Astrophotography workshop by Dale Liebenberg
posted: 2350 days ago, on Tuesday, 2012 Nov 13 at 12:56
tags: astronomy, astrophotography, ASSA, Dale Liebenberg.
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.