Lunar Observing program

March 8th, 2017

Beginning with the April 2017  a Lunar Observing program will be presented at the Regular monthly meeting of the club. The first will be a overview of the moon and the variety of lunar features that can be seen on the Moon with a telescope.

Lunar features visible beginning  2 days after New Moon ,and forward thru the lunar cycle, will be presented throughout the year. Listed here are the descriptions of  craters, Dorsa, Mare, domes, mountains, valleys and rilles in the PowerPoint presentations.

Sources for the features presented

The Moon Observers Guide – Peter Grego
The Moon and How to Observe It – Peter Grego
What’s Hot on the Moon Tonight – Andrew Plank
The Modern Moon – Charles Wood
Lunar Picture of the Day – http://www2.lpod.org

Recommended  books, software and websites

Maps and Atlas

Rukl’s Atlas of the Moon – Rukl
Sky & Telescope Field Map of the Moon – Reverse and Nonreverse
The Lunar 100 – Sky & Telescope – Charles Wood

Software maps

Lunar Field Atlas http://www.astronomylogs.com/pages/moon.html
Virtual Moon Atlas (VMA) free -  http://ap-i.net/avl/en/start
Lunar Map Pro commercial -

http://www.riti.com/prodserv_lunarmappro.htm

Photographic Moon Book by Alan Chu -

http://www.cityastronomy.com/moonbook-mirror.htm

Photographic Lunar Atlas for Moon Observers by Kwok C. Pau -

http://lunaratlas.blogspot.it/2016/09/photographic-lunar-atlas-is-returned.html

Online magazines

ALPO The Lunar Observer -

http://moon.scopesandscapes.com/ALPO_Lunar_Program.htm

Following are the lunar features in the PowerPoint presentations by Lunar Day.

Lunar Age Days  2 – 3

Cleomedes (78 miles)

Smooth floor.
2 smaller craters on floor.
Northern wall intrude by 2 craters.
Narrow rile running north of center.
Linear rille running 18 miles branching into two.
Small mountain massif.

Langrenus (82 miles)

Complex crater.
Smooth north floor, rougher in south.
Radial rides spreading northward.
Complex terraced walls.
Two central mountain massif.

Vendelinus (91 miles)

Old eroded crater.
Walls are heavily cratered
Smooth interior.
No central massive

Petavius (110 miles)

Spectacular complex crater.
Smooth north floor, rougher in south.
Straight wide rima, running SW to a deep trench.
Extensive terracing and deep groves.
Central mountain massifs.

Funerius (77.5 miles)

Dark floor in the center surrounded by a floor of sandblasted appearance.
Narrow 43 mile rima, NW to SE.
Extensive terracing and deep groves.
Large crater on floor.
Very small central massif.

Vaillis Rheita (310 miles)

Longest crater chain on the nearside.
Runs from crater Rheita (42 miles)

Lunar Age Days 4 – 5

Atlas (54  miles)

Rims have a very sharp look.
Fracture, rough floor with narrow sinuous rilles.
Radial groves on outer rim and floor.
Cluster of small hills in center.

Hercules (41.5  miles)

Sharp rims with interior terraces.
Smooth floor with large secondary crater in the south.
Very tiny central hill.

Proclus (17.4  miles)

Small bright sharp rim crater.
Very broad bright fan shaped rays that border Palus Somni, a light gray hilly terrain.

Plinus  (62 miles)

Sharp rims and terraced wall.  Large size ejecta brake outside rim.
Two or three central peaks.

Rima Plinus (62 miles)

Set of three linear rills, look like cat scratches.

Dorsa Smirnov ( 200 miles)

Serpentine Ridge.
Braided rope of ridges, some 9 miles wide.

Rima Cauchy (130  miles)
A wide long rille.

Rupes Cauchy (75 miles )

A fault changing to a rille.

Tau and Omega Cauchy

Two domes below Rupes Cauchy.
Look for a central pit in Omega

Messier A (4.3 x 8  miles)
Messier B (8.7 x 5.6  miles)

Two small neighboring craters.
B  is a deep oval  to the E – W
A  is deep circular bowl with a raised lip.
Under high Sun can be see two long linear parallel rays.

Fracastorius (76.8 miles)

A flood crater
Northern wall is obliterated.
Forming a bay on Mare Nectaris.
Small broken rille within.

