Mount adjustment: change mount-to-pier connection

Restarting after a year+ absence (various reasons, not for publication here).

In reviewing online support forums, I see another recommendation.  I’m using the SkyShed pier, and the supplied pier-to-adapter plate connectors, which is 4 long bolts the mount sits on, that enable leveling.

It’s pointed out that those are a (rather obvious) possible source of flexure. And they’re not really necessary – assuming the pier is reasonably level, proper polar alignment in the mount will overcome any fine-tuning of leveling.

So, new first step, that I’ll do on the first opportunity:  Remove those leveling bolts and mount the pier plate directly on the pier.  The only issue will be that the space between the pier and the plate has been a useful “cubby hole” for various cables, and those will have to be relocated.

Tune-up journal 3: more worm adjustments

Last night it was clearing up nicely, after a very hot humid day.  I opened up the dome about 1 hour before dark to let things start to cool, then used that time to continue tuning the mount gears.  I took the cover off both worm blocks, inspected belt tightness and pivot screw tightness.

The Declination pivot screw seemed a bit loose, so I tightened it slightly. (This is another screw that has a “right amount” of tightness and I can’t find any documentation that gives the appropriate amount for the MX+, so I just snugged it up a bit by feel.)   I also loosened the RA spring tension plungers another 1/2 turn, so they are now at 3.5 turns out-from-bottom, which is the loose end of the specified range.

To see what effect these changes made, I started to set up for another Periodic Error data gathering run.  I was having trouble finding the guide star, and was wondering why until it occurred to me to look up.  Sigh. The sky had completely clouded over.  In fact, shortly after I closed up the dome, it started to rain.  So no new sky data to test the results of these latest adjustments.


Tune-up journal 2: 1st worm adjustments

Evening of July 15, it’s moderately clear.

Before dark, I finished re-connecting all the cables to the mount and related gear without doing any through-mount cabling.  Ran the mount and the rotator through their full ranges of motion several times to ensure nothing snags on anything.

Adjust worm tension plungers

My plan said to start with a new, small, TPoint run to verify polar alignment.  However, I decided to change that and do an initial check if the worm tension plungers are in-spec.  I thought that, if they aren’t, I should discontinue running the mount around with out-of-spec gears as soon as possible.  So, I checked the cam stop and worm spring plunger settings of both gear boxes.

Declination gearbox

I adjusted the cam stop to spec (1/8 turn out from bottomed-out).  It was already in more-or-less that position, so I don’t think it was out of spec.

Then I adjusted the 2 spring plungers.  The spec for these is “3 to 3.5 turns out from bottomed-out”.  They were a bit loose so, wanting them tighter, I set them to 3 – the tight end of the spec range.

Both of these adjustments can be done without removing the gearbox cover, so I haven’t yet inspected other components for tightness and tension – that will come later.

Right Ascension gearbox

Again, cam stop seemed OK but I re-adjusted it to spec just to be sure.

Spring tension plungers were quite a bit too tight.  They were about 1.5 turns out from bottom, so I readjusted them to 3, per the spec.

Quick re-check of periodic error

Continuing to deviate from my carefully thought-out plan, I couldn’t resist doing a new Periodic Error data gathering run to see if those adjustments had any effect.

Guiding reference plate solveSo, I carefully checked all the various settings that I have got wrong so many times:

  • Camera rotation close to zero (used a plate-solved image to confirm this);
  • PEC, TPoint, and ProTrack all off;
  • Autoguider set to do no corrections;
  • Pointed to a star near zero Declination, and just West of the Meridian.

PE data capture in-progressI gathered 1/2 hour of tracking data, which is about 8 worm rotations.  I used the main camera, binned 1×1.  During this data gathering, I noted that the guide star was showing up as 100% saturated.  This might be a problem – if it is over-saturating it may be difficult to calculate the FWHM radius.  This goes into the list of things to check next time.

7.7 arc-seconds error peak-to-peak, uncorrectedA quick analysis of the results shows it has certainly improved things.  Uncorrected periodic error is now at 7.7 arc-seconds peak-to-peak, down from 12.  That’s almost within spec (which is 7-7).

