I'm one of the "hardcore" guys who wants to grind my own mirror. Any suggestions?
Grinding your own mirror is a very satisfying project and well within most folks' ability. First, get a good "how to" book and a quality mirror kit. Most amateur mirrors I see turn out poorly because they were never polished out completely and during the grinding process; the mirror edge was never "beveled" yielding a chipped edge.
During the grinding process, as you move from one grit to another, make sure you use a #120 stone and bevel the edge at 45 degrees for at lest 1/8". A smooth chip-free edge is one thing that sets a professional-looking mirror apart from a poor amateur attempt!
How do you know when to stop polishing? Take the mirror out into the sunlight and with a magnifier loop, examine the extreme edge. If you see any "sand pits," go back and give it another hour or two of polishing time.
How do I figure my secondary diagonal size for a reflector?
There are several things to think about when choosing a secondary mirror. Remember, when talking about secondary sizes, you always refer to the minor axis or shortest distance across the mirror.
Most people make the mistake of getting too large a secondary mirror. The larger the secondary, the less contrast and resolution.
The rules I follow are:
With a low- profile focuser, you can get by with a smaller secondary.
I have my tube, focuser, mirror cell, and main mirror. Where do I put the holes for all the parts?
Most books have a formula where you plug in all your numbers and they tell you where to drill the holes. I've tried these many times and always end up with problems focusing correctly. I finally settled on the old time-proven method of making a large 1:1 drawing of the optical system, taking into account the mirror's focal length from the mirror surface, tube diameter, secondary minor axis size, and height of the focusing mount in the racked-in position.
Rack your focuser all the way in, then back it out about ¾" to 1". This is the point you want to come to focus at. Drill the hole for the focuser first and mount the secondary holder. If you are using our mirror cell, back the tube bolts out and pinch the cell inside the tube, but don't drill the holes yet. Put an eyepiece in the focuser and look at an object at infinity. Carefully slide the cell back and forth until you get focus with the eyepiece. You are just doing a rough focus, so don't worry about collimation. When you can form an image, you can then drill the mirror cell holes.
What's the best size finder for my scope? What about the non-magnifying finders out there?
I think the most important accessory you can buy is a good finder. Even trying to find the moon with the tiny 5 and 6 power finders on most commercial scopes is an exercise in frustration! The so-called "one-power" finders are great as a quick guide, but you still need something that will reveal faint objects. In our part of the country, some nights looking up there, it seems like you can count the clearly visible objects on one hand!
A good quality 7 or 8X 50mm can act as a small rich-field telescope. An illuminated finder is nice, but a good quality unit will have enough contrast so you can see the crosshairs without difficulty. I can't include most of the Chinese finders out there as optical quality seems to be lacking. Look for finders using Japanese optics.
How can I clean my mirror?
First, don't clean your optics unless it's absolutely necessary. More damage is done to mirrors and oculars by aggressive cleaning than anything I can think of.
Invest in a can of pressurized air and blow off loose dust and dirt particles. For a reflector mirror, remove it from the scope, but if possible keep it mounted in the mirror cell. Wet mirrors are slippery little devils! Make a dilute solution (15% or 20%) using a mild dishwashing liquid. Use distilled water. Well water and city water contains tiny sand particles.
Using cotton balls, swab the mirror surface with the cleaning solution. Start in the center and work in a circular motion moving to the edge. Standing the mirror on edge, flush the surface with distilled water. If you did a good job, the water will "bead off", leaving only a few water spots. Take the edge of a paper towel and blot these up.
I have astigmatism and have to wear eyeglasses using my telescope. Any suggestions?
I have the same problem. Vixen makes several lines of oculars with a 20mm. eye-relief, but their good wide angle Lanthanum line is quite expensive, running well over $200.
Most folks can do well combining a longer focal length orthoscopic or Konig with a 2.8X Klee Barlow. Our 25mm Ortho has about 20mm e.r. so when you use the Klee, you get about 8mm. focal length. The 18mm Ortho also works well in this set-up.
I have read that astigmatism may effect only certain areas of the eye, so using a short focal length 5mm or 6mm eyepiece could "mask" the problem, and work for some people without using glasses. I have also found that when looking at a planet, even using a 6mm. Ortho, I can wear my glasses and use only the central portion of the field.
How much power can I use in my scope? How do I figure magnification?
Magnification is a function of the telescope's focal length and the focal length of the eyepiece. Somewhere written on your scope you should find the focal length in millimeters.
No markings? You can guess quite closely. If your reflector tube is about 48" long, convert to millimeters by multiplying by 25. This yields 1200mm. Divide 1200mm by the focal length of the eyepiece (example 12mm.) You have a 100X power or magnification.
I tend to be fairly conservative in how much power I use in my scopes. Some say 50X /inch of telescope aperture. I think that is a little high. I am more comfortable with 25X to 35X/inch. I know, some of you will say you can push far past those suggestions, but I say only on nights of exceptional seeing.
Is there a correct method of focusing a telescope?
You hardly ever see this discussed, but there is a correct way to focus a telescope with a traditional rack and pinion. Extensive tests by the military have show that you always start in the "out" position and then focus forward or "in." The image will be out of focus at first and as you move the eyepiece inward it will slowly come into focus.
When it's sharp, stop! Don't try to focus by using a "rocking" action. If you try to focus by the reverse method, that is by starting "in" and pulling out, serious errors can occur. The human eye tends to overcompensate or "accommodate" and the tiny muscles allowing the lens in your eye are under stress. Image sharpness is never as precise and eyestrain often results.
I want to look at the sun. Is there a safe way?
My advice is forget it! In the first place, without very expensive and complicated equipment, all you are going to see is a few granular spots without much fine detail.
When I bought my first Unitron refractor in the late 60's, it came with small dark filter that screwed into the bottom of the eyepiece. The entire light from the sun was concentrated by the objective onto the filter! I had one of these crack on me, but luckily I was fooling with the controls and not looking through the eyepiece. Just a simple "ping" and I recognized the danger.
I don't think any company today would go against the advice of their legal departments on this one!
I get sort of a "yellow" tinge to the image when using certain oculars. What gives?
This is becoming a problem as eyepieces become more and more complex, even showing up in fairly expensive oculars. The only explanation we can come up with is that quality of the optical glass now days is very inconsistent. Normally, you will not find this problem in good quality Japanese optical lens blanks.
Replacing your Newtonian focusing mount.
When you get your new telescope, the chances are it will have a plastic focuser made in China that will be loose, have sloppy tracking, and not hold the weight of your eyepiece. We have replaced thousands of these with our precision mounts. When you go to replace yours, we need to know the "racked-in" height in inches from the top of the mount to the tube surface. If your replacement mount does not duplicate this dimension closely, you will have trouble focusing your scope.