How To Guide: Astrophotography With a DSLR

Part 3 of 4  | Advanced Shots

In this section we will discuss shots that require the use of a tracking mount. This will require a bit of financial investment but there are some budget-conscious options like the home-made barn door tracker.

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Without having a way to track the stars you are limited to star trails and ~30 second exposures. While you have a huge variety of pictures to take with that restriction (as seen on the previous page) you open up a whole new set of possibilities if you have an equatorial tracking mount as seen above. There are 5 main components to a setup like this. The Tripod, the tripod head for your tracking mount, the tracking mount itself, then a ball head and your camera.

Picking a Tripod

You are going to want something more than you run-of-the-mill $20 tripod for this type of work. One suggestion would be the Manfrotto 055XPROB. This is a popular model and found for sale easily on Amazon.

Picking a tripod head for your mount

The next piece of the chain is the tripod head that your tracking mount will attach to. There are 3 main types of tripod heads – pan/tilt (usually used for video), ball-head, and geared head. Any type will work as long as it can support the combined weight load of your tracking mount, camera and lens but the best type by far is the geared heads as they make fine tuning your polar alignment much simpler. With a ball head you will have to hold your entire apparatus as you align it – definitely possible but probably frustrating.

A popular choice is the Manfrotto 410 Junior Geared Head which has a large weight capacity and very smooth and accurate fine tuning. It also has a built-in level which is helpful.

Ball head for your camera

The ball-head goes from your tracking mount to your camera. This is so you can keep your tracking polar aligned but freely move your camera in all directions to take different shots.

Any ball head will do as long as it can support the weight of your camera. I recommend something like the Manfrotto 496RC2.

Tracking Mount Options

Home-made Barn Door Tracker

With a couple pieces of wood and a few pieces of hardware you can make a tracking mount that you manually turn yourself (typically at 1RPM).

Tracking won’t be very accurate but if you build it to exact specifications and polar align decently you can have widefield exposures of around 5 minutes without any trailing.

I built one following the instructions in this guide here before moving onto other methods. It is probably too shaky and inaccurate for telephoto shots but for wide or medium shots it will definitely get the job done.

The downside is you will be manually turning it the whole time which means sometimes for hours at a time. Here is another guide for making one.

The Vixen Polarie portable tracker

I really love the Vixen Polarie which you can pick up on Amazon. It’s smaller than your camera body, runs off two AA batteries (2-3 hours of tracking on those) and is insanely easy to set up. It costs about $400 and that is priced very competitively compared to other alternatives.

They also make a polar alignment scope for it for precision alignment that costs almost as much as the Polarie itself  but I have had no problems getting good alignment using the little “polar sight” hole that is on the Polarie itself. I don’t think the scope is a necessary purchase unless you plan on doing a lot of 300mm+ long exposure shots.

The Polarie also features lunar and solar tracking modes as well as a special 1/2 speed “starscape” mode where it tracks at half speed to better keep landscape elements from blurring but at the same time offsetting star trailing somewhat.

It’s a cool compromise that lets you do nice 1 minute starscape shots instead of the regular 30 seconds while keeping both landscape and stars in decent focus.

I have now posted a more detailed review of the Polarie here.

The Astrotrac portable tracking mount

The Astrotrac or  was first to the market with an affordable and portable tracking solution for DSLR users. The Polarie wouldn’t exist without the success of the Astrotrac. Due to its design the Astrotrac is technically more accurate at tracking – accurate enough for people to use telescopes on it.

It also holds a lot more weight than the Polarie but the downside is its less portable and slightly more expensive. It also requires 8 AA’s to operate instead of 2 AA’s and must be rewound after tracking for a while.

It comes standard with a polar scope though at most places you can buy it and when you factor that in, its only slightly more expensive than a Polarie + polar scope combo.

It features solar and lunar tracking modes like the Polarie but lacks the half speed starscape mode. One feature it does have is a tracking port for autoguiders to further improve tracking accuracy.

 Piggy-back on a telescope

If you already own a telescope on an equatorial mount with a motorized tracker you can simply piggy back your camera onto the telescope (there are adapters sold for this purpose) or even directly attach your camera to the telescope. That is called prime focus but I won’t be covering that here. I want to focus on what can be done with just your camera and regular lenses.

