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Transcript
OBSERVING GALAXIES
RONALD E. MICKLE
Denver, Colorado 80211
©2000 Ronald E. Mickle
ABSTRACT
The objective of this project is to visually, photographically or by a charged
coupled device (CCD) image, observe galaxies within the capability of the
telescope used and its ancillary equipment and to compare observations with
normally published images and descriptions of those galaxies as relating to
surface brightness, visibility of spiral arms, etc. Observations for this project
were made visually and by CCD.
Galaxies come in many shapes, sizes and degrees of brightness, all divided
into four classes: spiral, barred spiral, elliptical and irregular. Many people
are familiar with variable stars, which can change in brightness from a
thousandth of a magnitude to as much as 20 magnitudes. It’s interesting that
certain galaxies, too, can change in brightness. But unlike variable stars,
which are observed from relatively close distances, galaxies are observed as
far out as our instrumentation will allow, over 15 billion light years. We
know today there are literally billions of galaxies. This paper humbly
includes observations of 21 of these galaxies and an appraisal of the
specifications of the two telescopes used and their capabilities. (Universe
1999; The Astrophysical Journal)
1. OBSERVATIONS AND COMPARISONS
Surface brightness is defined as “the luminosity per unit area on the sky, usually
expressed for optical data as magnitudes per square arcsec. It is a useful distanceindependent property to use in the comparison of low-redshift galaxies (where relativistic
corrections are unimportant) as the angular area subtended and the luminosity both
decrease with the inverse square of the distance”. ( The Facts on File Dictionary of
Astronomy)
A commonly misunderstood concept to most amateur astronomers is why a galaxy with
the same magnitude as a star is much more difficult find or see. For example, when
observing a galaxy and using the published magnitude, the observer must remember that
the surface brightness will not appear to be as bright as a star with the same magnitude.
The galaxy M51 (Table 1), which has a magnitude of 8.0, appears much fainter than
reference stars in its vicinity that have the same magnitude.
Both visual and CCD imaging observations were made. Visual observations were made
during six nights, between April 11 and May 23, 2002, for a total of 10 observations
1
(Table 1). During the 48 nights in the observing schedule, there were approximately 16
weather nights when no observations were possible.
1.1. VISUAL OBSERVATIONS
Visual observations were made from Denver, Colorado, United States: 40°N and 105°W.
1.1.1. VISUAL OBSERVATIONS:
EYEPIECE FIELD-OF-VIEW
To aid in locating the galaxies, especially from an observatory in a suburban setting with
light pollution, it was necessary to compute the field-of-view (FOV) for the eyepieces.
Using the 32mm eyepiece with a FOV equal to 14.1 arcmin to locate M83, which
subtends an angle of 13.1’ would have been difficult without first locating the correct star
field. The installed encoders are not accurate enough to place a galaxy in the field of
view.
The drift method was used to determine the FOV (AAVSO). As an example, computing
the FOV for the 32mm Plössl eyepiece consisted of locating a star on or near the celestial
equator. The star was placed just outside the east side of the eyepiece field of view and
the Right Ascension of the telescope disengaged. This allowed the star to “drift” into
view from east to west. The time it took the star to drift from the east side of the
eyepiece and exit the west side was measured in seconds using a stopwatch. Nine
measurements were made for the 32mm eyepiece. This time was converted to arcseconds
by using the Earth’s rotational speed of 1o /4-minutes. This is proportional to the FOV in
arcseconds (unknown)/FOV in seconds using the drift method. The formula is expressed
as:
1o/4’ = D/m
D = FOV in arcseconds,
UNKNOWN.
m = Measured drift of star across
eyepiece in seconds.
For the 32mm eyepiece:
D = FOV in arcseconds.
1o/4’ = D/m
3600s/240s = D/56.31s
m = Nine measurements averaged
to 56.31 seconds.
240(D) = 3600/56.31
1o = 3600 seconds.
D = 202,716/240
4’ = 240 seconds.
