Making an 8" F/6 Dob Mounted Newt

I wanted to try my hand at some of the ideas I have picked up from around the net. I have lurked (mostly) on the atm list for the last 3 years and came across some great concepts. This 'scope is a hodge-podge of what I though were the ones with the most potential for someone with my skills and limited workshop.

The Mirror Cell

I paid close attention to all the talk about ventilation and cooling rates. The cell is constructed of 1/2" plywood. The ventilation holes are 2 1/4" in diameter. The heads of the adjustment bolts are recessed and covered with silicone. The mirror was attached with 3 1" blobs of silicone directly over the bolt heads. Nickels were used to space the mirror from the cell. The tailgate is 12" diameter and attaches to the outside of the OTA with four 1/4-20 cap screws into T-nuts. The OTA and tailgate are marked to preserve collimation.

Late breaking change :-)
Never one to leave well enough alone, and cooldown times were fine at night, I just had to try out the fan thing. It does cool faster now, but where this really helps is during daytime solar use. I added 3 cpu fans from Radio Shack. They are suspended on 1/16" black craft foam rubber sheet. I sense absolutely no vibration at the eyepiece at 245x.

The Framework.

Construction Details.
  • 1. The rings are 9" ID, 12" OD plywood. Thickness varies: the end pieces are 1/2", the two near the altitude bearing 3/4", the others 1/4". This is overkill! I know now that the carbon fiber covering adds enough strength to use 1/2" and 1/16" next time.
  • 2. The stringers are 3/4" thick, 54" long. 1/2" thick would have been plenty. 2 more inches of length would make for better baffling though. (I forgot to account for the mirror thickness when laying out the baffles)
  • 3. The focuser is a 1 1/4" low profile helical. It sits on a platform of 1/2" ply, that is mounted flush to the ring ID. This allows me to use a 1.52" diagonal (19%) and still have a 1/2" diameter 100% illuminated field. I covered the bottom of the platform with self-adhesive flocking paper after assembly.
  • 4. The spider (Protostar, I love it) is mounted through 4 aluminum L brackets. These keep the Carbon tube from having to carry the load. Now that I know how strong it is, I would replace the brackets with washers and save a little weight and aggravation.

    The whole thing is shown here just before being brought outside to verify the focuser placement. I just propped it against the railing of the deck and aimed her at Polaris.

    Here is what the focuser looks like after covering the tube. I ground off the base to save an additional 1/4" of height. It is attached with (you guessed it) the ever popular silicone adhesive.

    The Mount.

    I am very happy with the way the mount turned out. The rack in front holds 5 1.25" eyepieces while preventing the tube from going more than 5 degrees past zenith. Note the cross braces. They are 3/8" threaded rod. They allowed me to get the base acurately parallel before screwing down the angle irons.
    You can just make out the turnbuckles that adjust the virtual counter-weight springs. The OTA is balanced with a video camera which make the springs necessary for my lightest eyepiece (a 10mm Ultrascopic). Thanks to Maj. Tom Kracji on the ATM list for this one. (His idea now has mass exposure: S&T 11/99 !!)

    The base is a triangular section that actually weighs less than a solid groundboard, while being enormously stiffer. That's an Astrosystems pivot (highly recomended). It could have been lower (or higher) but it brings the tube to a nice height for me.

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