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View Full Version here: : Powering Alt-Azimuth SCT Telescopes and the ‘Cord Wrap’ issue


PeterAnderson
07-07-2017, 04:25 PM
One of my telescopes, the 11 inch (28cm), is a Celestron CPC, an alt-azimuth unit with GPS and ‘go-to’. Like most modern telescopes it operates on 12 volt DC. In the field it can be powered by cable from a car lighter socket, or from a 12 volt battery. In the observatory I used AC power with a 12volt 2.5amp Celestron transformer.
My other recent telescopes (Celestrons C14 and C9.25) are mounted on German Equatorial mounts, and sweeping from one part of the sky to another does not involve much movement of the input power cable because the connection is near the fulcrum point of the axes of the mount. For these I use AC power and transformers and have had no problems. (Even better is my old C8 where the power inlet is in a fixed position on the underside of the base of the equatorial fork mount – so no movement occurs at all.)
However with the C11 CPC the altazimuth fork is set on a round turntable nearly 35cm in diameter with the power input connection in a spot near its outer edge. In order to track objects at all (as would be the case for all mounts of this type), the mount must be initialised by the identification of at least one object in the sky. Then other objects can only be successfully tracked after being located via a request through the telescope system. Put simply, being altazimuth and not equatorial, the telescope needs to know exactly where it is pointing to figure out which way the sky is going to move and then effect tracking. It can only do that if it locates the objects itself.
This brings me to the problem of ‘Cord wrap’. When you ask such a telescope to ‘go to’ an object, you must at all times have a fair amount of slack cable available and keep it well away from being fouled by the tripod legs. With a 35cm diameter plate a full rotation sweeps around 110cm so it is obvious that a metre or more spare cable would be necessary. The CPC operating system recognises the cable wrap problem in part and so will not go past a full 360 degrees azimuth rotation in an observing session to avoid ‘winding itself up’, but that doesn’t stop it from sometimes (for example) panning around 300 degrees of azimuth, when it might have gone just 30 degrees. All the while you are nervously paying out the cable. Occasionally you may miscalculate and the cable may get caught up, or, as happened to me, run out and wrench out a plug from the transformer. These little events do wonders in loosening the input plug and socket connection on the telescope mount and once loosened, you may lose power during another slew.
So what can you do? The first thing is to tighten up the input socket connection once this has happened. Generally easily done. Then, for me it was a matter of taking steps to ensure that the problem didn’t arise in the future. The moulded circular turntable supporting the fork is large and sturdy enough to hold a battery and with the position and even the battery I chose, it can still reach zenith easily. Combined with this I needed a short cable around 50cm in length for the input plug to comfortably reach the input socket. My unit is now completely self contained without the need for any external cables. I have since learned that the firm ‘Starizona’ have a very similar ‘piggy back’ battery system. The illustration they use to advertise it also shows a CPC telescope! However I came to this rather obvious concept independently and more cheaply.
Now for the detail. I decided to use a 12v 7amp battery, similar to home burglar alarm systems. (Cost $50). I chose this size because it would allow adequate power for a number of sessions if necessary. (Even with slewing motors operating the CPC should never exceed a usage of 1.5amps. (This power requirement comes from the CPC manual.) ‘Battery World Enoggera’ supplied the battery and also made up a cable with a small fuse and plugs for $7.50 – Yes $7.50! From Officeworks I purchased some large self adhesive Velcro strips for $11.98. (The Velcro is to ensure that the battery stays in place.) Everything works, and I can even recharge the battery without removing the connector plugs since my battery charger’s bulldog clips still make contact when clasping the battery terminals side on. Total cost? Just under $70.
As the battery is only attached by a Velcro strip it can simply be ‘peeled off’ if the telescope is being transported. The ‘cord wrap’ with its resultant problems are now a thing of the past for me. If you have a similar issue with some of your equipment, please consider this type of solution.
(Please note: Covers on unused ports on the mount - rubber adhesive pads - are to keep them clean.)

Allan_L
07-07-2017, 04:39 PM
Good solution Peter ! :thumbsup:
I lost count of the number of cordwrap disconnects and re-alignments I had with my previous CPC1100. No matter how carefully the battery was placed (on the ground), there was so often something to bring it unstuck :)

Now, having moved onto to a bigger Dobsonian light bucket, my solution is similar to yours, 18 amp hour battery with Velcro piggy backing on the mount.

Works for me. :thumbsup: