[AZ-Observing] How deep can you see? was: Re: Lunar observing tonight
- From: "Matt Luttinen" <mluttinen@xxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Fri, 13 Dec 2002 00:32:42 -0700
Jack,
I *know* I didn't see the G component, as it is about mag 15 or 16, I
think,
and thus way below the limit of my scope. I learned that night that
averted vision can fool you. I know it fooled me. I have since used
stars in M45 to estimate my limiting mag with the 120mm to be right at
12.4,
which is exactly the theoretical limit (~12.4 for my 6.3 mm dilated
pupil and a limiting naked-eye mag of ~6) for a 120mm objective.
BTW, the formula I found for limiting visual mag is:
m = Alpha - 5 log Delta + 5 log D
(assuming transparent dark-sky conditions and magnification > = 1D ,
D in mm)
where:
m is the approximate limiting visual magnitude
Alpha is the estimated naked-eye mag limit
Delta is the dilated pupil diameter in mm
D is the diameter of the objective in mm
Got this from Company Seven's site, with minor modifications by me.
I wonder about this formula due to the fact that Alpha and Delta are
almost always estimated. Company Seven inputs Delta = 7.5 (!) This
seems arbitrary, maybe to make the equation work.
It seems to me that finding the true limiting mag of your scope
is more dependent on your naked-eye limit than anything else, and that
is always subjective to each observer. It is safe to say, though, that
very few people have better than mag 7 eyes. So, if this equation is
right,
anyone who has seen a mag 15 object through a 120mm scope with dark-
adapted eyes (Delta >5, Alpha = 7, to be generous) is seeing a mirage.
Notice how increasing pupil size reduces the mag limit. This seems
counter-intuitive to me. Comments anyone?
I know I have gone off on a big tangent, but as Beavis says:
"I'm always thinking."
Matt
PS: I have seen many mirages in my life, and it's funny, they are always
dancing . . .
-----Original Message-----
From: az-observing-bounce@xxxxxxxxxxxxx
[mailto:az-observing-bounce@xxxxxxxxxxxxx] On Behalf Of Jack Jones
Sent: Thursday, December 12, 2002 11:12 PM
To: az-observing@xxxxxxxxxxxxx
Subject: [AZ-Observing] Re: Lunar observing tonight
No you saw it alright. I can hold it at 250x in the f/8 6" (a 4.8mm
Nag). Now there is some haze. Perhaps Rukl meant by "larger telescopes"
larger than maybe a 4" refractor. Strange, the off-center peak in
Pitatus is over 10,000 ft high, but the Straight wall isn't more than a
football field high. Maybe you didn't see the G component, someone at
Vekol last week thought they saw the Horsehead Neb in a 10". Never can
tell, it just depends on conditions and a lucky combination of optics.
Jack Jones
Saguaro Astronomy Club
Lunar List Awards and
Messier Marathon Co-coordinator
Phoenix AZ
spicastar@xxxxxxxxxxxxxxx
>
> heh heh heh,
>
> I somehow doubt I have better eyes! Perhaps I was seeing a
> floater. They become a nuisance the 0.53 mm exit pupil I am
> working at.
>
> But then again, I think (I know) I saw it. But this is the same guy
> who thought he saw the G component of the Trapezium at Flatiron
> last winter . . .
>
> Matt
>
>
> I have a better telescope but you have better eyes? Or vicey versae?
:-)
> Good practice for when we'll be seeing the canals on Mars in a few
> months.
>
> Jack Jones
> Saguaro Astronomy Club
> Lunar List Awards and
> Messier Marathon Co-coordinator
> Phoenix AZ
> spicastar@xxxxxxxxxxxxxxx
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