Sunday, 10 May 2020

Lucky Infrared Images of Jupiter

Back in 2011 I noted some IR shots of outer planets by Mike Brown at Caltech, at a wavelength of 1.5 µm.

Last week another fascinating set of images, this time of Jupiter, emerged. These were at 4.7 µm and rather than a demonic cricket ball this time the infrared image resembled a jack-o'lantern.

This wavelength reveals a glow from relatively warm deeper layers of the atmosphere breaking through upper cloud layers. In visible light this is obscured by even higher haze in the atmosphere. The image here is a composite built from a number of so-called 'lucky' images, captured from earth during brief pauses in our atmospheric turbulence. The instrument was the Gemini North telescope on Hawaii’s Maunakea volcano, at an altitude a little over 4,200 metres.

To find out more, I'll point you at the paper available on the Gemini Observatory web site.

[Image credit: International Gemini Observatory/NOIRLab/NSF/AURA M.H. Wong (UC Berkeley) and team Acknowledgments: Mahdi Zamani.]

Wednesday, 4 March 2020

Flying high ... for infrared

One of Nasa's longer-running infrared astronomy projects is the Stratospheric Observatory for Infrared Astronomy (SOFIA). It uses a pre-loved 747 with a suitable new aperture to fly high and catch infrared radiation from above 99% of atmospheric water vapour.

Not quite flying your Lear Jet to Nova Scotia for a total eclipse of the sun but still pretty cool.

Read about it on the Scientific American blog.

Tuesday, 11 February 2020

Spitzer gone ... waiting for Webb

I have to admit that I am not as au fait with what's happening in astronomy these days as I should be; especially since infrared astronomy, once the new kid on the block, is where most of the action now occurs.

The end of January saw the decommissioning of NASA's Spitzer Space Telescope. At about 1430 PDT on Thursday January 30th, JPL reported ...

...the spacecraft was placed in safe mode, ceasing all science operations. After the decommissioning was confirmed, Spitzer Project Manager Joseph Hunt declared the mission had officially ended.

We now have to wait until March next year for the James Webb telescope to launch and the next phase will begin.

NASA's Spitzer Space Telescope Ends Mission of Astronomical Discovery

Thursday, 16 January 2020

Filtering a full-spectrum camera

My full-spectrum camera, a FujiFilm IS-Pro, has no built-in filter and shoots from near UV to near IR unimpeded. This also means that when I put a filter over the lens it also affects what I see through the viewfinder. With, say, a 720nm filter I am unable to see through the viewfinder so would have to use a tripod. Sadly the Fuji's live-view is pretty-well useless.

Many people replace the high-pass IR-blocking filter over the sensor with a low-pass filter such as a 720 or 820 nm to get around this problem ... at the small expense of loss of versatility.

In recent years I have used a deep blue filter, which I can see through to frame. Blue filters usually pass a lot of near IR. You can use a red filter of course, as you might have done with film, but I find the blue filter sometimes produces an interesting colour balance with minimal post-processing. Interestingly, the auto-focus works most of the time, which helps.

I recently bought a specially-designed filter for a type of colour infrared photography from the American company, Kolari Vision. It's called their IR Chrome Lens Filter, which I'll come back to in a moment. This nudged me into looking at results of a number of filters with the full-spectrum camera.

First, here is the camera output without any filter. This is basically a 'normal' colour image but with infrared contamination. You can click on the images to make them larger.

Next is a minus-blue (ie yellow) filter. This can be used to emulate the old infrared Ektachrome film ... see this blog page for more details.

Next comes the red (#25) filter.

Now the blue filter. Different black and white results can be achieved by either removing saturation or by selecting individual channels. (This also applies with other filters of course.) The green channel is useful because, with a Beyer filter camera the green channel has twice as many pixels as the red or blue. With this filter I find I need to under expose (according to the camera) by 3 or 4 stops.

This is Kolari's IR Chrome Lens Filter, which gives a good approximation of the old Ektachrome images. However, it is not exactly the same so is not as useful for foliage health analysis: but it's not a bad approximation.

I had achieved good results using neutral density filters in the past, with Sony's Night Shot, since the ND doesn't apply at IR wavelengths. So I bought a variable ND filter, which is basically two polarising filters together. You rotate one with respect to the other in order to reduce the amount of visible light going through. In this case once I had frames with minimal density I simply rotated the outer filter until I could only just see anything then fired the shutter. Autofocus worked and by trial and error found the exposure change: in this case under by 4 stops. There is a little (false) colour information left but this method works best for a monochrome result.

Finally, for comparison, here is a 720 nm filter result.

One thing this experiment also showed me was how bad the chromatic aberrations are around the edges in the lens I am using, which are quite noticeable with colour shots but usually vanish when reducing to monochrome.

For more on this subject, here is Kolari's page outlining the characteristics of their various filters.