A silly question on how a newtonian telescope works

[Mountain_Wanderer]

Ok, this might sound silly and there's probably an easy explanation, but here is my question...

You know how at the sky end of the newtonian telescope where the light enters, the very centre is taken up by the holder for the mirror to the eye piece? Why then doesn't the image you see show a blacked out central region in the field of view, being the area obstructed by the small mirror for the eyepiece?

[Screwdriverone]

Hi Jowel,

No, you don't see any obstruction in the view as the primary mirror collects the light in a big tube shape coming down to it (minus the section blacked out from the secondary mirror) and then the parabolic shape of the primary mirror focuses the light all onto the secondary mirror which reflects it out to the focuser. As this is focused onto the flat 45% you dont see the secondary shadow.

Google a dobsonian design and you should see a light path diagram which outlines what I mean.

Cheers

Chris

ps. the only silly question is the one not asked.

[Geoff45]

Here is one way of visualizing what's going on. Defocus a bit and you will see the shadow of the diagonal in the defocussed image. Now watch what happens as you slowly focus. The star disc shrinks to a point and the shadow of the diagonal shrinks to a point within a point and you don't see it.

Every part of the mirror that gathers light from a star focuses it to the same point. The diagonal merely removes some of the potential light, but doesn't change the image.

[Mountain_Wanderer]

Cool. So if say there was an object right in front of the telescope that you wanted to see, but it was located directly in front of thwere the secondary mirror is situated, you'd still see it because the primary mirror would bend the line of sight around the secondary mirror so as to see the object?

[DavidU]

You can put your whole hand infront of a Newtonian and the image through the eyepiece will only dim a little.This is for a 10"+ scope.

[mbo]

I think another way of saying it may be that the secondary and spider are so completely out of focus that you don't see them in the image. Also, the main mirror easily sees light from a star around the both sides of the secondary mirror, as it is so much bigger.

I'm no optics expert, but this sounds like a fair analogy:
Hold a pen in front of your face, then focus on the house over the road, you can see all of the house, and the pen becomes much less visible.

[bird]

This is one of the misunderstandings that is common when starting out with an obstructed scope like a newt... everyone looks at the central obstruction and wonders why they can't "see" it when looking through the eyepiece at a focussed image...

It takes a bit of a thought experiment to sort this out, let's assume that you've got the scope pointed at a point source a long long way away, eg a star. Light from that star comes into the end of the scope, hits all the exposed parts of the primary (ie all except the central bit that's in shadow) and then bounces up to the secondary, is reflected into the focusser and comes to focus as a single point of light at the focal plane.

The first thing to realise is that all the rays of light that make up this point have come from all over your primary mirror. ie *all* of the unobstructed part of the primary contributes light into this single point that you look at through the eyepiece. It's not just coming from one part of the mirror.

Now imagine a second star, close to the first so that it's also visible in the same eyepiece. The rays of light from that star have also come into the scope and hit all of the unobstructed portion of the primary, but from a slightly different angle. As a result they bounce back up the tube and come to focus at a slightly different location to the first, but just like the first star *all* of the available mirror surface was used to form the final image.

Think about this for a while and it should become clear why you never "see" a black hole in the middle of any in-focus image, since every point in your field of view was formed the same way - ie by using all of the available mirror area, it's not possible for some parts of the image to have used a different bit of your mirror compared to any other part.

cheers, Bird

[Screwdriverone]

Nice Analogy Patrick, describes it very well. Welcome to IIS, what a great post to get you started!

Cheers

Chris

[Mountain_Wanderer]

It seems this all sounds reasonable for distant objects. But, say there was a little bee hovering right in front of the secondary mirror? Is it true to say that there is a certain distance from in front of the secondary mirror and further out that could be considered a "black hole" where the telescope cannot see? Therefore I wouldn't be able to see the bee? Sort of like having your nose so close to your eyes that it's partially missed in the view by the eyes?

[Screwdriverone]

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

[bird]

Almost all scopes have a limit for how close they can focus, since an object that is close will not have it's light rays coming in parallel. This is normally many metres (maybe 50m) so I wouldn't worry too much about trying to focus on anything closer :-)

cheers, Bird

[mbo]

Thanks Chris, I thought it was about time to 'de-lurk' in the forums ;-).
My wife (Kirsten) and I are puckering up to buy a scope soon, so we will be badgering you guys with plenty of beginner questions soon!

[erick]

Yep, just amble over to it and have a closer look! :P Oh that we could do the same thing with much more distant objects! Which explains why we send spacecraft with cameras out as far as we can - they will always give more detail (within the limits of equipment/experiment design) than pointing a lens or a mirror at the object from Earth.

[Screwdriverone]

Go for it Patrick,

That's what we are here for.

Cheers

Chris