Some Quick and Sloppy Procedures

Bruce L. Gary, Hereford Arizona Observatory (G95)
Last updated 2005.01.09

This web page was inspired by the realization that astrometry observations reported to the Minor Planet Center often include brightness estimates that are typically too faint by 1 or 2 magnitudes (for the faint asteroids). I'm accustomed to sweating long hours to produce photometry sequences with an accuracy of 0.03 magnitude, which is about 2 orders of magnitude better than these MPC submissions. Surely, I told myself, by deleting a few terms from my complete equations it should be possible to achieve an accuracy on the order of 0.15 magnitude. Indeed, if this accuracy is acceptable the "color terms" can be dropped, atmospheric extinction can be treated carelessly, subtle considerations in choosing photometry aperture sizes can be ignored, and a host of other details that consume the bulk of effort can be neglected. This web page is meant to guide the astrometrist along a path leading to quick and easy photometry results with accuracies no worse than 0.15 magnitude.

My only assumptions are that you 1) know how to measure a star's "intensity" (flux, etc) using an image processing program (such as MaxIm DL), 2) have a planetarium program for display of star fields and star magnitudes (such as TheSky 6.0), and 3) for some of the more advanced procedures that you are able to use a spreadsheet for simple tasks. I'm going to also assume that you're working with an asteroid (oops, "minor planet") that is fainter than about 14th magnitude, for which you've exposed long enough to register it with a good signal-to-noise ratio (i.e, SNR > 20).

Links Internal to this Web Page:

The following internal links are ordered from "sloppiest but easiest" to "best but hardest."

    Method #1: 0.25 mag SE, planetarium program using Tycho or UCAC2 catalog (3 minute effort)
    Method #2: 0.15 mag SE, planetarium program using several Tycho or UCAC2 reference stars (5 minute effort)
    Method #3: 0.15 mag SE, counts equation magnitude (5 minute effort, plus once/year system cal)
    Method #4: 0.15 to 0.30 SE, R-mags using several Tycho or UCAC2 stars (10 minute effort)
    0.07 mag SE, quick all-sky (4-hour effort, after practice)
    Method #6: 0.03 mag SE, all-sky with color terms, BVRI filters (2-day effort, after practice)
    0.02 mag SE, ask Arne (1 minute e-mail and good luck)
    Definitions and good observing practices
    Catalog Comparisons, UCAC2 Accuracy
    Additional Links

Method #1
0.25 mag SE, Planetarium Program Use of a Tycho or UCAC2 Reference Star (4 Minutes)

Order the UCAC2 3-CD set from USNO and copy them to your hard drive. Tell your planetarium program to support UCAC2 (easy with TheSky 6.0). Now you're ready for some easy and accurate photometry.

1) Look for a star in your image that's not seriously saturated (i.e., maximum counts <35,000).
2) Use your planetarium program to identify this star.
3) Use the planetarium program to determine the magnitude for this star (V-mag are normally displayed).
    If it's from the Tycho catalog, great. The star's V-mag is likely to have SE = 0.13 mag. Be sure the star isn't saturated and proceed to next step.
    If it's from the UCAC2 catalog, good. If V-mag >10.5, add 0.25 mag. Proceed to next step.
    If it's from the GSC catalog, try another star (unless you're willing to accept SE = 0.44 with the possibility of errors ~1.1). If you use it, subtract 0.33 mag.
4) Measure the star's "intensity" (set the photometry aperture so that most of the star's image falls within the signal circle, and check that no interfering stars are in the sky background reference annulus).
5) Measure the asteroid's intensity (using the same aperture settings).
6) Calculate the asteroid's magnitude: MagAsteroid = MagStar + 2.5 * LOG10 (IntensityStar / IntensityAsteroid)

Note 1: The accuracy of any measurement has two components: 1) SEs, the stochastic standard error (related to noise level), and 2) SEc, estimated calibration uncertainty. Add them together (orthogonally) and you get accuracy. Use the asteroid's SNR to calculate SEs [mag] = 1 / SNR. (MaxIm DL displays SNR in the information window for the star within the aperture circle.) Assuming your "reference star" magnitude was from the UCAC2 catalog, then SEc ~0.2 mag. If it's from Tycho, then SEc ~0.13 mag. Your final "accuracy" SE = SQRT (SEs2 + SEc2).

