Milky Way Imaging

Traditionally us astrophotographers tend to image the Milky Way with a wide angle lens on a DSLR. We’ll either do this with a static tripod, in which case we’re usually restricted to about a 15 second exposure due to the rotation of the earth introducing “trailing” into the image, or we’ll mount the camera and lens to a tracker, such as the SkyWatcher Star Adventurer, which will allow us to take longer exposures.

Milky Way over my kids house in Class 5 skies

Static Tripod Method

This is the age old method that quite literally ANYONE with a DSLR and a kit lens can do, and is often the route that the majority of us took into astrophotography, simply because it’s the easiest. There’s a few things to consider first off though such as the maximum exposure time before the effects of earth’s rotation becomes apparent, taking a long exposure image without introducing “shake” into the image from pressing the button, foreground movement in relation to the position of the sky to name a few things.

The first two are simple enough to deal with. When thinking about taking an image long enough to start capturing some sort of detail before stars start to trail, we take into consideration something called the “500 rule.” This is the formula that’s used to calculate the exposure length needed before trailing occurs, and is really simple. It’s 500 divided by the focal length you’re using. So for example, if you’re using a 14mm lens the formula is simply 500 / 14 which equals 35 seconds. HOWEVER, if you’re using a crop sensor camera then you also need to take that crop effect into account, which is different for both Canon and Nikon cameras. For Nikon it’s 1.5, and for Canon it’s 1.6. So that 35 seconds on a Nikon becomes 35 divided by 1.5, which equals 23 seconds, and for Canon it becomes 35 / 1.6 which equals 21 seconds. From there I dial it down to 20 seconds JUST to be certain I’m not going to get any trailing at all. Simple isn’t it?

If we’re not wanting to introduce any shake into the image when we take it, there’s a number of things we can do to mitigate it. The first one is, if the camera supports it, locking the mirror up. This option is so variable from camera to camera that I’m not even going to try, other than to tell you to look up the camera specific information applicible to you. Because mirrors are mechanical, they cause a small amount of vibration when they flip up for the exposure. Locking it up will eliminate this.

Assuming you don’t have an intervalometer or a remote shutter release, then we can get round the shake caused by pressing the shutter button on the camera by setting the timer. Again, the method for doing this varies between different cameras, so make sure you look up how to do it for your camera. On my Canon 450D I can set it to 2, 5 or 10 seconds delay. A lot of people will say 2 seconds is sufficient, and 10 seconds is just wasting time. Personally I play devil’s advocate and go with 5 seconds. If you’re able to program your camera to take multiple exposures before having to press the shutter button again, I would do this as well. Make sure you’ve set the exposure time to whatever you worked out using the 500 rule. I’ll run through more specific settings for different scenarios later.

Whether using a static tripod or a star tracker, any foreground you have in shot will move in relation to the night sky. You can take some amazing images with a good foreground interest, so what I’ll say is that if your image processing in photoshop is sufficiently good enough, then take a separate image for that forground so that you can layer it into the final image. I personally have yet to learn how to do this, but it’s something I’m hoping to be able to do at some point this season. Either way, a single 20 second capture on its own with both a good foreground and the Milky Way arching above it, can be a very effective capture.

Star Tracker Method

Right out of the bat you can ignore the 500 rule here because it simply doesn’t apply. With a star tracker, the motors and gears of the tracker itself are what counteracts the earths rotation. The effectiveness of this, barring mechanical issues, will be dependent on how good you’ve nailed your polar alignment. But with a 14mm lens, you should be able to quite easily manage 3 to 5 minute frames without things breaking into too much of a sweat. The method of polar aligning different between trackers, so again, I’ll advise that you look up how to accomplish that with your own one.

Everything else with regards to eliminating shake applies though, and you should read the above methods on how to achieve this.

Settings

The camera settings you use will be dependent on a number of factors, not least your location. If you’re under reasonably decent dark skies then these will differ significantly from those under city skies. There’s a number of good online tutorials on how to achieve both and a search on YouTube will provide a plethora of them.

The main idea though is to get as much light hitting the camera sensor as possible without introducing too much noise into the image. A lot of noise can be mitigated by taking calibration frames, something that I will cover elsewhere. For each camera there is a “sweet spot” with regards to achieving this. For my 450D I’ve found that an ISO of 800 is pretty good, although this alters under city skies. Bear in mind the exposure length as calculated either using a static tripod or a star tracker.

Under city skies (class 5 and above) you’re looking at using a method called ETTR, or exposing to the right. This essentially is when you look at the histogram and you’re pushing the peak as far to the right as you can without “clipping” the highlights. This is done in order to bring out as much of the darker areas as possible without oversaturating the lighter ones, and is the method I used in the image near the top of the page. If you’re using a star tracker and obtaining longer exposure lengths then you can feasibly dial back the ISO in order to avoid clipping the right side of the histogram.

Milky Way over kids back garden
35 x 60 seconds at ISO 1600

Milky Way Imaging Plans for 2020

Previously I’ve been restricted to just using a kit lens on my 450D, but this year I’m going to attempt a different appoach. My idea is to utilise the capabilities of the ASI178MC. I plan to mount this directly to a ballhead and then use either the Star Adeventurer or the EQ5 Pro in tracking mode (if I can’t figure out a way to have it guiding.) Instead of fitting the ASI178 to a telescope, which is what I would usually do, I plan on using a wide angle “meteor lens” fitted directly to the camera. Although this will give a very wide 170 degrees field of view, it will also give what appears to be a “fisheye” viewpoint.

Stay tuned as the Milky Way season progresses to see how I fair this year.

Update May 5th 2020

ASI 178MC with “fish eye” lens. 30 seconds at zero gain

An unusual perspective for me, thought I’d put the fish eye lens on and try an experiment in wide field whilst the moon is out. This is 30 seconds at zero gain with the ASI 178MC. No tracking, no guiding, just old fashioned static mounting. The plan is, if this works, to try for some REALLY wide field Milky Way around the next new moon.

Moon crossing the night sky
ASI178MC (non cooled) with “meteor lens”

First one of 2020…

Whilst out collecting Oiii data (which I’ve had to ditch) on my ongoing NGC 6888 Crescent Nebula project, and also imaging comet C/2020 F3 (NeoWise), I also took the opportunity to throw the modded 450D onto the underside of Miranda’s dovetail and collect some widefield of the Summer Triangle. Admittedly this was pushing the weight limits somewhat, not to mention the balance, but I’ve always wanted to attempt multiple targets using the same rig, so tonight I gave it a go and was pretty succesful. I used APT to capture the subs and full set of calibration frames (you can read here what calibration frames are used for and how to shoot them), and then carried out my standard workflow with APP, LR and PS. As a first go at widefield in quite some time, it’s been nice to do this again, and I’m looking forward to improving over the summer.

The Great Rift with visible Summer Triangle (Vega, Deneb and Altair) taken with modded 450D at ISO 400 using 30 second subs. 17 minutes total integration

Recently I visited Somerset and North Devon with my partner and also managed to get in a little widefield time on a couple of the very few clear nights we’ve had this summer.

Canon 60D at 18mm. ISO 6400, 6×15 seconds

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