Chapter 1: Introduction to GoodLook Suite

Purpose, file formats, and overview of workflow

Back up your computer before using.This suite remains experimental. Reliability is unknown. It could corrupt your system or personal records and data. It is offered for free, on the understanding that you will take steps to protect your system and data.

This document is set out as a tutorial. It is intended to be read in sequence, from start to finish, trying things out as you go. The manual is intended to be read on the screen, not printed. It is most convenient to use two screens, running the GoodLook suite on the primary screen, with the manual on the secondary screen (extended desktop) or on another computer.

The GoodLook suite (including Mushroom and Prometheus) together provide all you need for basic to moderately advanced deep sky astronomical image processing.

  • GoodLook displays, explores, and enhances astronomical images. Tools include wavelet noise reduction, background flattening, and deconvolution.
  • Mushroom creates and applies master bias, dark, and flat calibration frames. It converts Bayered images into colour images.
  • Prometheus registers and stacks images with sub-pixel accuracy.

Hardware

The minimum hardware set is not known, but GoodLook works fine on 32 or 64 bit versions of Vista or Windows 7. It works on a single core machine but much better on a machine with at least 4 cores. (Reason:Image update is multi-threaded, even for monochrome images. Wavelet noise filtering and deconvolution are multi-threaded for colour images). GoodLook itself only uses 2GB of memory but a machine with at least 8Gb is suggested. A 1920 x 1080 or larger screen is strongly recommended.

Supported File Formats

The GoodLook suite can read or write any of the following:

  • Uncompressed 16 bit TIFF. Excellent for capture.
  • Uncompressed 32 bit floating point FITS. Excellent for capture but twice the disc space.Good for just-stacked images.
  • Uncompressed 8 bit TIFF. Only good for finished images.
  • JPEG. Only good for emailing finished images.

If at all possible, capture your astroimages, darks, and flats as uncompressed 16 bit TIFF or as uncompressed 32 bit floating point FITS. Never capture as JPEG.

GoodLook can NOT read:

  • Canon CRW, Nikon NEF, or any other proprietary formats
  • Compressed images other than JPEG

If you cannot capture as uncompressed 16 bit TIFF or as uncompressed 32 bit floating FITS, you may be able to use software that came with your camera, or possibly a general purpose program such as Adobe LightRoomto convert to a format that GoodLook can read.For example, if you have captured as a Nikon NEF file, use LightRoom to export your entire batch of images as 16 bit uncompressed TIFF. Be aware that Adobe PhotoShop 5 out of the box appears to import NEF files as 8 bit, which is useless for astrophotography.

Why uncompressed?

  • There are too many compression algorithms out there to support them all.
  • Uncompressed images load hugely faster than compressed images.
  • Disc space is cheap but life is short.

Warning: Windows Photo Viewer will compress a TIFF file if you rotate it. Use GoodLook to view and/or rotate your images! If you accidentally compress a file, you must use PhotoShop or similar to uncompress it.

Suggested Directory Structure

Where should you keep your original images, your calibration files, your calibrated images, and your processed images?

The GoodLook suite works best if you keep files of the same type together. For example, keep all your shots of the Horsehead taken through an H-alpha filter on a particular night together in one directory. I use a deeply nested hierarchical structure. At the top level it looks like this:

  • Calibration
  • Galaxies
  • Clusters
  • Star-Forming Regions
  • Supernova Remnants
  • Wolf-Rayett
  • Planetary Nebulae

Within, say, the Galaxies subdirectory, I then add further subdirectories, as the occasion arises, for Grand Spiral, Dwarf, Irregular, and so forth. Then under Dwarf, I would have a further subdirectory for Barnard’s Galaxy, another for the Sculptor Dwarf, and so forth. And finally, under Barnard’s Galaxy, I would have separate directories for each filter used (because they will be stacked separately from the others, and because they require different flats), and for each camera temperature (because they will require different darks):

  • Galaxies
  • Dwarf
  • Barnard
  • Clear
  • R
  • G
  • B
  • H-alpha

Similarly, the Calibration directory has subdirectories like this:

  • Calibration
  • Unbinned
  • Bias
  • Dark
  • Flat
  • Binned
  • Bias
  • Dark
  • Flat

Then, within say the Calibration/Unbinned/Dark directory, I would have subdirectories for darks of different durations and temperatures:

  • Calibration
  • Unbinned
  • Dark
  • 1hr -20C
  • 1hr -12C
  • 30min -18C

The dozen or so 1 hour darks done at -12 degrees will all go together, and so on.

Reason

The reason for this approach is that it makes it easier to bulk-process two important steps: calibration (and possible Debayering) with Mushroom, and registration and stacking with Prometheus.

Chapter 2: Exploring images with GoodLook

This chapter concerns displaying and exploring images with GoodLook. Sophisticated enhancement (wavelet noise filtering, deconvolution, etc) will be discussed in a later chapter. For the moment, we will concentrate on just looking, so we won’t save our changes.

