The Astro-Tech AT50GS 50mm guidescope refractor is superb for guiding long-exposure astrophotography with refractors and imaging Newtonians up to 1200mm in focal length. However, it is also usable with longer focal length scopes, such as the Astro-Tech Ritchey-Chrétien astrographs.
Taking deep space photos used to require tedious and error-prone manual guiding through a crosshair eyepiece and a large aperture, long focal length guidescope. However, the guiding accuracy of modern CCD autoguiders allows hands-free automatic guiding of long exposure photos through smaller guidescopes with much shorter focal lengths. Using an optically "fast" guidescope, such as the 50mm f/3.6 Astro-Tech AT50GS, you save valuable time, space, and money, while still recording the pin-point star images that only autoguiding can bring to long-exposure astrophotographs.
How well does the Astro-Tech AT50GS perform as a guider? See for yourself. The feature image above of the Horsehead Nebula in H-alpha was taken by Dennis Wigley, John Dutton, and Kevin Davis. It used Dennis and Kevin's 80mm f/7 Astro-Tech apo refractor and John's mono Orion G3 camera. Dennis supplied the Meade LXD55 mount, Meade DSI pro guider, Orion 1.25" H-alpha filter, and Astro-Tech AT50GS guidescope. The 50mm AT50GS guided for nine 15 minute exposures and one 10 minute exposure – a total of 145 minutes of guiding. We invite you to check the roundness of the stars in the highly enlarged image above to see just how accurately the AT50GS can guide.
The Astro-Tech AT50GS guidescope is a 50mm aperture 180mm focal length (f/3.6) achromatic refractor with a helical focuser. It has dual mounting rings with nylon collimation screws (three per ring). The rings are attached to a dovetail mounting bar. Add an optional small-chip CCD autoguider (such as the Celestron NexGuide), and you ready for automatic guiding.
The guidescope’s 1.23” (31mm) wide by 3.5” (89mm) long dovetail mounting bar fits into the dovetail finder base found on most Astro-Tech telescopes. If your imaging scope doesn't feature a dovetail base or has one already occupied by a finder, the included dovetail bar has predrilled holes that let you attach the Astro-Tech guidescope to almost any optical tube. It can mount in the alternate finderscope position at the 1:00 o’clock position on the rear cell of most Schmidt-Cassegrain scopes. It can also mount on one of the tube rings holding a refractor or reflector optical tube (which are usually drilled and tapped to hold just such an accessory).
The Astro-Tech AT50GS guidescope weighs only 15.2 ounces (20 ounces with the mounting rings and dovetail mounting bar). Its light weight lets you replace a heavier guidescope to gain a little breathing room with imaging setups that are pushing your mount's payload capacity. It measures 8-1/16” (205mm) long with the focuser collapsed. With the focuser fully extended it measures 9-7/16” (239mm) long. A 48mm long fixed dew shield shields the fully multicoated cemented doublet lens from stray light.
The Astro-Tech guidescope’s helical focuser has a fixed-orientation design that does not rotate your CCD autoguider during focusing. The helical focuser simply moves the attached autoguider in or out while maintaining its set orientation. This ensures that the field of potential guide stars will not rotate when focusing.
The focuser has two parts. The first is a drawtube with 23mm of travel slides in and out of the guidescope’s body to provide a rough focus. The tube has a scale marked in 1mm increments so you can return to the correct extension distance whenever needed. A thumbscrew holds the drawtube in position.
The second part is the helical focusing mechanism itself. This has a knurled focusing collar and adds another 10.75mm of travel for fine tuning the focus. A 0-10 scale on the drawtube has sub-markings that show each 1/10th mm of drawtube extension. A thumbscrew holds the helical drawtube in position.
The focuser terminates in a 1.25” accessory holder for those guiders that have a fixed 1.25” nose piece for connecting guider to guidescope. There are also male T-threads for a more solid and secure direct attachment of a guider whose nose piece can be removed to expose female T-threads for attachment to a guidescope.
When not being used for autoguiding, the Astro-Tech 50mm can be used as a finder scope. Just insert an optional Astro-Tech 24mm illuminated reticle eyepiece (#AT24IL) into the focuser and the versatile 50mm Astro-Tech guidescope turns into a 7.5x50mm straight-through illuminated crosshair finder with a 6.6° field of view and a long 20mm eye relief.