After I get back on-plan by checking polar alignment, I’ll do another test with a couple of minor additional changes:

  • Shorten exposure or use a dimmer guide star to get saturation below 100%; and
  • Loosen those RA plungers another 1/2 turn, to the loose end of the specified range.  If “looser” improved things, maybe “more looser” will improve them more.

Mount tune-up Journal 1: July 13

First step in the plan is the removal of my added-in through-mount cabling.  I opened the access hatch and carefully removed:

  • USB cable to the QSI camera
  • Power cable to the QSI camera
  • RJ-11 cable to the focuser motor

The other cables in there were installed by the manufacturer and I assume they’re OK.  My added-in cables were tangled, and I could easily imagine that they were contributing to drag.

I started re-cabling the mount with external cables run along the outside, and held in place by velcro straps.  However, I didn’t finish the job, as I was driven indoors by mosquitoes.  I’ll finish this job on the next available night.

Starting Mount Forensics and Maintenance

It’s time to admit I have a mount performance problem

I am so impressed with the build quality of the MX mount that I’ve been reluctant to admit that there seems to be a performance issue showing up in my data. I think I’m ready to admit that now.

  • There is pretty good evidence of a problem;
  • I’m pretty sure it’s a rather basic problem, likely easily correctible (but only once I admit I have a problem and work on it);
  • The SB support forums show excellent responses from both manufacturer and other users, and every problem someone reports and works through seems to be solved.

Symptoms of one or more problems

There are a couple of things that I’m sure are indications of problems, although it remains to be seen if there is a single underlying problem or several. The symptoms are

Periodic Error (PE) is out of spec.


uncorrected periodic error 12 arc-seconds peak-to-peak

I gathered data incorrectly several times (mainly by forgetting to record the camera rotation angle, or to ensure it was 0 or 180), so it took me a while to see this. However, now I have reliable data showing I’m getting uncorrected Period Error of almost 12 arcseconds, peak to peak.

The spec for the mount is “maximum 7 arcseconds, peak to peak”. And this isn’t just marketing hype – most Paramounts do much better, and the SB support forums are full of examples of SB being horrified in the rare case when someone’s mount exceeds this spec, and of SB taking it seriously to correct it.


Corrected periodic error, 3.9 arc-seconds peak-to-peak

Note that with Periodic Error Correction (PEC) active, the mount tracks at 3.9 arc seconds, which is quite good. However, I think I should work on getting the uncorrected performance into spec.

    • Surely if I get uncorrected performance as good as possible, then PEC should improve corrected performance even more; and
    • Surely being out-of-spec means something is wrong, and that problem could have other effects that show up in results, or that cause wear and tear on the mount’s drive mechanism.

So, I’ve resolved to work on this, with the minimum target of getting uncorrected PE into spec.

Strange Dec oscillation when guiding

Next, I’m getting very strange behaviour in the Declination axis when auto-guiding. At this point I don’t have a good theory for whether this is the same problem as the PE above, or a separate problem.

When autoguiding, there is no reason why there should be Dec motion or Dec corrections at all unless Polar Alignment is off, or unless ProTrack is enabled and making corrections that it thinks are needed.

Simulation of strange behaviour seen in guidingWhat I’m seeing in Declination is large-scale, slow-period oscillation in the Dec axis. Dec shows as being several arc-seconds off, but multiple successive autoguiding periods don’t seem to be able to correct it. Then it suddenly swings over to being several arc-seconds off on the other side of the axis. At no point does it converge to a “no or small error” state. (This image is a simulation since I forgot to take screen captures the last time I did this test.)

Aside from the obvious – that this indicates something is wrong – this also means that I can’t use dithering when collecting a series of images. After the first image, guider “settling” back within a specified error range never happens. RA settles quickly, but Dec just keeps oscillating from far-off on one side to far-off on the other side.

Things that look odd but I’m not sure are symptoms of problems

The above two issues are things that I’m sure are problems. There are also a couple of other strange things going on that I’m not sure are problems, but they look odd.

TPoint-Scatter-Graph-2First, my TPoint model isn’t very good. The point-spread graph is quite spread-out, and quite oblong. This is probably just another symptom of the same problem causing the above issues. It could also be giving ProTrack ammunition to make bad adjustment decisions, compounding the problem.

