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#51
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Limiting Magnitude in Binoculars
Tony, this does not answer your latest post but provides additional
information about one critical topic. There is still plenty more to review. edz More Clarifications Methods I used for determining NELM in my Binocular Limiting Magnitude Study Certainly without complete knowledge of what my NELM chart sources were, any claim of non-standardization or that they are not reproducible is unfounded. I took great pains to develop NELM charts, some with points in small, localized areas so estimates would be more consistent. In my opinion, charts that require the eyes roving over large wide areas performing star counts leave the user open to far too much error, not only boundary error but count error. It may be difficult for some to visually project boundaries, even across small areas of the sky, with the intent to count or not count stars on either side of that projected boundary. Probably more than anything else, unfamiliarity with star fields in the charts would render the user prone to error. I prefer not to use the star count method, as it is my opinion there are better means to accomplish the task. For purposes of this study, I have identified five constellation areas or asterisms and by using sources such as published NELM charts, computerized star charting programs and AAVSO charts, verified a wide range of star magnitudes for use in NELM determination. These charts include only stars identified by a Greek letter or Flamsted number in and adjacent to the primary figure of the constellation or asterism. I corrected for any integrated magnitudes and checked variable magnitude for one important variable star, TX Psc, used in the study. For this study I used Sagitta, Delphinus, the Circlet of Pisces, Ursa Minor and the Cr399 asterism itself. The end result is I have at my disposal, several charts allowing NELM determination near continuous from 3.6mag to 6.4mag at 0.1 mag intervals or less with few gaps and often with check stars nearby to verify. All my NELM charts are not only reproducible, but contain readily available information. I believe the charts I am using are equally as accurate if not more so than any star count method of NELM determination. I almost always use Ursa Minor, as it has a wide variety of magnitudes, is almost always visible, provides for a good check even though it may not be in the vicinity of my observations and provides a good night to night comparison. I have learned it is relatively easy to know what to expect from Ursa Minor even after a short time outside to assess the sky darkness and transparency. Also after doing this so many times, I can now tell during the evening if sky conditions are improving, and I often am able to verify such by a re-observation of Ursa Minor. "The Backyard Astronomer's Guide" by Dickinson/Dyer provides very good LM charts for the Ursa Minor area. The suggested practice of using charts only in close proximity of 4° to the area observed seems unrealistically applied and somewhat impractical. This would require the observer to make critical observations and NELM assessment in an area no bigger than the size of a circle that would surround Sagitta. In fact, one website providing NELM charts uses 14 charts of 10°x10° to cover the whole sky and recommends observation from at least two charts. If one were to keep critical observations within 4° of any one side or corner of those charts, it would be possible to have a NELM observation within the chart boundaries easily at a distance of 15° from the target critical observation. Another website, which provides 30 charts for the whole sky, recommends using observations from several charts. In so doing, the observer may be making NELM estimates from an area easily as wide as 10-20°. This seems practical advice. My experience is this; areas of the sky in the same direction as wide as 10-20° may not show a significant difference in NELM. This does not say there will never be any difference. However, placing limiting restrictions on what method must be used by the observer to accomplish the task will do more to prevent attempts at observations than to assist attempts. I still recommend an observation at the critical location if at all possible and several others nearby to verify. I have found quite often, both thru the lens and naked eye, faint stars of a specific magnitude can sometimes be observed while apparently equal or brighter nearby stars go unobserved. Due to this phenomenon that I suspect can be attributed to color index, which I address in my article, I elect most often to observe NELM from several locations and in so doing often may record a variation of 0.2mag in observed NELM. Discordant advice is given to the user as to how much effort should be expended to acquire stars for determination of NELM. While some recommendations are given for observing stars at the averted vision limits, other recommendations suggest spending no extra effort at all to attempt acquiring stars by averted vision. The practice I use is to spend as much time, several minutes if necessary, to acquire stars at the limits of averted vision. edz |
