A nice portrait but ...

I was invited, along with a couple of other members of my camera club, to give a talk to nearby Neath Camera Club. I put together an array of portrait, fashion and editorial type work to wow people with and at the end tucked in a couple of astro photos to say how I was embarking on a new genre of photography. During the traditional mid-meeting tea break I was approached by several people that only wanted to talk about the astrophotography. I have also had several friends asking about how it all works. When I find out I will let you know, but for now I thought I would put together something about what I know so far.

One or two questions

The images I have taken and posted so far have sparked quite a few comments and questions from 'That looks complicated', to 'I'm intrigued by the process.' so this is my attempt to explain things with a little more space than Facebook provides.

As I mentioned in my previous blog, it is possible to get started in astrophotography with reasonable ease, especially if you are already a photographer. Images of things like the galactic core can be taken with a normal SLR or mirrorless camera on a tripod. I have even seen amazing results from a phone camera mounted on a tripod - an iPhone on a tripod can take a 30 second shot (actually 3 10 second ones stacked together in camera). With short focal length lenses on a DSLR or mirrorless camera, a 20-30 second shot will not create star trails but can be enough to capture something like the core. It is beneficial if you can get your hands on a fast lens to maximize light capture in this short time.

For something like photographing the galactic core, you will want to use a short focal length lens, typically something in the range 10-16mm. A shorter focal length lens will allow you to shoot for a longer time without getting star trails. My previous blog talks about things like the 500 rule for calculating how long you can shoot for before the rotation of the planet causes star trails. You also need to take into account any crop factor your sensor introduces. Unless your lens is specifically designed for a cropped sensor you will have to multiply the focal length by a crop factor. Understanding how longer focal lengths affect the length of time you can shoot an individual frame, or 'sub', for becomes important as you step up into the world of tracking mounts and guide cameras.

Your camera's sensor sensitivity is also something you need to adjust to maximize your light gathering ability. This is controlled by your camera's ISO settings. For astro work it is quite normal to be shooting at 1600 or 3200 ISO to allow you camera's sensor to capture as much light as possible in the time the shutter is open.

That is where it all started for me. I shot some images of the galactic core using a small mirrorless camera with a 23mm lens. Hold on, didn't I say in the range of 10-16mm. Well yes, but my 23mm lens was very fast at f1.2 so I traded field of view for longer exposure times. There are always trade offs in photography! This is also where, for the sake of my bank balance, I should have stopped.

Those pesky stars and things are a long way away!

Yes they are. I am now singling out deep sky objects 5,000+ lightyears away. To do that you need something with a bit more punch than a 10-16mm lens and a 30 second shot. This is where the first expense came in, buying a mount. The mount is the device that tracks the stars so you can shoot for longer without star trails. To photograph deep sky objects like nebulas, galaxies, and star clusters you need a longer focal length lens. I started by using my Fuji 100-400mm lens. My Fuji X-T5 has a crop factor of 1.5 effectively giving me a 600mm lens. This is way more than you actually need in many cases, those nebulas are indeed a long way away but some of them are huge - hundreds of lightyears across - and astrophotographers often use lens in the 250mm range. I have watched tutorials with people using 135mm lenses to capture some nebulas. Galaxies (excluding our neighbour Andromeda) and smaller nebulas do need much longer focal lengths.

We can start to see what the problem is though with using a longer lens. The 500 rule - which is very basic and not that accurate - says that you need to divide 500 by the focal length of your lens to get the number of seconds you can shoot for before you see star trails. Seeing the problem yet? 500/600=0.833 seconds, you are not going to get a very bright image even with the highest ISO settings. The mount is crucial for longer focal length lens because it does not limit us to these shutter speed rules. There are still limits, but we can shoot for much longer. I was able to start shooting for about 60 seconds at a time using the mount.

By now, Tina and Hannah had already decided they would like to see the stars as well and not rely on the back of my camera, so we purchased a proper telescope, a Sky-watcher Evostar 72ED Pro, and I added the additional bits required to connect it to my camera. In addition to the telescope I also needed something called a field flattener which was nearly as much as the scope itself. The field flattener sits between the telescope and the camera and optically corrects the image from the telescope so that the edges are recorded more sharply on the camera sensor. Camera lenses have a lot more optical components in them than telescopes that make these kind of corrections. I also needed a small T-Adaptor which connects the telescope to the camera and I added a camera rotator so that I could turn the camera to frame shots better.

