Deep-sky Observing

Amar A. Sharma

Bangalore Astronomical Society2011

ABOUT THE OBSERVER Started astronomy in 2002 (in 1st-PUC)

Tagged along with seniors of ABAA on observing trips to superb skies of Sivanahalli (Bannerghatta forest)

Immediately realized extremely fascinated by observing the celestial wonders beyond!

Started observing clusters from home terrace @ JPNagar with a 6” f/9 in 2003

Paired with new ABAA members, and formed observing team with guru Akarsh Simha, Shashank and Neetesh for out of city trips in late-2005 [Co-founded B.A.S in mid-2006]

Varied observing sites; Sivanahalli (35km) [2002-2005], Mysore Road (45km) [2003-2004], Devanahalli (55km) [2005], Hosahalli (70km) [2006-present]

Observed extensively from 2006-2008. Till date, managed to observe:

33 comets (half of attempted!)310 galaxies80 globulars (exact half of all Milky-way ones!)500 DSOs (75% of these with only an 8” scope!)

Love astro-sketching. Also binocular observing – 10x50 & 25x100 (currently 85 / 110 Messiers with just 10x50 binocs!)

Observing in the dark Basically the realm of Deep Sky Objects (DSO) appears

“boring” !

Shall never see color visually, as in photographs!

Only the best and brightest objects in the sky exhibit stark detail …yet minus the color

Eye (muscular camera) cant expose for looong like a mechanical camera, else its a very sensitive equipment

Generally DSOs appear colorless, featureless, invisible, without any distinct boundaries

1. Diffuse (low surface brightness) 2. Condensed (high surface brightness)

Observing conditionsSeveral factors determine visibility[Your log-book should ideally contain these parameters]

Light pollution (background sky glow: red, black, gray)(depends on distance from nearby largest civilization, dust-prone air

polluted areas) Transparency of sky Seeing conditions (steadiness of atmosphere) Various types of clouds (Upper and lower level) Haze, humidity, fog, moisture, dew Naked-Eye Limiting Magnitude (NELM) = 6.0+ mag very

good sky Structure of Milky-Way (with details like dark rifts seen

naked-eye)

Star Atlases

# Star Chart Objects / Highlights

Category of observer

1 Edmund Mag 5 Stars upto mag. 5 with some brightest DSOs

Beginner

2 Mag. 7 Star Atlas Stars upto mag. 7. Many celestial objects visible with an amateur telescope

Beginner

3 Collins Gem Night Sky Pocket sized book for basic celestial objects with info on constellations

Beginner

4 Norton’s Star Atlas 8800 stars upto 6.5 mag, 600 DSOs

Intermediate

5 Cambridge Star Atlas 8800 stars upto 6.5 mag, 920 DSOs

Intermediate

6 Sky Atlas 2000.0 81,312 stars upto 8.5 mag, 2700 DSOs

Intermediate / Advanced

7 Uranometria 2000.0 280,000 stars upto 9.7 mag, 30,000 DSOs (260 charts)

Advanced

8 Millenium Star Atlas 1,058,000 stars upto 11 mag, 10,700 DSOs (1550 charts)

Advanced

Softwares Eliminates the need to carry a hard copy and find

objects by turning pages in the dark. Also has a very extensive database, you can upgrade

comet elements, and take custom-field printouts:

1. Stellarium

2. Sky Map Pro

3. StarryNight Pro 6

4. Cartes du Ciel

5. Mega Star

6. The Sky X

7. DeepSky Astronomy (planner)

The Human Eye Rods & Cones

Rods in periphery, Cones in central fovae region

Cones sensitive to bright light, responsible for color vision while day

Rods active in low light and insensitive to colour

Dark adaptation Eye is an automatic camera, self-adjusting to incoming light

Eye pupil dilation ranges from 2mm - 7mm

Fully contracted pupil dilates in 5-10 mins in darkness

But the real process does not happen till 30-45 min

Rhodopsin (visual purple) chemical needs to be released, to increase photo-chemical sensitivity

If hard-earned dark adaptation is lost once by an instantaneous exposure to bright light, the process has to be re-started!

