Making Bicolour images from OSC LeNhance filter images using the Gimp 2.10.

The Optolong LeNhance filter is a duo-band narrowband filter with two bandpass regions; one that passes H-alpha light in the red and the other that passes both OIII and H-beta light in the blue-green.

Transmission curve of the LeNhance filter

The rest of the spectrum is effectively blocked which eliminates most light pollution and increases the contrast of the images.

Removing stars

Probably the most effective way to process astronomical images is to use star removal techniques, process the nebulosity and then add back the stars to the image. The advantage of this is that during the processing, the stars are not processed and possibly become bloated. Also, any false colours that may have been introduced into the stars by the filter can be reduced by reducing their saturation or otherwise correcting their colours.

The Gimp 2.10 is a cross platform image processor of the same calibre as Photoshop and the star removal tool Starnet++ version 2 is available from GitHub as a plugin for the Gimp 2.10 in Linux or Windows. (It probably works in macOS as well). This means that virtually all of the processing of LeNhance filtered images can be done in the Gimp (Gnu Image Manipulation Program). The Gimp does have noise reduction functions but at the moment they are not quite as powerful as for example, Neat Image. However, the Gimp’s noise reduction could still be used to good effect if no other is available.

The example used here is from data captured of NGC 7000 The North America Nebula, captured through an Altair Starwave 60ED doublet refractor using AstroDMx Capture and an SVBONY SV605CC OSC 14 bit CMOS camera with an Optolong LeNhance duo-band narrowband filter. The image is a stack of 22 x 3min FITS exposures. These data were captured when we were still working out the spacing for the field-flattener/reducer so the field was not yet flat.

Click on any image to get a closer view

The basic image of NGC 7000

The Processing workflow

ALWAYS duplicate an image and work on the duplicate. In this way any changes you make will not affect the original image and mistakes can be easily rectified.

Star removal

The image must be a 16 bit TIFF, Linear light.

Select Starnet++

Starnet++ v2 working on star removal will take a few minutes depending on the computer.

Stars are removed

Flatten and save the image

Copy the visible and paste it over the original image (with the stars) as a new layer and select Subtract as the mode (method of blending). This will give an image of the stars.

Flatten the image, reduce the saturation a little and apply the Gimp noise reduction. This will have the effect of reducing the noise and reducing the stars a little. Export the image of the stars that will be added back later.

Stars

Working on the starless image.

Export the starless image and denoise it with Neat image or other application, or the Gimp’s own noise reduction function. The denoised image will be worked on from now on.

Using Colours, Components and Decompose.

Uncheck Decompose to layers

The image will then be split into its three monochrome channels as separate images corresponding to Red, Green and Blue.

The three monochrome images can be seen above, the arrows have been added to show which channel is which.

We are going to construct a bicolour image using the red channel (H-alpha) and the sum of the green and blue channels (OIII and H-beta).

Copy the visible of the green-channel image and paste it as a new layer onto the blue-channel image, using Addition as the blending mode.

Flatten the image and then apply Colours and Curves to stretch the image

You may have to come back to this stage (to stretch more or less) depending on the result you obtain in the following steps. 

At this stage you can remove the green channel image to avoid possible confusion.

So we now have a red channel and a blue channel (actually the sum of the original green and blue channels).

Composing a bi-colour image

Using Colours, Components and Compose, we have to specify what will go into each of the 3 colour channels of the RGB bi-colour image we are about to construct.

Put red in the red channel, blue in the green channel and blue in the blue channel and the bi-colour image will be composed.

At this stage you will have to decide, based on the result obtained, whether you are satisfied with the stretching of the green+blue image in the previous stage, or whether that stage needs to be re-visited.

In Colours and Hue-Saturation, you can selectively adjust the saturation of the separate colours (in this case red and cyan) if required.

Starless bi-colour image

At this stage it is possible to use Colours and Hue-Saturation to selectively change the hue of one of the colours. In this example, the red has been changed to a golden hue often seen in bi-colour images.

Hue-changed bi-colour starless image

This is a matter of personal preference.

Adding stars back into the images

Remember that the stars now have less false colour and are slightly less prominent than in the original image.

Copy the visible of the stars image and paste it as a new layer onto the starless image selecting Addition as the blending mode. Then flatten the image.

