posted: 1656 days ago, on Tuesday, 2012 Nov 13 at 12:56
tags: astronomy, astrophotography, ASSA, Dale Liebenberg.

Tweet this

During the 9th ASSA Symposium in Cape Town, Dale Liebenberg presented an astrophotography workshop during which he discussed CCDStack and Photoshop for the processing of LRGB and narrow-band images.

A summary of Dale's workflows, as demonstrated during the workshop, follows:

Workflow: CCDStack

1. Load

2. Select frames

3. Calibrate

4. Register (align)

5. Normalize

6. Reject data

7. Combine

8. Deconvolution

9. If colour camera, you are done. For mono-camera, repeat the above for R, G and B images, and align

10. Open in Photoshop

Workflow: Photoshop LRGB processing

1. Convert to 16-bit mode

2. Levels to do a linear stretch

3. Curves to do non-linear stretch

4. Repeat (2) & (3)

5. Combine

6. Lab colour enhancement on colour layer

7. Flatten

8. Noise reduction

9. Sharpening the stars

10. Sharpening the nebula (with layer mask)

11. Adjust contrast and brightness

Workflow: Photoshop narrow-band processing

1. Convert to 16-bit mode

2. Levels to do a linear stretch

3. Curves to do non-linear stretch

4. Repeat (2) & (3)

5. Assign colour to each image (Hubble palette)

6. Adjust colour of each layer (curves, with clipping mask)

7. Adjust star colours

8. Bob Franke's re-colouring

9. Noise reduction

10. Sharpening

The accompanying video (3h30min) [watch it on YouTube] is an unedited recording of the workshop; the index below is time-stamped for easy reference.

Introduction

00:07:30 Calibration: Bias frames

00:09:25 Calibration: Dark frames

00:14:40 Calibration: Flat frames

Using CCDStack

00:19:44 Exercise: Using CCDStack to create an LRGB image of NGC 4945

00:20:55 Loading the image files

00:21:30 Question: DSLR/One-shot colour camera vs. mono-CCD

00:24:00 Question: binning

00:27:16 Question: luminance

00:29:30 Question: binning, DSLR, binning & calibration

00:30:50 Question: infra-red & DSLR, narrow-band imaging

00:36:00 CCDStack: Loading the image files

00:37:15 CCDStack: window: stretching

00:38:15 CCDStack: raw frames loaded, check with "Blink"

00:39:08 Dithering

00:40:20 Question: sub-pixel sampling, drizzle

00:41:40 CCDStack: Calibrate (dark, bias and flat)

00:42:54 Question: DSLR

00:43:35 CCDStack: Register the images ("Stack", "Register", FFT", "shift, rotate and scale")

00:46:40 Comment: Free software: Deep Sky Stacker

00:46:58 CCDStack: registration done

00:47:05 CCDStack: Apply the alignment ("Apply", "Nearest neighbor")

00:47:50 CCDStack: Check alignment with "Blink"

00:48:17 CCDStack: If alignment not OK, Register again (perhaps with different methods, different base image)

00:49:30 CCDStack: Check alignment with "Blink"

00:49:55 CCDStack: If alignment not OK, Register again (perhaps with different methods, different base image)

00:51:37 CCDStack: Check alignment with "Blink" (throw out problem images)

00:53:15 Question: loss of information during alignment

00:55:32 CCDStack: Loading the image files

00:55:05 CCDStack: Calibrate (dark, bias and flat)

00:57:00 CCDStack: Register the images ("FFT", "shift, rotate and scale")

00:57:30 CCDStack: Apply the alignment ("Apply", "Bicubic B-Spline")

00:57:55 Question: Using Photoshop for registration

00:59:20 CCDStack: Normalize ("Stack", "Normalize", "Control", "Both")

00:59:34 CCDStack: Select background area with few stars

00:59:55 CCDStack: Select highlight area

01:00:12 CCDStack has weighted the images (see "Window", "ImageManager", last column)

01:00:40 CCDStack: Data rejection ("Stack", "Data Reject", "Procedures")

01:00:52 Distributions

01:02:20 Select value for rejection factor (smaller = more critical)

01:03:47 Question: histogram

01:04:05 CCDStack: Combine the images ("Stack", "Combine", "Mean")

01:04:30 Combine done.

