM so a 10mm shaft is fine which will sit in ball races at each end. There are two shafts – one driven the other an idler responding to the motion of the driven one. I will mount the ball races in plywood blocks initially. The motor/gear assembly will sit between the platforms top plate and baseboard.
Next stage will be making up the ball race and motor mounting blocks…