After a career as a music educator, computer science teacher and keen amateur photographer I decided on astronomy as my next long-term project.
This page traces my efforts from purely visual, eyes and binoculars, through to fixed tripod shooting with standard digital cameras, and finally to tracked imaging using a Star Adventurer 2i mount.
My goal is to get the best photos I can from a digital camera, a few standard lenses, a tripod, a simple manually controlled tracker and free astrophotography software.
Before proceeding you should download Siril and GraXpert, and check that they can run on your computer.
You will also need the Command Line Tool version of Starnet. This can be tricky to install so that it works within Siril itself. Search for help videos on Deep Space Astro.
I also use GraphicConverter on an M1 Macbook Pro. This is a Mac only product. Any competent graphics program can be used to make final adjustments to the TIFF files produced by Siril and GraXpert.
I was inspired to start exploring the night sky up after meeting astronomer Michael Maher during a Vintage Rail Tour of the Riverina which lead me to the small town of The Rock, near Wagga Wagga in rural NSW, Australia. Here I received a warm welcome and great advice at the Regional Observatory.
I discovered that my shooting methods need LARGE chunks of free computer disk space and a fast multicore cpu to operate efficiently. More on this later, but be forewarned that you may need a high capacity external SSD.
The issue of disk space does not normally arise when processing deep space images where you are only stacking a few hundred shots from a digital camera. It is only the sheer number of images required for the extremely short shutter speeds that I prefer (tracked or untracked) that causes the problem.
If you have a modern digital camera then please try these ideas for yourself. No complex equipment is necessary and the software is free.
I have gleaned what I need to know from reading the extensive DSLR astrophotography site run by Dr Roger N. Clark, a NASA imaging scientist who worked on sensors for the Europa Clipper mission.
Although Clark uses tracking mounts, his ideas on shutter speeds, real colours, suitable targets, post-processing and other suggestions have been invaluable. It’s a convoluted site and a little difficult to follow without opening multiple windows, but full of top-rate information. The heavenly views have inspired me to get serious with this new hobby.
Rushing outside to snap your first night sky photo will highlight a non-trivial problem. Where do you point the camera? The Milky Way is obvious (at least in rural Australia), but where exactly are all the nebulae, star clusters and planets?
In my view, your first step ought to be learning the layout of the night sky. All you need is a phone app and your eyes.
The human mind loves to imagine patterns even when none are intended, like faces on toast. Grouping stars into asterisms helps. Start with the easy ones, depending on the season: Southern Cross, Scorpius, the Teapot in Sagittarius, Orion and Taurus.
At first, navigating the sky meant nights of confusion, disorientation and dizzyness, punctuated by sudden revelations when my mind “joined the dots”. The Stellarium app really helped.
Stellarium also has versions for computers and the web. Learn to read the sky outdoors on clear nights, or indoors on cloudy nights.
The brightest stars are Sirius, Canopus, Alpha Centauri, Arcturus, Vega, Capella, Rigel, Procyon, Achenar, Betelgeuse, Hadar, Altair, Acrux, Aldebaran, Antares, Spica, Pollux, Fomalhaut, Deneb, Mimosa and Regulus.
These are not all visible at once. Some only appear during a few months of the year here in the Southern Hemisphere. Stellarium has a Visibility feature that shows when each star, nebula, cluster or planet rises. There will be late nights and early mornings involved.
Use the Heart icon on the phone version to record deep sky objects as you discover them. With a dark sky and your phone you should be able to find most of the constellations as they move through the sky according to the seasons. With practice you will be able to recognise the constellations with your eyes alone. I was surprised to see how far they appear to shift from hour to hour and from week to week.
I also enjoy an old school approach using paper and pencil. I print the latest Skymap and cross off items I can see with my eyes alone or with binoculars. For each item I read the Stellarium description. I have found this to be an excellent learning experience.
Further night watching with binoculars was a most rewarding next step for me. I opted for the Canon 18x50 IS. These are powerful (maybe too much for beginners) but the Image Stabilisation feature is astonishing.
Laying back in my moon chair and gazing at unwavering views of individual stars, clusters, planets and the infinite complexity of the Milky Way provides me with hours of wonder on every clear night. I was surprised to discover that the Jupiter’s Galilean moons are crystal clear with these binoculars.
You do not need a set as powerful as these to get started, although I do recommend the IS models by Canon or Nikon. Even a small 8x20 pair (8 times magnification with 20mm front lenses) will expand your night sky vision in a meaningful way.
Ask around within your own family. Maybe a free pair of binoculars is waiting for you in a cupboard, ready to start your stellar adventure!
When you move to using a camera, any tripod you already own will do to get started. As you become more involved, your photos will improve if you can trade up to the sturdiest model you can afford.
For fixed tripod use I chose a Benro carbon fibre tripod, replacing the ball head with a Manfrotto MHXPRO-3WG 3-way geared head.
I recommend swapping the standard fixed or ball head for a geared one such as this. Moving your viewfinder in any direction in very fine increments is quite important if you don’t have a tracker.
A tracking mount moves continuously to keep your target image centred in the frame. On a fixed tripod you will need to adjust the framing manually throughout the session.
I found the ball head difficult to align, and it drifted off target with heavy lenses. For fixed tripod work the geared head has quick release grips for easy initial targeting and stays where I put it.
It only takes a small turn of the elevation and panning controls before I can set the intervalometer or wireless remote running again. Siril’s stacking algorithms are so good that, with care, you can approach the quality of a tracked shot.
