Tag Archives: telephoto

Hunting Comets and other faint objects in not-dark skies

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AirGlow Comet [5_070386]

It turns out the much hyped PanSTARRS C/2011L4 Comet is not living up to the hype. Unfortunately failure to meet the over exhuberant expectations  is common since predicting brightness and visibility of an object like a comet is a difficult science. In fact, it’s part science, part black art and part good guessing – mostly the latter.

The photo above was taken on March 12 when the moon and PanSTARRS nestled closely together. The close quarters made finding the comet much easier despite the bands of clouds passing by.  The strategy for finding the comet in that case was simple: use a telephoto lens, put the moon at the right edge of the photo and take different exposures periodically and at different settings (e.g. +2, 0, and -2 stops). Then hunt for smudges.

The IDEAL telephoto lens would be one that was a few angular degrees wider than the difference between the moon’s position and the comet’s position. How to determine the position of each is discussed in the last section below. Figuring out the angular view of your lens is easy using online tools like this one from Tawbaware, makers of Image Stacker (like that program!). If you know the field of view at your minimum and maximum zoom, you can use that information to your advantage.

Finding the Comet with a Nearby Moon

The point at the moon strategy made finding the comet easy because:

  1. There is no way you’d be able to see the comet if you were not able to find the much brighter moon nearby.
  2. On that one night, the comet and the moon were within 4 degrees of one another.  That’s quite close.

I know some people tried to find the comet using wide angle lenses. That strategy might work, but the comet is such a tiny thing and it’s visibility is so tenuous based on the atmosphere, light pollution, and sky brightness that you may only realize – as many did – that you captured the comet after carefully inspecting your photos at home.

Contrails and Comet Tails [B_050938]

The truth is you are unlikely to see PanSTARRS by eye or in your camera’s view finder unless your conditions are nearly ideal.  Hopefully ISON which is coming in December will be brighter and better.

Finding the Comet when the Moon is Farther Away

The following night, both the comet and the moon had moved relative to the sky. On March 13, the moon was 12.5 degrees above the comet and about 4 degrees farther west (again, how I knew this is coming in just a minute).  So one simple strategy for finding the comet would be to zoom your telephoto lens so that it has a field of view of about 14 to 15 degrees in the long direction which for me, is 80 millimeters focal length on a 1.6 crop factor camera.

On a tripod with the camera in portrait orientation adjust the view so that the the moon is in the upper left of the frame. Shoot bracketed shots. Check the lower right corner of each one for the tell-tale comet smudge.  Keep readjusting the view so the moon remains in the upper left for each shot. Zoom out a little bit too, in case your geometry is a little off. Eventually as it gets dark enough or the sky clear enough you should find it.

In fact the way I found the comet last night without using my camera but by using my telescope. The program Clinometer (on my iPhone) measures angles. I sighted the moon with my 8″ Dobsonian telescope and measured the angle along the telescope barrel using the inclinometer program. I then lowered the altitude (elevation angle) of the telescope by 12 degrees to match the altitude of the comet. Then I slowly rotated the telescope northward until I found the comet.  It wasn’t easy from my urban location, but it wasn’t impossible either.  By the time I was able to find the comet it was only about 6 degrees high in the sky – that’s way too low if you have trees, hills, and houses nearby to deal with.  In theory, this strategy would work with a telephoto lens or with binoculars, however, binoculars need to be steady and where I spied from last night had streetlights in the distance and the flare and glare from those streetlights made finding the faint comet nigh impossible.

What if there is no Moon to Find the Comet With?

Unfortunately starting on March 14th, the moon will be quite far from the comet, so the opposite strategy is required:  Use a landmark in a known direction as the starting point and look “upward” from the horizon.  In other words, zoom your telephoto lens so that the field of view covers the angle from the horizon to the comets altitude (angle) above the horizon.  Don’t forget that as the earth spins this angle changes every minute! Orient you camera in landscape mode and point it as close as you can to the correct direction (azimuth). Look along the top of the frame to see if you’ve captured the comet.

IMG_1622.PNG

SpyGlass’s view shows the direction the camera is facing (Azimuth) and the elevation angle (Altitude)

But what direction should you point your lens or telescope? Use a compass application or actual compass. BEWARE however as the compass applications have lots of gotchas and are only accurate to about 5-10 degrees.  And if you aren’t sure how to use a real compass your local magnetic declination might bite you. Better would be a GPS with a built-in calibrate-able compass.  And perhaps even better still would be to use an application like TPE (which I discuss in my Catching the Moon Webinars) to calculate the correct azimuth from the location you plan to stand.  An application that might help a lot is “SpyGlass”
however don’t forget that I found the directional accuracy of my iPhone and iPad to be pretty poor.  Being off by 5 degrees may mean looking in the wrong place.

