Eye & Pencil - Amazon S3...MLewis. BAA Deep Sky Meeting March 2013 4 The Observer’s Eye Much of the appeal of amateur astronomy comes from a visual appreciation of objects in the

Eye & Pencil

Martin Lewis

MLewis. BAA Deep Sky Meeting March 2013 2

•The Observer’s Eye

•Visual Observing Techniques

–Dark Adaption

–Averted Vision

–Use of Movement in Deep Sky Observing

–Use of High Magnification in Deep Sky Observing

–Use of Low Magnification in Deep Sky Observing


Looking and Seeing


The Observer’s Eye

Much of the appeal of amateur astronomy comes from a visual appreciation of objects in the sky

– Directly. Naked eye or through the eyepiece

– Indirectly. Looking at images from DSLR, CCD, webcam, Watec etc

http://www.telescopes.cc/m45.htm



• For a direct visual observer, the Eye, working with the Brain, gives a Visual Response when one looks at an astronomical object

• Many non-astronomers think that the rewards of visual astronomy come solely from the Aesthetic response to things that are inherently pleasing to look at


6

Aesthetic Treasures

• There are, however, only a few astronomical objects that are inherently aesthetically pleasing through the eyepiece

• If you didn’t know anything about the object it would still be wonderful to look at; eg. Total Solar Eclipse, Saturn, Moon, M42, M13/M51/M57 through a large scope -the small child test

Ngc246 with 25” Dob. M51 with 18” Dob.

M42 with 3” Refractor



Intellectual Response

• For astronomers there are other rewards apart from aesthetic ones that hopefully keep you looking on those long, cold, nights

• The list of rewards are personal to each observer

•Faint Fuzzy Blobs?

or

•Wonderful Compact Galactic Cluster?

Hickson 61 ‘The Box’ sketched through 18”

Dobsonian


©Max Alexander

MLewis. BAA Deep Sky Meeting March 2013

Why I Observe the Deep Sky

• A Sense of Awe

– An intellectual appreciation of the vastness of space and time… ‘that photon from Ugc9242 that just went in my eye was travelling across space for 70 million years’

• A Sense of Peace

– A calming effect of being under the silent and open heavens

• Meeting a Challenge

– Detecting faint objects or seeing difficult detail within objects eg all five members of Stephan’s Quintet or the central star of the Ring Nebula

• Honing my Observing skills

– Improving my observing skills to better enable me to see faint deep sky objects and details within deep sky objects

Ugc9242

Stephan’s Quintet

• A sketched record at the eyepiece

– A record of the observation as a reminder of what was seen that night

– Something to look at on those cloudy nights

– Invites you to spend time really studying an object

– A discipline that forces you to make decisions about whether you really can see particular details or not

– Helps to hone those observing skills

Why I Sketch the Deep Sky

Ngc1501 Ngc4565

Ngc2359 ‘Thor’s Helmet’

(OIII filter)


• The eye and brain break up the visual world into characteristics such as brightness, colour, shape, motion, and depth

• For visual observing of deep sky objects the key items are almost always brightness and shape

• For visual deep sky observers, understanding how the eye and brain work together best, will maximise the chance of seeing faint objects or faint detail

• When observing faint objects several techniques can be used to improve the likelihood of seeing the object or seeing detail in the object

– Proper Dark Adaption

– Averted Vision

– Use of Movement

– Use of Optimum Magnification



Dark Adaption in Deep Sky Observing


Dark Adaption

Two ways that the eye’s sensitivity to light improves in the dark;

• The unaided eye’s ability to see faint light can be improved by approx. 16 fold by the pupil opening up to its maximum size. Reflex takes 0.5 to 1.0secs

– for low power observations where the eyepiece exit pupil is large this is important, but for higher powers, where exit pupil may be 2mm or so, this pupil response has little benefit

• Chemical changes in the eye in dark conditions can increase the eye’s sensitivity to light by up to million fold. Dynamic range still 1000:1 - ‘black moves’

– The eye has two types of light sensitive cells in it. Cones can detect colour and luminance. Rods detect luminance only.