The Nikon Z50II is a modern mirrorless design with a powerful processor allowing silent, programmed shooting with its built-in intervalometer. Modern mirrorless cameras of any brand will operate in a similar way.
My Nikon D6 is a specialised DSLR (mirror reflex) camera with a high speed focus system, purchased for bird photography and car racing. Sadly, this feature does not extend to focussing on stars.
The D6 also has sensational low-light ability that has enabled me to enjoy museums, indoor exhibitions and night time events. The control buttons light up at night, greatly assisting my astro efforts. More details are available on my general photography pages.
If you have a DSLR then look for a silent mode in the menus. The constant clicking of an older style DSLR will annoy you and wear out the shutter. Shoot in Live View, using the rear screen only for focussing.
Cover the rear viewfinder eyepiece — most DSLRs supply a clip-in cover or have a small lever to lower a viewfinder curtain. A bright light from a torch or headlight entering the viewfinder won’t improve your images.
The D6 has a convenient side-loading battery with an official rating of 3580 shots. Mirrorless cameras generally have a rating of under 1,000. The Z50II is rated for 320 shots.
The good news is that the rear screen will be off for most of the night, greatly extending your battery life. I shoot well over 9,000 shots per battery on the D6 and 1,250 per battery on the Z50II.
Power supply will be a concern for owners of mirrorless cameras. If you are working late into a cold night you may need more than one battery. The battery warning lights on both cameras are somewhat pessimistic, flashing red while there is power for a few hundred more shots. Still, Z50II the needs two batteries to fill a 64GB card.
If the battery dies you will still have the images saved up until that point. Switch out the battery, recentre the image and start over. Siril can stack multiple sequences if the stars are roughly in the same position in each frame.
Newer cameras like the Z50II have USB charging from a portable power bank which is a huge bonus for this hobby. I hang a small bag containing a power bank under the tripod with a cable to the camera. This lasts all night.
The Star Adventurer 2i takes 4 rechargeable AA batteries that last over 5 hours which is more than I need.
In a world of almost total darkness the common wisdom regarding lens speed and exposure do not apply because you are NOT trying to get everything in a single shot with an average exposure across the frame. The sky is almost black, not light grey. Gather all the photons you can with the widest lens you can find.
Larger front elements always win, just like with telescopes. The aperture written on the lens is not as important as the physical diameter of the lens, found by dividing the focal length by the f-stop. A 35mm f/2 lens with a 17.5mm diameter will collect more light each second than a 28mm f/2 with a 14mm diameter.
Fast lenses are often prohibitively expensive. However, on a tripod in the dark you do not need autofocus, vibration reduction or fancy lens coatings. Suddenly a world of obsolete but very sharp lenses opens up on eBay. Often, superb lenses were only discontinued because autofocus or vibration reduction versions replaced them.
Sometimes a smaller manufacturer will produce an excellent astro lens at a relatively low price. The manual focus Samyang 135mm f/2.0 is a particularly well regarded entry level astrophotography lens.
The best lens at this focal length is the Sigma 135mm f/1.8 Art, at twice the price!
Really, any lens will do to get started, but a lower (wider) f/stop version is always better. Common lenses with approximate front diameters are given below.
Focal mm
f-stop
Diameter
300
4.0
75mm
200
2.8
71mm
135
1.8
75mm
105
1.4
75mm
85
1.2
71mm
50
1.2
43mm
35
1.4
25mm
24
1.4
17mm
21
2.8
7.5mm
18
2.8
6.4mm
14
1.4
10mm
Modern, fast zooms have the advantage of helping to find your target in the first place. Make sure you refocus after zooming in. Again, a lower f/stop rating is preferred. A 24-70mm f/2.8 and 70-200mm f/2.8 make a perfect pair for general Milky Way shots.
Most cameras made since 2015 should be suitable. Check the following:
Is your sensor clean? Take test shots of a clear blue sky at minimum and maximum focus distances then carefully inspect your images. Dust spots, cat hair, mould and goodness knows what may be sabotaging you.
If a DYI solution sounds scary then take your camera to a camera shop for cleaning. It’s one of those tasks which is really easy, the second time you do it… This is doubly important if you have a DSLR since one false twitch could break your mirror and spoil your whole day.
Have you extinguished all hot pixels? Take a dark frame at the ISO and shutter speed you intend to use. 1,600 seems popular but it depends on the design of your sensor.
Leave the lens cap on. If you have a DSLR then cover the viewfinder. If the result shows numerous spots then look for a menu item called Pixel Mapping or some such to cure the problem.
The dreaded walking noise
A limiting factor in astro work can be noise from the sensor. When images are stacked and stretched, low ISOs show fixed pattern noise. High ISOs show random read noise.
Find your optimum ISO value using dark test frames from ISO 800, all at 1ʺ exposure, with the lens cap on. Open the images in Siril, set the Display Mode to Histogram then click the Take a snapshot button (camera icon) to record each result. Convert the images to B&W in a separate graphics program.
In a perfect world you would see uniformly smooth images at all ISO values. In reality some shots will show bands, lines, squares or blotches. These are caused by a combination of fixed and random noise from the sensor itself.
What ISO is the smoothest? In your actual astro shots, fixed patterns will arch across your photos, looking like someone has scraped them with sandpaper, following the apparent motion of the stars.
Here are my D6 tests. ISO 12,800 to 25,600 is fine for astro work on my D6. I am happy using ISO 25,600 if I need to move my histogram a little to the right. Residual random read noise will be masked by stacking.
