RE: {Altair_B - 5306} -: Re: Orion's Belt -color spaces pt 2

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John Murrell

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Jan 30, 2015, 4:02:07 PM1/30/15

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Paul,

See http://www.cambridgeincolour.com/tutorials/color-management1.htm and the following two web pages linked to it for an explanation. However it is not specific to Astro Photography

John Murrell

Astronomer Without Portfolio

Website www.JohnMurrell.org.uk

From: alta...@googlegroups.com [mailto:alta...@googlegroups.com] On Behalf Of John Murrell
Sent: 30 January 2015 20:51
To: alta...@googlegroups.com
Subject: RE: {Altair_B - 5306} -: Re: Orion's Belt

Paul,

There is quite a lot on color spaces on wikipedia see http://en.wikipedia.org/wiki/List_of_color_spaces_and_their_uses and http://en.wikipedia.org/wiki/Color_space.

The diagrams on these pages show ‘all possible colours’ and the labelled triangles and pentagons show the limitations of particular colour spaces. Colours outside the boundary of the particular triangle or Pentagon cannot be reproduced correctly.

The diagrams are not true to life as they have to be produced using the colour space on the image + web browser + monitor. That combination cannot reproduce anything outside it’s own colour space.

What I am not sure about is the impact on images taken by a camera and the subsequent processing. I suspect that Adobe Photoshop will work in work in the Adobe RGB colour space and most other applications work in sRGB.

For astronomical images I suspect it does not make much difference as the final image is adjusted to be ‘pleasing’ rather than true colour. What would be interesting to see is where some of the common astrophysical emission lines sit on the diagram. H Alpha is obviously in the red corner but where. Are the numbers on the border of the figure below related to the wavelength (in nm) ?

I guess (but have no evidence) that colours outside the boundaries are reproduced as the ‘nearest’ colour within the boundary.

"CIE1931xy gamut comparison" by BenRG and cmglee - http://commons.wikimedia.org/wiki/File:CIE1931xy_blank.svg. Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:CIE1931xy_gamut_comparison.svg#mediaviewer/File:CIE1931xy_gamut_comparison.svg

John Murrell

Astronomer Without Portfolio

Website www.JohnMurrell.org.uk

From: alta...@googlegroups.com [mailto:alta...@googlegroups.com]
Sent: 30 January 2015 17:28
To: alta...@googlegroups.com
Subject: Re: {Altair_B - 5304} -: Re: Orion's Belt

Hi Graeme,

I also have a Nikon but a D3100 and in the settings there is a choice called color space, when selected you get two options (1) sRGB and (2) Adobe RGB can you explain the difference please. Couldn't find anything in manual.

Paul

-----Original Message-----
From: Roy Easto <roy....@btinternet.com>
To: altair_b <alta...@googlegroups.com>
Sent: Thu, 29 Jan 2015 23:43
Subject: Re: {Altair_B - 5301} -: Re: Orion's Belt

Hi Graeme,

John showed me an article about the median filter with example images. I thought the example they showed wasn't too bad and not bad enough for me to ditch my camera. I guess I'll just have to try the RAW files and see how they go.

Roy

On 29/01/15 10:40, Graeme Coates wrote:

Roy,

I believe the Nikons use a weak median filter in post-processing (even on the RAWs) across the range which will blat out a lot of the fine detail in an image (especially faint stars). You’re much better off using “Mode 3” as per Christien Buil’s page: http://www.astrosurf.com/~buil/d70v10d/eval.htm which details this on a D70s (it’s easy to try it and analyse it if there’s doubt as to what it does).

You’ll have to dark frame subtract from those RAWs, but you’re likely to get much better detail and a higher limiting magnitude. Flat fields might not be needed here, but shouldn’t be hard to take (point it at a blank screen) and you could easily compare.

GC

From: alta...@googlegroups.com [mailto:alta...@googlegroups.com] On Behalf Of Roy Easto
Sent: 29 January 2015 09:17
To: alta...@googlegroups.com
Subject: Re: {Altair_B - 5299} -: Re: Orion's Belt

ISO1600. To choose the ISO I take a photo at the longest exposure I can manage from my fixed tripod with acceptable trailing. I then choose the ISO where the sky background is visible at perhaps a level of 50 out of 255 so that I am getting the noise in the image and therefore the faintest stars. If I had a driven tripod I would increase the exposure and drop the ISO so the background was the same level but the noise would be reduced. My lenses are old and have no VR.

For stacking I have my own python app that will stack all the images in a folder. Taking 8 bit images, offsetting and rotating each one without intervention and creating a 16bit per colour fits image. I further have a simple routine to remove the sky background. It uses a gaussian filter to create a blurred copy of the image (radius 100 pixels). I then take a minimum of this and my image and create another blurred image of this. The resulting image is very much like the sky background without any detail. I then subtract 90% of this from the image removing the background. It is not as good as flat fielding but is close and requires no extra time on flat fielding. I should also then use the blurred image to remove the vignetting but it is not severe so I have not done this yet.

I have not used the raw images yet, I just use the fine setting which gives me 6Mb images, the compression seems not to leave noticeable effects. I should try raw images out but I probably need to understand more of what the processes images give me, perhaps if I use raw images I will have to do flat fields too?

Roy
On 29/01/15 08:25, Chris Green wrote:
Roy, thanks for posting the image.

I also have a Nikon DSLR, D70s, so I'd be interested to know what film speed you used and any other important settings. I assume you turned the lens VR off and used a tripod? Also, did you take RAW images and stack them in Registax?

Chris

On Thursday, January 29, 2015 at 7:36:56 AM UTC, Roy Easto wrote:
Just finished processing my images from last weekend. Attached is a
stack of 20, 2 second images taken from my back garden of Orion's Belt.

Camera Nikon D60 with 200mm telephoto at F4.

Faintly visible at the left is the Flame nebula. Faintest stars are at
magnitude 12.

John Murrell, I used Aladin to view the images as I don't know how to
get ds9 to display images in colour.

I now have to make by equipment portable to get away from the terrible
light pollution.

Roy
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