by John W. O’Neal, II
Atmospheric seeing is an often overlooked, yet critical aspect of solar imaging.
Without good seeing, your gear, camera, and even your techniques and skills don’t much matter. So, before we even start thinking about gear, telescopes, cameras, and processing software, etc., we need to discuss this frequently overlooked attribute of solar imaging — we need to discuss seeing.
For those of us with night-time astrophotography backgrounds, seeing may not have played quite as big a role as it does with solar imaging. Night-time astrophotography for a lot of us is actually quite easy with regards to seeing. We look at the sky and if it’s free of clouds, our seeing is good! Yep, I’m seeing stars, let’s git ‘er done!
Some nights our results were a little better than others, and sometimes a little worse. Why?
Well, sometimes the sky was a little hazy, sometimes a little murky, sometimes a little foggy, and with all that going on, seeing didn’t really seem all that important, did it? Plus, at night time we have much more important things to consider than seeing, right?
How am I going to block the light from that streetlight next door?
I wonder where the moon is going to be, what phase it’s in, and how much that’s going to affect the object I want to image?
Or, perhaps, what about the blanket of new snow on the ground? Is it going to be reflecting mercury or sodium vapor light up into the area I want to image?
All these considerations take a huge preference over something as seemingly insignificant as seeing.
In night-time astrophotography, I always considered good seeing a bonus, something that was great to have, but not a prerequisite. But in solar imaging, seeing cannot be considered a bonus, it’s absolutely a prerequisite. And, that being said, we need to define exactly what seeing is.
We need to learn how to quantify it, and even more importantly, how to adapt to poor seeing conditions. But, what can we do to quantify and optimize our seeing?
The image below (left) illustrates what poor seeing might look like. Everything looks soft, or out of focus and/or blurry. In the image on the right, illustrating good seeing, you can see plasma following magnetic field lines in the large prominence. Spicules adorn the solar limb and surface. Surface details include brushes, loops, and a filament otherwise invisible in the left image.
Seeing is quite easy to describe
Basically, seeing boils down to, (Get it? It boils down to…) how clear and transparent the sky is. Most solar imagers, pro and amateur alike, use the Mount Wilson scale to quantify their seeing conditions. The Mount Wilson scale ranges from 1 to 5 with one being the absolute worst conditions, and a five being pristine.
ONE on the Mount Wilson scale signifies very bad seeing. The absolute worst conditions you can imagine — with boiling so rampant it’s difficult to see or even focus an image, and with the limb resembling the teeth of a circular saw blade. If you have a ONE seeing, don’t even go outside. Stay home and do something constructive with your time.
FIVE on the Mount Wilson scale, on the other hand, represents almost perfect conditions. Video captures will look like still images. Focus will be tack sharp and there will be no scintillation whatsoever, anywhere throughout the image.
And a THREE on the Mount Wilson scale would pretty much signify average seeing conditions. A THREE is what many of us will encounter on a pretty consistent day-to-day basis.
The 5 stages of seeing as presented in the Mount Wilson scale
1Solar image looks like a circular saw blade. Completely out of focus. Limb motion and resolution greater than 10 arc-sec. Smaller sunspots will not be seen.
2Solar image is always fuzzy and out of focus. No sharp periods. Limb motion and resolution in the 5 to 10 arc-sec range.
3Solar image about half the time sharp and half the time fuzzy. Some short periods where granulation is visible. Limb motion and resolution in the 3 arc-sec range.
4Solar image more often sharp than not. Granulation almost always visible. Limb motion and resolution in the 1 to 2 arc-sec range.
5Solar image looks like an engraving. Extremely sharp and steady. Limb motion and resolution 1 arc-sec or better.
Note: Observers may use fractional values if the image quality falls between any of the above definitions.
If we can’t change seeing, can we diminish its effects?
We can’t change seeing. It is what it is! But don’t despair, we can learn what causes seeing conditions to improve or deteriorate, and we can change our conditions to better accommodate existing seeing.