Note 2: If you use a good observing and analysis program, such as MaxIm DL, the above procedure can be simplified using the "Calibrate" tool to force an intensity to display the corresponding magnitude (in an information window) as you move the cursor over stars in the image. Doing this replaces steps 4, 5 and 6, above. I assume that if you're not using MaxIm DL you can still read "intensity" (or flux, etc) using your analysis program (if you can't then you're out of luck for this and all the other procedures on this web page). Assuming TheSky6 and MaxIm DL are both open, and your image of interest is on display, the entire procedure for deriving an asteroid magnitude should take less than 3 minutes.

Note 3: If you're observing unfiltered then your asteroid magnitude (using V-mags for the reference star) can still be considered to be some version of a V-mag. It is only a good version of a V-mag to the extent that the asteroid's color is the same as the reference star's color. Since the advertised accuracy of this method is only 0.25 mag the color issue is unimportant.

Note 4: If your planetarium program is any good the Tycho magnitude that it gives for a star will be an adjusted version of the Tycho Bt and Vt measured magnitudes. If it doesn't make this conversion, you'd
better do it yourself: V = Vt +0.008 - 0.0988 * (Bt - Vt), which is Arne Henden's suggested correcting equation.

Method #2
0.15 mag SE, Using Several Tycho or UCAC2 Reference Stars (5 Minutes)

This method is a slight elaboration of the previous one and achieves it improved performance by using several UCAC2 or Tycho stars as "reference stars."

1) Look for at least 4 stars in your image that aren't seriously saturated (i.e., maximum counts <35,000).
2) Use your planetarium program to identify these stars.
3) Use the planetarium program to determine the UCAC2 magnitudes for these stars. If Vmag >10.5, add 0.25 mag.
4) Set the photometry aperture so that most of the "light" from the reference stars fall within the signal circle, and check that no interfering stars are in the sky background reference annuli.
5) Select the Photometry Tool (in MaxIm DL click Analyze/Photometry).
6) Set the mode to Object and left-click the asteroid.
7) Set the mode to Reference, and for each UCAC2 reference star left-click it and enter the UCAC2 V-magnitude for that star.
8) Click View, and save the results to a CSV-file.
9) View the CSV-file (using your favorite utility, such as Total Commander), and read the asteroid's magnitude.

Note 1: Since the UCAC2 catalog exhibits magnitude errors that appear to be random at a given star region it's possible to reduce the UCAC2 calibration uncertainty, SEc, by using N reference stars the calibration achieves a systematic uncertainty approximately given by 0.20 / SQRT(N). If you use 4 UCAC2 stars, then you may achieve 0.10 mag SEc. This overlooks the fact that color terms are still missing from the analysis.

Method #3
0.15 mag SE, Counts Equation Magnitudes (5 Minutes)

For this procedure you'll have to perform a once yearly calibration of your telescope/CCD system. If you change configurations, you'll have to repeat the calibration. I'll first show how to calculate magnitude under the assumption that a valid system calibration exists, then I'll describe that calibration observing session.

Procedure When System is Calibrated:

For my telescope system whenever the sky is cloudless I can convert a star's "intensity" to magnitude in a couple minutes without consulting a star catalog for a reference star's magnitude or observing a Landolt star field. The accuracy isn't very good, being ~0.15 magnitude, but it sure is quick! The procedure is straightforward, as summarized here (using MaxIm DL):

1) Move the photometer aperture pattern over a star and make sure the signal circle is large enough to "capture" most of the light from the object of interest (the asteroid),
2) Make sure the reference annulus does not have interfereing stars inside (if it does, either change the gap annulus width, change the sky background width, or use the pixel edit tool to delete them),
3) Note the star's "intensity" (taking care with faint objects to manually center the signal circle on the asteroid (and not the location yielding maximum intensity),
4) Note the time of the exposure,
5) Determine air mass using a planetarium program (such as TheSky 6.0),
6) Calculate magnitude using an equation of the following form:

