My 8" f/7 telescope is optimized for planetary and other viewing requiring high resolution and contrast. The tube is constructed of concrete formtube covered with Monokote. The inside is lined with crushed walnut and painted ultra flat black.

My scope includes an 8" long extension on the front of the scope to reduce glare, something that can be a problem with low-profile focusers. The end rings are each made of two 1/2" thick sheets of finish birch plywood that were cut with a router and then glued together. The finder is a Daisy BB gun reflex site. The telescope is shown with a homemade low profile helical focuser that has since been replaced with a JMI NGF-mini2 crayford focuser. When this picture was taken, the primary had been moved forward to give better access to the focal plane. Under typical operating conditions, the focuser has a profile of about 1.75 inches.

    In selecting and designing this scope, I gave careful consideration to what I wanted from a scope and other issues such as portability, etc. Essentially, I wanted a telescope that would give me high resolution images of the planets, yet would be practical for me to build and use. Of course, the telescope would have to fit within my budget. I was inspired by an article in Sky and Telescope by Gary Seronik on building a 6" planetary telescope. After careful consideration of all my options, I settled on an 8" f/7 or f/8 newtonian.  An 8" newtonian is a medium-sized instrument that has the potential of producing great images. It would also fit within my budget. A newtonian telescope has the potential of closely approaching the performance of an apochromatic refracter. Consider that a 6” apochromatic refracter of over 6” aperture by Astro-Physics costs thousands of dollars (the OTA of an AP 155, for example, costs $5,400), a newtonian can give quite a bang for the buck.  Testimony for this was recently given by Ed Ting who described a comparison between a high end refracter (an AP130EDT; 5.1” f/8 - $3650 for the OTA) a catadioptric (Intes MN61 Maksutov-Newtonian; 6” f/6), and a newtonian (a Starmaster 7” Oak Classic; f/5.6 - ~$1,000 on dobsonian mount). Five experienced observers in a careful comparison of optical performance, both on planets and deepsky objects, rated the Starmaster and AP130 very closely. The biggest criticism given the Starmaster (with superb optics) was that the diffraction spikes were distracting. Indeed, the main drawback to a newtonian reflecter (assuming that it is of excellent optical quality) is that the secondary and its supports create a slight loss of light and diffraction that can degrade the image. Also, there is usually reduced reflectance of the double mirror reflection as compared to the greater light transmission through the refracter’s lense. Also, an open tube with the optical path crossing the same space twice might result on a less steady image. However, some of these drawbacks can be addressed to bring them to a minimum.

    For Gary Seronik's 6" Planetary Scope, he ground, polished and figured a superb primary with smooth figure and of relatively long focus (f/9). He used this in conjunction with a small secondary (0.75”) (an obstruction of the diameter of the primary of only 12.5%) and a low profile focuser. In addition, he advocated thorough baffling of the tube to reduce contrast-robbing internal reflections and used a curved secondary to eliminate diffraction spikes. I wanted to emulate this to some degree.

    I have no experience in grinding and figuring optics. Therefore, I decided to purchase my optics from somebody who did. I ended up purchasing an 8" f/7 primary mirror from an experience optician in Toronto, Ric Rokosz. Ric had originally made the mirror for his own use and so put a lot of time and effort into it to produce and extremely smooth and well-corrected figure. Ric figured the mirror by way of the autocollmination test - against a 12 inch flat and using a ronchi grating (80 lpi). The surface was polished with rouge giving it an ultra smooth surface compared to most commercial optics (most commercial optics are machine-polished using Cerium Oxide. I have done extensive star testing (see Harold Suiter's book Star Testing Astronomical Telescopes) confirms that it has a very smooth and well-corrected surface with no obvious optical defects. I had the mirror recoated by H. L. Clausing inc. with a Beral™ coating. Beral™ is reportedly harder and more durable than aluminum and probably also dialectric films in enhanced coatings. It has a reflectivity of about 91%.

    The primary mirror is mounted in a homemade primary mirror cell. I used the program Automated Mirror Cell Optimization (PLOP) to determine how to best support my 8" primary. The mirror is a full thickness (1.5") pyrex mirror. Pyrex has better thermal characteristics than soda lime glass. Traditionally, designers have advocated supporting an 8" primary cell at three points spaced at 120 degrees and placed at 75% of the mirror's radius. Plop analysis indicates instead that the primary is best supported at only about 40% of the radius and so this is what I followed. The mirror is supported by about 1" diameter pads of silicone. The mirror cell is well-ventilated and contains a small 12 volt muffin fan that speeds cooling of the primary and helps reduce tube currents. The fan sucks air in from the front tube opening and blows it out the vent hole in the back, behind the fan. It is adjusted by means of three collimation bolts, compression springs and wing nuts. It maintains collimation remarkably well.

The primary mirror on its mirror mount. A central hole in the mirror mount promotes air circulation. Note the sticker on the underside of the mirror. It has been coated by H. L. Clausing Inc. with a beral coating.

The primary mirror is supported at 40% of its radius by pads of silicon adhesive.

The primary mirror is supported on a mirror mount that is separated from a disk of plywood by compression springs. Collimation is adjusted with wing nuts. Immediately below the center of the mirror is a 12 volt muffin fan. 

