When we first start out in astrophotography, we often buy a basic mount in order to get ourselves used to the basic principles behind producing those images we often sink a lot of time into. A german equatorial mount (GEM) is considered one of the best mounts for those longer exposures. But for some, all they have is what’s called an Alt-Az mount. The way this works is different from a GEM, and I’ll quickly attempt to explain the fundamental differences between the two types.
This is a combination of two words – altitude and azimuth. These are the two directions in which the mount moves; vertically in “altitude” and east-west in “azimuth.” One of the beautiful things about the Alt-Az mount is that it doesn’t need precise levelling or any of the tedious messing about with polar alignment like the GEM does. Instead, what you do is align on two or three known celestial objects. The mount then calculates what it needs to do based on this information. Of course it also needs to know where on Earth you are, and the current time. Unfortunately, with a basic Alt-Az mount one of the things you have to contend with is something called “field rotation.” However, if you keep your subs short enough, field rotation won’t show up in the image, although it will if you compare a set of separate ones. The way you get around this is to stack the short subs, whilst keeping the target in the centre of frame.
The other issue with field rotation is that the closer to zenith (pointing straight up) you are, the more field rotation becomes an issue, even with short subs. It’s best to pick a target that’s relatively low, although not so low as be fighting against atmospheric muck, and also due east or west.
Field Rotation Explained
If you watch the half moon rise in east, you will typically see the moon terminator at an angle not being vertical. If you look again later when the moon has reached south (or north), you will notice that the terminator now is vertical. When the moon reaches the western horizon, the terminator is again angled to the horizon and in the opposite direction to when it rose in the east. This phenomenon where the sky changes its angle to the horizon is called Field rotation.
The Alt-Az telescope mount moves around a vertical and a horizontal axis. Therefore, a camera placed on an Alt-Az mount will keep it’s angle in relation to the horizon and terrestrial objects. But if you point the camera at the moon and takes 3 images: in east, south and west the moon will have different angles on the pictures even though the mount has tracked perfectly and kept the moon centered in the pictures.
Source – telescopemount.org
The GEM, or German Equatorial Mount
A GEM works by aligning it with the celestial pole. It can then track in one axis, right ascension, keeping the target fixed. Because they tend to be more complex, they are inherently more expensive than the Alt-Az mounts as a rule. However, in my view, they’re worth the extra expense because you can shoot longer exposures. Longer exposures means less frames need to be captured and the signal to noise ratio (SNR) is improved. And because they move in right ascension instead of vertically and horizontally, you don’t need to deal with field rotation.
As I’m writing this I currently have both versions outside running. The EQ mount, in this case a basic SkyWatcher Star Adventurer is shooting two minute subs on the Iris Nebula, whilst my daughter’s Alt-Az, a NexStar SE, is shooting 20 second subs on the North America Nebula at 55mm. This is probably the limit for the NexStar SE.
Of course one way to get around this is to fit an EQ wedge to the Alt-Az mount, but these run into hundreds of pounds, and it can often be just as cheap to by a good secondhand EQ mount, such as the EQ5 Pro for close to the same amount. The other option with the EQ wedge would be to just build one, which is certainly not impossible. These need their own post though and perhaps I may do one if I ever build one myself.
So, with some of the above in mind, I headed out not long ago with my youngest to do some astrophotography with her NexStar SE mount. We kept it nice and simple to start with, and stayed on the moon for a while with the stock Canon 60D, where both her and my other astroheart, Sam, took turns getting some lunar time in.
Lunar imaging with this mount is easy, and quite frankly, so simple my 9, almost 10, year old little girl can do it…oh, wait…
Even initial alignment during the daytime was easy as you merely choose a solar system object, ie the moon, and then simply go to it.
Between them, Millie and Sam fired off several hundred images at varying ISO’s and shutter speeds, and in fairness I figured I’d probably give it a go as well as the moon passed Millie’s tree, although my own efforts were so embarrassingly bad I deleted them in disgust!
Deep Space Imaging
Lunar imaging is all well and good, but what about planetary, or, better yet, deep space?
At 55mm with the Canon 450D on the North America Nebula area I managed to confirm that 20 second subs were okay. In fact, the stars were that solid I could probably have pushed 30 seconds without too much trouble.
I had a clear forecast for the latter part of that night, so I planned on having three separate rigs running, including Millie’s NexStar SE. This time, the plan was to shoot at 250mm on the same area, and see how long it can go before I get apparent field rotation / trailing. If I could push 20 second subs again, I’d be happy with that. I should still be able to capture a fair amount of nebulosity as the 450D has been astro-modded. What this means is that all of the filters have been removed, which opens it up to collect more of that elusive red part of the spectrum, ie the prominent hydrogen-alpha gas that is so prolific in the area.
The above image, shot at 250mm was taken using 20 second subs. Using the idea of shooting relatively low and to the west has proven to me that this is something worth pursuing with this mount. Although when I shot this I had an almost full moon out, so the washout was quite severe, no matter how much I attempted to process it out. A quick online platesolve has shown that this is indeed the North America Nebula and Pelican Nebula region just below Deneb. As the season advances this will become harder to image, but I’m hopeful that other areas will rise in their place, notably the Pleiades and Orion later in the year.
Millie had the stock Canon 60D shooting 20 second subs in the Cygnus region at ISO 1600 and 55mm several weeks later. Having checked the back of camera during the session, the stars were nice and tight and there was a fair amount of detail in there. In all there’s 300 single frames, which equates to exactly 100 mins worth of data.
As you can see astrophotography with an Alt-Az mount is completely do-able, and although it won’t match the long exposures that you can get with a GEM, you can still obtain some worthwhile images. I’m certainly looking forward to the kids takes on the Pleiades and Orion Nebula later this year.
Many thanks for reading, and also thanks to Niels Haagh for allowing me to use his original material on field rotation. Please have a look and pay his site a visit.
Clear skies all!