Observation by Alex Pratt: Globular clusters Messier 13 and 92 in H...

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Alex Pratt


Alex Pratt


2019 Aug 11 - 20:22


2019 Aug 16 - 11:35


The Great Globular Cluster in Hercules (M13)

  • C11 f/10 @ f/3.3
  • Watec 910HX video camera
  • OccuRec (recording)
  • Tangra (playback)



Leeds, West Yorkshire

Target name

Messier 13 and Messier 92


Globular clusters Messier 13 and 92 in Her

About this image

Video is a medium ideally suited for recording transitory events such as meteors, occultations and astrometry of fast-moving NEOs, but I also like to use it to observe favourites in the Messier and Caldwell catalogues.

Having just monitored an asteroidal occultation I redirected my C11 onto Her, where the globular clusters M13 and M92 were riding high. Using an integration setting of x16 (0.32s) it was great to view these well-known objects in almost real-time to mag 15. The attached images are examples of individual video frames.  

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Jeremy Shears
Jeremy Shears, 2019 Aug 16 - 12:02 UTC

That's a really interesting method, Alex. So, do I understand correctly that each image was a 0.32 sec exposure? Remarkable. 

A short exposure does mean that you get good resolution even at the centre of the cluster.

Is there the possibility to integrate for longer? And can the image be processed to remove dark pixel or hot pixels should there be any?

Alex Pratt
Alex Pratt, 2019 Aug 16 - 12:44 UTC

Thanks Jeremy.

Analogue video cameras pump out frames at a constant rate of 25/sec (PAL standard, 50 interleaved fields/sec). Models like the Watec 910 can integrate, so they can be set to internally stack 2, 4, 8, 16 or more frames, then output a steady stream of these integrated frames. OccuRec can read this stream and individual frames from its resultant file can be saved as BMP, FITS.

In this example I used an integration setting of x16 (16 video fields = 16 x 0.02s exposures = 0.32s) and adjusted the AGC (gain control) to optimise the view, balanced between reaching a good lim. mag. and avoiding overexposure from local light pollution. The video integration and gain can be adjusted to bring out different features. This stacking from 8-bit video cameras doesn't match the well depth achieved by 16-bit CCDs, although expensive 12-bit video cameras are available.

Just like with standard CCD images, you can also take video darks, video flats, video bias recordings of a few seconds' duration. Tangra can process these and create dark and flat BMPs and apply them to the video. This is useful when recording an important event such as the Triton occultation, where Paris Observatory requested that darks and flats be included in our data submissions.

Jeremy Shears
Jeremy Shears, 2019 Aug 16 - 13:33 UTC

much appreciated. Hopefully you will share some more images taken this way in the future.

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