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

1. Power cables (to the QSI camera and the Pyxis rotator);
2. USB cables (to the QSI camera, the guider camera, and the rotator); and
3. 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.

Wednesday, July 3

Continuing from last night – now that polar alignment is close, I’ll do a first pass of Periodic Error analysis.

First, with all corrections off (no PEC, TPoint, Protrack, or guider relays), I collected 4 worm rotations’ worth of tracking data. Plotting this with TheSky’s PEC analysis, I get the following:

Peak-to-peak error, completely uncorrected, is 6.8 arc-seconds.  This is within spec (7 arc-seconds) for this mount, for the first time – so this is very good news, although I think the mount can do still better.

Using this data to program a Periodic Error Correction curve into the mount, I then did another 4-cycles worth of data collection, this time with PEC running.

Corrected Periodic Error is 2.9 arcseconds peak-to-peak.  That’s good, and I feel good about the improvements that the maintenance has generated.  But I’m left with a few questions:

1. Most important, why the corrected PE isn’t even better than that.  A good, pure, periodic error should be almost completely eliminated by PEC.  This wasn’t.
2. This makes me think that the error in the uncorrected data isn’t entirely composed of repeating periodic error, but rather that there are other factors still being injected.

On my next clear night, I’ll re-check the tightness of the pier mounting bolts, re-check equipment balance, re-check spring plunger settings,  then do a further refinement of TPoint/Protrack and polar alignment.  We’ll see what that does and, if the data are still puzzling I’ll look for other causes such as loose connections in optical chain.

Meanwhile, a few unguided test exposures with PEC and ProTrack running, just for fun.  These are all single 60-second exposures, binned 2×2, PEC running but no guiding, and corrected by dark and bias frames (no flats).

Since I was pointed right next to it for testing purposes, I started with M10:

Pretty nice, unguided.  That’s only a 60-second exposure, and the very slight star elongation I can see would no doubt get worse with a longer exposure.  It’s tempting to just throw guiding at it – but I want unguided performance to be as good as I can get it first.

Next, not far away in the sky, M12:

And, just to try something on the other side of the meridian, M27:

In all of these test photos, you can see the slight star elongation goes left-to-right, almost precisely along the horizontal axis.  Since the camera was rotated with polar North up, within a small fraction of a degree, this means the horizontal axis in the image corresponds almost exactly to Right Ascension.  So this elongation still corresponds to RA:  RA error, which is likely, but not necessarily, Periodic error.

There was also substantial Dec drift during my data gathering.  This doesn’t affect PE, but leaves me wondering why.  I’m guessing not-yet-perfect polar alignment is a factor.

A few days of clouds and thunderstorms are now forecast, so more testing later.

We finally had two clear nights of excellent conditions this past weekend. The sky was clear, the temperature was above freezing for the first time in months, there was no moon, and, being very early spring, there were no bugs. I have been waiting several months for an opportunity like this to finish installation and calibration of the mount.

First, I finished refining the polar alignment. This ended up taking four passes, when I thought only two or three would be needed. After the first polar alignment modeling session over a month ago, substantial polar error remained and a rather large correction was recommended by the TPoint software. Last night, I applied that large correction and ran a new modeling run. I didn’t expect the results to be perfect, just closer, and, sure enough, TPoint reported substantial improvement to the polar alignment but with a minor correction still needed. I must have misread the decimal point in the correction instructions because, on pass three, the polar alignment was still off by about the same amount and another minor correction was recommended. I made that correction, paying more attention to getting the decimal point in the right place, then ran a final TPoint model just to check the alignment.

The results were very good. TPoint is reporting my polar alignment to be off by 1/2 arcminute in azimuth and 3/4 arcminutes in altitude. That is certainly good enough for now.

Next, I wanted to do an initial, first cut, of the mount’s periodic error correction. I collected about four worm rotations’ worth of data using the autoguider camera. Even without PEC turned on, the Paramount’s impressive performance showed up. Uncorrected, I recorded a periodic error of ± 1.1 pixel, or about ± 3.8 arcseconds. That is better than the best I ever achieved, with PEC turned on, on my Losmandy G11 amount.

This generated a periodic error correction curve to be uploaded to the mount which applies a correction of ± 0.4 arcseconds.

I verified this by doing another PEC data capture, this time with PEC turned on. Sure enough, the corrected PEC is now showing at about ± 0.5 arc seconds. That is, frankly, incredible; especially since I’m not particularly impressed with my PE data capture yet. Seeing was quite poor tonight, probably in the range of 3-4 arcseconds, so my PE data capture was probably seeing the target star scintillating due to bad seeing more than it was seeing mount drift.

Just for fun, and as a more practical test, I decided to end the evening with a 10-minute guided exposure of the Leo triplet. I used five-second guide camera exposure intervals, with a low guiding aggressiveness to avoid “chasing seeing”. Although I forgot to take a screenshot of the guiding graph, guiding during this 10-minute exposure showed that it was making very small corrections, oscillating around a 1.5 arc second error. The single resulting image is shown here, without dark or flat fields. Nice pinpoint stars.

 Update: PE worse than I thought

Updated added later:  I have realized I made an error in recording the PE data above – I didn’t have the camera aligned “top-north”, so some of the error was masked as Dec motion.  Error is worse than recorded – in fact, it seems out-of-spec at about +/- 9 arc-seconds uncorrected.  Other posts will record the story of tracking this down and correcting it.