Brief summary of “Basics of Astrophotography by McWiki” 15 Januari 2010Posted by azmirul in Astronomy, Astrophotography.
for info, maklumat di bawah adalah summary (plus tambahan info aku skit) daripada website “Basics of Astrophotography – McWiki” by Richard McDonald, http://www.themcdonalds.net/~themcdo/richard/index.php?title=Basics_of_Astrophotography
Astrophotography: Fundamental Questions
Without a Telescope
- Star Trails -> use tripod & long exposure to celestial pole to get images of semicircular arcs centred on the celestial pole. set the focus to “infinity”, adjust aperture (f-stop) and the sensitivity (ASA or ISO) -> do not over-expose
- Constellations and Milky Way -> use wide-angle lens, 30- to 60-second exposures
- use telephoto lens
With a Telescope
- Bright Targets -> simple
- Dim Targets -> require long exposures (minutes or hours) & require a motorized mount
Visual Astronomy vs. Astrophotography
- balance between type of astrophotography and choosen visual astronomy equipment.
Astrophotography: Equipment Types
- Telescopes, Cameras, Mounts, Autoguiding & Computers and Software
Astrophotography Equipment: Telescopes
- Aperture is very important for visual observation but less important to astrophotography because you can use longer exposures to gather more light. Image with small aperture telescope can show more detail than using your eye /eyepiece.
- For beginners, faster focal ratio makes astrophotography easier since shorter exposure times are needed
- Telescope Optical Designs (Refractors, Reflectors, Catadioptric)
– Reflectors need a secondary mirror suspended in the tube near the top. The four thin support arms that hold the mirror cause bright stars in the image to have “diffraction spikes” (4 thin arms of light that protrude from the stars).
Astrophotography Equipment: Cameras
- “Point and Shoot” Cameras
– afocal photography = set to “infinity” focus, align camera behind eyepiece -> use mechanical jigs.
- Digital SLR Cameras
– Afocal mode = same as above
– Eyepiece projection mode = camera without lens is placed behind eyepiece
– Prime focus mode = camera without lens attached directly to telescope
- Webcams and Small Video Cameras
– Eg: Computer webcams, commercial “Lunar and Planetary” cameras, industrial low-light video cameras, astronomy live video cameras.
- T-Mount (most common mounting standard) – a threaded tube about 42mm in diameter, 0.75mm thread.
- C-Mount (less common) – smaller diameter than the T-Mount: one inch in diameter with a 32tpi thread.
- photo processing -> taking multiple images and “adding/stacking” them together on your computer
- take image of “dark frames” for noise cancellation.
- use extension tubes in prime focus photography to extend focus distance (not problem in afocal mode, just use infinity focus)
- use focal reducer to reduce telescope focal length. 0.6x reduction is very common. Eg, New focal length if using 0.6x focal reducer on 1000mm focal-length telescope is 600mm. focal reducer can also be used to:
– reduces the focal ratio -> 0.6x focal reducer, an f/10 telescope becomes an f/6 telescope
– as “flatteners” or “correctors” – reduce/correct out of focus at edge of the field
Astrophotography Equipment: Mounts
Mount type is important to take long-exposure imaging of deep space objects:
- Non-motorized Mounts – no electric tracking motor (eg non-motorized equatorials, alt-az, and Dobsonians).
- Motorized Fork Mounts
- Motorized German Equatorial Mounts
Most manufacturers supply mounts in a range of sturdiness. For example, the EQ series of mounts are rated EQ1, EQ2, through EQ6, with each being larger, heavier, and sturdier the one before. For astrophotography, your mount should be at least one grade heavier than what would be considered a good sturdy mount for visual use.