The best opposition of Mars until 2035 has just passed with the best times to view and image the red planet between late September and mid October. The weather cooperated (mostly) and we were all treated to a wonderful display (without the giant dust storms of 2018).
I was out every chance I got to image the planet when it was the closest and brightest. Mars was well placed, high in the sky. As reported back in early September I imaged Mars with my new ASI462 for the first time. Results were pretty good, but could be improved. Based upon what I've seen on the various astro-boards, my image was about a 4 on a scale of 1-10.
I imaged Mars on five nights, Oct 2, 13, 14, 21 and 22. Results were generally bad to awful. But on the 22nd I was able to get a respectable image.
Planetary photography is a tricky business. Lots of things can go wrong, and frequently do. The real culprit is the fact that we have a thick atmosphere above us. The twinkling of the stars on a crisp clear winter night is evidence of that atmosphere. Stars are very big, but they are incredibly far away. So far away that they appear as points of light, even in large telescopes. As the narrow beam of light makes its way to the observer it has to pass through various layers of the Earth's atmosphere. As it does the light is refracted and bent to and fro. What we see over time is that beam coming to us from various angles and that makes the star twinkle.
Since planets are close enough to us that they appear as a disk (although you'll need a telescope to see that), the twinkling effect doesn't occur. Since the disk is composed of lots of beams of light, each randomly refracting, but all in different directions at the same time, we see the combination of all them. Thus, instead of a twinkle, planets appear steady. But the disk image distorts over time because the beam combination is different as the air moves between us and the planet. These random fluctuations of the light are a big problem - they cause the planet's disk to 'bubble' and 'wobble' in the telescope's field of view. You may be aware of this if you've ever looked in a telescope or pair of high power binoculars at a terrestrial object during the heat of a summer day. The object appears to rapidly move about; sometimes appearing blurry but every now and then sharpen to crystal clarity (Outer Limit's fans will recognize the phrase).
So, planetary imagers use a technique known as 'lucky imaging'. This technique is used to record hundreds or even thousands of images over a short time span in order to capture some images when the seeing is momentarily steady. In order to capture this many images in a short period of time though we need a camera capable of high speed imaging. The ASI462 is one such camera - a video camera for astrophotography. But, just like with deep sky imaging, the process is rather involved.
On the evening of October 22 seeing conditions were pretty good, about 3 or 4 on a five point scale with 5 being the best. My location is not all that great for planetary imaging since my telescope location is within a 'bowl' of terrain surrounding by trees, and the denser air settles and swirls about around my property. I set up and waited for the scope to reach ambient temperature and for Mars to rise higher in the sky (the higher the better - less air mass to image through).
At 9:53 PM I set the system to capture Mars over a total exposure period of 6 minutes. The camera was set to take 45 frames per second, at 5ms per frame. I was able to capture 27,415 images on this run.
Here is a sample of 50 frames from the beginning of that run. This video was taken with my ZWO ASI462 through an EdgeHD11 telescope equipped with a Televue Powermate 4x teleconverter. This makes my EdgeHD equivalent to an 11" f/40 (11700mm) telephoto lens!
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As you can see the image moves quite a bit. But this is actually a nice run. But even in the short 1-2 second clip you can see areas of the image that appear sharper for a fleeting fraction of a second. With over 27,000 images all we need do is locate the best images from the set. That's where AutoStakkert comes in.
AutoStakkert is an image stacking software. As the author Emil Kraaikamp puts it:
"AutoStakkert! is all about alignment and stacking of image sequences, minimizing the influence of atmospheric distortions (seeing). Its goal is to create high quality images of the Planets, the Sun, and the Moon, without too much hassle".
AutoStakkert takes the video and processes the individual frames finding the 'best' of the bunch as per specific parameters set by the user. I usually ask for the 20% best. If the time is kept short, this is all that needs to be done to get a stacked image of the selected frames. But since I was exposing for a full 6 minutes, the rotation of the planet needs to be taken into account. After all, Mars rotates almost the same as Earth; 24 hours, 39 minutes, and 35 seconds to be exact. In a period of 6 minutes, Mars would rotate about 1.46 degrees, probably enough to cause some blurring of the image (although considering all the other factors I had to deal with, probably unnoticeable). To compensate for the planet's rotation, I use another software tool called WinJUPOS. This tool will take the video from AutoStakkert, along with the stacked image, and de-rotate it so that the movement due to planetary rotation is cancelled. This de-rotated version of the video is then reprocessed once more with AutoStakkert to produce a final stacked image. In my case, 5,483 frames of the original 27,415. The resultant image is shown below:
With the stacked image and de-rotation taking care of much of the seeing perturbations we now have a much cleaner image. But it's still not as sharp as we would like; the air turbulence is still causing some blurriness. The detail is trying to come out.
Next up, and the next to final step in the imaging process is to apply discreet wavelet transformation on the image to remove as much of the noise as we possibly can and sharpen the image, bringing out the details that are there, but covered up by the noise in the image. I do this with a tool called Registax. Once the proper parameters are set (which in itself is a time consuming trial and error process) the final image comes to life - and the results are magical.
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| Mars 10/22/2020 - EdgeHD11 fl=11,700mm ASI462 - 6min - 5,483 frames |
A final pass with Paint Shop Pro to enhance the color and adjust the brightness and contrast and I'm done. A rather successful night. Still not as good as what I see other amateurs accomplishing, but good enough for me. In the image you can see the polar cap, Mare Sirenum (the dark patch just to the right of center) and Amazonis the bright area on the bottom half. The most recognized region, Syrtis Major, is on the opposite side of the planet when this image was taken. It will be centered in early to mid November, and if the skies are clear I'll attempt to capture one more image for the record books. Mars and Earth will be separating rapidly during the next few weeks and so the image will be smaller.
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