Astrophotography – Deep Sky Objects

In this final edition of Astrophotography blogs, I cover the techniques involved in capturing nebula and star clusters.

Astrophotography – an Introduction discussed planning, equipment and apps that support astrophotography, so I’d recommend having a read of that blog.

If Astrophotography were seasons there would be two, March – October for Milky Way and October to March for Deep Sky Objects (DSO’s). The benefit of DSO’s during winter is dark nights come early so no need to stay up into the small hours of the morning. A very respectable 7pm to 9pm will be fine, just remember to wrap up.

DSO photography is more technical and time consuming than nightscapes and moon shots. There is the time to set up and take the shots, then there is the image review and processing time.

I allow 2 hours to set up and take photos. It’s a good idea to set the camera settings before you go out. Outside allow 30 mins to set up, find your target, take test shots for focus and refine settings. Allow 30 mins minimum to take the shots and an additional 15 mins to take calibration frames (I'll this cover later). The rest of the time is faffing and realising you forgot something in the settings!

Image review and processing time will vary, based on how many shots you’ve taken and the processing speed of your PC/ Laptop. For 200 photos I take 10 mins to review and delete any blurred shots. 20 mins for the stacking software to stack the photos, then about 30 mins stretching and post processing.

DSO photography can also eat into your pocket. You start with some nice images using a nifty 50mm lens but want to get closer, you need a bigger lens! Then you want cleaner images, so you want a star tracker! Then you discover astro cameras and telescopes! Before you know it, you have an observatory shed in your garden!

DSO Targets

There are many nebula and star clusters out there and a quick internet search will help you identify ones within the capability of your lens. Most targets are achievable from your back garden or nearest park, but some will require darker skies. Orion and Andromeda are great starter nebulae as is the star cluster Pleiades.

Andromeda 105 - 105mm (ff) 2.8 ISO 1600 2.5sec x 168 with crop

Andromeda 105 was my first DSO attempt, taken on a Canon 6d mkii (a full frame camera) with a Sigma 105 macro lens. It consists of 168 x 2.5 second images, which have been stacked, stretched and cropped. Below is what a single unedited image looks like. Whilst 168 images sound a lot it's only a 7 min exposure, which is quite short in astrophotography terms.

A single unedited image, Andromeda is slightly right of centre

Pleiades tracked – 135mm (200mm ff) f5 ISO 400 30 secs x 65

The star cluster Pleaides is bright on most winter nights and is a good option if you have clear skies but a full moon. Star clusters or constellations are the best options with a full moon as the moon's brightness can wash out nebulae.

You can get decent images using a zoom lens e.g. 70-300mm, at around 135-200mm.  I’d recommend an intervalometer if your camera does not have this function. This will automatically take any number of shots you set it to. You can use a remote trigger or even a 2 second timer, but you’ll be pressing the shutter button a lot!

Taking the shots

Orion was shot from out of my front bedroom window (its warmer inside!). I used https://stellarium-web.org/ to see where Orion would be, at the date and time I had planned to take the shots, also based on the clear sky weather forecast.

Orion – 200mm (350mm ff) f5 ISO 3200 1 sec x 180

I used Photopills (or https://www.lonelyspeck.com/advanced-astrophotography-shutter-time-calculator/ ) to get the exposure time for no star trailing.

With the camera on a tripod, I switch off vibration control/ image stabilisation, and manually focus on a bright star and take a couple of test shots to set the ISO. I generally try my aperture on the widest (lowest number) setting first.

I set the intervalometer to max 100 shots, once they’ve taken I then re-centre my subject in the frame and set off the intervalometer again to take the next 100. The more shots, the more detail and less noise.

The time set on the intervalometer must be longer than the exposure. If you have a heavy lens, you might want this to be up to 5 seconds to allow any shutter movement to settle. For Orion the exposure was 1 second and the intervalometer was set to take a photo every 3 seconds. It is important to turn off long exposure noise reduction in your camera settings as this may disrupt the intervalometer.

