The methane filter (CH4) is a filter whose very narrow transmission is centered around one of the CH4 absorption band in the near infrared at 890 nm. That filter is very interesting because in contrary to all others, it does not reveal information relative to color differences (the albedo) but to the relative altitude of the cloud structures! The very low amount of available light will none the less ask you to take much care of the imaging technique…
The left set shows how different are visible images (RGB) and CH4. On this last one, the brightness of a given detail is directly correlated to its relative altitude in the jovian atmosphere: the brighter it is, the higher. We observe that the red spots of the planet (GRS, BA) and the south polar hood are the highest structures… Such kind of information is very valuable.
Which filter to choose ?
If you are not equipped yet, you will have to choose one filter, the best (with the largest band) being the most expensive. This can be a hard choice because the low light is really a problem.
The best filter on the market today is the CH4 IDAS (sold by Astro Hutech in the USA). This is the one that delivers the largest band (15 nm) but it is expensive (more than 300 dollars) and if you leave in Europe you may have to pay additional fees. It is worth affording it though because you will have to buy it only once in your lifetime ;).
Then you find the Baader CH4. This one is noticeably cheaper (175 €). Its passband however is only 8 nm, twice less than the IDAS! It will be best used with instruments of at least 300 to 350 mm.
Finally one can get a CH4 filter from Edmund Optics. It is even cheaper (around 100 €) with a correct passband of 10 nm, but you would have to fit it in 31,75.
Find the correct sampling !
The difficulty of methane imaging should drive you to adjust your usual settings. The goal is to keep the exposure time below the second, and even if possible below half a second. For this you must change the sampling to match the needs of the methane filter. You have two ways to do this:
You can lower the effective focal length (with another barlow lens, or by using the prime focus of the instrument)
Or you can keep the same focal length but you will use the binning mode of the camera.
But aren’t you going to lose some resolution ? Not at all, because this methane band is found deep into the near infrared (almost near the end of sensitivity of the CCD!) and the resolution of the telescope is also reduced. It’s just not relevant to use the same sampling as usual. The diameter of the Airy disk is enlarged by almost 40 % (550/890… remember that the resolution is lower when the wavelength increases!)
My personal advice would be to keep the same optical imaging train as usual but to use the camera in binning 2x. This will save disk space, processing time, and you won’t have to unset anything.
Try to get an exposure around 0,5 to 0,7 at maximum, the limit being the maximum acceptable gain of the camera. It is not really a problem if the raw is under exposed, because we are going to use WinJupos (see below!)
Take a Dark frame
This is unavoidable. With an exposure of several hundredth of a second and a high gain, the image will show all the weird stuff with hot pixels and horizontal or vertical lining, that will be visible on the final image if you don’t remove a dark frame.
At the end of the CH4 Jupiter video, just shut the telescope and launch another video with the same parameters, but with only around 200 raw frames. Use the “Create Dark frame” function of Registax and save the result. Beware that you will have to adjust the luminosity of this dark image in order to avoid loss of real signal on the planet itself.
Absolutely essential !! WinJupos will allow you to use 1000 to 2000 raw frames instead of 100/200… the difference on the final result will be huge. The de-rotation technique to use is the video de-rotation.
Take a very long video. 10 mn is really a minimum to make profit of the advantage given by the software, but you can go even further. I’m routinely getting 15 mn SER files in CH4 but last year, F.Colas and M.Delcroix at the Pic du Midi went up to more than 30 mn !!
For the final image, choose the “stacked image” option, not the “corrected video”. Results look better, even if there is no possibility to select frames for the final stack (but in methane you will be likely to take them all anyway). Don’t forget to use the dark frame :).
When processing the image, use the processing module of Registax 6 Spline resizing to 200 %. (click on the image to enlarge)