Twilight, minor planets and the problems of  astronomical observing in general

Fig 4: Towards the Simmer Dim: Venus looking NW by W at 22h 00m UT, April 26 2007. Gemini far left, with Auriga (and Capella) above Venus and Perseus far right. Strong twilight and a Moon in Leo (phase 75%) well to the left outside the frame. A 30 second exposure 28mm f/3.5 ISO400. Springfield, Rousay.

Fig 1: Asteroid (16) Psyche imaged March 22 2007 at 20h 45m UT. A 40 sec. exposure 160mm f/1.8 ISO 400. Psyche is at the centre of the field between the two lines.

Fig 2: Asteroid (16) Psyche imaged April 4 2007 at 21h 15m UT. A 50 sec. exposure 160mm f/1.8 ISO 400. Psyche is at the top right of the field between the two lines. For reference the two stars mentioned in the text may be identified towards the top left in both figs. 1 & 2.

Fig 3: Leo imaged April 16 2007 at 21h 49m UT. The area occupied by fig. 2 is shown by the white rectangle. A 95 sec.exposure 36mm f/2.8 ISO 400.

Commencing April 25, and until August 19, astronomical twilight from the latitude of Orkney will last all night.

This may be of less account these days since there is now so much light pollution from the ground.

I sometimes think one has to be a little mad to attempt observational astronomy from this locality. Looking over my records for the past fifteen years I see that on average we have had fewer than two dozen good nights per year and far fewer first-rate nights in which it has been possible to observe from dusk to dawn. Looked at another way, the expenditure on equipment seems hardly justifiable when set against the actual number of hours during which it is used.

But of course it is not as straightforward as that. There are many instances in which one is able to grab the opportunity to see something outstanding or unusual by dint of good luck, such as when the sky decided to remain clear for most of the duration of the total lunar eclipse in March this year.

And there are many projects one may undertake that require a relatively short time at the telescope or with the camera. Transitory phenomenon such as the aurora, meteor showers or passing comets may yield rewarding and interesting result from a few minutes work where a break in cloud cover permits.

Minor Planets (or Asteroids) may be conveniently imaged these days using digital cameras. This too may only require a period of fifteen minutes or so to achieve a number of observations in a selected region of the sky. I give as an example the work I have done recently on Parthenope, Psyche and Thalia, three asteroids currently in the constellation Leo. All three objects were bright enough (at around magnitude 10) to be imaged with a fast lens in less than 50 seconds. I managed on several nights, with the cloud coming and going, to secure images of all three within the space of twenty minutes.

 Such observations do however require a relatively dark sky. One may tolerate astronomical twilight but once into high summer, with Nautical Twilight lasting all night from mid-May to the end of July, little may be done requiring longer photographic exposures.

In the two examples for Psyche given in Figs. 1& 2, stars approximately 1.5 magnitudes fainter than the asteroid are recorded. The scale may be assessed from the fact that the two bright orange stars to the left (TYC 849-136-1, mag 7.7 and TYC 842-1369-1, mag 7.1) are a little under 32 arc minutes apart. (Note the star TYC 842-922-1, mag 11 which is 62" below the latter.)

JV 16/04/07