Re: Solar Transit Observations - Difficulties Thereof

Robert Sheaffer (sheaffer@netcom.com)
Sun, 4 May 1997 11:12:08 -0700 (PDT)

Bruno Tilgner
> 
> Recently, several possible solar transit observations have been reported here.
> 
> Five unsuccessful attempts for such observations, undertaken in the last few
> days by a couple of friends and myself lead me to the following conclusions:
> 
> 1. A satellite or rocket body in the 6m to 10m size class at typical distances
> between 1000 and 2000 km has an angular size in the order of 1 arcsecond,
> give or take 25%.

Yes, this is correct.
> 
> 2. A rule of the thumb for the resolving power of a telescope objective lens
> is "13.8 arcseconds/objective diameter in cm". To resolve objects 1 arcsec
> apart, the minimum size of the objective lens is thus around 5 inches.
> 
> This assumes perfectly collimated optics and does not account for the effects
> of bad seeing which in the case of solar transits is inevitable due to air
> turbulence.

Yes, that applies to separating two objects of equal brightness. However,
it does not apply to determining the visibility of a dark object against
a bright background, or vice-versa. To pick an obvious example: the
apparent size of a bright star is some tiny subfraction of a second of
arc. However, we can easily see it with our naked eye, owing to the
sharp contrast with the background. Likewise, a human hair can be
detected against a bright background when it is far smaller than the
above parameters suggest.
> 
> 3. Even if the object can be resolved by the telecsope optics, the human eye
> possibly cannot. Another common rule of the thumb says the eye's resolving
> power in arcseconds is "720/eye pupil's diameter in mm". The eye pupils'
> diameter is usually assumed to be 6 mm (depending on age), and this leads to
> the figure of 120 arcseconds often found in the literature.
> 
> However, 6 mm corresponds to a dark-adapted eye. Under bright illumination,
> which can certainly be assumed for the observation of the sun (through a filter
> or by projection), the eye's pupil is closer to 2 mm in diameter. The eye can 
> then only resolve objects of at least 360 arcseconds size.

"Resolve" is not the same as "detect". Remember, "resolve" in this
context means the ability to distinguish as separate two closely-joined
identical objects.

> 
> 4. The typical satellite/rocket body at a typical distance, observed through
> a telescope of suitable opening, must therefore be magnified at least 360 times.
> 
> At this magnification it is exceedingly difficult to aim the telescope at a
> certain point on the solar disk, assuming it can be calculated at all which
> region of it will be crossed by the object. Magnifications which still show
> the entire solar disk in the field of view (i.e. in the order of 50 to 80
> times) are clearly not sufficient no matter the size of the telescope aperture.

It is indeed difficult to scan the solar disk for a tiny, fast-moving
object. It is not, however, impossible. Only very difficult.

> 5. When staring for a minute or more at a bright surface like the sun or
> the moon in a telescope, strange objects tend to appear, all of which are
> inside the eye. Medical doctors call them "mouches volantes". Most of them
> cannot possibly be confused with satellites because of their thread-like
> appearance, but sometimes a more round or square object crosses the field of
> view.

Yes. They have their own characteristic type of motion, floating specs
in a fluid. A satellite's transit will last about 2 seconds; these will
hang around considerably longer. One must learn to recognize these
"flittering flies".

> 
> 6. In view of all the arguments above I have reluctantly conluded that the
> observation of solar transits of anything below the size of Mir is not
> feasible with amateur equipment. 

Rob Matson sent me a GIF file of the transit path of Cosmos 1763 R/B
(#16864) across the sun, predicted for 15:55:54 UT here Friday morning.
Its predicted size was 1.2", making it one of the larger such objects.
It exactly matched the path of the object that I earlier observed crossing
the sun, whose passage I had clocked at 15:55:56. I have to consider that
a definite "hit". I have "probably" glimpsed a few others, but was
unable to confirm them by tracking them all the way across the face
of the sun. Still, a transient spot was observed at about the correct
time, and no similar spots were seen at other times. I believe that
I glimpsed a transit of Copernicus, but only briefly. The seeing 
needs to be favorable, and when observing the sun it seldom is. There
was a predicted favorable transit on Saturday of #7284 at 20:26:52 UT, and 
I could not see it. However, the seeing was not great. On Friday morning
I could see the contrast on the solar surface between the lighter and
darker regions; that was not visible on Saturday afternoon.

-- 

        Robert Sheaffer - robert@debunker.com - Skeptical to the Max!
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