Second, I note, when gathering uncorrected data for doing a PE calibration, that I’m getting a large measured declination drift, in addition to the expected periodic error in Right Ascension. I haven’t thought through the mechanics enough to know if this is normal or an indicator of a problem.

Research in support group

I spent quite a bit of time searching through the SB support forums and documents (which are not well organized, and not easy to search, by the way). Here are some miscellaneous points I’ve noted – in no particular order.

  • Polar alignment is important, of course. If it’s off, drift will occur in both axes, compounding error. (I’m quite sure mine is quite good.)
  • Camera rotation is critical when gathering data to analyse PE. It must be close to 0 or 180 degrees. If it’s in-between, PE is masked by being distributed between the RA and Dec axes. (I made this mistake several times, gathering PE data with a poor camera angle.) I note that the software could be coded to correct for this (if given the rotation value), but it isn’t.
  • ProTrack affects tracking. That’s its job; but, when diagnosing problems, start by turning it off so we’re looking at “raw” performance, and to help determine whether ProTrack (or the TPoint model that drives it) are part of the problem.

The most common causes of problems seem to be:

  1. Flexure in system (mirror flop, loose physical mounting, etc.) Externally, I’m pretty sure I have this under control, but there could be something loose inside the mount.
  2. Problems with worm gears, any of:
    • Spring tension plungers needing adjustment;
    • Cam stop needing adjustment;
    • Pivot screws being loose;
    • Various other mounting screws being loose;
    • Rarely: gears or gear assemblies needing service or replacement.
  3. There are some reports of through-the-mount cabling contributing to problems, if the cables tangle, don’t have the necessary slack, bind against internal shafts or surfaces, or “wind and unwind”, contributing force to the expected movement of the shafts.
  4. Tension on the drive belts can also be checked and adjusted. It’s not clear how incorrect tension would cause the problems I’m seeing, but it’s easy to check and it would presumably contribute to longevity of the belts to ensure it’s correct.


To be confirmed by experimental corrections and measurements, my hypotheses for my problems are:

Higher-than-spec PE

I think the spring plungers on my RA worm gear are too tight. If they were too loose, it wouldn’t affect normal RA tracking (but would show up as backlash in slews, and in TPoint). Too tight, however, might be causing gear friction, stiction, or over-sensitivity to minor machining imperfections.

Through-mount cabling could also be contributing to uneven PE performance if cables are binding.

How to test this hypothesis:

  • Check TPoint model for RA backlash, which would suggest “too loose” and disprove this hypothesis;
  • Make a new PE data-gathering run after making the following checks and adjustments.
    • Temporarily remove all in-mount cabling, routing cables on the outside;
    • Adjust the RA spring tension plungers to spec;
    • While I’m in the RA gear box, adjust cam stop and confirm other mounting screws are tight.

Erratic Dec during guiding:

I think the spring plungers on my Dec gear are too loose. Backlash could be causing the non-response to guiding adjustments, and is probably also polluting the TPoint model and causing ProTrack to make adjustments. Occasional ProTrack “adjustments” could be responsible for periodically throwing the Dec error to the other side of the zero axis.

Through-mount cabling could also be contributing to uneven performance if cables are binding.

How to test this hypothesis:

  • Check TPoint model for Dec backlash, which would confirm “too loose” and support this hypothesis;
  • After fixing PE as planned above, do a new guiding test after making the following adjustments:
    • o Temporarily remove all in-mount cabling, routing cables on the outside;
    • o Adjust the Dec spring tension plungers to spec;
    • o While I’m in the Dec gear box, adjust cam stop and confirm other mounting screws are tight.

Plan to deal with it

So, here is the plan to work through this problem. The plan will need adjustments as data come in.

