Limiting magnitude areas - telescopic and naked-eye - for northern hemisphere observers Rev. 8/6/2006

Go to: 44 Northern NELM Fields 34 Northern TLM Fields Extinction correction factors Bortle Dark-Sky Scale (External link) Schaefer TLM Calculator (External link) Sky Quality Meter (External link) NELM to B Calculator TLM and NELM area database for Cartes du Ciel Excel worksheet

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:

  1. 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.
  2. Table 1 - Clark and RASC Charted Open Cluster Telescopic Limiting Magnitude Areas
    Cat_Id_________ J2000_Position Con HR_HD Comments___________________________________________________
    NGC225 J004342.00+614648.0 Cas Clark note: 20 stars mag 9+
    NGC1647 J044554.00+190636.0 Tau Clark note: 25 stars mag 8 to 13; OMeara10
    NGC2129 J060106.00+231836.0 Gem Clark note: about 50 stars
    NGC2422 J073636.00-142848.0 Pup
    RASC M67 TLMA J085006.00-115300.0 Cnc Northwest quadrant of M67; mag range: 10.6-21.3; coordinates per HEARSAC
    NGC6494 J175700.00-185848.0 Sgr
    NGC6823 J194309.60+231724.0 Vul Clark note: 30 stars mag 11+
    NGC6910 J202307.20+404612.0 Cyg Clark note: 40 stars mag 10+
    NGC7031 J210712.00+505248.0 Cep 50 stars mag 11+
    NGC7235 J221224.00+571536.0 Cep about 25 stars


    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

    RASC. 2005. RASC Observing Handbook (Annual)

  3. 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.
  4. 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.
  5. A paint program or a copier with a reversing feature can be used to make single or double reversed copies of your charts that will be easier to use at the eyepiece.
  6. 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:
  7. Table 2 - Sample table of predicted telescopic limiting magnitudes for a 10 inch Newtonian telescope at the zenith - assumes 50 year old expert observer on stars with color index = 0
    EyepiecePowerNELM 5.1 5.6 6.1 6.6 7.1 7.6
    17mm68x 14.3 14.7 15.1 15.4 15.9 16.4
    10mm114x 14.9 15.3 15.6 15.8 16.2 16.7
    6mm200x 15.3 15.7 15.9 16.1 16.5 17.0
    6mm 2x Barlow488x 16.0 16.2 16.2 16.3 16.7 17.2
    Bortle 654321

    Optionally, graph Table 2 in the following forms:

    Figure 1 - Effect of applying increasing magnification to a 10 inch telescope on TLM under a mag 6.1 sky

    Figure 2 - Schaefer TLM curves for a 10 inch telescope - various sky brightnesses and constant observer and star characteristics

    To aid in preparing these estimates, an Excel worksheet is provided that incorporates a Schaefer limiting magnitude calculator.

  8. Determine which naked-eye limiting magnitude ("NELM") field, listed in Table 6, will be nearest your zenith and your open cluster TLM field.
  9. 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:

  1. Estimate and record the naked-eye sky brightness near your TLM field (Table 1) from the NELM area you identified from in Table 6.
  2. Estimate and record the number of degrees from your zenith your naked-eye limiting magnitude field.
  3. 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.
  4. 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:

  1. 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.
  2. 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
    Adjusted NELM magnitude
    Adjusted V-band magnitude
  3. Adjust your readings for extinction using Table 5.