Observing 101: Dark Adaptation

Dark Adaptation: “Looking” Your Best

March 14, 2008
To best see faint deep-sky objects like galaxies and nebulae, you have to make sure your eyes are “dark adapted”. Here’s what you need to know.
The Basics
• The human eye evolved to operate in two modes, photopic for seeing in well-lit conditions and scotopic for seeing faint objects in the dark.
• As you learned earlier, your retina has two types of cells, rods and cones. In photopic mode, the cones detect bright light and colors. But in scotopic mode, the rods detect faint light. Both types of cells contain dyes that undergo a chemical change called “bleaching” when hit by light.
• In light-adapted or photopic mode, the dyes in your rods are fully bleached, so they can’t detect faint light… they’re out of action. Turn the lights off and the rods to return to dark-adapted mode, but it takes a long time, about 20-60 minutes. That’s why astronomers get so angry when someone carelessly shines a white light in their eyes… they have to wait a long time to recover their dark-adapted vision.
• Going from a dark to light adapted state happens much faster, in only a few seconds.
A Deeper Look
• Each eye reacts separately to light, so you can keep one eye dark adapted while using your other eye to read star charts and slew your telescope. An eye patch is ideal.
• You can keep unwanted streetlights out of your eyes by throwing a towel over your head when looking through the eyepiece of your scope with your dark adapted eye.
• You often see astronomers using bright red LED flashlights when looking at star maps and gear around the telescope. That’s because red light cannot bleach the dye in the rods if the wavelength is > 620 nanometers. So the chemical structure of the dye in the rods is completely unaffected, while the dye in the cones still enables scotopic vision.
Good To Know
Your body cannot by itself make the dyes for the rods and cones in your retina. It needs an external chemical-beta carotene-to synthesize the dyes. A good source of beta-cartone? Carrots. So carrots really can be good for your eyesight.
Personal View
You can imagine the reaction I get from the police officers who occasionally find me in the local park wearing an eye patch with a towel over my head to block out the nearby street lights. “Really officer, I’m just trying to see the faint structure in M97. Want a carrot?”

The Atmosphere and Observing - A guide to Astronomical seeing.

Introduction

An observer, be they at a mountain top observatory, or in their own back yard must, at all times contend with the Earth’s atmosphere. It is a notoriously unpredictable and limiting factor in obtaining fine views of the Planets, and close binary stars. Many often comment, especially here in the UK that seeing is all too often mediocre on most nights, but what are the factors that contribute to this?. Are there ways and signs, which indicate whether the atmosphere, will be stable or turbulent on a given night?.

What is “seeing”?

So what exactly is atmospheric seeing? - it is high frequency temperature fluctuations of the atmosphere, and the mixing of air “parcels” of different temperatures/densities. This behaviour of the atmosphere is seen at the eyepiece as a blurred, moving, or scintillating image. There are roughly 3 main areas where Atmospheric turbulence occurs. Near ground seeing (0 – 100metres or so.) central troposphere (100m – 2km), and High troposphere (6-12km.) Each area exhibits different characteristics, which are explained in more detail below. 

• Read more about "Astronomical seeing"
  
   
• Seeing in Holland  

Beware of the Jetstream

  Clouds along a jet stream over Canada.

Jetstream can influence your "seeing" 

3. High Altitude effects.

Effects at this altitude are caused by fast moving “rivers” of air know as Jet streams. Wind shears at around the 200-300mb altitude level can cause images to appear stable, but very fuzzy, and devoid of fine detail. There isn’t anything the observer can do to prevent these effects, but forecasts are available, to help predict weather a Jet stream is present over your area. Areas of the Northern hemisphere most affected by the Polar jet stream are the Central US, Canada, North Africa, and Northern Japan. The Jet stream’s position varies with the seasons, tending to move further South during the winter and spring months.

Southern hemisphere Jetstream

Africa Jetstream

Source: Wikipedia > Jetstream

Jet streams are fast flowing, narrow air currents found in the atmospheres of some planets, including Earth. The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with altitude) and the stratosphere (where temperature increases with altitude).[1] The major jet streams on Earth are westerly winds (flowing west to east). Their paths typically have a meandering shape; jet streams may start, stop, split into two or more parts, combine into one stream, or flow in various directions including the opposite direction of most of the jet. The strongest jet streams are the polar jets, at around 7–12 km (23,000–39,000 ft) above sea level, and the higher and somewhat weaker subtropical jets at around 10–16 km (33,000–52,000 ft).