    R-mag = (20.0 +/- 0.1) - 2.5 * LOG10 ( INTr / g ) - (0.13 +/- 0.01 ) * m

where INTr = "intensity" of the asteroid using the R-band filter, g = exposure time, m = air mass. The constant 0.13 +/- 0.01 is my site's extinction {magnitudes per air mass). The constant 20.0 +/- 0.1 is unique to my telescope system and is the number that must be calibrated once per year (or whenever the configuration changes). Each filter has a different pair of constants, and I'm just illustrating the equation for converting intensity to magnitude for one filter.

Note 1: Notice that it was not necessary to consult a star catalog for obtaining a reference stat's magnitude. Reference stars are not used with this procedure.

Note 2: This procedure only works when the sky is cloudless. To assure that your observations are unaffected by clouds it is useful to check the sky background level on images taken at about the same air mass. This background level (after subtracting 100 counts) will be proportional to air mass, and the slope of this dependence will be greatest for the B-filter. Whenever I'm doing precision photometry I monitor the sky background level (comparing it with charts) and I also monitor an internet satellite IR image of my area to see when cirrus clouds are moving toward my site.

Note 3: The first constant term in the equation, above, is somewhat dependent upon the signal circle size. If the circle is smaller than the size used for the annual calibration (and that's a temptation, since your SNR for a faint object is best for smaller signal apertures), then you'll have to determine the response ratio. This is done using a bright, nearby star, where you note the ratio of intensities for a large aperture that captures >99% of the star light, and the intensity corresponding to the smaller aperture you want to use for the faint asteroid. Use this response ratio to adjust the asteroid's intensity before using the above equation.

Note 4: Notice that your asteroid magnitude will be more accurate when observing at high elevations, where air mass is small and the extinction term is smallest. It is difficult to know the exact value of your extinction coefficient on a particular night, and the greater the air mass the greater the systematic error this uncertainty will produce. At my site the clear sky extinction at zenith probably varies from 0.12 to 0.15 during the year. An observer at a lower altitude than my 4700 feet will have slightly higher extinction. For my site extinction values for the filters BVRIC (where C = unfiltered) are typically 0.28, 0.16, 0.13 and 0.09 magnitude per air mass.

Procedure for Annual System Calibration

The calibration observing session is meant to evaluate the first constant in the above equation, and possibly to check your assumed extinction per air mass coefficient. Whenever I conduct a system calibration I cycle through my 5 filters, BVRIC. Probably most asteroid searchers observe unfiltered, so you may ignore references to changing filters in the following description.

For the system calibration choose only a completely cloudless night with no greater than calm winds. When these two conditions are met the night is referred to as "photometric."

Start the night with dusk flat frames near zenith. I use a double T-shirt "diffuser" placed in front of the telescope aperture to eliminate the possibility of stars showing up on the flat frames. Choose exposure times that produce a maximum counts value for the entire image of no more than ~30,000 counts (in order to avoid saturation). Don't use images with exposure times <1 second (because shutters can introduce their own component of vignetting for short exposures). I use only flat frames with exposure times between 1 and 20 seconds. Using dark frames is a good idea, but some people consider this optional. Don't use a cooler for the CCD chip (or if you do, be sure the chip temperature is very stable). After the flats are complete for all filters, turn on the CCD cooler to a vlue that the TEC can reach in a reasonable time (a few minutes) and start focus observations.

Select Landolt areas to observe that are at low and high air mass. Try to position your FOV so that several Landolt stars are present. (As a convenience, I've produced a TXT-file of the Landolt stars for import into TheSky. During the import process TheSky creates a SDB-file for fast loading on subsequent uses of the program. Whenever TheSky is run it automatically includes the Landolt stars and displays their magnitudes when a star is clicked.) Choose exposure times that avoid saturation for all Landolt stars. For me 10 seconds works well. Make at least 4 exposures of each Landolt area, and make at least that many dark frame exposures. Calibrate the light images using many darks (median combined) and your flats (averaged).

