Achromatic Refractor Telescope

Overview:
This project involves the design and construction of a small refractor telescope. For those familiar with building telescopes, and more specifically the equipment needs of an astrophotographer, much of the following will be old information, but I wanted to include some background on how this scope was built and also explain the role that it plays in my imaging setup.

About the optical design:
Refractors are what most people see when they think of a telescope - a long tube with a lens at one end and an eyepiece at the other. Modern refractors have at least two individual lenses that are stacked together to form the 'objective lens'. An objective with two lens elements is called a â€˜doubletâ€™, making the telescope an â€˜achromaticâ€™ refractor. An objective with three lens elements is called a â€˜tripletâ€™, making the telescope an â€˜apochromaticâ€™ refractor. APOâ€™s as they are sometimes called, offer superior color correction over doublets. They are also much more expensive due in part to their performance but also because they require that a total of six optical surfaces be figured and coated (three lenses, both sides).

Application:
My purposes in building a small refractor is to have a well-made guidescope for my astrophotography rig. The role of a guidescope is literally to guide a larger telescope while the attached camera takes a series of long exposures. From our perspective on Earth, the stars are constantly moving across the sky while our planet rotates. Astrophotographers use a computerized mounting system capable of tracking the Earthâ€™s rotation. The mountâ€™s axis is aimed at the celestial pole (near Polaris) and the accuracy with which the mount tracks any given target is a function of how carefully the mount is aimed by the user. Because no one is perfect, human error is always present. Over the course of several minutes the target will slowly leave the cameraâ€™s field of view due to a slightly imperfect polar alignment. The solution is to use a process called â€˜autoguidingâ€™. A second telescope with a second camera is mounted piggyback to the main imaging scope. The guidescopeâ€™s camera captures a live feed of a small patch of sky very close to the target being photographed by the other camera. This feed is processed by software running on a laptop where the user can choose a star to guide with. If the chosen star strays from the crosshairs, pulses are sent to the mountâ€™s stepper motors to compensate. The result is a target that stays framed on the main imaging cameraâ€™s CCD chip for hours at a time, thereby allowing for long exposures with nice tight stars and rich detail.

Design:
For various reasons I chose to build an achromatic refractor using a doublet objective with an aperture of 80mm. This was influenced by the availability of cheep glass. In my opinion, high-quality optics arenâ€™t necessary for a guidescope to do itâ€™s job perfectly well. Specs like color correction and field flatness arenâ€™t a concern unless you plan to use your guidescope visually. I donâ€™t, so I purchased an inexpensive objective from Surplus Shed (www.surplusshed.com) that has a focal length of 400mm. This focal length will offer a fairly wide field of view from which to select a star for guiding. In my experience with past guidescopes, an aperture of 80mm is wide enough to reveal a good selection of stars with varying brightness (also a function of the cameraâ€™s sensitivity). This aperture also makes for a fairly compact and lightweight telescope that will not add to much weight to my mountâ€™s load.

Construction:
Several components were purchased prefab, such as the 80/400 objective and the GSO single-speed crayford focuser. I machined the lens cell, focuser adapter, and dew shield from 6061 aluminum stock purchased online from Speedy Metals (www.speedymetals.com). The tube was purchased from Hastings Pipe Co (www.hipco-ne.com).

For those who are interested and/or savvy about related astronomy equipment, this guidescope will ride piggyback on an 8â€ f/5 astrograph (see my other project page). Both instruments will be mounted to an Atlas EQ-G with EQMOD and CdC at the helm. A QHY8 CCD camera will be used to image with the astrograph. I autoguide uisng an Orion StarShoot Autoguider and PHD Guiding v.1.9. My astrograph is currently a tandem ATM project to the guidescope and should be completed around the same time.

Thanks for checking out my project!


Boring out 6061 aluminum stock	I purchased several lengths of aluminum piping in various inner and outer diameters. Each section was bored out and turned down on a Cummins 7x12 lathe. Here the focuser adapter is being bored out to match the GSO focuser's outer diameter.

Here the focuser adapter is being turned down to fit snuggly inside the hastings pipe.	Turning down 6061 aluminum stock

Completed Focuser Adapter	The finished piece will be anodized red.

I'll be the first to admit this isn't the most ideal way to design a lens cell. Mine is basically a pressure fitting held together by three 4-40 machine screws. The depth of the lens well in the left-hand piece is slightly less than the thickness of the objective. This design was actually a last-minute improvisation after I realized my lathe wasn't cutting threads properly. I hope to replace this cell with a more traditional screw-down style where the interior of the lens well is partially threaded. The finished pieces will be anodized black.	Completed Lens Cell

Installed Objective (front view)	(Insert New Description)

(Insert New Description)	Installed Objective (rear view)

Machining the dew shield	Another confession - here I'm doing it wrong. My lathe isn't intended to turn out something this large, so I used a fully extended boring bar to turn the outer diameter of the dew shield. After several passes I was able to gradually eliminate the amount of chatter in the tool. In the end I got a nice smooth surface that looks like it was machined with a standard turning tool.

This is a homemade template I created to help me drill perfectly verticle holes into the curved surfaces of the focuser adapter, dew shield, and hastings tube. The jig is held in place by a milling vise installed on my drill press. The vise allows fine positioning of work under the bit.	Homemade drilling jig

Focuser adapter & drilling jig	Here I'm using the drilling template to pop three mounting holes int he focuser adapter. These holes will line up with equally spaced ones in the hastings tube.

(Insert New Description)	Complete view of parts

Lens cell & dew shield installed	(Insert New Description)

I recently had first light with this bare-metal OTA on February 21, 2009. My autoguider came to focus at about two thirds travel, a bit further than I'd planned but not an issue at all. Stars came to a nice points in the PHD viewer. That'll work! Collimation is....well, imperfect. I attribute some of my problems to the hastings pipe being less than perfectly round. The cross-section is actually a very slight oval with some excentricity that various about the circumference. Andy at HPO tells me that their piping, while very popular among ATMers, is not manufactured to be perfectly round because it's not necessary to perform perfectly well as irrigation pipe. Makes sense. At some point in the future I may seek out a better tube for this scope. In the meantime it does it's job exceptionally. The final appearance of this OTA will be black powder coating for the hasting tube and dew shield. The focuser adapter will be anodized red for a subtle color accent that matches those found on the astrograph. The lens cell will be anodized black.	Assembled OTA

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