Posts Made By: Dennis Faith

After regretting the sale of my first real home-built 12.5 dob about 5 years ago (I recently saw it up for sale here in the reflectors section). I finally put the finishing touches on a 12.5 inch dob I have been working on for over a year now. It all started around a 12.5 f/5 mirror that I got a great deal on in the classifieds. I wanted it to be a classic dob but wanted to do something to make it a bit unique so I decided to go with the contrast of walnut on maple. The corners of the mirror box and base are OLD black walnut and the maple is cabinet ply.

I finally put the finishing touches on the finish this week and took it out for a thorough first light. Everything went great. The balance it right on and the views were awesome, man is it nice to have a larger aperture scope again. Took a nice tour of the summer highlights and must say it passed with flying colors. Thought I would post a few pics here.

Now onto the next project...a bigger (17.5") twin to this one....the mirror still needs to be finished so it is a long way off.

Well, to make a long story short.

Two basic types of photoreceptors are present in the eye, rods and cones. Cones are used to receive color information and rods for contrast and dim light.

Typical color-blindness resides in the photopigment of one of the 3 cone types...one of which is likely deficient in your case.

Astronomy relies on low light and contrast.. Which is processed by the rods. So when it comes to astronomy you are more than likely just like everyone else on the forum.

As for the coating, I am not sure of the wavelenght reflectivity versus regular coatings, but you sill see the smae results as everyone else.

Hope this helps.

Here is another pic showing some of the detail...

Is your mirror full thickness? If it is then you should really be pretty good with a 'normal' depth mirror box. If it is a lightweight mirror then you may be a bit top heavy.

I agree that balance seems like an untamable beast...I have always used a spreadsheet to layout the torques for the major components ahead of time. The trick with truss dobs are the length of the tubes. In the past, I have always gotten some graph paper and made a scale drawing. This last time I used MS visio to do the detailed layout..it worked like a charm everything came out very close.

After you have a pretty detailed plan the upper cage is a good place to start. Get it as close to done as you can and get an accurate weight on a postal store scale (they'll give you some great looks). If you do that, then you can cut the parts for the lower box with complete confidence.

I for one have had success cutting poles by measurement alone. I used the basic formula you did except I added an adequate amount of focus travel to focus all of my eyepieces. In order to do this I looked through my eyepieces and found the one that focused at the shortest distance (for me, since they are not parfocal, I think it was the 27mm panoptic), put it in the focuser and racked it out a 1/4 inch for breathing room. When I took the eyepiece out I used this as the minimum focuser height. As it turns out the remaining out travel will focus all of my eyepieces.

After getting this number I measured from there to the inner ring of the upper cage to set the height of the focal plane including focuser board and shims to level the focuser. Then I added the radius of the upper cage and the distance from the center of the focuser to the bottom of the cage....