FreeLists / az-observing / [AZ-Observing] Re: Recording Seeing Conditions (was WOW!)

["William R Wood" <w.wood@xxxxxxx>]

----- Original Message ----- 
From: "Brian Skiff" <Brian.Skiff@xxxxxxxxxx>
To: <az-observing@xxxxxxxxxxxxx>
Sent: Tuesday, July 12, 2005 1:00 AM
Subject: [AZ-Observing] Re: Recording Seeing Conditions (was WOW!)



>
>>>  ...standing by a telescope I need something, and it needs to be quick,
>>>  simple and need not be horribly accurate.  The only  important thing
>>>  that it be reasonably consistent.
>
>>>  Looking at a star at high mag and
>>>  looking at zenith with a star chart in hand fits the bill.
>
>     In case others besides Andrew have misconstrued what I'm trying
> to get at, rephrase the above quote as follows:
>
>     Looking at a [double] star at high mag and
>     looking [through the telescope] at zenith with a star chart in hand...
>
> \Brian
>

Hi Brian,

I believe I got my head chopped off for saying this before, so I hesitate to
stick my neck out again but..... :)

I must agree with Andrew about the speed and practicality of using a regular
star near zenith to measure seeing.

With all due respect and certainly no offense intended, I feel that the
close double star method is impractical  for most folks and simply will not
get used.  As Andrew said we need something quick and simple.  To use double
stars, one needs a big list of them and one needs to find a suitable pair at
least somewhere near zenith when the observing session begins.  I am a
double star person and I do not use doubles to measure seeing because: (1) I
can't easily find a tight double near zenith when I begin observing even
using your excellent list and (2) I still can't figure out how to measure
seeing in arc seconds using tight doubles.

The practical answer for me is to use any medium bright star (about mag 4)
that I can see near zenith and rate it using the 1-10 Pickering scale as
described by Damian Peach:

http://uk.geocities.com/dpeach_78/pickering.htm

I keep the above visual depiction of the scale on my laptop so I can refer
to it frequently right at the telescope and thus keep my seeing estimates
consistent.  This scale is arbitrary but using it is super fast, super easy
and absolutely consistent for me.  This approach is way faster than using a
tight double.  I have your list of doubles on my laptop and I still struggle
to find a pair that is near overhead every time I go out.  Then after I pick
a pair I must actually find it with the telescope.  I am a starhopper which
makes that part a little harder than for the Goto folks but even with Goto
it takes time to get the target star in your eyepiece.

Then, after you get the target star in your EP, what does it mean?  Lets say
the target double has a 2 arc second separation (2".0).  OK, so I look at it
with my 10" Cassegrain and it is a major fuzzball, both stars basically
merged in blur with no diffraction rings discernable at all but I can see,
sort of anyway, the separate brighter blobs of the two Airy discs bouncing
around.  If I look at the pair long enough I can call a split because the
air has moments of stability.  So what is the rating in arc seconds?  I have
no idea.  I can split the double if I try hard enough, so, is the seeing
rated at 2"??  Or is it rated at something more than 2" since most of the
time the pair is merged in blur?  And isn't this rating just as subjective
as the Pickering scale since it depends on the individual's evaluation of a
debatable situation and perhaps the individual's observing skill?  Using my
Pickering scale I would call it 3-4/10 and be done with it in about 30-60
seconds.  This (3-4/10), by the way, is the typical rating I give when I use
my 10".  Seeing generally stinks in my backyard and at the desert sites I
have visited.

The problem with doubles has another aspect.  If I use my 5" or 3.5"
refractors, I get much better views of stars at high mag vs using the 10".
In my above example, 3-4/10 seeing as measured by the 10" would look more
like 4-5/10 seeing in the smaller refractors.  But smaller refractors don't
split a 2" double as easy as the 10".  Thus, if I used the 3.5" refractor
instead of the 10" in my above example, I might not be able to split the 2"
double at all.  Again, what is the seeing rating in arc seconds if your
target star is a 2" double and you cannot split it?  And, again, I can split
it with the 10" if I am persistent but not with a 3.5".

Does the inability to split the 2" double mean that I must now pick a wider
pair until I find one that I can just barely split with the scope in
question?  Another complication is star magnitude.  With the 10" I might
measure seeing at 3-4/10 on a mag 4 star near zenith but when I actually
start observing my real targets I might find that I get decent images of mag
7 doubles as long as they are also pretty high.  What does this do to the
arc second method of rating seeing?  Must all rating targets for the double
star method be mag 6-7 or dimmer??

As noted above, I like the Pickering method because I can look up and
actually see mag 4 stars in my backyard and all I have to do is pick the
first one I happen to find somewhere near zenith.  There is no way I can see
mag 6-7 stars (let alone find close doubles at that mag) due to light
pollution which means I have to hunt such targets up using star charts.
Even though I am a double star person, there is no way I am going to do this
every night because all I want is a consistent, fast rating method that I
can put in my log before I start observing.

I also feel that the Pickering method is valid for sharing with others if
one uses a standard reference scale like the Damian Peach rendition.  And
Pickering scales ok with scope size but, if sharing, one should give the
scope size and star magnitude used to make the measurement.  I believe scope
size and star mag is also necessary to know if sharing data using the close
double method.

Regards,

Bill Wood
Fountain Hills, AZ

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