01:04:44 In ImageManager, compare the new (stacked) image to the others

01:05:30 Question: alignment biased to the centre of the image

01:05:55 Success! Final image

01:06:10 Image stretching

01:06:40 Deconvolution ("Process", "Deconvolve") to make the stars round

01:09:12 Question: ImageManager FWHM column

01:09:44 Check that deconvolution has modified only the stars and not the target too

01:10:40 In ImageManager, compare the new (deconvolved) image to the previous one

01:11:41 Chris Forder explains how to detect black holes

01:11:54 Histogram ("Window", "Histogram") and screen stretch ("Window", "Adjust Display", "auto scale")

01:13:20 Stretching 32-bit images & Photoshop

01:14:24 Open L, R, G, B images and align them

01:14:40 Could use Photoshop to combine the four images IF they are aligned

01:15:32 CCDStack: create a colour image ("Color", "Create")

01:16:06 CCDStack: select background colours, adjust colours

01:16:40 Question: filter factors

01:17:16 Colour image done!

01:17:25 Colour and Luminance files now ready for Photoshop

Using Photoshop for LRGB processing

01:18:45 FITS file format

01:23:30 Opening the sample 32-bit L and colour files in Photoshop (NGC4945-Lum.psd & NGC4945-RGB.psd)

01:24:30 "Window", "Histogram"

01:25:10 Convert to 16-bit ("Image", "Mode", "16-bit", Preset=Default, Method=Exposure & Gamma)

01:27:15 Examining the histogram

01:27:40 Stretching

01:28:04 "Image", "Adjust", "Levels"

01:28:20 Linear stretch, adjust only black and white points, without clipping

01:29:30 Question: DSLR

01:29:41 Question: Levels midpoint

01:30:25 Start a series of curve (non-linear) stretches

01:30:28 "Image", "Adjust", "Curves"

01:31:05 (Curves displayed from dark to light: select "Show Amount of - Light (0-255)")

01:31:24 Question: Curves not enabled - check that you are in 16-bit mode

01:32:02 Stretch dark and bright part of the curve

01:33:10 Adjust the other image

01:33:43 Check and adjust black point with "Levels"

01:35:42 Adjust "Curves" again

01:36:12 Question: why this approach?

01:36:53 Question: different strategy for L and RGB images?

01:37:23 When to stop?

01:38:02 Importance of stretching of L vs. RGB

01:39:05 Question: When to stop based on histogram?

01:41:50 Histogram spiking = stop

01:42:45 Question: spikes at the bright end of the histogram?

01:43:27 Goal: high contrast in L image, keep stars small, background dark

01:43:49 Question: why not use "Contrast"

01:44:23 Rename layers to avoid confusion when combining

01:45:01 Combine the separate L and RGB images

01:45:22 Place L on top of the colour, close the L file

01:46:26 One file, two layers

01:46:56 Check that the layers are properly positioned

01:48:01 SaveAs

01:48:24 Question: undo, stepping back with History

01:49:28 Assign layer blending to each layer ("Layers", "Luminosity") and ("Layers", "Color")

01:50:37 Before merging layers, examine Colour layer, modifying it with Curves on the Colour layer only

01:51:39 Adjusting colour using Lab colour ("Image", "Mode", "Lab color" -> "Don't Merge")

01:52:26 Open "Curves", "Channel", "a"

01:53:33 Adjust red vs green balance (channel a)

01:54:37 Adjust yellow vs blue balance (channel b)

01:55:27 Revert to RGB colour ("Image", "Mode", "RGB color")

01:56:27 Question: How Photoshop's transforms work; use of Adjustment Layers

01:59:20 Recap on colour manipulation; why not to use Hue and Saturation; Lab manipulation

02:04:16 Flatten the image (right click on any layer in the "Layers" palette, select "Flatten Image")

02:05:07 Examine the noise; zoom in on the image

02:05:33 Noise reduction with Photoshop: "Filter", "Noise", "Dust & Scratches..."

