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Old 11-03-2008, 04:47 PM
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iceman (Mike)
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Question Getting star colours with an unmodded DSLR

So what's the best way to get/preserve rich and deep star colours with an unmodded DSLR (my 350D, to be exact)?

I was of the mindset that long exposures helped to reveal the deep star colours - but in a recent image, the long exposure just saturated all the stars and they're almost all white (ISO800, 5 minute exposure on an ED80 (480mm,f/6)).

Should I use a lower ISO? Should I use shorter (1-2 minute) exposures? Obviously we want long exposures for the object itself - so then would you blend (in processing) the starfield from one set of exposures and the object from the other?

Or do I need to go the whole HDR/LDR process that Bert does to such great effect?

Thoughts and suggestions welcome.
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Old 11-03-2008, 05:28 PM
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I had the same thoughts...
HDR technique may be of some help here but it is very hard to avoid saturation..
Also, I was thinking of trying to add to stack the intentionally unfocused images, with stars that are not saturated, to get colours.
I am afraid that this is one of the shortcomings of electronic cameras....
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Old 11-03-2008, 05:31 PM
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[1ponders] (Paul)
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I think you've got it in 1 Mike. The unless the star is particularly red then, yes the long exposures saturate the stars.

You could try for a HDR process, but initially I'd suggest experimenting on the longest exposure you can go for and still maintain star colour and use that as your baseline for image capture. Of course if you are wanting to go for really faint stuff then you will most likely need to sacrifice your star colour for faint detail.
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Old 11-03-2008, 06:43 PM
tnbk00 (Daniel)
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Im not sure with regards to shooting stars (pardon the pun) but in order to get good colour saturation with terrestial things such as sunsets etc I have always found under exposing by a stop on the light meter works a treat. This is a practice that I have been told works with slide and film, and should translate directly to digital. I dont know how this will translate to astrophotgraphy.

Daniel
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Old 12-03-2008, 07:54 AM
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Quote:
Originally Posted by tnbk00 View Post
Im not sure with regards to shooting stars (pardon the pun) but in order to get good colour saturation with terrestial things such as sunsets etc I have always found under exposing by a stop on the light meter works a treat. This is a practice that I have been told works with slide and film, and should translate directly to digital. I dont know how this will translate to astrophotgraphy.

Daniel
It will translate directly, as you indicated.
The problem with digital is the linear response of the sensor, and the finite amount of electrons each pixel can hold. When this finite amount is reached, thats it, saturation is here.
Film suffers from the same problem but because the response is logarithmic,
it takes a lot more light to reach the saturation.

The problem with digital sensors is also scattering, which means the red light, for example will also saturate the green sensor next to it. The end result: stars with no colours.

The only way to help this situation is to use monochrome cameras with filters, and not DSLR's. The sensitivity will also be way better, up to 3x....
Or, to treat DSLR as a monochrome camera, and shoot with filters.
This will result in a need of prolonged exposure times, but we have a lot of that . Somebody should try this and let me know how it went.. I will not sue for the breach of patent rights for this idea


BTW, DSLR's are not astrocameras, they just happen to deliver acceptable results when used as such. So we have to compromise (unless we can spend k$ on REAL equipment, and many of us just have other priorities in their lives)
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Old 12-03-2008, 08:49 AM
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I wonder if you could to an LRGB arrangement with layering, with a shorter exposure for the colour values of the high intensity areas and a longer exposure for the luminance and dimmer region colour?
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Old 12-03-2008, 10:43 AM
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I find if it's a shot of a globular or open cluster you can boost up the saturation in photoshop a little and it gives stars a little extra colour, but if there is a nebulae or galaxy in the shot it can affect it, so you'd have to mask it out.
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Old 13-03-2008, 09:24 PM
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The only way to get all the star colours is to have a high dynamic range. The sixteen bits of an astro CCD will give you 64k levels. The meagre twelve bits of a DSLR will give you about 4k levels. Both will saturate very quickly with the brighter stars if you are trying to collect dim nebular data with a reasonable signal to noise.

The pixels just outside the brighter stars have their colour information as a 'ring' of colour. This method may help if you don't want to go the HDR track. This would work better with larger image scales ie about one second of arc per pixel.

http://www.waidobservatory.com/tutor...tar-color.html

I have found that using HDR gives me 20+ bits of dynamic range ie 1M of levels before converting to LDR. This handles most star colours even with widefields where there is a very wide dynamic range. Just check out

http://members.optusnet.com.au/~avandonk1/rho.jpg

or any of my last few posts in deep sky at IIS.

Bert
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