In theory you can stop after obtaining exposures of a Landolt area at high air mass and another at low air mas, but it's prudent to repeat the pairing to be sure temporal trends of extinction are not present. Two pairs of Landolt area observations should take no more than 4 hours.

Analysis of these observations might be considered difficult by observers not used to the joys of spreadsheets. I make manual readings of intensity for all Landolt stars using a carefully chosen set of photometry dimensions. The signal circle must be large enough to accommodate >99% of the photons registered by the largest FWHM Landolt star for the night. Each image must be inspected to assure that nearby stars are not present in the sky reference annulus. If interfering stars can't be avoided, then edit them away! Don't be afraid of pixel editing, but be careful in using this tool to select new pixel values that are representative of the sky background level for that part of the image.

Record all intensities, and mid-exposure UT times, and use a planetarium program to determine air mass values for each image. Enter these data into a spreadsheet, and also enter the Landolt magnitudes. To veiw a sample spreadsheet layout, click SampleSpreadsheet. The concept to be implemented in the spreadsheet is to perform a least squares solution for that first term, unique to the telescope system, such that the equation produces magnitudes that agree with the Landolt magnitudes. With this as a guiding concept you are invited to create your own spreadsheet to solve for the best value for this one constant. You are also invited to solve for your site's extinction coefficient, making use of the fact that you have high and low air mass measurements of known stars. If you're ambitious, you can also try to detect a  temporal trend for extinction for this calibration observing session.

When the results of this telescope system calibration are used for the analysis of images taken on other nights there may be systematic errors of 0.1 magnitude. I don't understand these changes yet, but beware of systematic changes that might be even larger for unknown reasons. It is always prudent to use another method to check the magnitude determinations from this method if you want to be assured of 0.15 magnitude accuracy.

0.15 to 0.30 mag SE, R-Filter, Using Several Tycho or UCAC2 Reference Stars (10 Minutes)

This method is the same as Method #2 except that catalog magnitudes have to be converted to an R-magnitude. As stated before, be sure you have UCAC2 files on your hard disk and configure your planetarium program to support this catalog.

1) Locate up to 4 unsaturated stars in your image that have Tycho magnitudes displayed in your planetarium program. Avoid Tycho stars with Vt > 10.5.
        Note: You may not find any Tycho catalog star in your FOV if it's smaller than ~15 x 25 'arc.
        If you have many images at about the same air mass try those and use the "zero offset" value for analysis of the image of interest.
2) Convert Tycho magnitudes Bt and Vt to R using Arne Henden's conversion equation: R = Vt -0.014 -0.5405 * (Bt - Vt)
3) If you have to use UCAC2 stas, convert UCAC2 V-mag (which I'll refer to as Vu) and J magnitudes to R using:
        R = 0.17 + 0.83 * Vu + 0.14 * J. (Don't use the Vu 0.25 mag correction mentioned elsewhere.) Estimated accuracy = 0.30 mag.
4) Set the photometry aperture so that most of the "light" from the reference stars fall within the signal circle, and check that no interfering stars are in the sky background reference annuli.
5) Select the Photometry Tool (in MaxIm DL click Analyze/Photometry).
6) Set the mode to Object and left-click the asteroid.
7) Set the mode to Reference, and for each Tycho or UCAC2 reference star left-click it and enter the Tycho-based or UCAC2-based R-magnitude for that star.
8) Click View, and save the results to a CSV-file.
9) View the CSV-file (using your favorite utility, such as Total Commander), and read the asteroid's R-magnitude.

Note 1: The more Tycho stars you use the smaller your systematic error. With just one Tycho star you can expect SEc = 0.30 mag. With 4 Tycho stars it should be ~0.15 mag.

Note 2:
This procedure is a lot more work compared with the procedure for V-filter or unfiltered observations, and the results are less accurate. This is due to the fact that star catalogs don't include  R-magnitudes (except for the very brightest, which will always be saturated for CCD users). Converting B and V magnitudes to R involves additional errors, so the R-mags you calculate are less accurate than the catalog V-mags. Here's an example of Tycho-based R magnitudes versus "true" (Landolt-based) R-magnitudes for a region chosen at random (the Landolt Area at RA = 00:55).

