Quick start for finding Telescopic Limiting Magnitude ("TLM")
The following is a basic procedure. Alternative, but more labor intensive methods, are detailed in the section titled "Discussion".
Before you go into the field:
From Table 1 determine which open cluster will transit or be at a 60 degree or higher altitude above your local horizon during your planned observing frame.
Go to a local university library and obtain R.N. Clark's chart of that open cluster(s) from Appendix C of his book Visual Astronomy of the Deep Sky. Alternatively, go online and prepare a facsimile chart using Jean Mermilliod's Webda Open Cluster Database. From the Webda homepage, click Navigation | Enter the NGC designation in the Display the Page box | Select Query - From cluster chart (plotted). Once plotted, the chart will look like Figure 19.
Make one copy of your open cluster chart for a range of eyepieces in your equipment. Label each chart copy with an the eyepiece focal length.
Using an online Schaefer TLM Calculator prepare a table of predicted telescopic limiting magnitudes in the form of Table 2 for your telescope and your personal characteristics:
To aid in preparing these estimates, an Excel worksheet is provided that incorporates a Schaefer limiting magnitude calculator.
Determine which naked-eye limiting magnitude ("NELM") field, listed in Table 6, will be nearest your zenith and your open cluster TLM field.
Go to the Nine Planets website or the International Meteor Organization website and printout the NELM finder chart for your naked-eye limiting magnitude area and the star count table that you will use to estimate sky brightness.
On the observing night, do the following:
Estimate and record the naked-eye sky brightness near your TLM field (Table 1) from the NELM area you identified from in Table 6.
Estimate and record the number of degrees from your zenith your naked-eye limiting magnitude field.
Estimate and record the number of degrees from your zenith to the open cluster from which you will take a TLM reading with your telescope.
Using your telescope and your charts of the open cluster that are labeled and paired with your eyepieces, start with the lowest magnification. Circle and letter the faintest stars visible on the chart. "X"-mark stars that are not visible. "S"-mark those stars you cannot separate because they are too close together. Record the faintest star visible on your chart for that eyepiece. Then repeat, working your way down to your shorter focal-length, higher magnification eyepieces.
After the observing night, reduce your data:
Go back online to the Webda database chart. Confirm the photometry of your limiting magnitude decision stars. If the limiting magnitude stars you circled are not O, B or A, you may need to reevaluate their true Johnson V band limiting magnitude using a Schaefer TLM Calculator.
Table 3 - Table for recording measurements
Observation_Id__________________
________1________
________2________
________3________
________4________
________5________
Date UTC
Op description
Telescope description
Observer description
Eyepiece fl
NELM Area
NELM Area Zenith Distance
TLM Cluster
TLM Cluster Zenith Distance
Extinction factor
Star Chart Id
Webda Id
Chart V-band magnitude
Color index
Spectral class
Notes
Magnification
Adjusted NELM magnitude
Adjusted V-band magnitude
Adjust your readings for extinction using Table 5.
Amateur astronomer asks to the question "what's your limiting magnitude
?" in order to:
Communicate the condition of parts of the night sky to other amateurs;
Record the condition of the night sky for their observing records;
Predict the visibility of objects either for visual observing or astrophotography;
Measure the performance of their eyes without telescopic assistance; or,
Measure the performance of their telescopes when employed as an aid to enhance or extend human vision.
The ability to measure the performance of one's eyes and of one's telescopes under varying night sky conditions
is a basic amateur astronomy skill. It's part of getting know your new scope.
Because performance of the telescope depends on the background sky brightness, the beginning step for measuring the performance of your telescope is to find the naked-eye-limiting magnitude of the sky near your object-of-interest. NELM is an indirect estimate of sky brightness.
Source and photo credits: Night Sky Team, Canyonlands National Park, U.S. National Park Service. 200_. All Sky Night website. http://www.nps.gov/cany/nature/allskyimages.htm accessed Aug. 2006
In part because of these common light pollution variations in North American night skies, in 2001, John Bortle developed the Bortle Dark Sky Scale:
The Bortle scale is very useful for communicating the general nature of totally or partially light polluted skies, but it is not used as a parameter in determining telescopic limiting magnitude performance.
