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A Two Comet Night as a Lesson in Air Temperature and Humidity

2021 January 8

Three nights ago on January 5th, I caught two comets in one imaging session. The first, Comet 156P/Russell-LINEAR, was in the constellation Triangulum. The second, Comet 398P/Boattini, was in the constellation Orion.

Comet 156P/Russell-LINEAR

Comet 156P/Russell-LINEAR on January 5, 2021, 02:54-03:26 UT. At the time of this 30-minute animated sequence, the comet was 78 million miles away from Earth and heading just beyond the orbit of Jupiter where it will turn around to make another inbound pass through the inner solar system. With an orbital period of 6.4 years, Comet 156P/Russell-LINEAR should be back this way in 2026. In this image north is up and east is left. The field of view is 16’x12.’ [1]

Both comets were in the sweet zone for my telescope and camera combination. I estimated Comet Russell-LINEAR at magnitude 12.3 and Comet Boattini at magnitude 12.8. These magnitudes are 300 times fainter than the faintest celestial object that can be seen by a pair of very good eyes from a very dark location.

On the images posted here Comet Boattini is noticeably fainter than Comet Russell-LINEAR. Both were captured using similar exposure times and gain settings. Under these circumstances, they should appear of similar brightness. At the time, however, I was imaging Comet Boattini in comfort from inside my warm house, and failed to notice that the outside air temperature and the dew point temperature were converging near 31° F. This was serious negligence.

I didn’t realize anything was amiss until I moved on from Comet Boattini to my next target, the Pinwheel Galaxy (M101) in Ursa Major. This large galaxy fills the whole field of view of my imaging set up, but no matter how I worked the camera controls, I just couldn’t get anything better than a pale noisy ghostlike pinwheel to show up on the monitor.

Comet 386P/Boattini

Comet 398P/Boattini on January 5, 2021 06:39-07:19 UT. At the time of this 38-minute animated sequence the comet was 38 million miles away from Earth and heading outbound to somewhere between the orbits of Mars and Jupiter where it will turn around and head back toward the inner solar system. With an orbital period of 5.5 years, Comet 398P/Boattini should be back in our neighborhood in 2025. In this image north is up and east is left. The field of view is 17’x13’. [2]

Thinking that clouds had rolled in, I went outside to check. No, the sky was still beautifully clear. But upon reaching the telescope rig in the back yard the situation became apparent. The entire rig was covered in thick crystalline icy frost. Even the foam dew shield over the front of the telescope was covered with the stuff.

After removing the dew shield the source of the problem became even more painfully obvious. The corrector plate, the large flat glass lens that covers the front end of my Schmidt-Cassegrain telescope, was completely frosted over like a heavily frosted car windshield on a cold winter morning.

Needless to say, this ended the session. But, although I was tempted, I didn’t take an ice scraper to the corrector plate.

Temp-Dew Point Graph

This graph shows air temperature and dew point temperature from my back yard weather station on January 4-5, 2021. The area inside the red circle encompasses the time from midnight on the night of the 4th through approximately 01:30 a.m. on the morning of the 5th. This was when I was observing Comet 398P/Boattini. During this time the air temperature dropped below freezing, hovering around 31° F and the dew point temperature hung around 29° F. This temperature differential put the relative humidity in the 85%-89% range, a fraught range for astronomical imaging. With the air temperature and dew point temperature within two degrees of each other, moisture condenses out of the air to form heavy dew on all exposed surfaces, especially telescopes and their optical components. But in this case the situation was made worse by the subfreezing temperatures causing the liquid condensate to freeze as frost. This was the death knell for what started out to be a promising observing-imaging session.

I didn’t realize it at the time, but the frosting event had already started while I was imaging Comet Boattini. This explains why Comet Boattini is fainter than Russell-LINEAR in these images despite both being at nearly the same visual magnitude.

Nevertheless, the dew shield did its job. It delayed the inevitable onset of frozen dew collecting on the optics, but it obviously was overcome by the onslaught of humidity and below-freezing temperatures and succumbed.

Having once again learned a lesson about humidity, temperature, and telescope optics I now have a telescope dew heater on order and am anxiously awaiting its arrival! There are a few months of winter left. I don’t want to get frosted again.


[1] This animated sequence was captured with a Celestron C8 telescope (203mm f/10) operating at f/5 with a ZWO ASI224MC camera. Each of the eleven images in the sequence is made up of a stack 11-13 sub-images, or frames, each exposed for 15 seconds.

[2] This  animated sequence was captured with the same equipment described in note 1. The sequence is made up of nine images, each of which consists of a stack of 11-13 frames, each exposed for 15 seconds.

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