Some Goals

  • Very rapidly explore images full-screen, in the pitch dark, with the minimum of intrusive clutter, making GoodLook suitable for presenting astronomical images at a meeting.
  • Flick easily between the full image and actual-pixel views (and beyond) of salient features.
  • Keep underlying images unstretched for as long as possible. Apply strong nonlinear onscreen stretch in real time while exploring.

Finding an image

  • Run GoodLook.In the middle of the screen, you will see a standard Windows Explorer navigation panel. It will wake up pointing to whatever directory you last visited using Mushroom, Prometheus, or GoodLook.
  • Navigate to a directory containing some images that you would like to look at. For simplicity, they might be JPEG holiday snaps, or finished astrophotos in one of the formats that GoodLook can read (JPEG, 8 or 16 bit uncompressed TIFF, or uncompressed 32 bit floating point FITS).
  • If required, use the Windows Explorer View Menu icon to set the view to Large Icons. Windows 7 can show icons of TIFF images, but not FITS. Vista is even less obliging.
  • At this point, you can cut, paste, drag to another place, or delete files one at a time, in the usual Windows Explorer ways.
  • Double click on the icon of the image that you wish to view (or single click and then [Open]).GoodLook will now show you the image, full screen. GoodLook is designed to be fast at displaying an uncompressed 16 bit TIFF from a cold start.
  • Note: If you only have unprocessed astrophotos to look at, you will have to jump ahead to the section on enhancing the darks, or you will probably see only blackness.

Navigating within an image

  • Zoom: Zoom in and out at any time using the mouse wheel. You can zoom in to 8:1 beyond individual pixels. GoodLook interpolates very smoothly between pixels.
  • Full Image: Press the Spacebar at any time to go back to the full image.
  • Pan: Use the mouse to Left-Drag the image at any time left, right, up, down. While dragging, GoodLook shows a rough and ready preview.
  • Centre Object at Actual Pixels: Better than dragging is to Double-Click on any item of interest in the image. This will go to actual pixels, centred on the item. Use the mouse wheel to zoom in and out, or the space bar to go back to the whole image.

Toolbars

GoodLook always runs full screen, and on your primary monitor only. Across the top and bottom are full-width but very narrow toolbars. These fade in slowly if you hover the mouse over them, and fade out again when you move the mouse away. That way they don’t distract from your image.

  • Press F1 to lock the toolbars in place, and activate context sensitive hint mode. In Hint mode, hovering over any item on the top or bottom toolbar displays its purpose. Press F1 again to go back to normal mode.

Top Toolbar

Bring up the top toolbar, either by pressing F1 or by hovering the mouse at the very top of the screen. Starting from the left, are:

  • A green box marked [1] that when clicked will apply a double stretch. See below.
  • A checkbox to automatically clip the black point to the second centile. Images straight out of the camera may have a milky gritty background due to dark current, skyglow, and airglow. The checkbox will fix this.(In real life, use partially stacked edge clipping, wavelet noise reduction and background flattening before clipping the black point).
  • A slider to progressively enhance the darks, at the expense of bright contrast. The slider applies an arcsinh (pronounced arcsynch)stretch. This is probably the best stretch for most deep sky images. It hugely brightens the darks without too much loss of contrast in the brights. Hitting Enter resets it. Images straight out of the camera are likely very dark (after suppressing milky background). The stretch slider will fix this, at the expense of making them much more gritty again.
  • Double-clicking on the knob of this slider brings up a graph of the actual arcsinh stretch, with a numeric display of the dark boost, mid-tone contrast, and bright contrast.
  • Another checkbox to dynamically rescale whatever is displayed on the screen. This changes in real time as you zoom and pan. Try it.
  • [Hold]. This button commits the existing dark point, stretch, balance, and saturation, resets the relevant sliders and checkboxes, and recalculates histograms – a kind of Apply button for these controls, to make what’s on the screen the basis for moving forward.
  • Sliders to linearly increase or decrease red, green, blue. On a monochrome image, any one of these sliders will double the image brightness. All three will therefore increase brightness 8 times. Hitting Enter resets them.
  • [A]. This button performs an automatic colour balance. See final chapter.
  • A slider to adjust saturation of a colour image. Hitting Enterresets.
  • Various other controls not relevant now: [*] removes boxes you have placed with Ctrl-Left-Click around stars etc, [D] deconvolution, [C]cropping, [H] removing hot and cold pixels, measuring FWHM, [B] removes background gradients, and so forth. These will be discussed later.

