How faint a star can one see?

Originally posted to vsnet-chat in Feb, 2003, by Brian Skiff.

Brian Skiff writes:

Here are my two notes with results about naked-eye magnitude limits, which have posted around the Net for about as long as there's been the Web. Note that these results are entirely consistent with the limit Mike Linnolt gave, but simply apply to lower detection probabilties. They are also consistent with other personal anecdotal experience. For instance, several times at the Texas Star Party, I would get different folks oriented with the sequence in eastern Coma mentioned below, and giving them general directions and not saying anything about magnitudes, have them try to spot progressively fainter stars. Even the relatively inexperienced observers, people you wouldn't think of as being hot-shot deep-sky observers, typically start having problems only once you got close to V=8.0. Similarly, observing the same region very casually at a dark site in southeastern Arizona, a friend pointed out a star I hadn't mentioned---one he didn't know the location of and certainly didn't know the magnitude of, but which I did: V=8.1. It's a star I know the location of, but have never been able to see.

Anyway, on to Dave Nash and Heber Curtis...

In re naked-eye magnitude limits, here is a post I made to the sci.astro.amateur newsgroup several years ago. At that time, the Hipparcos/Tycho photometry had not been published, so the observer not only did not know where the stars were but also could not have known their correct V magnitudes, since they had not been measured, and traditional astrometric catalogues contained values up to 0.5 mag. in error.

A couple of months ago, in the s.a.a. thread of discussion comparing the merits of sky quality and darkness of the Texas Star Party versus the Nebraska Star Party, Dave Nash claimed to have seen some mag. 8 stars from NSP last August. The test was "blind" in the sense that he made a sketch of a small area of the sky without looking at an atlas to select stars. (As will be seen, however, Dave is far from being blind.) The region is near the head of Draco, the asterism sometimes called the "lozenge". Upon comparing with the Tirion "Sky Atlas 2000", he was surprised to find that the most difficult stars were marked with the smallest symbols on the chart, corresponding to roughly mag. 8.

At my request, Dave supplied IDs for the specific stars involved. Only the brightest star involved had any reliable published photometry, so I was curious to find out just how bright these stars really were. I promised to observe them photoelectrically at the first opportunity. I was able to do so about 3 o'clock this morning, and the results are summarized below.

For the record, I used the Lowell 53cm photometric telescope, Stromgren b and y filters, and a 29" diaphragm. I observed Dave's stars together with nine primary and secondary Stromgren standards, whose colors extended beyond the range of colors of Dave's stars. The y filter observations are transformed to standard Johnson V magnitudes, as is usual in this sort of work. The Stromgren y filter has the same central wavelength as a V filter, but is only ~200A wide, instead of ~1000A wide. For what it's worth, my several hundred observations of Landolt equatorial standards show that I match his V system systematically to within a couple thousandths of a magnitude over the color range -0.28 < B-V < 1.75. As a further historical aside, the original primary standards for the UBV system were measured on the same 53cm telescope in the mid-1950s by Harold Johnson and Daniel Harris. This morning the rms errors of the linear fits to V magnitude and b-y color were +/-0.009 and 0.008 mag., respectively. I would have preferred to use more standards, but these nine were entirely adequate for the purpose at hand.

During the course of a general survey of mag. 6-8 stars, I had previously measured the two brighter stars three and two times each, respectively. This morning's results agree with those observations well within their mutual errors, so I am confident that the single observations of the fainter stars are reliable to within about 0.015 mag. or better.

Name         RA  (2000)  Dec       V     b-y   spec  SAO         HD     BD
HD 160520*  17 37 14  +55 44.5   7.027  0.710   K0   7.2(GC)    7.18   7.4
                                  .006   .003
HD 162131   17 46 30  +53 34.7   7.609  0.088   A2   7.5(AGK1)  7.34   7.5
                                  .005   .001
HD 163010   17 50 31  +57 26.6   8.01   0.01    B9   7.5(AGK1)  7.9    7.7

HD 161353   17 42 13  +52 03.4   8.06   0.58    K0   7.8(AGK1)  8.1    7.9

HD 161179   17 41 23  +51 28.3   8.15   0.77    K2   8.0(AGK1)  8.1    7.9

* for HD 160520, Kornilov et al. (1991) give:  V = 7.050, B-V = 1.158, from
  four observations.

