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Spiral Galaxy NGC 6744 in Pavo

NGC 6744 is a Milky Way like barred spiral galaxy in the constellation Pavo.  Visible only from lower latitudes, the light we see now left this galaxy around 25 million years ago. 


NGC 6744 in Pavo ( please click / tap on image to see larger and sharper )


Capture Details:

North is up.

Orion Optics CT12 Newtonian ( mirror 300mm, fl 1200mm, f4 ).
Corrector: ASA 2" Coma Corrector Quattro 1.175x.
Effective Focal Length / Aperture : 1400mm f4.7.

Skywatcher EQ8.
Guiding: TSOAG9 Off-Axis-Guider, 
Starlight Xpress Lodestar X2, 
PHD2 .

Nikon D5300 (unmodified) 
(sensor 23.5 x 15.6mm, 6016x4016 3.9um pixels).

Blue Mountains, Australia 
Moderate light pollution 
( pale green zone on darksitefinder.com map ).

Capture ( 16, 17, 19 Sept. 2017 ).
9 sets of sub-images with exposure duration for each set doubling ( 1s to 240s ) all at ISO800.
85 x 240s + 5 each @ 1s to 120s.

Processing ( Pixinsight - 5-17 Nov 2017  ).
Calibration: master bias, master flat and no darks.
Integration in 9 sets.
HDR combination 
Arcsinh stretch ( function written by Mark Shelley )

Image Plate Solver - NGC 6744 - Sept 17, 2017
Resolution ........ 0.586 arcsec/px ( full size image )
Rotation ............ 0.001 deg.
Focal ................. 1372.24 mm.
Pixel size ........ ..3.90 um.
Field of view ..... 58' 30.3" x 38' 59.0".
Image center ...... RA: 19 09 46.591  Dec: -63 51 13.44


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The new Pixinsight Arcsinh stretch function ( written by Mark Shelley ) was used to stretch the HDR linear image whilst keeping colour in the stars.  Thanks Mark :icon_salut:


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13 hours ago, Victor Boesen said:

Beautiful image!!!


12 hours ago, cfpendock said:

Very nice image.  Sadly this one is not available here....



6 hours ago, joelshort said:

Very nice!  I really like the new ArcsinStretch too.  It works really well on RGB data.

Thanks guys, much appreciated.

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Very nice, not one I am familiar with. What’s the benefit of the range of 1s exposures from 1-120s?



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7 minutes ago, tooth_dr said:

Very nice, not one I am familiar with. What’s the benefit of the range of 1s exposures from 1-120s?



Thanks Adam - NGC 6744 is way down here in the South and is a little compensation for the wonderful objects you have up there in the chilly North :)

The range of exposures is based on my HDR capture and processing workflow...

The shortest exposure is chosen based on the brightest star in the frame; that is, the longest exposure such that the pixel value of the centre of the brightest star < 16383 ( the 14bit maximum ).

Then I use a range of exposures ( doubling every time ) so as to smooth out the tones in the HDR image of the bright stars.  Previously, with an HDR image based on sets of exposures every 2 or 3 stops, the brightest stars had an “onion” like appearance with rings of differing exposure values.  Having said that, my processing workflow has changed quite a bit since then ( in particular I now use Pixelmath instead of the build in HDR module in Pixinsight ) and it may be worth checking to see if I can get away with a bigger step size than the 1 stop I am using now.




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Fascinating Mike. Thanks for the detailed explanation.  It certainly works well!

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3 hours ago, MikeODay said:

A deeper look ...


NGC 6744 in Pavo ( negative )

I like that tidal stream coming from the main galaxy. Reminds me of Gabany's work. Do you have any plans of enhancing that in the colour image?

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10 hours ago, wimvb said:

I like that tidal stream coming from the main galaxy. Reminds me of Gabany's work. Do you have any plans of enhancing that in the colour image?

I will have to have another look at it - the challenge is the noise; even desaturated it looks pretty bad in the highly stretched colour image.

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On 20/11/2017 at 10:01, Allinthehead said:

Very Nice Mike.


On 20/11/2017 at 10:09, toxic said:

thats a very nice image Mike :thumbright:

Thanks Guys, much appreciated.

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