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visual limiting magnitude

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#1 tboss70

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Posted 18 November 2012 - 01:29 PM

Last night I viewed through my very first Nagler. A 13mm T6.
Other than last night I have always used cheaper eyepieces. Wow! Why did I wait so long?

Anyway, I found myself renewed in my observing. With a better eyepiece I suddenly became so much more interested in viewing visually. Color, doubles, open clusters, magnitude!

At one point I found myself trying to find the dimmest star I could. I was looking at Pleiades and with the use SkySafari Pro I found my limit at 11.8. There were a few stars that would come and go and occasionally I could see them with direct vision.

Is this pretty typical (seeing 11.8 magnitude) using a Skywatcher Pro 100ED refractor, a 13mm T6 for 69x, and a light polluted location thats in the yellow/red area.
Thanks,

#2 Sasa

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Posted 18 November 2012 - 02:15 PM

Hello Todd,

100mm ED is indeed very nice telescope. And could do much more if you put it to higher magnifications. Under similar conditions (backyard in small town just on the boarder of 1.5 million city), I could see stars down to magnitude 14, if I push the magnification to 350x (Pentax XO2.5).

Cheers,

Alexander

#3 Tony Flanders

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Posted 18 November 2012 - 03:35 PM

Is this pretty typical (seeing 11.8 magnitude) using a Skywatcher Pro 100ED refractor, a 13mm T6 for 69x, and a light polluted location thats in the yellow/red area.


Do you mean yellow/orange or orange/red? There's a 10-old difference in sky brightness between the red and yellow zones!

I'd say 11.8 seems plausible for a 100-mm refractor at 69X in the red zone. However, 69X is way below the optimal power for seeing faint stars in a 100-mm scope; you would probably see at least a half magnitude deeper at twice that power.

#4 tboss70

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Posted 18 November 2012 - 03:44 PM

Sorry, orange/red. I will have to bump the power up to see what I can get.
Thanks everyone for commenting.

#5 TexasRed

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Posted 19 November 2012 - 04:17 AM

http://scopecity.com...-calculator.cfm

#6 tboss70

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Posted 19 November 2012 - 05:36 PM

Thanks for the link TexasRed. That calculator showed 11.9 for my setup/conditions.

#7 Starman1

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Posted 19 November 2012 - 06:00 PM

Using the same calculator, I see mag.14.85 as the theoretical limit, so your results will vary a fair amount depending on where your scope is used and how many years you've been doing this.
I wasn't into seeing the limits when I had a 4" SCT, but I do remember catching Pluto from nearly pristine skies when it was mag.14.2 back in the '80s.
Look up a used copy of Roger Clark's, "Visual Astronomy of the
Deep Sky," In it you'll find some charts with star clusters and the magnitudes of each star in the cluster. These charts are ideal for scopes smaller than 10" or so to determine an ultimate limit for a scope.
I've never been able to get to the theoretical limit of any instrument (variety of reasons), but I have found that though my home's night skies are 4+ magnitudes brighter than my observing sites, it only seems to shave 2 magnitudes off the limit, where seeing faint stars is concerned. Possibly because, at high powers, the sky in the scope seems pretty dark.

#8 Aleko

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Posted 19 November 2012 - 07:42 PM

I believe that calculator is flawed. It shows the older I get, the deeper I can see! :-)

Alex

#9 blb

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Posted 19 November 2012 - 07:52 PM

I believe that calculator is flawed. It shows the older I get, the deeper I can see! :-)

Alex


Very interesting but it still gave me a magnitude limit near to what I saw with my 10" dob on a good night from a dark sky site (mag. 15.6 at 300x). Could it be that the older you get, the more experanced you are. That may be more important than any loss in vision.
:question:

#10 Starman1

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Posted 20 November 2012 - 01:30 AM

I believe Schaefer's work showed that the decreased exit pupil in older observers actually led to, in a way I'm not familiar with, greater contrast for faint point images on the retina.
But certainly, experience is a critical thing, too, and that is an element in the calculator.
In my experience, the difference can be up to 2 magnitudes difference for visual acuity and experience combined. We verified that with a fairly large group of people looking at a naked eye limit. Just as an aside, the magnification of about 25X/inch of aperture seems to give the best point source resolution, since higher magnifications do darken the sky background, but also enlargen the Airy disc and make it visible as a greater-than-point-source object. Yet, the calculator shows increased visibility of point sources as magnification goes up, though the curve does level off at some point.
My best and darkest limit achieved with my 12.5" has been at 304X, close to that 25X/inch. So far, though I haven't really tried to see how absolutely deep I can go, I've pushed to about magnitude 17.3, about a magnitude brighter than the theoretical limit under optimum conditions.
Speaking of optimum conditions, those using the calculator should use a figure of 0.15 magnitude of extinction (common to a high altitude, transparent sky) and a color index figure of -.11 on the stars, and a zenith distance of zero degrees to get a limit near that of perfection, and a magnitude 8 NELM.
A more typical result would be obtained with extinction of 0.25 magnitudes, color index of 0, a zenith distance of 15 degrees, seeing of 1.5", and a NELM of mag. 6.2

#11 Astrojensen

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Posted 20 November 2012 - 04:38 AM

The calculator shows that I should be able to go to roughly 13.5 with my 63mm Zeiss, which is right about what I've been able to push it to. Maybe I've gone a little deeper, since I've seen G1 with it, at mag 13.7 but the calculator does say that I should be able to, if I use high magnification (which I did) and have excellent skies (which I also, occasionally, have). Setting the NELM to 8.0 indicate that I should be able to go a whole magnitude deeper or an astounding 14.6! That will not be from sea-level Denmark, that's for sure!


Clear skies!
Thomas, Denmark

#12 Sasa

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Posted 20 November 2012 - 10:07 AM

I believe that calculator is flawed. It shows the older I get, the deeper I can see! :-)

Alex


With your age, you should also fix limiting magnitude. If you keep the naked eye limiting magnitude at the same level while increasing your age, you are basically saying that you have better and better eyes (your entrance pupil decreases with the age, so the sensitivity must go up in order to keep the same limiting magnitude). That is why you go deeper and deeper with telescopic visual magnitude...

#13 Sasa

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Posted 20 November 2012 - 10:13 AM

The calculator also works quite well for me. It is telling me that under my poor conditions I should get with 100mm refractor down to magnitude 14. This is about right.

Concerning the dependence on magnification, it is a function of seeing. The idea behind the formula is simple. The source is "point-like" up-to 15' (900") and until that size the contrast between star and background increases. After that, there is no gain in further increasing magnification. So, if your seeing is 3", you gain only up to magnifications of 300x. With 1" seeing, you can push the magnifications up to 900x.






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