FAll Semester 2005 Project

The fall semester 2005 of Astronomy 101 Lab Class was the first class to fabricate parts and assemble a full 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 semester later. 

The 6”f5 Dobsonian kit was supplied by amateur telescope builders in the area.  The 6”f5 mirror and diagonal was purchased finished.  The AVC has no facilities to actually fabricate a mirror to completion.  The Dobsonian kit included all wood parts for the base, the support box, the telescope tube, mirror cell, and miscellaneous bolts and screws and pvc pipe.

A printed handout provides some guide to assembly and preparation of all the parts.  It was up to the students to find more information about Dobsonian telescopes and to perform any modifications they thought would help the scope function.

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 be started. 

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

Fine grinding will require grinding from 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 surface has a smooth glassy finish.  To polish the mirror until all grit pits are smoothed out 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 pitch lap. The mirror must be polished to a spherical surface before continuing to "figure" the mirror.  Once the spheroid surface is smooth, the next step is to give the mirror a parabolic 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 spherical 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 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 sonotube, the Dobsonian base AZ rotator wood plates, and the Dobsonian EL elevation wood box, and rotator pvc cups.  Miscellaneous parts were, flat black spray paint, Teflon slide pieces, fiberglass epoxy, pvc plastic pieces, bolts and nuts. The eyepiece rack and pinon 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 parts hasto 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 can be used to magnify the stellar images.

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

Upon testing the purchased 6”f5 mirror it has 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 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 viewing image.  Eyepieces are lenses that will magnify the stellar 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.

 REMEMBER!

TELESCOPE MIRRORS and LENSES ARE POWERFUL CONCENTRATORS OF LIGHT.

Sunlight reflected off the face of a telescope mirror can cause BLINDNESS

or START A FIRE!

Always handle your mirror indoors or in the shade!

The telescope described in these plans is for

NIGHT USE ONLY. NEVER set up your telescope in a

location where it can reflect sunlight, and, NEVER

NEVER EVER EVER !!! point your scope at the

sun and try to view.

NEVER LOOK AT THE SUN THROUGH YOUR TELESCOPE!