Look to the stars. God isn't hiding anything.

Intro to image stacking

A beginners exercise in a simple image stack using Deep Sky Stacker (DSS)


In order to capture smooth, detailed and noiseless long exposure astrophoto’s, a number of identical images is required to be combined or “stacked”. This has the effect of averaging out all the differences to produce a clean image, which can then be exported to various editing software programs.

Deep Sky Stacker
is a freeware program available for download on the internet. I take no credit whatsoever for the writing of the DSS program. The following step-by-step procedure is very basic and simple, but should get you well on your way to turning out your very own beautiful astrophotos.


There are many different cameras and lenses and even more methods of capture. For the purpose of this exercise, we will be using a Canon EOS 400D DSLR with the standard 18-55 EF kit lens.

The camera was pointed and focused on the centre of the Milky Way Galaxy on a typical tripod. Because the stars move and the tripod does not track this motion, the lens was set at 18mm and the shutter could only be open for a maximum of 30 seconds or the stars would become too elongated. ISO was set at 1600. A shutter release cable or timer should be used to avoid shaking the camera. To get this part right, see the instructions on Widefield Astrophotography page. (Part way down the page)

Fourteen identical photos were taken for this exercise. Each one advanced slightly in the frame because the sky moved over the series, but the camera did not. This is not a problem over a relatively short period of time as the stacking process will align on the stars and the uneven edges can be cropped off later. Below is a single subexposure (sub) unedited from the camera.


Note: for the purpose of this exercise, we will not be getting into stacking of offset, flat or dark frames. We will be stacking real-light images only.


1.    Open DSS and click on “open picture files”. A dialogue box will open where you can direct DSS to your images and select them.

2.    For this run we will be using JPEG images, so go to “Files of type” and use the drop down box to select JPEG files. If you were using RAW, TIFF or FITS then you would select that instead.

3.    Select all the images you wish to stack by clicking on the first one, hold down shift and click on the last one. Click open.


4.    Click on “Check all”. You will see all the images in the bottom pane of DSS will be checked with a tick.


5.    Click on “Register checked pictures” and make sure “Register already checked pictures” is ticked.


6.    Click on “Stacking parameters” at the bottom of the same dialogue box.
7.    Under the “Result” tab, make sure “Mosaic” mode and “Align RGB Channels in final image” are selected.


8.    Select the “Light” tab and make sure “Median” is selected. Leave the other tabs alone for now and click OK.


9.    Click on “Recommended settings” and select “Use median combination method” it will turn green. Click OK, then click OK again.


10.    A new dialogue box will appear, showing the number of frames, ISO setting and total exposure time once stacked. It will also indicate if you have added offsets, darks or flats, but as stated earlier, we will not be using them this time. Click on OK and DSS will begin registering and stacking the images. Let it run its course and go have a coffee break.


11.    Now you have had your coffee, you should return to a rather grey looking final image. Note the three tabs on the bottom left. These will be very important.

12.    The RGB/K Levels is a histogram graph of the Red, Green and Blue primary colours in all images. You will notice that the towers are all aligned together. In all your adjustments to follow, you can come back here and use the sliders to make sure they stay together. If the towers occupy different positions, the colour balance of your image will go wrong.


13.    Now click on the “Luminance” tab. Here you can adjust the sliders, particularly the midtone sliders. Have a play with this and click on the “Apply” button.


14.    Now click on the “Saturation” tab. Here you can increase or decrease the amount of colour in the image. This one has to be treated with subtle hands. Too little colour and your precious data you spent all that time capturing is wasted. Too much and the image starts to look fake and painted.


Export to Photoshop

Once you are happy with your image, keeping in mind that more can be done in Photoshop or some similar editing program, click on “Save picture to file”. Select where you want to save it, give it a name that you can recognise. I find that the details of its make-up are handy for later. Make sure you save it as a 16 bit per channel TIFF image. Now this image will be a HUGE file. This one was 62MB. Once you have finished editing in other programs you can resize it as needed.


Once you have exported to Photoshop or other editing program, you can have a play with levels, curves and saturation until you are happy with it. Be careful not to overcook it!


Congratulations, you have just stacked your first simple image!


Barry Armstead.
ASIGN Observatory II

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