The human eye's response to a star's color index effects the visual determination of NELM and TLM
Naked-eye limiting magnitude and telescopic limiting magnitude are measured by observing stars. A star's MK spectral class, as measured through its color index (B-V), effects your measurement of NELM or TLM. The eye has many more faint-light sensitive rods that are used for dark-adapted scotopic vision than light-adapted photopic cones (120M rods v. 6-7M cones). Furthermore, the more numerous rods are more efficient than cones in the blue-white wavelength around 507nm. As a result, brain interprets light from blue and white stars as being brighter than a photometer measuring in the V-band would report. Conversely, red color index stars are reported to the brain by the human eye as fainter than would be measured by a photometer. This is also known as the Purkinje effect. As the brightness of a scene decreases, the brighntess of red colors decreases faster than the brightness of blue colors.
The eye over reports the V-band magnitude of an O,B and A star. Stated conversely, the brightness of red color K and M stars is underreported by the eye. The one-magnitude range of this effect shown in Figure 8 explains why for two stars shown on charts with the same apparent V-band brightness, one star may be invisible to the eye while the other is not. Rough compensating factors for a stars color index are shown in Figure 8.
Best practices when measuring NELM and TLM includes trying to use, whenever possible, stars of a similar spectral class, e.g. - O,B and A stars, in order to obviate the eye's differing sensitivity to color. When making measurements determine the spectral class of the star that defines, in particular, your TLM measurement, so you are not up to a magnitude off. When selecting stellar fields by which you will measure TLM, try to use fields with an relative abundance of O,B and A stars.
To visually determine naked-eye limiting magnitudes or telescopic limiting magnitudes is less a question about "how" and is more of question about
"where" to look. Visual determination of naked-eye limiting magnitude and telescopic limiting magnitude involves finding a suitable pre-measured star field as close to the zenith, in the case of determining a night sky's zenithal limiting magnitude ("ZLM"), or as close to the object-of-interest as possible. When measured off-zenith near and object-of-interest, ZLM is called NELM.
To make the measurement for the naked-eye, one needs a wide stellar field with pre-determined standard magnitudes where the stars are in the range of 2 to 6.5 magnitudes and have narrow step intervals between stars. If the minimum interval of available steps between stars in the limiting field is 0.3 or 0.4 magnitudes, which is the maximum accuracy for your estimate. The International Meteor Organization ("IMO") has pre-measured the magnitudes of star fields suitable for northern hemisphere observers. The International Meteor Organization limiting magnitude fields have very finely 0.1 magnitude stepped fields - the best visual accuracy available.
Table 6 is a consolidated list of naked-eye limiting magnitude fields for north hemisphere observers and includes a description of the naked-eye limiting magnitude fields of the IMO and from other major sources. However, the IMO's limiting magnitude field charts are a simpler and easier way to view the location of these fields.
Use of these NELM fields is self-explanatory. Print and download the charts and limiting magnitude tables. The Nine Planets website has a complete list of charts of the IMO fields along with tables showing the magnitude of stars by count for each field. The Nine Planets website is recommended over the IMO website for printing hardcopies of IMO limiting magnitudes charts and tables. After assembling the charts and tables into an observing book and becoming familiar with main fields applicable to your favored observing points, stepping out of your car and looking up to determine the ZLM or NELM on a particular night becomes second nature.
Visual determination of ZLM and NELM has an accuracy of about 0.2 magnitudes for average observers - assuming that the available minimum stellar magnitude steps in your limiting magnitude field are 0.1 or 0.2 magnitudes. A magnitude field that has steps of 0.5 or 1.0 magnitudes cannot accurately measure magnitudes in smaller steps.
