Re: [APML] NGC7822, Ced214, and Sh2-170

[Chuck Vaughn <aa6g@aa6g.org>]

Loke,

I understand what you're saying but max S/N and optimum exposure
time for color balance may not be mutually exclusive. Max S/N
occurs once the film is exposed enough so that the sky fog level
is above the toe of the curve and the brightest parts haven't
reached the shoulder.

This whole thing is object dependent but objects that are all
low brightness like the Witch's head, should allow for some latitude
in exposure without severely impacting S/N since I doubt their
brightness range would fill up the entire linear part of the curve.

If you're going to expose each channel for max S/N then you might
as well start calling them "Maximum S/N False Color Images."<g>

Chuck <aa6g@aa6g.org>

-------------------------------------------------
 
> Chuck,
> 
> I see what you mean about the color of the sky fog effecting the
> exposures. This never occured to me <g>. I agree that the exposure
> ratios should be set according to your technique of the graduated
> grayscale for correct color balance. However, given that the exposure
> ratios have been determined in this manner, my thoughts were that if I
> exposed to the sky fog limit for each filter then I would obtain the
> best signal to noise ratio for each channel at the expense of making
> color accuracy secondary. Later on during Photoshop processing, color
> can be somewhat post-compensated. This way I get the best overall SNR
> with reasonably accurate color.
> Alternately, if I had stuck to the correct color ratios all the time,
> there will be two channels which will be exposed past sky fog limit and
> might be too dense. I am assuming all 3 RGB channel have a different sky
> fog level after normalizing with the correct exposure ratios.
> Alternately, I can expose so that only one of the channels is at the sky
> fog limit, then I would sacrifice SNR for the other two channels. At any
> rate, these were my thoughts concerning the different exposure ratios I
> had used in the past.
> 
> Loke
> 
> 
> 
> Chuck Vaughn wrote:
> 
>> Loke,
>> 
>> The correct exposure ratios are not necessarily where the sky fog is the
>> same on all three. The color of the sky fog influences that.
>> 
>> The correct exposure ratios are those that create the same densities
>> of a grayscale step chart through each filter. With my filter set that
>> means the blue and green require twice the exposure as the red.
>> 
> <snip>
> 
> 
> 
> --  APML Archives at <http://astro.umsystem.edu/apml/>  ---
> Unsubscribe at <majordomo@seds.org>


--  APML Archives at <http://astro.umsystem.edu/apml/>  ---
             Unsubscribe at <majordomo@seds.org>