Sunday, July 24, 2011

Car roof carrier - transformer (Part I, The Cradle)

We have a relatively small car (Subaru Forester). Even though the OTA and the mount fit OK inside, when one of the rear seats is folded, there is a very little space for the family and camping inventory left. The 4-night GSSP star party was coming soon. So, it's all started from the building of a dirty car roof rack carrier for my Dob OTA using some 3/4 inch plywood leftovers I had at hands. I called it the Dob Cradle.

The first image depicts the onsite setup. The scope is placed on the tarp used for its transportation. The cradle is behind it (behind the red "Astro" bag) and serving as an observer's table with 2" eyepieces holders and integrated slip/roll preventing "bins".

When on the roof rack, the cradle is fortified by two ski-like beams secured on top of the semi-box. These beams are used for chair transformation and "sled" transformation as well later (see the Part II article). The entire open box, with the OTA, secured inside by two heavy-duty bungee cords, is finally wrapped with the watertight tarp, using several wraps around and with the overhanging ends sealed by wrapping them inside and over for an airtight sealing. The sealed opening of the tarp's front end must face backward as on the picture to prevent any chance of the fast oncoming air stream forcing the road dust in. The tarp is secured in place with a single ratcheting tie-down tightening the bundle along the sides of the box. It is placed on the rack making sure the special bottom ridges (legs) of the cradle are sitting between the cross beams of the rack, and finally secured there with two other ratcheting tie-downs.

Construction basics

The construction is extremely simple. It's a 3 wall semi-box (front, back, and the bottom base), utilizing the styrofoam inserts of the original cardboard shipping box of the scope. Plus, two detachable narrow beams forming the top frame, plus - that huge waterproof blue tarp from Costco.

I've used some wood glue and most importantly the Kreg drill guide to join all of the walls - very sturdy, and reliable for the task perpendicular joints making option, and an extremely time-saving tool. I believe it's dedicated to the plywood projects because the multi-layer structure of the joined materials involved holds the screw at an angle extremely well. The only constructional requirement is the precision cut edges of joined surfaces. The glue is actually optional, but it fills the small gaps nicely, sealing wood screws a bit, and adding a bit of strength to the joint anyway.

Later these elements will have some reinforcement measures implemented to extend the versatility of this cradle (See the Part II. Transformer)

Other important elements of the design

  • the bottom "legs" (a). They are not only for raising the cradle above the ground when loading or unloading the telescope, but also for holding it between the roof rack's crossbars, which prevents it from shifting to the front or back. Also, they are mounting points for "skis" in the "sled" conversion,
  • My cross bars have an arc shape. So in order to prevent the side to side rocking of the base board I have 4 wooden blocks (b) mounted on the sides in proper locations. In fact, they are "cross bar legs". 
  • The front and rear panels of the cradle are mounted slightly lower than the base board providing comfortable hand grips (c) on both ends for easier handling through the tarp's layers for safer rising, lowering, and carrying by two persons.
  • I've drilled five 2" holes (d) in the base board. 4 to accept industrial-strength bungee cords hooks and 1 for the ball handle near the rim of the OTA (they are also accepting 2" eyepieces for at hand storage when the cradle used in a table mode).
  • Two top beams (skis) are also made of 3/4" plywood and are about 2.5 inches wide. They have aluminum L-shapes on the ends which are secured over the front and rear panels in the dedicated cut-offs by 4 screws.
  • I've decided to add an aluminum L-bar (e) under the base board to increase the rigidity of the construction. It's working well held in place by 12 small wood screws (however it turns out to be an excessive measure, and removed by now).
  • All of the construction's details are smoothed well enough in the way to prevent an accidental puncture or tearing through of the outer shell (the wrapping tarp).


  1. The cradle is placed on the tarp near the side, but such as to make the first full wrap around easy to make.
  2. The bottom halves of styrofoam inserts are placed in proper locations on the board and the OTA is carefully lowered on them.
  3. OTA is secured in place by bungee cords through the dedicated openings.
  4. The top halves of styrofoam inserts are placed on top of the OTA side. The middle piece is covered by a small matching piece of 1/8" plywood (protection from styrofoam shattering by top beams carving into, pictured on the second image, used as a notepad board otherwise).
  5. Two beams are lowered in place on top of the front and rear panels and secured by screws through holes in L-shapes.
  6. The box is carefully wrapped in the tarp using some aid of a duct tape for tension at the end. 
  7. On the front and rear sides, the tarp is then folded in a way forming neat and almost airtight closures.
  8. The ordinary ratchet strap is used to secure the ends in place along the sides of the resulting "mummy".
  9. Now the "mummy" can be carried to the garage and mounted between properly adjusted crossbars. The "legs" will protrude from the tarp down, and the entire construction will snugly sit between crossbars.
  10. Secure it in place by another 2 long or 4 short ratchet straps to prevent it from shifting to the sides (red bands on the right of the last image).

  • The piece I had for the bottom was too short, so I had to join 2 pieces together (still works fine 5 years passed). And that's what actually triggered the chair transformer idea, as that additional piece already looked like a chair seat.
  • Had one of the legs torn off while driving through the low gates. Bent the roof cross beam too, but the OTA stayed unscathed. Mounted it back and reinforced with two additional screws going into the side from the back of the base, and two more going perpendicular into the crossbars' legs.
To Do (improvements)

  • Utilize the upper beams as bottom rails for the cradle. They should help a lot with loading and unloading the cradle to/from my hatchback (instead of using it on the roof, good for short or solo trips). (Done).
  • The observing chair transformation designs are plentiful. Again, plan to use those upper beams (Done).
  • Protective paint (Done)
The second part is about converting the cradle to an observer stool.


  1. Who is that boy? On the first picture.

  2. Hey, Ozzy. You're my first commenter, huh? That's my son, Kirill. He liked to spy on our neighbors on the GSSP star party evenings, shortly before the sunset.

  3. This is my first time i visit here and I found so many interesting stuff in your blog especially it's discussion, thank you. cơ khí thành tuyết

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