EQ Platform for Planetary Scope

Wanna make a scope? Or better still, grind a mirror yourself. Or, you have some good tips in making a really useful accessory? This is the place to show what your hands can do...
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geyes30
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Joined: Sun Jun 04, 2006 10:07 pm

EQ Platform for Planetary Scope

Post by geyes30 » Sun Sep 30, 2018 8:01 pm

My DIY planetary scope has fast become my favorite scope to use, not least because this summer boasted, on some nights, up to 4 easily visible planets (Venus, Jupiter, Saturn, Mars). Coupled with the DSCs, it was easy to find the planets at 133x, and bumping up to 240x when I felt like it. However, it has always bugged me that the scope had to be re-positioned frequently due to the non-motorized nature of the scope. I’ve thus hoped to work on an Equatorial Platform for it. Equatorial platforms have to be custom-built for every telescope, since the center-of-mass of the scope plus moving part of the platform ought to coincide with the center of the circles making up the North and South segments. Therefore, it’s taken me much longer to overcome the inertia, and finally get down to it. I ordered the plywood on September 14th, and started the build on the 20th, completing it on the 23rd. This is a pretty fast build owing to the simplicity of the design.

Design of Platform
For the planetary scope, I estimated the entire mass to be 13.5 kg, and the center of mass to lie about 34cm from the base of the scope. Adding an additional 6.5cm for the arc of the segments, and giving some allowance for heavier eyepieces, that brought the diameter of the segments to 84cm. I briefly considered cutting them with a router, but cutting such a large circle is hard for me, since I don’t have a large enough workbench. Therefore, reasoning that any inaccuracy will probably be insignificant, I resorted to the same trick as before, that is, pasting a print-out of a precisely sized arc, generated using SolidWorks, and cutting with a scroll saw. The end results were pretty good.

ImageSegments glued to new platform top by Cyrus Beh, on Flickr
The new platform features a much simpler cradle design, for ease of construction and improved stability.

For this build, I decided to work with a cradle-like design, with two equally-sized segments. This is to simplify the build, as well as to take advantage of the greater stability, which proves to be important for the much larger scope (at almost 2x mass), compared with the Ultraportable 10″.

The bottom of the platform proved a bit more challenging. I didn’t start off with a good plan for the bearing surfaces. I had wanted to use some roller bearings, but couldn’t come up with an easy way to mount them without excessive turning moment. In the end, I found these hollow rollers I had bought for the Portabowl, but which were never used. Instead of having these run over a 6mm rod, I simply put a 70mm M6 screw through them, supported on either end by a small wooden block. For the direct drive system, I used a rigid shaft coupler, which was fortuitously the same diameter as the rollers at 14mm. The coupler mates directly with one side of the North segment, which has a length of 120 grit sand paper glued onto it for friction. This worked quite well, but to minimize slippage, I put a heatshrink sleeve over the coupler, which added only 0.4mm to the diameter of the coupler.

ImageBearing surfaces before sanding. by Cyrus Beh, on Flickr
My planetary scope base board had three rubber feet.

ImageUntitled by Cyrus Beh, on Flickr
Feet at the bottom of the (unremovable) base board of the scope necessitated holes to be drilled in the platform.

To accommodate these as well as the center screw, I cut out three holes on the top of the platform. This created an easy way to seat the scope on the platform. Two days after the build first started, I had a more-or-less finished platform. What cannot be seen from the image below is that there are 4 M6 screws in T-Nuts that serve as adjustable leveling feet for the platform.

ImageEq for planetary scope - testing for clearance by Cyrus Beh, on Flickr
Holes in the platform allow the scope to be securely seated.

The NEMA17 stepper is controlled using the same controller as the scrapyard EQ platform, with adjustments made to the drive rate. Given the high magnification expected for this scope, aligning and calibrating the drive rates were expected to be critical for the platform’s performance.

ImagePlanetary scope on eq platform by Cyrus Beh, on Flickr
Scope sitting on the platform.

Testing, Tuning and Using
As it turned out, the platform started cutting off power every 1-2 mins or so, for periods of 5 seconds, during tracking for some reason. This resulted in drifts, as well as crazy vibrations (sometimes so violent, the scope moves a bit in the Alt-Az) when the power kicks back in. A bit of reading suggested that the culprit was the TMC2130 clone, which cuts off when overheated. A quick lookup indicate that the driver cuts off at 95 degC. Adjusting the current brought the running temperature down to 65 degC, while maintaining a usable amount of torque to move the big scope. In retrospect, I should probably try driving with a DC motor – much simpler wiring, and none of this overheating nonsense.

Using the platform turns out to be quite easy. Usually I’d just do a rough align, turn on SkySafari, align to one object, and start tracking. However, I decided to place pretty stringent requirements on the scope. SkySafari has the option for modeling Basic Encoders on EQ platforms, which basically assumes that the platform is perfectly aligned, and locks onto objects. However, in truth, the platforms are often slightly off-alignment. This means that with the platform, the object drifts over time in the eyepiece, but not in the Planetarium. Note that, without a platform, the software is able to create a good model of the alignment, and allow the objects to drift in the software as well.

So, the conundrum here is that, without the platform, the eyepiece and software match up well; with the platform, the objects remain within the eyepiece field of view (FOV) much longer, but the model mismatch can be quite large.

‘Drift’ Alignment
To solve this, one performs something like drift alignment, where a star or planet is centered in the eyepiece, and after some time, the position of the object is noted. We expect the object to stay centered, but if it were to move North- or South-ward we can then adjust the platform such that the object is closer to the center again. After a few iterations, the platform is pointed in the right direction. The Altitude measurement in this case is set using a spirit level or a level app on the phone, both of which seem to work well for my latitude (1 degree North).

Using these methods, I was able to keep a star within the eyepiece, drifting less than 0.1 degrees in 10 minutes. In practice, that translates to keeping an object within the view of a 5mm orthoscopic eyepiece for 10 minutes. This really is more than enough for the purposes of visual tracking! I’ve been observing the planets at 133x and 240x routinely, and marvel at the quality of the image.

Conclusion
One thing that continues to amaze me is how easily one can improve a Dobsonian, with the platform built in only 2 days, and made from plywood and motor costing just 20 bucks each.

The scope has undergone quite a number of improvements. Firstly, I had encoders installed on it. Then, I solved an astigmatism problem with the scope by replacing the secondary mirror with one that is better than 1/10 lambda. And now, with the EQ platform, the scope is nearly perfect for planetary viewing. The only issue I have now is that there are 3 things that need to be powered – the Encoders, the mirror cooling fan, and the platform. Cable management has not been a huge problem, but I need to figure out a better way to power everything with a single battery.

ImageCooling fan behind primary mirror. Scope is sitting on an eq platform by Cyrus Beh, on Flickr
All the things that need powering – Encoders, Fan, Platform.

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Airconvent
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Re: EQ Platform for Planetary Scope

Post by Airconvent » Sun Sep 30, 2018 10:23 pm

Wow...very nice effort and thanks for the detailed design and construction report!
Hope you are able to resolve the power issue. :)
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