3.5-inch travel scope
Modifications
[March 2005] I've made a few adjustments to this scope and now it works just
beautifully! I've added a second aluminium strut as the previous
version, with only a single strut, flexed a bit. Having two struts
also makes it straightforward to mount a pair of altitude bearings
and use a Dobsonian-style cradle. The only drawback to
mounting this scope as a Dobsonian is that the heavy zoom
eyepiece makes the scope a bit more complicated to balance. This
can be fixed with a couple of springs, as outlined below, and I
think it's worth the extra effort for the beautiful simplicity of
use...
I've also upgraded the tripod to a nice Manfrotto 190. This
threads on to the Dobsonian
cradle with a 3/8" threaded insert in the underside of the
cradle and can be seen in this
photo. This makes for a rock solid mount!
Note also the bulldog clip stuck to the top of the cradle
with industrial velcro. This is a really nice addition, as the
top of the cradle can be used as a small table to hold a book,
with the bulldog clip holding it open at the correct page.
Note the two springs visible in some of the photos above. The heavy eyepiece makes the telescope top heavy at low elevation, and tends to drag the scope in the other direction at high elevations. This is counteracted with a spring forward of the altitude bearing on the left strut and behind the right alt bearing. These are attached to the struts with cable ties, which allows the spring to be slid about for fine balancing. The struts attach to the PVC rings using eyebolts (easy to turn with gloves on) and these are held firmly against the strut with wingnuts. Detail in this picture. The bolts thread into threaded inserts without prongs (which I believe are called blind-well nuts) stuck into the PVC pipe with automotive Goop. The insides of the pipe and struts are lined with "peach fuzz" black felt from a local art store. A small section of craft foam lined with felt extends below the upper ring to fully baffle light entering the focuser. A perspex mirror cover can be seen in the previous photo, and two similar covers fit above and below the upper ring for protection during transport.
Older version
This travel scope is a 3.5-inch f/5.7 Newtonian based on the "Bare-Bones
travelscope" by Gary Seronik in the December 2003 Sky & Telescope.
This in turn was based on the Coulter CT-100. I
started building the scope after picking up the parts at Home Depot,
one Sunday afternoon, and the scope was finished by the following
Thursday, working about a couple of hours each evening. The only
really time-consuming step (apart from waiting for the paint to dry!)
was filing a curve into the focuser plumbing fitting to match the shape
of the PVC tube upper ring. I considered buying a commercial 1.25"
focuser, but that would have more than doubled the cost of building the scope!
The scope is shown disassembled for travel
on the left. It comprises:
- an aluminium C-channel tube with two captive eyebolts (held in the rod with a nut above and below). The upper bolt sits in a hole, and the lower bolt sits in a slot. The slot allows the mirror "box" to be slid up and down for coarse focus adjustment. I now no longer use this sliding feature as I only use the scope with one eyepiece.
- A Rigel Systems Quikfinder a great, lightweight unity power finder. This is all that is needed on a scope like this under a dark sky.
- A ScopeTronix 7-21mm zoom eyepiece provides a good range of magnifications (~25-75x) within the small amount of focus travel provided by the PVC plumbing fitting focuser.
- Upper ring
(from 4-inch PVC drainpipe) with PVC
plumbing fitting focuser, curved spider (made from bent aluminium
stock) and Quikfinder bracket. - Lower ring (or mirror box) with simple wooden mirror cell held on three spring-loaded bolts, a perspex mirror cover, held by 2 two flaps of velcro (this cover is just visible in the open position, in the photos above, if you look hard enough). Also notice two small wooden blocks at the 7 o'clock position, above and below where the eyebolt threads into the box. These are cut to the size of the channel, to prevent rotation when sliding the box up and down for coarse focus.
First light for this setup was on 13th July 2004 (on the Jewel Box cluster!) from Las Campanas Observatory, 2800m up in the Andes, Chile. This scope already has over 15000km on it. It's great to have even a relatively small scope at a really dark site. I pack the upper and lower tube rings in a clear plastic bag, and together with the finder and eyepiece, these go in my hand luggage. The aluminium bar and tripod go in my checked baggage. I've been through around 10 different airport security checks and never yet had my hand luggage opened to look at the scope components (I think being made of plastic helps a lot). Amusingly enough, at La Serena airport I got through with my scope unnoticed, while my travelling companion had his backpack searched for carrying an ethernet cable...
My only real complaint with this arrangement is the large amount of backlash from using the camera tripod head. I'm working on fixing this by making some sort of Dobsonian-type head, like on my binocular mount head. You can see how this instrument used to look on the old scopes page.