The traditional method of finding ZLM or NELM from a star chart suffers from this accuracy limitation. Rarely does a star field align itself near your local zenith that has a sufficient number of steps between stellar magnitudes to permit an accurate measurement of ZLM.
and then adjust the off-zenith but more accurate NELM reading for atmospheric extinction. Adjusted (or reduced) for atmospheric extinction, your off-axis NELM yields an estimate of ZLM.
As a supplemental aid in finding the IMO NELM areas in a planetarium program, markers for the IMO Limiting Magnitude Areas have been ported to a Cartes du Ciel compatible external database - tlmnelm.zip (8kb).
Photoelectric determination of NELM
Visual determination of ZLM has been largely supplanted in modern amateur practice by a consumer device - Welch and Tekatch's Sky Quality Meter. The Sky Quality Meter is small hand-held device that examines a 60 degree cone of the sky around the zenith and that reliably reports an "average" brightness for that area of the celestial sphere.
The Sky Quality Meter reports in the B (MPSAS) scale, not the magnitude scale. SeeFigure 3, above. Equations to convert magnitudes NELM (V) to MPSAS (B) and B back to V are given in the Math Appendix, or use the provided NELM to B Calculator
The Sky Quality Meter does not completely replace the older and less accurate method of visual determination of NELM. The Sky Quality Meter is designed to measure a set cone of the sky around the zenith. It cannot be used off zenith or describe small areas near the horizon. Many of dark skies now available to northern hemisphere observers are compromised by light pollution. One part of the horizon may have a 4.5 magnitude sky and opposite side a 6.2 magnitude hole. The Sky Quality Meter cannot accurately estimate such skies.
For skies with significant light pollution caused variations in sky brightness, the visual NELM method will remain the method of choice for amateurs.
Adjust the NELM measurement for extinction
Raw chart and catalogue values, such as those plotted from the Tycho-2 catalogue and the Hipparcos Mission, need to be adjusted for atmospheric extinction.
A simple table of extinction correcting values is provided in Table 5, below. Depending on the distance to your local zenith to your NELM limiting magnitude area, stars will be fainter than the catalogue value listed above due to atmospheric extinction. In general, add the Table 5 correction value to your NELM measurement.
Telescopic limiting magnitude
To determine your visual telescopic limiting magnitude,
you need to find a star field that contains a good selection of stars near your telescope's limit. To select a good star field, you first need some prediction or estimate of what the limit will be. Predicting telescopic limiting magnitude cannot be divorced from the questions of the background brightness of the night sky or the from the magnification applied by the observer.
Pre-estimating your TLM in order to select a good star field
Historically, TLM
has been estimated using variations on the Steavenson-Sigdwick light-grasp model:
TLM = Background Sky Brightness + Light Grasp of the Eye + Light Grasp of the Telescope {Eq. 1}
TLM = 6.5 - 5*Log10(eye pupil size_in) + 5*Log10(Telescope Aperture_in) {Eq. 2}, assuming a 6.5 ZLM and simplifying for an assumed 0.3 inch eye pupil
TLM = 9.1 + 5*Log10(D) {Eq. 3}, where D is the aperture of the scope in inches.
There are many published variations of this light grasp rule based on measuring the eye pupil and telescope aperture in meters, centimeters or millimeters:
TLM = 16 + 5*Log10(D_m) {Eq. 4}, same as Eq. 3 in meters (after Kitchin 2003 at 168)
TLM = 7.5 + 5*Log10(D_cm) {Eq. 5}, same as Eq. 3 in centimeters
TLM = 1.8 + 5*Log10(D_mm) {Eq. 6}, same as Eq. 3 in millimeters.
The simple light-grasp model does not account for the application of magnification and it assumes a uniform sky brightness of 6.5 mags. Astronomers of the late 1800s and early 1900s realized the inadequacy of the model. Referring to Steavenson's data from the late 1800s, Sidgwick noted that, "these curves do not agree well with the results of observations, yielding results which are consistently high by about 1.5 mag through the aperture range from 2 to 20inch." Sidgwick (1971 3d at 27).
