[APML]: Risp: measuring seeing with ST4

["Alfredo Zanazzo" <alferd@mbox.lol.it>]

Hi all

weeks ago I had posed a question about this subject, but I have been
ignored.
Maybe because of too few people on the list.
In the book AstrophotographyII pat Martinez shows a method that
consists in measuring the width of a star trail photographed at long
focal lenght (10 meter at least).
Turbulence is producing a drift of the star and seeing should be
equal to the maximum thickness of the trail.
To my opinion explanation is not sufficient, as you should consider
the drift on both left and right sides, and you should (I believe)
subtract star's diameter, as this would vary depending on star size.
Personally I evaluate differently.
Knowing image scale of my instrument (around 7 microns per arc
second) and considering that theoretical resolution is impossible
(I believe..) to achieve on a long exposure, at least I know for sure
that seeing must be LESS than smallest stars diameters achieved
on the negative divided by 7.
I measure diameters putting the negatives on a microfiche reader
with 40X and a graduated low power loupe on the screen.
A 100x microscope would do also.
Diameters decrease a little when I use red filters.
Turbulence can be measured also with ST4 by computing the
Average error, provided that CCD focus is perfect.
With my focal lenght (1600 mm) a constant A0 correspond to a
seeing better than 1,5 arc/sec A0-A1 to 2 arc/sec, A1-A2 to 2,5 a.s..
I consider A3 too high for a good photo.
All these parameters with short FL setting and SA setting to standard
position.
I would like to have comments about my experiences.
Am I right in your opinion or you could show me more accurate
measurements tecniques?
I hope Brad Wallis and Covington read this message and say their
opinion
Thanks
--
Regards, Alfredo Zanazzo ( alferd@mbox.lol.it )

Professional home pages:   http://www.lol.it/aziende/music/zanazzo
Astronomy home page (Off/axis guider-camera project):
http://www.lol.it/~alferd




----------
> Da: Steve Bell <sb635@delphi.com>
> A: astro-photo@nightsky.com
> Oggetto: Re: [APML]: Questions
> Data: mercoledì 18 settembre 1996 19.47
> 
> 
> Brad Wallis wrote:
> > 
> > 
> > 2.  Can hypered film be re-hypered?
> > 
> > NO .. you'd only add more fog ...
> 
> I have a related question. If some 2415 turned out to be not very
hypered, say, 0.05 above base (ie, not very 
> fogged at all), can you then stick it back in the tank and hyper it some
more?
> 
> 
> -- 
> Clear skies,
> Steve Bell
> sb635@delphi.com
> http://people.delphi.com/sb635 - Astrophoto page