An approach to star removal and replacement using the Gimp2.10

Click on any image to see it full size

Depending on your computer screen size, it may be necessary to click on each image in turn in order to be able to read the details in the menus etc. on the screen

One thing to emphasise is that star removal is one of the first things to do in the processing of an image; it is not an afterthought carried out when thee processing workflow is fairly well underway. The stars are replaced much later in the proceedings after the starless image has been enhanced.

We present here in detail, one approach to the removal of stars using the Gimp 2.10 with the Starnet++ plugin.

The aim of the processing of the starless image is to make the nebulosity brighter and stand out from the background. This must be done without making the darks too black (clipping at the black end) or making the bright end too white (clipping at the white end). 

(It should be noted that if the stars were present at this stage of the processing, they would dominate the image and would be bloated and over brightened by the stretching process and the nebulosity would not be adequately processed. This is why it is essential to have a starless image early in the process and with its full bit depth and therefore dynamic range.) 

We shall assume here that the data have been stacked into a 32bit tif image (although it could just as easily be a 32bit fits image)

Create a new folder and put a copy of your 32bit stacked tif image into it. 

The unstretched 32bit stacked tif should be loaded into Gimp 2.10 from the folder in which it was placed. In this case, it is an image of The Forsaken nebula in Cygnus.

Note that the image is virtually black with just a few stars showing through 

Use Colours > Curves to stretch the image a few times as shown. until the nebulosity just starts to show. (The number of times the image will need to be stretched will depend on the image. Objects with brighter nebulosity will need stretching fewer times). We don't want to stretch the image too much at this stage because we don't want to overstretch the stars.

Remember, if you make a mistake and have Clicked on OK, you can immediately undo it by clicking on Edit > Undo Curves if, for example, Curves has just been used.

Click on Image > Duplicate to produce another copy of the image.

Click on Image > Precision, to check that it is Linear Light. 

If it has Perceptual Gamma (sRGB) then click on Linear light to change it to Linear.

This because Starnet++ requires a Linear image in order to work properly.

Click on Filters > Starnet++ which will launch the Starnet++ plugin 

Click on OK and Starnet++ will proceed and show a terminal window showing progress., 

When the Starnet++ process finishes the starless image will be shown. However, it can be seen that this is an image with two layers that can be seen at the right-hand side of the window.

Click on Image > Flatten Image, which will flatten the image to a single layer image; which can be confirmed at the right-hand side of the window. 

The flattened image single layer can be confirmed at the right-hand side of the window. 

Click on Edit > Copy Visible, which will copy the starless image.

Then Click on the original starry image and Click on Edit > Paste as > New Layer. 

This pastes the starless image as a layer on top of the starry image as can be confirmed at the right-hand side of the window. 

At the right-hand side of the window under Mode, from the drop-down menu select Subtract. 

This produces an image of just the stars. However, as can be confirmed at the right-hand side of the window, it is an image with two layers. 

Click on Image > Flatten Image, which will flatten the image to a single layer image; which can be confirmed at the right-hand side of the window.

Click on File > Export As..

A dialogue box will appear and at the top: Name the file as Stars.tif and Click on Export 

This will launch a further dialogue box. Make sure that Save thumbnail is unchecked and click on Export.

Click on the starless image. 

Click on File > Export As... 

A dialogue box will appear and at the top: Name the file as Starless.tif and Click on Export 

This will launch a further dialogue box. Make sure that Save thumbnail is unchecked and click on Export.

So now, in our original folder we have the original unstretched image, the Stars image and the Starless image.

Click on the starless image  

Click on Image > Precision > Perceptual Gamma (sRGB) to turn the starless image to a non-linear image, which, note is still a 32bit image. 

Click on File > Export to Starless.tif (which replaces the linear starless image with a non linear image) 

This is not essential but may be helpful if the next part of the workflow involves other software such as GraXpert. However, in this example, we are going to stick to the Gimp 2.10 so as not to confuse the issues.

It is now time to work on the starless image which is where virtually all of the work is done. 

The aim of the processing of the starless image is to make the nebulosity brighter and stand out from the background. This must be done without making the darks too black (clipping at the black end) or making the bright end too white (clipping at the white end). 

(It should be noted that if the stars were present at this stage of the processing, they would dominate the image and would be bloated and over brightened by the stretching process and the nebulosity would not be adequately processed. This is why it is essential to have a starless image very early in the process and with its full bit depth and therefore dynamic range.) 

Some simple, illustrative stretching to illustrate the ideas.

The following is not prescriptive, but serves to show the type of things that can be done to achieve the aims.  

Click on Colours > Curves

The Curves can be pulled by the control points as shown below to keep the dark areas dark, but to brighten up the lighter areas of nebulosity.

Click on Colours > Levels

Adjust the Levels as shown below; again to darken (but not so much to clip the dark end) the dark areas and to brighten the lighter areas (but not too much)

Click on Colours > Hue Saturation

Increase the saturation a little and Click OK

Click on Filters > Enhance > Noise reduction

Left at the defaults the noise can be seen to reduce

It should be noted that there are far better ways of doing noise reduction using other software, such as GraXpert, but we are sticking to the Gimp 2.10 to avoid confusing the issues.

Click on Colours > Levels

Adjust the levels as shown below, to darken the dark areas and lighten the light areas (but neither too much)

Click on the stars image

Click on Edit  Copy Visible

Click on the processed starless image

Click on Edit > Paste as > New Layer

The star image is pasted as a new layer on top of the starless image. The two layers can be seen at the right-hand side of the window.

