[Screwdriverone (Chris)]

Hi Jowel,

Yes, that is true, the bee could have landed on the secondary holder and would be invisible to the primary. Also, if the bee flew in a straight line (perpendicular from the base of the primary and concentric with the tube) away from where it landed there would be a cone of distance with the base the same width as the secondary diameter which diminishes in diameter the further the bee flies from the telescope. BUT, once the bee reaches a certain distance away from the secondary's shadow, then the edges of the primary mirror "see" the bee and because they are parabolic, will bend the light that hits them and focus it onto the focal point on the secondary diagonal and you will end up seeing it in the focuser.

Think of it this way: a car's headlight mirror (on a ROUND headlight obviously) works in the OPPOSITE way to a telescope. The illuminated bulb is at the focal point of the headlights mirror and all of the light generated by the bulb (minus the light blocked by the REFLECTOR behind the bulb) hits the mirror and is reflected OUT in parallel lines (not counting the lens) of the headlight toward the road. If your car has a central mounted bulb with a reflector behind it in the design of the headlight, turn it on and then look at it. A certain distance away from the car, the headlight will look completely solid round ball of light. IF you move closer and closer to the front of the headlight (and in line with the reflector) you will start to see the reflector hiding the bulb as the light coming out of the headlight cannot overlap the "shadow" provided by the reflector and you will quite clearly see a donut of light.

After a certain distance, the reflector's blocking amount starts to seem minor compared to the amount of light you are actually seeing.

Certainly, smaller secondary mirror obstructions, let more light in than larger ones, but due to the fact the primary mirror is MUCH MUCH cheaper to make than a large complicated unobscured LENS made of quality glass, then the trade off is worth it. For example, my 12" dobsonian newtonian reflector specs for the secondary are below;

Secondary Mirror obstruction = 70mm, secondary obstruction by diameter = 23%, Obstruction by area = 5%.

So in my case, I am losing a grand total of 5% of the light coming in the end of my scope. Not much for such a LARGE mirror and a scope costing around $1500. If you tried to get a REFRACTOR (which has no central obstruction at all) of the same size (12"), it would cost tens or even hundreds of thousands of dollars to buy. For Example, a decent 127mm (5inch) ED refractor costs over $2500-$3000. TWICE the price and still 7 inches SMALLER.

Also, as the focus of the telescope is most often BILLIONS of Km away, (technically infinity) then you aren't really missing anything in the extremely short distance between the top of the secondary and the point where the primary can see the proverbial "bee" again.

And what David says is exactly right. You can cover the ENTIRE 12" aperture of my dobsonian with the tube cover and only pop off the off axis cap (about 2" across) and STILL see what is coming in that relatively tiny little hole as ALL of the light coming through that hole is hitting the primary and being reflected to the focus point. Its just very, very dim. (unless you are looking at say the moon which is VERY bright to start with). FYI - NEVER look at the sun without a special solar filter.

Sorry if I rambled on a bit.

Cheers

Chris