NGC 6888 - Crescent

[WolfieGlos]

I wasn't going to image this one this year, but a change of heart with this weeks clear skies following the Full Moon led to 16 hours of data collected over 3 nights. This is more time than I have put into any image before but I was surprised at just how much noise still remained. I think with the heatwave, the DSLR seems to have suffered quite badly and what I was seeing was possibly thermal noise despite dithering and matching dark frames also applied. After running Starnet, I realised it was actually walking noise...which I now need to look at, since it has not shown up in my images since I started dithering when I got the HEQ5 in January. Despite the continued heat, we have been completely clouded over here for 2 nights now so I'm calling this one done now.

After night 2, I noticed a faint bubble just below-left of the centre of the image. I thought I had incorrect flats and that it was a dust spot.....turned out I had captured the Soap Bubble Nebula! Although I have heard of it, I didn't realise it was located here so that was a nice bonus! It shows up a bit more defined in the image before noise reduction, but I suspect it will need a lot more time to bring it though.

The third night I accidentally ran NINA capturing 6minute frames instead of 5minute, so I had to take yet more darks for that noise....

One thing I can't work out....why is it called the crescent? To me, it looks like a space brain.....

Canon EOS 800Da + Starfield 102ED + 0.8 Reducer on HEQ5
ISO - 400
Frames - 128 x 300s + 52 x 360s (Total = 15:52:00)
Calibrations - 50 bias, 35 flats, 26 darks (300s) + 12 darks (360s).
Stacked in SIRIL
Edited in SIRIL, Starnet, AstroDeNoisePY and GIMP

Comments welcome 🙂

VERSION 1:

 

VERSION 2:

 

 

Edited September 9, 2023 by WolfieGlos
Revised processing

[simmo39]

Nice!

[alacant]

14 hours ago, WolfieGlos said:

surprised at just how much noise still remained

Hi

Lovely shot.

After 16 hours, the noise is most likely introduced by using in-camera bias and dark frames. The best way we find with an eosxxx is to simply subtract the offset -the 800 uses 2048- instead of applying the former. That, along with -at least- a 5 pixel dither between frames and stacking with a modern clipping algorithm should obviate the need for smoothing/denoise.

Cheers and HTH

Edited September 8, 2023 by alacant
Spel

[WolfieGlos]

12 hours ago, simmo39 said:

Nice!

Thanks simmo!

8 hours ago, alacant said:

Hi

Lovely shot.

After 16 hours, the noise is most likely introduced by using in-camera bias and dark frames. The best way we find with an eosxxx is to simply subtract the offset -the 800 uses 2048- instead of applying the former. That, along with -at least- a 5 pixel dither between frames and stacking with a modern clipping algorithm should obviate the need for smoothing/denoise.

Cheers and HTH

Thanks Alacant! 

I don't fully understand what you are saying. I don't use LNER on the camera or any form of in-built camera noise reduction. Using bias frames I thought were required for flat frame calibration? I don't understand the technicalities of this, it's more just "do this, and don't do that!". Stacked with Siril, I would have thought it would have taken care of all of this?

But I am tempted to restack without darks to see what happens.

[alacant]

36 minutes ago, WolfieGlos said:

Using bias frames I thought were required

Subtraction of the bias, yes. Bias frames, no. On Eos we find that they introduce artefacts, including noise.

Siril makes offset removal easy. I explained how here;

There's also a link to the excellent  Siril documentation.

Cheers 

[windjammer]

>>To me, it looks like a space brain.....

Like something out of Mars Attacks!

>>I noticed a faint bubble just below-left of the centre of the image. 

I looked and looked but can't see it!

Simon

[WolfieGlos]

1 hour ago, alacant said:

Subtraction of the bias, yes. Bias frames, no. On Eos we find that they introduce artefacts, including noise.

Siril makes offset removal easy. I explained how here;

Oh, synthetic bias. I remember reading quite a heated debate over on CN suggesting never to use them....but then again, I've never tried it so happy to give it a go.

45 minutes ago, windjammer said:

>>To me, it looks like a space brain.....

Like something out of Mars Attacks!

>>I noticed a faint bubble just below-left of the centre of the image. 

I looked and looked but can't see it!

Simon

Yeah, exactly  

Here is the soap bubble, the whole noise reduced version of it. Nothing exciting on my image sadly. In fact, zooming in I really don't like how the nebulosity looks, much prefer it when the image is viewed as a whole. I might reprocess this again.

[windjammer]

Thanks, must get some new specs....

[WolfieGlos]

Haha, to be fair it's not really that obvious.

I've had a few goes at re-processing this image tonight, I feel it is an improvement on the original but still struggled to bring out the soap bubble. I think it just fundamentally needs more time (and aperture) to bring the faint structure out.

[GalaxyGael]

Visual observation and many earlier images showed the outer edge of the brain structure, and resembles a crescent. There are still a bundle of decade or more images around where the crescent part shows to be dominant. Nice image too

[ollypenrice]

Very good! Simple as that.

Olly

[WolfieGlos]

On 09/09/2023 at 07:14, GalaxyGael said:

Visual observation and many earlier images showed the outer edge of the brain structure, and resembles a crescent. There are still a bundle of decade or more images around where the crescent part shows to be dominant. Nice image too

Thanks! Makes sense actually, the individual subs only show the brighter parts so yes, I kind of see it. 

14 hours ago, ollypenrice said:

Very good! Simple as that.

Olly

Thanks very much Olly! 

[ollypenrice]

BTW, the 'Crescent' name makes sense if you observe this visually. Only the bright outer rim is visible and it does make a crescent shape.

Olly