From: Milton Aupperle <milton@outcastsoft.com>

Date: November 10, 2005 2:35:11 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: Stacking and Sharpness estimation.

Hi Folks;

One thing I'm working on is a pictorial guide to interpreting the Histogram sharpness curves, what it means with image sharpness and some idea as to how to set your cutoff value. Here is a quick verbal run down of it. I don't have the images put together yet, but here's more or less the text for it.

Note that in all the cases described below, the base assumption for estimating sharpness is that some frames will be sharper than others. If the image is out of focus and consistently blurry to start with, it may just give you the "Type 1" or "Type 2" histograms. Also this applies to stacking for Planetary, Lunar or Solar imaging where sharpness is important. For DSO's or longer exposures you normally stack just to suppress noise and sharpness of each is of much less concern.

You get an indication of how consistent the frame sharpness is is by looking at the Min - Max value range, the STD68 or STD96 standard deviation values and how "skewed the curve is. The larger the Min - Max range, the more variance in frame sharpness and the higher you want to increase the Confidence Interval Slider. The Skew of the curves is indicated by comparing at the Mean and Median values and if they are the exact same, the histogram is not skewed. Visually a skewed histogram has a long tail or long nose and the peak of the hump is shifted to towards the low or the high end of the histogram. The skew direction tells you whether you have more sharp frames than unsharp frames. Generally you want to use a Confidence Interval that selects (ie hilights in red) the sharpness bars that are to the right of the central hump.

Basically you will encounter the following main types of histograms (they will grade into each other):

1) Single bar histogram, which means all frames are almost the same sharpness. That's virtually impossible because of turbulence and the fact that image gain will also be interpreted to a small degree as an indication of sharpness. In this case you can accept a very high (i.e.100% Confidence Interval) of the frames, depending on how many frames you want to use in your stack.

2) A single unskewed symmetric "hump" with a narrow range of sharpness values and a small STD96 value of probably 0.05 and lower. This is the curve you get on good imaging nights or if you take lots of sample under consistent skies. With this you should select a Confidence Interval percentage that is to the right of the central hump in around the 50 to 75% range, depending on how many frames you want to use in your stack.

3) Skewed hump with longer left tails or longer right noses. The direction of skewing indicates whether you have a larger percentage of sharp frames or a larger percentage of unsharp frames. If the Median is greater than the Mean value then it's skewed to the right (has a longer left tail) and this means it has more sharper frames than unsharp frames. If it's skewed to the left, then it has more unsharp frames than sharp frames. With this type of curve you want to be more restrictive in how many frames are used and probably want to go with a 65 to 80% Confidence Interval. If it's skewed to the right, I tend to increase the Confidence Interval value and if it's skewed to the left, I decrease it.

4) The double or "bimodal" hump which may have almost no "Skew" at all, high STD96 value > 0.1 and a large range in the Min Max range. It means means you have a percentage of really blurred frames (lower left hump) and a percentage sharper frames (upper right hump). In this case you'll want to use CI percentage that selects frames that are to the right of the upper right hump, usually that means it's in the 70 to 80% range.

5) Undulating hump, with multiple peaks, wide range of values and very wide STD96 value. I generally throw these out as just "bad" and don't process them.

Hope that helps someone.

Milton J. Aupperle

President

ASC - Aupperle Services and Contracting

Mac Software (Drivers, Components and Application) Specialist

#1005 - 815 14th Avenue. S.W.

Calgary Alberta Canada T2R0N5

1-(403)-229-9456

milton@outcastsoft.com