D = 844.65 arcseconds
⇒
844.65 arcs/60 = 14.1 arcminutes
(Variable Stars)
2
1.1.2. VISUAL OBSERVATIONS: EYEPIECE
MAGNIFICATION AND FOV
The following table shows the magnification and FOV, as explained in section 1.1.1. for
each eyepiece measured. (Variable Stars)
Eyepiece
Power (x)
Field-of-View
(arcmin)
25mm
32mm
40mm
65mm
80mm
312x
244x
195x
120x
98x
(not available)
Focal Length
7800 mm
14.1’
14.7’
28.9’
19.9’
I have been observing for more than twenty years, and still consider the task of
identifying the star field correctly a bit tricky. There were times that I spent up to two
hours properly identifying a star field. My wife says I should get a life!
1.1.3. VISUAL OBSERVATIONS:
20” REFRACTOR
All visual observations were made using the University of Denver’s (DU) historic
Chamberlin Observatory Alvan Clark-George Saegmuller 20-inch f/15 refractor
telescope. The Clark 20” is capable of yielding images beyond 16.5 Mv. Of course, this
is limited by the sky conditions and its location in Denver, Colorado. The five galaxies,
all Messier objects, were chosen due to familiarity, relative brightness and ease of
recognition.
Some visual observations for this project followed the Public Night outreach at
Chamberlin Observatory, co-sponsored by DU’s Physics and Astronomy Department and
the Denver Astronomical Society (DAS). As Lead Observer during Public Nights, I am
responsible for opening and closing the observatory, and presenting the lecture to the
public prior to moving upstairs to the telescope for viewing.
As with any celestial object, visual observations do not bring out the same detail as
photographic or CCD images, but having the photograph available to reference during the
observation aids in seeing certain detail, which otherwise could be overlooked.
Table 1. Visual Observations
Designation
Coordinates
Date/Time1
mmddyyyyhhmm
Mv2
M51
11h 29m, 47° 11’
042220020440
052220020450
8.0
3
1
2
M66
11h 20m, 12° 58’
051420020351
042920020415
10.0
M82
9h 56m, 69° 40’
041920020530
042920020435
9.19
M83
13h 37’, -29° 52’
041920020630
042020020548
8.0
M104
12h 40m, -11° 37
042020020518
052420020450
9.50
All times Universal Coordinated Time.
Published magnitudes using Starry Night Pro Astronomy Software.
For quicker and more accurate identification of each galaxy, a graduated approach,
starting with the widest FOV eyepiece, and moving to the narrower FOV, was used.
Note also that the 32mm eyepiece is a super Plössl of good quality.
1.1.4. COMPARISON TO PUBLISHED
IMAGES: VISUAL
M51 was observed on two separate nights (Table 1). The seeinga condition was
logged as III with a note of a bright sky. As mentioned earlier, Chamberlin Observatory
is located a short distance from downtown Denver, therefore, light pollution is a constant
hindrance. M51, also known as the Whirlpool Galaxy, is an Sc class spiral galaxy
located in the constellation Canes Venatici, 8.5 Mpc (million parsecs) from Earth. The
galaxy is approximately 20 kiloparsecs in diameter, with an angular width of 0.21201°.
When compared to published images some spiral could be seen, but this was after
allowing the eyes to totally dark-adapt. The spiral arms of M51 contain hot, blue stars
and are star forming regions, but no color was visible. NGC 5195, the companion galaxy
to M51, was barely visible. I question whether or not it would have been noticed if not
for prior knowledge and a photograph to reference. (Universe 1999, Sky & Telescope,
Astronomy)
M66 was observed on two separate nights (Table 1). The seeing condition was
logged as III with intermittent haze. M66 is an Sb class spiral galaxy located in the
constellation Leo, approximately 10.7 Mpc from Earth. The galaxy has an angular width
of 0.14663°.
When compared to published images, no spiral could be seen. The 65mm lens was used
first to locate M66, then replaced with the 40mm and 32mm. The 40mm yielded the best
a
For visual observations, “seeing” is defined by the Antoniadi Scale of I - V, with I=perfect seeing and V=appalling.
4
view, allowing some nebulosity to be seen, but no detail structure. No color was visible.