The "missing" 1.5 magnitudes results from the application of increasing magnification from the lowest useable (3.7x per inch of aperture) to extreme magnification (75x per inch of aperture). The same telescope applied at a higher magnification sees "deeper" than the same telescope used at a lower magnification, as shown Figure 1, above, for a 10 inch telescope and in the following graph for various aperture sizes:
The simple light grasp model does not consider the response of the human eye to faint light.
Spurred by the need to spot planes and ships against the horizon at dusk, World War II advanced research into the ability of the human eye to see faint stellar and extended objects against backgrounds of varying brightness. Knoll (1946), Hecht (1947) and Weaver (1947) characterized the response of the human eye to stellar points against bright backgrounds. (Blackwell (1946) examined the eye's ability to see extended objects (patches) against varying levels of brightness. Here, we are only concerned with stellar points.)
Between 1940 and the 1990s, a new problem arose for the modern amateur that Sidgwick and Steavenson did not face - light pollution. Between the 1940 and 1999, the United States population increased from 131.7 hundred million in 1940, to 248.7 hundred million persons in 1990 and to 272.9 million persons in 1999. The average density of United States residents increased from 16.9 persons per square mile in 1880, to 44.2 persons per square mile in 1940, to 70.3 persons per square mile in 1990, and to 77.1 persons per square mile in 1999 - a 74% increase over 1940. (U.S. Statistical Abstract of the United States, 2006 online edition). Electric lighting came into widespread use in this era of increasing density. Population density is correlated with light pollution.
Increasing light pollution spurred professional research to characterize the background brightness of the sky as population encroached on major observatory sites. Garstang (1989). The best skies around observatories have a sky brightness of about 21 or 22 MPSAS. Cinzano (2001a, 2001b) expanded Garstang's light-pollution sky-brightness model into space-based satellite imagery of the Earth at night. This led to many of the tools that amateur astronomers use everyday to predict whether the sky will be good for a particular night, including the ClearSky Clock, the Meteorological Service of Canada's North American Seeing Forecast and the International Dark Sky Society Dark Sky Map.
In the late 1980s and early 1990s, then NASA/JPL research Bradley E. Schaefer synthesized these developments into a new model for telescopic limiting magnitude - one that considered applied magnification, light-grasp (aperture), the physiological ability of the human eye to see faint point sources, and background sky brightness. Schaefer (1990). This improved model predicting telescopic limiting magnitudes is called the Schaefer TLM algorithm or in this note - "Schaefer curves".
The Schaefer TLM algorithm is mathematically ponderous. Schaefer algorithm models star brightness based on the product of a series of corrective factors:
Using the modern Schaefer TLM model, you can graph your scope's expected performance either for one expected NELM sky brightness, as shown in Figure 1, above, or in a broader form that records expected telescope performance for varying sky brightnesses, as shown in Figure 2, above. The following charts also show this broader form of graphic presentation. They were prepared for an amateur 30" aperture telescope and binoculars from data organized as shown in Table 2 and plotted using a spreadsheet program. The charts show the effect of both background sky brightness and applied magnification on the limiting magnitude of a telescope.
Telescopic limiting magnitude usually refers to the faintest stellar point observable using averted vision at the highest applied magnification useable on the telescope. But in modern practice, each TLM measurement is limited to and paired with the stated ZLM sky brightness under which the TLM measurement was made.
To practically apply these techniques, using a 10" scope as a working example, if you are going to dark sky site and reasonably know it will be a magnitude 6.1 sky, and you usually use at most a 6mm eyepiece at 200x, then Figure 1 and Table 2, above, indicate that you need to find an open cluster or other telescopic limiting magnitude field that at least maps down to magnitude 15.9. If you are going to site with variable brightness that may be anything between magnitude 5.1 to magnitude 6.6, referring to the broader TLM chart form like Figure 2, above, will give you the needed TLM estimate.