Bi-colour image with stars back

Hue adjusted bi-colour image with stars back

RGB image with stars back

The images in which the stars have been put back have a more pleasing star field with less false star colour and with the stars being less dominant in the image in which the nebulosity can be more readily appreciated.

Bi-colour images can give more insight into the qualitative composition of nebulae than can be deduced from the original RGB image captured through a duo-band narrowband filter such as the LeNhance filter.

Live-Stacking with AstroDMx Capture and stacking software

Live Stacking of images is a technique of value for Electronically Assisted Astronomy (EAA). EAA is of particular value for outreach work where members of the public are shown astronomical objects in real time.

Although it is possible to use EAA during astro-imaging sessions, it is not normally used in these circumstances.

EAA uses Live Stacking of images, that is, as each image (often Deep Sky images) is captured, it is stacked with the previously captured images and displayed on the computer screen. In this way, the viewer can observe the image building up on the screen and becoming less noisy with more details becoming visible.

AstroDMx Capture may well have EAA Live Stacking built into the software at some future date, but it is possible to do EAA and Live Stacking with AstroDMx Capture by using it in conjunction with Live-Stacking software in a completely seamless way.

Three programs have been tested and found to work simply and seamlessly with AstroDMx Capture for Live Stacking. They are:

Deep Sky Stacker Live 4.2.6

ASTAP version 2023.01.21

Siril-1.2.0-beta1

It is likely that one of these programs would be used routinely to stack and maybe partially process the images captured during an imaging session.

Deep Sky Stacker Live is a Windows only program.

Siril-1.2.0-beta1 is cross platform and works on Linux, macOS and Windows.

ASTAP version 2023.01.21 is cross platform and works on Linux, macOS and Windows.

All three programs work in the same way. They monitor a user specified directory (folder) and when a new image appears in that directory, it is stacked with the previous images that have appeared there. The current stacked image is displayed.

AstroDMx Capture saves its images into date-time-stamped folders by default.

Click on an image to get a closer view

The directories (folders) generated by AstroDMx Capture during an imaging session

So, for example, AstroDMx Capture created a folder on 2023-03-02.

Within that folder AstroDMx Capture created other folders that would contain the various sets of images captured. One of these folders was created at 19-48-15 when its contents started to be captured. When opened, it can be seen that it contained image files called S2HHead_000001_19-48-15_data.fits; S2HHead_000002_19-53-15_data.fits etc.

This folder was created when AstroDMx Capture started to capture the data on this object (SII on the Horsehead nebula).

It is this folder (19-48-15) that would be specified as the folder to be monitored as soon as the first image had been captured.

Whichever program that was being used would proceed to Live-Stack the images as they were captured by AstroDMx Capture and placed in the folder.

Deep Sky Stacker Live

The directory (folder) to be monitored is selected

Select the Stacked image tab. Click on the red Stack button and stacking begins.

When just the first image is captured and displayed

When all 20 of the images have been stacked and displayed

It can be seen that the 20 image stack is more defined and less noisy than the first, single image. In fact, the Signal to Noise ratio increases as the square root of the stack size.

Siril Live Stacking

The Home folder is set to the folder to be monitored.

Live Stacking is started by clicking the red button.

When the first image is captured it is displayed

After all 20 images have been captured and stacked

Again, it can be seen that the stack of 20 images is more defined and less noisy.

ASTAP Live-Stacking

The folder to be monitored is selected

The button is clicked to start Live Stacking

When the first image has been captured and displayed

After all 20 images have been captured, stacked and displayed

As before, it can be seen that the stack of 20 images is more defined and less noisy. This, of course, is why EAA is so valuable for outreach.

With ASTAP it is then important to click the button ‘Rename all files back to original’. This is because during the stacking process, the files have an underscore added to the filename extension as they are stacked. Clicking on the button removes the underscores from the filename extensions.

In each case it is a matter of moments to start the Live Stacking process. The window in which the Live-Stacked image is displayed is quite separate from the preview window of AstroDMx Capture. It is best to display this window on a separate desktop or even an external Monitor.

In this way, it is possible to simply and seamlessly Live-Stack the images being captured by AstroDMx Capture, whatever operating system is being used.

Exploring tri-colour narrowband palettes with an SV605MC, a William optics 81mm APO refractor and AstroDMx Capture

Tri-colour false-colour images of the Horsehead and Flame nebulae

Note that FWHM is Full Width Half Maximum as defined below.

