It’s been a while since my last instalment, so here’s a fresh one.
I thought for this challenge it would be good to look at a few Observer’s Tricks to pull out every photon we can out of our gear. Large or small gear, scope, binos or just naked eye. Some of these “tricks” I’ve been using for a very long time. Others I’ve adopted recently myself from the sticky thread ‘Honing observation skill’ in the Obs’ forum.
The targets I’ve selected this time may not all be new and unfamiliar territory. Most are actually quite popular targets. But the level of detail that they all offer is outstanding, if you are equipped with a few visual tools.
Naked eye
This is one of my favourite times of the year to observe. The early evening sees the Milky Way arc overhead from horizon to horizon east to west. The detail that this splash of illuminesence offers to the naked eye is bewildering. A little insight and what seems like a flat ceiling becomes a 3D experience. What is needed though is a dark sky, at least away from the worst of urban light pollution.
Picture the classic image of a spiral galaxy. It’s a round disk with arms made of billions of stars radiating out from a bulging core. The arms loop around the core, overlapping other arms. Our position in the Milky Way sees us placed on the outer arm of one of these overlapped sections. Between us and the core lies another arm. This inner arm called the Cygnus arm, and it shields the core of the Milky Way from our view. The core lies in the constellation Sagittarius, behind what also happens to be the brightest star cloud in the sky, The Cloud of Sagittarius. Jump into your Ford, or Holden, hit the gas, and at the speed of light it will still take you 30,000 years to reach the centre of our Milky Way galaxy.
Also from our imagined spiral galaxy, we note that there are gaps in its structure. It isn’t one smooth whirlpool. Rather it is turbulent, with lumps of bright and dark rippling through the arms. These dark areas are dust and gas that hasn’t been absorbed by other stars, and is also the feeding brew in the formation of new stars. We see these dark structures of dust and gas cutting into the Milky Way overhead. One of these is the Coal Sack to the left of the Southern Cross. Another is the Dark Horse of Sagittarius, and The Emu. And their shape isn’t flat, but twisted, erratic, billowing and dynamic as it is pulled and pushed by the gravity and energy of the surrounding stars and the Milky Way as a whole. Imagine a cloud, only black!
Binos and scopes
First, a rundown on a few “tricks”: Averted Vision: the most important one. The most light sensitive part of our eyes surrounds our central vision. Our central vision is blessed with colour sensitive receptors. But this has come at the price of actual light sensitivity. Ever notice how you manage to always catch something “out of the corner of your eye”. That’s our eyes light sensitivity registering the slightest quick change between light and dark. In astro, we take advantage of this by looking just to the side of the object we want to examine. Just to the side, as there is enough detail recognition to make out detail in these faint subjects. It takes a little getting used to, but the reward is staggering.
Movement: a static scope loses light! Not literally, but because our eyes become saturated quickly at low levels of light to the extent that we no longer can see the faintest of detail. Our eyes are made to pick up movement (corner of your eye stuff again!). Every now and then, giving the scope a little tap introduces vibration and movement, and all of a sudden we actually see MORE. This is just restimulating our eyes’ sensitivity.
Magnification: varying magnification helps greatly. A wide field is great to capture an expansive view, but we also get a brighter background. Upping the magnification darkens the background and increases the contrast with the object in question, and can greatly aid in picking up faint details. Globular clusters and galaxies can benefit greatly with a little oomph. Not always though, as the prevailing seeing conditions will dictate how high in magnification we can go. Other times a wide field view just pleases us more at the time too.
Filters: this isn’t the place to go into details about the in’s and out’s of filters. Instead it is about the ways in which they can be used. They can be used wound into your eyepiece, or mounted onto a paddle (like in the picture below), which is then flicked between your eye and the eyepiece, effectively to “blink”. Tiny planetary nebulae that would be otherwise invisible then POP out from their surrounds as the filter is flicked into view.
Nebula filters can also be used with larger aperture scopes on galaxies. Anything over 8” can be used to tease out OB star concentrations and nebulous regions in the closest of our galaxy neighbours. “Blinking” with an OIII filter is good for this too.
Binoculars benefit most from the first two points, though an OIII or even just a “light pollution filter” mounted on a paddle to “blink” with will prove beneficial.
Mixing up your aperture: This is my most recent observing revelation. Telescopes and binoculars compliment each other in ways you can’t imagine. The large aperture of a scope shows up faint detail, but binoculars, being a pair of rich field telescopes, benefit from our brain’s capacity of amplifying the image that one eye alone can see. As an example, the enormous nebulous expanse of M8, the Lagoon Nebula, that is seen in photographs is not easily visible in a scope. Binoculars on the other hand will quite literally show this enormous expanse of glowing gas, way beyond the bright Hourglass core of M8.
July’s targets
Table of Scorpio: This is a massive ‘OB association’ of the Milky Way. O & B stars are the most massive stars. An OB association is a concentration of these giants that are all related to the original molecular cloud that they were formed in. What makes these associations significant is as these stars’ nuclear furnace burns so fiercely they are short lived stars, so their dispersal away from their point of origin is relatively short. Many of the stars in the Scorpio OB association span down to Sagittarius and across to Ara, even some in Centaurus. Another massive OB association is the Orion association, identified by the high concentration of blue and blue white (O & B respectively) stars, including Rigel.
The Table of Scorpio reaches north from zeta1 & zeta2 Scorpii (ζ1 & ζ2) over to mu1 & mu2 (µ1 & µ2). Here lies one of the most impressive collections of open clusters, dark & emission nebulae, coloured stars and variations in magnitude. No less than 5 open clusters lie here, with NGC 6231 being a brilliant naked eye 2.6 magnitude cluster. 6231 is as illuminous as the giant globular cluster Omega Centauri! Panning the scope north slowly reveals each cluster in turn, and at a dark site subtle variations in background glow accentuates the overall appeal. An OIII or UHC type filter will reveal the emission nebula within the largest of the clusters. Binoculars will easily show the many dark nebulae that riddle this conglomeration of clusters.
