Saturn and the Curse of (Astronomical) Seeing
Last night, a clear night got me out into the garden to attempt a proper imaging run on Saturn. Unfortunately, the ringed wonder isn’t best placed at the moment for imaging from my garden, as my clear sky is very limited due to various obstructions. Being low down means that the time I get on it is very constrained. Knowing this may be my last proper chance to attempt get a good run, I set about giving it a shot. The shot below (following processing), is the fruits of the session.
My basic setup procedure for planetary capture goes like this:-
- Set up mount without scope
- Polar align using ‘PolarAlign’ on the iPhone to reasonably accurately align the mount to the pole star.
- Add the weights and scope.
- COLLIMATE THE SCOPE using the laser collimator.
- Connect everything up (power to the mount, laptop to the mount, laptop to the camera, laptop and iPad to the WiFi)
- Align on a single star near my target planet.
- Point the scope where required
From beginning to end, it usually takes me about 20-25 minutes. The caps above are intentional. With a reflector, I’d say it’s vital (at least on a setup that gets broken down and rebuilt each session), to collimate the scope before capture on planetary targets. It only takes a couple of minutes with a laser collimator, and seems to make all the difference.
Once you’ve done the above, you’re ready to go. However, you’re still at the mercy of the absolute bane of planetary imagers lives – astronomical seeing. The ‘Seeing’ that astronomers talk about is basically the stability of the air. Everyone’s seen the shimmer caused by the heating of the air above a hot road on a sunny day. It’s caused by currents of differing temperature air causing the light to be refracted at varying angles, acting like lots of mini lenses and causing the ‘shimmer’. The more atmosphere there is for the light to pass through, the worse the effect is. This is why objects lower in the sky (closer to the horizon) are generally harder to observer effectively than those that are high up. The light from those objects has had to negotiate a lot more of the earth’s atmosphere before reaching your eye or camera sensor.
The problem with this when applied to astronomy is that you are seriously magnifying the things that you’re trying to see/image. This has the side effect of also magnifying the shimmering air. This is an extra pain with planets, as the detail on them is all important, and that’s exactly what gets knobbled by bad seeing (read unstable air). There are other sources of instability – for example tube currents. These happen when the scope hasn’t cooled to the ambient temperature properly before you start, and results in a similar effect to bad seeing, except this time, it’s actually happening inside the telescope tube itself. I always get my OTA (Optical Tube Assembly – the tube bit of the scope) out to cool down about an hour before I start. That way its down to the right temp by the time I get around to starting to observe/image, and tube currents are minimised.
Despite all best efforts, the seeing in the UK is generally mediocre at best. For example, the image above was actually produced from the following captured video file…
People are amazed when they see the before and after for planetary imaging. The capture is often so disheartening, that many don’t even bother to try processing, as they expect the outcome to not be worth the effort.
As for the look of the video above…
There are few things to bear in mind about this video. Firstly, it’s not ‘debayered’. This means that it’s the raw data straight from the camera’s sensor. This data hasn’t been converted to colour yet. This is done within the stacking program itself. The reason for not debayering on the fly is two-fold. Firstly, it results in capture files that are approximately a third of the size of their debayered counterparts. Secondly, the debayering process itself takes up CPU time which could otherwise be used to capture more data. The result of this is a lower framerate.
The big requirement during planetary capture is capturing the raw data from the camera at the highest framerate possible. The more frames you haven the more chance you have of getting a glimpse each and every part of the planet in good focus in the tiny spells of good seeing that you get in each capture. The stacking program takes the frames, and finds which regions of the planet look good on each frame, discarding those frames that have no decent quality at all. It then takes all these ‘good’ parts and combines them into a single master stacked frame. This can then be sharpened to bring out the detail. Without this process, and the input of thousands of frames, you would likely never see the planet like we do in the top image from Earth.
The best thing? The software for doing this is free! I use :-
Autostakkert!2 (AS!2) – http://www.autostakkert.com
Registax – http://www.astronomie.be/registax/
I also use Adobe Lightroom to finish off the images, but that’s by no means a necessity, and there are plenty of free alternatives that will do the job very well. Even the straight stacked and sharpened image out of the above software is perfectly good.
Moral of the story? Something I’ve mentioned previously – don’t be put off by a bad-looking capture – it could be hiding a gem! On the other side of the coin, there are nights, probably once every few years or so in the UK, where seeing is fantastic. If you manage to get out and do some visual planetary observation on one of these nights, the experience will never leave you. Just a shame it doesn’t happen more often in old Blighty!