When you want to see the stars, find someplace dark
Sky events
With this new Moon, an annular solar eclipse will sweep across the North Pacific from China and Japan to the west coast of the United States. In Hawai’i this will be seen as a minor partial eclipse with only a few percent of the Sun obscured by the Moon.
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.
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
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| 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.
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.
You have heard it before, but it really is true! Looking at the Sun for longer than a few brief moments with an unprotected eye can lead to permanent damage. Looking at the Sun with any sort of optical deceive that has not been properly filtered can lead to instantaneous eye damage.
Discussed below are the only safe methods I am aware of to view the Sun. There are some dodgy methods out there. Take chance with your irreplaceable eyesight? I think not. Be careful and do it right if you want a look.
For the unaided eye there are a couple options to view the Sun safely…
A much better view of the event can be seen if modest magnification is used. Do keep in mind that any soft of binoculars or telescope also concentrates much more light that the eye alone. Remember those childhood “experiments” involving a magnifying glass and ants? The best options here are the use of solar filters designed to be used on a telescope, or indirect means such as image projection.
Keep in mind that the beam of light out of the eyepiece is quite intense, potentially hot enough to burn a careless finger placed near the exit from the telescope.
The method works best with small optical systems, binoculars or the smallest of telescopes. You do not need big optics to project a very nice solar image.
You do not have any optics? Use the simplest optical arrangement of all, the pinhole camera!
If you do not have a safe means of viewing the transit and do want a look, you can go to one of the many public events being organized. Here on the Big Island there are quite a few options, one should be close to you.
Everyone is gearing up for the Transit of Venus. Starting at just after noon on June 5th, Venus will cross the face of the Sun. This will be our last chance to see such an event in our lifetimes, as a result, many skywatchers are making an extra effort to see this transit. This includes a number of folks who are traveling to Hawai’i this June, just to be here for the transit.
There will be solar telescopes stationed at the MKVIS, at the summit, and at a number of other locations around the island. With many options to choose from, you do not need to go to the summit to get at least some view of this event.
The diehards, those wanting to see the entire transit, will probably go to Mauna Kea. I really do not know what sort of crowd to expect on the mountain. While it could be substantial, I really expect a more modest showing than some folks are predicting. In any case it will be a fun event.
Myself? I will be on the summit. I will be running the Keck live webcast of the event using one of my telescopes. From a vantage point behind the Keck 1 telescope we should be in position to witness the entire event from start to finish.
As you might notice, the times change by a mere two seconds between Kona and Honolulu. These times will be pretty close for the entire island chain. That last number is the Sun’s altitude, the angle above the horizon. Note that this is near 90° for Hawai’i, nearly straight up for the start of the event. Final contact will occur with an altitude of about 5°, just above the horizon near sunset.
| Location Name | External Sun Ingress Alt h m s ° |
Internal Sun Ingress Alt h m s ° |
Greatest Sun Transit Alt h m s ° |
Internal Sun Egress Alt h m s ° |
External Sun Egress Alt h m s ° |
| Anchorage, AK | 14:06:30 51 | 14:24:04 51 | 17:26:53 38 | 20:30:46 16 | 20:48:32 14 |
| Honolulu, HI | 12:10:07 85 | 12:27:46 89 | 15:26:20 49 | 18:26:38 9 | 18:44:38 5 |
| Kona, HI | 12:10:09 86 | 12:27:48 87 | 15:26:16 47 | 18:26:33 7 | 18:44:33 3 |
| Los Angeles, CA | 15:06:26 58 | 15:24:02 55 | 18:25:33 18 | — – | — – |
| Phoenix, AZ | 15:06:03 54 | 15:23:40 50 | 18:25:32 13 | — – | — – |
| Portland, OR | 15:06:05 57 | 15:23:40 54 | 18:25:53 23 | — – | — – |
| San Francisco, CA | 15:06:29 61 | 15:24:04 57 | 18:25:39 22 | — – | — – |
| Seattle, WA | 15:05:58 56 | 15:23:32 53 | 18:25:57 23 | — – | — – |
All times are local time. Keep in mind that this event occurs on June 5th for viewers in the US and Hawai’i. Some sources show June 6th for a date, and so it will be for viewers on the other side of the date line in Australia, Japan and China.
There is only one more chance in our lifetimes to see a Transit of Venus. If you are curious about this event, where do you go to see it?
For those folks who just want a nice view of the event, and do not necessarily need to watch for the entire seven hours, there are a number of easier options. To accommodate the public, there will be solar telescopes setup and manned by volunteers at a number of locations around the island…
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!
Today, Jupiter will pass through superior conjunction at 02:46HST. The planet will remain lost in the Sun’s glare for the remainder of the month, appearing in the dawn during the first days of June. The planet will arrive at opposition on December 2nd.
Throughout June, Jupiter will climb higher in the dawn. Towards the end of June the planet Venus will emerge from the Sun’s glare to race after Jupiter. The two will meetup for a brilliant morning conjunction during the first week of July.
This week Mercury will be exiting the dawn sky, sliding closer to the Sun in our sky each day. It will pass through superior conjunction on May 27th, heading for an evening apparition beginning in the first days of June.
This is your last chance this lifetime to witness a Venus transit! On June 5th the Earth, Venus and the Sun will line up correctly so that earthbound observers will see Venus cross the disk of the Sun.
To witness this event you only need to be on the daylight side of our planet when it occurs. The 2012 event favors the Pacific, Australia, Japan and eastern Asia.
For Hawai’i the event will begin at about 12:10pm with the Sun high in the sky, mid-point will be about 3:26pm and will end about 6:44pm, just before sunset. These times are only approximate, exact times will depend on the observer’s location and can vary by several minutes across the islands.
As always the single best source on the web (or anywhere) for eclipse and transit information is Fred Espenak’s eclipse website at NASA. Stop by whenever you have a question on upcoming events as well as viewing and photography tips.
Keck Observatory will be making an extra effort for this transit as Mauna Kea is perfectly positioned to observe the event. We will be webcasting the transit from a small telescope located at the summit. The webcast can be seen on your computer, or come over to our headquaters in Waimea to see the webcast and a live view in a solar telescope if the weather cooperates. there will be extra staff on hand to answer your questions.
Stay tuned to DarkerView for further details as the transit draws near.
1) Transit of Venus, Wikipedia Article, retrieved 8 Feb 2009
2) 2004 and 2012 Transits of Venus, Fred Espenak, NASA/GSFC, NASA Eclipse Website, retrieved 8 Feb 2009