Once I’d taken my shots of Orion, I take calibration frames. These images are used in the stacking process to reduce noise, get rid of small artifacts, and remove vignetting.

Calibration frame terminology:

• Lights – these are the images of the DSO. For Orion I took 200.

• Darks – with all settings the same, take photos with the lens cap on. I take 10% of the number I took with lights, so 200 lights equal 20 darks. These remove thermal noise and so should be taken just after your lights.

• Bias – increasing the shutter speed only (say to 1/4000) and with the lens cap still on I take the same number of shots as darks. 20 biases. These remove the read-out noise.

• Flats – I take these back home, although it is important to use the same ISO, focal length and focus point. I use White screen | Online Tool and take half the number of the darks, pointing the camera at the screen and adjusting the shutter speed so I get a histogram in the centre. 10 Flats. These remove vignetting.

It’s worth noting that you can stack with just light frames.

Review Images

This is an important step to ensure the stacking process outputs the best possible image. For every light image taken I zoom in 100% and ensure the stars are round and sharpish. Often the first couple of images are a little blurry, I also find some mid-way through are, probably caused by a little gust of wind or traffic or movement creating vibration on the tripod. Andromeda 105 had 168 images stacked, but I had taken 180 and had to delete some during image review.

Stacking

There is lots of software out there to stack your images.

• Affinity Photo (paid for) – if you have Affinity photo this has Astrophotography Stacking capabilities already and does a decent job. Depending on your PC and number of images it can take time e.g. 15-30 mins. You can apply post stack adjustments all in one place.

• Sequator (free) – this is great for Milky way and Nightscapes as it aligns the stars and the landscape. It will stack DSO’s and it is quick and easy to use. The image output is certainly usable, but you may not get as much detail as some of the other software.

• Deep Sky Stacker (DSS) (free) - this is one of the original stackers. The user interface is not great, and it takes the longest to stack, sometimes hours depending on your PC. But it does deliver great results.

• Siril (free) - this is one I am using currently and is a good compromise of time and quality. Its relatively quick and produces great results but maybe not quite as good as DSS. I’m still getting used to it, the interface is better than DSS, but it still takes time to understand. There is a simple workflow to follow on its website to get you started.

• Pixinsight (free) – I have not used this, but supposedly it has a steep learning curve; however, users have reported the results being better or on par with DSS.

If you already own Affinity Photo use that, if not I’d try Sequator first and see how you go.

The principle of stacking is the same across each piece of software. Load your (preferably RAW) Lights, Darks, Biases (no need for these in Sequator), flats, and select start. The output will be a very large file (150,000 mb) either in tiff or fit file format. Fit files can be used in Affinity photo or Photoshop.

Stretching

Once the stacked file is output, I apply some stretching and final adjustments in my photo editing software. Stretching is a technique to bring out more details in the image and is achieved by applying level adjustments and curve adjustments.

A stretched image after applying a levels adjustment and a curve adjustment

After stretching, I apply standard adjustments of brightness and contrast, saturation, a crop and rotation.

Andromeda 380 tracked - 380mm (570mm ff) f5.6 ISO 1600 20 sec x 8

Star Trackers

A quick mention on star trackers. These support your camera to track the rotation of the stars allowing you to take a longer exposure at a narrower aperture. This has the effect of reducing noise and star bloating (stars can appear larger and softer with wide apertures). It also reduces the impact on your camera shutter count. Star trackers come in a wide range of sizes, weights and prices starting from around £170 and up to astronomical amounts.

Andromeda 80 tracked – 80mm (120mm ff) f5 ISO 2000 30 sec x 5

Andromeda 80 tracked was my first attempt using a lightweight star tracker (MSM NOMAD). With a total exposure time of 2.5 min on an APS-C camera this image was only 5 shots (lights only) vs the 168 on the Andromeda 105 image.

That completes my blog series on Astrophotography. The main objective was to share my experience starting out in astrophotography and demonstrate how accessible it can be.  I hope you were able to take something from these and I wish you clear skies!

Simon