PhotoShop for AstronomySession #1 – Levels & Curves

AstroImager’s Special Interest GroupApril 2007

Image Processing Workshop Outline

i. Levels & curves

ii. Sharpening & blurring

iii. Zone selection

iv. Star shaping

v. Color balancing

vi. Mixing Ha & color data

vii. PS add-ins for gradient & noise reduction

Workshop Goals

One-hour length with room for discussion

Files distributed in advance

PowerPoint file uploaded after meeting

Hands-on processing of basic process covered

“On-topic” discussion encouraged/needed

Suggested Resources

Zone System for Astro Imaging, by Ron Wodaski

PhotoShop Astronomy, by Scott Ireland

Making Every Pixel Count, by Adam Block.

Image Processing Workflow

File Preparation: Image calibration & combination, pixel cleanup

Basic Histogram Stretching – Levels & Curves

Smooth the Dim Zones

Sharpen the Bright Zones

Special Problems – Star Shaping, Gradients, Artifacts

Color Combination

Final Adjustments, Printing & Publishing

Wodaski’s Zone System

“Each image is broken down into multiple zones. By applying the appropriate type of processing to each zone, you can bring out the best in any image”

Dark zone – Noise is extreme & should be eliminated from imageDim zone – Significant noise but can preserve detailsMiddle zone – Noise is moderateBright zone – Noise is minimal

“The lower the noise level in any part of the image, the more sharpening you can perform. The higher the noise level, the more you need things like smoothing”

Definitions

Histogram – a graph that shows the relative number of pixels at each brightness value

Noise – random uncertainty in brightness values

System noise – repeatable noise that can be eliminated

Sharpening – image processing technique that empasizes details

Smoothing – image processing technique to reduce noise

Black point – sets the darkest value in the image

White point – sets the brightest value in the image

Definitions

Clipping – when a curve reaches the top line of the graph before reaching the right side of the graph

Curves – PS tool for making non-linear adjustments to the histogram – fixed number of levels so if you stretch one image section, you must contract another section

Levels – PS tool for making linear adjustments to histogram

Linear – adjustments that preserve internal brightness relationships

Non-linear – adjustments that do not preserve brightness relationships – part of image may get a boost in brightness where another may actually be dimmed

PS Processing Checklist – Basic Histogram Stretching

1 – Levels: set white point

2 – Levels – set initial black point

3 – Curves: use Standard Curve to expand dim and compress bright areas

4 – Levels: refine black point

5 – Repeat last two steps until you can see dimmest details

6 – Curves: determine the value of the dimmest visible details

7 – Curves: Boost dimmest details

8 – Curves: balance brightness levels between zones and make any inter-zone contrast adjustments

“The Standard Curve contains a point to brighten the Dim Zone; a point to establish the boundary between expansion and contraction and if necessary a Control Point to yield a flat line to the top right corner (eliminates clipping)”

Curve Point 1 – place where at dimmest area of image and use a steep slope to brighten Dim Zone aggressively

Curve Point 2 – place where there is visible but moderately bright detail and use a less aggressive slope to reduce the impact on the Middle Zone

Curve Point 3 – place at bright spot still not saturated and make the curve “flat to finish” to eliminate clipping

Unprocessed TIFF file showing little or no image detail

Steps #1 & #2 – Levels: set white point & initial black point

Curve Point 1 – place where at dimmest area of image and use a steep slope to brighten Dim Zone aggressively

Step 3 – Curves: use Standard Curve to expand dim and compress bright areas

Curve Point 2 – place where there is visible but moderately bright detail and use a less aggressive slope to reduce the impact on the Middle Zone

Curve Point 3 – place at bright spot still not saturated and make the curve “flat to finish” to eliminate clipping

Step 4 – Levels: refine black point

5 – Repeat last two steps until you can see dimmest details

5 – Repeat last two steps until you can see dimmest details – this can take several iterations and adjustments become finer

Other tools can adjust the final histogram but take care to prevent clipping

Brightness/Contrast command makes the same adjustment to every pixel in the image (linear adjustment)

Shadows/Highlights enables separate controls of the shadows and the highlights and is useful where astronomical targets have “burned out” areas

Step 8 – Curves: balance brightness levels between zones and make any inter-zone contrast adjustments

Where do you go from here?

Experiment with aggressiveness in settingsPost JPEGs on Yahoo for reviewFollow-up questions & discussion on groupTry other data files

Practice, practice, practice , practice, practice , practice, practice , practice, practice , practice,

practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice, practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice , practice, practice

And practice some more!