In a recent article I explained how you can bump up the brightness of stars using a simple layering trick in Photoshop. In this article we will be going in the opposite direction. Instead of increasing the contrast of stars our goal will be to reduce the streaking that results from long exposures or exposures at a long focal length. For background on streaking, please refer to the article The 600 Rule which as rules go is a pretty bad rule.
Streak Reduction Procedure
The original image, cropped.
For the streak reduction procedure we only need one image.
Open the document in Photoshop.
(Noise reduce as needed – method to be described in a column coming soon)
Duplicate the background (Ctl-J).
Change the new layer blend mode to Darken.
Select the move tool to “nudge” the top layer in the direction of the predominate streak (in this case up one and right two). It helps to zoom (Z) pretty far before making the nudge.
Apply levels and curves to brighten the image and correct the color. The color correction process is described in this article. The easiest way to apply an adjustment layer is to view the Layer tool (key F7) and click the adjustments icon at the bottom as shown here:
To make it easier to see what you are doing, you can apply a curve to lighten the image. Make sure to drag any temporary adjustment layer to the top of the layer stack.
The nudge operation may result in foreground elements being noticeably duplicated like the tree tops below:
Background plus nudge foreground, unmasked.
Fortunately it is easy to undo that duplication by masking off the area by painting black on a layer mask. Here is the same darkened layer combination, but I have created a mask and painted out the area around the duplicated tree tops. To create a mask click the icon that looks like a camera on the Layer Tool – it’s just to the left of the adjustment icon shown earlier.
Tree duplication has been masked off here.
This image looks better. Now we might want to apply the bump operation using the combined background and nudged darken layer. There are three ways to get the layers combined: use flatten image, select the two layers and select layer-merge, or – and this is a really useful trick, use the keys: Ctl-Alt-Shift-E (that’s how I got Layer 2). On the Mac you use Option rather than Alt. Two ways to get to the flatten or merge operations include clicking on the tool bar under “Layer” or clicking the tiny little menu icon at the upper right of the layer tool window.
After a little bump:
Caveats
This destreak method works best if you are not using a wide or ultra wide view. With a really wide view, the star streaks are not uniform. The closer the stars are to the north or south poles, the more curved their streaks will be.
In another article I will walk through how you can composite together images taken at different times or exposures, as was done below.
We have a once-in-a-while webinar on beginning Astrophotography. The purpose of the webinar is to get people acquainted with the tools and techniques required to delve into this interesting genre of night photography. As we teach in that webinar the single most important piece of equipment you can buy is an Equatorial Mount. An Equatorial mount is an apparatus that counteracts the rotation of the earth so that your camera can peer at the same place in the sky for long enough to capture an image without streaks. There are many equatorial mounts that range in price from almost nothing (and not even worth nothing) to more expensive than logic would dictate. For more background please see our survey of Astrophotography Gear.
One of the newer pieces of equipment in the arsenal is a less-than three pound piece of gear called a Polarie. Here is what it looks like with a ball head attached to its face.
Polarie – Close Up
What Polarie Can Do
As noted earlier, the primary purpose of Polarie is to counteract the effect of the earth’s rotation so that objects in the night sky can be exposed longer without getting streaking. Below are examples of 42 second exposures using an effective focal length of 215 mm. The image at the left is with the Polarie turned on in normal mode, the middle image is the same length exposure but in 1/2 speed mode, and the right is what you get if you use no tracking at all.
Tracking is less critical when shooting with wider angle lenses. I ran a test with a telephoto lens because it is a more difficult scenario. For example when shooting the Milky Way, an effective focal length of 10 to 50mm makes more sense.
A Critical Look At Polarie
I purchased only the Polarie unit (about $400 USD) not any of the accessories. The unit is deceptively heavy at almost 3 pounds but at that weight it is still – and by far – the lightest equatorial mount you can find. The only other device in its weight class at present is the Astrotrac with a starting price about twice as much. The Astrotrac does come with a better tripod mount, however at a total cost of around $1300 USD. I paired up the Polarie with my Canon 50D and the 70-200 f/4 lens. The addition of a Giottos ball head brings the total weight of the equipment attached to Polarie to about 6 pounds.