– In first 5-10 mins, the colour-sensitive cones adapt most quickly and are more sensitive

– After 10 mins the rods take over and reach maximum sensitivity after about 30 mins

Rod response

Cone response

Combined In

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wh

ich

fai

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• Time for full dark adaption and speed of adaption depends on the brightness of the environment before proper dark adaption commences

• Also depends on how long you spent in that environment

– If your night vision is spoilt by a careless torch etc try and minimise the length of exposure- it’s not a lost cause!

– Rhodopsin destroyed in one eye by unintentional light can be preserved in the other eye, if kept closed

Dark Adaption Light level at start of adaption

Duration of exposure at 333mL

Troland= unit of retinal illuminance!


• Spectral sensitivity of rods and cones is different and changes with time as eye adapts to dark. More sensitive to blue as eye adapts. Colours change as dusk progresses Purjinke Effect

• So dark adaption times are wavelength dependent;

• At very long wavelengths (>650nm) rod

and cone sensitivities are the closest

– eye is relatively insensitive to red

but for at these wavelengths if you

can see red light you are likely to

see it as coloured as cones are

stimulated as well as rods

– green most frequently seen in

DSOs because nebulosity often

significantly brighter in green-

enough to stimulate the green

cones

Dark Adaption


• If the eye is more sensitive in the green why not have green torches at night and then they could be lower power and still allow you to read maps

• They would not be coloured, however, at dimmest setting and that is important

• For good visual acuity for reading maps and seeing fine detail you need to use your cones

• Cones are crowded at the centre of your retina (photopic vision) whereas rods are more widely spaced and are away from the centre of your eye (scotopic vision).

• Red light allows you to stimulate the red cones with minimal stimulation of the rods so preserving your night vision.

Red Torches


• Treasure your hard-gained Dark Adaption

• Red torches affect dark adaption less than other colours but still must be dimmed to minimum practical level

– “just cos its red doesn’t mean it’s totally safe”

• The eyepiece field of view will generally be darker than the night sky seen naked eye. Looking at a bright sky can affect your adaption as can too low an eyepiece power or a bright field star

– if you can seen an ‘after-image’ of the eyepiece field of view when you blink, then it is affecting your dark adaption

– close your observing eye when you need to use a bright torch to find something

– do your planetary observing after your deep sky observing or use the other eye

• Very good link on the performance of the eye can be found here; http://webvision.med.utah.edu/

Dark Adaption

http://webvision.med.utah.edu/

http://webvision.med.utah.edu/


• Be comfortable. Straining affects your ability to see faint objects so use a chair where possible and avoid standing on tiptoe etc

• Keep breathing. It is important to keep the brain properly oxygenated to have the eye working at optimum

Dark Adaption- ADDED after BAA TALK


Averted Vision in Deep Sky Observing


Averted Vision in Deep Sky Observing

• Dark adaption is different for different distances from the centre of vision

– At centre of vision the dark adaption curve tails-off

– Here cones only so does not benefit from adaption of rods

• Cone peak at centre of vision (Fovea). 5 million cells overall

• Rod peak several degrees away from centre. 100 million cells overall


Blinking Planetary Ngc6826 508x


• To make the most of the potential sensitivity improvement, you need to ‘point’ your eye to one side of the object of interest but focus your attention on it. This is called Averted Vision

• Best distance approx. 10° from fovea

• Look above or below object or place object on nose side of fovea - avoid blind spot on other side

Averted Vision


• Look above or below object or place object on nose side of fovea - avoid blind spot on other side

• Best ‘gaze positions’ for object at centre of 60° field of view eyepiece

and left eye

• Keep the eye dancing around- use of eye movement

Averted Vision


Use of Movement in Deep Sky Observing


Use of Movement in Deep Sky Observing -Eye Movement

• If an image, especially a low contrast one, is static on the retina then eye fatigues to the image very quickly and it fades away from view

• The eye/brain filters out the object and treats it as noise if it is stationary on the retina

• The brain recognises a fixed moving pattern amongst the noise as something real

• This is all contrary to oft stated 6 sec integration time of eye!