Polar Alignment

North America Nebula – Nikon D700, 105mm, f/2.8, 10x3min (18min total), ISO3200

In order to track the stars you must align your tracking mount to the celestial pole. Then the mount rotates at the same rate as the sky to keep the stars in place. For the barndoor tracker you would sight along the hinge and for the other mounts they have either a scope to use or in the case of the Polarie a small hole to look through. Alignment is different in the northern and southern hemispheres.

Northern Hemisphere Alignment

The star Polaris is almost exactly at the northern celestial pole. If you know how to find “The Big Dipper” then you can find Polaris. Look North and use this diagram.

Polaris is unmistakable once you first learn to find it.

Southern Hemisphere Alignment

The star closest to the southern celestial pole is Sigma Octantis in the constellation Octans. It is a dim magnitude 5.5 compared to Polaris at magnitude 2. There are 2 main methods of finding it.

If you can see both Magellanic clouds, forming an equilateral triangle with the third point being the celestial pole gives you an easy way to find Sigma Octantis. You can also use the Southern Cross method as shown in the diagram to get you close to the pole.

M51 The Whirlpool Galaxy – Nikon D300, 300mm, f/5.6, 6x2min

Stacking, stacking and more stacking

Now that your camera is tracking the stars you can fully take advantage of exposure stacking. Stacking is when you take a bunch of images of the exact same thing and then stack them on top of eachother to improve the signal to noise ratio.

This can drastically reduce the amount of noise in your final picture and all that extra signal will let you stretch the faint details of deep sky objects much better in post. In addition to regular frames (called light frames) there are other frames you should be taking to maximize the quality of your final image.

Light frames

These are your regular pictures. Take as many as you can! Eventually you will hit diminishing returns for adding more exposures but more never hurts. A number I often use is 6. There’s nothing special about this number, I just find it to be a good compromise of quality and the amount of time it takes to capture it all.

Dark frames

I discussed these in previous parts of the guide but dark frames are identical to light frames except you put the lens cap on your camera when taking them. They are then subtracted from your light frames to remove all that unwanted junk. It is VERY important to take dark frames at the same temperature as your light frames as temperature has a big effect on noise and hot pixels. Like light frames you want lots of these to stack. I like to usually get as many dark frames as I do light frames.

Flat frames

Flat frames are used to remove vignetting, dust spots and other optical system anomalies. Flat frames should be shot at the same f-stop and focal length as your Light frames. It’s best to use Aperture priority with the lowest ISO setting of your camera (usually ISO 100 or 200). The goal is to get an evenly illuminated frame. If your camera has a histogram you want the peak to be right in the middle.

One way I like to take flat frames is a clear twilight sky before you can see any stars. Aim the camera away from the sun so there’s no gradient and its evenly illuminated across the frame. You can also use a light box, or throw a few white sheets in front of the lens with a bright light source behind it. It’s good to take a few of these for stacking.

Great, so you have all the frames. They will be useful later on in the processing section. One tip to remember is to shoot your darks and flats at the same orientation as your lights (portrait or landscape) – it just makes things easier when you are processing everything.

The Orion Nebula – Nikon D300, 1000mm, stack of 10x1minute frames

What should I shoot?

Here’s a few targets to get you started. You can use a program like Stellarium (free) to find these. There are also lots of books, iPhone/Android/tablet apps and other ways to help you find objects easily.

• Hyades and Pleiades star clusters
• Barnard’s Loop and the Orion Nebula
• Large and Small Magellanic clouds (southern hemisphere only)
• Andromeda Galaxy (best for 200mm+)
• North America Nebula
• The Whirl Pool Galaxy (300mm minimum)
• Lagoon Nebula and Trifid Nebula
• Omega Nebula and Eagle Nebula
• Flaming Star Nebula
• Horsehead Nebula

 
4 nebula in one shot – from top to bottom: Eagle Nebula, Omega Nebula, Trifid Nebula, Lagoon Nebula. Shot at only 70mm!

So hopefully you have some shots to work with now. Let’s move onto the final part which is processing in Photoshop.

Continue Reading Part 4 Of The Guide…..