The Positives
Inexpensive
Good instruction manual
Mostly easy to set up and to use
Suitable for a beginner
Good power for the price.
Can be powered with mini USB (or two AA batteries). Claimed life is 4 hours on AA batteries but mine lasted at least 6 hours using rechargeable batteries.
Compact and MUCH lighter than almost everything else.
Can be used in Northern or Southern latitudes.
Tracks at star, solar or lunar rates (and yes, they are all different) as well as a 1/2 speed rate which should be good for Landscape Astrophotography.
The Negatives
The back plate can be unscrewed to peer through the axis of the motor and also houses a built-in magnetic compass but the plate is almost flush to the Polarie body and it is quite hard to grip.
The inclinometer (angle measurement device on the side) seems like a good idea except that the markings are so small and coarse that to my eyes it is illegible. The lighted inclinometer *might* help if the North Star is obscured by trees or such.
The front plate has a 1/4″ retractable bolt and attaches awkwardly to the motor plate with two thumbscrews that are hard to reach once a head is on the motor plate. I would have preferred that Polarie supply a 1/4 to 3/8″ adapter since most good heads attach via 3/8″ bolts.
The battery compartment door is a nail buster to open.
Since Polarie will certainly be used with a DSLR camera, Vixen really missed an opportunity to add a remote release cord – I see no jack for one.
Not sure what the point of the flash shoe is. I do see the Vixen has another (much larger) inclinometer that can be attached there, but you may be able to do better using an application on your smart phone.
There is a sight hole to line up Polaris – the North star. I used only that method to align Polarie and got fair results. To get really long exposures one of two methods will need to be undertaken to increase the alignment accuracy: either invest in a Polarie polar alignment scope at almost double the cost or do drift alignment. Drift alignment is not simple and probably would frustrate the aspiring astrophotographer. The Polarie can be purchased with an optional ratcheting tripod base which might be a good idea, however the stated load capacity of the bundled tripod seems too low to use with a heavy camera.
Noteworthy
Remember that you will need at least two heads and you’ll want them both to be ball heads for optimum configurability. The head on the tripod should be sturdy – see below for why. Below I refer to tripod head - the apparatus that joins the tripod to the Polarie, and to the Polarie head – which is the hardware used to attach a camera to the Polarie.
Problem Areas
In addition to the negatives listed above, there are several other sources of problems including every point where one element attaches to another. For example: the Polarie base if not attached securely to the tripod head can rotate. If using quick release plates the attachment point creates another source of rotation. If the camera is not securely attached to the Polarie head rotation can occur there, too. All the pieces together may severely tax a cheap tripod head making it difficult to hold up or adjust the load. In my configuration I found I had to allow some slouching – meaning I had to adjust the camera so it was pointing slightly above my target and then tighten the head so that it would settle to the right place.
What Can You Do With A Polarie?
Maybe we should have put this section first! Some of these things can only be done with a Polarie are highlighted in RED.
Align Polarie and take a series of shots of the night sky – the sky will stay in the same place in every shot – and any minor movement can be compensated for using Astrophotography procedures.
Outfit your lens with a solar filter and track the sun (e.g. for photographing eclipses or solar activity)
Track the moon e.g. to catch the space station flying across its face, the slow creep of the terminator, or just to get a time-lapse as the moon sets or rises.
Double your exposure on a landscape astrophotography shot by using 1/2 speed mode.
Polarie – Close Up
The inclinometer is just too small to read about the size of a US nickel.
Top and Back of Polarie
Looking down on the Polarie
Tripod Head + Polarie + Ball Head, Camera and Telephoto lens
For more hints tips and examples on how to use Polarie, stay tuned to this channel!
My first test of the Polarie was to track the radiant point of the Orionid Meteor shower. My attempt was mostly a bust due to clouds, however note how stable the time-lapse is – and remember this spans almost 14 minutes of real-time.
Here are two more ways I’ve used the Polarie – as a horizontal panning device
![DeStreaking [C_049387+406]](http://farm9.staticflickr.com/8057/8190903031_c63b373533.jpg)
