USB over Cat5/6 cabling and general hints.

Forums Telescopes USB over Ethernet USB over Cat5/6 cabling and general hints.

William Bristow


I used to use medical rated versions of the Startech USB over Cat5 cabling extenders in medical imaging systems and have some experience with them.  

In some circumstances they do work well but the achilles heel is that all the data from your four devices at the client end flows through a single USB 2 port at the host end and is therefore limited to 480Mbs split four ways. These devices work well where the total data throughput from all the client devices never reaches that 480Mbs limit and that all the devices are roughly balanced in data throughput.

Problems can occur where a modern large sensor CCD/CMOS camera hogs all the available bandwidth for thirty seconds or more while downloading to the host and regular polling of the mount by your observatory software can not occur, in some cases the mount will “time out“ and require a reboot to reestablish USB communication.

This problem with the main camera taking all the available bandwidth has been addressed to some extent with the latest manufacturers offerings that include an onboard memory buffer in the camera that allows for a reduced data rate on the USB link however at this time I have only seen this with USB 3 CMOS cameras.

It is also the case that you will most likely need many more than just 4 USB ports in the observatory as you will find as you progress along the automation route.

In my own observatory (dome) I have (up to, depending on OTA configuration) 12 USB ports in use, main camera, guide camera, fiter wheel, AO unit, mount, focuser, rotator, dew controller, dome controller, cloud/rain/wind monitor, UPS supply for the PC and observatory power switch controller.

Depending how far along the remote control/automation path you plan to go if you begin adding in plate solving and refined mount pointing/closed loop slewing plus an all sky camera, well, a single USB over Cat5 cabling extender just won‘t be sufficient, you could be looking at two or even three separate extenders and at that price you might as well use a dedicated laptop or PC in the observatory controlled remotely by TeamViewer or Microsoft Remote Desktop from a separate laptop/tablet/PC in the house.

In the past my observatory was controlled by a dedicated desktop PC, that blew up due to damp, the ATX power supplies of conventional desktop PC’s are not rated for use in a condensing atmosphere and one dreary damp November the computers ATX power supply let go and damaged several connected devices. This was replaced with a laptop powered by a weatherproofed 19v supply. The laptop worked well until the temperature fell below freezing where the RAM memory clocks would drift and the dreaded ‘Blue Screen of Death’ would appear as Windows gave up. That problem was resolved by placing the laptop in a ventilated cabinet on top of low voltage pet warmer pad that kept the laptop above freezing. Eventually when that laptop came to the end of life it was replaced by an industrial fanless sealed PC with specially rated components for use below freezing, this works perfectly but at a high price. This type of industrial PC was a priority for my observatory because it is run fully autonomously for much of the year with no one in attendance to step in and deal with failures.

From the house to the observatory I ran three Cat5 cables, two in use and one spare, a multicore cable that is used for the alarm and halon gas fire supression system and an armoured power cable connected back to the main house fuse box. One of the Cat5 cables connects to a network switch in the observatory, the other end to the house router. The second Cat5 cable connects to a second separate switch and a pair of surveillance CCTV cameras, one inside the oberservatory the other outside, both connected back to the main house alarm system. Wireless networking is just not reliable enough at my location, it was not an issue with distance so much as too many other users nearby using the same wireless channels plus a requirement by my house insurer that only wired CCTV networking was used as many ne’er do well’s carry wireless network jammers while engaged on their activities. Networking over the powerline was considered but a line interactive UPS in the observatory does not pass the signal downstream of the UPS and severely attenuates the signal upstream.

All the low voltage signal cables from house to observatory run in a single 90mm diameter plastic pipe, buried a metre below ground, with a couple of spare pull-through draw wires in case I need to add or replace in the future. The armoured mains cable runs in the same trench but buried a further 450 mm below the pipe carrying the signal cables to prevent mains bourne interference and comply with local planning rules where minimum depth for a buried mains cable was stipulated at 1.2mtrs below ground level. The mains supply is configured as a TT system, that is, only live and neutral leave the house protected by a dedicated RCD in the main house fuse box and a two metre earth rod was driven into the ground right next to the observatory connected to a single earth bus bar installed inside the observatory. All metal work, including the steel pier, and observatory device power supply earths run back to that single point to avoid possible earth loops. The observatory computer is protected and maintained by a line interactive UPS, the dome shutter is battery operated and its supply is recharged by a solar panel on the dome roof. In case of loss of communication between shutter and dome controller or PC failure the shutter closes automatically. As well as a cloud, wind and rain monitor talking to the observatory computer and initiating a shutter closure command, a separate, independent rain sensor on the observatory roof triggers a shutter closure irrespective of the observatory computer.

In use, the observatory is controlled by ACP software that I can log in to from anywhere via TeamViewer but apart from giving it a list of targets ACP is in control of all the observatory functions including turning off/on frost protection heaters and the dehumidifier, observatory startup and shutdown, weather monitoring and target sequencing, switching on the flats panel etc. At the same time I can log into the house alarm and security system via a browser and watch the live video stream from my security cameras. When a planned target observation is complete, however many hours, days, weeks or months that may take, ACP sends the folder of images over the internet to my dropbox account as well as an email notification for me to post process wherever I might be.

Some astronomers are reporting successful control of small observatories using ’micro’ or ’stick’ computers run ‘headless’, that is without monitor keyboard or mouse, mounted up on the OTA with an additional 12v powered USB hub and 12v power distribution bus. Usually these micro stick PC’s only have a couple of USB ports plus either a cabled network port or wireless LAN and Bluetooth. As long as the main camera is assigned one of the available USB ports alone while everthing else goes via the second USB port and hub they do seem to work. The slightly larger mini PC‘s usually have four USB 2 sockets and are still light enough to be mounted alongside the OTA for short cable runs. Communication and control is only via TeamViewer, Remote Desk Top, or similar, from another computer but with the ability to run a full version of Windows and very short USB cables to everything else bar the dome/roof which would probably need an active extension from the USB hub up on the OTA to reach. Though not very fast and small memory storage I have read they will run observatory software including plate solving quite successfully with a minimum of cabling running up the mount. If you are not planning a fully autonomous observatory and are happy to attend to switching heating/ventilation and dehumidifying on off plus flats panels as necessary and don’t require a cloud/rain/wind sensor or weather station then these little ’Micro Stick’ or Mini PC’s may offer you all you want.

Do remember to check with your insurance company, I found it impossible to get insurance cover for the observatory without providing the connection into the house alarm and CCTV systems, when you look carefully at the wording of many household policies high value items used outside the home are not covered if you are actually away from home and out of sight of the items while general outbuilding cover is limited to a fraction of the cost of a typical observatory, I also discovered that in the case you are underinsured the insurance company can refuse to payout anything in the case of a claim!

Rather a long reply but hopefully of some use.