Posidonis (60 miles  )

Bowl shaped, crater inside west of center.
Very complex floor.
Linear rilles on east floor.
Curving ridge on east wall.

Theophilus (62 miles)

Broad terraced walls.
Impact melt floor.
Group of  4590 ft. peaks form central mound.

Cyrillus (61 miles)

Older eroded neighbor of Theophilus
Disorderly and lower wall.
A ridge along the east wall.
Three long central peaks.

Catharina (61 miles)

Smooth light color floor in the east.
Submerged crater on the south floor.
Mountain ridges with the north floor that connect to Cyrillus.

Rupes Altai (496 miles)

Giant fault cuts a path of 496 miles.
Eastern drop is 74 miles below  the scarp rim.
Runs north of Picolomini.

Picolomini ( 54.6 miles)

Prominent crater.
String internal terraced walls.
Cluster of 6561 mile high central peaks.

Lamont Dorsum 46.5 miles

Inconspicuous feature outlined by wrinkle ridges.

Lunar Age  Days 6 – 7

Aristotles (54 miles)

Very impressive broad terraced wall.
Complex crater with exterior radial ejecta blanket.
Smooth floor with two small covered peaks

Vallis Alpine (120 x 6 miles)

A graben, rift valley, 11 miles at it widest point.
Steep walls up to 6561 ft.
Smooth floor, darker than surrounding mountains.

Eudoxus (41.5 miles)

Terrace walled with complex interior.
Smooth floor in the west.
Grouping of four small central peaks.
Very small sinuous rille in the center.

Aristillus (34 miles) rim 10,824 ft.

Substantial outer radial ejecta blanket.
Collection of central mountain peaks in center.
Complex inner terrain.

Archimedes (50 miles) rim 6232 ft.

Walls display terracing.
Flat flooded floor, light rays deposited on floor.
Central peak is covered by flooded floor.

Autolycus (24 miles) rim 11,250 ft.

Small crater with outer radial ejecta blanket.
Complex floor of extruded lava.
No central peak.

Apennines Mt
.   (370 x 62 miles)

Of the 300 peaks the highest measured peak is over 18,000 ft.

Albategnius (85 miles)

Complex crater with broad inner walls.
Dark, smooth flooded floor.
Central peak, 487 ft., is west of center.
SW wall intruded by 27 mile crater Klien.

Maurolycus (68 miles)

Eastern wall displays terracing.
Western wall rough and disorientated.
NE hilly with small craters.
Central peak, 487 ft., is west of center.
Ragged mountains in North
Broad, deep, curving trench in NW

Ptolemaeus (101 miles) rim 7872 ft.

Circular Plain, Striated walls, Flooded floor.
Six mille crater on floor in NE corner.
No central peak.

Alphonsus (73 miles) rim 8954 ft.

Solitary central peak 9840 ft in height.
Small rilles around edge of floor.
Dark patches around rilles.

Arzachel (59 miles) rim 12.496 ft.

Inner walls have a intricate terracing.
Narrow rima, Rima Arzachel, parallel to east wall.
Central peak, 4900 ft. high, is offset to west from center.

Alpetragius (24.8 miles)

Small prominent crater.
Very large rounded mountain massif that takes up to 1/3 of the crater floor,
that rises up to 2170 feet above crater floor.

Mare Serenitatis

Well defined sea with tonal variations of the albedo
of the different lava flows.

Rima Ariadaeus (136 x 6 miles)

A graben – elongated depression between two parallel faults.
Ground between has fallen away.

Rima Hyginus  (130 miles)

An interlocking chain of craters, or pits, that appear to connect
to crater Hyginus, itself may be a rimless pit.
Each ends are smaller rilles.

Rupes Recta (86 x 16 miles)

(Straight Wall)
A classic fault in  east edge of Mare Nubium.
It is a 7 degree slope, at a height of 800 feet.

Lunar Age  Days 8 – 9

Plato (68.2 miles)

Very prominent Dark floor crater.
The floor is 6561 feet below rim.
Shadows cast by rim onto the floor is a spectacular sight.

Timocharis (21 miles)

A remarkable small crater.
Substantial sharp terraced wall.
Without a central mountain peak.
Has a 3.7 mile crater in middle of floor.

Eratosthenes (36 miles)

Complex crater with sharp rim and wide internal terraced walls.
Floor shows impact sculpting  surrounding the 3 mountain peaks.
Outside is a thick ejecta blanket.