Now that you know who deep the field needs to be, you can start looking for a suitable TLM field to use on your observing night.
Selecting a good stellar field to measure TLM
To make the visual measurement of telescopic limiting magnitude, one needs a stellar field that:
Will comfortably fit in the true-field-of-field of an eyepiece;
Has a pre-determined standard magnitudes where the stars are in the range of 9 to 16 magnitudes depending on the aperture of your telescope or binocular;
Generally has O,B,A type stars of a similar color index;
Have tight interval steps between the stars of a similar color index. Ideally, you would want 0.1 to 0.2 magnitude interval steps between the available O,B and A stars in the measuring field; and,
The available stars need to be spaced far enough apart that they do not merge and their brightness integrates together in the eyepiece.
Few stellar fields meet all these requirements.
Often the available step intervals between stars are greater than 0.5 magnitudes. This means the maximum visual accuracy that you can obtain using that TLM field cannot be lower that 0.5 magnitudes. The human eye sees stars with a red color index (G,K,M,N,S) fainter than stars with a whiter color index (O,B,A,F). If you field has mixed spectral types, care needs to be taken that you have not overestimated the faintest star magnitude because it is a red color index star.
For pre-measured visual TLM fields, there are three primary sources: 1) 9 open clusters measured by R.N. Clark in Appendix C to his 1990 book Visual Astronomy of the Deep Sky (plus a 10th cluster, M67, charted in the RASC's Observer's Handbook), 2) ZLM charts generated by planetarium software, and 3) Landolt's 24 photometry star fields spaced along the celestial equator.
Finding TLM using Clark open cluster stellar fields
Table 7 is a list of telescopic limiting magnitude areas. The NGC objects in the list are Clark's 9 charted open clusters with a 10th plot for M67 in the Royal Astronomical Society of Canada's annual Observer's Handbook. This amateur author has also charted NGC1647 in detail to magnitude 13 for use with smaller scopes.
Clark's charted open clusters generally go down to magnitude 15 or 16. For persons living in rural areas travel to a major university that will have a copy of Clark's out-of-print text is not practical. However, facsimiles of Clark's open cluster charts can be prepared using Jean Mermilliod's online Webda Open Cluster Database at the University of Vienna. The Webda interface quickly plots a cluster chart by a limiting magnitude cut-off value for any open cluster with an NGC number.
From the Webda homepage, click Navigation | Enter the NGC designation in the Display the Page box | Select Query - From cluster chart (plotted).
reasonable facsimiles of Clark's open cluster charts can be prepared using Jean Mermilliod's online Webda Open Cluster Database at the University of Vienna. The Webda interface quickly plots a cluster chart by a limiting magnitude cut-off value for any open cluster with an NGC number.
From the Webda homepage, click Navigation | Enter the NGC designation in the Display the Page box | Select Query - From cluster chart (plotted).
Finding TLM using planetarium program generated ZLM charts of stellar fields
The traditional method of finding TLM is to use a star chart. Modern planetarium programs extend this tradition by allowing the easy generation of user prepared charts. Cartes du Ciel is one such program.
There are three limitations of the traditional charting method in the modern planetarium program age.
First, you must have the Hubble GST and Tycho-2 or the USNO A2 catalogues installed in your planetarium program to generate charts down to a limit of magnitude 15. These catalogues are not easily downloaded since they can exceed 500mb in size.
Second, in the post-2000 era of low-cost DOBs with apertures larger than 10 inches, amateur telescopes now frequently reach below magnitude 15 at their highest useable magnification. Even with the USNO A2 reduced catalogue installed, you may not be able to generate charts that reach a sufficient depth.
Third, even where you can generate a chart, rarely will there be a sufficient number of stars in the field of the appropriate 0.1 or 0.2 magnitude intervals within the O,A and B spectral classes and around your predicted TLM.
The zenith chart method has the attraction in that a TLM measuring field can always be generated in or near the zenithal hole. You do not have to wait for a charted open cluster to transit at a sufficient altitude.
