Three Comets in the Dawn

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 Astrophoto Rig
The ZEQ25, TV-76mm and Canon 6D setup and taking photos of comet ISON
Comet 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.

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Watching for ISON with Cloudcam

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.

Latest Cloudcam Image
The latest Cloudcam image

Good Flat, Bad Flat

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.

Bad Flat
A bad flat field taken at 1/4000 sec with the EL back-light flicker interacting with the camera shutter
What 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.

Good Flat
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.

Postcard from the Reef – Posing Dascyllus

Hawaiian dascyllus are a favorite of ours. I would expect that these fish are a favorite of many divers and snorkelers. These bold little fish will often challenge even a diver who invades their territory. Floating nose to nose with a little black fish is just too comical…

eb Photographing a Dascyllus
Deb wielding a camera at a defiant Hawaiian Dascyllus (Dascyllus albisella)

Mercury at Maximum Elongation

Today Mercury reaches maximum elongation, the furthest point it will reach from the Sun in the sky and the highest it will be above the sunrise for this morning apparition. The planet is easily visible as a bright, starlike object about 19° above the rising Sun as the twilight begins. Over the next couple weeks Mercury will slide back into the sunrise, heading for superior conjunction on December 28th.

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