Tycho R-mag vs true

Figure 1
. Tycho-based R-mags versus "true" R-mags, where "true" R-mags were determined using an R-filter image that contained 21 Landolt stars for establishing a zero-shift value.

Note 3: I'm unsure about the accuracy of using UCAC2 stars for deriving R-magnitudes. If the V-mags have SE = 0.2 mag, and the R-mags should be even better (since I think they're derived from JHK magnitudes). I did a quick correlation that indicates 0.15 SE for R-mags derived from the catalog Vu and J mag's (using the formula given in step 3, above).

0.05 mag SE, BVRI Equation Magnitudes of Landolt Area and Region of Interest (4 Hours)

I will use the term "photometric group" to refer to an observing sequence of a Landolt Area (LA) and the unknown star field, or Region of Interest (ROI) that are at the same approximate air mass (m). A photometric group consists of BVRI images for the LA and BVRI images for the ROI. There are a total of 8 images that must be measured, 4 for the LA and 4 for the ROI. Each of these 8 images, which for my usual observing sequence is obtained by median combining 10 frames for each star field and filter combination, which have been calibrated (raw frame that has been dark subtracted and flat field corrected).

I will use an example observation from 2004.12.10, when I performed two "photometric group" observations, one at m = 2.4 and the other at m = 1.8.  Each photometric group produces a complete all-sky photometry solution for the ROI. The remainder of this web page describes my attempt to achieve an all-sky photometric solution from the first photometric group.

The first step in my procedure for producing an all-sky photometric sequence is to solve for one unknown term in an equation for magnitude as a function of star intensity, exposure time, air mass and star B-V color. This is done using the Landolt stars, with known magnitudes (and colors). For this first photometric group I obtained the following equations:

Equation mags for 1st grp

Figure 1. Equation magnitudes that account for measured intensity (INT) for Landolt stars, using their known magnitudes, B-V colors, air mass (m) and exposure time (g) for BVRI filter observations. Other more extensive observations have produced values for the coefficients related to extinction, star color and the product of air mass and star color; only the first term, a constant, is solved for using the Landolt stars.

When these magnitude equations are applied to the Landolt star measure intensities they yield a set of magnitudes that can be compared with the true magnitudes, shown in the next figure.

BVRI vs Landolt BVRI

Figure 2. Comparison of equation magnitude solutions with Landolt magnitudes for the LA0540 (Landaolt Area at RA = 05:40, Dec ~0). RMS scatter is 0.023, 0.021, 0.054 and 0.030 magnitude (for B, V R and I). Observations were made on 2004.12.10 at an air mass of ~1.8. 

These 4 panels show that the equation magnitudes can produce acceptable fits with typical SE of ~0.03 mag by adjusting only one constant (zero offset) in each filter's equation relating B-V, air mass, exposure time and measured intensity.  The next set of 3 graphs were produced by comparing colors for the unknown star field (E0540) using the magnitude equations and constant solutions. If the 4 constants that work for the Landolt fields also work for the unknown star fields, then the color/color scatter diagrams for the unsknown stars should be in approximate agreement with the corresponding color/color scatter diagrams for the landolt stars.

B-V vs R-I etc for 4C10 #2

Figure 3. Star colors scatter diagram for 707 Landolt stars and the 24 E0540 stars that were "solved" using my equation algorithm.

This graph is a "sanity check" meant to uncover systematic errors in the solutions for any of the 4 magnitude bands for the unknown region E0540. The E0540 stars exhibit small offsets with respect to the Landolt stars. This could be caused by changes in extinction between the time the Landolt area was observed and the time E0540 was observed. It could also be caused by an error in my assumed extinction combined with the fact that the E0540 ROI was observed when it was at a slightly different air mass than the Landolt area. A third possibility is that my filter wheel doesn't stop at the same location each time a filter is selected, and the slight mis-alignment causes a slightly different flat frame (indeed, I found evidence for this when exposing the flat frames: the appearance of the flat frame image depended on the direction from which I approached a specific filter). Whatever the explanation, it is appropriate to make use of the extra information about star colors to apply empirical adjustments to the equation magnitudes in order to achieve agreement with the Landolt color/color scatter diagrams. When this is done we obtian the following color/color scatter diagrams.