Red wavelength least hampers night adaptation; hence use dim red light while observing faint

Also use an eye-patch and/or hood (blanket) to shield from stray light while observing

Averted Vision You might run over faint objects un-noticingly hundreds of

times with direct vision! While observing, look “away” from the central focus,

however, being aware. Photons WILL catch the attention of your peripheral vision Requires practice; the more the better. Rest is instinct. Mastery over averted vision is a key to seeing those horridly

low surface brightness objects

Slight wiggling the telescope helps. Eye + Brain catches (slow) moving fuzzies

Graphic created by Akarsh Simha

Naked-eye Averted vision

Charts created by Akarsh Simha

A.V. applies for DSO observing in eyepiece as well as naked-eye observing

Star hopping Locating a DSO using pathways of star patterns, either

in eyepiece, or naked-eye

Create patterns of stars to reach your destination DSO

Trains the hand-eye-mind coordination; increases observational skill

You can flaunt your DSO pointing skills to rival a GOTO telescope’s pointing skills! :-P

Lets find Andromeda Galaxy…

Charts created by Akarsh Simha

Charts created by Akarsh Simha

Charts created by Akarsh Simha

Lets find a DSO; telescopically

M4 is a globular cluster near Scorpius’ brightest star, Antares

The view through finderscope, one eye closed

Antares is centered, but M4 is outside the f.o.v

Place the centered Antares now at the edge of f.o.v in eyepiece

Pick any pattern of stars, from where you will move ahead

There…just one field away was the misty patch. This was a simple one-step star hop.

Hope you are now ready to star hop multiple fields

Aperture Foremost…you do not need larger apertures to do good

visual astronomy

Use whatever aperture you have…just don’t let it gather dust!

An experienced observer with a self-made 8-inch can see hundreds of objects in a dark sky, upto 12th magnitude

Suit your equipment; Observing with your time-spent, accustomed equipment will help you “see more”

On really best moments of sky conditions, with small apertures you can even resolve details, like spiral arms and features in certain NGC galaxies!

Ultimately the visibility of a DSO for an observer depends on a combination of all factors put together:

(Vary one and the observability changes)

1. Sky quality / Local atmospheric conditions

2. Aperture of equipment

3. Observer’s experience + eye sensitivity (proper dark adaptation)

Surface brightness Very important! Least understood and often over-looked

Apparent magnitude of DSO is not sufficient alone, to judge an object’s brightness.

S.B. coupled, will define the visibility of the DSO

Magnitude is mainly for point sources, S.b. is for extended objects

Take the total light from the object and divide by the area. S.b. is a gradient of the magnitude of the DSO spread over

each square arc minute

Not yet made sense? Good ! This example will…

Take M31, the famous Andromeda Spiral Nebula

Its magnitude is 3.4 But its surface area is spread out over a

large area [3 degrees (189'x 62') or 6 Full Moon widths]

Which means the apparent magnitude of 3.4 is evenly stretched out to over 6 Moon widths

Or vice versa: if the 189'x 62’ big galaxy is compressed to a point, M31 would shine like a star of 3.4 mag.

Well, thats the definition of its magnitude

But how much of each square arc unit area of that stretched 189'x 62‘ large M31 will be, is measurable as a low surface brightness of mag 13.5

Another example, UGC 9749, is a 10.9 magnitude galaxy in Ursa Minor. But…

…it has a surface brightness of magnitude 17.8!!

DSO Taxonomy Galaxies

1. Spirala) Face onb) Barred spirals c) Edge on

2. Elliptical

3. Irregular

4. Galactic Mergers

Clusters1. Open2. Globular

Nebulae

1. Planetary2. SNR3. Diffuse4. Reflection5. Dark

Extra-galactic features

1. E.G. nebulae2. E.G. globulars3. Supernovae

(A) Galaxies(i) SPIRAL FACE-ON

Appears overall uniformly fuzzy & circular

Sometimes with a concentration toward the center and spiraling arms

Some face-ons are moderately visible, rest are l.s.b.

Some are extremely faint: M101, M74 (Phantom galaxy), NGC 6946 (Fireworks galaxy)

TIP: Observe with the lowest power / widest eyepiece You need contrast with background sky Use averted vision Then switch over to medium power for further examination Use the largest aperture + dark skies to extract most detail within the

galaxy; like arms, knots, emission nebulae regions

(ii) Barred spiral Gals

Same as spiral galaxies, except for the stretching bar in the center

Some bars seen with 8-inch aperture;NGC 1365

Sketch by Akarsh Simha

Faint streak, maybe with a cutting dust lane

Will be challenging to find, needle-like, because of their thin-ness

(iii) Edge-On Galaxies

NGC 7640, NGC 4945, NGC 891, Integral Sign galaxy

TIP: Find with medium power then switch overGood finder chart will help, using nearby star patternsLook for the slightest thinnest elongation

NGC 4565 sketch by Amar A. Sharma

(2) Elliptical Gals

High surface brightness, round

Basically appear like only cores of spiral galaxies

Will see NO feature in them, except that bright fuzz!