02:06:05 Noise reduction with Photoshop: "Filter", "Noise", "Reduce Noise..."

02:07:12 Noise reduction with Photoshop: Blurring: "Filter", "Blur", "Gaussian blur"

02:08:18 Use Lasso Tool (generous Feather) and blur just the outer regions

02:10:52 Noise reduction with NeatImage plugin

02:13:03 Check black point of Levels in case noise has been introduced

02:13:35 Sharpening using a filter (two steps)

02:14:11 Sharpen whole image, using a high-pass filter

02:14:15 Select duplicate layer, "Filter", "Other", "High Pass", selecting a small radius

02:15:10 Set duplicate layer's blending mode to "Overlay"

02:16:02 Check that halos around stars haven't been created

02:16:19 Merge the layers

02:16:37 Repeat

02:16:49 Do high-pass (large radius) to bring out the galaxy

02:17:50 Using a layer mask ("Layer", "Layer Mask")

02:18:34 Now selectively colour in the galaxy on the mask, with the paintbrush

02:20:53 Question: star removal methods?

02:21:20 Painting on the mask to reveal sharpening of selected parts

02:23:36 Flatten the image

02:23:57 Question: Use of unsharp masking?

02:26:53 Adjusting the contrast and brightness ("Layer", "New Adjustment Layer", "Curves")

02:27:14 Brightening the image by pulling the curve up

02:27:27 Increasing the contrast by using an S-curve

Using Photoshop for narrow-band processing

02:30:40 Narrow-band image processing: open three files (SII Deconvolved, OIII Deconvolved, Ha Deconvolved.psd)

02:31:00 Question: Luminance images?

02:32:25 Change images to 16-bit

02:32:34 Question: Always use "Exposure and Gamma" method when changing to 16-bit?

02:33:40 Principle: when stretching, try not to make the stars too bright

02:34:21 Stretch ("Levels") the H-alpha image (black point = 5, white point = 29)

02:36:04 Oxygen filter, star colours, moonlight

02:36:43 Stretch ("Levels") the O-III image (black point = 5, white point = 81)

02:38:07 Stretch ("Levels") the S-II image (black point = 4, white point = 64)

02:39:00 Apply "Curves" to H-alpha (use detailed small-block grid: centre 1 block left and up; tail 1 block down)

02:39:47 Apply "Curves" to O-III (use detailed small-block grid: centre 1 block left and up; tail 1 block down)

02:40:33 Apply "Curves" to S-II (use detailed small-block grid: centre 1 block left and up; tail 1 block down)

02:41:15 Stretch ("Levels") the H-alpha image (black point = 1)

02:41:36 Stretch ("Levels") the O-III image (black point = 5)

02:41:56 Stretch ("Levels") the S-II image (black point = 10)

02:42:24 Apply "Curves" to H-alpha (use detailed small-block grid: centre 1.5 block up, tail half-block down)

02:42:51 Apply "Curves" to O-III (use detailed small-block grid: centre 1.5 block up, tail half-block down)

02:43:06 Apply "Curves" to S-II (use detailed small-block grid: centre 1.5 block up, tail half-block down)

02:43:34 Examine three images for star brightness

02:44:02 Stretch ("Levels") the H-alpha image (black point = 2)

02:44:17 Stretch ("Levels") the O-III image (black point = 3)

02:44:30 Stretch ("Levels") the S-II image (black point = 6)

02:44:50 Apply "Curves" to H-alpha (use detailed small-block grid: centre 1 block up, leave the tail pont as is)

02:45:17 Question: what does pulling down the right-hand side of the curve do?

