Plugin Operation

These are some basic instructions in using the plugin.

Installation

Extract the file picolour_synth.dll to the Astroart directory just like all other plugins. Start (or restart) Astroart so that it recognises the new plugin.

Startup

Load an image into Astroart that you want to convert or take an image direct from the camera. Then select Tools->Plug In command->Mike's MX Colour Synthesis vX.YY to star the plugin. The plugin window will then appear in the middle of the screen, attempt to identify the image format and convert the image to a colour one within a window. This window is the Preview Window. Drag on the widow borders to expand it so you can see the whole picture.

The three menu items visible are 'Edit', 'Update' and 'Save'. 'Update' causes the image to be recalculated and drawn in the Preview window. 'Save' bring up a dialogue box to save the colour image to a bitmap file. 'Edit' brings up a dialogue box which allows the user to control how the colour image is created.

Five tabs are visible.

'Image Format' tab selects the input and output image format. Currently the plugin supports 4 input formats and normally tries to detect the format automatically. If it fails, click the 'Auto Format Detect' tickbox to ungrey the 'Input Format' group and then try one of the other options.

The 'Output Format' group allows you to select the format of the output image i.e. black and white only or colour. This selection also control whether the plugin generates a single Luninance window or 4 LRGB windows with the 'FITS Update' button (see later).

The 'Camera' group selects which type of camera was used to take the image. This affects the RGB settings when 'Default' button is clicked on the 'Colour Adjust' tab.

The 'Bits/pixel' group selects the number of bits/pixel in the image. If set to 12-bits, all the pixels in the image will be multiplied by 16 before processing.

The 'Always on top' tickbox when ticked forces the plugin to always appear over other windows. This can be a nuisance, however. If so, untick the box. However, now, clicking any part of the Astroart window will hide the plugin windows. Reselect the plugin from the Tools->Plugin Commands menu to get it back.  The state of this tickbox is recorded between sessions.

The 'Colour Adjust' tab lets you control the colour balance in the preview window.

'Update' updates the Preview window to show the effect of the current settings.

'White Bal' tries to determine RGB multipliers to give an image with no colour casts. You can control the area of the image used for colour balance by selecting part of the raw image. If no selection box is present, the colour balance is taken from the whole image. Automatic colour balancing doesn't always work though, especially on astronomical images or images containing strongly coloured subjects.

'Default' sets a default colour balance. This give close-to-true colours when using an MX7C camera with an IR blocking filter and a camera lens. The same setting should work for astronomical and gives you a starting point.

The Red, Green and Blue boxes allow you to adjust the multiplier values and adjust the relative amounts of red, green and blue in the output.

The 'Luminance' slider controls the image brightness. Moving the slider to the left decreases the brightness. The default centre position means that no luminance adjustment is applied.

The 'Saturation' slider controls how intense the colours are. Moving the slider to the left reduces the amount of colour in the image. The default slider position gives an unadjusted image.

The 'Altitude' slider compensates for colour variation due to atmospheric refraction and absorption. The 90 degree setting disables the adjustment. Any other position applies an adjustment based on the angle of the object above the horizon when the image was captured. The effect is quite slight until you get to low altitudes where the blue level becomes increased.

The 'Synthesis' tab contains settings to adjust other parameters of the image.

'Update' updated the preview window using the current settings.

'Pedestal' specifies the value to be subtracted from each pixel in the image before colour synthesis.  This removes a value added to each pixel by the camera.  If a dark frame of bias frame is used, the Pedestal value is greyed out because dark or bias frame subtraction will do the same thing.

'X offset' and 'Y offset' are there to compensate for errors in earlier versions of Astroart and Star2000. Try these if you fail to get a correctly coloured image. You will certainly need to click 'X offset' if importing pictures taken with MaximDL - for some reason it produces images with a 1 pixel shift in the X axis.

'Colour LPF' enables or disables the low-pass filter applied to the colour planes. Disabling this degrades the image, so it may be removed in future versions.

'VBE Filter' tries to compensate the the venetian blind effect that some images have. This degrades the vertical resolution of the image, so use only if necessary. In my experience, a correctly exposed image doesn't suffer from VBE.

'Lum Stretch' specified the stretch function used on the luminance frame. 'Gamma' defines the gamma value used by the gammalog stretch. Linear stretch is the best option for daylight images (e.g. when testing the plugin or the camera). Gammalog is best for deep sky images because it enhances the faint parts of the image without burning out brighter parts of the image like stars.

'White Clip Level' defines the pixel value above which the image is clipped to white. This gives a more natural appearance to overexposed highlights and bright stars.  Adjust this value if bright stars or highlights have strange colours.

'Luminance HPF' controls the amount of sharpening applied to the luminance image to compensate for the blurring caused by the sysnthesis operation.

'Lum Histogram' control the black and white endpoints for the stretch function applied to the luminance image just before the image is updated in the Preview window. The endpoints are determined by calculating a histogram and then finding the black and white endpoints based on the percentage of pixels which exist above and below the endpoint. The boxes define the percentages used and hence the percentage of pixels which will be discarded by the stretch. To brighten an image, increase the value in the 'White EP' box. To remove more of the background, increase the value in the 'Black EP' box. This stretch function is not applied to the luminance image output via the 'FITS Update' button. This allows a better stretch function to be used either in Astroart or other programs like AIP4WIN.

'Single LRGB' is the same as the 'Update' button in that it causes the Preview window to be updated. However, it also causes 4 windows to be opened containing the LRGB planes. The naming convention is the same as used by the Astroart colour plugin so that the Astroart Trichromy tool will read the correct planes.

The 'Batch LRGB' button allows the current image settings to be applied to multiple raw frames. Each input frame is processed to four LRGB frames which are written to the same directory as the raw input frame. This allows the image to be processed within Astroart which has more filters available or by other image processing programs.

The 'Calibration' tab allows dark frames and flat frames to be defined. Click the 'Browse' button to find the relevant file. The 'Clr' button deletes the frame.

If no dark frame is given, the plugin removes a pedestal value applied to the pixels by the camera (see the Synthesis tab). This improves the colour saturation in the absence of a dark frame.

If a Dark Flat frame is omitted, it is assumed that the Flat frame has already had a dark frame subtracted from it.

The 'Combine LRGB' tab allows 4 separate images to be combined into a single colour image.  First, open the luminance, red, green and blue images in AstroArt.  Click on each of the 'Luminance', 'Red', 'Green' and 'Blue' buttons in turn to select the correct images.  Then click 'Update' to create the preview image.  Each LRGB image can me multiplied by a value to get the correct colour balance.  The other controls are the same as similar controls on the other tabs except they only apply to the 'Combine LRGB' tab.