E.g. – M87, M49, NGC 1316

(3) Interacting Galaxies

Merger of two galaxies, either before or after

M51 (Whirlpool galaxy) shows the galactic mass transfer with even a 4-inch.

Most are very faint & need larger apertures.

Antennae galaxies (NGC 4038 & 4039) are just visible in 8” as heart-shaped

Antennae galaxies sketch by Amar A. Sharma

(i) Open Clusters Loose and scattered, or more dense

Generally they cover a wider field, hence binoculars needed to appreciate them

Telescope could be used to count stars

Some stars show stark colors within the cluster; e.g. in the Double Cluster

(B) Star Clusters

(ii) Globular clusters Only 150 GCs known to exist in MWIn Messiers, 7 exist in Ophiuchus & 8 in

Sagittarius alone

Observationally two types:1. Concentrated toward center with stars at edge

resolved2. Uniform, low surface brightness

With 10x50 binoculars, they all appear as a fuzzy spot.

Best known to mimic new comets! Especially when comet hunting in MW regions.

Few of the farthest MW globulars are visible even with an 8” aperture

Intergalactic Wanderer (3x MW!!!), Palomar 9 & NGC 7006

Sketch by Amar A. Sharma

(C) Planetary nebulae Like GCs, even PN’s can appear similar.

1. Highly condensed and stellar2. Low surface brightness and uniform, like face on spirals

LSB are hard to observe, like Owl Nebula, Skull Nebula

Stellar ones generally show vivid blue, green colour in medium aperture telescopes!

E.g. Blue Snowball, Ghost of Jupiter, Blinking Planetary, Cats Eye, Eskimo

Other genre of nebulae require special Nebula Filters to enhance their view. They are generally photographic and not observational objects

Photo by Nikolas Hericks

Extra-galactic objects Even other galaxies obviously do possess their own

DSOs like our MW does Some nearby galaxies showcase rare sights within

them, at that distance

Ever observed the bright & easy M54 (Sagittarius) and M79 (Lepus) globulars?

-- They belong to Sagittarius Dwarf & Canis Major Dwarf Galaxies resp, and not our MW !

-- Technically, these are extragalactic globular clusters!!

1. Globulars in M31, more difficult is within M332. NGC 1049 g.c. is relatively easy to see while the

parent galaxy Fornax A is ultra-high faint!

Wealth of clusters and nebulae can be observed in our neighboring companion dwarf galaxies; LMC & SMC

Globulars in Andromeda Galaxy

Credit “Atlas of Andromeda galaxy” website

Large Magellanic Cloud (LMC) Galaxy

Binocular v/s Telescope

Theory that 2 eyes are better than one:

Enhanced signal perceived by brain since both eyes are employed

Increase in resolution, contrast, color perception by 20 – 40 %

Feels to render a 3-d visual image

Less strenuous than one eye observation, more effective

Portable, small and light-weight. Hence can compensate for hardships of telescope portability

Wide fields (2* - 6*) & low magnifications (7x – 25x) offer stupendous views for scanning starry regions of the night sky

Disadvantages of binocular:

1. Fixed low magnification2. Lower apertures3. Hence restricted magnitude limit & resolution4. One of the largest commercial binoculars 25x150

(Fujinon) cost more than a large computerized scope @ $7,000 & weigh-in 10 kilos !!

What can binoculars show me/you?10x50 (~ 6* f.o.v)Stars & DSOs to 9th magAll 110 Messiers under very dark skies, by a very experienced

observer; lots of other NGCs (as basically smudges)Wonderful wide-field Milky-Way vistas

20x80 (~ 3* f.o.v)All Messiers by a good observer, more of NGCs

25x100 (~ 2* f.o.v)A mini-telescope (5 kilos! Never for hand-held observing, need a

very sturdy mount)Mind-blowing Milky-Way and starry sky vistas!All Messiers; many visible more than mere fuzzesGalaxies & comets upto 11th magnitude with good experienceFor a more experienced observer 12th magnitude galaxies!

Battling false positives Glares and internal reflectionsExternal un-shielded lighting sources Tap the telescope; DSOs will stay fixed w.r.t stars, while

glares will not, they will stay fixed in their location Move your head; DSO will move, glare will not. Use a hood / blanket to avoid external lights

AsterismsClose groupings of stars whose collective glow mimics dso’s Resolve using higher magnification

Imagination & Hallucination!Lack of sleep and oxygen circulation / Over will power! Call for another observer

Visual impressions of the skySketches by Amar A. Sharma

Thank You

Happy Deep-sky observing!