02:45:41 Apply "Curves" to O-III (use detailed small-block grid: centre 1.5 block up, tail half-block down)

02:46:10 Apply "Curves" to S-II (use detailed small-block grid: centre 2 blocks up, tail half-block down)

02:47:00 Rename the layers of the three files to the names of the filter, to avoid later confusion

02:47:36 Stacking; Hubble palette (RGB = Sulphur, Hydrogen, Oxygen)

02:47:56 Use hydrogen as base layer

02:48:44 Shift+drag oxygen layer onto hydrogen layer, then Shift+drag the sulphur. Close the oxygen and sulphur files.

02:50:00 Change image mode to RGB

02:50:36 Colourize the images using adjustment layers.

02:50:58 Select the hydrogen layer, "Layer", "New Adjustment Layer", "Hue/Saturation", "Use Previous Layer"

02:53:20 Select "Colorize"

02:55:13 Hue = 124 (green), Saturation = 100, Lightness = -50

02:56:07 Select the oxygen layer, "Layer", "New Adjustment Layer", "Hue/Saturation", "Use Previous Layer"

02:56:50 Hue = 240 (blue), Saturation = 100, Lightness = -50

02:57:23 Question: hue setting: wavelength value, colour wheel

02:58:30 Select the sulphur layer, "Layer", "New Adjustment Layer", "Hue/Saturation", "Use Previous Layer"

02:58:49 Hue = 360 (red), Saturation = 100, Lightness = -50

02:5927 RGB image: Select the "sulphur" layer and set its mode = "Screen"; same for "oxygen" layer

03:04:40 Save

03:05:26 Push the blues and reds

03:05:42 New adjusment layer, "Curves", "clipping", on the sulphur

03:06:10 New adjusment layer, "Curves", "clipping", on the oxygen

03:06:33 Adjust the curve of the sulphur layer

03:08:02 Adjust the curve of the oxygen layer

03:09:29 Merge the image

03:10:07 Modify the colour of the stars

03:10:20 Remove the smallest stars with "Dust and Scratches"

03:10:59 Attempt to select the stars by using highlights: "Select", "Color Range", "Highlights"

03:11:15 Picks up stars as well as bright regions of nebula; subtract nebula region with lassoo tool

03:12:09 Increase the selection size ("Select", "Modify", "Expand") to include outer parts of the stars

03:13:21 Prevent a hard edge to the selection by applying a feather ("Select", "Modify", "Feather", radius = 2)

03:13:45 Take the colour out of the selected stars ("Image", "Adjustments", "Hue/Saturation")

03:14:25 Clean up the odd magenta stars ("Image", "Adjustments", "Selective Color")

03:15:13 Desaturate the odd magenta stars ("Image", "Adjustments", "Hue/Saturation", dropdown box: "Magentas")

03:16:40 Another way to remove star colours, by Bob Franke: ("Filter", "Noise", "Reduce Noise", Reduce Color Noise = 100%)

03:17:30 Bob Franke's re-colouring approach, with selective colour

03:17:35 ("Image", "Adjustments", "Selective Color") Green: Cyan=-100, Magenta=-25

03:17:35 ("Image", "Adjustments", "Selective Color") Yellow: Cyan=-100, Magenta=+25

03:19:55 ("Image", "Adjustments", "Selective Color") Cyan: Magenta=-25, Yellow=-100 {Magenta=0% ?}

03:20:30 ("Image", "Adjustments", "Selective Color") Cyan: Magenta=-25, Yellow=-100

03:21:11 Use History to review the changes

03:21:52 Alien civilization detected

03:23:03 Sharpening with an unsharp mask ("Filter", "Sharpen", "Unsharp Mask")

03:24:50 Sharpening with a high-pass filter

03:26:12 Rotating the image using Straighten with the Ruler tool

03:28:38 Done!

nothing more to see. please move along.