Re: [APML] NGC 7023 aka Iris Nebula

Tony,

I've been doing it that way for years. Actually most CCD imagers

do exactly what you did. Only a small minority do 1:1:1 and then

rescale the exposures. You've rediscovered the wheel <g>.

Rob Gendler
Email: robgendler@att.net
Web site: http://www.robgendlerastropics.com/

----- Original Message -----

From: Tony Hallas

To: Discussion of Film Astrophotography

Sent: Thursday, October 23, 2003 9:20 PM

Subject: Re: [APML] NGC 7023 aka Iris Nebula

Rob,

    Let's say that you shot 10 each 600 second exposures of the red, the green, and the blue with an ST-10E camera and typical SBIG filters ... (I think the ratio of exposure times to get "normal" color is something like R=1.0 G=.9 B=1.7 or 1.8 ... this assumes that you are not going to do any stretching of the data to achieve neutral color). So ... if you were to combine the equal exposure times your image would come out really yellow ... not enough strength in the blue.

    To get normal color out of those equal exposure times, you are going to have to stretch the blue values ... multiply what you have by 1.7 to get the correct strength. Uh - oh ... when you do that, your NOISE just went up 1.7 times !

    If the S/N of the red and green are approximately 1.0, then to get the same S/N out of the blue when it is stretched, it needs to have 1.7X the S/N compared to the R&G. Now when the blue is stretched and the S/N decreases, it comes back to 1.0 just like the red and green. So we need to shoot a bunch more of the 600 second blue exposures to increase our S/N ... we know that later we will be stretching the blue data to get neutral color.

    The huge advantage with this is having only one master dark frame ... oh that is soooo nice!

    Hope this makes sense ...

           Tony

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