[This is just one of many articles in the author’s Astronomy Digest.]
This was an exercise relating to an astrophotography on-line course being given by the author where the students were asked to use a DSLR mounted on a fixed tripod to image a constellation.
As the camera is fixed in a particular direction, the stars will move across and rotate with respect to the sensor. This problem is worst at the Celestial Equator and becomes less so as one images towards the North Celestial Pole. To avoid ‘star trailing’ exposures must therefore be kept short.
The simple rule for determining the maximum exposure length is to divide the effective focal length of the lens into 500. To determine the effective focal length one multiplies the focal length specified on the lens by the ‘crop factor’ of your sensor.
Full Frame 1
Canon APS-C 1.6
Nikon APS-C 1.5 (and all other APS-C)
Micro 4/3 2
So a 35 mm focal length on a Nikon APS-C camera is 52.5 mm and thus a maximum exposure is 10 seconds is suggested.
Exposures can be longer nearer the Celestial Pole so this exposure time can be divided by the Cosine of the declination [Cos(Dec)]. For example Cassiopeia is at +60 where Cos (60) is 0.5. So one can multiply the exposure time by 1/Cos(Dec) which in this case is 2 so that, with this lens and camera, one could then increase the exposure time to 20 seconds.
In fact with the latest high resolution sensors, 500 is somewhat on the high side and many are now using 300 instead. However, in the example that follows using the 500 rule, the star trails were very short and, as will be explained below, can be easily removed in post processing.
Equipment and exposures
The lens used was 50 year old, 50 mm, Helios Lens attached to a Sony Full Frame camera so the effective focal length stays at 50 mm and thus 10 second exposures were employed. 115 frames at ISO 800 were taken saving both raw and Jpeg frames. [Even if raw frames are to be used for processing, it is far easier to inspect the Jpeg frames to spot problems – in this case it became obvious that the final 15 frames were partially cloud covered and so were deleted.]
The image below is a comparison of the first and last Jpeg frames (somewhat stretched) showing how the image has moved across the sensor during the period of the exposures and also that light pollution was present – increasing, as expected, towards the horizon.
Plane trails could just be seen in both these frames with the worst affected frame shown below. Happily, these can be removed in the image processing.
Aligning and Stacking in Deep Sky Stacker (DSS)
The free DSS software will take the individual images and align the stars in each frame before stacking them to give the result of a longer exposure. The 100 usable raw frames were input into DSS (‘Open picture files’ top left), the frames checked (Check all) and the ‘Register Checked Images’ clicked on. Then, in the window that is opened up, all boxes are ticked and ‘Advanced’ is clicked upon. One then sees a ‘Star detection threshold’ slider and the ‘Compute the number of detected stars’ tab is clicked upon. DSS analyses the first frame and shows the number of stars detected below. One then adjusts the slider so that ~100 to 150 stars are detected. [Too great a number will just slow down the alignment and stacking process.]
Clicking on ‘OK’, the stacking control window then opens and the Stacking Mode may well say ‘Standard’ and the Stacking is likely to say ‘Average’. Click on this and, instead, select ‘Kappa- Sigma clipping’. This rejects pixel values which lie away from the average for that pixel and so removes satellite or plane trails.
If you then click on ‘OK’, DSS will begin to align and stack the 100 frames to give a total exposure of 1,000 seconds – nearly 17 minutes.
The output seen after DSs completes the process shows no more than a single frame. However the data is there but the result produced by DSS has to be ‘Stretched’.
The ‘best buy’ program to process your astro images is ‘Affinity Photo’. It normally costs around £50 but is quite often on sale at around £30. There are other articles about it in the Astronomy Digest.
However you can download GIMP for free and this can be used to stretch the image but does not have one key filter that enables the light pollution to be removed as easily. [I will add a note where appropriate below.]
Using Affinity Photo (AF) to work on the output from DSS.
The Tiff output from DSS is loaded in to AF, the ‘Levels’ adjustment tool selected (click on ‘Adjustment’ then ‘Levels’ and ‘Default’. In the window that opens, the ‘gamma’ slider is moved over to the left which brightens the image and shows up the light pollution. The ‘Merge’ tab is clicked upon to complete the process.
Removing the light pollution
The layer is duplicated (‘Duplicate’ in the ‘Layers’ drop down menu) and the ‘Dust and Scratches’ filter applied (select ‘Noise’ in the Filter drop down menu to find it) with a radius of 30 pixels. Clicking on ‘Apply’ removes the stars leaving just an image of the light pollution. [In GIMP, the Dust and Scratches is not present, but one can uses the Median Blur filter instead.]
If the two layers are now merged down (‘Merge Down’ in the ‘Layers’ drop down menu) using the ‘Difference’ blending mode, the light pollution is removed from the image – which then shows very little!
An alternative and more controllable method of stretching an image is to use the ‘Curves’ adjustment tool and two applications were made to bring up the fainter parts of the image to give the results shown.
The HSL adjustment tool can then be used to increase the saturation somewhat and bring out the star colours.
Removing Star Trailing
Observing the image at 100%, some very slight star trailing can be seen.
To remove this, the image is duplicated and the ‘Darken’ blending mode selected. The ‘Move’ tool is then used to move the top layer over the bottom layer using the ‘arrow’ keys. In this case it only had to be shifted by a single pixel to the left to give round stellar images.
A slight crop of the final result which is quite pleasing.
By selecting the brighter stars using the selection brush tool, applying a little Gaussian blur (which makes the stars larger but fainter) and then bringing up the brightness, one can make the main constellation stars more prominent.
A 100% crop of the ‘Sword of Orion’. A little of the H-alpha light can be seen in the Orion Nebula as well as a hint of the ‘Flame Nebula’ that lies to the left of Alnitak, the lower left star of Orion’s Belt.