Tag Archives: smearing

Astrophotography – The Polarie

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.

Polarie Test - Telephoto

Tracking is less critical when shooting with wider angle lenses. I ran a test with a 200mm 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.
  • The optional polar alignment scope is expensive, and bulky. It’s also complicated to operate because you must remove the ball head and camera from the device. BUT the weight of the camera and ball head is likely to create enough “sag” that the careful measurements will be wasted.  We like the SkyTracker much better in this regard.

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.

  • Point the Polarie straight up and use it as an automatic panning motor for a time-lapse.
  • 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.

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

As a sky tracking device

600 Rule?

You may have heard it elsewhere as the “600 rule”.  I first heard about the rule while visiting the Looney Bean in Bishop, California in 2008.  Five photographers sitting in a coffee shop poring over their laptops reviewing what they recently bagged are bound to start talking.  It was my good fortune that one of those present was the very talented Brenda Tharp who first quoted the 600 rule to me.

I, however, have repeated the rule as the “500 Rule” because I think 600 is overly optimistic.  What is the rule?  The rule states that the maximum length of an exposure with stars that doesn’t result in star streaks is achieved by dividing the effective focal length of the lens into the number 600.  A 50mm lens on a 35 mm camera, therefore would allow 600 / 50 = 12 seconds of exposure before streaks are noticeable.  That same 50 mm lens on a 1.6 crop factor camera would only allow 7.5 seconds of exposure.

But Wait. The Rule Isn’t All That Great!

The real number is quite subjective.  A little math reveals that on the Canon 5D Mark II (a full frame camera), with a 16mm lens a pin point star on the celestial equator moves from one pixel* to the next in 5.3 seconds.  But the 600 rule would allow 37 seconds of exposure and the 500 rule 31 seconds.  Both rules will produce streaks on the sensor! The visibility of those streaks will depend on the finished print size and viewing distance.  Print it large and stand close and the streaks will be obvious.

So what does a 30 second exposure look like at the pixel level:

3 Stars at 30 Seconds, 16mm on 35mm Sensor

Clearly those stars are streaking across about 5 pixels* just as the math would bear out.

What is going on here?  The Canon 5D Mark II images are 5634 x 3753 pixels* from a sensor that measures 36 x 24 millimeters. Dividing 36 by 5,634 reveals that the distance from the center of one pixel* to the next is a scant 0.00639 millimeters (or 6.4 microns).

The formula for calculating the distance in millimeters (d) that a star travels across a sensor due to the earths rotation looks like this:

d = t * f / 13750

Where t is time in seconds, f the effective focal length and 13750 is, well 13750.  Is the math scaring you a bit… don’t worry… we’re almost done. Earlier we calculated the pixel* to pixel distance as 0.00639, what we want to find is how long (t) it takes for a star to move that far on the sensor.

0.00639 = t * f / 13750

Solving for f = 16mm we get a t value of 5.3 seconds as I asserted earlier.

But how does that calculate out on a different sensor, the Canon 50D, for example?

The Canon 50D has 4770 pixels across 25.1 mm or an inter-pixel* distance of 0.0053 millimeters.  Substituting into the earlier equation we find that a star marches across a pixel on the 50D with the same 16mm lens in 2.83 seconds.  With a 50mm lens on the same camera… the bad news is the star is speeding from one pixel* to the next in less than a second!

What does an image look like with a 30 second exposure at 16mm on a full frame camera? Remember that the streaks will be 40% longer on the cropped Canon 50D.

Milky Way over Black Rock Desert, Nevada

30 Second Exposure – a close look shows elongated stars.

Scaled down to only 16% of the original image size or seen from a distance no streaking is obvious! We will try not to twitch knowing – because we pixel peeped – that the stars are really dashes not nice round pinpricks of light. And indeed only the eagle eyed are likely to notice the dash-like nature of the stars until the photo is printed large, say at 20 x 30 inches.

What Can We Conclude?

  1. Streaking starts a LOT sooner than any rule you may have learned.
  2. The time it takes to streak depends on the inter-pixel* distance (sensor density / mm) and the focal length.
  3. How much streaking to allow depends on your aesthetic tolerances.
  4. You can not get more or brighter stars by exposing longer; starlight has already given up on one pixel* and moved on to the next in just a few seconds.
  5. The longer the focal length, the more impossible it becomes to prevent streaking.
  6. Gaps in your star trails may be unavoidable if the inter-shot delay (normally 1 second) is long enough to skip pixels*.

Final Note

I carefully added asterisks* to every location where I wrote the word “pixel” in a way that might imply your camera collects light in pixels. You might be wondering why I did that. The answer is: your sensor is comprised of sensels, not pixels. It takes 4 to 9 sensels to create a single pixel depending on the de-mosaic-ing algorithm your camera uses. Maybe you aren’t that picky, but I didn’t want to hear complaints from the purists.

I particularly relish this epiphany because I reported long ago that “longer exposures do not result in more stars“.  I just never got around to doing the extra bit of math – or the experiments – to prove out my assertions.

Real Final Note

A commenter has rightfully taken me to task by pointing out that the perception of a streak is dependent on many things other than just the actual sensor values recorded. In particular, if the image is not enlarged much some streaking will be scarcely or completely unnoticeable because the feature will be too small for the eye to perceive.  The problem with this assertion is that it assumes a lot of preconditions: e.g. how large the print is, how far from the print a viewer stands, and the subjective experience of the viewer.  My real world experience has led me to conclude that it is a reasonable goal to keep the streaking to 2 to 3 pixels or less because that will provide the greatest possible usable magnification (finished viewing size).  There would be no point to collecting a high megapixel image if you can not produce a print proportionately larger or more detailed than a lower megapixel image!

Here is an example that makes my point. I love this image captured on a Canon 5D Mark II. When printed at 20×30″ and viewed at 4 feet there is some streaking. Perhaps only a critical eye would notice, but even an untrained eye will notice when viewed from two feet away.

Famous III [C_035478]