Copernicus (57.7 miles)

Young crater with impressive radial ray.
Secondary impacts from excavation of crater.
Complicated terraced walls and internal landslides.
Floor is 12,464 feet below rim.
Two major mountain massive peaks.

Reinhold (30 miles)

Smooth floor containing small hills instead of a mountain massif peaks.
Strongly terraced walls.

Bullialdus (37.8 miles)

Very prominent crater with an indent on the SE wall.
Intricate terraced walls.
Tight group of central peaks

Pitatus (60 miles)

Eroded crater with complex rills system on flat floor.
Low terraced walls.

Tycho (52.7 miles)

Has the largest and complex system of rays.
Impressive sharp rim terraced walls.
Interior floor filled pool of impact melt.
Large pair of mountain massif peaks.
Dark collar of impact melt outside of the rim.

Clavius (140 miles)

Large flat floor crater with a scalloped rim.
Heavily cratered floor with a distinct small arc of unconnected craters.

DOMES

Milichius dome – an isolated  3 mile diameter dome with a central pit.

Hortensius domes – Six dome cluster field, 1 to 3 miles in size, with central pits.

Lunar Age  Day  10 – 11

Sinus Iridum (160 miles)

Very prominent Dark floor crater.
Southern half of the rim is overrun by lava flow of Mare Imbrium.
The floor is 6561 feet below rim.
Shadows cast by rim onto the floor is a spectacular sight.

Mons Gruithuisen

Second largest dome with a small summit crater.

Mon Rumker (43 miles)

Broad lumpy plateau.
Largest dome complex.

Aristarchus
(24 miles)

Brightest ray crater on the moon.
Western wall show radial banding of alternating
dark and bright features.

Vallis Schroteri (3 x 6 wide  x 100 long miles)

Impressive sinuous rille, a collapse 3000 ft deep lava tube.
It starts at the “Cobra Head” rim and runs 100 miles to Oceanus Procellarum
Exterior mound shows radial structure.

Reiner Gamma (37  x 90 miles)

Young crater with impressive radial ray.
Secondary impacts from excavation of crater.
Complicated terraced walls and internal landslides.
Floor is 12,464 feet below rim.
Two major mountain massive peaks

Kepler (20 miles)

Terraced walled crater with small hills
instead of central mountain peak.
Sun aged rays span a good distance,
interacting with rays of Copernicus.

Gassendi (68 miles)

Magnificent crater on the North boundary of Mare Humorum.
Complex floor containing small hills, mountains, ridges and rilles.
Has three of a mountain peaks.
Walls are partial submerged by Mare Humorum.

Schiller (111 x 42 miles)

Elongated flood crater.
Southern is very smooth, north has an prominent central mountain Ridge.
May have been formed by 3 or 4 impacts.

Schickard (140 miles)

Low circular walls with around a smooth floor.
Darker floor in the north, brighter floor in the south
that a darker patch near the wall rim.

Wargentin (52 miles)

Strange sight of a crater filled up to the top of it’s
rim with extruded lava. Y shaped ridge on center.

Lunar Age  Days 12 – 15

Pythagoras (80 miles)

Broad scalloped rim walls.
Hilly interior floor,
with a central mountain peak.

Seleucus (27 miles)

Sharp rim crater with broad inner terraced walls.
Dark inner floor with a small central hill.
Has exterior ejecta blanket.
Bright broad rim and dark interior floor under high sunlight.

Struve (105 miles)

Large elongated flood crater with sharp rims.
North wall  all is eroded by lave flow.

Russell (64 miles)

Shares a eroded wall with crater Struve.
Flooded floore is smooth, does not have a central peak.

Cavalerius (36 miles)

Deep crater with a sharp rim with signs of terraced walls.
Small central hill.

Hevelius (66 miles)

Eroded wall crater with rilles on the interior floor.
Rima Helevlius rille crisscross to form an X pattern
with smmaler sinious rille before it continues outside
of the crater wall.

Grimaldi (27 miles)

Broad dark lava plain.
Broken hills surround the plain.
Very prominent

Riccioli (90.5 miles)

Well defined low wall crater.
Complex interior floor,small rilles and hilly in the south.
Large dark patch in the north.