Here is a horrid example of walking noise from my very first astro pic, taken at ISO 800. The curvature and direction of the scrapes will depend on where you were pointing and your target’s distance from the celestial pole. In my case the walking noise was caused by fixed pattern noise from my sensor. Increasing the ISO solved this issue.
Another noise problem can arise by leaving the camera in the same position for too long. On a fixed tripod I move the camera to re-centre my target star every three minutes with a 90mm and every 60 seconds with a 300mm. With a tracker I normally reset the aim every 10 minutes, assuming I was succesful in pointing the rig somewhere near the southern celestial pole.
In astrophotography many sensor noise problems are greatly reduced simply by raising the ISO. Random noise in particular is mostly eliminated during stacking. The correct astro ISO for your camera will be much higher than you use during daylight photography.
Collecting enough light
Another form of sensor noise is caused by insufficient exposure in each of the light frames. Very short exposures with long lenses appear as almost pure black frames with severe clipping of shadow detail. Extreme stretching in Siril brings back some stars but with wide vertical stripes of random colour across the entire image.
A single test shot can help before you start collecting hundreds of frames. Look carefully at your camera’s histogram. Check for space between the left hand edge and the start of the data. If the data touches the left edge you will need to raise your ISO or use a slower shutter speed.
I need at least 45 minutes of total exposure to avoid this when using sub one second exposures, with my D6 on a fixed tripod. Here, with the stars removed, is the ghastly effect with only 20 minutes of 0.5ʺ exposures at ISO 16,000 and f/8.
Siril’s Banding Reduction helps reduce this vertical banding although some trial and error is needed for best results.
Calibration frames
I do not use darks, biases or flats. Living in a Bortle 3 zone, perhaps edging towards 4 in the south low on the horizon, I have no problems with city light pollution. Nikon NEF files, with background extraction tools in GraXpert and Siril are all I use for results I like, including repair of any vignetting that my lenses may have.
It is vital that the sensor be kept scrupulously clean if you are not using flat frames.
If you use calibration frames there can be problems introduced by the stacking process. Walking noise can appear due to a mismatch between the ratio of dark frames to light frames, or from temperature differences between calibration frames and light frames.
This is a complex topic but well explained in this article by Roger N. Clark
ISO Invariance
At Photons To Photos the tireless William J. Claff conducts exhaustive tests to find how every camera chip actually responds to changing ISOs.
The chart below shows data for the Nikon D6 and Z9, my older D500 (similar sensor to the Z50II) and the Canon 7DII. The Canon was a highly regarded astro camera in its time. Your camera is likely to be in the list, so start some research of your own.
Empirical evidence above shows the least random read noise on my D6 between ISO 12,800 and 25,600. I was interested to know whether Photons to Photos could shed more light on this.
The first chart showes the Dynamic Range Shadow Improvement which, as I understand things, indicates at what ISO a camera becomes ISO invariant. That is, from what ISO can you achieve the same result either by further dialling up the ISO in the camera, or by later increasing the exposure and shadow sliders in post-processing.
ISO invariance begins at the point where values stop stepping up and begin a slow upward gradient or flatten out altogether. That point is clearly different for each camera here.
In astro work, all photos are massively underexposed and must be adjusted (stretched) in software. Click this chart to find where your own camera becomes ISO invariant.
Set your ISO above the value where this invariance begins, depending on the lens and exposures you need. Fixed tripod work with long lenses (above 300mm) is likely to need even higher values to compensate for the short shutter speeds required.
The D500 and Z9 are invariant from ISO 400 and 500 respectively. Above those settings the photographer may choose to deliberately underexpose in the camera to gain shorter shutter speeds then adjust things later in software.
The Canon D7II line settles down and begins a steady climb from ISO 1250. Roger Clark often used that camera at ISO 1600. View Clark’s nebula gallery. If you are looking for an older affordable, second-hand camera then this would be a good choice. You will need fast lenses, f/2.8 or wider. Even a simple 50mm f/1.8 kit lens will get you started.
The D6 has a highly optimised chip that works best from ISO 800 and up, way up… I visit car museums, model train shows, indoor events and art galleries with my ISO at 12,800. DxO Photolab produces excellent noise-free images at this ISO.
This graph shows that D6 ISO invariance begins at ISO 10,000.
Input Referred Read Noise
The trick is to find the ISO where the camera is producing the fewest possible random electrons so that photon data from the stars is not compromised. Read noise is only relevant at long exposures. A good value is anywhere below 1 log2(electrons).
The D500 (similar sensor to the Z50II) and Z9 (similar to the Z8) dive below 1 log2(electrons) at ISO 400 and 500 respectively.
The Canon D7II drops below 1 log2(electrons) at ISO 2500.
The D6 bounces down its last “stair” at ISO 10,000 with its lowest value at ISO 16,000.
These two charts plus dark frame testing show that for astro work the D6 generates the least random sensor noise at ISO 16,000. I shoot the Z50II between ISO 3,200 and 8000.
With enough individual exposures, sensor noise becomes somewhat irrelevant. The stacking process cancels most random pixels caused by the camera’s electronics.
Click this chart to find your own camera’s response.
The shutter speeds necessary to capture clear, pinpoint stars are much shorter than I originally thought. This time-lapse video shows the Milky Way and Magellanic Clouds as they appear to rotate around the Southern Celestial Pole over a single night.
The calculation of short shutter speeds to halt this stellar motion is a prime concern for astro photography. Near the celestial equator everything moves much faster than you might expect.
