Reflecting Telescopes: Hole in Picture at Focus?

[skiboka33]

Was wondering why reflecting telescopes (with a secondary mirror) do not have some kind of hole in the picture at the focus of the telescope. My guess would be that the obstruction is not near enough to the focus. Anyone know?

 

[SpaceTiger]

Was wondering why reflecting telescopes (with a secondary mirror) do not have some kind of hole in the picture at the focus of the telescope. My guess would be that the obstruction is not near enough to the focus. Anyone know?

The confusion here would seem to be in identifying a position on the image with a point of entry on the aperture of the telescope. In reality, the light ray impacts the focal plane at a point determined by its angle of incidence, not its point of entry. For example, the center of the image should nearly correspond with light rays coming parallel to the sides of the telescope. It doesn't matter where these lights rays enter because they're all focused to the same point (or close to it). See this image, for example. Therefore, you shouldn't see a hole, but the obstruction will bring down your total light-gathering power a little bit (some will be absorbed or scattered by the equipment in front of the mirror).

 

[skiboka33]

yeah, that's kind of what I figured. thanks very much.

 

[turbo]

The size of the obstruction of the secondary is fundamental. The smaller the obstruction the more accurate the image (in most cases). You should know that the obstruction posed by the vanes supporting the secondary miror is also important, as high f:l planetary astrophotographers can tell you. Curved vanes will give less-pronounced optical spikes than staright ones.

 

[Labguy]

Was wondering why reflecting telescopes (with a secondary mirror) do not have some kind of hole in the picture at the focus of the telescope. My guess would be that the obstruction is not near enough to the focus. Anyone know?

Any obstruction between the entrance pupil (scope's mirror in a Newtonian or any type Cassegrain) will cause the image to degrade due to diffraction (scatter) as turbo-1 stated. And your question about the "hole in the picture" is a good one and is correct in some cases. A decent Newtonian will average no more than a 20% secondary obstruction and 15% is considered excellent. Most Cassegrain designs have obstructions in the 32% to 38% range, and at low powers they do, in fact, show a "blackout" in the image. This is a common subject on the Astro-equipment forums. The percentages are given in terms of linear proportion, but by area would be a simple conversion. The following is just one of hundreds posted by a knowledgeable amateur:

The obstruction on the 8" is huge, and I was starting to see the center blackout if I went too low in power. Here is a shot of the front of the 10" which is also pretty big to give you a rough idea of the mirror size. The 10" did not show the blackout like the 8" did. If you can figure out a way to reduce the 8" obstruction, I'd be very interested, as otherwise the optics on these scopes seem very good.

This is because low powers result in large "exit pupils" where the secondary black-out can be seen, in focus. It would be best if you could go to a site with telescope terminology, but I would be happy to post any explanation of terms you might need.

 

[Chronos]

Yep, that's why APO refractors and catadioptic designs yield the highest quality images. I would consider spiking turbo's coffee with Rohypnol to have my way with his APO for an hour.

 

[SpaceTiger]

Most Cassegrain designs have obstructions in the 32% to 38% range, and at low powers they do, in fact, show a "blackout" in the image.

Do you happen to know why this happens? In general, there should be light entering the telescope that corresponds to the center of the field of view. If it's not appearing on the image, I would be prone to suspect the optics rather than the obstruction presented by the secondary.

 

[russ_watters]

Do you happen to know why this happens? In general, there should be light entering the telescope that corresponds to the center of the field of view. If it's not appearing on the image, I would be prone to suspect the optics rather than the obstruction presented by the secondary.

I think it would make sense for close-by objects since the light rays would not be parallel (it would block light trying to cross from one side to the other), but for astronomical purposes, with the light rays exactly parallel, it shouldn't matter at all. Otherwise, these new observatories with multiple mirrors would have problems. It was my understanding that they work just fine even if you install the mirrors one at a time.

 

[DaveC426913]

BTW, it is the same principle that allows you to place a penny on the lens of a(n old-style) camera and still be able to take a picture. you can look at it as if the penny is 100% out of focus, thus spreading its apparition across the entire image (which is perceived as a simple dimming of the entire image).

 

[SpaceTiger]

I think it would make sense for close-by objects since the light rays would not be parallel (it would block light trying to cross from one side to the other), but for astronomical purposes, with the light rays exactly parallel, it shouldn't matter at all.

Yeah, that's what I was thinking. I think that spherical aberration can cause blackouts, so perhaps the low-power blackout is from a bad secondary.

 

[turbo]

BTW, it is the same principle that allows you to place a penny on the lens of a(n old-style) camera and still be able to take a picture. you can look at it as if the penny is 100% out of focus, thus spreading its apparition across the entire image (which is perceived as a simple dimming of the entire image).

It's not simply a dimming of the image-the real pain in the butt is the loss of contrast. No optical system is perfect, but an "ideal" telescope should show you a star as point-like as possible. What you get in a real-world telescope is a small but measureable primary image called the "Airey disk" surrounded by symmetrical diffraction rings. Central obstructions (even very small ones) have the effect of smearing light from the Airey disk into the diffraction rings, causing a loss of contrast. The effect is worsened when there are vanes holding the secondary mirror, since these cause diffraction spikes, further distorting the star's image.