This image of the Orion and Running Man nebulas was shot using this exact equipment, a Sky-watcher Star Adventurer GTi mount, my Evostar 72ED scope, and my Fuji X-T5. I was absolutely elated when I saw the first 60 second shot appear on the back of the camera. In the death I only got 21 usable images giving me what is known as 21 minutes total integration time, and my initial edit did not look like this but more on that later!

Clear skies

Cloudy skies are the curse of the astrophotographer, along with street lights, moon light, wind, Starlink satellites, .... Actually there are a lot of curses for the astrophotographer. I was fortunate enough though to get a few clear nights, shortly after getting all the new kit, so made the most of it. Here is a shot of the Horsehead nebula. This was my second attempt because the first time I think my focusing was off. This was 61 minutes of integration time (see earlier if you are not sure what that means) but introduces the next issue - periodic error. I actually took over 200 images but a lot of them were not usable because there was slight blurring in the stars. There are a couple of reasons for this. Firstly in some cases the scope can be affected by wind or vibrations. A bigger issue with the mount though is something called periodic error. The mount tracks the stars using mechanical gearing and occasionally the mechanism, in simple terms, is not accurate enough to guide the mount and it has to play catch up. This slight jarring can cause stars to blur.

Okay, I'm all in ...

I'd heard about unguided and guided images but was not really sure what that was all about so it was time to resort to everybody's friend Google and YouTube. Simply put, guided exposures are ones that use a special camera to track stars and provide more information to the mount to improve its tracking capabilities. Simple tracking really just takes into account the speed of rotation of the Earth and moves the mount to counteract that rotation. Guiding uses a guide camera to lock onto a star and monitor its position. As the position changes the camera can provide information back to a computer, which in turn tells the mount to adjust its position to keep the stars sharp.

So now I need a computer? Well yes, but there are options. You could use a laptop computer. That involves running cables from the guide camera to the computer, run some astrophotography software on the computer to interpret the guide camera information, and then another cable back to the mount. Most people have a laptop of some form, and the software, PHD 2 is commonly used for example, is free so not a big deal. The problem is all those flipping cables! Also, do you want to be sitting around outside on a cold night with a laptop running? Well this used to be the norm but thankfully there are other options now that do not involve a massive laptop. There are small dedicated astro computers that often run something like a Raspberry Pi operating system in a small box that can sit on top of the telescope or nearby. They connect to the guide camera and mount and run the software required to control the mount. In addition they can provide a whole host of other features such as sky maps to allow you to find deep sky objects and the likes, polar align your scope, and control additional hardware you may have in your setup.

There are now several companies that make the 'magic box of tricks' I referred to in the opening paragraphs of this blog. I opted to go for one called an ASI Air Mini made by a company called ZWO Optics and the reason for that is because it seems to be the most commonly used one. I was also looking at a ZWO guide camera so it made sense. The problem is that the ZWO ecosystem is very limited to only supporting ZWO products. The ASI Air can control some DSLRs and mirrorless cameras but they tend to be Canon and Nikon models. My little old Fuji just does not cut it so now I had another problem, only use some of the functionality of a the ASI Air or get a supported camera to go with it.

DSLRs and mirrorless cameras can work well for astrophotography but there are a couple of issues with them. Firstly they are susceptible to a lot of sensor noise for long exposures. Secondly they tend to filter out a lot of infrared light frequencies because they have a filter in them specifically to do that. You can get cameras 'modded' to remove the filter but it is a specialized thing the camera has to be sent away for. There is a lot of red light in space (caused by Hydrogen alpha star forming gas) so ideally you want to capture it. Below, jumping ahead a little, are two shots of the Rosette nebula. The first is taken with my Fuji, the second with my new dedicated astrophotography camera. Although some of the difference is down to different integration times and processing, you can see the amount of red light the astro camera has caught is far greater.

I decided that I was going to go all in. If I wanted to make the most of the ASI Air functions I would need a different camera and I saw little point in buying a Canon or Nikon one and getting it modified when I could buy a dedicated astro camera. The camera I went for was the ZWO ASI533 MC Pro. The MC bit means it shoots in colour and the Pro bit means it is a cooled camera. Cooling greatly improves sensor noise and helps with what we call calibration frames which we will discuss later. it is very much an entry level camera. It has a tiny sensor compared to something like my Fuji - just 9Mb (3008x3008 pixels).