How Do I Know the Altitude and Azimuth for the Comet?

Stellarium_MoonMarch14

Unfortunately, that’s a tough one.  I use the free program Stellarium. I then added the comet to the “Solar System Data Base” (search around on the web and you’ll find instructions). I selected my viewing location, dialed in the time, did a search for good ‘ol C/2011 L4 and let it tell me the azimuth and altitude.

PanStarrs_March14

Above I’ve dialed up the time and clicked the moon. The highlighted line shows me the azimuth (direction) and altitude (angle above the horizon) for the moon which at that time are 264 degrees or just a little south of west, and 30.5 degrees high.  Clicking on the comet shows 272 degrees – a tiny bit north of west and 9.5 degrees.  So now we know that the comet will be 8 degrees north and 21 degrees south of the moon – and that won’t change significantly for the rest of the night.

Since we also know the direction for the comet is about due west at this time, we can apply the telephoto-lens horizon trick I described earlier.

Another way you can find the azimuth and altitude is by checking my animation HERE – note that the animation is correct for San Francisco  (and most places nearby).  There is also a table of the azimuth and elevation in the text of the Flickr post.

 

By the way, one way to find the right spot on the horizon is to use the sunset location as a guide.

CometIllustration

Alignment (Part 1 of 2)

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Sometimes alignment is everything. As an amateur astronomer at heart I am fascinated by the course of celestial bodies through the heavens and how they coincide with terrestrial features (is that strange?). Buildings, lighthouses, arches, and yes, observatories all beg to be photographed as they are kissed by the moon, the sun, or the Milky Way.

One example is shown in Photo 1, below. The moon is rising behind the Lick Observatory. The observatory is perched on the top of Mount Hamilton overlooking San Jose, California.  The photo was taken from the shore of Halls Valley Lake in Joseph D. Grant Park County Park. But it was not serendipitous – it was purposeful.  Days and weeks worth of planning were required.

Moon Lick [5_009717]

Photo 1: The full moon rises over Lick Observatory, Mount Hamilton, San Jose, California

An alignment of man-made artifacts and the moon occurred on the night of a total lunar eclipse.  It also was not completely accidental – but I can not claim I knew I would capture it:

Airplane Transits the Partially Eclipsed Moon

Photo 2: An airplane transits the partially eclipsed moon.

But sometimes the moon just happens to be in the right spot, as in this photograph by May Wong which captured the moon in an interesting alignment while hiking up a trail in Mission Peak Preserve.

Photo 3: (May Wong) The moon teed up on Mt. Allison's Tower

Many fascinating views of the sun and moon can be found in books by Harold Davis one particularly interesting example is “100 Views of the Golden Gate Bridge“.

Planning Moonshots

Ignoring happy accidents for a moment, getting the moon to align with some terrestrial object involves quite a bit of calculation. While there are some great tools to aid the lunar photographer (The Photographer’s Ephemeris, for example), it helps to understand why the moon is a difficult object to catch.  Starting with the first problem:

The Moon is BRIGHT

Jewel [C_029690]

Photo 4: Long exposure for details during a total eclipse - notice the few stars.

Indeed the moon is a very bright object as most people discover when they try to capture any of the details of the moon. Typically the full moon requires settings of f/9, ISO 100, and 1/100 of a second to preserve detail; but at night, those  settings result in everything else being a deep black, therefore to get moon details and foreground details there must be some illumination.  The best time is before sunrise or after sunset and more specifically the very best time is on the cusp between nautical twilight and civil twilight.  I will explain what those are in Part 2.  Of course the moon also makes planning harder by the changing daily illumination. In 29.53 days the moon completes one full cycle from new where the moon is in line with the sun and not illuminated; to full – opposite the sun in the sky and fully illuminated; and back to new. Surprisingly, however, the exposure needed to capture moon detail does not change very much until the moon becomes a slender sliver. When in the sliver phase longer exposures can capture moon detail in the darker (unlit) portions of the moon though this effort comes at the cost of blowing out detail from the lit edge.  In the extreme case, as when eclipsed (Photo 4) longer exposures are needed.