• Read more about Jetstreams

Total Lunar Eclipse of June 15 - Also in Bloemfontein

Total Lunar Eclipse of June 15

The first lunar eclipse of 2011 occurs at the Moon's ascending node in southern Ophiuchus about 7° west of the Lagoon Nebula (M8). The Moon passes deeply through Earth's umbral shadow during this rather long event. The total phase itself lasts 100 minutes. The last eclipse to exceed this duration was in July 2000. The Moon's contact times with Earth's umbral and penumbral shadows are listed below.

   Penumbral Eclipse Begins:  17:24:34 UT
   Partial Eclipse Begins:    18:22:56 UT
   Total Eclipse Begins:      19:22:30 UT
   Greatest Eclipse:          20:12:37 UT
   Total Eclipse Ends:        21:02:42 UT
   Partial Eclipse Ends:      22:02:15 UT
   Penumbral Eclipse Ends:    23:00:45 UT

At the instant of greatest eclipse [5] the umbral eclipse magnitude [6] will reach 1.6998 as the Moon's centre passes within 5.3 arc-minutes of the shadow axis. The Moon's southern limb will lay 54.2 arc-minutes from the edge of the umbra while the northern limb will lay 22.3 arc-minutes from the umbra's edge. Thus, the northern regions of the Moon will probably appear brighter than the southern regions that lie deeper in the shadow. Since the Moon samples a large range of umbral depths during totality, its appearance will change dramatically with time. It is difficult to predict the exact brightness distribution in the umbra so observers are encouraged to estimate the Danjon value at different times during totality (see Danjon Scale of Lunar Eclipse Brightness). Note that it may also be necessary to assign different Danjon values to different portions of the Moon (i.e. - north vs. south).

Nearly 30 years ago (1982 Jul 06), the author watched another total lunar eclipse with the Moon in the same part of the sky. I was amazed at how brilliantly the summer Milky Way glowed since it was all but invisible during the partial phases. Observers will have a similar opportunity during June's eclipse. In this case, the totally eclipsed Moon will lie in southern Ophiuchus just 8° northwest of the brightest Sagittarian star clouds. The summer constellations are well placed for viewing so a number of bright stars can be used for magnitude comparisons with the totally eclipsed Moon.

Antares (mv = +0.92v) is 15° to the west, Shaula (mv = +1.63) is 14° south, Epsilon Sgr (mv = +1.85) is 15° southeast, Arcturus (mv = -0.05) stands 55° to the northwest, and Altair (mv = +0.77) is 46° northeast of the Moon.

Figure 3 shows the path of the Moon through the penumbra and umbra as well as a map of Earth showing the regions of eclipse visibility. The entire event will be seen from the eastern half of Africa, the Middle East, central Asia and western Australia. Observers throughout Europe will miss the early stages of the eclipse because they occur before moonrise. Fortunately, totality will be seen throughout the continent except for northern Scotland and northern Scandinavia. Eastern Asia, eastern Australia, and New Zealand will miss the last stages of eclipse because they occur after moonset. Again, the total phase will be seen from most of these regions. Even observers in eastern Brazil, Uruguay and Argentina will witness totality. However, none of the eclipse will be visible from North America. At mid-eclipse, the Moon is near the zenith for observers from Reunion and Mauritius.

Table 3 lists predicted umbral immersion and emersion times for 20 well-defined lunar craters. The timing of craters is useful in determining the atmospheric enlargement of Earth's shadow (see Crater Timings During Lunar Eclipses).

The June 15 total lunar eclipse is the 34th member of Saros 130, a series of 71 eclipses occurring in the following order: 8 penumbral, 20 partial, 14 total, 22 partial, and 7 penumbral lunar eclipses (Espenak and Meeus, 2009a) spanning 1262 years. Complete details for Saros 130 can be found at:

eclipse.gsfc.nasa.gov/LEsaros/LEsaros130.html

ISS & Endeavour visible from Bloemfontein

Watch out for the International Space Station and the space shuttle Endeavour passing over this evening (Tuesday).  Not sure how far apart they'll be - could be a minute or two - but they should both look like bright stars moving upwards from the north-western horizon at 6:57pm.  Both will "disappear" about halfway up the sky as they pass into the shadow of the Earth.

Endeavour launched Monday, and will dock with the ISS just after midday Wednesday.  The "crew hatch opening" at around 2:30pm (and other mission events) can be watched live at www.nasa.gov/multimedia/nasatv
This is the last flight of Endeavour.

