The Dogs' Globular

Tucked under the Big Bear's tail is the tiny constellation of Canes Venatici, Boötes' hunting dogs. For such a small constellation it might be accused of celestial gerrymandering by having its borders claim such deep sky masterpieces as M51 (Whirlpool galaxy) from Ursa Major and M3 from the herdsman. 

Messier 3 is a wonderful globular boasting half a million suns packed into its perimeter. Even from Towson you can sweep up this dandy DSO as a fuzzy 6th magnitude star roughly midway between Arcturus and Cor Caroli (the α star of Canes Venatici). My 6" RV-6 Newtonian at medium magnification and averted vision shows a brighter core and some of the members of the cluster winking in and out with averted vision. The 10" Cyrus reflector exposes many more suns and begins to hint at the true majesty of this object. 

When planets aren't around for imaging I will give the deep sky a go. In this regard I'm not very hard core, using my  unmodified Canon EOS t6i rather than a dedicated unit, and I have not invested in things such as a field flattener that the more serious imager might do. I also do not currently have a guide camera, relying on a good polar alignment and shorter ~30 second shots to keep stars from trailing. I use a simple illuminated tracing pad to serve as light source for my flats. I have invested in a copy of PixInsight for processing, and will probably be plumbing the depths of its functionality for years to come. So when at the end of April we had a pleasant evening, a little chilly but with pretty good transparency, I rolled the mount out of the garage, attached the Vixen, and set off to see if I could get a nice scrapbook photo of the globular.

One of the things that I wanted to try out was my recently purchased Baader Moon & Skyglow filter to see if it could cut down on the nasty light pollution in my stacked image that I so often encounter when doing deep sky. The company claims "it darkens the spectral region which is particularly marked by street lamp light, which is the biggest contributor to the nightly Skyglow". The second thing I was interested to evaluate was BackyardEOS, a software package that allows you to automate a sequence of exposures taken with the camera. In the past I had turned on the camera's built in Wi-Fi and used the Canon app on my tablet to take the exposures. But that gets tedious fairly quickly, firing off an exposure every 20-30 seconds. 

The time spent on polar alignment was worth it as the scope dutiful slewed over to Arcturus for a quick focus check with the Bahtinov mask in place. From there we slid up to M3 and began the session shortly after the passing of astronomical twilight. The BackyardEOS performed quite well as I set up a series of 30 second exposures over 15 minutes. At the end of each run I would check that the globular was positioned near center and that no star trailing was evident. After 6 such runs I then set about acquiring the dark and flat exposures.

Of course, collecting the data is only half the game. The stack in Deep Sky Stacker looked pretty good when I did an auto-stretch in PixInsight. The light pollution detritus was much reduced, so high marks for the Baader filter doing its job fending off the neighbors' lights. This time, in my search for a good PixInsight workflow, I followed along with a YouTube video made by Richard Bloch. It was one of the best I've watched so far, easy to follow along and enough rationale provided for why you are taking certain steps. It ended up helping me produce a pretty nice image of this cluster of suns that was the first faux comet entry that Messier himself is thought to have observed. 

It is always satisfying to see your work improve, and I was happy to find that my small investment in the two new tools did help me in my desire to occasionally snatch a deep sky portrait from the driveway of my home.

Lessons From an Owl - Part I

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?

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.

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!

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.