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Mercury in infrared, June 3rd


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About this observation
Chris Hooker
Time of observation
03/06/2018 - 13:30
Observing location
Didcot, Oxfordshire
254mm F/6.3 Newtonian
TeleVue 3x Barlow lens
Baader IR pass filter (685nm)
ZWO ASI120MM-S camera
1 ms
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Mercury imaged near superior conjunction, in excellent transparency but poor seeing (Ant III-IV). The final image is from a stack of approximately 0.3% of the captured frames, selected initially in software and finally by eye. The features present are in excellent agreement with a degraded Messenger image; however, the wavelet processing in Registax has slightly exaggerated the contrast around the planet's limb.

Note on safety and observing precautions

I hope this note will address any safety concerns readers may have about this observation. Be aware that I have developed my own techniques for finding and imaging Mercury, and using them reduces the risks involved to a level that I find acceptable. I would not encourage anyone to attempt similar observations, but in the interests of safety I would be happy to discuss observing techniques and precautions with anyone contemplating doing so: in that case please leave a comment with your contact details. If you think the risk involved would be too great then do not try it.

For imaging Mercury when it is close to the Sun, a Newtonian telescope is the only safe choice, for the reasons explained below. I will only attempt it if three conditions are satisfied:

1: Mercury must lie within a specific angular range from the Sun, such that the cone of sunlight reflected from the primary mirror emerges fully from the top of the tube. The tube must be rotated so that the sunlight cone does not hit any of the spider vanes.

2: There must be no wind or only a slight steady breeze, so there is no shaking of the telescope.

3: The air must be extremely clear. The slightest haze will scatter a lot of sunlight and make the planet very hard to locate.

The angular range is the most important. It is essential that the reflected sunlight does not hit the secondary mirror, and this defines the smallest usable angular separation from the Sun. The angle at which the sunlight cone starts to hit the inside of the tube defines the largest angular separation: any larger and the tube will heat up, generating tube currents, and there will be a lot of scattered light. For the telescope used in this observation these angles are 2.25 and 4.2 degrees respectively, but for safety the smaller angle is taken as being 2.5 degrees, i.e. one additional solar radius away from the Sun. These values must be calculated for each individual telescope, based on the tube dimensions, the focal length and the diameters of the mirrors.

Catadioptric telescopes and refractors are not suitable for this type of observation because the Sun’s heat and light will be deposited either on the secondary mirror, its mounting and the corrector plate, or inside the tube. Both of these scenarios are potentially damaging or dangerous.

If the air is calm and clear and Mercury is within the acceptable range of elongations, I will attempt to find it. On this occasion, Mercury was at an elongation of 3.2 degrees, i.e. well within the acceptable angular range. It was located, as usual, by aiming the filtered telescope at the centre of the Sun, then making an offset in RA and Dec based on coordinates from Stellarium. On removing the solar filter, the planet was seen near the centre of the field of view after a few seconds. When the elongation is small, minor errors in polar alignment have very little effect on the offset position, thus provided the offset is made correctly the planet will certainly be somewhere in the field of the eyepiece. If the sky is clear enough, slightly jiggling the telescope will be enough to reveal Mercury as a tiny bright speck.


geoflewis's picture

Hi Chris,

I've never imaged Mercury and am unlikely to be able to do so from my location as I have my house to the east of my observatory and trees to the west, so with Mercury never getting far from the Sun, I'd have to find it in full daylight. Your technique and cautionary advice are exemplary. The resultant images are superb and are a complete match to the Messenger data images, which to me seems remarkable.

Very well done.


Chris Hooker's picture

Hi Geof,

I'm glad you like the image, and I appreciate your kind words. If you wanted you could find Mercury in daylight when it's well away from the Sun, and it's completely safe provided no sunlight enters the telescope. It's been so well mapped by Messenger that there's no great benefit in imaging from Earth, but I enjoy the challenge and it's satisfying when everything comes together like this.

Kind regards,


Fossil Light's picture

This is a very impressive image Chris given the elongation and hats off to you undertaking such a challenge. Thanks for including the details of the technique which requires very careful planning and steely nerves,



Chris Hooker's picture

Hi Martin,

I wouldn't claim to have nerves of steel but you're right about the planning. Even so it was good luck that the right conditions occurred just as Mercury was in the right place so I was able to take advantage of them.



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