With the ultra-wide 14mm lens used in this video, 30 second exposures slowed the stars enough for this demonstration. Perfect individual stars were not needed for each frame of the video.
Shorter times are needed for single high resolution shots of the same sky. For closeups of small deep sky objects with telephoto lenses longer than about 135mm, speeds of less than one second are needed from a fixed tripod.
The Internet is littered with useless information about The 500 Rule, a film-era concept and therefore entirely obsolete. An equation easy to calculate in your head but always wrong is no use to anyone. I ignore it.
The shutter speed to freeze stars (assuming mythical, perfect seeing conditions) depends on Focal Length, Pixel Pitch and Declination. The physical size of the sensor is irrelevant.
I use an equation derived from the Plate Scale of any sensor and lens combination.
There are 206265 arc-seconds in one radian. An equatorial star travels 15 arc-seconds per second. These two numbers never change and can be used to determine the speed of a star across a single pixel of your sensor.
A workable rule of thumb for ideal, pinpoint stars is:
(206265 x pixel pitch mm / focal length mm) / 15
Simplifying this is easy. 206265 / 15 = 13751
13751 / 1000 gives a µm value for pixel pitch.
Now plug in your own pixel pitch and focal length.
13.751 x pixel pitch µm / focal length mm
For a 21MP full-frame D6 with a pixel pitch of 6.43µm and a 90mm lens, this becomes:
13.751 x 6.43 / 90 = 0.98 (set camera to 1ʺ)
The 21MP APS-C Z50II has smaller pixels at 4.22µm. At 1ʺ each star will cross more than one pixel per second so I need a faster shutter speed to avoid star trails.
13.751 x 4.22 / 90 = 0.65 (set camera to 1/1.6ʺ = 0.6ʺ)
Nearer to the celestial pole your shutter speeds can be much longer — divide the initial calculated value by the cosine (in degrees) of the declination.
Worked example
The number of degrees in a radian / arc seconds traveled in 1 second = 13751
Divide 13751 by 1000 so I can use µm for pixel pitch.
The Nikon D6 pixel pitch is 6.43µm. For a 135mm f/2 lens the full equation for stars on the celestial equator is:
13.751 x 6.43 / 135 = 0.66 (set camera to 1/1.6ʺ = 0.6ʺ)
If every star in my field of view is higher than 60° then:
13.751 x 6.43 / 135 / cos(60) = 1.32 (set camera to 1.3ʺ)
If every star in my field of view is higher than 80° then:
13.751 x 6.43 / 135 / cos(80) = 3.8 (set camera to 4ʺ)
Online calculator
A more complex Nightscape Photography Formula (in French unless your browser can translate) gives fixed tripod exposures which are slightly longer than my equation, assuming you enter an aperture of f/1.
Aperture (f/2 in the example image below) does make a difference in this formula. Very small aperturers allow slower times. If you use slow telephoto lenses or telescopes at f/8 or f/11 then you may safely increase your shutter speed a little.
Scroll down to the second form. This asks for the horizontal pixel count of your sensor (5568 on my D6) which makes no difference to the FULL NPF results but can be used to calculate your approximate pixel pitch. Changing the sensor size also makes no difference to the result.
Enter zero as the declination for a value to freeze stars at the celestial equator. Select a formula accuracy for pinpoint stars
Cautious astrophotogaphers would use a shutter speed as close as possible to the lowest number on this page.
Try different values for declination to see how exposures may be greatly lengthened as you aim higher in the sky. For example, if every star in your field of view is higher than 70° then exposure times may be tripled with no ill effects.
If in doubt, temporarily turn up your ISO to maximum and take a single shot of your target at your intended shutter speed using a remote release or self-timer. Magnify your image and you will soon see if the stars have trails!
Effective star-stopping shutter speeds will always be faster than the Dodgy 500 Rule.
Magic Numbers
In practice I do not use online calculators to find my best shutter speeds for fixed tripod shooting. I use a constant value for my own camera (pixel pitch 6.43µm), derived from the Plate Scale equation.
206265 / 15 = 13751
13751 / 1000 = 13.751
13.751 x 6.43 = 88
My slowest speed at the celestial equator is my Magic Number (88) divided by the focal length of my lens. e.g. A 200mm lens needs 88 / 200 = 0.44. (set camera to 1/2.5ʺ = 0.4ʺ)
My shutter speeds are MUCH shorter than advice given on most astro sites, allowing a maximum star trail of only 1 pixel. I have a very low tolerance for oval stars! I have enough issues with ground vibrations from passing trains, wind catching my large lenses, upper atmospheric conditions and lens abberations. I don’t want to contribute to weird star shapes by using slow shutter speeds.
This method of shooting requires more images for stacking but any home computer less than about 3 years old should cope, given enough RAM and free SSD space for Siril to use.
Below are the calculated and actual shutter speeds I use on my 21MP full-frame Nikon D6. For example, on a fixed tripod using a 135mm f/2 lens I set 1/1.6ʺ = 0.6ʺ for stars on the celestial equator.
Lens
Calc.
Actual
300
0.29
1/3ʺ (0.33ʺ)
200
0.44
1/2ʺ (0.5ʺ)
135
0.65
1/1.6ʺ (0.6ʺ)
105
0.84
1/1.3ʺ (0.75ʺ)
85
1.04
1ʺ
50
1.76
2ʺ
40
2.20
2.5ʺ
24
3.67
3ʺ
21
4.2
4ʺ
18
4.9
5ʺ
14
6.3
6ʺ
The 500 Rule NEVER works. I use my camera-specific 88 Rule which works for my D6.
