Category Archives: Imaging

M13 – Hercules Cluster

I have a guilty pleasure: I like globular clusters. I should put more effort into more challenging classes of objects – diffuse nebulae and galaxies, but I’ve always loved GCs, even though they are easy to see. M13 is my favourite.

This was a test image last night, after a couple of days of maintenance, collimating the scope and then re-doing the mount polar alignment and pointing model.

Exposure Luminance: 60 minutes (20 x 3 minutes)
Colour: 15 minutes (5 x 1) of each of Red, Green, Blue
Camera QSI583wsg, with Astrodon LRGB filters, running at -15°C
Image scale 0.69 arceconds/pixel (luminance), 1.38 (colour)
Telescope AT8RC
Mount Paramount MX+
Guiding Autoguided with Starlight XPress Lodestar
Processing 32 dark frames per sequence
32 flat frames per filter
Sigma-clip mean combination on the darks and flats
Sum-combination on the light frames
Aligned, combined, and deconvolved with CCDStack2

Added Colour to NGC 4565

Here is the first round of adding colour data to the previous NGC 4565 image. Colour doesn’t come through my light-polluted sky very well – I could use a lot more time on the colour channels, but will have to wait until next year because there isn’t much time now between darkness and the target setting.


Exposure Luminance: 90 minutes (18 x 5 minutes)
Colour: 20 minutes (10 x 2) of each of Red, Green, Blue
Camera QSI583wsg, with Astrodon LRGB filters, running at -15°C
Image scale 0.69 arceconds/pixel (luminance), 1.38 (colour)
Telescope AT8RC
Mount Paramount MX+
Guiding Autoguided with Starlight XPress Lodestar
Processing 32 dark frames per sequence
32 flat frames per filter
Sigma-clip mean combination on the darks and flats
Sum-combination on the light frames


NGC 4565

Finally a couple of beautifully clear nights, not cold, and not busy with other things.  I started data gathering for NGC 4565.  This is 90 minutes of Luminance only (18 5-minute frames).  It’s still a bit noisy in my poor quality skies, so I plan to do another hour or so next clear night.  Haven’t gathered any colour yet.

NGC 4565

Flat Frame Acquisition App

A sibling to the Dark Frame app announced below, this application orchestrates the collection of Flat Frames. It is simpler, designed to be used at the end of an imaging session when the scope and camera are already running – so it does not contain delayed start and finish logic.

It’s available for download, in both Mac and PC binaries, here.  Use at your own risk and don’t build commercial dependencies on these tools – I’ll continue to work on them best-effort.

The program is written in python (v3.8) and is open-source.  View or modify here.

Start M1 with Luminance

Last night was not only clear, it was excellent.  Clear, moderate seeing, no moon, and temperature only about -3°C.  It’s late enough in the year now that the Orion area is now above my high horizon.  With the AT8 scope presently mounted, I started on M1.  Mount was performing very well – tracking very well on its own, and autoguiding to around 1 arcsecond (which is less than seeing jitter, and so just a guess).

Here is the start of M1, the Crab Nebula.  This is 2 hours of luminance as 24 5-minute subframes.  It’s going to be clouds for many days now, so I’ll add colour later if and when the sky permits.

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.

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.

Cold Night; Started B33

It was clear last night, the first truly clear night in a long time, but very cold.  Ambient temperature was -18c. I opened the dome and started things up, to have a series of mount problems.  Guiding was very ragged, jumping around a lot.

I suspect this was Periodic Error Correction being unhelpful – that the PE curve has changed since I adjusted and rebalanced the mount, and PEC was now working against me.  I used an hour to gather new PEC data but never did get to the point where I was happy with the correction.  However, the uncorrected PE was reporting as fairly small (+/- 3), so I decided to just do some guided capture without the PEC.  Turns out that worked very well.

B33-Sum-Red-2x2-11x300This is the time of year that B33 (Horse Head) is visible over the house, between the trees, so I started that for the season.  Unfortunately the previous problems meant that I only had an hour left and, sure enough, after 55 minutes image capture started to fail as the target began to encounter tree branches.

So, just as a sketch, this is the start.  This is 55 minutes of Red light, as 11 5-minute subs.  Guided but no PEC – I’m quite pleased with the guiding.  More light and other colours as the season progresses – it’s forecast cloudy again for several nights now.