You can buy mono cameras. They only shoot black and white images and you buy red, green, and blue filters. Shoot a load of images with each filter and combine them in software. Mono cameras provide a greater level of detail because of the way the sensors are made but the process obviously takes a lot more time to shoot. With a one shot colour camera you could take 12 5 minutes shots to get 1 hour of integration time but to get the equivalent image with a mono camera you would have to shoot 12 5 minute shots for each colour filter - so 3 hours in total. There is also something called a luminance filter which is typically used with RGB filters increasing the integration time still further as all 4 are combined.

There is a trade off between detail and time. There is however another advantage of shooting mono and using (L)RGB filters, and that is the images are not affected by light pollution from city lights etc.

All the gear, still no idea

Okay, so that is all the kit and theory but how does it all work in practice? All this kit has ultimately led me to a situation where I can shoot for much longer lengths of time and increase the total amount of light the camera can gather in an individual shot, but that is not where the story of light collection ends. As I have already said, our total integration time is not just a single shot. It can be hundreds.

After doing the polar alignment of the telescope you can ask the ASI Air Mini to go to a specific target. This is a really handy feature. The ASI Air has an inbuilt map of the sky and can advise on what is currently visible (and for how long) or you can just browse a catalogue. Once you have selected a target the ASI Air will slew the mount to that target and then 'plate solve' an image. This involves the ASI Air analyzing the stars in the picture it has taken and making sure it is exactly on target. Without the ASI Air I would have to perform a 1, 2 or 3 star alignment process. This involved me selecting a target and asking the mount to go to it. Then I would have to manually align the target to ensure it was in the centre of the telescope view using buttons on the mount, and confirming the position. The process had to be repeated for however many star alignments I had selected. The more you did, the more accurate the mount would be when slewing to an actual target to photograph. You are essentially training the mount to know where it is pointing and then accurately target your actual subject. The ASI Air takes care of that learning for you.

I connect to the ASI Air, using a wireless connection it creates, with an iPad (any tablet or phone can be used), which allows me to control a lot of the functions from the comfort of my sofa via the obligatory app!

Lights, flats, darks and biases??

Earlier I mentioned calibration frames. You might think that you just take a bunch of images and stick them all together to create the final image but it is not quite as straight forward as that. The images we have talked about so far, the shots of an actual target, are called light frames in astrophotography land. There are some other shots we have to take as well to get the best out of our light frames.

With long exposure times, sensors can get hotter leading to more sensor noise. To counteract this in our final image we take what are called dark frames. These are shot using exactly the same camera settings in terms of ISO (called gain on dedicated astrophotography cameras) and duration but with the lens or scope cap on. If I shoot my normal light frames at a gain of 100 for 3 minutes each, I shoot 10-20 dark frames at gain 100 for 3 minutes each with the lens cap on. The dark frames are used by the stacking software to average out the digital noise created by the camera circuitry and effectively subtracted from my final image. This takes care of a type of noise call dark current noise. The more dark frames you take the better but 20-30 is usually enough.

We also shoot a bunch of frames called bias frames. For these we still have the lens cap on but use a very short exposure time. These types of calibration frames take into account something called sensor read noise. The noise from the circuitry created by simply reading the pixel values. Again these are averaged out and subtracted from the final image.

Lastly we take flat frames. For these we put a constant light source in front of the lens (now with the lens cap removed!) and take as series of shots using the same gain (sensitivity) and allow the camera to select an exposure duration. I use an iPad with an app that generates a pure white screen to create my constant light source. The flat frames are used to detect lens and sensor abnormalities such as sensor dust and lens vignetting. Again these are averaged out and subtracted from the final image.

The ASI Air allows me to set up a list of shots I want to take so, for example, I can can say take 50 lights at Gain 100, 300 seconds long each, 20 darks at Gain 100, 300 seconds each, 20 bias frames at Gain 100, 0.001 seconds each, and 20 flats at 100 gain and whatever the calculated exposure time needs to be - the ASI Air has an auto-exposure mode a bit light aperture priority on a DSLR or mirrorless camera. Hit the play button and let the ASI Air get on with it all night long.

I reality you cannot do that because you have to put lens caps on or place light sources over the lens for some of the required calibration frames but you get the idea. Some people choose to take their calibration frames the following morning, so can run these automated plans, but I cannot leave my scope out all night.

With a cooled camera you can create what is known as a darks library. The darks which are shot at the same gain and duration as the lights are supposed to be done each time you do your lights to account for differences in temperature. With a cooled camera the sensor temperature is known so you can shoot them anytime providing you cool the sensor to the same temperature that was used for the lights, and use the same gain. This is really useful because you can spend more time capturing the actual lights at night and not waste time capturing very dark images!

