From: "Tim" <tjp314@pacbell.net>

Date: November 10, 2005 3:35:29 PM MST

To: Astro_IIDC@yahoogroups.com

Subject: Re: Stacking and Sharpness estimation.

Thanks for this, Milton! I need to go back and look at some of my better videos and see

whether I might be able to pick them off at a more sensible CI value.

Oh, and I've had the bimodal histograms a few times. I thought I might be losing my

mind, so it's good to know what that indicates (basically what I thought).

-Tim.

--- In Astro_IIDC@yahoogroups.com, Milton Aupperle <milton@o...> wrote:

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@o...