  1. Change mount-to-pier fastening (see post of 2018-4-14)
  2. Remove through-mount cabling.
    (Temporarily – I like it, and hope to be able to restore it once I understand its role, if any, in the problem.)
  3. Check and adjust Cam-Stop on both axes
  4. Check tightness of worm-end pivot screws, both axes
  5. Check that RA and Dec worm plungers are adjusted to spec.
    It wasn’t easy to find out what the spec is: the SB documentation isn’t well organized. The specs are:

    • Cam Stop 1/8 turn back from bottomed out (measured with clutch in “run mode”);
    • Spring tension plungers: 3 to 3.5 turns back from bottomed out. (measured with clutch in “balance mode”)
  6. Check belt tightness, both axes
  7. Do a new 20-30 point TPoint run to verify polar alignment is good
  8. Carefully record uncorrected Periodic Error
    • double-check camera angle is 0 or 180
    • double-check PEC, TPoint, ProTrack, and Guiding Relays are off
    • keep a plate-solved image to verify camera angle and image scale
    • keep the gathered guiding log, and a screen shot of the data gathering graph
    • hoping, with above adjustments, PE will fall in spec; get help if not
  9. Upload PE Correction and record results
    • double-check “west side of mount” checkbox is correct
    • do a PE recording with PEC on but TPoint and ProTrack still off
    • keep the gathered guiding log, and a screen shot of the data gathering graph
    • residual error should be on order of 1 arcsecond
  10. Carefully record an autoguiding run
    • compute autoguider correction parameters with CCDWare calculator
    • hoping strange Dec issues gone; get help if not
  11. Reset TPoint
    • do a new large run (150+ stars) and supermodel
    • verify if it still thinks I have good polar alignment
    • test unguided tracking, with PEC and ProTrack on
    • test guided tracking with PEC and ProTrack
  12. Restore through-mount cabling
    Replace through-mount cabling, one cable at a time, verifying PE and guiding after each. This will be a slow and painful process, that I’ll work at as other reasons to take OTA off mount arise.

A test sketch of M81

Friday night was clear with very good transparency.  I finished re-doing my TPoint calibration and, as part of that, confirmed that polar alignment is still good.  Then did a new, larger, data collection run for periodic error correction.  Something still not right there – the uncorrected error was higher than it should be (although not high).  Corrected it’s fine, but I still hope to get to the bottom of what’s going on there.

Autoguiding is also working fine now – not incorrectly calibrating on hot pixels – since I set up dark-frames and hot pixel elimination for the guide camera.

Messier 81

Messier 81

So, last thing before the sky went hazy, I did a test run of M81, just for fun.  This was 6 5-minute luminance frames.  Interesting: the mount is so stable with PEC and guiding running that I have a problem with dust spots that I’ve never had before.  In my former setup, the error and wobbling in the mount provided a sort of “auto-dither” that smeared out the noise (and the non-noise). Now it’s so stable that the pixel errors are always in exactly the same spot, and so are visible.  So, next time, I’ll start adding a several-pixel dither to the autoguiding.

Anyway, it’s a pretty image.  I think I’ll keep it, add data and colour, and work it into a finished product.  Not right away though: this object has almost rotated behind my tree, and I’ll soon lose it until autumn.

Changed scopes for summer

It was clear and warm last night, but a full moon.  So no serious imaging but a good night to adjust things.  With winter skies basically done, I decided to take off my widefield refractor and go with a longer focal-length OTA for the summer season of galaxies and nebulae.  So, the SV80S is put away, and I mounted the AT8RC 8″ Ritchey-Chretién, and rebalanced the mount accordingly.

M67-3x3-10secs--15C-LuminanceThat creates new re-adjustments that are needed, of course.  I re-built a V-Curve model for FocusMax, and got nice round stars on a quick 10-second test image of M67.

I was going to do a new TPoint pointing model, and also use that to re-check the polar alignment, but I became distracted.  I tried a quick test of autoguiding, intending to confirm that my previous autoguiding problems were field curvature on the SV80S.  They were – the AT8 produces a nice flat field, and the guide starts weren’t too badly distorted to use.

The problem was that I tried doing a guider calibration, and it didn’t work.  That ended up distracting me for the rest of the night, as I experimented with the parameters that control the amount of movement the calibration routine injects before measuring direction and distance.  No matter what I specified, even absurdly long displacements, I got the same error:  “Error, insufficient motion in X-direction during calibration”.

Guider-calibration-motionWhich is clearly not the case.  As these two guider frames show (click for animated gif), there is good motion happening in both axes between calibration images.  I puzzled over this for an hour or so, then it clouded over so I shut down and went to bed.