You can actually set up the ASI Air to 'live stack' images so you can see how your final image will look. I usually do this but make sure the ASI Air is keeping each individual light frame so that I can stack them manually in an astrophotography software package. It's nice to see the image building up though. You should create your calibration frames first when using the live stack feature to see the best possible image but I rarely do this as I am stacking it manually later. As we shall see shortly, a large part of astrophotography is the post processing and this live stacking does not do any of that so it only gives you an idea of how things will look.

There are two things to note about the above image. It looks a lot different from the live stack image and this is down to processing the various light, flat and bias frames in a software package called PixInsight. This does the heavy work of stacking all the images together, applying the calibration frames and allowing me to do other adjustments like removing any gradients caused by street lights, sharpening the stars (in fact removing them altogether and blending them back in) and making sure star colours are accurate (they are not all white!) Secondly, what happened to the dark frames? Well there seems to be a lot discussion at the moment as to whether they are really needed when you have a cooled camera. So far, from my experience, they are not. I have yet to process a final image using dark frames so it will be interesting to see if they do make much difference.

What about all the guiding stuff?

If you look back at the image of the live stacking you will see a small graph up in the top left. This is showing how accurately the guiding information is and thus how well the stars are tracked. Seasoned astrophotographers will be dying on their backsides at the amount of total error the graph is showing (2.16 degrees) as they can become obsessed with getting that little graph as straight as possible. I am not going to lie 2.16 degrees is good for me! Guiding is affected by polar alignment accuracy, how well the guide scope is detecting the stars, configuration parameters for how information is sent to the mount and doubtless many other things (even wind, and vibrations caused be walking too close to the telescope). The ASI Air has a mode for setting up the guiding but you have to get some stars in focus first. If I tap on the guiding graph it opens the full guiding screen where I can focus on stars and adjust some parameters to improve the guiding.

This information is fed back to the mount to make small adjustments in its position, and keep the mount pointing exactly where it needs to be to ensure your target does not fall out of the frame (ever tried to see the moon in your view finder with a long lens?) and that things stay nice and sharp.

With good polar alignment, well calibrated guiding and preferably no wind you can greatly increase the exposure times of each light frame. I am now getting exposures of 5 minutes at a time with very little to no blurring of stars.

This image of the Jellyfish nebula below is 30 light frames, each 300 seconds long, at Gain 100, with 20 flats each about 2 seconds long, and 30 bias frames. It was stacked and processed in PixInsight and some final tweaks to sharpness, colour, and exposure in Lightroom.

How do all those individual frames create something like the Jellyfish nebula?

A major part of astrophotography is the post processing, taking all those individual frames and combining them to make that final image. There are numerous software packages out there that will stack photos for you, even Photoshop and Affinity Photo can be used. The latter has a special astro mode allowing better use of the calibration frames unlike Photoshop. There are also specialized software packages that are designed to get the best possible results out of your frames such as Siril (which is free) through to PixInsight (which is most definitely not!) There are additional plug-ins that can assist further for AI enhanced noise reduction and star enhancement. I use PixInsight and 3 plug-ins - RC Astro Blur, Noise and Star Exterminator.

PixInsight does the heavy lifting of combining all the different light, bias, flat and dark frames (if used) into a single image but you might be a little disappointed with what it throws out. Initially we get what is known as an 'un-stretched' or 'linear' image.

There are various tools in PixInsight and other image manipulation software packages that can stretch the data. In Photoshop you can use things like Levels and Curves adjustments, in PixInsight you have the elaborately named Generalised Hyperbolic Stretching tool, Curves tool, and Histogram Transformation tool. There is also a tool in PixInsight to show a preview of a kind of automatic stretch called the Screen Transfer Function.

Here is what the Flaming Star nebula looks like after doing an initial stretch.

The Screen Transfer Function only shows me a preview of what the image might look like when stretched - we are still showing what is known as a 'linear image'. To convert the preview into an actual stretched non-linear image I use a tool called the Histogram Transformation. You effectively copy the information in the Screen Transfer Function into the Histogram Transformation tool and apply it to the image.

Then I do something a bit strange. I use another plug-in called StarXTerminator to remove the stars from the image and create a second image with just the stars in it. The reason this is done is so that we can further stretch the nebula gases separately from the stars. This stops the stars from becoming blown out as they are already much brighter than the gases in the nebula.