Bigger pixels are better for sharp stars from a tripod. A full-frame 24MP sensor allows slightly longer shutter speeds for a given lens if compared to a full-frame 50MP sensor. Longer shutter speeds collect more photons for better images.
The 21MP AP-C Z50II has a pixel pitch of 4.22µm so I use a 58 Rule. For example, my fixed tripod speed for a 135mm lens is 58 / 135 = .42 (set camera to 1/2.5ʺ = 0.4ʺ)
Search for the pixel pitch for your camera. Enter that value in the box below to find your personal exposure rule for pinpoint stars, on a fixed tripod, at the celestial equator.
Using your Rule, the shutter speed will be the orange number divided by your focal length. Use the next slowest speed available on your camera.
These extremely short speeds must be balanced by the need to collect enough light to avoid fixed noise problems, and your own tolerence for oval stars.
Siril’s Full Resynthesis feature can improve the appearance of misshapen stars.
Astrophotography stacks multiple, short exposures using a computer. While human eyes see landscapes in shades of grey on moonless nights, the stars and nebulae themselves are most colourful. These saturated hues appear when the images are aligned then added together. Your final shot shows what was really there, if only we had better eyes.
A huge bonus is that stacked images naturally reduce random noise. Normally accepted photographic ideas such as “High ISOs always give you more noise.” do not apply in astrophotography.
If possible, use a remote release that can be locked so that the camera keeps on firing. My wireless remote locks on if I hold it down for two seconds.
An alternative is to use an intervalometer, programmed to take multiple shots automatically. Check if your camera has one hidden in the menus. If not, then plug-in devices can achieve the same goal.
The menu pictured here shows a camera ready to take one shot every 4ʺ, repeated 50 times. Obviously, this will only work if the shutter speed is less than 4 seconds long.
I can start the process by touching the Start button on the screen.
Have a clear plan
I use a standard procedure every night:
Set the shutter speed that applies to my camera and lens choice. If I calculate 0.49 at the celestial equator my shutter will be 0.5ʺ, the closest available speed.
If using the wireless remote, I start my timer then press the transmitter button. Cable releases with a lockable button are also available. Take care not to pull on the cable as you start a sequence. Moving the camera will ruin the first shot, at least.
If using the intervalometer, I set it to fire 9 shots per interval (the maximum on my Nikons). The interval itself must be longer than the computer needs to fire and save 9 shots. For example, an interval of 6ʺ allows 10 sequences of 9 x 0.5ʺ shots per minute. Entering 25 x 9 gives 225 frames over a period of 3 minutes. This is a suitable time wait before adjusting the framing with a 90mm lens.
Each time my timer rings, or the intervalometer stops, I check the aiming point, returning the target star to the centre of the magnified focus box, then start again. On a fixed tripod I move my camera using the tilt and pan controls on my geared tripod head. On my Star Adventurer mount I reset my aim every 10 minutes using the illuminated arrow buttons and the fine declination adjustment. Siril will take care of all the jumping around so long as movements are within a few degrees.
I shoot as many frames as I can during a session, sometimes as many as 5,000 RAW files for stacking. On a Mac computer, Siril will handle these huge numbers easily. Windows users may be restricted to 2,080 RAWs. My Siril script will reject frames with meteors, aircraft and Musk Orbiting Objects then select the best 90% of the rest.
I find that two or three hours of photon gathering with very short exposures is plenty to create images that I enjoy. I dislike the soft, fuzzy stars that often result from poor weather conditions during long exposures. I prefer to work with thousands of very short exposures, and let Siril sort out the best images to stack.
My methods are relatively inexpensive if you already own a modern camera and a good computer. Just add a geared head and/or tracker to a sturdy tripod.
We shall see what we shall see as I build up a gallery. I am not trying to win prizes here or impress people I have never met.
Nothing I learned in my fixed tripod explorations of sky imaging was wasted. I am glad that I went through all the stages described above before I bought the tracker.
In optimising my early astro technique I built skills in:
Learning star charts and phone apps to find areas of interest.
Star hopping my way around the constellations with my eyes alone.
Knowing the Milky Way, constellations, asterisms, major star clusters and nebulae using binoculars. I may even be persuaded to get a telescope. The deciding factor will be how much I can lift. I am leaning towards a Goto Dobsonian.
Understanding the apparent movement of constellations around the celestial pole.
Determining optimum shutter speeds for a range of lenses at various declinations
Understanding and controlling the interval shooting features of my camera.
Using Siril and GraXpert at increasingly proficient levels.
However, as I become more experienced I can foresee physical limitations — walls that I will need to accept when working from a fixed tripod.
Shutter speeds are extremely short, and must be calculated carefully to avoid egg shaped stars or trails near the celestial equator. Use this equation: 13751 x pixel pitch mm / focal length mm / cos (declination)
Low light processing. Siril gives weird artefacts with massively underexposed shots. The test shot histogram must not be touching the left side of the chart.
Long lenses are difficult to aim and focus. I would put my own limit at 300mm f/4 for a prime and 150-600mm for a zoom. Larger scopes can double or triple this focal length. You won’t be counting Saturn’s rings with a camera lens.
Small pixels and long telephoto lenses do not mix well in fixed tripod photography. If you have a magic number lower than 45 then 135mm @f/2 is probably the limit for a telephoto lens, with shutter speeds no slower than 1/2ʺ.