Even if you do not have a planetarium program, useable zenith TLM measuring charts can be generated using the CDS Strasburg Simbad web application.Figure 24 is a sample chart generated using Simbad:
When the Simbad Query Result screen displays, under the section labeled "Plot and Image Tools", use "Query and Plot" to plot a chart equal to the size of your TFOV. Simbad provides options for limiting the display to stars only.
Right-click the Simbad generated chart to download the chart to your local computer.
While connected to the Simbad chart, you can click on any star to get more information.
Finding TLM using stellar fields in Landolt photometry areas and Skiff's LONEOS catalogue
For smaller scope owners whose limiting magnitude is between 9 and 12 magnitudes, Landolt's 24 pre-measured photometry fields provide another option for measuring TLM. In the 1970s, Landolt performed photo-electric measurements on 226 stars in charts 24 areas spaced at about 15 degree intervals along the celestial equator. Unlike open clusters, Landolt fields are just collections of fortuitously optically aligned stars. Initially, Landolt's photometry fields ranged from magnitude 9 to 12. In the 1990s, approximately another 500 stars were measured in those same fields, extending some fields down to magnitude 16. Online catalogues of stars in the Landolt fields are available through databases at CDS-Simbad (1973, 1983 and 1993) and at the University of Hawaii.
Landolt fields suffer from limitations similar to planetarium program generated zenithal charts. Most Landolt fields do not contain a sufficient number of O,B and A stars of a close intervals to make accurate TLM measurements.
The depth and charting clarity of Landolt fields can be improved and supplemented with 34,000 stars in Brian Skiff's (Lowell Observatory) LONEOS photometry catalogue. This catalogue compiles about 34,000 stars with known photometry from around magnitude 12 to 19 for use in the Lowell Observatory Near-Earth Object Search (LONEOS). Skiff's catalogue incorporates Landolt's photometry stars. A Cartes du Ciel external catalogue of Skiff's catalogue is available for download from Cartes.
Table 7 provides a list of 24 of Landolt's stellar photometry fields from his 1973 paper. Where the brightest central star of Landolt's field is a star with a Henry Draper catalogue designation, the HD number is also listed in Table 7.
As a supplemental aid in finding the Landolt areas in a planetarium program, markers for the Landolt celestial equator photometry fields have been ported to a Cartes du Ciel compatible external database - tlmnelm.zip (8kb). The Landolt area markers can be supplemented by adding Skiff's LONEOS catalogue of 34,000 stars to Cartes du Ciel. Once loaded, charts similar to Figure 26 can be generated.
Even if not used as TLM fields, intermediate amateurs interested in astrophotography and photometry should become familiar with Landolt's fields. They are used to calibrate the photometry readings in your CCD camera software.
Adjust the TLM measurement for extinction
As with NELM measurements, apply the simple table of extinction correcting values in Table 5, below, to adjust the catalogue value of the star's apparent brightness for extinction.
An International Comet Quarterly table (Green 1992) provides rough
TLM and NELM correcting values for atmospheric extinction based on the kilometers of
the observing point above sea level:
Table 5 "Average" Atmospheric Extinction in Magnitudes for Various
Elevations Above Sea Level (h, in km) (Excerpts from Green 1992)
The "z" value is the degrees from zenith to the celestial object. So
z=80 is 10 degrees altitude above the local horizon.
For example, if the "V" band catalogue magnitude of your comparison
star is 7.5, the star is located 30 degrees above the horizon (or 60
degrees from the zenith), the observer is at sea level, then the true zenithal brightness of the comet is v7.5 and its
apparent brightness is v6.9 (7.5-0.56).
Practical measurement of telescope/binocular performance using NELM and TLM readings
The evolution of the modern Schaefer TLM algorithm along with increased light pollution variations in sky brightness experienced by modern amateurs, suggests three methods for examining the telescopic limiting magnitude of a telescope and of comparing telescope performance against TLM model predictions.