Bottom Toolbar

Bring up the bottom toolbar, again either by pressing F1 or by hovering the mouse at the very bottom of the screen. From the left, are:

  • [Open]. Clicking here will bring up the Windows Explorer navigation panel, enabling you to click on a different image icon, or to browse other directories.
  • Boxes showing the number of the current image (in alphabetical order of file name) and the number of images in the directory, eg [5] of [9].
  • The name of the file, eg [Tarantula.TIF]. If you press F1 to lock the toolbars, this box says [Hint Mode]. Just click on it to get the file name back.
  • [<Prev] displays the preceding image in this directory, in alphabetic order of filename.
  • [L] locks zoom and pan across images. This is very useful for comparing a small area of interest (eg a tiny background galaxy) under high zoom across a number of similar subframes. The button goes yellow when locked. Click again to unlock.
  • [Next>] displays the next image in the directory.
  • [FILTER] is not relevant here, but applies wavelet noise filtering to an image. This will be discussed later.
  • [::] debayers an image from a one-shot colour camera. See below.
  • The next three boxes show the raw underlying RGB values for the point under the mouse.
  • The bar graph shows the black-point clipped and stretched RGB values for the image as displayed on the screen. To show unstretched values, hit the Enter key. To show unclipped values, uncheck the relevant check box on the top toolbar.
  • [L] and [R] rotate the image 90 degrees.
  • [F], [T], [t], and [j] save the image in various formats (see below).
  • Two white boxes showing the pixel coordinates in the underlying image.
  • [Exit] closes GoodLook.

Navigating between images

As described above, the [<Prev] and [Next>] buttons on the bottom toolbar move to the alphabetically previous or next image in the current directory. The RIGHT ARROW and LEFT ARROW keys do the same thing.

Also as discussed above, the [Open] button on the bottom toolbar opens the directory navigation dialog box which lets you select a file manually or go to another directory. The ESC key does the same thing.

Saving an image

On the bottom toolbar, [F], [T], [t], and [j] save the file in various formats. The image is always saved with original filename, plus the letter E (for Enhanced), plus a number indicating the degree of stretch.

  • [F] 32 bit uncompressed floating point FITS. Lossless, but twice the size of a 16 bit TIFF.
  • [T] 16 bit uncompressed TIFF. Generally, intermediate work should be saved this way. If you want to explore your image at a meeting using GoodLook, save as a 16 bit TIFF.
  • [t]8 bit TIFF. Half the size again. It is not possible to do much further enhancement or even to stretch it further. Not so useful now we have cheap fast terabyte drives.
  • [j] Very lossy but hugely smaller file. Great for emailing a copy to a friend, but of no use for anything else.

Bayered images from a one-shot colour camera

Before we can use or look at an image from a one-shot colour camera (whether it is an actual photo of a faint fuzzy, or a flat, dark, or bias frame) we must understand about debayering.

A bayered colour CCD sensor has each pixel made up of four sub-pixels in a square array, typically like this:

R G R G R G

G B G B G B

R G R G R G

G B G B G B

Is your one-shot colour camera image bayered?

Open an image taken with your one-shot colour camera in GoodLook. If the image is bayered, you will see a monochrome image. If you zoom in, you will very clearly see the above 2x2 pattern in the grey.

Debayering with GoodLook

Debayering in GoodLook is only for a look-see or to assuage curiosity. (Normally, we will painlessly debayer images in bulk, at calibration time (see Chapter 3), using Mushroom.)

GoodLook has a button with four dots on it [: :] on the bottom toolbar, representing the 2x2 pattern. Clicking this button debayers the image, i.e. shows it on-screen as a colour image. Clicking [T] on the bottom toolbar would then save as a regular 16 bit colour TIFF.

There are many bayering schemes. At present, GoodLookSuite only correctly debayers the RG/GB pattern and the BG/GR pattern. Use Mushroom to select the correct pattern for your camera.

Enhancing dark detail

Above is a stack of 1-hour exposures of NGC 1365 through red, green, and blue filters. Notice the dark enhancement slider on the top toolbar. The knob is at far left, which leaves the image unchanged. At first glance you can see only stars. The super-bright active galactic nucleus is just visible to right of centre.

To bring out dark detail, drag the knob of the Dark Enhancement Slider progressively further and further to the right:

The little graph shows the applied arcsinh nonlinear stretch. The three boxes at the bottom of the graph show the resultant change in contrast in the darks, mid-tones, and highlights respectively. The darks have been enhanced (brighter, more contrast) a whopping 39.3 times, as indicated by the curve being extremely steep at the left of the graph. Moving the slider yet further to the right will increase the dark enhancement up to a maximum of 100 times. This makes the background start to look noisy and gritty and is probably excessive.

Necessary loss of mid-tone and highlight contrast

If the curve is steeper at the left, it must get flatter elsewhere. The mid-tone contrast is down to 32 percent of that in the original image, and the highlight contrast is down to 16% of that in the original image. This seems unfair, but it is a mathematical necessity. Assuming that even one star in the incoming image is fully exposed, then any stretch that enhances the darks must reduce the mid-tone and highlight contrast.

Double stretch

Click the little button marked [1] at the left of the top toolbar. This will apply a double stretch, as shown above. As in the earlier example, the slider was set to enhance the darks 39.3 times. This is still the case: the double stretch applies two weaker stretches to achieve the same final enhancement of faint features such as background galaxies. The noise in the background will also be unchanged.However,the mid-tones will becomebrighter. This can produce a more striking image, but the unavoidable cost is a further reduction in highlight contrast. This will result in near burn-out of bright stars and galactic centres. Again, it is a property of any nonlinear stretch, in any program whatsoever, that if you make the mid-tones brighter, you will lose contrast in the highlights.