The Horsehead and Flame nebulae were imaged with AstroDMx Capture version 2, a William Optics Super Zenithstar 81mm ED Doublet APO refractor at f/5.5 with x 0.8 reducer/flattener, (F=445.5mm) and an SVBONY SV605MC monochrome, 14 bit, cooled CMOS camera fitted with a 7nm FWHM H-alpha, or a 6.5nm FWHM OIII filter or a 6.5nm FWHM SII filter or an Altair Quadband filter whose transmission curve covers all relevant wavelengths with two, wide, 35nm FWHM bandpasses.

The Altair Quadband OSC narrowband filter transmits two spectral bands: 

FWHM spans 477.5nm - 512.5nm at the blue-green end of the visible spectrum and FWHM spans 642.5nm - 677.5nm at the red end of the spectrum.

The first band contains the emission lines of H-beta at 486.1nm and OIII at 495.9nm and 500.7nm and is centred on 495nm.

The second band contains the emission lines of H-alpha at 656.3nm and SII at 672.4nm and is centred on 660nm.

The Filter is called ‘quadband’ because it contains the emission lines of the elements H-beta, OIII, H-alphaa and SII.

Each of the two transmission bands has a FWHM of 35nm and the rest of the visible spectrum is essentially blocked.

Imaging procedures

As usual, the mount was placed on marks on the concrete base which give a fairly good polar alignment. AstroDMx Capture passed the time, altitude and location coordinates to the hand controller via the INDI server. The hand controller which now contained all of the correct information was set to its previous alignment and was unparked by AstroDMx Capture.

AstroDMx Capture was used to send the scope/mount to a bright star (Sirius) to check focus with a Bahtinov mask. 

Then AstroDMx Capture sent the scope/mount to the mag 7.5 star HD37805, which lies roughly central within the Horsehead-Flame nebulosity. This is a preferred way to compose the image, than sending the scope/mount to the published coordinates of the Horsehead or the Flame nebula.

AstroDMx Capture plate-solved the field of view and centred the selected HD37805 star

6 x 5min exposures were captured with each of the OIII and H-alpha filters, 8 x 5min with the SII filter, plus 15 x 2min exposures through the Quadband filter (for luminance data) giving a total of 2 hours 10 minutes exposure time.

AstroDMx Capture v2 capturing data through an SII filter

The images were stacked and partly processed in Siril and post processed in the Gimp 2.10 and Neat Image. The colour channels were composed and luminance blended into the images in the Gimp 2,10.

Narrowband image Palettes

The Additive nature of light

From this diagram it is possible to see the colour that will be produced by combining the primary colours Red Green and Blue.

Constructing False-colour narrowband astronomical images

To construct a narrowband false colour image, the object of interest must be imaged with a monochrome camera through three narrowband filters:

H-alpha                    656.3nm                            Red                        7nm FWHM bandpass

OIII (Oxygen 3        500.7nm                            Blue-green             6.5nm FWHM  bandpass

SII  (Sulphur 2)        671.7nm & 673.0nm        Deep Red                6.5nm FWHM bandpass

Transmission curve of the three filters  

The width of the transmission bandpass determines the contrast in the resulting image; the narrower the bandpass, the greater the contrast.

Typically, available narrowband filters have FWHM bandpasses of 3nm, 6.5-7nm or 12nm.

We shall refer to H-alpha as H, OIII as O and SII as S from now on.

It must be remembered that monochrome images are captured through the narrowband filters. Colour (in this case false colour) images can be composed by assigning an image taken through a given filter to Red, Green or Blue; R, G or B. 

These are the monochrome images of the Horsehead and Flame nebulae region captured through each of the filters:

H

  

S

  

O

  

In addition, an image was captured through a quadband filter to provide extra luminance detail in each image at a later stage of the image processing and applied equally to each palette produced.

Quadband image for luminance  

Exposure times were 30 min for H and O, 40 min for S and 30 min luminance; giving a total exposure time of 2 hours 10 minutes.

We shall now assign H, O and S each to one of the primary colour channels, R, G and B to produce a false colour image palette.

The first palette we shall construct is named for the Hubble Space telescope and has often been used in its images. In this palette, S is assigned to R, H is assigned to G and O is assigned to B.

Therefore SHO maps to RGB.

SHO Hubble palette with the luminance data blended in.

The second palette is named for the Canada, France Hawaii telescope. In this palette HOS maps to RGB.

