# Advice to Beginning ATM’ers - From the Mailbox.

### by Tom RyanPrinted in Reflections: January, 2005.

“I acquired a mirror and I’d like to make a telescope with it. It is an 8 inch mirror and has “F-8.5” written on its back side. A quick check would seem to show its radius as being about 138 inches; very close to the 136 inches a true 8 inch F-8.5 mirror would in fact possess. That would make its focal length 68 inches. Am I correct??”

It is true that a mirror’s focal length is exactly one-half of the radius of curvature, but I’m concerned about the 2” difference in measurements. It is really worthwhile to measure this distance correctly, either by making a radius of curvature measurement, or by making a focal length measurement (perhaps by sticking a tape measure through a 4”x4” piece of cardboard, aiming the mirror at the sun on a semi-cloudy day, placing tape over the end of the tape measure so as not to harm the mirror coating, and, with the tape measure held against the center of the mirror and the mirror pointed at the sun, slide the paper along the tape measure until the sun is in focus.) The measurement should be good to within 1/8”. Otherwise, you could have problems that result in your needing to drill extra holes in your telescope tube to get everything spaced correctly.

“From my notes from some years ago, my diagonal mirror size starts at a minimum of 1.5 inches (minor axis) to about 1.7 something inch (minor axis) depending on how large I want the 100% illumination circle to be. Do you agree, or disagree??”

The exact formula for finding the diagonal size appears in How to Make a Telescope by Jean Texereau, on page 375 of the second English edition, and is:

Diagonal’s Minor Axis Length “MA” = (L/M) + (L/N)

The diagonal Offset “Delta” = [(L/M)-(L/N)]/2

(Note: For 8” mirrors f/6 and longer - your mirror, in particular - delta can be assumed to be zero. This “delta” is the distance between the scope’s optical axis and the diagonal’s center (offset because the diagonal’s lower edge intercepts the converging light cone from the primary where it is closer to the primary, and hence larger, than where the top of the diagonal intercepts it), but is only important on very fast scopes.)

where L, M, and N are calculated as follows:

L = A(F-e) + H(D-A)

M = 2(F-e) - (D-A)

N = 2(F-e) + (D-A),

where “e” is the mirror’s maximum depth, calculated from the formula:

e = r2 / 4F

“Parks sells some nice looking fiberglass tubes & end rings that would look to match the telescope shown on the cover of the Novak catalog I have dated 92-93 edition. They also list some kind of “rotation rings.” I assume that allows a person to rotate the telescope tube in the cradle mount for easier access to the focuser and eyepiece. Would you recommend those choices, if I am indeed correct?? -or- should I just buy some sonotube for now, as it is a lot cheaper when I have an “oops” the first time around??”

Sonotube is good. Fiberglass looks terrific, but my own Parks fiberglass tube, made in the early ‘60’s, still smells like fiberglass. It’s your choice.

When I put my own first scope together and started looking at objects in the sky, I quickly realized that the eyepiece usually ended up in some awkward place, so I made my tube rotatable (it slides on felt glued to the cradle), and have continued to do so ever since. Most guys I know try to make all or some of the tube able to rotate. However, the very early rotation rings sold by Parks or Cave were too loose. They would rotate too freely, and wouldn’t keep an object in the field when the tube was rotated. Maybe no reasonably priced rotation system will do this perfectly, or maybe Parks has improved the product since I last looked at it, 30 years ago. Call them and ask about this. A rotating tube can be a great convenience when you’re using a Newtonian telescope on an equatorial mount.

Incidentally, my first homemade telescope, a 6” f/8, had a three-vane spider, a helical focuser, and a cork-lined aluminum tube which was closed off to ventilation at one end by the solid mirror cell. Obviously, I was trying to get every bad move out of my system at once, forever.

Whatever you decide, keep in mind that the Parks rotation rings will probably only fit the Parks fiberglass tube. You might want to take a look at the Lowbrow club’s 8” F/8 Cave Astrola out at the observatory to get an idea of how the whole thing works, and whether or not those features are important to you, personally.

“My next “guess” is that my tube length will be close to the mirrors focal length of 68 inches. I would think that mirror cell construction would affect that. How much distance from the mirror cell to tube mounting fasteners -to- the mirror surface; how far from the tube end to the cell mounting fasteners; how far from the secondary mirror centerline to the spider mounting fasteners; how far from the spider mounting fasteners to the open end of the tube?”

Yes, you are right. All of those things affect the tube length. Try to get all the parts together before you get the tube, but certainly before you start drilling holes. Take measurements, and remember to allow some tube length out past the diagonal. That not only shields the eyepiece from stray light, it also acts as a dew shield and keeps the diagonal from dewing up. A good amount to add past the diagonal is a length equal to the diameter of your mirror, which means your tube will end up being closer to 76” long. If in doubt, ask Parks for a recommendation. They’ve been doing this for a long time.

“It would seem to be a tough measurement to figure out without having most of your mounted parts to obtain measurements from in order to do some layout!! Am I correct, or am I just making it a lot harder than it really is?!?!”

No, you’re not making it harder for yourself. You’re just trying to prevent the “extra set of holes” syndrome, which affects about half of the first telescopes out there. Wait until you have all of your parts before you start drilling holes. Lay the whole thing out first on a full size piece of paper or a CAD system, if possible. It’s even worthwhile to get some eyepieces first, too. I have a very expensive Nagler eyepiece which won’t reach focus on my 8” Cave Astrola, because that particular eyepiece needs to be a lot closer to the mirror than do most other eyepieces. If you know this beforehand, you can better plan the placement of all of the components.

For specific eyepiece advice, I recommend you talk to Doug Scobel. He does a lot of observing, and he has tried a lot of eyepieces.

“The equatorial mount I have contains the word “Astrola” cast into the housing. As I remember it, that was a high quality product line for it’s time, wasn’t it??”

Astrola was one of the best manufacturers in its day. The mirrors were and are top quality, but the mounts are not as good as a really expensive (\$4000) mount can be today. Still, they’re light years ahead of anything that is a mass produced equatorial available today. Surprisingly, a Dobsonian mount is better (for just looking around the sky) than even an Astrola mount. The Astrolas had ball bearings on the shafts, which made them easy to move, but ball bearings don’t have enough friction to dampen vibrations, and the Astrola mounts without gear drives tended to oscillate too much for my taste. If you’re just looking around the sky, you want to have big Teflon or Delrin bushings, like the Dobsonians. If you’re taking pictures, you want to have a big, heavy mount with ball bearings, like the Astrola. The Astrola mount, combined with a modern gear drive, like a Byers, would be a great instrument for imaging. Heavy, but great.