FAll Semester 2005 Project

The fall semester 2005 Astronomy 101 Lab Class was the first class to fabricate parts and assemble a fully working 6”F5 Dobsonian (Dob) telescope from a parts kit.  Each student was required to hand grind on a standard 8” pyrex mirror blank that will be completed several semesters later. 

The 6”f5 Dobsonian AZ-EL kit was supplied by amateur telescope builders in the area.  The 6”f5 mirror and diagonal were purchased already finished.  The AVC has no facilities to actually fabricate a mirror to completion.  The Dobsonian kit included all wood parts for the Azimuth rotation base, the Elevation support box, the telescope fiberglassed cardboard tube, spider, mirror cell wood, and miscellaneous bolts and screws and pvc pipe. The primary mirror and the diagonal secondary mirror, and focuser were purchased already finished.  

A handout provides some guidance to assembly and preparation of all the parts.  It is up to the students to find more information about Dobsonian telescopes and to perform any modifications they feel necessary to help the scope.

For experience the class members are required to practice grinding on a standard 8" mirror blank.

A pyrex  8” mirror blank is being hand ground in class on a glass tool of the same size using various grinding grits. Rough hogging out of the 8” mirror to make a concave curved surface requires 20 to 30 hours. The first grit used in hogging the 8” mirror toward a concave spherical surface is 80 grit. If and when the 8” mirror is ground concave enough to give a 50” focal length then fine grinding can begin. 

(Note: the focal length, FL., of a telescope mirror is ½ of the concave radius of curvature, Rc, of the concave surface)

Fine grinding will require grinding using the following grit sizes: 120 grit, 220 grit, 320 grit, 400 grit, 500 grit, 600 grit, and finally a very fine grit like 800 grit. Grinding takes about 2 hours per fine grit.

When the 8” mirror is successfully ground through the fine grits it will be time to make a pitch lap so that the mirror can be polished smooth. 

Polishing the concave surface of the 8” mirror requires hours of pushing and pulling until the mirror glass surface has a smooth glassy finish.  Polishing will take about 1 hour per square inch of mirror surface ( pi * r * r = 3.14 * 4 * 4 = 50.2 square inches).  Polishing can take another 50 hours.

Once the 8” mirror is ground and polished it will be time to “figure” the concave surface of the mirror into a smooth mathematical concave curve. The mirror surface is “figured” by further polishing on the same pitch lap. The mirror must be polished to a spherical concave surface before continuing to "figure" the mirror.  Once the spheroid surface is smooth, the next step is to give the mirror a parabolic concave surface.  The parabola has only one focus point when light from very far stellar objects is focused.  A sphere will not focus far away objects to one focus but rather to many foci.  Figuring the mirror may take about 5 hours.  A Foucault or Ronchi tester has to be made in order to measure the concave surface figure.

The fall class did not fabricate the first mirror, which is a 6” F5.  The 6” F5 has a focal length of 30 inches ( 6” * 5).   This type of mirror is considered a wide field of view or rich-field telescope. This is because the focal length is short.  The shorter focal length has a wider field of view at the focus point. The F ratio of a mirror is the ‘focal-length, FL., divided by the mirror-diameter, D.: calculating 30” focus /  6” mirror, this is an F5.

The class performed some fabrication on all of the Dobsonian telescope kit parts.  These parts were the wood mirror mount and cell, the fiberglass 8” cardboard tube, the Dobsonian base AZ rotator wood plates, and the Dobsonian EL elevation wood box, and rotator pvc cups.  Other parts needed to work on the telescope are flat black spray paint, Teflon slide pieces, fiberglass epoxy, pvc plastic pieces, drywall screws, bolts and nuts. The eyepiece rack and pinion holder, the single vane spider with secondary mirror holder were pre-fabricated for the kit.

The inside of the telescope tube and all non optical components have to be painted flat black to minimize reflections from stray light.  The mirror and the diagonal mirror are the only parts in the telescope that reflect the star light up the tube into the focuser where eyepieces are used to magnify the stellar images.

Figure 2 is a layout of a Newtonian telescope optical system.  Note that primary mirror is installed at one end of the tube. The secondary reflector mirror held by the spider is installed near the opposite end of the tube. The stellar light is then reflected up through a hole in the tube wall to the focuser which holdes the eyepieces.

The Dobsonian telescope is a Newtonian telescope with a simple AZ-EL mount.

The class students were free to come up with modifications to the Dobsonian.  Modifications were performed on the Dobsonian design to allow the AZ and the EL rotator circles to rotate more smoothly. The class chose the painting scheme for the tube and Dob box assembly.

Upon testing the purchased 6”f5 mirror it was found that the mirror is a spherical concave surface and not the required parabolic telescope surface needed for good quality stellar images.  A future class may actually refigure the 6” mirror to its correct parabolic conic curve.

To keep the dust out of the front end of the telescope place a shower cap over the tube and focuser opening.  The mirror cell back plate keeps dust from entering from the bottom of the tube.

The Dob does not have a finder or pointer device to help the viewer point the telescope at stellar objects.  The viewer must kneel down behind the scope tube and point the whole Dob in the general direction of the stellar object.  Then the viewer looks though the eyepiece and scans to and fro, and up and down, until one finds the object.  One quickly learns that a finder scope would be very helpful in locating stellar objects in the dark sky.

The Dobsonian Azimuth-Elevation rotation methods can be difficult for a beginner to learn.  The viewer has to learn to pull the telescope around the Azimuth direction while moving the tube up and down in Elevation to keep the stellar object centered in the eyepiece.

Telescopes use eyepieces like a microscope to magnify the image.  Eyepieces are lenses that will magnify the image created by the telescope optics.  Eyepieces range in price and in quality.  The cheaper $25 eyepieces are not designed to correct for telescope aberrations that arise from the poor quality of the telescope mirror.  Unfortunately the best eyepieces with the best correction range in price from $300 to $600 in today dollars.

After you have used a Dobsonian AZ-EL telescope you begin to appreciate the problems with the design. The Dob is quick to make and easy to assemble, but has many inherent viewing problems.