For a while now I have been revisiting the objects on the Astronomical League's Urban list using my 80mm Vixen refractor from my driveway. This began out of my curiosity as to what an observer with a modest scope amid significant light pollution might be able to see. Certainly the big & bold stuff would be easy pickings, but after that, where's the limit as to how subtle an object can be bagged?
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| Bortle 8 Skies from Towson |
Of course, the first benchmark on visibility that any amateur astronomer invokes is the brightness of the target, i.e. it's magnitude. There are handy charts and online tools that calculate the faintest magnitude star that one can possibly see for a given telescope aperture. Out in the field there will be a host of other factors that influence that value - the most obvious one being just how aggressive the artificial sky glow is at your location. But things from the condition of the scope (clean optics, alignment) to your age will also affect your limiting magnitude.
As the observer quickly learns, magnitude is only half the story when predicting visibility of a non-stellar target. Size is a critical consideration since the given magnitude is going to be more or less spread out across the object. This is why although galaxy M33 is one of the few galaxies above 6th magnitude, it is also notoriously hard to spot from suburban settings because it has that light spread out over 60'x30'. Surface brightness, the average per arcsecond (or arcminute) magnitude for the target, attempts to level the playing field when ranking the relative brightness of a deep sky object.
Based on some of my observations thus far (below), M97, the famed "Owl" planetary nebula located in the bowl of the Big Dipper, presented as a DSO that might be doable. Some objects fainter than its 21.88 surface brightness had been visible, while one brighter specimen - reflection nebula M78 in Orion - had failed to materialize. Could I capture the Owl?
The first spring evening of 2021 turned out clear and with reasonable temperature. I decided to set up for an observing session, even with a waxing fat crescent Moon hanging in the sky. Since this evening coincided with the first HAL public virtual star party I set up my phone nearby and listened in as I went through the alignment process. Jim Johnson provided some celestial mechanics relative to the Moon as it traverses the ecliptic throughout the year while getting it centered for viewing. Victor Sanchez took a turn and shared a colorful image of the Orion Nebula using his equipment. It was as if I had the company of fellow observers as I embarked on my quest for the Owl.
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| https://freestarcharts.com/messier-97 |
With a good alignment obtained I drop in my 9.7mm Plossl with UHC filter and bid the mount to slew over to Dubhe to ensure sharp focus, and then slip over to the nearby Owl. As expected, there was nothing there at first glance except a few dim field stars. Time to apply the tricks and techniques that I've picked up over the years. First up is draping a dark pillow case over my head to shield my eyes from neighbors' house lights. Then it is a matter of just relaxing and allowing my dark vision to gradually improve. I shift my focus to various points in the field so that my averted vision has multiple opportunities to notice anything. When that fails I turn to using the controls to nudge the field a bit, hoping the well camouflaged Owl face will give itself away if it moves slightly.
After perhaps 10 minutes of effort it is time for the next tactic and I boost the magnification slightly with an 8mm TMB eyepiece. I again start off at Dubhe for a focus check and then swing over to where M97 should be. The same now familiar field stars are there to greet me while the planetary still refuses to emerge. I begin my stalking routine again and then it happens. I think I see something there to the right of the three brighter field stars! I spend the next five minutes directing my focus to various points in the field in an effort to give my averted vision its fullest advantage. Finally I am convinced enough that the faint, shapeless specter is indeed M97 and not just imagination. Success!
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| The elusive Owl Nebula captured! |
As I work up a sketch of the Owl I notice that it becomes a little easier to see. This is likely because drawing an object involves some study, allowing your brain & eye collaboration to bring out the more subtle features of the field. But even given that, M97 remains an averted vision only object, impossible for me to honestly comment on size or form given its tenuous nature.
Out of curiosity I decide to push things a little and add a 2x Barlow into the optical path. A quick refocus on Dubhe and I'm off to the Owl again. This time I am able to see the glow a bit more readily, enough that I can judge that it is more or less round in shape. It's still an averted vision only object, but the higher magnification seems to darken the background a bit and make the planetary slightly easier to see.
The Owl had been a challenging target, possibly at the limit of what I can do in terms of this equipment and location. I saw yet again that surface brightness does not provide an infallible predictor of whether an object can be seen, only an estimate (something I plan to explore in part II). But without question this observing session served to reinforce some of the best practices when trying to pluck a DSO from skies bathed in city lights:
- Guard your dark adaptation, even if your neighbors may think you're a bit weird sitting in your driveway with a pillow case over your head.
- Averted vision should be applied thoroughly and systematically when inspecting the field.
- Just as a camouflaged animal gives itself away when moving, inducing a little movement in the field can sometimes unveil a faint target.
- Contrast
is crucial. To that end a light pollution filter (UHC, OIII) can
improve the view if the target is an emission nebula and bright enough to overcome the light loss caused by the filter. Boosting the magnification
is another way to enhance the contrast and will work regardless of
object.
- Patience - above all, patience! Study the entire field carefully, giving each section a few moments of attention and then review them again. While our eyes don't build up an image over time like a CCD, your eye & brain do collaborate to gradually reveal dimmer objects as you spend time at the eyepiece.
