For serious astrophotographers, an optical tube that has quality optics and suitable specifications is key to producing successful images. Astrographs come in many different specifications and designs, so each model can be oriented or preferred for a different type of imaging (eg. solar, terrestrial, deep sky etc). Some common designs include the Ritchey-Chrétien, Riccardi Honders, and Catadioptric optical tubes. Choosing an astrograph depends on how the astro-imager wants to observe, what focal length and focal ratio it has, aperture, optical coatings, and how suitable it is for the camera they intend to use.
A key specification very important to consider when choosing an astrograph is focal ratio in relation to aperture. This is an important to remember because focal ratio determines what the magnification and field of view the observor will get. Therefore, if the observor intends to mainly view deep sky objects with lower surface brightnesses, a shorter focal length will reduce magnification and increase field of view since most deep sky objects tend to appear very large but not bright. Astrograph telescopes with slower focal ratios such as an f/12 will provide higher magnification and smaller field of views; better for planetary observing.
The 20" AG Optical astrograph has a medium focal ratio of f/6.7. This optical tube has a two element <0.5% reflective corrector which makes this possible by reducing many of the common light distortions such as coma.... Learn MoreFREE SHIPPINGModel#: 20IDK(48 States)
A great benefit behind the Dall Kirkham design is excellent off-axis performance. The 12.5 astrograph from AG Optical is in a carbon fiber tub and has a medium focal ratio with a large aperture to support a variety optical and astrophotography combinations.... Learn MoreFREE SHIPPINGModel#: 125IDK(48 States)
Imaging Dall Kirkham (iDK) astrographs are high resolution instruments with outstanding on and off axis performance across a 60mm field of view.... Learn MoreFREE SHIPPINGModel#: 145IDK(48 States)
The AG Optical 17" iDK Astrograph is an excellent astrograph for a wide variety of astrophotography. The telescope has a medium focal ratio of f/6.7 with 6 micron RMS spot size. This means that stars appear very pinpoint while still at large field of view.... Learn MoreFREE SHIPPINGModel#: 17IDK(48 States)