• Subconsciously the eye will dance around to keep the image moving on the retina

• This is just a natural part of how averted vision works just a matter of paying attention to the whole of the field as the eye dances around

Hickson 68

Abel 262 (12.9 to 14.6) Ngc3718 & Ugc6527A/B (11.6 to 15.8) Hickson 51 (14.6 to 15.8)

Minkowski 2-51 (mag. 13.6) Abell 2 (mag 14.5) Hickson 57 Copeland’s Septet (13.7 to 15.2)

Dark Adaption and Averted Vision with Eye Movement to pick up targets


• Even with the eye scanning around the field of view the brain tends to ignore large low gradient contrast variations relative to edges of the eyepiece field and see this as a noise gradient

• If these low contrast areas are moved relative to the eyepiece field edges they become much easier to see!

– use of lower eyepiece powers will help by increase the gradient of the edge

– movement of scope by sweeping back and forth

– tapping focusser works well

• Method works particularly for large faint areas of nebulosity with very low contrast with background sky

– California Nebula, North American Nebula, outer regions of M31

Use of Movement in Deep Sky Observing -Field Movement

28


Use of High Magnification in Deep Sky Observing


Use of High Magnification in Deep Sky Observing

• Several Advantages of using higher magnifications

– Higher magnifications can be used to make it easier to see low contrast detail by making it larger

– Higher magnifications can be used to see fainter stars/small galaxies by darkening the background

– Higher magnifications will make eye astigmatism less apparent by reducing the exit pupil size


• Rods are well spaced out and resolution is poor

• Increasing magnification extends the image of an object over several separately connected areas of rods in retina and the brain believes it has then seen something real - like finding a pencil in the dark- contrast does not change!

Use of High Magnification for Low Contrast Details

32

• Visibility of faint extended objects depend on surface contrast with background and on angular size

– Plot of the contrast at which objects of different sizes are just detected for different background brightnesses

– Graph shows that for given background brightness, larger objects are easier to detect than smaller ones

– Fainter the background the larger the detail needs to be to be seen for given contrast


Background of 24 mags/sec2 equivalent to very dark Country sky

At 15mag/sec

A contrast of 1.1 to 1

can just be detected

if 3.6’ across

At 24mag/sec

A contrast of 1.1 to 1

can just be detected

if 6° across

At 24mag/sec

A contrast of 100 to

1 can just be

detected if 3.6’

across


• For more info on how the eye works in low light and discussion of the optimum magnification to bring out details in deep sky objects see;

Use of High Magnification


• As you increase magnification the object gets larger which makes it easier to see

• As you increase magnification the background brightness and the object brightness get lower which makes it harder to see even though contrast is the same

• As magnification increases up to an optimum magnified size, the benefit from making the object larger, outweighs the fact that both the background and the object get darker. As a consequence the object, or detail within, becomes easier to see

• Beyond the optimum size, the decrease in brightness with increasing magnification, more than nullifies the increase in visibility due to increase in size. As a consequence detail actually becomes harder to see



• Graph shows the Optimum Magnified Visual Angle as a function of background brightness in the eyepiece.

• For reasonably dark background in eyepiece OMVA is about a degree or two

• OMVA smaller for brighter backgrounds

OMVA is the angle that an object, or detail within an object, needs to be magnified to in order to maximise the chance of detection



• To see make it easier to see detail in Deep Sky objects progressively increase the magnification until the detail or object subtends a degree or two

• Experiment with different powers increase power until image deteriorates then back off

• In a complex object like a large spiral galaxy or emission nebula detail is of different sizes and brightnesses and effective background brightness can vary considerably. Hence at different magnifications different detail becomes more visible


37

Use of High Magnification

M51 127x to 290x M106 185x and 290x

Ngc6946 290x


• For some higher surface brightness objects more detail is seen at much higher magnifications than would be used for say planetary observations

• Need steady skies as well as good transparency

Eskimo Nebula (left) and Cat’s Eye (right) both observed at 762x to bring out all the objects’ details. 40x per inch of aperture

Use of Very High Magnification

39

Blinking Planetary Ngc6826 508x

Blue Snowball Ngc7662 508x

Ghost of Jupiter Ngc3242 508x

Humaston 1-2 508x


• Optimum Magnified Visual Angle applies to extended objects like nebulae, but what about objects like globular clusters which are comprised of just stars? Are higher magnifications useful here too?

• The eye sees very faint star discs as point sources because the size is below the critical resolving size for the eye for faint objects (<0.3°)

• Consequently to the eye the surface brightness of a very faint star appears not to change with increasing magnification.