Bailly (189 miles)

Largest crater on the Moon best seen during favorable librations.
Low multi ring impact basin.
Eroded outer walls, rough crater floor.

South Polar Region

Numerous mountains around lunar rim near the pole.

Mare Orientale (580 miles)

Very large Multi Ring Impact Basin.

Resources for Observing Mars

November 27th, 2015

In the year 2016 the planet Mars will be at opposition and close to our planet Earth.

Listed below are some sites on how to observe the planet Mars and detail information on the 2016 opposition.

This information is from the presentation at the LAS regular meeting

Software and web sites with information on observing the planet Mars.

Mars Preview II

http://www.skyandtelescope.com/astronomy-resources/freeware-from-sky-telescope/

Websites about observing Mars

Association of Lunar and Planetary Observers (ALPO)

Download observing forms http://alpo-astronomy.org/

British Astronomical Association (BAA)

Mars Section https://www.britastro.org/section_front/17

Links to other sites are found on the ALPO and BBA sites.

Using filters to view Mars:

Yellow (W8) Enhances Martian cloud details.

Yellow (W12, W15) to brighten desert regions, darkens bluish and brownish features.

Orange (W21, W23A) further increases contrast between light and dark features, penetrates hazes and most clouds, and limited detection of dust clouds.

Red (W25, W29) gives maximum contrast of surface features, enhances fine surface details, dust clouds boundaries, and polar cap boundaries.

Yellow-green (W57,W11) darkens red and blue features, enhances frost patches, surface fogs, and polar projections.

Blue-Green (W64) helps detect ice-fogs and polar hazes.

Blue (W80A, W38, W38A) and deep blue (W46, W47) shows atmospheric clouds, discrete white clouds, and limb hazes, equatorial cloud bands, polar cloud hoods, and darkens reddish features.

Wratten Filter Numbers and color guide

Wratten Color

#8 light yellow

#11 yellow-green *

#12 yellow

#15 dark yellow

#21 orange

#23A light red

#25 red

#30,#32 magenta

#38,#38A light blue

#46,#47 violet or deep blue

#56 light green

#57 yellow green *

#58 green

#64 blue-green

#80A median blue

#82A light blue

Neutral density These filters simply darken bright objects and reduce glare for better resolution of details.

More information on color filters below:

http://lackawannaastronomicalsociety.org/?p=1254

Searching For Comet C/2011 L4 PANSTARRS

January 15th, 2013

Expectations of the appearance of Comet C/2012 L4 PANSTARRS is running high since the first solution it’s orbit. Magnitudes are expected in the minus range, sporting a long broad tail after sunset. I’ve seen the preview of this picture before, withthe repeat to follow at the end of the year.

First item to consider is the comet will be close to the Sun and the horizon when it reaches peak brightness. The comet will slowly move from the southeast of the setting Sun, moving northward and gradually higher each night. It will share the twilight sky with a crescent Moon and later light of the First Quarter Moon.

And that’s a description of the path the comet will take. This all happens in mid March in North Eastern Pennsylvania, not the most forgiving of weather around here most of the time. Dark clouds silhouetted against a light blue sky, winds whipping from the west, and with snow on the ground or sub freezing temperature. The regional weather can hinder viewing this comet, do you recall looking for comet C/2006 P1 Mc Naught in January 2007? Searching the western horizon for the coma or tail between broken layers of clouds each night.

Comet Hale-Bopp came around in March and April of 1997, it was in the northern sky, farther from the setting Sun and moving towards the north west. Weather conditions cooperated that year, moderate temperature, no snow on the ground and many clear nights without clouds made for many clear views of Hale-Bopp. March weather the year before and the year after Hale-Bopp were more typical of the start of spring. Cloudy skies, cold temperature and snow on the ground.

C/2012 L4 PANSTARRS is moving in the opposite direction, hence it will be viewed in twilight skies for a while as it quickly sets before the sky becomes completely dark. To rub it in, Daylight Savings Time begins as the comet appears. Make any required adjustments to your computer, planetarium software, or computer controlled telescopes.

If you must travel to a location to get a good view of the western sky, then binoculars would be the first instrument to use when searching the twilight sky for C/2012 L4 PANSTARRS. A digital SLR camera or a Point and Shoot on a tripod will give results with exposure of less than 20 seconds for focal length of 50 mm. Take many pictures as the every changing twilight sky will affect the exposure time. This would be good practice for the next comet at the end of the year.

