J. Brit. Astron. Assoc., 108, 2, 1998, p.111-112
J. Brit. Astron. Assoc., 108, 2, 1998, p.111-112
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From Dr F. W. Thompson
For permanently mounted telescopes the usual method for accurate polar alignment involves observation of a star near the meridian and the celestial equator, and another star close to the eastern horizon, adjusting the telescope in azimuth and elevation respectively during these observations. This can be a tedious process, particularly since adjustments in azimuth and elevation interact. Furthermore, the adjustment in elevation is performed at a fairly extreme position of the telescope, and accurate adjustment of telescope elevation in that position does not guarantee good adjustment of telescope elevation in more frequently observed areas of the sky, given that few amateur telescope mounts approach perfection. I have found that, for my equipment, an alternative method which makes use of the rapid response of some CCD cameras, is quicker and better. My camera is a Starlight Xpress CCD colour camera, with a frame store. My telescope is a JMI NGT18 Newtonian, with a 460mm (18") f/4.5 mirror, and a split ring equatorial mount. The method is as follows.
As with other methods, the telescope should first be levelled. Secondly the optical axis and the polar axis should be adjusted as closely as possible to coincidence, by standard procedures. Lock the telescope securely in this position. Switch off all drives, set the RA approximately to the middle of its range, and lock it in this position. Take a CCD image. Since the drive is switched off, this will give star trails. Trails of at least 1" in length are suitable. If necessary, take another image, at an increased exposure, to achieve suitable trails. For my equipment an exposure of 8½ minutes, at minimum gain, is satisfactory. Examination of the image will reveal:
1. There is a gradient in length of trail, in a direction perpendicular to the trails.
2. The trails are slightly curved.
3. The direction towards the centre of curvature coincides with the direction of decrease of trail length.
The centre of curvature is, of course, the celestial North Pole, and the objective is to bring this to the centre of the field of view. To achieve this, increase the gain on the camera system to maximum, and find the minimum exposure which allows detection of a few of the brightest stars in the field. For my equipment this is an exposure time of eight seconds. Now cautiously alter either the azimuth or the altitude of the telescope, and take another short-exposure image, to observe the effect. The aim is to move towards the centre of curvature, in the direction previously established. Ideally, adjust azimuth and/or altitude until the last of the stars in the original image is about to leave the frame. Then take another long exposure, to produce a fresh set of star trails.
Two effects will be observed:
1. The trails are shorter.
2. The curvature is more pronounced.
Use this image to re-establish the direction of the centre of curvature. Then revert to short exposures and further telescope adjustments. Continue in this fashion until the centre of curvature is reasonably close to the centre of the field of view. The telescope is then very accurately polar aligned, at an RA near the middle of the range. Success is imminent when some star trails become perpendicular, or almost perpendicular, to others; the final image will show stars near the centre with virtually no trails, on a long exposure, surrounded by short, curved trails lying on concentric circles. The method has the advantage that, after the initial trial adjustments, it is at all times obvious whether azimuth or altitude adjustment is the more required, and both azimuth and altitude are corrected in one operation.
F. W. Thompson
109 Beltoy Road, Carrickfergus, Northern Ireland BT38 9DA
Oxfordshire's sky-friendly lighting policy From the Local Officer of the BAA Campaign for Dark Skies, Oxfordshire
Oxfordshire astronomer John Napper has an observatory and planetarium at his home in the village of East Hagbourne, with which he guides the public around the night sky. His only problem was the street lights outside his house which were not fully shaded. As CfDS Local Officer for Oxon., I put John in touch with Mr Peter Wilsker who is the principal lighting engineer for Oxfordshire County Council. The offending lights were promptly changed to full cut-off ones.
The CfDS gratefully presented Mr Wilsker and his team with a Good Lighting Award. Oxfordshire County Council lighting department is fully sympathetic to the CfDS cause and has a policy of only fitting full cut-off lights on all of its new roads and when renewing old lights - if only all councils would do likewise.
R. C. Dryden
21 Cross Road, Cholsey, Oxfordshire OX10 9PE
From Herr Dietmar Staps
I refer to the letters in the Journal on the above topic (107(1), 1997 February, p. 48, and 107(2), 1997 April, p. 107).
There are two sources in the literature which show earthshine and some features of the moon during total solar eclipses: W. T. Peters, 'Unusual solar eclipse photograph', Sky & Telescope, 40(2), 1970 August, p. 119-120; and J. Durst, 'Multicolour Photometry of the Corona', Proc. Indian natn. Sci. Acad., Vol. 48A, Suppl.No.3, p. 53-56 (1982).
Dietmar Staps
SONNE-eclipse and Corona Section, V.d.S., Schonbergstr.28, 65199 Wiesbaden, Germany. [dietmar.staps@wiesbaden.netsurf.de]
From Dr Allan Chapman
I wish very much to express my thanks and appreciation to those people who kindly made contact to supply me with information about Sir John Herschel and the Leeds Astronomical Society in 1863, following my article in the 1996 October Journal (106(5), p.252). I wish particularly to thank Professor Michael J. Crowe of Notre Dame University, Illinois, USA, who is preparing a collection of Sir John's letters and was able to supply me with valuable information. But most especially I wish to thank Ray Emery and other members of the modern Leeds Astronomical Society for their generosity in giving me access to the Society's Victorian 'Album' of documents, supplying me with copies of documents, and being extremely helpful in so many ways. My work on Sir John Herschel and the origins of the LAS will soon be published, with full acknowledgments, in my forthcoming book The Victorian Amateur Astronomer. Thank you.
Allan Chapman
Wadham College, Oxford OX1 3PN
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