FreeLists / az-observing / [AZ-Observing] Re: Measuring Seeing

[Brian Skiff <Brian.Skiff@xxxxxxxxxx>]

     Perhaps I should post this annually so that Bill Wood remembers that
he asked about this "last year"---last July 17th in fact.   B-)

[this time from 14 June 2004]

>>  The dim, close pairs are hard to find...

     The pairs in the list are mostly mag 5-6, so are not "dim".  Many in fact
are Greek-lettered or Flamsteed-numbered stars, i.e. naked-eye stars.

>>  A big list of pairs would be necessary...

     I'm open to suggestions to add to the list described and linked below.
I've used it for 20 years with no problems.

>>  One must guess where to start regarding the degree of separation of the
>>  test pair.  If you guess wrong you might have to test 2 or even more pairs.

     Heaven forbid that you might want to look at more than one object
in a whole observing session....  sorry, the sarcasm was too hard to avoid.
     OK, yes, the _first_ time you do this, you'll have to look at a few
of the pairs to get an idea of the scale involved and so on.  What's the
big deal?  (Your comment makes it clear you haven't bothered to try this
even once, which makes your complaint kinda "theoretical".)  But after that
first time, you can take one look in the eyepiece and from the appearance
of the images say "oh, I need one around one-and-a-half arcseconds", then
check the list.  I think you'll find that the range of images sizes is
in fact relatively narrow, so you'll be checking the same pairs repeatedly---
you'll have 'em memorized pretty quickly.

>>  Like the Pickering test, there is a degree of subjectivity inherent...

     Agreed.  If you'll read the post (again), I address this.  In brief,
whatever you do, do it consistently with whatever telescope.

>>  I cannot find any rules/guidelines about how to handle typical varying
>>  conditions.

     Write something like this in your logbook:  "about 1".5 in steady moments,
but puffy up to 4" when a breeze kicks up."

\Brian

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[re-re-re-post from July 2003]


     There was in fact some text that went with the seeing-checker
double stars when the list was originally built in Feb 2000 and posted to the
'amastro' mailing-list.  It describes how the list was compiled and how
I intended it should be used.  This should answer a lot of the questions
that Bill Wood asked.  The original post is copied below.
     Since that time the speckle observers have decamped to USNO-DC, and the
Web pages have gone from Georgia to DC with them.  The list has also been
updated at least once with more recent observations for the pairs.  The
revised list is kept at:  ftp://ftp.lowell.edu/pub/bas/dbls.fil

\Brian

===============================================================================

[Feb 2000]

     Attached below is a first revision of a list of double stars that should
be useful for estimating "seeing" (image size) in moderate-aperture amateur
telescopes.  Most of the stars are drawn from the list Chris Luginbuhl and I
prepared for our "Observing Handbook", with additional objects found as a 
result of the double-star viewing survey I did roughly ten years ago.  A few
special cases are included for pairs that appear near deep-sky objects.
     The first column shows the ADS number; an asterisk indicates a note at the
bottom of the table.  The ADS (Aitken Double Star) catalogue is the double-star
equivalent of the NGC/IC, since it contains essentially all the pairs one can
readily expect to resolve without special techniques, and is the preferred name
for pairs found up to about 1930.  Next comes the RA/Dec to 0'.1 accuracy.  The
V magnitudes are the best that I could find without a lot of work; careful
scrutiny of the Hipparcos data might yield more consistent data for some pairs.
I tried to include only pairs with magnitude differences less than 1.0, but
there are several exceptions.
     For current separations and position angles I used the on-line catalogue
of speckle interferometry maintained by the group at Georgia State University
(http://www.chara.gsu.edu/CHARA/DoubleStars/Speckle/intro.html).  Nearly all
the pairs have post-Hipparcos speckle observations, usually within the last
three years.  Separations closer than 1 arcsec are given to two decimals.  Most
of the pairs are not changing rapidly, but for the closer ones, revision (or an
ephemeris from an orbit) will be needed on timescales of a few years.  The date
has been rounded (they are commonly truncated in the double-star literature).
Finally a common name (mostly Bayer/Flamsteed names or HD numbers) is shown in
the last column.  This is mainly to facilitate literature searches, since in
SIMBAD (and thus ADS---Astrophysics Data System) many ADS (Aitken) numbers are
poorly indexed.  (For political reasons SIMBAD is larded with double-star
information from "CCDM", which is about the most unreliable stellar catalogue
that I know of.)
     I apologize for the northern bias, but this results largely from my modest
amount of southern double-star eyeball experience.  I welcome suggestions from
southern observers to add to the list.

     Okay, that's how I built the list.  My intention is that as part of just
about any sort of visual observing, be it hunting down Himalia or mere deep-sky
observing, it is useful to know what the seeing is on an absolute scale.  As
I mentioned previously on 'amastro', I found that deep-sky viewing, where
limiting magnitude threshold is important, required seeing no worse than about
1".5, at least with my 15cm refractor.  The old myth that deep-sky doesn't
require good seeing, is, well, a myth, and like all myths, is false.  So just
as one judges transparency by limiting magnitude estimates, one would like to
know the seeing as well.
     When I was doing a lot of double-star viewing with various apertures up
to the Lowell 60cm Clark, I found that the best pairs for estimating the
size of the images scaled fairly well with aperture.  Pairs whose components
were between mag. 5 and 7 were about ideal for the 15cm (6-inch) refractor.
A terrific pair of this description available in winter months is ADS 6263, the
brighter of the two pairs immediately east of Procyon (really easy to find!).
At a current separation of 0".9 (slowly closing), this is just right for seeing
checks for this aperture.
     With larger apertures one needs to go to progressively fainter in order to
avoid problems with scattered light in the eye (called "irradiation" by old-
time visual double-star observers).  Thus at 10-inches, you need to choose
pairs roughly a magnitude fainter, another magnitude fainter at 16-inches, etc.
Obviously my list is tailored for telescopes up to only 12-inches aperture or
so.  And if you get a lot of good seeing, at larger apertures you'll also need
much closer pairs to get accurate estimates.  However, I suspect many folks
will need those wider pairs as often as not.
     Without some objective measurement, it is difficult to say what
constitutes "resolution" or the actual image diameter.  From Flagstaff with my
refractor, usually the Airy disc is well defined, but moves bodily by some
small amount.  Thus my seeing estimate includes that image motion in addition
to the image size beyond the Airy disc.  Thus for example on a pair like
pi Aquilae, the total shift over an interval of many seconds might amount to
roughly their 1".4 separation, even though the Airy discs (0".85 across in the
6-inch) are well-defined.  Larger apertures will usually not show so much
bulk motion, but instead simple blurring since there are several seeing cells
in front of the aperture at any moment.  Here you need to decide where the
edge of the image is.  So long as you do this consistently (having test pairs
of similar magnitude all around the sky is one way of gaining consistency),
then with a certain aperture your estimates will become pretty reliable with
practice.
     I hope folks will try out this list.  If you know of "nice pairs" that
are omitted, I'd like to know about them.  My goal is to replace if possible
the larger delta-mag pairs with more nearly equal ones, and to add southern
pairs (gamma Circini comes to mind, for instance---gorgeous object!).
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