Re: Solar Transit Observations - Difficulties Thereof

jean monseur (
Tue, 6 May 1997 11:16:42 +0200


Some questions about this most interesting debate on satellites
solar transit...

At 11:12 04/05/1997 -0700, you wrote:
>> 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.

Also the rope of TiPS can be seen on the dark sky while it is much
smaller than a human hair at that scale, because it is violently lit
by the lateral sun.

But how do you intimately explain that a satellite can be seen in
front of the sun while it could not  against the simple sky
during daylight ?  Please, explain the role of filters.

>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.

Dowloadable ? Uploadable ?

Many thanks  
Lat +49d 05'    Long 359d 37' E