WARNING: Centeral obstructions and focal ratio help needed.

[ollypenrice]

This was kind of the answer I was looking for to put my mind at rest. Sorry for others who have basically already said this but as I said I needed it in layman terms.

I don't think the whole FOCAL LENGTH FOCAL RATIO confusion helped

What matters about aperture is what we might call the diameter of the effective incident beam. You can limit this incoming beam either by using an aperture mask or tube or whatever, OR you can have a tube as big as you like and limit it by the diameter of the primary mirror. But it is the diameter of the beam reflected by the primary that is the only thing that matters. That is the aperture. Nothing else is the aperture.

Nothing else should be fed into the equation focal length over aperture equals focal ratio.

Honestly, that is all there is!

Olly

[terrym2002uk]

Hi Spaceboy,

This is my understanding:

Parallel rays of light hit the concave primary mirror which is curved (either sperical or parabollic), these rays form a light cone, and the point at which the light rays converge to a point (not truley a point) is the focal length of the mirror.

The diameter of the mirror is the effective diameter of the cone of light. Obstructions have no effect on this diameter.

The f number of the system is the focal length / the diameter of the mirror.

The obstruction caused by the secondary will not effect these parameters, just the amount of light forming the cone. You could stick a blacked out circle in the centre of a refractors primary lens and all this will do is reduce the light entering the system. It wont effect the diameter of the cone if you see what I mean?

The secondary in a newt is there to bend the cone off so you can see the focus point at 90 degrees. The minor axis of the seconday is effectively the diameter of the secondary you see through the focus tube? The secondary is sized at a trade off against seeing more of the primary mirror against blocking off light enetering the system.

So the shadow of the secondary is only reducing the amount of light entering the system, more light - better at seeing faint objects. So this aspect is more about the light gathering abilities of the system.

The possible light collection area of you primary = pi x r x r where r is the radius (d/2) and pi is approximatley 3.142

The shadow of the scondary = pi x minor axis x minor axis

So the usable area of the primary is its area less the area of the secondary shadow.

Hope that makes sense ?

[EDIT]

If you blacked out the edges of the primary or primary lens, then I believe this would affect the diameter of the light cone and hence f number of the system. (not 100% certain of that fact tho)

[terrym2002uk]

Spaceboy,

Have a look at this site. They have a calculator for designing scopes. Playing with the numbers may help.

Oldham Optical Telescope Design page

[spaceboy]

Thanks again to everyone for your help.

So a larger secondary mirrors will only effect contrast. Dose blackening the rear & edges of the secondary help increase contrast or have no effect at all??

[terrym2002uk]

So a larger secondary mirrors will only effect contrast.

I would say brightness rather than contrast.

Dose blackening the rear & edges of the secondary help increase contrast or have no effect at all??

As for flocking and blacking lens/mirror edges etc, that helps to reduce reflected stray light from reaching the focal plane, this will "improve" the contrast and I mean improve so what you see at the eye piece is similar to the bit of sky you’re looking at. It won’t make it better than that.

It can become a bit of an obsession trying to make every photon count. Its another astro disease similar to collimation OCD I flocked my tube, and blacked all the shiny bits including the screw heads, focuser outside tube and secondary mirror sides. Does it make a difference? Yes but it depending on your observing environment. A good dew shield will practically achieve the same thing I suspect.

[AweSIM]

very interesting discussion i must say.. =).. just wanna contribute my understanding to the discussion..

- it is true that FL for a mirror is FIXED.. central obstruction or not..

- it is true that F# for a mirror is also FIXED.. given that we dont stop down the aperture..

- BUT if it is stopped down it WILL change.. depending on where exactly have we stopped down along the tube..

for instance.. if we stop down right next to the primary mirror by painting a black circle along the edge of the mirror.. it is literally like replacing that mirror with a smaller mirror.. it would be unwise to say that effective aperture wont be reduced in this case.. and this would lead to a larger F#..

but what would happen if we dont stop adjacent to the primary mirror? like the secondary mirror which is far from the primary mirror.. does that reduce the aperture of the scope? like someone suggested on this thread.. does it render the area at the center of the primary mirror as wasted? id say no it doesnt..