For sightings of the Space Station from Southern Africa later this week, go to www.planetarium.co.za  click on "In the Sky" and then "Satellites", and follow the instructions

Source:  

Claire Flanagan

Wits Planetarium

011-717-1390

086-521-4273 (fax)

www.planetarium.co.za

Info for Bloemfontein from Heavens Above
 

ISS - Visible Passes

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Observer's location:	Bloemfontein, 29.1330°S, 26.2000°E
Local time zone:	Universal Coordinated Time -2 (UTC + 2:00)
Orbit:	343 x 346 km, 51.6° (Epoch May 16)

Date	Mag	Starts			Max. altitude			Ends
		Time	Alt.	Az.	Time	Alt.	Az.	Time	Alt.	Az.
17 May	-1.8	18:58:03	10	NNW	18:59:32	23	N	18:59:32	23	N
18 May	-1.2	19:21:31	10	WNW	19:22:57	24	WNW	19:22:57	24	WNW
19 May	-2.8	18:10:11	10	NNW	18:12:52	34	NE	18:14:47	16	ESE
20 May	-2.6	18:33:35	10	WNW	18:36:22	42	SW	18:37:56	21	SSE
21 May	-0.7	18:58:46	10	WSW	19:00:06	12	SW	19:00:56	11	SSW
22 May	-2.6	17:45:30	10	WNW	17:48:17	42	SW	17:51:06	10	SSE
23 May	-0.7	18:10:36	10	WSW	18:11:56	12	SW	18:13:15	10	S

Images: 3rd Karoo Star Party

Images of the 3rd Karoo Starparty

ScopeX op kykNet se nuus

"Dis heavy interessant!"

Werskwinkel by Boyden-Sterrewag - Saterdag, 9 April 2011

Foto by Boyden geneem tydens die Venus/Maan okkultasie

Kom sluit die vakansie af met `n Sterrrekunde-belewenis!

Die Bloemfontein Amateur Sterrekundige vereniging hou op 9 April `n Werskwinkel by Boyden-Sterrewag, by Bloemfontein.

Dit gaan `n praktiese dag wees, waar deelnemers gaan leer om met ander oë na die uitspansel te kyk.

Datum: Saterdag, 9 April 2011
Tyd: 14:30 vir 15:00 - 22:00
Plek: Boyden-Sterrewag, by Bloemfontein.Werkswinkel: Sterrekunde vir beginners

Van die lesings is onder meer: `n Zoemreis van 100 meter bo Boyden tot `n paar miljoen ligjare verder; Astrowaarneming vir beginners; Wat kan ons alles in die naghemel waarneem en later gaan kyk ons daarna; Inligting oor Teleskope en verkykers voor jy gaan koop.

As dit donker word gaan ons met `n verskeidenheid teleskope en verkykers kyk na al die voorwerpe waarvan ons vroeër gehoor het. Voor die maan teen 20:00 sak gaan ons ook na die hemelliggaam kyk. Jy gaan ook die geleentheid kry om self na iets in die lug te soek.

Pak vir jou `n piekniekmandjie vir die aandete en kom geniet die hemelruim. Ons sal sorg vir koffie en koeldrank.

• Hulpmiddels vir die Werkswinkel (Bo-aan)
  

Die koste vir die werkswinkel is:

1. Per gesin (3) = R240; Ekstra kind = R60; 
2. 1Volwassene = R 120; 
3. Skoolkind en volwassene R180;
   (Kinders graad 7 tot 9 moet deur ouers vergesel word.
   Elke inskrywing ontvang `n Sterrekunde DVD propvol inligting en nuttige sterrekunde sagteware.
4. Klublede van die Bloemfontein Sterrekundevereniging is R60.

• Besoek die webtuiste www.assabfn.co.za/2011_werkswinkel.htm vir `n meer volledige program.
•  Om te bespreek
  - E-pos assabfn@gmail.com
  - Bel vir Yolande Fick by 051 4019751 (Soggens)

Earth Hour 2011

Are you burning candles this Earth Hour?

Click here to find out how to do it safely. Keep your children and home safe.

• Sign Up for Earth Hour:

Saturday 26 March 8:30 - 9:30PM

Sign up now to show your support for Earth Hour and to receive regular updates and information.

Earth Hour is the largest mass participation environmental event in the world and every year, WWF encourages people all over the world to turn off their lights for one hour in a symbolic gesture to pledge their commitment to combating climate change.

This year is especially important for South Africa as we will host the crucial COP17 climate change conference in Durban in December and will once again be in the global spotlight. This annual conference is where world leaders meet to assess progress in dealing with climate change and negotiate obligations for reducing greenhouse gas emissions. The significance is huge: South Africa will play host to a point in history at which humanity has the opportunity to prevent runaway climate change. We encourage everyone in South Africa to take a stand against climate change as it affects all of us, our country and our planet.

This year, Earth Hour is asking everyone to ‘go beyond the hour’ and commit to an on-going personal action to benefit the environment. With COP 17 on the horizon, it is especially important for people in South Africa to make their voices heard and to go beyond the hour.

GASLAND Trailer 2010. Stop these cowboys from fracking our Karoo!