This brings us to the second problem:

The Moon’s Path through the Sky Changes Daily

As if the changing illumination were not enough the moon’s path through the sky  dramatically changes from day to day. At my latitude (39 degrees north) the moon rises about 42 minutes later each day.  The compass direction (azimuth) at which the moon rises and sets also changes significantly from day to day.   Capturing the moon near the horizon during twilight ALWAYS means attempting a shot of either a slender crescent moon or a full moon.  In most months at most 2 days near the full moon provide full moon capture opportunities. What about the other phases? During the first quarter, the moon is highest in the sky near sunset. During its last quarter the moon is highest in the sky at sunrise. So in short, at first and last quarter you have to shoot nearly straight up to get the moon.

NOTE: First quarter refers not to the amount of the moon that is lit – it is half lit – but to the phase. Similarly at last quarter the moon is also half lit.

Determining the rise and set times of the moon is not hard. Many sites feature the sun and moon rise times.  www.sunrisesunset.com is one site I like. sunrisesunset.com can generate a calendar for a whole month. With a little experience it is often enough to know what phase the moon is in. For me a calendar that does not feature moon phases is useless!

Once I choose which direction I will be shooting, I then know whether I must shoot near sunrise or sunset. Pigeon Point Lighthouse – my nemesis – is on the west coast. To capture the moon behind it the full moon must be setting – which means the sun is rising.  (It also means a 3:00 AM wake up to allow me time to drive to the coast!) Conversely when  attempting to capture the moon over the San Francisco Bay Bridge, the best viewing locations face east – meaning an evening (sunset) shot is best. One advantage to attempting the full moon is that the sun’s glow illuminates the face of the foreground whereas when shooting a crescent the sun and moon are on the same side of the sky so the foreground is in silhouette.

Now we face problem three:

The Moon is Tiny

In this wide angle shot, it is difficult to even see the moon! It’s there in the upper left, but with the 10mm lens the entire moon occupies about 467 pixels out of the 15,154,290 (15M) total pixels. That’s a paltry 0.03 percent of all the pixels in the image. Of course the moon is not tiny, it is very large but it is so far away that its angular size is 1/2 of a degree or about the width of your pinky finger at arms length.

When the Lights Go Down in the City [5_018683]

Photo 5: 20mm Focal length = tiny Moon... did you spot it?

Often my goal is to include a moon in a way that shows it large and well featured relative to the foreground. There is no practical way to get closer to the moon, so the way to make the moon larger in the frame is to use a telephoto lens (as in photo 1 and 2).

Putting the moon near some foreground element allows me to exploit the large moon phenomenon as shown in Photo 1. But it is not enough to use a telephoto lens – I must also be far enough away from the object in question so that the apparent (angular) size of the moon is nearly equal to the angular size of the foreground object. The proper distance can be measured with the pinky fingernail at arms length, or calculated with some trigonometry. In Part 2 I’ll supply a simple formula that works well. Meanwhile Figure 1 illustrates the challenges involved in positioning and “sizing” the moon relative to a foreground object.

Figure 1: Relative sizes of the moon based on distance from the foreground object. See notes.

NOTE: To keep the lighthouse the same size as shown in images A, B, and C above the focal length must be increased. Alternatively, using one fixed focal length pictures B and C can be cropped from a larger photo.

And there is another complication, too, depth of field. The longer the focal length the harder it is to keep the foreground and the background in focus. And one last complication:

Near The Horizon, Atmospheric Conditions have a Significant (Negative) Effect

Looking straight up there are about 50 kilometers of atmosphere to diminish the quality of a photo. Looking toward the horizon, that number is effectively 38 times as much! The sky must be clear of clouds and haze through the entire distance. And a more sinister thing occurs, too. The atmosphere bends the light. When objects like the sun or moon approach the horizon the atmospheric distortion can become quite noticeable as a vertically flattened object. And finally, due to refraction when the sun or moon appears to be setting, it in fact has already fallen below the horizon and remains visible only because of  refraction.  The take away here is that trying to capture a detailed moon at the horizon is not as effective as capturing the moon at least a few degrees above the horizon.

In Summary

To capture the moon near a terrestrial feature:

  • The moon’s current illumination must be managed.
  • The moon’s rising (or setting location) must be accurately calculated.
  • Exposures to capture moon detail require the right amount of foreground illumination (near twilight)
  • The location chosen must have an unobstructed view of the sky toward the desired direction.
  • To get a “big moon” it is necessary to get far enough away from the foreground to get the relative moon size as desired. If too close, depth of field problems arise.
  • A well supported telephoto lens is required.
  • Capturing a shot of the moon near the horizon means the atmosphere must be relatively clear of clouds, dust and haze.
  • Too low in the sky means there will be significant distortion from the atmosphere.

So there it is: all the complications that must be overcome in order to capture the moon. I just have not written HOW to overcome all those obstacles, that information is coming in the next installment.