photography – Page 8 – A Darker View

Photographing the Transit of Venus

With the Transit of Venus looming on the calendar, a discussion of solar photography is in order. Taking good photos of the Sun is not that difficult, but can be aided with a little information. There are some unique challenges in solar photography.

The one obvious problem is dealing with the sheer intensity of the Sun. An intensity that can easily damage a camera if placed behind unfiltered optics. A proper solar filter is the easiest way to reduce the light to a safe level.

A solar filter will also produce the most pleasing images of the Sun. Indirect techniques like projection can be used. But for good solar photos, a proper filter in front of your optics is the single best method.

Solar filters for optics are constructed with a thin film of metal such as aluminum or stainless steel vacuum deposited on a substrate. This substrate is usually glass or a thin mylar film. The resulting filter allows only a small fraction of the light through, about 0.01% or 1/10,000 of the unfiltered value. Importantly, the filter blocks the ultraviolet and near infrared part of the spectrum as well. The result is a safe filter than can be used on a telescope or telephoto camera lens.

Sufficient magnification is needed if details of the Sun’s surface are to be well recorded. A few hundred millimeters focal length, found in common telephoto lenses will produce a reasonable solar image. The image will still be fairly small. To fill the sensor requires more. For an APS-C sized sensor (Canon T2i, 60D, 7D, Nikon D5000, D3200 or similar) a telescope with 1,000mm focal length will create an image filling a good portion of the image.

Image sizes for APC-C Sensors
Focal Length		Image Size (arcmin)
100mm		760×510
400mm		190×128
800mm		95×64
1000mm		76×51
1500mm		51×34
2000mm		38×25

The table to the left shows the resulting images sizes, in arc-minutes, given various focal length lenses, on an APC-C sized sensor. Recall that the Sun is about 30 arcminutes across as seen in our sky. With 100mm the resulting image is 510 arcminutes from top to bottom in the frame. This is 17 times the width of the solar image, a pretty small image indeed. With 400mm this improves to about 4, thus the Sun will reach about 1/4 the height of the image. At 1000mm this is about ideal, the Sun will reach more than halfway across the frame.

1500mm will just fit the solar image. While this may seem ideal, there is an issue. A small amount of drift will put part of the Sun out of the image, cutting off part of the disk. Sizing the image to fit in the frame with a good margin will allow some drift, while still giving a good image scale.

If you have a full frame camera (Canon 5DMkII, Nikon D800, etc.) a larger image can be used to fill the larger sensor, thus a longer focal length can be used. A telescope with 2000mm focal length will produce an image 17mm across, neatly fitting in the area of a full frame sensor.

Few compact cameras can boast a lens that will zoom far enough to produce an image of the Sun filling the frame. For these cameras another technique can be used, afocal photography. This can also produce good images, but will require experimentation to find the right combination of telescope, eyepiece and camera to produce a correctly sized image.

Solar Framing — The full frame solar image with about 1000mm of focal length and a Canon 60D

If you want to calculate the image scale for your optical combination, lens and camera, I suggest downloading the CCD Calculator from New Astronomy Press. You can enter the optical parameters and see exactly what the resulting image will look like with a sample image of the Sun, Moon or other selected objects.

Another issue is resolution. Our atmosphere usually limits the practical resolution to about one or two arcseconds, blurring any finer detail through atmospheric distortion. This can be much worse in the daytime with solar heating of the ground and air around the telescope. Thus the limit for resolution will be reached with about 1000mm focal length and a modern 10-15 megapixel camera. Any further magnification beyond about 1000mm will simply result in magnifying the blur. There are techniques for overcoming this (image selection and stacking), but if you know how to do that, you already know what you are doing.

Just a bit of summing up… You need a proper solar filter or other method of safely reducing the solar intensity. A long telephoto (400mm or more) will produce a reasonable solar image. A small telescope with about 1000mm of focal length is ideal for photographing the entire disk of the Sun with a DSLR camera.

Preparing for the Venus Transit

Time to start preparing the gear for Venus Transit! This means dismantling the astrophotography rig in the garage and reconfiguring for solar work. Taking the the AT6RC telescope off and remounting the 90mm APO. The APO has just the right focal length to produce a nicely sized solar image on an APC-C sized sensor, such as the sensor in the Canon 60D.