HOS Canada, France Hawaii telescope palette with luminance data blended in.  

The other palettes follow. These palettes are simply named by their RGB mappings.

HSO palette with luminance data blended in.

  

OHS palette with luminance data blended in. 

 

OSH palette with luminance data blended in.

  

SOH palette with luminance data blended in.

It is possible to change the colour saturation of the images to enhance the information in them as can be seen in the montage below.

Table showing details of the RGB mappings in the montage

With a knowledge of the additive properties of light and the mapping of the filters onto RGB it is possible to gain insight into the chemical composition of the clouds of gas that make up the nebulae.

The SVBONY SV605MC monochrome, cooled, 14 bit CMOS camera performed flawlessly with AstroDMx Capture, and as the results show, proved to be a very capable camera.

Release of Version 2 of AstroDMx Capture with advanced functionality

 We are pleased to announce that Nicola has released Version 2.0.2 of AstroDMx Capture.

Mutatis mutandis

Aside from bug fixes, camera implementations and UI redesign, AstroDMx Capture is now much more capable.

This release contains major advances in functionality that astro-imagers will find useful for their imaging.

The code base now exceeds 98 KLOCS (thousands of lines of code) with 19 KL (thousand lines) of internal documentation, giving a total of 117 KL The code is written from scratch and does not use any GPL licensed code or contravene any other Open-source licences.

To put these 117 KL in an understandable and realistic perspective I will use a method that I have used before: 

The book 'iWoz', the autobiography of Steve Wozniak, co-founder of Apple, is a fairly typical book in terms of size, if not content. It has 30 lines of text per page and 342 pages.

Therefore, if the whole of AstroDMx Capture was to be printed book fashion; to hold all of the lines of source code and internal documentation in AstroDMx Capture, it would require more than 11 printed volumes, each the size of 'iWoz' to contain everything; and the code base is growing. The code is multi-threaded and uses polymorphism when needed, which is, in part, responsible for the efficiency and responsiveness of the software.

To say that AstroDMx Capture now has support for INDI would be an understatement. Controls for using an INDI server have been baked into the AstroDMx Capture UI. This includes INDI configurator controls that allow the configuring of an INDI server directly from AstroDMx Capture.

The INDI server used can be on the same computer as AstroDMx Capture or it can be located on a separate computer such as a Raspberry Pi. With Linux and macOS an INDI server can run natively on the imaging computer. With Windows, which cannot natively run INDI servers, we have demonstrated that it is a simple matter to set up an Oracle Virtual Box running Linux on the Windows machine and to run the INDI server within the virtual machine; thus achieving the end of running the INDI server on the same Windows computer as AstroDMx Capture for Windows.

The INDI server can be used by PHD2 to auto-guide the mount, either from another computer, or from the imaging computer.

This new functionality allows AstroDMx Capture to do a number of things in addition to the capturing of images. The following list is not exhaustive, but is indicative of the increase in functionality found in this release.

AstroDMx Capture can:

(1)    Populate the mount's hand controller with all of the required information such as location, altitude and exact time.

(2)    Control the mount fully. The mount can be sent to a huge number of Deep Sky objects or catalogued stars. The latter allow the mount to be sent to a star very precisely and optimise the composition of the image to be captured.

(3)    Plate solve and then send the mount precisely to the position intended by correcting the mount's position.

(4)    Use a previously captured image to plate solve and centre the field of view in the same place as in the image.

(5)    Control an electronic focuser such as the Pegasus fucuscube V2.

(6)    Control an electronic filter wheel.

(7)    Control INDI driven cameras. At the moment this is a work in progress.

(8)    Save profiles of settings and also equipment used.

It should be noted that all of the functionality from previous releases remains; such as the native implementation of cameras that will achieve much better frame-rates than an equivalent INDI driven camera. INDI cameras should only be used if there is no native implementation of a camera.

The AstroDMx Capture UI is more complex than that in previous versions, however, it is very intuitive to use and sections of the UI can be hidden if they are not required.

The AstroDMx Capture UI in action

Extra non-destructive preview visualisation controls have been added and many of the controls can be used in conjunction with each other to optimise the visualisation of an object being imaged.

Nicola will be putting some documentation on the download site  shortly. This should help people to get started with the new functionality. In the meantime, explore the software.

AstroDMx Capture for all platforms can be downloaded HERE.