The first traditional method, shown in Figure 1 above, involves using the highest useable magnification under the best possible dark sky to measure the faintest possible star.
The second method, also shown in Figure 1 above, involves using a range of eyepieces under a single sky brightness condition to see if the overall performance of the telescope follows the predicted Schaefer curve.
The third method, shown in Figure 2 above, involves accruing TLM data in a variety of sky brightness conditions to see if the telescope performs as predicted as shown in the various Schaefer curves in Figure 2.
This third option is well-suited to the modern urban-suburban amateur.
To aid in recording TLM & NELM measurements, an Excel worksheet is provided that incorporates a Schaefer limiting magnitude calculator.
Mag range: 10-13; N=12; Landolt1992 adds 41 stars to mag 16; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
SA93
J015450.16+004658.8
Psc
Mag range: 9-12; N=29; Central star is BD+00 307. HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
SA94
J025556.16+003057.6
Cet
Mag range: 6-13; N=31; Landolt1992 adds 7 stars to mag 14; Central star is BD-00 617. HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
SA95
J035415.12+001720.4
Eri
HD024537
Mag range: 8-13; N=26; Landolt1992 adds 44 stars to mag 16; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
NGC1647
J044554.00+190636.0
Tau
Clark note: 25 stars mag 8 to 13; OMeara10
SA96
J045250.40+000701.2
Ori
HD031073
Mag range: 6-12; N=34; Landolt1992 adds 6 stars to mag 13; HR1574 is v5.97 star in field.HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
SA97
J055725.92+000140.8
Ori
HD040210
Mag range: 7-12; N=30; Landolt1992 adds 7 stars to mag 14; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
NGC2129
J060106.00+231836.0
Gem
Clark note: about 50 stars
SA98
J065209.60-001742.0
Mon
HD050209
Mag range: 8-13; N=30; Landolt1992 adds 46 stars to mag 18; HR2530, v5.78, is bright star in field. HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manually corrected declination plotting sign error.
NGC2422
J073636.00-142848.0
Pup
SA99
J075440.80-003722.8
Mon
HD064605
Mag range: 8-12; N=29; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manual correction of coordinate sign
RASC M67 TLMA
J085006.00-115300.0
Cnc
Northwest quadrant of M67; mag range: 10.6-21.3; coordinates per HEARSAC
SA100
J085357.60-003643.2
Hya
HD076082
Mag range: 8-14; N=31; Landolt1992 adds 6 stars to mag 13; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manually corrected declination plotting sign error.
SA101
J095638.40-002739.6
Sex
HD086135
Mag range: 8-12; N=31; Landolt1992 adds 35 stars to mag 16; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manually corrected declination plotting sign error.
SA102
J105524.00-004846.8
Leo
HD094616
Mag range: 8-12; N=29; HR4245, v6.3, is bright star in field. HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manually corrected declination plotting sign error.
SA103
J115500.00-003321.6
Vir
HD103486
Mag range: 8-12; N=28; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manual correction of coordinate sign
SA104
J124304.80-003216.8
Vir
HD110572
Mag range:8-12; N=21; Landolt1992 adds 34 stars to mag 16; galaxy in FOV is probably NGC4632; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manually corrected declination plotting sign error.
SA105
J133745.60-003730.0
Vir
HD118579
Mag range: 7-12; N=47; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manual correction of coordinate sign
SA106
J144138.40-002558.8
Vir
Mag range: 8-14; N=31; central star is BD+00 3224; galaxy NGC5719, Bmag 13.1 is in field; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
SA107
J153900.00-001839.6
Ser
HD139590
Mag range: 6-12; N=32; Landolt1992 adds 28 stars to mag 16; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manually corrected declination plotting sign error.
SA108
J163719.20-002446.8
Oph
HD149845
Mag range: 8-13; N=33; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manual correction of coordinate sign
SA109
J174450.40-000802.4
Oph
HD161304
Mag range: 9-14; N=17; Landolt1992 adds 7 stars to mag 14; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field; manually corrected declination plotting sign error.