At the right-hand side of the window under Mode, from the drop-down menu select Screen.

Screen combines the stars with the starless image

Before the image is flattened, it is important to display the stars to the desired extent so that they are present but do not overwhelm the image.

To this end Click on Colours > Curves  (this is only going to affect the top layer, i.e. the stars)

If the curve is pulled up, the stars will become more dominant

If the curve is pulled down, the stars will become less dominant

When the desired degree of prominence of the stars has been achieved:

Click on Image > Flatten Image

The image is now complete, however, it is still a 32 bit image.

Click on Image > Precision > 8 bit Integer

This will spawn a dialogue box to confirm the conversion of the image to an 8bit image.

Click on Convert

We now need to save the 8bit image so that it can be shared on social media or posted on a blog.

Click on File > Export As...

Give it a name and the filename extension .png

In this case we have called the image file Forsaken nebula.png

Another dialogue box is spawned. Make sure that the Compression level is set to 0

The processed image of the Forsaken nebula has been saved in the same folder as the original 32bit stacked image.

The final image of the Forsaken nebula

It is not suggested here that this is the best processing of the image that could be achieved for it is not. This is just an example of how The Gimp 2.10 can be used to remove stars (using the Starnet++ plugin), process the starless image (which is the most important part of the processing), and the final replacement of the stars with the desired degree of prominence.

Implementing and testing the SVBONY SC715C prototype planetary camera with AstroDMx Capture.

Nicola has been implementing the prototype camera in AstroDMx Capture.

The SC715C camera

The SC715C camera is built around the IMX715 sensor which shows no amp glow and has small pixels of 1.45 μm x 1.45 μm size, and a sensor size of 5.6mm x 3.2mm; 8.4M pixels.

It is a 12 bit camera and offers:

24 bit RGB (8 bit)

48 bit RGB (16 bit)

8 bit RAW

16 bit RAW

In this case the RAW is true RAW straight from the sensor and not changed by any processes such as white balance, gamma etc. Which are destructive processes. We believe that this is the best approach to RAW data and that as a principle, all changes should be made in processing, not by the camera. 

Comparing the SC715C with the SV705C

The SC715C camera is well suited to capturing high resolution lunar, solar or planetary images, more so than the SC705C which has 2.9μm x 2.9μm pixels and a sensor size of 11.2mm x 6.3mm which, whilst it can capture quite high resolution images, is much more suited to EAA and Deep Sky imaging than the SC715C because the SV705C’s  pixels are 4 times the area of those of the SC715C and have a much greater full well depth.

The SV705C and the SC715C are cameras with different purposes and therefore complement each other.

The SC715C was tested with AstroDMx Capture and an Ekinox 80mm ED, F=440mm refractor with a Skywatcher Autofocuser and an IR/UV cut filter for lunar imaging and Deep Sky imaging only with the bright DSO, the Orion nebula.

Ekinox 80mm ED f/5.5 refractor

To achieve optimum focus AstroDMx Capture sent the scope with plate solving, to the star Deneb which was brought to focus using a Bahtinov mask and using a 4tronix Focus RF Autofocuser controller to remotely control the autofocuser.

4tronix Focus RF Autofocuser controller

Screenshot of AstroDMx Capture focusing on the star Deneb using a Bahtinov mask

Capturing Lunar data

AstroDMx Capture was used to capture two overlapping 1500 frame 16bit RAW SER files of the 59% waxing Moon

Area 1

The advantage of capturing RAW files is that they are only 1/3 the size of RGB files, so they are captured quicker and use only 1/3 of the storage space of RGB files.

Area 2

The best 60% of the frames in each of the two SER files were debayered and stacked in Autostakkert!4. The two resulting images were stitched together in Microsoft ICE, wavelet sharpened in waveSharp and post processed in Gimp 2.10 and ACDSee.

Final image of the 59% waxing Moon

Full size (scroll to explore the image)

This is a very good result from a small, F=440mm refractor. We shall soon be testing with a much longer focal length Maksutov telescope to further explore the high resolution capabilities of the SC715C.

In order to illustrate the high resolution results, 5 equal areas were cropped out of the image and independently optimised for levels in each area.

Mare Imbrium, Mare Serenitatis, Montes Apenninus, Archimedes, Aristillus, Autolycus region

Palus Somni, Mare Tranquillitatis, Mare Crisium region

Herschel, Ptolomaeus, Alphonsus, Arzachel, Albategnius region

Fernelius, Stofler, Faraday region

Mare Nectaris, Theophilus, Cyrillus, Catharina, Rupes Altai, Piccolomini region

Capturing data on the bright (mag 4.0) DSO, the Orion nebula

100 x 10s exposures of the Orion nebula were captured along with Flats and Darks.

AstroDMx Capture capturing 10s RAW Fits files of the Orion nebula

The data were debayered, calibrated and stacked in PixInsight processed with SPCC and StarXterminator and further processed in Siril, Gimp 2.10, GraXpert and Seti Astro Suite Cosmic Clarity.

The final Orion nebula image oriented in a more familiar way

The small pixel size of the SC715C enabled the resolution of the trapezium stars.

The SC715C is so far shaping up to be a very capable high resolution camera with TRUE RAW imaging capabilities, that will complement other cameras in the imager’s collection.