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Magnitude Check. 6" in a white zone.

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#1 Cotts


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Posted 29 March 2013 - 10:32 PM

I'll be setting up my SkyShedPod this spring so I thought I would check just how deep I could go in my mag 3.5 white zone skies, just 15 miles from the center of Toronto.

I used my 6" f/8 Intes MakNewt with a 5mm Nagler. NGC 2129 in Gemini was very high in the south. I use Roger Clark's book 'Visual Astronomy of the Deep Sky. He presents about a dozen excellent charts of open clusters with magnitudes down to as much as 15. The charts are presented in pairs, one with magnitudes and one without. I take a photocopy of the no magnitude chart out to the scope and simply circle every star I can see at the eyepiece. Data reduction is a simple matter of making a list of the magnitudes of circled stars. I also note when averted vision was used.

My results? Faintest star with steady looking - 12.5. with averted vision, 12.9.

The SQM redings averaged 18.35 over the half hour I took to do the test.
4C - 39F temperatures.

Excellent results for a big city location, I'd say...... Bodes well for my observatory being useful.....


#2 azure1961p


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Posted 29 March 2013 - 11:03 PM

That's pretty impressive given a magnitude 3.5 sky! Under 6.2v with my 8" Ive hit an even 15.


#3 buddyjesus



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Posted 29 March 2013 - 11:42 PM

that should be very encouraging to people that have a phobia against city observing.

I regularly observe in an red zone that has a big light dome to the west. I can attest to seeing a good number of targets in the Messier and Caldwell lists and some H400 targets from this spot. I obviously don't go deep in Virgo or anything, but certainly not a waste of time.

#4 Astrodj


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Posted 30 March 2013 - 03:04 AM

A very thorough and informative test, Dave.

There are WHITE zones, and there are white zones, and you inhabit one that is all CAPS!

Here in the burbs of STL I am barely in the white, but my ZLM is about 5.1 on the very best nights (after 2am Su-Wed.) A more normal measure is about 4.7 magnitude skies at the zenith.

My observing location is completely shielded from nearby light glare. I am only illuminated by the skyglow.

I tried a much more informal/less rigorous test than yours last summer with my 5" Celestron. I used M57 as a target overhead, a 6.6mm Cave Ortho, and the comparison chart referenced with Brian Skiff's (photometry table) at Lowell.

Keeping in mind the reduction in aperture due the CO of the SCT, I was able to hold the magnitude 13.02 star just to the east of the ring with direct vision on a nearly perfect night of seeing and transparency. The 13.36 magnitude star to the lower left of the chart was in and out with averted vision only.

I seriously doubt I could achieve anything close to that from Toronto's WHITE zone!

I would like to repeat this little experiment this summer with some larger scopes to see how faint I can go. I think this is a great chart for this kind of experiment.

#5 Cotts


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Posted 30 March 2013 - 08:40 AM

I wouldn't want to encourage visual work on faint fuzzies from the inside of a WHITE zone* but there is still a decent list of observing types that suffer little in that environment.

Solar, Planets, double stars, lunar, open clusters etc. I have seen some decent Photos , too, taken with very narrow filters.

* a possible new subdivision of the light pollution scale suggests itself. From least bright to most bright:


and, finally,



#6 VanJan



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Posted 30 March 2013 - 09:11 PM


#7 johncmcd


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Posted 23 September 2013 - 09:34 PM

I can second the concept that one should never assume that their skies are too bright for deep sky observing. I live under very "white" skies just over the border from Chicago in one of the very close suburbs. My balcony faces SE directly over the city with heavy glow from O'Hare airport to the WSW (the city itself is blocked by a building so I don't have the lights shining directly in my eyes).

On very good nights I get something slightly better than a limiting magnitude of 4, but most nights are more in the 3.5 range. With my old Intes 150mm Mak I regularly observed the brighter open clusters and a good portion of the Messier globulars.

I can also vouch for the fact that aperture makes a large difference under these kinds of skies since I just finished building a 12.5" Dob, and tonight (about mag 4 skies) I was able to catch NGC 7006 (a.k.a. Caldwell 42) in Delphinus. Its an extremely remote globular cluster at magnitude 10.6. O'Meara gives it a surface brightness of 13.2 in "The Caldwell Objects". It was clearly visible with averted vision at 210x and I could catch it from time to time directly as well. (No resolution of course, but a 12.5" isn't really capable of resolving stars in this glob as it is around 135,000 LY away :) NGC 6934 also in Delphinus was easier to catch at mag 8.8 and seemed grainy as if it wanted to resolve (I'll come back to it on a better night.)

So, suffice it to say that I'm very satisfied with what I can see looking thought this kind of soup, though like everyone, I long for trips to truly dark skies. The interesting thing is that I find that star hopping in heavy LP has made it a LOT easier for me to find objects when I am in dark skies because I have had to struggle so hard to follow guide stars in the finder here.

Sometime I'll have to do a test to see how faint I can go with either the 6" or the 12.5" here.

Clear Skies!


John McDonald
Morton Grove, IL
4.5" F/5 Newtonian (home-built)
6" F/12 Intes MK66 Maksutov
12.5" F/3.9 Truss Dobsonian (home-built, mirror ground with Dan Joyce)

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