This morning an idea hit me which I think is the solution.  The guide stars aren’t in focus. That, alone, shouldn’t be a problem, as the guiding software should calculate the centroid of each star.  However, there are bright hot pixels in the image.  I bet the guider software is selecting and guiding on a bright pixel, which doesn’t move, not on a star.

I’ll test this next clear night:

  • Adjust focus of guide camera to get nice bright pinpoint guide stars; and
  • Use a dark frame with the guide camera to calibrate out the hot pixels.

We’ll see.

PEC, Autoguiding stars, and a test image of M81/82.

It was clear last night, quite warm, although seeing and transparency were poor. More testing and calibration, and some real progress was achieved.

Better PEC

Now that I have focus perfect, and given my suspicion that I may be doing unguided imaging with this short refractor, I decided to re-do my PEC calibration. This time I used the main camera to get cooling and higher resolution. I captured about six worm cycles of data and recalculated the model.

Gathering data like this can be done before it is completely dark, so it was a good way to spend the hour of dusk while waiting for the sky to become completely dark. In the process, I noted two important things:

  1. Somehow, my TPoint model is off, even though it was near-perfect the other night. I needed to use a closed-loop slew to get to what should have been easy targets. I imagine I bumped something the other night while I was disconnecting and reconnecting the guide camera multiple times. Next testing evening, I will first try doing a re-synchronization and then, if necessary, a completely new TPoint model.
  2. SV80S-Field-CurvatureAlso, in the process of doing full-frame images with the large QSI camera, I can clearly see the field curvature. The extreme edges of the field are badly distorted, confirming this as the likely cause of the distorted stars I’m seeing in the guide camera, which is even farther from the center of the field.

So, I think I’m not going to worry about correcting those guide camera stars now, but, rather, see what I can do with unguided imaging. After all, this is a very stable mount, and I’m presently using a very short focal-length imaging scope.

Testing Unguided Imaging Limits

So, I slewed to a nice test image – a spot midway between M81 and M82 – and took test images at 10, 60, 100, 200, 300, 400, 500, and 600 seconds, with PEC on but no guiding. Those images, calibrated, are here:

Ignore the badly distorted stars at the upper left edge of the field, and look at the center. While I think I may have a minor collimation issue with the camera, I can see no difference between the stars in the 10-second image and those in the 10-minute image. With this mount and scope, I clearly don’t need Autoguiding for exposures in the 5-10 minute range. So, I’m going to shelf my calibration of the Autoguider until later in the season when I switch back to a longer focal-length imaging scope.

It also occurs to me that I have a field flattener for short scopes in a drawer somewhere.  If I come back to serious imaging with this scope later in the season (which I might want to do for a very wide field shot, such as M31) I’ll try that flattener.

Now, since I have a nice image framed anyway, I thought I would do some trial imaging of that M81 – M82 shot. I took 10 five-minute sub-exposures, unguided, and then a series of flat fields using the light panel mounted in the observatory. I already have a calibrated dark frame for use with this camera.

There sure are a lot of satellites in that part of the sky.  Lots of satellite trails to be removed.  Fortunately, the data rejection procedures in CCDStack make that fairly easy.

2015-04-17-M81-82-monoHere is the result, calibrated, combined, and cropped. (Still just a test – not a finished product, which would need better noise removal, scaling, etc.)

I would not have imagined I could get results like this unguided. This truly is a wonderful mount.

Static Electricity

A final note.  I’ve had several major static electricity shocks over the last couple of days, and I don’t remember this being a big issue in the past.  Twice tonight, a piece of equipment actually reset after a static spark.

What’s changed?  I think the answer is the new vinyl cover on the dome – I used to have a woven polyethylene cover, but switched to this vinyl one last year.  Maybe dragging the vinyl cover off the plastic dome is generating the charge.  Running a ground wire to the dome is now on the to-do list.

Hmm. Guide Collimation doesn’t look like the problem.

Another clear night – that’s 3 in a row, possibly the only time I’ve had such a chance in a decade.

I spent it investigating my guider collimation problem.