To even begin to improve our seeing conditions we first have to understand what external factors determine seeing and what detractors are involved in making seeing good or bad. Once we understand how it works, we can make plans to enhance our favorable conditions or correct our local detrimental conditions.
Probably the most important consideration of seeing is the air we breathe, our life-giving atmosphere. When we look at the sun, we’re looking through miles and miles of sometimes boiling, sometimes quiescent atmosphere. Understanding the role our atmosphere plays will help us to improve our seeing. The old adage of “zenith works best because we’re looking through less atmosphere” works great at night, but not quite so well during the day. The sun is not overhead in zenith until at or around local noon time, which gives us the least amount of atmosphere to look through, but by then, the sun has had plenty of time to heat up the sky and the ground, and to get heat and convection currents rising up into the atmosphere, which we don’t want.
So, if noon is not optimal, when is? Overnight the earth and the atmosphere cools, and as the sun rises each morning it begins to heat the earth and the atmosphere. Each clear morning there’s a short window of opportunity where the earth or ground temperature and the atmosphere reach an equilibrium. The sky becomes stable and still, and during that short period of time seeing is at its optimal best.
Sometimes a similar window appears in the evening as the sun’s heating rays begin to diminish but we haven’t reached the point yet where the earth begins cooling. That small window of good seeing is usually not as significant as the morning, but it can present opportunities for short periods of excellent, if not fleeting seeing.
To take it a step further we must learn to control our ground conditions to a certain extent by recognizing features of our terrain that can enhance and/or detract from our seeing conditions. We’ve all seen the shimmering effect of heated roadways and parking lots. So, obviously, setting up our gear in the middle of a parking lot is not going to be the best scenario. But, large grassy fields and open waters and large lakes are things we might want to look for to improve our seeing. As you travel around your area keep an eye out for locales that will fit your needs. Look for a reservoir nearby, a golf course, a park, or maybe even your local astronomical club’s observatory grounds.
Having a plan to utilize the best time of day and controlling the local ground and terrain conditions around your setup will go a long way toward improving your seeing to the point that your images are going to be tack sharp without smearing, and atmospheric effects and anomalies will be mitigated with satisfaction.
Only you can decide what’s best for you and your local seeing conditions, but controlling as many factors as possible will only make your images better. That’s not to say we should all move to the Chilean mountain plateaus, or even to Arizona. Although my friend did just that and his seeing and imaging improved dramatically when he did. But if you have a job and/or family that requires you to remain in an area that’s not exactly conducive to solar imaging, rest assured that improving your seeing conditions is going to go a long way towards getting you those “WOW” images you see in pictures and that your friends in Arizona and Chile are getting.
We must adapt our setup to seeing conditions
In the full disk image above, seeing conditions were terrible, so I used an short focal length 80mm refractor to decrease the effects of bad seeing.
In the image to the left, under good seeing conditions, I used a 100mm refractor and a 2.5x TeleVue Powermate to capture a high-resolution closeup.
Learning to see the seeing and optimize your setup will go a long way in solar imaging.
The very first thing I do after filling my coffee cup each clear morning is check my local seeing conditions (North American readers can use the Clear Sky Clock). Once I learn about my seeing conditions I can decide if I’m going to shoot hi-res, large aperture, long focal length images, or if I’m going to keep the aperture down and the focal length short. Learning to adapt to local conditions will go a long way to assuring a worthwhile imaging session.
NOTE: Images in this article were shot by the author using a DAYSTAR FILTERS QUARK Chromosphere on an 80 or 100mm refractor and a ZWO ASI174MM CMOS imager.
About John W. O’Neal, II
John O’Neal has been an amateur astrophotographer for over 40 years and began solar imaging in 2011. He is co-founder of the Facebook SOLARACTIVITY group, a worldwide social networking and imaging group with 25,000 members. Recently retired, John plans to travel the United States promoting solar outreach and pro-am collaborations.