| I'm one of the "hardcore" guys who want to grind
my own mirror. Any suggestions? |
| Grinding your own mirror is a very
satisfying project and well within most folk's
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.
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
polishing time.
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" of an
inch. A smooth chip-free edge is one thing that sets
a professional looking mirror apart from a poor
amateur attempt! |
| 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 to large a secondary mirror.
The larger the secondary, the less contrast and resolution.
The rules I follow are:
6" F/10 - 1" secondary
6" F/7 or F/8 - 1.3" secondary
8" F/6 - 1.52 Secondary
10" F/6 - 1.83 or 2.14 depending on the focusing mount
you are using.
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 3⁄4
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. First 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. |