Using movement (panning) brings up brilliantly the background glow and its mottling. Blinking the emission nebula identifies its location and its expanse. Increasing magnification reveals just how many stars are involved in each cluster, particularly the bright 6231 with its overpowering OB stars. Zeta 1 & 2 are also a part of 6231.
NGC 6744, in the constellation Pavo. A little challenging for smaller scopes. It is a large galaxy, which must make it fairly close by, but it is of low surface brightness (use movement to show it up). But it is also one of the few galaxies that benefits from the use of nebula filters to reveal the HII and OB regions within its spiral arms.
M83, in the constellation Hydra. This is one of the largest and brightest galaxies visible in the sky. It is easily visible in binoculars and even in a 30mm finder scope. M83 is thought to resemble the structure of the Milky Way, with its barred centre and the arms extending out from the bar. M83 is also called the Southern Pinwheel. Its spiral form can begin to be seen in an 8” scope (very difficult though). Larger instruments will show more details in the arms. Both M83 & NGC 6744 benefit from large aperture to make use of filters to spy out the HII and OB concentrations.
NGC 7293, the Helix Nebula, in Aquarius. The Helix is the largest and probably the closest planetary nebula to us. Binoculars easily reveal this “smoke ring”, and a telescope will show its ring like structure. The larger the aperture the more mottled and filamentary structure will be revealed. An OIII filter is of great benefit with the Helix to subdue the background glow and show up more detail.
I deliberately made the target list short this time. Have a go at utilising the various "tricks" noted above in your own explorations. Even revist your old favourites. You'll be surprised just how much more you'll be able to see by employing some of these techniques.
Please feel free to add more objects to this target list. Please don’t be shy to ask for more targets too.
Mental.
Last edited by mental4astro; 16-07-2012 at 02:13 PM.
Thought I'd add one more object to the list as an exercise on using the blinking technique.
I've been raving for some time about M7 being my favourite deep sky object. This colossus of a cluster is at least 1.3 degrees in diameter. At this size, it is no surprise that there lie some other DSOs in the same line of view. M7 boasts no less than three planetary nebulae, three open clusters and one globular cluster within its line of view.
The GC I use as a quick guide to gauge to seeing conditions. It is just visible with an 8" scope from Sydney. The open clusters' stars appear ruby like against the diamonds of M7. The PNs are invisible without the aid of an OIII filter, and blinking is really the only way to spot them from against their background as they are tiny.
The chart below was forwarded to me by the observational guru NGCles. I spotted one of the PNs noted on my very first go at them using the blinking technique from home. Tip: use some decent magnification in order to bring up the size of the disks, otherwise it'll be too difficult. Something in the order of 120X will help. Aperture too helps, but we do what we can with what we've got.
You may also attempt to track down the other PNs noted in the chart too.
M7 is also a fantastic binocular target from a dark site as dark lanes and nebulae run through it. Just beautiful.
Mental.
PS, I'm hoping Suzy will add her challenge to this one. Please do. Mine's a bit light on targets this month.
Last edited by mental4astro; 16-07-2012 at 09:08 PM.
Nicely put together Alex and I like the challenge of the various objects in M7. It is certainly a very rich field. I'm a bit concerned that at the moment the naked eye challenge in this neck of the woods will be seeing stars at all! I now know how those poor Victorian souls who live south of the Great Divide feel most of the time.
An interesting mix of targets. I find the Helix nebula notoriously difficult in light-polluted suburban skies, even when a lot of fainter objects show up quite easily. I guess it's a good one to try out those filters.
Oh, and thanks Alex for reviving the monthly challenge.
Hi Garry, no, filters are not necessary, but they can help. In a light polluted sky the can be the only way to bring out detail or even see them. At a dark site filters help tone down the background or improve contrast.
Which filter is best depends on what you are chasing, where you are observing from, & on the size of instrument you are using. Just on the last point some filters transmit so little light that unless your scope is large enough there is just not enough going through to actually see anything!
Before you spend money on filters, do some reading. There is a good article by John Brambury about them:
I also suggest you go to a star party where you can see these things in use & even get to try them out in your scope BEFORE you buy. You can burn a lot of dosh with filters!
Thanks Alex for the list.
We looked at all of them on Sunday night with a 16".
The nebulae IC 4628 near NGC 6231 is pretty bright, well worth a look with a UHC filter. https://picasaweb.google.com/1100488...90692797592754
Lacaille found M83 with a 13mm refractor in 1751. It was the only galaxy that he found and I think it should be called Lacaille's galaxy.
Suzy has started a separate thread where she gives an extensive list of objects to chase down. The list compliments the techniques I've noted here very well.
It would have taken Suzy a lot of work to put her list together, and with some colaboration from some other very experienced observers.
I set about meeting the challenge with a 103mm refractor on 14th July - report is rather late! The night was clear with good seeing (3/5) and very clear transparency (6/7).
I managed to locate all targets that evening. However, it did take some of your observing hints to tease out the observations. Some of my notes:
NGC7644: Faint hazy spot, no structure. Required patient motion to find, then clearly visible.
M83: Faint patch with bright nucleus. Averted vision showed occasional hint of spiral structure
NGC7293: Very faint diffuse glow. Hint of helix structure blinked in and out with averted vision
Table of Scorpio: Panning from Zeta1&2 over to mu1&2 revealed rich mixture of stars, open clusters and some dark lanes visible. Needs more time to explore.