Use software to calibrate, if you also take Darks and Flat images,to process and to stack the individual images. The list of software to examine included, RegiStaX, Deep Sky Stacker both freeware. Photoshop, CCDStack, MaximDL are commercial software, the later two astrophotography packages.

Plan ahead if you are setting up a telescope to view or take pictures. If it is a computer controlled telescope mount then, you may need the use the Moon or Sun to fix the scope’s position before you can slew the telescope to the comet. Arrive early to set up and wait for the comet to appear.

Here are the position of the comet at 7:30 p.m. Eastern Daylight Time using a conservative magnitude estimate from Guide8 software. Remember the comet will be seen in a twilight sky, not against the dark night sky. Magnitudes will be difficult in a twilight sky, frustrating with moving clouds on the horizon.

Addition Jan 20, 2013

New magnitude estimate by Sechii Yoshida  put the comet at magnitude  2 or magnitude 3 at brightest.  http://www.aerith.net/comet/catalog/2011L4/2011L4.html

This reminds me of the appearance of comet Bradfield 1974 b (C/1974 C1) in March  1974.  Comet Bradfield  perihelon to the Sun was 0.5 AU,  higher in the sky at sunset and seen better as darkness fell. It traced a path in the western sky similar to what C/2012 L4 PANSTARRS will due this March. Comet Bradfiled reach 5 th magnitude back in 1974,  C/2012 L4 PANSTARRS will be closer to the horizon than comet Bradfield.

PANSTARRS (C/2011 L4) Magnitude listed are assumptions

Date                                    RA          declination       r     delta mag Elong Alt Azim    Sun el

—-                                       –             ———–          -       —–    —   —–  ——   ——     —–