NGC6494
J175700.00-185848.0
Sgr
SA110
J184216.80+000918.0
Aql
HD172829
Mag range: 7-12; N=17; Landolt1992 adds 39 stars to mag 16; HD172651, v7.4, is bright star in field. HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
SA111
J193821.60+002042.0
Aql
HD185297
Mag range: 7-13; N=20; Landolt1992 adds 8 stars to mag 13; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
NGC6823
J194309.60+231724.0
Vul
Clark note: 30 stars mag 11+
NGC6910
J202307.20+404612.0
Cyg
Clark note: 40 stars mag 10+
SA112
J204221.60+002642.0
Aqu
HD197232
Mag range: 9-12; N=20; Landolt1992 adds 7 stars to mag 12; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
NGC7031
J210712.00+505248.0
Cep
50 stars mag 11+
SA113
J214226.40+002642.0
Aqu
HD206488
Mag range: 7-12; N=18; Landolt1992 adds 42 stars to mag 15; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
NGC7235
J221224.00+571536.0
Cep
about 25 stars
SA114
J224221.60+004612.0
Aqu
HD215044
Mag range: 7-12; N=26; Landolt1992 adds 9 stars to mag 12; HD215129, v. 6.9, is bright star near in field; galaxy PGC0069505, Bmag 15.5, is 1 deg s.w. of central star. HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
SA115
J234314.40+005414.4
Psc
HD222733
Mag range: 8-13; N=19; Landolt1992 adds 7 stars to mag 12; HD col is central star other name; common name is Landolt central star id; either is region marker; Bayer or Flamsteed, if present, is bright star adjacent to field.
NELM=7.93-5*log(10^(4.316-(Bmpsas/5))+1) {Eq. 9} per Olof Carlin (1990)
In order to be visible, an extended object, like a galaxy or nebula, has to have a surface brightness brighter than the background sky brightness. A list of the brightness of approximately 600 deep sky objects in both the magnitude and MPSAS scales is available in Clark's Visual Astronomy of the Deep Sky. Clark lists the MPSAS and integrated magnitude for 616 common DSOs in Appendix E to the Visual Astronomy, available online at http://www.clarkvision.com/visastro/appendix-e.html.
L = is the response of the human eye to light
F_b = transforms the binocular eye perception equation of Knoll (1946) and Hecht (1947) to monocular viewing
F_e = extinction factor
F_t = transmission factor of the telescope
F_p = correction factor for the observer's exit pupil size based on the observer's age
F_m = correction factor for the dimming of the image resulting from magnification
F_r = extended object size correction factor
F_sc = correction factor for the Stiles-Crawford effect in the human eye
F_c = correction factor for the color of the star observed
F_s = correction factor for the experience of observer
Acknowledgements
Centre de Données astronomiques de Strasbourg - Simbad: This note has made use of the SIMBAD database, operated at CDS, Strasbourg, France.
Centre de Données astronomiques de Strasbourg - Catalogue Service: This note has made use of catalogues redistributed through the CDS Astronomer's Catalogue Service, operated at CDS, Strasbourg, France.
NASA Astrophysics Data System/Computation Facility at the Harvard-Smithsonian Center for Astrophysics - NASA ADS Abstract Services: This note has made use of NASA's Astrophysics Data System.
Webda: This note has made use of the WEBDA database, operated at the Institute for Astronomy of the University of Vienna.
Clark, R.N., 1990. Star Clusters for Finding Your Limiting Magnitude. Appendix C in Visual Astronomy of the Deep Sky, Cambridge University Press and Sky Publishing.http://www.clarkvision.com/visastro/
Olof Carlin, Nils. About Bradley E. Schaefer: Telescopic limiting Magnitudes . . . . Web page discussion of brightness in Schaefer (1990) and Clark (1994). http://w1.411.telia.com/~u41105032/visual/Schaefer.htm (accessed 7/2003)