(The Facts on File Dictionary of Astronomy, Starry Night Pro)
M82 was observed on two separate nights (Table 1). The seeing condition was
logged as III with haze on one of the nights. M82, also known as the Cigar Galaxy, is an
Irregular class galaxy located in the constellation Ursa Major, 3.5 Mpc from Earth. The
galaxy has an angular width of 10.5’. (Starry Night Pro)
When compared to published images, nebulosity could be seen. As with M66, the 40mm
eyepiece yielded the best view. No detail structure was observed, nor was color visible,
even though M82 is a starburst galaxy with prolific star forming regions. (Universe
1999, Astronomy)
M83 was observed on two separate nights (Table 1). The seeing condition was
logged as II-III, with intermittent clouds on one night. M83 is an SBb class galaxy
located in the constellation Hydra, 4 Mpc from Earth. The galaxy has an angular width
of 13.1’. (Starry Night Pro, The Facts on File Dictionary of Astronomy)
When compared to published images, nebulosity and spiral structure could be seen.
Details of the spiral were visible with the 65 mm eyepiece, but better views were possible
with the 40mm and 32mm. Some blue color was visible after the eyes were dark
adaptive. The most visible feature was the bright nuclear center. (Universe 1999,
Astronomy)
M104 was observed on two separate nights (Table 1). The seeing condition was
logged as II-III, with intermittent clouds on one night. M104 is an Sa class galaxy
located in the Virgo supercluster, 15 Mpc from Earth. The galaxy has an angular width
of 13.1’. (Starry Night Pro, The Facts on File Dictionary of Astronomy)
When compared to published images, nebulosity could be seen, as well as a bright core.
No spiral structure could be seen due to its edge on orientation to our line-of-sight. M104
was best viewed with the 65mm and 40mm eyepiece. No color was visible. (Universe
1999, Astronomy, Sky & Telescope)
M51 and M83 were the brightest of the five galaxies viewed.
1.2. CCD IMAGES
CCD images were made from atop Mt. Evans, Colorado, United States: 39°35’N
latitude, 105°38’W.
All CCD imagesc were made at the University of Denver’s (DU) Meyer-Womble
observatory atop Mt. Evans. The observatory is equipped with the dual-aperture
b
c
Starry Night Pro list M83 as a SB class spiral, while Facts on File Dictionary indicates Sc class.
Images are in JPEG format and are available electronically upon request.
5
binocular mounted 0.72 meter, Ritchie-Chretien telescopes, with the south scope fitted
with an SBIG ST7 CCD. More information on the Mt. Evans Observatory (most
commonly referred to name) is discussed in Section 4. (University of Denver’s
Observatories)
Images were taken over the last two years during my visits of August 7-11, 2000 and
August 13-17, 2001. The Mt. Evans Observatory has imaged objects fainter than 20th
magnitude. The high altitude puts the observatory above most of the atmosphere’s water
moisture. The 16 galaxies were chosen from more than a hundred CCD images captured
during the weeks I spent at the Mt. Evans Observatory. (Table 2)
Table 2. CCD Images
1
Designation
Coordinates1
Exposure Time
in Seconds (s)
Mv2
Angular
Width3
BL
Lacertae
22h 03m, 42° 17’
30s
--
--
NGC 6207
16h 43m, 36° 50’
15s
11.6
3.0’
NGC 672
01h 48m, 27° 27’
60s
10.8
6.60’
NGC 3115
10h 05m, -7° 44’
30s
9.19
3.0’x1.0’
NGC 157
0h 35’, -8° 23’
60s
11.0
4.1’
NGC 7723
23h 39m, -12° 58’
60s
11.1
3.6’
NGC 7331
22h 37m, 34° 25’
60s
9.5
10.7
NGC 7318
22h 36m, 33° 58’
60s
13.1
1.9’
NGC 253
0h 48m, -25° 17
5s
7.1
25.1’
NGC 470
1h 20m, 3° 25’
60s
11.9
3.0’
NGC 7253
22h 19m, 29° 24’
60s
14.0
0.0’
NGC 7436
22h 58m, 26° 09’
60s
14.0
0.0’
NGC 7479
23h 05m, 19° 19’
60s
11.0
4.1’
NGC 7549
23h 15m, 19° 02’
60s
13.9
2.8’
NGC 7640
23h 22m, 40° 51’
60s
11.8
10.0’
NGC 7678
23h 28m, 22° 25’
60s
12.2
2.3’
Coordinates are either from Digital Sky Survey or Starry Night Pro, and are
rounded up to the nearest minute in RA or arcmin in dec.
