Multi wavelength observations of Noctilucent Clouds

[Bill Ward]

Hi,
I conducted a version of this experiment 14 years ago(!) when I was interested in polarisation effects in NLC. https://youtu.be/I26xnuxUtVE?si=A3vVNrCC73uEVeLG

The other night I took a quick sequence of shots of the same part of the display through a series of Schott GG, OG and RG filters using an ATIK 314L CCD and a 25mm f1.4 lens at f2.

The conditions were pretty poor with low cloud and fog blowing in all the way from the east coast.

However the results at the two extremes, 400nm and 1000nm were quite remarkable. The only thing I’ve done with these images is a contrast stretch.

First image is a 1 second exposure through the 400nm filter. The NLC is extremely distinct. The low cloud/fog is clearly seen silhouetted at the bottom of the frame. Close inspection shows some darker areas in the NLC. These were invisible to the naked eye.

The second image is remarkable, The NLC have vanished, they are completely trasnparent at 1000nm. This gives a Mie scattering limit to the size. But the most interesting thing is what I think are cirrus clouds stronly scattering near IR light. As our eyes are completely insentsitive to this wavelength we don’t seem them but there they are, glowing away… They must be scattering light already reddened by passage through the atmosphere as I don’t think they were getting direct sunlight yet (exposures were at 0154 and 0155UT respectively)

Their presence is only hinted at in the darker areas casued by them obsuring the scattered shorter wavelenght light from the NLC. Approximate areas marked as A and B.

Hopefully there will be more decent displays to get better images and more of the intermediate wave bands.

Cheers,
Bill.

Attachments:

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   nlc400b-annot-2025_07_12_13-0154UT.jpg
   

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   nlc1000b-annot-2025_07_12_13-0155UT.jpg
   

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