On 3/11 - 3/20, the order of plotting of the stars was tested in good rural skies east (Mirror Lake Highway) and west (Grassy Valley) of Salt Lake City, Utah, using 20 x 70 "Little Orion" tripod mounted binoculars with 20x magnification. Testing locations varied varied from approx. 4,500 to 7,500 ft MSL, NELM v5.2 to v5.7 skies, temperatures of 0 deg to 10 deg C. (A full type-up of observing session testing notes has not been done due to time constraints.)
The chart was reviewed in order of magnitude. At each step, the target star was observed for about 30 seconds of relaxed viewing. It's brightness was compared with the star preceding and following the target star. If the target star was between the preceding and following star in brightness, it was accepted. A web photograph of NGC1647, with a photographic limiting magnitude reaching below v13.0, was used as an identification aid. Useable stars are marked with a "Y" in the useability column.
If a star could not be viewed separate from a nearby neighbor, it was marked as unacceptable in the useability column with an "N". Comments, such as "SP" = split problem, indicate why the star was rejected.
The chart was field tested to approximately v11.0. At v11.3, stars clearly shown on the photographic image of NGC1647 were not visible. The chart was worked through three times on three separate nights in skies an NELM of v5.2 to v5.7.
An internet request for assistance in field testing the chart was solicited. The following comments were received:
| Date | By | Chart | Stars | Problem | Response | Action |
| Mar-04 | TF | v7.0 to 13.0 NGC1647 cluster chart | 048 049 | At 60x magnification, "#048 (10.7), but hard to split from #049 (10.3) | Stars 48 and 49 have a split problem which is marked in the supporting tables. | Rather than regenerate all the charts, the problem was noted on the supporting tables for each chart. |
| Mar-04 | TF | v7.0 to 13.0 NGC1647 cluster chart | 065 066 | In light polluted urban skies viewing off-zenith In light polluted urban skies viewing off-zenith and near the scope's limiting magnitude, "60X, #065 [9.6] seemed little brighter than #066 [10.3]." | In star testing on 3/11 and 3/17 in better skies, these stars appear to be unequal. The initial problem is attributed to viewing in light polluted skies. | None. Chart appears to be correct. |
| Mar-04 | TF | v7.0 to 13.0 NGC1647 cluster chart | 094 099 | These stars do not appear to be properly ordered. | In star testing on 3/11 and 3/17 in better skies, 094 and 099 appear to be properly ordered. But star 042 is intermittently visible and confounds the perceived order of brightness. | Note made in supporting table indicating that star 042 is not useable and can create observing problems for stars 094 and 099. |
During 3/11 - 3/20, 2004, this chart was used without testing to estimate NELM. The results were cross-checked with the International Meteor Organization (IMO) limiting magnitude area 8 chart. Results were the same within +-0.2 magnitudes - the difference being attributed to the wide-field in the NELM chart presented here.
Because NGC1647 has fallen off zenith (in March 2004), formal star testing of this NELM chart is deferred until September 2004.
Split problems were apparent in the small asterism of stars above Orion's body - star nos. 035,036,038,039,041,042,043, 111 Ori. The two brightest stars in this asterism, 039 and 044, preliminarily are still considered useable.
As an initial impression, the NELM chart presented here was felt to provide equivalent results in near zenith positins to the IMO Limiting Area 8 Chart. Each chart provides a good cross-reference for general amateur observers, familiar with estimating ZLM off of general purpose star charts, but who are less familar with the IMO star-count-method NELM charts. In the NELM chart presented here, the variance in the photometry is presented in the supporting tables, unlike the IMO chart and supporting table. As NGC1647 descends into the western sky, each chart has its strengths and weaknesses. Because IMO Limiting Magnitude Chart 8 is oriented east-west and the measuring field sticks "up" rectangularly from an extinction horizon, part of the measuring area is a heavy extinction zone, while most of the measuring stars are in distinctly darker skies. The NELM chart presented provides more north-south measuring options.