The table lists the stars along with 0'.1-precision positions. The new magnitudes and colors come next. For the two multiply-observed stars, I give the rms scatter of the data on the second line of each entry. For the fainter three stars, I give the results to just two decimals. The spectral types are listed directly from the HD catalogue, which is the only source of types for these particular stars. My b-y colors combined with the spectral types show that the sample consists of two early-A type stars (probably dwarfs) and three garden-variety K-giants.

The final three columns are "visual" magnitudes from three sources: the SAO star catalogue, the HD catalogue, and the BD. The original source for the SAO magnitudes is given in each case. As you can see, the magnitudes in the early catalogues aren't _so_ bad, but nevertheless usually report the stars to be brighter than they really are, and scatter around "truth" by up to half a magnitude.

In summary, Dave saw three mag. 8 stars from Merritt Reservoir, which shows not only that NSP has a great site, but that Dave is a skilled visual observer and also probably has acute (sharp) vision. He probably can see similarly faint from similarly-good sites, such as TSP, or other high and dry dark places. I've been observing from "perfect" sites in the desert Southwest US and Chile for twenty years and haven't been able to go fainter than about V=7.8, so my hat's off to ya, pal!

[posted to the 'amastro' list]

This second note deals with Heber D. Curtis' observations of some faint stars from Lick at the turn of the (last) Century. In this case, the stars were selected ahead of time, and a large black mask attached to a telescope (the 12-inch refractor at Lick). The mask contained a hole through which Curtis viewed from the observing floor, and swept across the star location in search of it. This isn't a good test of finding faint stars at unkown locations, but as with the Nash test the star magnitudes were very poorly determined, so he could not know how faint he was seeing on the now standard scale.

For some years Brent Archinal and I have wondered about the naked-eye limiting magnitudes determined by Heber Cutis at Lick Observatory at the turn of the Century. He claimed to see down below mag. 8, but we wondered what the modern standard V magnitudes of those stars were. I've dug out the relevant publication (1901 Lick Obs. Bulletin, 2, 67), and have looked up the stars (fortunately a short and well-identified list). The names are given below along with V and B-V from the Hipparcos/Tycho catalogues, and the magnitude Curtis gave for the same stars. I use the H/T data for convenience; they ought to be reliable to within a couple percent, and comparison with data collected in SIMBAD from ordinary sources suggest these numbers are fine for the present purpose. Curtis's comments are also shown for most of the stars.

As can be seen, the faintest star he saw reliably is V = 8.44, and two others below V=8.0 and one at 7.98 were seen as well. As might be expected, he did rather better overall on the near-overhead field around T UMa than on the T Vir field, several degrees south of the Equator (Lick is at about latitude +37.4). Just that alone tells you a lot about observing other than close to the meridian.

These in essence reproduce the results of Dave Nash at the Nebraska Star Party several years ago, when he did a double-blind test using stars in the head of Draco, and saw down to about V=8.2. On winter and spring nights at our Anderson Mesa site, I use a star in Coma at V=7.8 as a transparency test, and usually see it.

group near T Vir

Star          V    B-V   Curtis  remarks
HD 106384    6.56  0.28   6.52   [FG Vir, sl var]
HD 107830    7.19  0.43   7.20   seen easily
HD 105654    7.23  0.40   7.31   seen quite easily
HD 106515    7.34  0.82   7.42   seen easily
HD 106622    7.47  0.93   8.1    seen without difficulty on last two nights
HD 106579    8.44  0.44   8.3    seen with considerable difficulty; perhaps
                                 one-fifth of trials failed
group near T UMa

HD 110275    7.98  0.24   8.1    seen; one or two failures
HD 110408    8.08  0.53   8.2    seen
HD 110104    8.21  1.12   8.3    seen with difficulty
BD+60 1415   8.98  1.35   8.5    glimpsed at intervals; very doubtful