The back end of the OTA shows the rear of the plywood disk that supports the mirror mount. Note the three wingnuts for adjusting collimation. A 12 volt muffin fan cools the mirror and reduces tube currents. It may also eliminate a boundary layer of air from forming immediately above the optical surface that might degrade the image (see "Thermal Management in Newtonian Reflectors" by Alan Adler, Sky and Telescope, January 2002, p. 132 - 136). Since this picture was taken, I've added a rheostat for controlling the mirror fan speed to minimize vibration. A jack (on the right aluminum panel) is for plugging in a 9 volt power supply to the fan.

    In order to help this mirror reach its full potential in the finished telescope, I decided to use a low profile focuser with a relatively small secondary. According to Newtwin, a low profile focuser allows me to reduce my secondary to just over about 1" and still get a 100% illuminated field at the eyepiece. However, secondaries often show edge defects and so some recommend that a slight larger secondary be used to stay clear from the edge. I decided to use a 1.3" minor access secondary giving an obstruction of 16%. This obscures only 3% of the area of the primary. For my secondary, I purchased a flat from the Protostar ULS Quartz series with a 1.3” minor axis. These are made from a fused silica quartz. These are very smooth and reportedly polish 2 to 4 times smoother than Pyrex glass and are coated with an enhanced metallic/dielectric coating. Each flat is provided with an interferogram. For comments on rating optics, see Mel Bartel's site "Rating Mirrors." Here is the interferogram provided with my flat. Note that my flat is very smooth (0.099 peak-to-valley surface, 0.02 [1/50] RMS at 633 nm wavelength of light) but it does have a slight "potato chip" figure. There is a high spot near the top edge. The secondary is oversized enough that I can stay off of that edge for the 100% illuminated field (approximately the inner 1.1" diameter, minor axis) for high resolution imaging. If I neglect the edge, the flat has a peak-to-valley surface of about 0.08 (better than 1/12) wave and less than 1/50 RMS surface flatness. According to R. F. Royce of Precision Optical Components, this flat would have a wavefront error of 0.0428 at 587 nm giving a strehl ratio of about 0.93 (including the edge) and so this flat would rate between Good and Very Good (strehl ratio between 0.92 and 0.94).  If I neglect the high top and bottom edges, this value would be somewhat higher.

   For a secondary support, I initially used a homemade curved vane secondary. However, I found that it was difficult to collimate with this so I purchased Three-vane micromount secondary support from Protostar. A three-vane support results in 6 diffraction spikes spaced at 60 degree intervals rather than 4 diffraction spikes spaced at 90 degree intervals (see Protostar for simulation of diffraction spikes). A three-vane spider results in a 25% reduction in light scattering from the vane supports relative to a four-vane spider. 

I use my 4.25" f/4 newtonian as a large finder scope. The mounting rings are made from finish birch plywood cut on a router. Brass inserts held in place with epoxy hold nylon bolts for alignment of the scope. A 40 mm eyepiece provides a magnification of under 11x and a 3.7 degree field of view (but a large, 10 mm exit pupil).

The JMI NGF-mini2 crayford focuser. Smooth and precise motion and low profile. 

    In high resolution observing an imaging, proper focusing can be extremely crucial to obtaining the best results. I used a homemade helical focuser for several years. This gave me a very low profile of only 1.5 inches.  However, when collimating with an Orion laser collimater, I discovered that the laser beam would describe a circle over the primary when I rotated the focuser. I also found that there was a bit too much slop in the thing as it would jiggle if not clamped down with nylon bolts. Focusing my various cameras was also problematic as the drawtube would rotate with the focusing ring. I decided to replace this focuser with a crayford focuser from Jim's Mobile, Inc. (JMI). I went with an NGF-mini2, a 1.25" focuser rather than a 2" focuser because the small secondary would not provide a well-illuminated 2" field. I don't currently own a 2" eyepiece anyway. The crayford provides a very smooth movement and maintains perfect alignment of the eyepiece or camera during focusing. I plan to eventually motorize the focuser with a stepper controlled by my PCB and handpaddle.

    Proper collimation is vital for best optical performance. I use a Cheshire eyepiece and a LaserMate™ Collimator  from Orion. I've recently begun to collimate with a barlowed laser as described by Nils Olof Carlin in the January, 2002 issue of Sky and Telescope. This techniques makes collimation extremely quick and easy and can be easily accomplished in the dark. I can't recommend this technique more strongly.

    Under excellent seeing conditions, I typically use my Nagler 7.0 mm with my Televue 2x barlow giving me 400x (50x per inch of aperture) and don't see any significant image breakdown when viewing low contrast features on Jupiter and Saturn. In other words, based on my qualitative observing criteria, I would rate my 8" optically as good to excellent.

Links 

Newtonian Telescope Design Planner and NEWT software provide excellent help in newtonian telescope design.

ATM archives

Books

"Build Your Own Telescope" by Richard Berry

"How to Make a Telescope" by Jean Texereau

"The Dobsonian Telescope" by David Kriege and Richard Berry

Articles

"An Optimized Newtonian Reflecter" by Gary Seronik, Sky and Telescope, June 1997, p. 83 - 86.

"Sizing up the Newtonian's Secondary" by Gary Seronik, Sky and Telescope, August 2000, p. 120 -123.

"Diagonal Calculations with Sec" by Alan Adler, Sky and Telescope, August 2000, p. 123-125.

"Flexing Spheres into High-Quality Telescope Mirrors" by Alan Adler, Sky and Telescope, November 2000, p. 131 - 140.

"Thermal Management in Newtonian Reflectors" by Alan Adler, Sky and Telescope, January 2002, p. 132 - 136.

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