Another Jupiter image

I’ve already posted this on facebook, so apologies to friends who’ve already seen it! Jupiter taken 12th January 2013, 22:17 :

This is my best image to date which is due to several factors.

Firstly is the timing. Jupiter is now high in the night sky meaning there is less atmosphere to see though, and this leads to clearer seeing. You need good seeing to get decent planetary images.

Next is that I’ve got more familiar with the camera settings. For capturing avi’s I use WXastrocapture. The camera settings seem to work best when using a low frame rate, noting above 10fps. Auto white balance seems to work well. Exposure time, brightness, gamma, and gain are all inter-related. I use an exposure type typically of 1/25s. Brightness I set to around 50-60. This leave gamma and gain, I try to keep the gamma near to 0 and bring the gain up until the histogram shows that I’m getting about 200 (out of 256) on all colours.

Focussing is the next step. I have a Borg Helical Focuser on the scope. This lets you fine tune the focussing to 0.1mm. Much finer than you can do with the regular SCT focuser, especially with the problems associated with the design of SCT focusers and not being able to fix the mirror on the scope. An image will noticeably go out of focus in even 0.3mm. I had been using a Bahtinov Mask, but the above image was focussed by patiently and slowly looking at the image on the laptop.

Lastly for the telescope you need very good collimation. Having concentric circles on a defocussed start with a 4mm eyepiece using a 2x barlow seems to be good enough. Though I need to study the optics of collimation and resolution in a lot more detail to work out how good collimation really needs to be.

A big problem with imaging Jupiter is that the planet rotates very fast. Here are two images taken about 30 minutes apart:

For a decent image you need at least a few hundred good avi frames. And that means taking at least a thousand frames. At 5 fps, that means 3 minutes of footage – at a minimum. Ideally you want a couple of thousand useable images and that means a 10 minute avi which will have lots of planetary rotation in it. There is software which can derotate your avis called WinJupos. I’m only starting to use this software so when I’ve got the hang of it I’ll write about it some more.

Now that you have your derotated avi you can put it through Registax. Registax has been doing the rounds for years and is quite user friendly at this stage, certainly for basic use. It will align the frames from your avi, compare which are the best ones, and stack them to produce a more detailed image. As a rough guide, start by selecting (limiting) the 50% best images for stacking. Experiment with more and fewer as this number will depend greatly on the seeing. The one thing about registax that always confuses people is the wavelet processing. I don’t understand it either. However I find that using the following values gives a good starting point: Layer 1: 1, Layer 2: 1, Layer 3: 5, Layer 4: 15, Layer 5: 35, Layer 6: 70. Registax is very simple to get decent results with default settings, but it also can be a very complicated program if you want to get into the details.

Finally the image can be processed in gimp. First take a gaussian blur using of the image. Yep, blur it. If you try sharpening the image straight from registax you may get some strange artifacts. A gentle blur then an unsharp mask will give better results imo. Then tweak the levels and saturation etc. until you get something that looks good. I probably overdid the processing in the top image, but it still looks good I think.

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