The shape of the obstruction is crucial to the distribution of light in the final image. You who own telescopes can prove this to yourselves by cutting out a disk of black construction paper that will just fit over the telescope's tube's aperture. Find the center of that disk, and construct a hexagon within that shape and carefully cut out that hexagon with a razor knife (leave enough material so the mask won't flop over). Install this mask over your aperture and look at a bright star. The Airey disk of the star will appear compressed (pinched) in 6 places separated by 6 sharp points. What's the purpose of this? If you have a sufficiently large and contrasty telescope, you can aim your telescope at Sirius and by rotating the mask you may be able to suppress the Airey disk of Sirius to allow Sirius B to come into view.

A small scope with high-quality optics and no central obstruction (like my 6" AstroPhysics APO) can show you objects that are difficult or impossible in much larger 'scopes with central obstructions. Inch for inch, APOs are pretty darned expensive, but when you take optical performance into account (instead of raw aperture) they are a pretty fair deal.

 

[Labguy]

Do you happen to know why this happens? In general, there should be light entering the telescope that corresponds to the center of the field of view. If it's not appearing on the image, I would be prone to suspect the optics rather than the obstruction presented by the secondary.

It all depends on the exit pupil delivered by the telescope / eyepiece and the pupil diameter of your eye's pupil at the time.

A low-powered eyepiece always gives a larger exit pupil. Exit pupil is simply the diameter of the beam of light produced by the scope / eyepiece combination. It (exit pupil) can easily be figured by dividing the focal length of the eyepiece in mm by the focal ratio of the scope's objective. At the eyepiece, your eye is seeing everything that the primary "sees" , including the central obstruction. If the exit pupil of the scope is large (low power) and the dilation of your eye is small, the image of the secondary will appear larger since most of the light of the exit pupil doesn't enter your eye.

Example;
(1) Telescope is f/5 (diameter doesn't mean anything here) and you have an eyepiece of 10mm fl. So, the exit pupil (EP) = 2mm, the 10mm eyepiece divided by 5, the scopes f/ratio. Even in daylight all of the light from the scope would enter your eye (about 2mm-2.5mm) so you would see all that the objective sees and the light from the whole image enters your eye and the secondary can't be seen. It is there but it is "hidden" by the brighter light from the image. Light to form an image in the center of the field comes from "off-axis" light entering the scope.

(2) Same telescope with a 32mm (wide field) eyepiece. The EP here is 6.4mm. In example (1), the secondary image is very small since the eye sees the entire image of the primary. But, in this case the EP of the scope is 6.4mm, but your daylight eye is only opened to about 2.5mm Max. So, the whole image of 6.4mm doesen't enter your eye; only the central 2.5mm. Since you only see the most central part of the image in this case, you are seeing mostly the central obstruction and very few of the off-axis light that would otherwise form the central parts of the image. IOW, most of what you see is the magnified secondary, so you get a black spot in the center.

Another short reference from an amateur user:

the kidney bean effect is not the same as what is often referred to as a "black spot" or "blackout". In a reflector, a low powered eyepiece with a large exit pupil produces a large image of the secondary mirror obstruction. When the pupil of the eye is small during daylight and the size of the secondary obstruction image approaches the size of the pupil, it will appear as a darkened region in the center of the field. At night the darkened region is not so noticeable, provided the pupil of your eye is able to dilate large enough.

Note the last sentence where he states that if your eye is dialated wide enough to "take-in" the whole EP of the scope, the blackout effect can't be seen.

Also note that this blackout effect is mentioned only when the scope is used at low powers. In any given telescope, a lower power means that a longer focal length eyepiece is being used. As we saw above, a longer fl eyepiece will result in a bigger exit pupil since EP = eyepiece fl / scope's f/ratio. As an aside, it always bugs me when we see drawings of telescope designs when only light rays are shown entering parallel to the tube and at the outer edges of the primary (lens or mirror). If it wasn't for off-axis light hitting the primary, we wouldn't be using telescopes today.

 

[SpaceTiger]

It's not simply a dimming of the image-the real pain in the butt is the loss of contrast. No optical system is perfect, but an "ideal" telescope should show you a star as point-like as possible. What you get in a real-world telescope is a small but measureable primary image called the "Airey disk" surrounded by symmetrical diffraction rings. Central obstructions (even very small ones) have the effect of smearing light from the Airey disk into the diffraction rings, causing a loss of contrast. The effect is worsened when there are vanes holding the secondary mirror, since these cause diffraction spikes, further distorting the star's image.

Yeah, in professional astronomy we call this the "point spread function" (PSF). Basically, we have some mathematical function that describes how the light from a point source is smeared on the image plane. With a perfect circular aperture (and no atmosphere), this function would be equivalent to an Airy disk, but the secondary makes it so that the effective aperture -- and thus, the PSF -- is considerably more complicated than that. However, this is different from creating a "hole" in the image, as suggested by the OP, which would imply that one wasn't seeing any light from some of the sources in the FOV.

