1. Practical Telescope Demo Evening Classes Week Five Downstairs in Physics Green

2. Telescopes Evening Classes Week Five Presented by Oisín Creaner

3. Pupil

4. The Eye at Night The pupil widens to collect more light.

5. The Eye at Night 7mm

6. 50mm x 50mm = 2,500mm 2 7mm x 7mm = 49mm 2 50mm binoculars collect 50 times as much light as the naked eye.

7. Aperture Typical values are 5cm for modest binoculars to 30cm or more for large telescopes

8. Focal Length FocalPlaneFocalPlane

9. A second lens is used to re-diverge the rays.

10. The eyepiece is placed at the focal plane

11. Binoculars Point and look Adjust for each eye Useful on land, sea or sky! Relatively inexpensive

12. Refractors

13. Refractors - Pros & Cons ADVANTAGES Simplicity - no maintenance. Suitable for terrestrial viewing. No obstruction = high contrast. Sealed tube - no air currents. DISADVANTAGES Very expensive in large apertures. Long, heavy & uncomfortable. Residual chromatic aberration. Long f-ratios unsuitable for photography.

14. Telescopes Evening Classes Week Five Presented by Oisín Creaner

15. Reflectors

16. Newtonian Reflector

17. Open-end of a reflector

18. Reflectors - Pros & Cons ADVANTAGES Low cost for size No chromatic aberration. Compact & ergonomic. Short f-ratios possible. Optically simple. Can be built very large. DISADVANTAGES Obstruction = loss of contrast. Only suitable for astronomy. Open tube = air currents. Mirrors tarnish & need re-coating. Need maintance, esp. collimation. Off axis coma.

19. Schmidt-Cassegrain Reflector

20. Light-path in a Schmitd-Cassegrain Reflector

21. Catadioptrics - Pros & Cons ADVANTAGES Best of both worlds Large mirrors available Suitable for terrestrial viewing. Extremely Compact Sealed tube - no air currents. DISADVANTAGES Can be expensive. Obstruction = loss of contrast. Long f-ratios unsuitable for photography.

22. Alt-azimuth Mount Dobsonian Fork Point and shoot

23. Equatorial Mount Manual Motorised GoTo Photography

24. Eyepieces

25. SWAApparent field 82° Plossl Apparent field 52° KellnerApparent field 40° Real field = Apparent field/Magnification

26. 1° 39 arc min30 arc min

27. Barlow Lens 40mm - 20mm 32mm - 16mm 26mm - 13mm 20mm- 10mm 12mm - 6mm 9mm- 4.5mm

28. What can I see?

29. More like this

30. Appearance of Venus (1)Inferior conjunction (2)Greatest western elongation (1) (2)

31. Detail on the moon Craters Maria

32. Deep Sky Objects Clusters Galaxies

33. Thank You www.astronomy.ie/handouts www.stellarium.org Useful websites