#52
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Limiting Magnitude in Binoculars
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#53
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Limiting Magnitude in Binoculars
Tony,
In practice, your curve depends utterly and totally on those two estimates of 4.0 and 4.5 near the full moon in September Well that's not entirely accurate, so this will help clarify. See the clarifications dated Oct 6. I refer to the observations on Sept 25 that I have included in my analysis, but did not include any written text for you to know the data exists. I clarified that in Oct 6 notes. This is a summary of what I have for bright skies listed as mag5.0, total observations and total critical observations (stars no more than 0.3mag below max) Sept 10. NELM 4.37 approx 40 observations 17 critical Sept 12 NELM 4.43 approx 32 observations 13 critical Sept 12 was mag 5.0 Sept 25 NELM 4.68 approx 80 observations approx 20 critical Because the analysis uses only the maximums it may seem like there is not a lot of data, but there is atleast 50 critical observations from 3 bright nights and that's not really a small amount. I suggest that before you can draw any conclusions about NELM vs. BLM, you need to refine your bright-sky NELM estimation methods, and you need a bunch more data from bright nights. No question there is always room to refine. As I said, In reality I already have included in the analysis the maximums from 50 critical bright night observations, but only over three bright nights. I did go out and collect some more data on another recent opportunity to check viewing under bright sky. Here is that data. Oct 5. conditions deteriorated from 4.68 at 8pm to 4.43 at 9:30pm Recorded NELM several times throughout session. In vicinity of observations started out with 4.68. No 5.0 or 5.05 or higher stars were ever visible and NELM never got back up above 4.43Del and 4.37Sge. Cr399 (5.15) was not visible. In Ursa Minor I observed one 5.0 star. Oct 5 NELM 4.43dir, 4.68avr, approx 60 observations 14 critical Limits reached 8x42 = 9.5, 16x70 = 10.6(didn't try any higher) Oct 5 NELM 4.43dir, approx 70 observations 15 critical Limits reached 8x42 = 9.1(also 9.3/9.1 Pair), 12x50 = 10.23, 16x70 = 10.76 (tried many more for all three binocs) For comparison, Best BLM observed under mag5.8 skies, reported in article 8.42 = 9.75 12x50 = 10.5 16x70 = 10.83 For comparison, Best BLM observed under mag4.4 skies, data used in article 16x50 = 10.1 16x60 = 10.3 15x70 = 10.3 16x70 = 10.5 Oct 5. NELM 4.43, 125mm/1250mm G5 SCT scope, testing for maximum Lim Mag for the night. mag12.0 star in Cr399, not vis at any mag 100x, tried (50x, 60x 70x 80x 100x), at 140x mag12.0 star glimpsed averted, at 170x mag12.0 star seen constant but just barely. This scope has been tested many times on the Brian Skiff M57 photometry, on a few occasions of absolutely best local conditions (NELM 5.8, I have never ever recorded a mag 6.0 NELM star locally) has reached 13.1mag, have never recorded anything deeper. Oct 5 NELM 4.43, 78mm/480mm AT1010 Ref scope, testing for maximum Lim Mag for a 78mm scope at low power. Used a 15mm TV plossl for 32x magnification. Stars of 10.5, 10.52 and 10.56 seen readily, stars of 10.76 and 10.8 seen but not easy, 10.86 barely seen, 10.93 only glimpsed, 11.0 and 11.2 not seen after many tries. The 78mm scope is closely equivalent to the 16x70 binoculars. In NELM 4.43 skies, It reached a max Lim Mag of 10.93 at a magnification of 32x (more powerful than binocs). In NELM 4.43 skies, the 125mm scope reached 12.0 mag but only at 140x to 170x. For comparison purposes in mag 5.8 skies, using the 78mm scope two mag11.0 stars were seen at 40x. A mag 11.3 star was seen at 40x. A mag 11.4 star was not seen at 40x, however it was just barely seen at 65x. For comparison to the 125mm scope, when it was used in mag 5.8 skies the same mag12.0 star was not seen at 50x, but was glimpsed at 76x and was seen steady at 110x. Under mag best ever skies, the 125mm scope reaches mag13.1 only at very high powers about 175x to 200x. Scopes show a much wider range in Lim Mag vs NELM, but only at higher magnifications. Scopes show scope Lim Mag follows much more closely in line with NELM. Binoculars show almost the same results recorded on earlier bright nights. BLM does not drop nearly as much as NELM. So from this it can be seen that scopes follow what you are saying Lim Mag closely mimics NELM. But the binoculars still prove to not be following that rule. I'll keep trying, but I keep getting the same thing. edz |
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