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NGC6744 - What to do?


hjw

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Hi,

I've been imaging a galaxy (NGC6744) in the constellation of Pavo over the last few nights. The object is rather big (20'x13') but faint (surface brightness 14.79). I started too late on it because I can only get about 1 hour shortly after sunset. So far I've got 2.5 hours of exposure time with 2-4 min at ISO 800 or 1600. The image is still extremely noisy and I can only just make out the outer spiral arms. My question is, how do you improve imaging on very faint targets. Obviously, more exposure time - but seeing the improvements from one night to four, I am not sure it will get me where I want to be. Darker skies later at night would also help but that is not an option until next year. I am just wondering whether there is anything I am missing.

 

1578159993_NGC6744cropped.thumb.jpg.c835a7ee97e230fded819e1da0d5124f.jpg

Equipment: Saxon 200DS on a HEQ5, SW ST80 with ASI120MC for guiding, Pentax K30 with GSO coma corrector, 120-240sec exposures ISO 800 or 1600, 20 flats and 150 bias frames for each ISO.

Thanks!

HJ

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No, you haven't missed anything. The only way to improve this image is more time on target. (Oh, and possibly a cooled camera ?).

If you image just after sunset, the sky isn't dark yet and will add a lot of noise. You already know the remedy: dark skies and lots of data. Unfortunately there's no shortcut in AP. Personally, I would move on to another target, higher in the sky, and revisit this under better conditions.

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2 hours ago, wimvb said:

No, you haven't missed anything. The only way to improve this image is more time on target. (Oh, and possibly a cooled camera ?).

If you image just after sunset, the sky isn't dark yet and will add a lot of noise. You already know the remedy: dark skies and lots of data. Unfortunately there's no shortcut in AP. Personally, I would move on to another target, higher in the sky, and revisit this under better conditions.

Thanks! I was hoping I missed something and hope dies last... Well, there is always next year.

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I've never yet been able to properly image the Orion nebula. At my locations it was either not clearing the trees or above a neighbour's house where the air was unstable. Now that I've moved to a dark location with a clear sky line, I can finally add it to my list.

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There is certainly quite a bit of detail coming through there!

The only thing I can add is that the signal-to-noise ratio only increases with the square root of the exposure time. Quadrupling the exposure time would therefore only halve the noise with respect to the peak signal. I spent 7 hours on the Orion Nebula at F/4.8 last winter and still the background detail could be improved, and the Orion nebula has a far higher surface brightness than your target.

M42USM3expcropcurves3.thumb.jpg.a2c6ed3e05a7e8aec8e995b2774e2289.jpg

The core of the nebula is fine, but the faint outlying regions are pretty noisy. I will add a lot more this winter, if the weather allows

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If you are only able to image for an hour before the target drops below the horizon, then this will be another factor, as you are imaging through a thick bit of atmosphere.

You would be best to wait until it is higher in the sky.

Carole 

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