First up? just setup the ‘scope in the driveway and take a few photos of the Sun. Just checking the photographic setup, the necessary parts and pieces. Nothing misplaced? Where did I store the solar filter? A nice focus on the camera? Perhaps take some nice photos of the large sunspots that current grace the surface of the Sun while I am set up. I hope we have some nice spots during the transit, they make focusing so much easier!

Next step is to get autoguiding operational, this will be a seven hour event and I really do not want to manually guide for the entire duration. Particularly with a telescope that was setup in the daytime and is not properly polar aligned.

A couple other steps remain in the preparation. Automate the camera to take photos at a regular interval. Insure I can provide a good video feed to the computer sending out the webcast. I do have a few more weeks to accomplish this. I am certain those weeks with speed by surprisingly quickly. Time to get ready!

iPhone Moon

Despite numerous attempts, I had never managed a decent shot of the Moon using an iPhone. When showing people how to do afocal photography, I have leaned how to make just about any compact camera perform nicely, but routinely seen nothing but trouble with cell phone cameras.

Lunar photography is incredibly popular with folks using the telescope, a great activity for a night with a bright moon. Long ago I found that an inexpensive 20-25mm Plössl is a good match for the lens of most compact cameras. On a 1-2m focal length telescope this combination can produce very nice lunar photos. The setup does not work with cell phone cameras. Though people do try, the results have been routinely disappointing.

Working a resort star party recently, I discovered a combination of telescope and eyepieces that works very well. A C11 telescope, an f/6.3 focal reducer, and a 20mm Nagler type 2 eyepiece produced very nice photographs with several different cell phone cameras. The result was a very happy audience and a lot of great lunar photos. As people walked away from the telescope they were rapidly replaced by a crowd holding glowing screens, all wanting to get a nice lunar shot for themselves. I will have to explore other telescope/eyepiece combinations to find another solution that does not involve a $500 eyepiece.

iPhone Moon — An eleven day old Moon, taken with an iPhone 3GS, a C11 and a 20mm Nagler 2 eyepiece

Afocal Photography

When doing any sort of public astronomy, showing folks the beautiful sights available to a telescope, I often hear the question “Can I take a photo of that?” The person asking the question is usually holding the ubiquitous compact digital camera. They are often surprised when my answer is “Yes”.

It is indeed possible to manage hand held shots of bright astronomical objects by simply holding the camera up to the eyepiece. There are a few tricks to making it work, but nothing that can not be demonstrated in a minute or two. The resulting photographs can be quite pleasing, definitely worth showing to friends and family along with the rest of the Hawai’i vacation shots.

The method of positioning a camera with a lens in front of an eyepiece is called afocal photography, or sometimes digiscoping. Afocal has been around for a while, but was not considered a practical photographic method by most. The advent of common digital cameras without removable lenses has changed this.

Keck in Motion Scene Guide

I have been getting a few questions about the video. To answer a few of them I have compiled a guide to the scenes. Some quick explanations to what you are seeing, information on the camera used as well as the exposure information.

The video is a combination of two techniques. Many scenes were filmed as standard video then accelerated during editing to allow the motion to become clear. Examples of this are scenes of telescopes slewing and the interferometer delay lines moving.

Slower subjects, such as clouds or the stars moving across the sky, were photographed as time lapse. Here a large number of still images were taken. These are then processed and converted to video using Photoshop CS5 before loading into the video editing software, Adobe Premiere Elements. To construct the time lapse sequences sometimes required thousands of separate images, quickly filling memory cards and exhausting batteries. After dark it is long exposure time lapse that is used, with individual exposures often 15 seconds to one minute long.

Continue reading “Keck in Motion Scene Guide”

Laser Return Photometery

A different use for amateur astrophotography gear.

An amateur CCD camera can do more than take pretty pictures. There is no reason why any decent telescope, however small, and a CCD camera can not be used to do real science, or real engineering in this case.