The last plug-in I use is NoiseXTerminator that removes the digital noise that may still be in the image.

Using the Generalised Hyperbolic Stretching tool and the Curves tool you can increase or decrease the dark and light areas of the image to taste. This is still very much black magic for me and I never seem to get the same results twice! You get the idea below though, using the tools you start to pull out more details from the stacked image data.

I hate maths but ...

When you are happy with the nebula stretching you can re-introduce the stars back into the final image. You may want to stretch the stars a little as well if they are too dark. I use a tool call Pixel Math which has the catchy formula Combine (Nebula Photo, Star Photo, op_screen()). Photoshop users may catch on that this process is using a screen blending mode to add the stars back in. Pixel Math is not the only tool you can use, free plug-ins to PixInsight provide other options like Image Blend. This is not only true for blending the stars in but many other functions, for example there are about 4 different ways I know of to removed colour gradients in images caused by light pollution.

My eyes are not that good

The stacked frames pick up a lot more stars than you could ever hope to see with your eyes even through a telescope. It is quite common practice to reduce the amount of stars by removing the ones that are faintest. There is also a trend for removing them completely in some nebula and galaxy images but I like to see a few of them. I can use a star reduction tool to remove some of the smaller stars.

Finally you can export the image as something like a TIF file and do final edits in Photoshop or Lightroom. I tend to use Lightroom to boost clarity, maybe play around with the levels and colours a little to produce my final image.

Any other gems you want to share Iain?

Well, yes. Google any of the nebulas I have shown here, or indeed any deep sky object, and you will see that the colours in any images of them can vary greatly. Is any of it real? A lot of astrophotography is kind of art, open to interpretation or, in my case, how much the editing tools want to play ball for me! I have re-edited the same data and ended up with completely different looking versions of something - sometimes for the better and sometimes not!

Another question would be 'which way is up?'

It is not just the editing tools either. I mentioned earlier that some people capture images using red, green, blue and luminance filters to avoid light pollution. How those images are recombined can affect the final colours of an image.

There are also 'broadband' and 'narrowband' filters that can be used to try and filter out the effects of light pollution for colour cameras. A broadband filter, as the name suggests, allows a broad range of light frequencies through but tries to cut out pollution from things like street lights. The narrowband filters are also used to filter out light pollution but target more specific wavelengths of light. Typically they concentrate on allowing Hydrogen alpha gases, Oxygen III gases, and Sulphur II gases to be captured. Hydrogen alpha and Sulphur II gases are red in colour, and Oxygen III is bluish in colour. I use a Ha/OIII filter a lot of the time but because this is filtering out all but the red and blues it can make the star colours incorrect. You can shoot some subs with the Ha/OIII filter and then some more using a broadband filter and combine them in a similar way to somebody shooting with red, green, blue and luminance filters. Again how they are combined affects the final image.

Everybody knows the Hubble Telescope right? Well there is a thing called the Hubble Colour Palette. This is where the Hubble telescope takes images using a Sulphur II filter, a Hydrogen Alpha filter, and an Oxygen III filter and combines them so that Sulphur II maps to red, Hydrogen Alpha maps to green, and Oxygen III maps to blue. Go check out the iconic 'Pillars of Creation' shot taken by Hubble in 1995.

I am considering getting an SII/OIII filter to go with my Ha/OIII filter so I can combine the two for a Hubble palette effect.

The bottom line is, colours are pretty subjective and very much depend on the method used to capture them - shooting something with an infrared filter would show a very different image to one shot without but they are both still real.

As for which way is up - that is a different kettle of fish entirely. With some telescopes and camera flipping and mirroring images and some correcting for that your guess is as good as mine!

Simple

I hope this gives a brief idea of what is involved in creating astrophotography images. I hasten to add that this is just my very simplistic understanding and what works for me. You can quite literally have a brain melt down if you read into the more technical issues around the genre. The nice thing is the more you do it, the more you start to understand and the scope (no pun intended) for improvement is huge. I do think though this is an area of photography where kit really makes a difference. Something like the ASI Air or equivalent really does make capturing images so much easier than in the past. A key component is the mount and in some ways I wish I had invested in a better quality mount to take heavier payloads, for example bigger telescopes, but at the same time I wanted a portable setup and the Adventurer GTi I have is great for that.

Better mount, longer telescopes, mono cameras, cottage in the middle of the Elan Valley. Where will it all end!

Until next time, clear skies.