if the scope is directly pointing towards a star.. the light from this star will enter the tube parallel to the axis.. this light would be blocked by the secondary mirror and will not reach the central portion of the primary mirror.. but if there is another star in the FOV.. the light from this star would enter the tube at an angle.. and might reach the central portion of the mirror.. so the central portion of the mirror is not wasted..

we know why does vignetting occur.. it happens cuz off-axis rays have a smaller effective aperture than on-axis ones.. this means less photons for regions near the edge of the film and hence a dimmer image in this region.. refer to the "image" below which depicts this.. it also shows that off-axis ray CAN fall on the central mirror region directly behind the secondary mirror..

hope it helps.. =)

 +--------------------------------------------------------------
||.                   .                    .                    .   --+
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||                                                   ||        .      | effective aperture for on-axis rays
||                                                   ||        .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .   --+
 +--------------------------------------------------------------


 +--------------------------------------------------------------
||                                                              .   --+
||                                         .                    .      |
||                    .                    .                    .      |
||.                   .                    .                    .      |  effective aperture for off-axis rays
||.                   .                    .         ||        .      |
||.                   .                              ||        .      |
||.                                                             .   --+
||                                         .
||                    .
||.
 +--------------------------------------------------------------

[terrym2002uk]

AweSIM

Thats really interesting. I never considered the off axis stuff. So that means I have a doughnut on good mirror That said, tis a very handy doughnut.

[AweSIM]

@terry2002uk.. pal i wont say your efforts were wasted by blackening the edges.. the diagram assumes no stray reflections.. but they do happen.. since they happen inevitably and since the inside of the scope is not mirror smooth the reflections it causes only decreases the contrast of the final image by flooding it with stray diffuse light.. so yes blackening the sides, insides, and maybe just a little along the outer perimeter of the mirror should help.. i dont own a scope as yet.. what im presenting is bits and pieces of things ive learnt on this forum and what my knowledge of physics tells me.. i may be wrong.. and if i am.. senior members will definitely correct me.. but till they do.. i believe the argument ive presented is sound.. at least to me.. ..

[Macavity]

Re. Secondary shadows. My TS/GSO 8" (D=200mm) F4 (f=800mm) "photo-newt" as a worked example (HTH):

BRIEFLY! With a 20mm eyepiece, the magnification would be 800/20 = 40x. Subjectively, I find this (40x) about the minimum usable daylight visual magnification. The exit pupil is (200/40) = 5mm. The large (70mm minor axis!) secondary casts a shadow of 70/40 =1.25mm, as the "black spot" central to the exit pupil. The PRACTICAL usability is determined by the relative size of secondary shadow and the EYE pupil. The above limit is thus consistent with the conventional idea of the (1.25mm) shadow becoming comparable with my ~1.5-2mm (probably!) daylight-adapted eye pupil.

Of course a dark-adapted (younger) pupil would be larger, making the use

of a 20mm eyepiece easier and even lower powers possible... maybe.

[spaceboy]

very interesting discussion i must say.. =).. just wanna contribute my understanding to the discussion..

- it is true that FL for a mirror is FIXED.. central obstruction or not..

- it is true that F# for a mirror is also FIXED.. given that we dont stop down the aperture..

- BUT if it is stopped down it WILL change.. depending on where exactly have we stopped down along the tube..

for instance.. if we stop down right next to the primary mirror by painting a black circle along the edge of the mirror.. it is literally like replacing that mirror with a smaller mirror.. it would be unwise to say that effective aperture wont be reduced in this case.. and this would lead to a larger F#..

but what would happen if we dont stop adjacent to the primary mirror? like the secondary mirror which is far from the primary mirror.. does that reduce the aperture of the scope? like someone suggested on this thread.. does it render the area at the center of the primary mirror as wasted? id say no it doesnt..

if the scope is directly pointing towards a star.. the light from this star will enter the tube parallel to the axis.. this light would be blocked by the secondary mirror and will not reach the central portion of the primary mirror.. but if there is another star in the FOV.. the light from this star would enter the tube at an angle.. and might reach the central portion of the mirror.. so the central portion of the mirror is not wasted..

we know why does vignetting occur.. it happens cuz off-axis rays have a smaller effective aperture than on-axis ones.. this means less photons for regions near the edge of the film and hence a dimmer image in this region.. refer to the "image" below which depicts this.. it also shows that off-axis ray CAN fall on the central mirror region directly behind the secondary mirror..