• However, background brightness of the sky does decrease with increasing magnification.

• As a consequence the contrast between faint stars and the background seems to increase with increasing magnification making faint stars more visible

Use of High Magnification on Small Objects

41

• No benefit once the sky background 27mags/arc-sec2 as eye detects this as ultimately black to the eye

• Try increasing magnification until this limit is reached to see faintest stars- field will look very black

• Very apparent effect with globular clusters- many more stars visible with high power than low power

• Applies to small galaxies too (<0.3°). Which can pop into view as magnification increases and background darkens

• If you are not operating at a magnification where the filed is essentially black then very wide field eyepieces without the dark edge of the field next to the object being observed seem to help with spotting objects. Some odd eye rescaling effect. You would think it would help by making the field darker

Use of High Magnification on Small Objects

Hickson 10 254x


Use of Low Magnification for Deep Sky Observing


Use of Low Magnification for Deep Sky Observing

• Use Low Magnifications to see as much as possible of very extended objects and frame them nicely in the eyepiece field

– Veil Nebula = 2.8° so want 3.5° FOV

– Pleiades = 1.5° want 2°

– M31 = 3° want 4.5°

– Double Cluster = 1° want 1.5°

– Sword of Orion 1.7° so want 2.5°

• Encircling objects with dark sky makes them stand out better and can make them more readily detectable;

– California Nebula = 3.2° want 4.5°

– North American Nebula = 3° want 4.5°

– Rosette Nebula = 1.5° want 2°

http://www.telescopes.cc/ngc1499large.htm



• Lower magnification increases the intensity gradient at the edge of soft diffuse edged objects such as M31 or California Nebula - especially useful in conjunction with the use of field movement

• Use Low Magnification to maximise surface brightness and improve likelihood of seeing colour in objects like M42- SB can never be brighter than that of unaided eye!

• Low magnification does not maximise level of detail seen and does not improve contrast between object and background

•

Use of Low Magnification


• Lowest Useful Magnification is when the ‘waist’ of the cone of light leaving the eyepiece (exit pupil) is the same as the size of your dilated pupil - if exit pupil was any larger then some of the light would not enter the eye

• When eyepiece exit pupil matches the diameter of the dilated pupil this is called Richest Field operation. Object then has same surface brightness as seen with naked eye

• Not necessarily the most comfortable views at Richest Field. Eye works best with exit pupil 1-3mm. Edge flaring of stars especially if eye slightly off-centre



Putting It all Together

47

Omega Nebula 156x

Horsehead with 25” Dob. 100x (Hbeta)

Hubble’s Variable Nebula. 340x

Jones Emberson 1 with 25” Dob 143x (OIII)

Ngc1514 156x (OIII)

48

M97 Owl Nebula 102x (OIII) Sharpless 2-71 127x (OIII)

Jones 1 102x (OIII)

Ngc1365 with 25” Dob 143x M33 102x

49

Saturn Nebula 340x

Ngc4762 156x

M101 102x

M81 127x

Western Veil with OIII Eastern Veil with OIII


• Your pupil gets smaller as you get older. To check you pupil size hold a ruler close to your eye in darkness then photograph the arrangement with a flash camera (5.5mm for me)

• 30 year old 7mm 40 year old 6mm 50 year old 5mm 60 year old 4mm

• So lowest useful magnification for 5.5mm exit pupil and different scopes will be; 18” scope =83x 150mm scope =27x 11” scope =51x 100mm scope =18x



• In Richest Field mode the object has same surface brightness as that seen by unaided eye- eg for 150mm scope and 5.5mm pupil at 27x we get 730x the light but this is spread over 730x the area!

– You cannot increase the surface brightness of an object above that of the brightness seen by the naked eye.

– Higher magnifications take 730x the light but will spread it out over more than 730x the area

– Next step to get more light is to use a binocular telescope

• Scope magnification = Dia. of the objective ÷ Dia. eyepiece exit pupil. So, for Richest Field Operation; RF Magnification = Dia. of objective ÷ dia of dilated pupil


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Eye & Pencil - Amazon S3...MLewis. BAA Deep Sky Meeting March 2013 4 The Observer’s Eye Much of the appeal of amateur astronomy comes from a visual appreciation of objects in the