Going Bigger, or Learning to Love Your Ladder
Submitted: Tuesday, 22nd October 2013 by Frank Wright
This all started as aperture fever took hold. I decided to have a go at making my own mirror as it was a cheap method of moving up the aperture ladder. And yes, at f 6 and 17" you need to get a ladder, and the bigger the mirror, the bigger the ladder! I started off with a 17 inch piece of glass and ended up with 16.5" of useable mirror, with a 1/4" "support ring" of glass around the outside.
I should point out that I already have a trolley for moving telescopes around after moving up to a 12 incher a year or two ago, this is another essential item if you are planning to invest in a larger scope that has no attachable wheels. The trolley cuts in after moving past about 8 inches, depending on your strength and telescope design.
After the mirror polishing for the 17 inch scope was completed, I needed a testing frame for the mirror to check the parabolization. So it was time to build the telescope assembly. Why build a test unit when a proper telescope is needed anyway.
The first thing about building a telescope with a mirror over 12 inches in diameter is that the dimensions just seem to be monster size. When you start thinking about the design, you start to notice how low the door frame heights are from the house to the yard, and they aren't very wide either! Is that some steps to get down to the back garden? Hell, I need a new house just to fit the new scope - and on flat land. My 17 incher started out at approximately f 6, but it was so close to the ceiling that our light fittings were in danger every trip out to the yard and back, so I reground the mirror to f 5 which gave me another 17" of clearance. You need to think of these things before you commit to something this size. Even after I shortened it, I then copped a lot of flack from the beloved because of chips and dents appearing in the door frames. This was caused by various collisions while bringing the beast in with the trolly when tired after a long session late at night in the back yard.
When you have the beast outside, you require a LARGE and LEVEL piece of ground. There is nothing worse than trying to see an object not only at the limits of vision, but also at the teetering edge of balance up a ladder. Some objects that could be observed with the telescope have to be left for another time because you just can't get that ladder in the right position. You also need to be careful descending the ladder as it is real easy to think the next step is ground level. Am I up two steps or three, and as you step off, to what you think is ground level, gravity takes over. This happens not all that infrequently but does keep you alert during the night.
It looks like this whole big scope idea is awful, but overall it IS worth it. After you get used to using a ladder, you find it is better than taking out a short dob without one. I have a 12 inch f5 dob and do not need to use a ladder with it. I now find it harder to enjoy using this telescope because there is no support for me to lean on. This is a bigger problem at higher magnifications. Once you get used to a ladder, and learn how to position it, you find that it helps you enjoy the view comfortably. The scope can be moved around smoothly with your elbow on the top step of the ladder for steadiness and I have had no problems pushing the big one around at over 800x. Holes can be drilled into a step for the eyepieces as well as for other accessories, which saves an it annoying climb up and down for every eyepiece change.
Have you read about how a big scope packs down in a small car? So next time you look at a picture of a big scope in a magazine or on the internet, have a closer look. The user is probably looking through the eyepiece while standing on terraferma. The scope is probably at an elevation angle of only 45 degrees or less. No ladder in sight? It's probably still on the roof racks on the car! They forgot to mention that bit. You never see a picture of the roof racks for the truss tubes and ladder. These parts could possibly get longer than the small car that holds the rest of the gear. So if you are about to move up the aperture ladder, remember to add some extra dollars to the budget for accessories.