1. PhotoShop for Astronomy Session #1 – Levels & Curves AstroImager’s Special Interest Group April 2007

2. 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

3. 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

4. Suggested Resources Zone System for Astro Imaging, by Ron Wodaski PhotoShop Astronomy, by Scott Ireland Making Every Pixel Count, by Adam Block.

5. 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

6. 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 image Dim zone – Significant noise but can preserve details Middle zone – Noise is moderate Bright 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”

7. 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

8. 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

9. 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

10. “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

12. Unprocessed TIFF file showing little or no image detail

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

14. 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

15. 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

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

17. Step 4 – Levels: refine black point

18. 5 – Repeat last two steps until you can see dimmest details

19. 5 – Repeat last two steps until you can see dimmest details – this can take several iterations and adjustments become finer

20. 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)

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

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

23. Where do you go from here? Experiment with aggressiveness in settings Post JPEGs on Yahoo for review Follow-up questions & discussion on group Try 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!