The goal of the night was to perform proper photometry on the laser returns with independent equipment. We want to quantify the performance of the Keck adaptive optics laser systems. We launch two powerful lasers into the sky, one from each telescope, to allow analysis of the atmospheric distortions through which the telescope is observing. Using the data the system can correct for this atmospheric distortion and create much sharper images of distant stars and galaxies.

The lasers pass through a layer of sodium atoms about 90km (55miles) above the ground. There the 589nm yellow light excites these sodium atoms creating a glowing beacon, what we call the laser return. This return is what we look at to analyze atmospheric distortion. A brighter return allows better data and better performance of the system.

Both the Keck 1 and Keck 2 lasers in operation under the light of a nearly full Moon

Amateur astrophotography gear is perfectly capable of doing this task. A portable telescope, a proper CCD camera, combined with care to acquire calibrated images. All that I needed to add to the setup was a photometric V filter.

It was a perfect night for it, clear, dry and cold. Best of all, there was no wind to bounce the telescope around and chill anyone working outside. The winds are nearly constant atop at 14,000ft peak, calm nights are unusual, I was lucky indeed.

I setup the telescope atop a crust of ice and snow. The snow was convenient as it allowed me to set down gear on a cleaner surface than the gritty volcanic cinder underneath, keeping everything quite a bit cleaner. The altitude and cold made setup and breakdown a slow, laborious process, and added unique difficulties. I had to be very careful moving the heavy gear, so as not to slip on the icy snow. When I went to move the telescope tripod I found it frozen into the snow and cinder! I had to heave hard to break it free.

Continue reading “Laser Return Photometery”

The ACLU Publishes a Guide to Photography Rights

A nice guide to your right to photograph by the ACLU. It is interesting to note the current restrictions to the audio portion of videography that some states have attempted to enforce using wiretapping laws. Fortunately Hawai’i is not a “two party consent” state, removing that issue here.

The Canon 60D

A box was waiting for me when I got home. A long awaited box. A box that represented hours of reading, weighing and wrestling with the question…

A new camera!

I now have a replacement for my venerable Canon 20Da that I have used for over six years. Not that I will be getting rid of the older camera. It is still invaluable to me for astrophotography, a role it is specifically modified for. Nor will it replace my Canon G11, a camera I have carried every day for well over a year now. The G11 will remain my day to day camera, a role for which a compact is well suited.

The Canon 60D DSLR camera, image credit: Canon USA

No, the 60D will be there when the smaller camera is simply not enough. There have been a few recent instances when I had opportunity for a good photo. An image I knew the camera in my hand simply could not capture. There was that pueo sitting on a lichen covered boulder last week. Or the summit under a blanket of fresh snow, lit by the full moon. Or… To many instances.

Another primary reason for the 60D… High quality HD video capability. This is something I have come to truly miss in my existing cameras. There have been a number of occasions when I really could have used that capability! Unfortunately now that I have a camera capable of truly good HD video, our backyard volcano has stopped producing photogenic lava flows. At least I know that will not last.

The decision was made more difficult by the choice of cameras available. A dizzying array of options now exist. A number of very capable DLSRs, the new mirrorless designs, this was a decision without a simple answer. In the end it came down to a choice between the Canon 60D and the very similar 7D. The newer 60D sports a flip out screen (something I love to have), better movie controls, and while it gives up a metal body it is also much lighter to carry. Both cameras use the same sensor and feature essentially the same image quality.

My thanks to Baron. I ran into him at the ROV competition last week. And lo… he was carrying both the 60D and a 7D. Even better, he let me fondle his gear while we chatted about the relative merits of the two cameras. Nothing like a hands-on look at the gear and the opinion of someone who uses the cameras extensively.

Even when holding a brand new camera I am wondering what will replace it in a few years. Maybe a mirrorless compact? That is a market segment to watch. What about my veteran G11 camera? Deb is making less than subtle suggestions about my getting a G12 so she can have my G11, mostly for underwater I suspect. Cameras are one place the technology is still changing rapidly enough to make these decisions difficult.

For now I need to learn a new camera and find its limits. A good low light session is in order, and I have a night on the summit coming up… with lasers!