All sky-watchers are hoping that comet ISON is spectacular when it emerges from the solar glare. there is no guarantee on this, we just do not know. But it could be as pretty as comet Ikeya-Seki or comet McNaught, both of which became far brighter after perihelion passage.
An ohia tree silhouetted against the dawn in the SaddleIf this does happen the question is where to go to photograph the comet. A week ago I found that ISON was slightly behind the ridge from the Mauna Kea VIS. Not badly, but enough to delay when I could acquire the comet and start taking photos.
This recent Saturday I only went partway up the Mauna Kea access road, just high enough to be clear of the clouds and haze. There is a turnoff on the east side of the road just above the cattle guard at about 8,000 ft, one mile below Hale Pohaku. Plenty of room to park a vehicle or two and plenty of level ground to take photos from.
Plotted below is the path of comet ISON through perihelion. The image is zenith up on the morning of November 28th from the island of Hawai’i. The actual moment of perihelion will be Nov 28.77501UT (18:36UT or 08:36HST).
A few things are notable… The obvious one is how close the comet will get to the Sun. Not just in absolute terms, which is really close. But rather how close it will look to us. The comet will be under 30 arc-minutes from the center of the Sun, recalling that the Sun is about 30 arc-minutes across. The comet will not pass behind the Sun from our point of view. While we may not be able to see it while lost in the solar glare, it will remain in the view of those solar monitoring spacecraft that are near the Earth.
Separation will help in trying to spot the comet during the day. During the 27th, 28th and 29th the comet will be very close to the Sun. On the morning of the 29th the comet will be only 4.5° from the Sun. Best bet to attempt a daytime peek may be on the 30th or later, when the comet will again be more than 7.5° from the Sun. Look for the magnitude estimates and be prepared to give it a look.
The orbit of comet C/2012 S1 ISON as it passes through perihelion on November 28th, 2013
Astrophotography is not normally a daytime activity, but there are exceptions. If a comet is bright enough, about magnitude -2 or brighter, it is possible to spot the comet in the middle of the day. Comet C/2012 S1 ISON may very well be visible near the Sun in the middle of the day.
C/2006 P1 McNaught while 5° from the Sun on Jan 14,2007The comet will pass through perihelion on November 28th. At a mere 1,860,000km (1,150,000miles) this will be a close pass indeed. As perihelion is measured from center to center, the distance is even closer if you consider the 695,500km (432,200mile) radius of the Sun. Subtracting the solar radius you realize the comet will pass a mere 1,165,000km (724,000miles) above the surface of the Sun. At this distance the intensity of the solar radiation will be nineteen thousand times more intense than a sunny day on Earth.
This sort of solar intensity will cause the comet to emit enormous amounts of gas and dust. It is this cloud of material around the comet, the coma and tail, reflecting the sunlight that makes the comet bright.
Not quite the dramatic comet in the dawn shot I was hoping for. The comet is just barely able to compete with the dawn glow. Still, a beautiful morning.
Waiting to see what fate holds in store for this dirty snowball as it travels through the hell of the solar corona. I will try another photo session after perihelion.
Comet C/2012 S1 ISON, Mercury and Saturn in the dawn over Hilo
While comet C/2012 S1 ISON is getting all of the attention, it is not the only comet currently visible. There are a couple other good comets available to observe or photograph. On Sunday morning I tracked down and photographed three comets.
The ZEQ25, TV-76mm and Canon 6D setup and taking photos of comet ISONComet ISON is sharing the stage with comet 2P/Encke and comet C/2013 R1 Lovejoy. All three are bright enough to be spotted in a small telescope. Encke was about magnitude 8, Lovejoy and ISON were both about magnitude 5 on Sunday morning.
Despite following comet ISON for the last year, I had never actually observed it myself. Attempts to change this repeatedly ran into problems, either work commitments or bad weather. Over the last month cloudy skies have been more common than clear, even the big ‘scopes on the mountain losing a large amount of time to poor conditions.
Wondering what comet C/2012 S1 ISON is looking like today? There is a very good camera aimed at the eastern horizon from the summit of Mauna Kea. Just what you need to take a look for yourself.
The CFHT Cloudcam is a DSLR camera that is programmed to take exposures constantly through the night. Used by the telescope operators to monitor oncoming weather, the camera shows the sky conditions over the eastern coast of the island and the city of Hilo. The images are live during the night, each morning you can load a timelapse video of the entire night.
The camera has quite a following, quite a few people check the camera constantly. This includes quite a few UFO consipracists. If anything odd shows up on the camera the video quickly shows up on YouTube and linked to postings on the UFO sites. When the launch of a missile from Vandenberg AFB in California created a glowing sphere of light these websites went into overdrive with wild speculation.
Most of the mornings this week have been too cloudy to see the comet. I have been checking the video each day. Currently at magnitude 5 the comet will be a small dot in the imagery. As the comet brightens it should appear nicely in Cloudcam.
Another little lesson in astrophotography, one more in a very lengthy list… Do not use too short an exposure with the flat source.
Flat frames are used to calibrate out any uneven field illumination or dust in the field. This ever more important with the new camera, the larger, full frame sensor shows some vignetting at the corners. I use an electroluminescent source to do my flat frames, actually an old laptop back-light that has been re-housed in an acrylic frame. It provides an even illumination across the aperture of the telescope or lens I am using to acquire the flat field calibration frames.
A bad flat field taken at 1/4000 sec with the EL back-light flicker interacting with the camera shutterWhat I have discovered is that the EL backlight flickers. This is too fast to see with the eye, but if the camera exposure is fast enough it will create issues. This shows up as horizontal structure across the field as the flicker interacts with the camera shutter.
I discovered the effect as I took flats the first time with the Canon 6D. I had the ISO set to 6400 which resulted in a 1/4000 sec exposure. Fortunately I looked at the last flat and saw the problem before I dismounted the camera from the telescope, it was pretty obvious. Once the optical setup is disturbed it is not possible to re-shoot the calibration frames.
A proper flat field calibration frame, Canon 6D and AT6RC telescope.It took a few seconds of thought to realize what the problem was. I never realized an EL light flickers, or rather I had never thought about it. Given that EL lights are driven with a high voltage switching power supply, flicker should be expected. The switching frequency of the supply should be above the human threshold of hearing to avoid an annoying whine, at least 20kHz. It can not be too much higher than that or it would not have shown up in a 1/4000 second exposure.
Slowing the camera down removes the effect. As I could not change the aperture I simply reduced the ISO to minimum, this slowed the shutter speed to 1/15 of a second for the nice mid-scale exposure needed for flats.
The corrected flats reveal the usual things that a flat is takes to correct. The dark corners reveal the expected uneven field illumination. the dust doughnut reveals at least one notable speck of dust on the cover glass of the sensor. There is a dark band at the bottom of the frame which I believe to be shadowing from the edge of the mirror. Looking at the flat I realize I will need to be conscientious about taking flats with the Canon 6D.