B-V vs R-I etc for 4C10 #2

Figure 4. Star colors scatter diagram for 707 Landolt stars and the 24 E0540 stars that were "solved" using my equation algorithm. Adjustments of +0.04, -0.03, 0.00 and 0.00 were applied to the E0540 BVRI magnitudes.

These color/color scatter diagrams were obtained by applying empirical offset corrections to the B- and V-band magnitudes for E0540. The corrections are +0.04 and -0.03 magnitude. No corrections for the R- and I-band magnitudes were necessary. When Arne Henden makes available his photometric sequence results I expect to find that my star magnitudes exhibit accuracies of ~0.03 magnitude (based on past such comparisons).

I should be able to achieve more accurate results using this technique (as Brian Skiff poitns out), but I'm still learning. I suspect I have a flaky color filter wheel, that doesn't end-up at the same location each time (since I've seen sky flats look different depending on which direction I approach a filter from). All-sky photometry must get easier the more of it you do. I'm counting on that!

work in progress

Definitions and Good Observing Practices


SE = standard error uncertainty; the orthogonal sum of SEs and SEc; SE = SQRT (SEs2 + SEc2)
SEs = stochastic SE; SEs = 1 / SNR in magnitude units (when SNR > 10).
SEc = calibration SE; usually estimated. Check stars offer quick estimate of lower limit to SEc, as do multiple measurements of a single star. For faint asteroids SEs > SEc.
Photometer signal circle = the circular aperture containing star light to be measured
Sky background reference annulus = the outermost annulus, with a gap annulus between it and the signal circle, used to determine an average level for subtracting from the signal circle readings
Intensity = sum of counts above background level; sum is performed within a circular photometry aperture; background level is established using an annulus (outside a gap annulus)

Good Observing Practices

Median Combining

It is a good practice to work with images that are a median combine of 3 or more images (that have been dark frame and flat field calibrated). This usually is not done for asteroid search programs since triplets (or quadruplets) of images are taken of a specific star field, each image >20 minutes after the preceding one, and this image set is used to search for moving targets. Photometrists median combine before doing photometry because this process removes cosmic ray artifacts. Fortunately, it is still possible to remove the effect of cosmic ray artifacts using a set of 3 or more images of an asteroid. This can be done by median combining using the asteroid for image alignment. In such an image the background stars disappear leaving only an asteroid feature. There are two advantages for doing this: 1) the asteroid feature has improved SNR, and 2) nearby stars are much less likely to interfere with photometry since they are removed by the median combining process. Combining images can also be done by averaging, but in this case you actually increase the chances of having nearby stars interfere with photometry. Here's an example comparing averaging and median combining (using the asteroid for alignment in both cases).

 Median combine aligning w/ asteroid

Figure 1. Two versions of combining 14 images of asteroid 2004 MN4 (V-mag = 16.88 +/- 0.15). Left panel shows averaging (using the asteroid for aligning), and the right panel shows median combining (also using the asteroid for alignment). [Celestron 14-inch, prime focus f/1.86, SBIG ST-8XE, red filter, total of 14 60-second exposures, unfiltered; 2004.01.08; Hereford, AZ]

Median combining may incur a ~15% SNR penalty, but it removes cosmic ray defects and it reduces the brightness of star tracks due to the asteroid's motion. Notice that a cosmic ray is visible in the upper-left region of the left panel. The reduction of star track brightness can be an advantage when doing photometry with a large sky reference annulus.

Pixel Editing

Professionals would be "horrified" to learn that someone was suggesting "pixel editing" to recover an image for photometric analysis. But that's what I'm going to do. Here's the situation where I recommend it.

 Animation (26 frames)

 Figure 2. Animation of Asteroid 12753 passing by a bright star. This 26-frame sequence shows motion during a 5-hour period (2004.12.31).

