After a star tracker, the amateur imager reaches a point where choices have to be made on the future direction of his/her hobby.  When the limits of the tracker have been fully explored, and more advancement is sought, one must begin to think seriously about acquiring a telescope. 

Generally, there are 3 different styles of amateur Astronomical study; including Observation, Planetary (including the Moon), and Deep-Sky Objects ("DSO" - nebulae, galaxies and star clusters).  Each carries with it a different approach toward equipment, including both the telescope and its mount style.  The following table outlines recommendations for equipment that best fulfills each astronomy style:

                                                                                                                                                                                    Good for
     Astro style     Common telescope style        Typical FL       Mount style                                              AP?            Tracking
     Observation    Dobsonion                                   > 1000mm      Altitude-Azimuth ("Alt/Az")                         No           Typically manual - either push-pull, or geared control knobs
     Planetary         Schmidt-Cassegrain ("SCT")      ≥ 2000mm      Altitude-Azimuth ("Alt/Az")                         Yes           Internally computer-driven "GoTo"; No guiding needed to assist
     DSO                  Refractor                                      < 1200mm      German-Equatorial ("GEQ" or just "EQ")  Yes           Either internal or remote computer-driven "GoTo"; With guiding to assist

Observation - Many observers tend to favor this aspect of the craft; because, they love the thrill of literally seeing with their own eyes the wonders the night sky has to reveal.   Because of their large apertures, Dobsonian telescopes offer the ability to see details not apparent in other types of telescopes.   (Their owners often refer to them as "light buckets".)  Imagers, on the other hand, also tend to favor their aspect of the craft; seeking enjoyment from learning and successfully using advanced photographic and computer controlled technical equipment, and of seeing the colors and details revealed in the resulting images.  Generally, most amateur telescopes can be "rigged" for one (observing) or the other (imaging); but not both.

Planetary - Being some of the most beautiful objects in our nighttime skies, the planets may be enjoyed thru either Observation or AP.  For those who want to pursue AP, equipment and skills are required that are very unique, and differ significantly from DSO imaging.  The biggest reason for this uniqueness is that, except for the Moon, the apparent size of any one of the planets is extremely tiny.  And, they are also very bright - lending themselves well to short-exposure imagery.   Because of their tiny size, the planets require a very long focal length to bring them "close" enough to either observe, or photograph adequately.  While observers enjoy the ability of similar equipment to serve well for both Planetary and DSO observations, AP is where the similarity diverges.  ...because such long focal lengths are required, the "shimmering mirage effect" of Earth's atmosphere (...also called 'Seeing conditions') comes strongly into play.  The effect is only somewhat irritating for observers, but for AP, it is devastating.  To mitigate the effect, experienced imagers have developed a very unique approach to acquisition and processing sometimes called, "Lucky Imaging".  Similar to (but distinct from) the 'stacking' of DSO sub-frames (below), the Planets require that hundreds (even thousands) of short exposures (recall the planets are bright; so short exposures work well) are captured within minutes through the use of a camera capable of capturing motion video.  The best choice of camera to enable this technique is either a DSLR with internal video capture capabilities, or a dedicated planetary astro-camera, also with fast frame capture capability.  Through the law of statistical probabilities, a small number of captured frames will 'catch' the target during extremely short periods when the seeing conditions are at rest, resulting in a few good, "Lucky" captures.  Typical capture retainer rates are only 10-20% of the total number of captures.  The rest are discarded.  To obtain the sought-after sharp, detailed final image, imagers use very specialized planetary processing software capable of quickly sifting-through and analyzing those thousands of sub-exposures, and selecting a small sampling of best quality for inclusion into the final stacked (combined) image.  Then further sharpening and color adjustment of the stacked result are typically needed.

DSO's (nebulae, galaxies and star clusters) - Except for the very brightest ones, many DSO's typically do not lend themselves well to visual observation; because, they are simply too faint to see with the human eye; even when magnified through a telescope.  Arguably the most popular style of AP, it is this 'faintness factor' where the capturing and processing of DSO images requires long-exposure imagery techniques to be successful.  Today's best technique for imaging these objects involves capturing dozens of long duration "sub-exposures", and then "stacking" (combining) them to improve contrast and reduce electronic digital noise.  Total integration times usually extend into hours (even days).  ...and because of these long exposure durations, the use of guiding equipment is recommended to assist the telescope mount with tracking.  Further recommended is to use a German-Equatorial style of mount.  EQ mounts were developed specifically to counteract the effect of 'field rotation' inherent with the Alt/Az style of mount.  Field rotation is easily demonstrated by taking note of the apparent orientation of a star pattern or object when it first appears in the early evening above the eastern horizon.  Then as the night progresses, it slowly turns into an upside-down orientation just before it sets in the west.  Unlike an EQ mount, an Alt/Az mount (without a 'wedge') is unable to compensate for this rotation.  An Alt/Az is therefore inherently restricted in both, the length of exposures it can capture, and the framing of the target with respect to the image frame.  ...thru the course of an evening, the target is constantly rotating, thus adding a complicating factor to the ability to process sub-exposures.   An EQ mount eliminates both of these deficiencies by virtue of its inherent design.  Most amateurs engaged in DSO imaging quickly acknowledge the advantages of an EQ mount; but, there are some 'trade-offs',  including the need for very accurate polar alignment of the RA axis parallel to the rotational axis of the Earth, and the need to perform a 'meridian flip' sometime mid-way through an evening's imaging session.  Nonetheless, most DSO photographers readily accept these 'shortcomings' in order to secure the quality of images that can be achieved with an EQ mount.
200731 - ES102CF Refractor
