Monthly Archives: September 2019

Start NGC 891

I started a new imaging portrait, with about 90 minutes of data capture before the almost-full moon rose and lit up the sky.  This is 16 300-second subframes of NGC 891, calibrated with darks and flats, luminance only, stacked and processed in CCDStack.

On the next few clear nights I’ll add more luminance data and colour.  This image is cropped – about the inner 75% of the full frame – to eliminate distortion I’m getting around the edges.

This was my first “real” use of the mount, as opposed to testing and calibration, since all the refurb work.  It’s performing quite well.  Despite the testing suggesting unguided imaging at 300 seconds would be fine, I used guiding – why not, and for practice.  Seeing was poor – about 2 arcseconds of jitter, so I took longish guide exposures (5 seconds) to average out the seeing and avoid having the guider chase it around.Guiding corrections were within +/- 1 pixel, which is 1.05 arcseconds on the guide camera, and usually within +/- 0.5 pixels, or .5 arcseconds – consistent with seeing scintillation of 2-3 arcseconds.  The large spikes in guiding are artificial – that’s the software deliberately throwing the guide position off by 3 pixels after each subframe, then letting the guider drag it back in line, to create subframe dithering.

Testing unguided performance

Sunday night I used the removable dome cover (via the PZT) to enable two automated TPoint data-collection runs.  (The PZT makes this practical because I can start the run and then go indoors while it operates.  Before the PZT I had to stay in the observatory and adjust the dome position for every “sweep” of data.  This was both inconvenient, and also probably affecting the results through the vibration of moving the dome.)

The first run was 56 data points, just used to refine polar alignment, which was reported as “Excellent” in azimuth and needing small adjustment in altitude.  After doing the polar adjustment, I did another, large, TPoint run of 360 data points. This confirmed polar alignment was now excellent in both axes, and I then used this to build a high-quality TPoint “supermodel”.

What’s all that mean?  It means that

  • The scope control system now has a detailed model of how theoretical and actual pointing of the telescope differed at 360 different locations around the sky.  The recorded differences indicate where alignment is off, where something is loose or sagging, where non-linearities in the optical system affect pointing, etc.
  • Using this model, the scope control system can compensate, resulting in extremely accurate go-to pointing anywhere in the sky.
  • A feature called ProTrack also uses this model to adjust the drive tracking of the scope while it is in operation, keeping tracking accurate without the use of Autoguiding.  (This doesn’t prevent autoguiding, or PEC;  all 3 can be used together, and each contributes something to the accuracy of tracking.)

With a good TPoint model and ProTrack, the mount is supposed to be capable of unguided imaging of fairly long exposures.  I decided to test that, by taking unguided images of M15 of various exposure lengths.  In the following images, PEC, TPoint, and Protrack are turned on, but not autoguiding.  These are unprocessed.

Before PEC, TPoint ,and Protrack, 20 seconds would be the longest I could go without noticing distortion in star shapes.

To my eye, the above are showing no star distortion up to and including the 300-second (5-minute) exposure, but the 600-second (10-minute) one is starting to show distortion.  So it would seem I can do unguided imaging for exposures up to, and slightly over, 300 seconds.  Wow.

Zenith Table complete

Except for a final coat or two of paint, the PZT (Pod Zenith Table) is complete. Now, in addition to the usual rotation, the dome can be pushed backward, entirely off of the observatory base. This opens up the whole sky, allowing the scope to look straight up, and eliminating the need to move the dome opening around as targets move.  I probably won’t use it all the time – no need for shorter sessions targeting just one part of the sky. But it will make more sky available, and will make things like automated TPoint data collection easier.

I’ve opened it for practice in daylight, but haven’t used it under dark skies yet.

Confirmed, the new RA worm is working well

Finally another clear night presented itself last night, and I replicated my testing of PE with the new RA worm block in place.

I tried data gathering with longer exposures, but didn’t like what it was telling me, then realized I was on the wrong track anyway;  I should keep my data gathering to short exposures and let the fact that I am averaging the PE over multiple worm cycles handle smoothing out the seeing.  This is consistent with all the advice in the Bisque support forum too.  So I increased to gathering more worm cycles – 7 cycles.