Badly distorted, elongated stars in Lodestar guider in WSG guider port.Imaging with an SV80S (480mm f/6 triplet refractor) I’m getting very elongated stars in my guide camera, seeing through the off-axis guider port on the QSI583WSG camera. So elongated that I doubt guiding will be accurate. And considerably worse than I remember seeing a couple of years ago. What’s going on?

My assumption was that the pick-off prism or the guider camera mount was somehow out of collimation – that it wasn’t orthogonal to the incoming light path. That would seem consistent – the distortion in the guider stars is exactly vertical, as though the prism was out of alignment in the vertical direction, but fine horizontally.

However, before that it occurred to me that the entire camera/guider system might be out of alignment. Better check that first.

Using CCDInspector, I discovered a minor out-of-plane error in the main camera, which I was able to correct by snugging it better into the focuser drawtube.

Main camera field well centred, no collimation errorThen, a re-check shows a field with no skewing off orthogonal, although with significant field curvature (back to that in a moment). So, it’s not the overall alignment of the system.

Guide-Camera-CurvatureNext, I did a field analysis of an image from the guide camera, which confirms what I could see with my own eyes – there is major distortion, almost exactly vertical, toward the bottom of the image field.

Now we come to the problem. I think I need to tweak the alignment of my off-axis guider port’s “pick-off” prism. However, it doesn’t seem to have any such collimation adjustments. It’s solidly machined, as is the threaded guide camera mounting port. It’s not supposed to be out of alignment, and there doesn’t seem to be any way to adjust it if it is.

By now I have run out of evening time and patience, and I have to get up for work tomorrow, so I’m stopping, stumped, for now.

On the weekend I will use daylight to take the camera off and apart and look more closely for some way to adjust or shim the pick-off prism or guide camera mount.

My fall-back plan if I can’t adjust that will be not to worry about it. I’m pretty sure the problem is greatly exaggerated by the rather tight field curvature in this scope – evident in the curvature map shown above. The pick-off prism is very close to the edge of the field, where curvature is strongest. Maybe that’s why I don’t remember having this problem before – because I was using a different scope, with a famously flat field.

With this short focal-length scope I should be able to take 5- to 10- minute images unguided and, if guiding is needed, at this short focal length a separate guide scope should work well.

So, I’m going to have a look at the mechanics on the weekend. If I can find a way to adjust the prism alignment, I will try that. Otherwise I’m going to declare “don’t care” and use this short scope without the off-axis guiding. Its season is nearly over anyway – I’ll be returning to a longer focal-length, and flatter-field, scope for the summer. If I have a guide-star problem with that scope I’ll pick up this problem again, but I’m guessing I won’t.

So, assuming I don’t find an adjustment, next clear night will be spent refining my PEC training now that I have perfect focus, and then I’ll do some tests of unguided exposure length to see what the Paramount can do at a short focal length. I expect to be impressed.

Upgraded to FocusMax v4, Guider Collimation

Since switching to TheSkyX as my primary control application, I have tried its built-in @focus2 autofocus routine a couple of times, but I missed FocusMax, which I used to use and really liked. Last night it was clear enough for focus testing, and I reinstalled FocusMax.

To my surprise, the software has been acquired by CCDWare, and is now a commercial offering. It’s no longer free, but I have found CCDWare’s other products to be excellent and well-supported, and I don’t mind paying a bit for reliable, low-hassle software.

focusmax-vcurve-5As hoped, the commercial V4 is a major improvement. The original free package worked well when it worked at all, but it was always a challenge to install, integrate with various versions of the OS, configure drivers, etc. With the new version, after figuring out the initially complex user interface, it just worked, no hassles. Using the convenient “first light wizard” I did several V-curve calibration runs, and it’s giving me great focus easily.


Next problem.

Now that I have excellent focus it’s clear that I have a Collimation problem with my guide camera. When I get pinpoint stars off the main camera, an image through the attached guider is giving very elongated stars. Calling them “ovals” would be an undeserved compliment – they are long streaks. I assume this indicates that the pick-off mirror in the QSI camera’s WSG guide port is not properly collimated to be exactly 90 degrees to the light path.

While stars don’t have to be pinpoint for good auto guiding, since the software calculates the centre of each circle, I am pretty sure they should be round, or at least ovals. It must challenge the software to guide on little line segments. Next chance, that will be my next maintenance project.