6
2
3
Published magnitudes using Starry Night Pro or Space Explorer II.
Angular Width values derived from Starry Night Pro Astronomy Software.
1.2.1. CCD IMAGES: COMPARISON TO
PUBLISHED IMAGES
Comparison images to Table 2 were obtained from primarily from the Digitized Sky
Survey. In addition, other sources were used such as Astronomy and Sky & Telescope
magazines, Starry Night Pro software and the textbook Universe.
The images I obtained from the Mt. Evans Observatory generally provided more detail of
spiral structure, and allowed processing of the images with software such as CCDSoft
from SoftWare Bisque. For example, in the DSS image of NGC 7678 seen in (A) below,
spiral structure can be seen as well as the nuclear core.
(A) NGC 7678, DSS
(B) NGC 7678, Mt. Evans Observatory,
University of Denver
Astronomy Program
The same structures can be seen in the Mt. Evans image (B), but the active star forming
regions in the spiral arms are clearly visible, as well as the two spiral arms.
When viewing the DSS images (C) of NGC 7318 (Stephan’s Quintet), five galaxies can
be seen, with some nebulosity. In image (D), distortion can be seen in four of the five
galaxies.
7
(C) NGC 7318, Stephan’s Quintet, DSS
(D) NGC 7318, Mt. Evans Observatory,
University of Denver
Astronomy Program
Without accurate distance measurements, it is difficult to tell if there is gravitational
interaction. Spiral structure is clearly visible in three of the galaxies, with some
similarity to bars in two. All five appear to exhibit a bright nuclear core suggesting
AGN.
One of the most intriguing images from Mt. Evans Observatory (although not realized at
the time), was BL Lacertae. At 280 Mpc, this is the most distant object I had ever
imaged. As explained in the textbook Universe, BL Lac is an ultraluminous galaxy,
referred to as blazer, which has an incredibly bight and active nuclei. BL Lac varies in
brightness by a factor of 15 over a period of several months.
(E) BL Lac, DSS
(F) BL Lacertae, Mt. Evans Observatory,
University of Denver
Astronomy Program
In the DSS image (E), BL Lac is in the center of the image. The image is unremarkable
in that it resembles an ordinary star field. In the Mt. Evans Observatory image, BL Lac is
8
the upper right star. Due to the distance and luminosity of BL Lac, no contour or shape is
distinguishable. Only from the research conducted in the 1970’s do we know BL Lac is
an elliptical galaxy. (Universe 1999)
2. THE TELESCOPES
2.1. UNIVERSITY OF DENVER’S CHAMBERLIN
OBSERVATORY
The University of Denver’s Historic Chamberlin Observatory was constructed in the
early 1890’s, with the telescope seeing first light in 1894. Alvan Clark and Sons, of
Cambridge, Massachusetts, United States, manufactured the 20-inch objective lens,
which was cast in France at a cost of $11,000. This is the same Alvan Clark who first
glimpsed Sirius B in 1862. Although the lens of the Clark 20” telescope is considered of
excellent quality, especially when it was manufactured over 107 years ago, it does exhibit
curvature at the edges. Under ordinary viewing situations, this presents no problem. The
lens also exhibits some coma, but this is confined to the edges. Near the center of the
field of view, there is little aberration. The University of Denver and the Denver
Astronomical Society installed encoders to the telescope in early 2001, and routed them
to a computer running The Sky astronomy software. This aided tremendously in
identifying the star field and locating the target galaxy. (University of Denver
Observatories)
The resolution of the Clark 20” is unknown; however, with the following measurements
made in February 2002 (Denver Astronomical Society). I believe the resolution is better
than 0.8”.
Primary Wave : 0.55
System Aperture : Entrance Pupil Diameter = 508mm
Eff. Focal Len : 7797.516 (in air)
Working F/#
: 15.34165
2.2. UNIVERSITY OF DENVER’S MEYER-WOMBLE
OBSERVATORY, MT. EVANS
The Mt. Evans Observatory is located atop Mount Evans at 4,303 meters elevation, 70
km west of Denver. This is the second highest operating astronomical facility in the
world; access is via a paved state highway that extends all the way to the summit,
although winter access is by snow machine only, usually for emergency maintenance.