Notice that about 1/3 of the way through the sequence the asteroid appears to fade. This set of observations was made for the purpose of establishing a rotation light curve. But what can be done to minimize the effects of the bright interfering star? Should this data be rejected simply because it would spoil the signal aperture reading, or spoil the sky background reference annulus reading? Here's the problem, using Frame #10 as an example.

problem star

Figure 3. Frame 10 (enlarged) from the above sequence, showing the "problem star" east of the asteroid (at center of aperture circles).

The solutionis to "pixel edit" the problem star away.


Figure 4. Pixel editing was used to "remove" the problem star, permitting the use of a better quality "sky background reference annulus."

The final rotation light curve appears to be unaffected by this problem star and a few other fainter ones that were dealt with in the same manner, as the following graph shows.

Rotation light curve

Figure 5. The red squares are from the 26-frame sequence from 2004.12.31, using an R-filter. Only one frame could not be "rescued" from the effects of nearby stars. The pixel editing example in the previous figure led to the data point at UT = 4.1 hours. The 2004.11.11 data (green circles) are V-filter observations and they have been adjusted by -1.6 magnitudes to achieve agreement with the R-filter rotation light curve. A full rotation occurs each 12.85 hours.

The rescue work appears to have been successful since the rotation light curve is consistent with data from a month before and a year before that. I therefore recommend pixel editing for the rcovery of images that are affected by nearby interfering stars.

Catalog Comparisons

Not all catalogs are created equal when it comes to photometry. So far the best ones are Tycho and UCAC2 (a 3 CD set, free from USNO), as the following graph illustrates.

 V-mag catalog comparisons

 Figure x. Comparison of V-mag for 3 catalogs available in planetarium programs. Landolt magnitudes are the standard (truth).

This graph was constructed from a few stars chosen at 5 random locations (well, 5 Landolt area locations). Given that the Landolt magnitudes are a primary standard, biases and scatter are quite different for these three catalogs. Maximum errors of ~1.1 magnitude are possible for the much-maligned GSC, which was created as a guide star catalog for Hubble Space Telescope and doesn't claim to have good photometry. Now that the UCAC2 catalog is available (for free) on a 3-CD set, and considering that planetarium programs can be easily confgured to support this catalog, there's really no excuse for anyone serious about photometry to not upgrade for use of this catalog. This brief analysis suggests that an additional improvement can be had by adding ~0.28 mag to UCAC2 magnitudes for stars fainter than ~10.5. When this is done we can expect the following performance. The Tycho catalog is somewhat better, but it doesn't go any deeper than ~11.3 V-mag, where most stars are saturated for asteroid searches.

 UCAC2 adjusted vs Landolt

 Figure x. UCAC2 V-magnitudes versus the Landolt starndard V-magnitudes, after adding 0.28 mag to the UCAC2 stars fainter than 10.5.
The UCAC2 catalog is complete for declinations south of +40 degrees (some stars go as far north as +55 degrees). The USNO web site modestly states that the photometry is poor, being 0.1 to 0.3 mag. For astrometrists, that's good photometry. To order the 3-CD set containing 48 million UCAC2 stars from USNO:  send a short e-mail message to with "request UCAC2" in the subject line and give your mailing address in the main part of the message. Please use upper case letters for your name and address in a format directly usable for mailing lables.

To install it on your hard disk, and configure TheSky 6.0 to support it, follow the Help instructions for TheSky. If you have trouble, you may e-mail me.

Additional Links

    Brian Warner's Guide to Minor Planet Photometry

    AAVSO's CCD Observing Manual

    Arne Henden's brief text tutorial (pdf-file)

    All-sky photometry tutorial (one of my tutorials)
    CCD Transformation Equations explained (my derivation from first principles)

    Photometric Sequence Chart Creation for a 2004 nova (my example of typical session)

    2004 MN4 rotation light curve (Jan 8 & 9) 

    CCD Imaging Tips

    AstroPhotography Tips

    Bruce's AstroPhotos

   You may e-mail me at B L G A R Y @ u m i c h . e d u


This site opened:  January 1, 2005 Last Update:  March 30, 2006