SSD free space is a major concern when Siril needs to deal with thousands of images. Siril generates several FITS files for every Raw image. I have successfully stacked 7,500 files overnight on a Macbook Pro with an external 4TB SSD but there is a 2,048 file limit on Windows computers.
Set the shutter speed to match the lens. e.g. 1ʺ for 85mm on a 6.43µm sensor for a fixed tripod shot, or 10ʺ on a tracker.
If using an intervalometer, check that the settings allow the camera some extra time to save your shots and reset the electronics before each series. Test this indoors to save time later.
Focussing is a major problem with reflex cameras where using the rear screen in magnified mode is the only choice. I love my D6, but it is quite difficult to manually focus on stars. You will have the same focus problems when using the Z50II with the FTZII adapter and older styles F Mount lenses.
Set your lens to manual focus, turn the focus ring to infinity, then slowly move backwards until the main star is as small as possible. As you reach this point, nearby lower magnitude stars will suddenly become visible. Now concentrate on those dim stars as they blink in and out of existence.
Changing the ISO will alter the brightness of your Live View screen, helping to zero in on the exact point of focus. Try a darker screen to see more contrast. Don’t forget to reset your ISO before starting a new sequence.
For longer lenses with larger stars a Bahtinov mask clipped to the front of your lens may help. You can see diffraction lines moving as you focus, giving you a better sense of where the optimum point really is. These masks are available from 3D printer shops on eBay.
There is no such thing as Depth-of-Field when you are shooting stars. If you are not 100% in focus you will be trying to stack a fuzzy mess, with predictable results …
The good news is the Nikon Z50II with Z mount lenses will focus on stars! The camera automatically focuses at infinity as you turn it on, giving the focus system a head start for astro work. Each time I check the tracking I zoom the focus box onto my main target then tap the rear screen or press the AE-L button, now repurposed as an AF-On button in CUSTOM MENU SETTING, menu f2. This leaves the shutter button free for test shots as required or to wake up the camera without focusing.
Infinity focus changes with temperature. I refocus every 15 minutes.
The Z50II also has a side-hinged, fully articulated screen. I can swing it into position for easy viewing no matter what direction I have pointed the camera. On a tracker it is easy to adjust the screen angle as the camera rotates.
With much testing still to do, I am very pleased with the Z50II as an astro camera. It is small, highly capable and relatively inexpensive.
With the 50-250mm kit lens I can mount it on the tracking mount using only the ball-head adapter — no L-bracket with counterweight required.
As a (small) bonus its 21MP sensor gives me a few more pixels on my nebula than the 19MP DX crop of the full-frame Z8.
Test frames
If using a tracker, turn it on, focus and watch the target star for a few minutes. If it drifts off course then you need to realign the celestial pole and declination settings. I use PS Align Pro.
Shoot a single 30 second frame to check the camera is pointed where intended. With long lenses it’s easy to see star patterns that look like your target but are light years away. I often find new nebulae or clusters in the test frame that I can then include by moving the camera slightly.
Check for a healthy histogram, with the data for every channel well away from the left side of the chart.
Erase all test frames before starting the session. Always start with a blank storage card. Every thumbnail on your computer screen may be black, depending on your ISO, so you won’t know if you have extra frames before you start stacking.
Check aperture and shutter speed before starting the intervalometer or remote release. It’s easy to bump something in the dark then later discover you just shot for two hours at the wrong settings.
Astro processing requires non-trivial computer resources. I currently use a Macbook Pro M1 with 32GB RAM, 1TB internal SSD and an external 4TB SSD. All cpu cores are fully utilised by Siril. I cannot launch other programs after Siril has started.
With thousands of RAW and FITS files shuffling back and forth to the computer, the external Scandisk SSD overheats after just a few minutes and slows down. I have not experienced an operational failure but this is a definite choke point in my current workflow. I look forward to a new computer with enough internal disk space to operate more efficiently.
You cannot have a computer too big or too fast for this type of photography. I bought a 4TB external SSD because the 500GB of free space on the internal drive was clearly inadequate to process the many thousands of files needed.
In addition to my RAWs, Siril generates several thousand more work files. If space is available it will create a 2TB scratch file for intermediate calculations.
If you have an older computer then you may need to plan your life around extended processing times. I read Astronomy books while Siril works. My library is growing every week!
Thanks to videos by Deep Space Astro and Nebula Photos I have gained a basic understanding of how things work and which software tools will get me images I enjoy sharing.
Along the way I picked up useful tips about astronomy in general from AstroBackyard.
Given the large number of RAW files you need to stack, data compression is necessary at the start. Switching to uncompressed processing before the stacking command will retain image quality.
Siril rejects images with clouds, meteors, aircraft and satellites. The Full Width Half Maximum (FWHM) algorithm will also reject the worst 10% of images where the stars are ovals, caused by wind, bumping the tripod, poor tracking etc.
If you are keeping an eye on the console as the processing data whizzes by, an FWHM score of 2.5 is good for small round stars. The lower the better.
You may download my script OSC_TripodAstro_1-1.ssf. I do not use darks, biases or flats. I only use a single ‘lights’ folder containing my data. Save the script in the same folder as your other scripts, then restart Siril.
If you copy this code directly into your text editor then leave a blank line at the end.