9 Mar 2013 19:30:59 00h21m52.73s -05 34′ 33.2″ 0.3016 1.1092 0.5 15.2 4.31 258.73 -6.1

10 Mar 2013 19:30:59 00h25m12.39s -02 55′ 44.1″ 0.3026 1.1144 0.5 15.1 5.98 260.79 -5.9

11 Mar 2013 19:30:59 00h27m56.28s -00 19′ 00.0″ 0.3068 1.1203 0.6 15.2 7.51 262.95 -5.7

12 Mar 2013 19:30:59 00h30m07.40s +02 14′ 25.0″ 0.3141 1.1266 0.7 15.5 8.89 265.16 -5.5

13 Mar 2013 19:30:59 00h31m49.45s +04 43′ 33.4″ 0.3240 1.1333 0.9 15.8 10.13 267.43 -5.2

14 Mar 2013 19:30:59 00h33m06.40s +07 07′ 45.3″ 0.3364 1.1402 1.1 16.4 11.21 269.71 -5.0

15 Mar 2013 19:30:59 00h34m02.20s +09 26′ 37.9″ 0.3509 1.1474 1.3 17.0 12.17 271.99 -4.8

16 Mar 2013 19:30:59 00h34m40.49s +11 40′ 02.6″ 0.3671 1.1548 1.5 17.7 13.00 274.26 -4.6

17 Mar 2013 19:30:59 00h35m04.52s +13 48′ 01.7″ 0.3848 1.1624 1.7 18.5 13.72 276.51 -4.4

18 Mar 2013 19:30:59 00h35m17.06s +15 50′ 45.1″ 0.4035 1.1700 1.9 19.4 14.34 278.72 -4.2

19 Mar 2013 19:30:59 00h35m20.43s +17 48′ 27.1″ 0.4232 1.1778 2.1 20.3 14.87 280.90 -4.0

20 Mar 2013 19:30:59 00h35m16.52s +19 41′ 24.9″ 0.4436 1.1857 2.3 21.3 15.33 283.03 -3.8

21 Mar 2013 19:30:59 00h35m06.89s +21 29′ 56.8″ 0.4645 1.1936 2.6 22.2 15.73 285.11 -3.6

22 Mar 2013 19:30:59 00h34m52.77s +23 14′ 21.0″ 0.4858 1.2016 2.8 23.2 16.08 287.15 -3.4

23 Mar 2013 19:30:59 00h34m35.18s +24 54′ 55.5″ 0.5074 1.2097 3.0 24.2 16.38 289.14 -3.2

24 Mar 2013 19:30:59 00h34m14.90s +26 31′ 57.2″ 0.5293 1.2178 3.2 25.2 16.64 291.08 -3.0

25 Mar 2013 19:30:59 00h33m52.57s +28 05′ 42.1″ 0.5513 1.2259 3.4 26.2 16.87 292.98 -2.8

26 Mar 2013 19:30:59 00h33m28.70s +29 36′ 25.0″ 0.5734 1.2341 3.5 27.2 17.07 294.84 -2.6

27 Mar 2013 19:30:59 00h33m03.69s +31 04′ 19.6″ 0.5955 1.2423 3.7 28.2 17.25 296.65 -2.4

28 Mar 2013 19:30:59 00h32m37.84s +32 29′ 38.5″ 0.6176 1.2506 3.9 29.2 17.41 298.42 -2.2

29 Mar 2013 19:30:59 00h32m11.40s +33 52′ 33.4″ 0.6398 1.2588 4.1 30.2 17.56 300.15 -2.0

30 Mar 2013 19:30:59 00h31m44.55s +35 13′ 14.7″ 0.6619 1.2671 4.2 31.2 17.70 301.84 -1.8

31 Mar 2013 19:30:59 00h31m17.42s +36 31′ 52.2″ 0.6839 1.2754 4.4 32.2 17.84 303.50 -1.6

1 Apr 2013 19:30:59 00h30m50.13s +37 48′ 34.9″ 0.7058 1.2837 4.5 33.1 17.96 305.12 -1.4

2 Apr 2013 19:30:59 00h30m22.72s +39 03′ 30.9″ 0.7277 1.2920 4.7 34.1 18.09 306.71 -1.2

3 Apr 2013 19:30:59 00h29m55.23s +40 16′ 47.6″ 0.7495 1.3003 4.8 35.0 18.21 308.27 -1.0

4 Apr 2013 19:30:59 00h29m27.68s +41 28′ 32.0″ 0.7712 1.3086 5.0 36.0 18.34 309.80 -0.8

5 Apr 2013 19:30:59 00h29m00.05s +42 38′ 50.2″ 0.7927 1.3169 5.1 36.9 18.47 311.30 -0.6

6 Apr 2013 19:30:59 00h28m32.31s +43 47′ 48.1″ 0.8142 1.3251 5.2 37.8 18.60 312.77 -0.4

7 Apr 2013 19:30:59 00h28m04.42s +44 55′ 31.0″ 0.8355 1.3334 5.3 38.7 18.73 314.21 -0.2

8 Apr 2013 19:30:59 00h27m36.31s +46 02′ 03.6″ 0.8567 1.3417 5.5 39.6 18.88 315.63 0.0

Color Filters for the Planets

May 11th, 2012

This PDF file contains the tables and notes used the PowerPoint program that was given at the regular club meeting this year. The first section list each planet and the filters best suited to improve or enhance various visual planetary features. A narative description of the visual performance is present by Wratten filter number and color type desciption. 

A table format is next, listing each Planet, planet feature and filter most suited to enhance the visual viewing of the feature. This is good to use at the scope since it fits on on 8 x 10 sheet of paper.

The last section list the Wratten number, the planet it is to be used on and the feature on the planet the filter will enhance. This is the most common method that suppliers and dealers display their filters.

Download the pdf file Useful Filters for Planets

 

Here are Internet web sites that provide more information Color Filter for the planets.

The Abbey Road Observatory – http://www.karmalimbo.com/aro/index.htm

Observing the Planets with Color Filters – http://alpo-astronomy.org/mars/articles/FILTERS1.HTM

Meade Filters – http://www.meade.com/catalog/meade_4000/meade_series_4000_filters_02.htm

The Meade 4000 Series – http://www.analyticalsci.com/Astronomy/Filters/Filters_Described_Meade.htm

Celestron – http://www.celestron.com/astronomy/accessories/filters.html

Celestron Kits – http://www.celestron.com/astronomy/accessories/eyepieces/accessory-kits.html

 

These three include Solar and DeepSky Filter, which I did not cover in te PowerPoint slide show.

The Use of Filters – http://zeca.astronomos.com.br/pratica/filtros/The_Use_of_Filters.htm

Optical Filter Guide – http://www.myastroshop.com.au/guides/filters.asp

Nebula Filter comparison on Deep Sky Objects – http://www.prairieastronomyclub.org/filtercomparisons.htm