EQ Platform for 10-inch Dobsonian

[fwm891]

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…

 

 

 

[Stu]

Excellent project! I had always planned to try to build one but knew that is it would take me forever so bought one in the end. I shall watch with interest though

 

[Alan White]

Look forward to seeing the build come along, clearly you have fine woodworking skills Francis.
I however do not, I just butcher it.
So as Stu says above, watch with interest.

[westmarch]

Francis,

looks like the work of a craftsman. Good luck and as my old gaffer used to say, “measure twice, cut once”.

John

[fwm891]

Currently the segments are stuck to the top plate with double sided tape while I measure up heights, angles etc for the drive and idle shafts. I will also add a couple of safety blocks at the end of travel to prevent accidental overbalancing in either direction. These will probably incorporate cut-off switches to prevent the drive running but going nowhere (I have days like that!). I also need to disengage the drive shaft to re-set the platform - probably use a lift roller to accomplish that rather than a clutch... More thought required.

[westmarch]

Francis,

One of the things I love about this hobby is the never ending list of problems to be overcome:  it makes success all the sweeter.

I would not be without my EQ platform now.    I must admit that my build took the simplest route wherever possible but I admire your adherence to first principles.

John

 

[fwm891]

My thoughts at the moment on the drive shaft and motor. Mounted it with the worm vertical so I can rotate the drive shaft around the worm shaft to fine tune the shaft contact with the curved segments. The motor come below the ground board so I will need a waterproof cover to protect the electrics (not shown)

Edited November 16, 2018 by fwm891
text added

[fwm891]

Tried the motor and PWM controller to confirm the speed range. On max it didn't quite get to the 2 rpm stated (though driving direct from the battery it did!) so I turned the controller back to it's mid point and made a few measurements. I now get at this setting 1 rev in 70 seconds. Given that I'm running through a 40:1 I can increase the drive shaft diameter to compensate for the slower revs. Drive shaft now 23.14mm OD which is good as it will allow better (shallower) contact between the drive shaft and the segment. Pleased I didn't assume the PWM controller would give me full speed.

[fwm891]

Set-up a temporary pivot and edges to simulate the drive and idle shafts so I can see the extent of the platform tilt in either direction. Initially I wanted a 45 min travel but I have enough for 60 mins plus. The level is set at ±8° the pencil marks show there is a cushion either side I can set switches on to cut power should it be left un attended. The aperture for the motor has been cut allowing some wiggle room. The holes to take the baseboard's feet have also been bored. Waiting on some metal rod now to make the two shafts and 2mm sheet to make the motor mounting plate.

TBC...

[fwm891]

Sorting out the electrics today while waiting for the metal to arrive. I'll run this off a small 7Ah 12VDC battery - need a couple of push-to-break switches. There will also be an on/off switch and a (dim) red led. Not sure whether to put the led in the hand speed control or at the power entry...

Make some short legs to go under the baseboard to bring the eyepiece height up a bit. Then sort out a seat, then...

[Alan White]

and then.... made me smile Francis, made me smile ? and today I needed a smile

[fwm891]

Aluminium rod arrived today so I've got the two main rollers machined up. The brass stub shaft's need finishing to length but I wanted to have a dry fit first. The motor drops into the recess and will have a fixing plate to hold it in position and fine adjustments.

Few photos:

 

[fwm891]

Just had to try it out....

 

[fwm891]

Electrics now in place and tested. I've timed it over measured distances on the sectors - waiting for some clear sky now to try it in anger.

The push-to-break switch is very sensitive and stops everything as soon as pressure is applied. Two blocks come together to set the beginning of travel point. Still debating the pivot point, whether to stay with the ball bearing or switch to something more positive. The flashing led is quite dim so shouldn't cause any headaches.

 

[fwm891]

It cleared for a while and things started well @ circa x180 (14mm + x2 barlow & 1250 mm fl) I started making pot adjustments, turning drive off then back on, increasing speed to recentre stars, adjust down to track ? - then there was this distinct 'POP' from the PWM controller ? speed output went to max regardless of the pot's position. Mmmm back to *bay for another controller or something better?

TBC...

[Stub Mandrel]

2 hours ago, fwm891 said:

 

Kudos! I was proud of my 3D printed holder to avoid reverse connecting a 12V battery but that is great bush engineering ?

[fwm891]

Just run some calcs on the controller - it's giving me x1.3 the correct PWM pulse width required for sidereal motion whether set to max or min potentiometer settings... Kaput I think. It was a cheapie of the bay. Just ordered something more substantial.

TBC.

[fwm891]

New controller arrived today and installed. Works fine, from mega slow to see it rotating! ?

Soon as I could see a couple of stars I set the platform up, rotated to Vega being the brightest to get a initial speed indication. Let it drift then pull it back in with the controller. Adjust the controller to give me a tracking speed. Kept Mars in the centre of fov for a good while at  x180 - strated to rain so further test will need to wait... ?

[westmarch]

Congratulations!  ?

[Alan White]

I was wondering how this build was going Francis,

the trigger was driving through Kingstone earlier today on my way to visit relatives.

Looking great, what great wood, but then I would expect that to be the case having seen pictures of you guitars.

[fwm891]

Thanks ALL.

Out again briefly, again rain stopped play but at least I was able to confirm the variable drive rates through an eyepiece. When (and if!) there's a longer spell of clear stuff I'll pop my Lodestar in and capture a short animation. Biggest problem I think is going to be getting a good north when I put the platform down. Might try and make a little jig to take the right-angle finder, or a wooden tripod to fit the rocker-box foot holes with a finder on top...

Life would be so much simpler if I didn't keep thinking up things to do!

[Chriske]

Hi,

Made a platform myself, and what I did was knurled the driving axis. It improves grip on the platform itself.
 

Chris

[fwm891]

Managed to get a short run with the Lodestar installed last night (need to look at SharpCap settings!), anyway sequence of 50 frames shot with a gap of 4 secs between frames.

I've stacked the first and last frames in Photoshop CS3 with an image of M1 to approx. same scale (not my image). The arrow shows the drift in 4 mins 7 sec (from timestamp). Drift is mainly due to poor north alignment. I made no adjustments later when viewing M1 at x62 (1250 fl with 20mm ep) for 5 mins.

So need to sort the north alignment and fine tune the tracking speed (Oh and remember to re-set the platform when it ends a run... ?)

[Chefgage]

On 15/11/2018 at 11:36, Stu said:

Excellent project! I had always planned to try to build one but knew that is it would take me forever so bought one in the end. I shall watch with interest though

 

I know this post was from last year but can i ask where you bought your platform from. I do fancy making my own but i have that many other things on the go i do not know when i would find time

[Stu]

15 minutes ago, Chefgage said:

I know this post was from last year but can i ask where you bought your platform from. I do fancy making my own but i have that many other things on the go i do not know when i would find time

I bought mine used, but it is this type. It looks like David Lukehurst may have taken these over as I think the original owner retired.

http://www.dobsonians.co.uk/Equatorial Platforms.html

Sumerian also do one that looks good

https://www.sumerianoptics.com/eq-platform

TS stock these as I don't think Sumerian sell direct any more.

https://www.teleskop-express.de/shop/advanced_search_result.php?keywords=Eq+platform&suchdas=OK