###############################
# Script for Siril 1.2.5
#
# Tripod Astro 1.1
#
# December 2024
# Gregory Lewis
###############################
requires 1.2.5
set32bits
setcompress 1 -type=GZIP2 16
cd lights
convert light -out=../process
cd ../process
calibrate light -debayer
register pp_light -2pass -noout
seqapplyreg pp_light -framing=cog
setcompress 0
stack r_pp_light rej 3 3 -norm=addscale -filter-fwhm=90% -out=../$_$STACKCNT:%d$x$EXPTIME:%d$s_$DATE-OBS:dm12$
close
Create a folder on the external SSD, with details relevant to your filing system. Use something like Eagle_Nebula_2024-08-31
Inside this folder create another folder named lights; and move your RAW files into it. There may be multiple folders on your camera card(s) containing this RAW data.
Open Siril, set the Home directory to the new target folder then select my script.
Read an astronomy book.
OR
Go outside for more stargazing or photos.
OR
Wait … Make a toastie and coffee … Wait …
Check that the Display Mode is set to AutoStretch then open result.fit to see the initial stack. This will first be processed in GraXpert before continuing with Siril.
Typical green screen after stacking by Siril
Open GraXpert and load the result.fit file. From the ADVANCED menu on the right side of the screen, select the latest version Background Extraction and Denoising AI-Models. Background Extraction with zero Smoothing removes most of the gradients caused the moon and faint city lights where I live. I run Denoising with a strength of zero in preference to using the Siril denoise tools.
Save the Processed image and return to Siril. Open the new GraXpert file and Run Background Extraction from the Image Processing menu. Since most of the background gradients have already been removed by GraXpert I use these quick settings to tidy up. Click Generate then Compute Background.
Select Image Processing -> Color Calibration -> Photometric Color Calibration (PCC). This only works if you have enough stars for plate solving, so I leave cropping until after this step. Enter your target name into the Image Parameters dialog then click Find. If focal length and pixel pitch have not been automatically entered then do that (Origin specs are 334.8 and 2.4). If the plate solving fails, try turning off Auto-crop and/or Auto Catalog Limit. Also try cropping around your target. The sample image here was unsuccessful when searching for Swan Nebula but worked with Eagle Nebula.
Eagle Nebula region unstretched
Select Desaturate Stars from the Star Processing menu to improve the colour of stars.
Deconvolute the image if you think it needs sharpening. This is well explained in the Siril manual. I click the Stars option then Apply. If you see no difference then try a larger number of iterations. Origin TIFF images need high numbers above 50.
Save your updated file with a different name so you can come back to this version if required. Use the Save the current image button that looks like a download icon
If your target is a nebula or the Milky Way, separate the stars from the background. It helps to enhance the colours and shadow details without the stars. Run Image Processing -> Star Processing -> StarNet Star Removal... then Execute.
Open the new starless file with Generalised Hyperbolic Stretch Transformations (GHST). Set the Stretch Factor to 10 and the Local Stretch Intensity to -5. Click Apply to start moving the data away from the left edge of the histogram. Repeat the step above until the peak of the histogram is about 1/3 of the way across the chart.
Close the GHST tool and select Histogram Transformation. Repeatedly click the Apply Autostretch (cog wheel) and Apply buttons until the channel values stop changing. The Midtones, Shadows and Highlights will then read 0.5, 0.0 and 1.0 respectively.
Run Image Processing -> Star Processing -> StarNet Recomposition.... On the left half of the screen open your new Starless file. On the right side of the screen open the Starmask. Slide the Star Stretch factor slider to the right until all your stars appear but not so far as to cause bloating and haloes. A Stretch factor of between 5 and 8 generally works. Click both Apply buttons and Close.
Find the Save the current image button that looks like a download icon (next to the Save button at the top of the screen) to save a TIFF or PNG for further tweaking and cropping in your favourite graphics program.
Eagle Nebula region – final version.
For difficult images I use the Curves tool in GraphicConverter but I usually try Preview first as the controls are simpler. My final Adjust Colour tool in Preview usually looks like this, with the Shadows, Sharpness and the center Levels slider under the histogram moved to the right. Some additional saturation may be also needed.
Siril creates a Process folder containing many thousands of intermediate files. Unless you are planning additional manual processing it can be safely deleted.
I went for a simnple equatorial mount available from the local camera shop. The skills I have acquired from my fixed tripod experiences still apply when I use the Sky-Watcher Star Adventurer 2i Pro. I still re-aim the camera at regular intervals but now only every 15 minutes with long lenses. All the stacking and processing steps are identical.
My only issue is getting the mount pointed exactly south. I use this phone holder attached to the mount and phone apps.
I found that the declination reading in PS Align Pro is accurate but the compass bearing often drifts off course. The Apple Compass app was often much worse.
I get a better south bearing with CrowdMag. Its compass seems the most reliable.
On clear moonless nights I can see Octans through the mount’s polar scope, in the correct position as shown in the Console app supplied with the mount. However, I find that using only the compass apps is close enough.
With focal lengths longer than 200mm I limit exposures to 6 seconds. The mount itself may well be more accurate than that. However, a 4K crop on the Z50II 50-250mm kit lens gives an effective focal length of over 900mm. A 400mm lens, cropped, gives the equivalent of 1,500mm. It doesn’t take a very strong breeze to give me star trails.
I use a Nikon DF-M1 Dot Sight, mounted on the camera hotshoe to get an approximate bearing. Starting with the widest setting on my zoom lens, I take test shots with progressively more zoom to line up my object for the night. Although this gadget saves a lot of time, the red dot in the sight is quite bright even at its lowest setting. I may remove the battery and just use the glass sighting frame to find my targets.
