In a continued effort to get the Periodic Error down lower (now just at spec, but can do better), I next decided to tackle the mass of dangling cables. These are a snag risk and I imagine that, even if not snagged, the fact that they are dangling and swinging back and forth is probably affecting PE – especially during times when a dangling cable is rubbing against some stationary part.
I did not want to route the cables internally through the mount – I did that some time ago and believe that internal rubbing contributed to the worsened PE at that time. Instead, I want to make maximum use of the mount’s instrument panel and existing internal wiring to eliminate cables or shorten them.
There were 3 classes of cable to deal with
- Power cables (to the QSI camera and the Pyxis rotator);
- USB cables (to the QSI camera, the guider camera, and the rotator); and
- The special telephone-wire cable that feeds both power and control signals to the focuser.
1. Power Cables
The mount instrument panel provides 3.5mm power jacks supplying 12V and 5V. The 12V provides enough current for the Pyxis rotator, but not enough for the QSI camera. So I made a short jumper cable to power the rotator off the instrument panel’s 12V jack.
To get power to the QSI camera, I decided to use the mount’s “Aux Power In” and “Aux Power Out” jacks on the control and instrument panels. As described in the manual, these jacks do not have any power supplied – they are simply a 4-conductor cable route through the mount. If you want power to come out at the instrument panel, you must send power in to the jack on the control panel.
You can buy pre-made cables from Bisque that have the appropriate jumper jacks and plugs to do this for a variety of devices. However, it’s very expensive, especially for a Canadian. $169 for the cable and $95 for UPS shipping, for a total of US $264, or about $375 Canadian.
Instead, I made an appropriate cable set. This is not hard if you are comfortable doing fine soldering, and I certainly recommend it. I already had wire, bought the appropriate mount connectors from Mouser Electronics, and bought male and female 3.5mm power jacks from a local electronics shop. A couple of hours work and a cost of about $30.
2. USB Cables
I had 3 USB cables to deal with:
- USB to the QSI camera (data only);
- USB to the Pyxis rotator (data only); and
- USB, data and power, to the LodeStar guide camera.
The mount’s Instrument Panel provides two USB ports that are hubbed, internally, to the same USB cable that controls the mount. I needed a third, so I mounted a tiny USB hub on the side of the instrument panel with velcro. I ran power to this hub from the unused 5V power jack on the instrument panel, so it would have the power available to handle USB-powered devices.
Then I ran short USB cables as follows:
- USB jack on instrument panel to QSI camera (avoiding the hub, because this pathway is carrying imaging signals and needs to be clean and stable);
- 2nd USB jack on instrument panel to the input of the mini USB hub;
- USB hub to Pyxis rotator;
- USB hub to Lodestar guide camera. (This cable also powers the guide camera, which is why the powered USB hub.)
3. Focuser Cable
This specialty cable is very long, so I just routed it up the body of the mount, bending at the mount’s pivot points, and leaving some slack where needed.
Here is the result – no dangling cables.
I re-ran a Periodic Error test, thinking things might get very slightly better.
Nope. They got worse. I’m now at 7.7 arcseconds peak-to-peak, which is out of spec for this mount.
I’m running out of ideas. Before I go to the online forum for advice (and might end up replacing the RA drive gear), I thought I’d try one more thing. I noted in the recent tests a “grumbling” sound coming from the RA gear box that I am pretty sure was not there before. Something in the drive not running smoothly.
When I cleaned and regreased the gears a month ago I used MolyKote 33 grease, because it’s rated for lower temperatures. I’m now second-guessing that decision. The MolyKote is really thin and light and my winter temperatures aren’t that low. Meanwhile, I’m wondering if it is too thin for the very hot temperatures we’ve been having this summer.
So, I disassembled the gear boxes again, and replaced the grease with Lubriplate 105, which is the specific grease recommended by Bisque for the mount.
I also changed the adjustment on the mount’s spring plungers – which set the tension holding the belt-driven worms to the worm gear. There are two Bisque documents that contain information on setting these plungers, and they are inconsistent on one matter; so I tried setting them according to the other document, the one I hadn’t been following.
It’s due to be cloudy for a few days, but next clear night I’ll test again and see where we stand.
Meanwhile, I was a little discouraged by this, so I thought I’d see how well guided imaging would do at this point. Not that I’m going to use guiding to make up for out-of-spec PE, but just for fun.
That’s pretty nice. It’s a single 300-second exposure, guided. No flats, but darks and bias frames used. Looking closely, especially near the edges, the stars are not perfectly round, but I suspect that’s optical (collimation) not guiding error. Nice to know I could image if I needed to; but I’m going to keep working on the raw mount performance first.