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Dale Keller built this 5 inch F15 refractor using a
D&G Optical objective. Almost
everything else is homemade.
The 66 inch long tube is 6 inch aluminum irrigation pipe from
Hastings Irrigation Pipe Co. The wall thickness is
0.058 inch (approximately 1/16 inch). The tube is one inch larger than the objective
lens diameter, making it easy to baffle. (It takes fewer baffles than if the tube is
very close to the lens size.) This scope uses four wooden disks (to help maintain a
round tube) with thin sheet metal baffles
tacked to them. The holes through the wooden disks are a little larger than the hole
through the metal baffle, and are tapered away from the baffle, so the actual baffle
is very thin. This gives less reflection from the inner edge of the baffle. The baffle
sizes and positions were calculated using a scale
drawing
on a simple (old, cheap) CAD program. The method used was from the book All About
Telescopes by Sam Brown, available from Edmund Scientific.
The objective lens cell supplied by D&G has a ring which
mounts to the outside of the tube with three screws and nuts. The cell itself mounts
to this ring, and is push-pull adjustable for fine collimation. The outer end of the
cell has an outside diameter which matches the inside diameter of the tube. A 10 inch
dew shield made from the same tubing slips over the lens cell.
The finder scope is machined from aluminum, and has a 50mm binocular objective. The
focuser is a simple screw-in drawtube with 1.5 inch 12 pitch threads. The eyepiece is
a Meade 25mm Modified Achromat with cross hairs glued to the field stop. The finder
mount uses two thumbscrews on each ring, along with a steel spring plunger. This makes
alignment very quick and easy - there is no need to loosen one screw to tighten
another. The plunger takes up the slack and applies pressure against the thumbscrews.
The downside is that the finder can get bumped easier, and needs adjustment slightly
more often.
The focuser is machined from aluminum. It uses a threaded
rod which moves the drawtube in and out when the external knurled knob is rotated.
This allows very precise focusing, but it does take a while to move the drawtube
through it's full travel. The drawtube has a 2.5 inch inside diameter and has 5.4
inches of travel. The drawtube was baffled by placing a cylinder of steel window
screen in it's barrel. Nylon screws are used to lock the star
diagonal and eyepieces in place.
The Lumicon 2 inch star diagonal is well made, but
has shiny internal surfaces. Black velvet-like cloth was glued to these surfaces to
reduce reflections and increase contrast. The barrel of the star diagonal which goes
into the focuser has a shallow groove around it where the lockscrew presses against
it. The lockscrews are nylon. This allows the diagonal to be rotated to a comfortable
position without loosening and getting sloppy.
The Dobson style alt-azimuth mount is made from birch plywood. The
tripod is extremely stable. To test it I stood on top of it
and bounced up and down. It felt like I was standing on concrete. The rocker box is
not as stable, and still needs some reinforcing. The wood is unstained, and coated
with a water-based polyurethane. I would very strongly recommend using water-based
poly in spite of it's expense, because it dries very hard and very quickly. You don't
have to wait for weeks for it to lose it's tackiness, and it can be used as a bearing
surface (used here on the altitude disks).
The telescope delivers excellent performance. On nights of good seeing, stars show
hard little Airy disks, with several distinct diffraction rings. It has very easily
split double stars down to 1.4 arcseconds, with lots of black sky between them. I will
be doing more testing in the near future. Dawes limit for this telescope is 0.9
arcseconds, and I expect it to perform to that limit easily. Details on Jupiter are
noticeably better than when viewed through larger reflectors. High contrast details
in the cloud bands are easy to make out, but some of the low contrast things, like the
Long Enduring White Ovals around the Great Red Spot haven't been spotted yet.
It took well over a year and many long distance phone calls to receive the right lens
from D&G. The first one they sent was uncoated (see the link
D&G Trouble). According to D&G, this lens
design has a fair amount of internal reflection, which is why this lens really needs to
be coated.
Of course, a homemade telescope is never really finished. The next project for this
scope is a german equatorial mount with a clock drive. This mount will be made from
machined aluminum, thinwall steel tubing, and steel shafts, with tapered roller
bearings. I plan to place the equatorial head on the existing wooden tripod, since
it is extremely stable. The clock drive will consist of a Byers 7" ring gear and worm,
driven by a stepper motor. I'll post pictures and a description here when it is
completed.
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