I perform an initial test on tracking accuracy by focussing on a single star, magnifying the view and watching to see if it moves. Even a small nudge of the camera or twist of the mount on the tripod will ruin the setup. Any obvious mistracking needs another check with the phone apps. Ensure that every possible adjustment has been tightened.
As the mount inevitably drifts off course during the night I check my aim and focus every 15 minutes. I use this time to stargaze and cross off items on my paper Skymap.
In addition to small errors in aiming south or setting the correct declination, the slow swinging of the counterweight also alters the tracking. I often find that I need different amounts of aiming correction through the night.
Keeping the target in the centre of the rear screen reduces field rotation in the final stack, allowing more of the frame to be used. My script includes the -framing=cog command for a good general alignment. When my tracking has gone well with only tiny recentering adjustments needed I use -framing=min instead. This automatically crops out field rotation artefacts around the image borders.
I now run a Celestron Origin smart telescope alongside my camera rig. I use the Nikon Z50II from 14mm to 400mm and the Origin for smaller targets. With an effective focal length of 1,600mm @ f/2.2 the Origin collects and stacks images on its own. The manual is clear and there are several online groups to help.
Here are my observations about Nikon’s newest camera. Included here are some points from previous sections on this page, and opinions about its suitability as an astro camera.
The green "AUTO" setting is the most competent I have ever used in a small camera for general photography. Straight out of the box this camera thinks for itself, with advanced subject recognition, button control for common features and excellent quality images.
A full range of modern imaging options are available as custom settings, allowing photographers to develop their own style in advanced stills or movie work.
The Expeed 7 electronics are also used in Nikon’s full-frame Z6III, Z8 and Z9. Although some high speed professional features are limited by the battery size and the reduced light gathering area of the smaller sensor, the Z50II is a modern design in every respect.
While the Z50II is in no way a replacement, or even a distant contender, for the high-speed action and low light photography that I enjoy with my D6, it is a tiny powerhouse of cutting edge technology. Highly recommended.
I think the Z50II is also an ideal camera for deep sky imaging. My experience with the tiny 16-50mm, 12-28mm and 50-250mm kit lenses was so impressive that I purchased a full-frame 400mm f/4.5 VR S lens specifically for deep space photography.
Some astrophotography oriented features are:
This is Nikon’s best small camera. 21MP (5568 x 3712) on a 23.5mm x 15.7mm sensor gives a 1.5x crop advantage for long telephoto lenses. In effect this gives more “pixels on your nebula” compared to a full-frame Z6III. It also has a slightly higher pixel density than the professional level Z8 and Z9, at a significant cost saving.
Older F Mount lenses can be used with an FTZ adapter. I have tried these lenses, all with excellent results. Manual focusing on stars is time consuming but I regularly get FWHM values under 2.5 in Siril.
Sigma 14mm f/1.8
Sigma 24mm f/1.4
Tamron 45mm f/1.8
Tamron 90 f/2.8 Macro
All Z lenses focus near infinity instantly — just recycle the power! This is not perfectly in focus, but close enough for the autofocus to lock on to stars easily. It is literally a three second operation, an immensely useful feature.
The fully articulated rear screen (shared only by the Z6III) allows me to see my target no matter how awkwardly the tracker orients the camera during the night. This is a definite plus compared to more basic tilt screens.
The small lightweight design allows beginners to mount the camera and any kit lens directly onto a ball-head and adapter on the Star Adventurer 2i, without needing the declination bracket and counterweight. Smaller more portable tripods may be used.
Sequences may be programmed with the built-in intervalometer. I often set bursts of 9 x 6ʺ exposures, restarting every minute. 15 repetitions makes an even 15 minutes, when I refocus and check my aiming point.
Low levels of fixed and read noise combined with GraXpert’s excellent AI background extraction and noise reduction give me pleasing, high quality images. This is a hobby, not a competition. While there are undoubtedly better specialised astro cameras for use with large telescopes, I like the simplicity and quality of the Z50II combined with the flexibility of switching focal lengths as needed.
Astronomy Australia is an annual magazine published by Quasar. This should be on every star gazer’s desk.
My favourite books are here. All have filled my mind with profound ideas and/or exquisite imagery. This list includes only those I have read. All are recommended.
I am currently working through a free Astronomy textbook (161MB) from Rice University in Texas. End-of-chapter questions are tough for non-mathematicians but the information is well presented, taking you up to the launch of the James Webb Telescope.
Home Observatory images. 335mm with a 4.8 crop gives an equivalent 1,600mm @ f/2.2.
The RAW files can be saved and processed as normal in Siril, or you can use a pre-stacked TIFF created by the scope itself. Processing this single file is only slightly different.
In GraXpert open the pre-stacked file. e.g. Tarantula_Nebula_03-07-25_21_17_41.tiff. Run Background Extraction and Denoise then save.
Open Siril and load the new file —
Tarantula_Nebula_03-07-25_21_17_41_GraXpert.fits.
View in Histogram Display Mode and crop to remove edge artefacts. Reset the display to AutoStretch and follow my Stacking and Processing routine from step 9 to 16.
Is your sensor clean? Take test shots of a clear blue sky at minimum and maximum focus distances then carefully inspect your images. Dust spots, cat hair, mould and goodness knows what may be sabotaging you.
I use a Nikon DF-M1 Dot Sight, mounted on the camera hotshoe to get an approximate bearing. Starting with the widest setting on my zoom lens, I take test shots with progressively more zoom to line up my object for the night. Although this gadget saves a lot of time, the red dot in the sight is quite bright even at its lowest setting. I may remove the battery and just use the glass sighting frame to find my targets.
