Tag Archives: MX

Tune-up journal 2: 1st worm adjustments

Leave a reply

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

Leave a reply

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.

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

Leave a reply

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.