Uncorrected, the worm is averaging out at 1.2 arcseconds peak-to-peak, and even looking at the raw data, the worst case swing is 2.9 arcseconds. That difference shows the seeing jitter.  This is really good performance – the guaranteed spec from Bisque is 7 arcseconds, so the worm is ‘way outperforming that, and is below seeing.

Then another 7 cycles with PEC turned on:0.4 arcseconds peak-to-peak residual error with correction running (averaged out over the 7 worm cycles), with max including big seeing spikes of 2 arcseconds.


I tried a quick image with guiding turned on.  I’ve lost the guider log, but with 3-second guide exposures and light aggressiveness (40%) it was holding the image to under 1/2 pixel.  The resulting image in this 5-minute exposure has nice round stars:

This seems to be avoiding an obvious question: what does an unguided image of some substantial length look like? I didn’t see the point of doing that test yet, since unguided imaging is substantially enhanced by ProTack, which I haven’t configured yet.

Next clear night (several days away) i’ll do a small TPoint run to re-check polar alignment, then a large TPoint run to build a good model, then do some experiments with PEC, TPoint, and ProTrack all turned on, to see how long unguided exposures can be.

These TPoint runs will be much easier, and more accurate, now that I have installed the SkyShed’s PZT so I can roll the roof right out of the way.  Previous TPoint runs involved having to chase the scope around the sky with roof rotation – both inconvenient, and also likely introducing vibration.

New RA gear: looking good

The new RA worm block is installed, and I had time for a brief test.  Testing was cut short because of a sudden cloud-over, but looked pretty good.  Next clear night I have a bit more testing to do.

Recall that I was unable, with all previous adjustments, to get uncorrected Periodic Error consistently below about 7.5 arcseconds peak-peak.  For example:

With the new worm block in place, here is a first uncorrected PE data collection:

This is incredible – 1.5 arcseconds peak-peak, with no correction being applied.  More in a moment on why the curve is so jagged, but this is certainly great performance.

Then I loaded the above as a correction curve, and did another data collection with PEC turned on:

1 arcsecond, peak-to-peak.  The wild fluctuation that is just visible on the right edge of the graph is when the guide star was lost when the clouds suddenly moved in.  I deleted those data points to get this graph – but I guess I missed one.

Both of these – uncorrected and corrected – are, in fact, error smaller than the seeing fluctuation I have on a typical night.  I think that’s why the curves are jagged – that’s seeing bouncing around.

I was going to test this theory, when it suddenly clouded over.  So next clear night, I’ll do another set of runs (uncorrected and corrected) but with guiding exposures of 3 or 4 seconds.  The long exposures should “smear” seeing jitters out and, if I’m right about the effect of seeing, should produce smoother curves.  I’m looking forward to testing this.

I’d also like to do another data run just to convince myself that this isn’t “too good to be true”.  Did I do something wrong that resulted in this excellent performance?   I don’t think so, but I’d like to replicate.  Adding guiding to a PE of 1 arcsecond should produce a very stable image.  In fact, I’m interested in seeing how long an exposure I can use unguided – that would certainly be convenient.

Standby for another clear night.  Looking out the window, that’s not tonight.

Two Big Changes underway

Two things are in progress for the observatory:

  1. A new Right Ascension worm block. It has arrived from Bisque and I’ve installed it, but haven’t yet had a clear night to re-do PE measurements and see what effect it’s had.
  2. A PZT (Pod Zenith Table) – a DIY project, supported by hardware and plans sold by SkyShed, that allows the clamshell dome roof to be rolled off to the side.  Doing so will open up the whole sky above the dome – where, right now, I can’t point straight up or, indeed any higher than about 70° altitude.  Considering how high my horizons are (hedge, house), opening up the sky above will make a huge difference.  The PZT woodwork is assembled and I’m in the process of painting;  installation in the next few days.