The Mt. Evans Observatory consists of a dual-aperture binocular mounted 0.72 meter,
f/21 Ritchie-Chretien telescopes. The optics were fabricated from Zerodur by Contraves
USA, and each system has a measured total wavefront error <0.050 at 633nm. All optical
surfaces are coated with a multi-layer dielectric enhanced silver, providing high
reflectance from below 350nm to beyond 26æm.
9
The Meyer Binocular Telescope is of an equatorially-mounted, English yoke design. The
overall mass of the telescope is 9,000 kg with a moving mass of 4,100 kg.
For all CCD images referenced in this paper, the south tube of the Meyer Binocular
Telescope was used, fitted with an SBIG ST7 CCD, 9-micron pixels camera usually run
at -20C, and a 3.3x telecompressor. (University of Denver Observatories)
3. SUMMARY
The purpose of this project was to observe visually, photographically, or by a CCD,
galaxies within the capability of the telescope used and its ancillary equipment, and to
compare observations with normally published images and descriptions of those galaxies
as relating to surface brightness, visibility of spiral arms, etc. Through observations, I
was able to visually observe five galaxies and report on 16 CCD images of other galaxies
while Mt. Evans Observatory.
Visually acquiring and observing a galaxy is in itself rewarding. Looking through the
lens of the telescope and waiting until enough photons have gathered on the eye to see
structure in the spiral arms is gratifying. After acquiring the subject galaxy, I compared
what I saw to published images. I am constantly amazed at how much more detail I can
see when I have a photo in hand to reference. Every image in this paper was imaged
using a CCD.
In summary, while visual observations are important to the amateur astronomer, it is the
CCD chip which is the tool of the astronomical community for research. It far surpasses
the photographic plate, which itself once revolutionized astronomy.
Lessons learned from this and other observing projects: collecting the data through
observations is the fun and easy part. Analyzing and number crunching is where the
work starts!
4. References
Starry Night Pro Astronomy Software, V 3.0, Sienna Software, Inc. 1991-1999.
Space Explorer II Astronomy Software, V 2.1, 1997, Meade Instruments, Corp.
American Association of Variable Star Observers (AAVSO), http://charts.aavso.org,
http://charts.aavso.org/howtouse.stm, http://www.aavso.org/links.stm
Variable Stars, Mickle R., HET603A, No. 20 Variable Stars
Sky & Telescope Magazine, January 2002.
Astronomy Magazine, March, April & June 2002.
Dickinson, T. & Dyer, A. 1995, The Backyard Astronomers Guide, pp. 150.
Kaufmann & Freedman 1999, Universe, , pp. 624, 641, 657, 684, 690.
Denver Astronomical Society, Measurements on Clark 20”, February 2000,
http://www.du.edu/~rstencel/Chamberlin/20inlens.dat
Facts on File Dictionary of Astronomy, The 1994, Third Edition, pp. 9-10, 260, 286, 422-424.
10
University of Denver’s Observatories, http://www.du.edu/~rstencel.
Astrophysical Journal, The University of Chicago via Swinburne University.
Software Bisque, The Sky Astronomy Software, Level IV, Version 5.00.012 http://www.bisque.com/
MacMillan Online Encyclopedia of Astronomy and Astrophysics - Guest Access, http://www.encyastro.com/
Digitized Sky Survey, The copyright © 1994, Association of Universities for Research in
Astronomy, Inc., http://archive.stsci.edu/dss
University of Denver, Chamberlin Observatory, http://www.du.edu/~rstencel/Chamberlin/
ACKNOWLEDGEMENTS
This paper was prepared by the author as part of the curriculum requirement of
©Swinburne Astronomy Online (SAO). Thanks to Melissa Hulbert (SAO) and Joanie
Mickle for editorial comments. Special thanks to Robert “Dr. Bob” Stencel, Director of
Observatories, University of Denver, Department of Physics & Astronomy.
11