hope it helps.. =)

 +--------------------------------------------------------------
||.                   .                    .                    .   --+
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||                                                   ||        .      | effective aperture for on-axis rays
||                                                   ||        .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .   --+
 +--------------------------------------------------------------


 +--------------------------------------------------------------
||                                                              .   --+
||                                         .                    .      |
||                    .                    .                    .      |
||.                   .                    .                    .      |  effective aperture for off-axis rays
||.                   .                    .         ||        .      |
||.                   .                              ||        .      |
||.                                                             .   --+
||                                         .
||                    .
||.
 +--------------------------------------------------------------

Now this is interesting as I only flocked my scope as I thought it would help reduce stray light from light pollution but from your image it will also help light bounce from off axis stars.

[Moonshane]

very interesting discussion i must say.. =).. just wanna contribute my understanding to the discussion..

- it is true that FL for a mirror is FIXED.. central obstruction or not..

- it is true that F# for a mirror is also FIXED.. given that we dont stop down the aperture..

- BUT if it is stopped down it WILL change.. depending on where exactly have we stopped down along the tube..

for instance.. if we stop down right next to the primary mirror by painting a black circle along the edge of the mirror.. it is literally like replacing that mirror with a smaller mirror.. it would be unwise to say that effective aperture wont be reduced in this case.. and this would lead to a larger F#..

but what would happen if we dont stop adjacent to the primary mirror? like the secondary mirror which is far from the primary mirror.. does that reduce the aperture of the scope? like someone suggested on this thread.. does it render the area at the center of the primary mirror as wasted? id say no it doesnt..

if the scope is directly pointing towards a star.. the light from this star will enter the tube parallel to the axis.. this light would be blocked by the secondary mirror and will not reach the central portion of the primary mirror.. but if there is another star in the FOV.. the light from this star would enter the tube at an angle.. and might reach the central portion of the mirror.. so the central portion of the mirror is not wasted..

we know why does vignetting occur.. it happens cuz off-axis rays have a smaller effective aperture than on-axis ones.. this means less photons for regions near the edge of the film and hence a dimmer image in this region.. refer to the "image" below which depicts this.. it also shows that off-axis ray CAN fall on the central mirror region directly behind the secondary mirror..

hope it helps.. =)

 +--------------------------------------------------------------
||.                   .                    .                    .   --+
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||                                                   ||        .      | effective aperture for on-axis rays
||                                                   ||        .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .      |
||.                   .                    .                    .   --+
 +--------------------------------------------------------------


 +--------------------------------------------------------------
||                                                              .   --+
||                                         .                    .      |
||                    .                    .                    .      |
||.                   .                    .                    .      |  effective aperture for off-axis rays
||.                   .                    .         ||        .      |
||.                   .                              ||        .      |
||.                                                             .   --+
||                                         .
||                    .
||.
 +--------------------------------------------------------------

great stuff. this explains the logic in my mind and I get it!

[AweSIM]

@spaceboy and @moonshane.. thanks guys.. but i may be wrong.. till some senior member like ollypenrice (who definitely have a lot more knowledge and experience than myself) can confirm this.. ..

@moonshone.. yes youre right too.. flocking should help kill reflections from the inside of the OTA of off-axis stars too.. and this should lead to a better contrast.. since otherwise the inside of the OTA diffusely reflects light that forms no clear image and "washes" out the image..

[John]

My understanding is that starlight is traveling effectively parallel to the optical axis when it reaches and enters the scope.

Flocking, light shields etc reduce glare from terrestrial off-axis light sources such as street lamps, neighbours lighting etc. This does help to improve contrast.

[spaceboy]

DOH! SORRY READ YOUR POST WRONG JOHN!!

[John]

But surly in doing this it will increase contrast as the image isn't awash with useless light ?? Not that I have a clue what I'm talking about but in my head it seems to make sense.

Doing what ?

Sorry if I'm being dense !

I think light shields, shrouds, flocking etc (particularly the former) are worthwhile to increase contrast.

[John]

DOH! SORRY READ YOUR POST WRONG JOHN!!

No probs - it's a complex subject and I'm far from 100% sure of my ground here anyway