Incidentally, weird apertures are not always a problem -- in fact, sometimes they can be quite useful. At the moment, there is considerable effort being devoted to designing an aperture that will redistribute the light from a star in such a way that off-center intensity peaks will be emphasized in the final image. Why? Imagine you're sitting in the Alpha Centauri system and looking towards the sun. What off-center intensity peaks might be of interest?

 

[Labguy]

It's not simply a dimming of the image-the real pain in the butt is the loss of contrast. No optical system is perfect, but an "ideal" telescope should show you a star as point-like as possible. What you get in a real-world telescope is a small but measureable primary image called the "Airey disk" surrounded by symmetrical diffraction rings. Central obstructions (even very small ones) have the effect of smearing light from the Airey disk into the diffraction rings, causing a loss of contrast. The effect is worsened when there are vanes holding the secondary mirror, since these cause diffraction spikes, further distorting the star's image.

Any obstruction will cause image degradtion (diffraction). Roland Christen, who's company made your fine APO said this in an email:

If you're talking visual, I would put my money on a high end reflector. Reason is Central Obstruction. Two years ago I brought two high end 10" Mak-Cass scopes down to the Florida Star Party. One was an F9 with 31% obstruction, the other an F14.6 with 23% obstruction. The scopes were set up side by side. On things like delicate detail on Jupiter, you could see the difference quite readily in the amount of subtle detail and contrast visible. When we aimed both scopes at Sirius, the companion was easily visible in the 23% F14.6 obstructed scope, and very difficult in the 31% F9 scope.

The C9.25 is even worse than 31%, something on the order of 38%, I believe. As such, it would not stand a chance against a well made high end Newt with 20% or less obstruction.

Just because there are great images coming out of this scope does not mean that you will visually see this kind of contrast and detail with your eye. In order for the eye to discern subtle detail the inherent contrast must be very high, and a largish central obstruction will definitely kill it real quick, no matter how well the optics are figured. An imager can stack thousands of images and crank up the contrast until even the minutest detail pops out. There is no such contrast enhancement feature in a human eye/brain system.

Roland Christen

Of course, APO's were not in the subject at the time...

 

[SpaceTiger]

It all depends on the exit pupil delivered by the telescope / eyepiece and the pupil diameter of your eye's pupil at the time.

Ok, so basically the answer is that, at the center of the image, you're not seeing all of the light collected by the telescope, only that from the inner parts. Since the inner parts are blocked by the secondary, you won't see any light from outside sources coming in at this angle.

At the eyepiece, your eye is seeing everything that the primary "sees" , including the central obstruction.

Well, you're not really seeing the obstruction in the way one normally means it, since the rays are not coming in parallel and, therefore, are not being brought to a focus by the mirrors.

As an aside, it always bugs me when we see drawings of telescope designs when only light rays are shown entering parallel to the tube and at the outer edges of the primary (lens or mirror). If it wasn't for off-axis light hitting the primary, we wouldn't be using telescopes today.

I think the idea it's supposed to communicate is that all light coming in from the same angle is (in a perfect world) focused to the same point. This would be the same for off-axis rays, but is a bit more difficult to visualize. However, good optics textbooks will give a lot more than just on-axis rays in their diagrams.

 

[Labguy]

Ok, so basically the answer is that, at the center of the image, you're not seeing all of the light collected by the telescope, only that from the inner parts. Since the inner parts are blocked by the secondary, you won't see any light from outside sources coming in at this angle.

Exactly!

Well, you're not really seeing the obstruction in the way one normally means it, since the rays are not coming in parallel and, therefore, are not being brought to a focus by the mirrors.

Also true!

I think the idea it's supposed to communicate is that all light coming in from the same angle is (in a perfect world) focused to the same point. This would be the same for off-axis rays, but is a bit more difficult to visualize. However, good optics textbooks will give a lot more than just on-axis rays in their diagrams.

Exactly again!..

 

[turbo]

Yep, that's why APO refractors and catadioptic designs yield the highest quality images. I would consider spiking turbo's coffee with Rohypnol to have my way with his APO for an hour.

Come to Maine. I'll share the 'scope if you promise not to drug me.

After I've got my other house sold, I'm going to build an observatory at this one, so I don't have to drag everything out, set up, etc every time. The equatorial mount alone has got to be at least 50#, and the tube and oak tripod are no lightweights, either. Set up with my 90mm APO finder/guide scope, etc, that's got to be 150# or so. With a permanent observatory, just walk to the building, open the roof and start observing or making images. More looking, less lugging and tugging.

 

[Chronos]

Come to Maine. I'll share the 'scope if you promise not to drug me.

Surely we could raid a few lobster pots while awaiting sunset.

 

[skiboka33]

So even though there is not a hole in the picture in a general case, there still is some loss in effective collecting area correct? would this just be equal to the area of the main mirror - the area of the smaller? or is it more complicated?

 

[Chronos]

Diffraction is the name of the game. Mirror supports degrade images due to diffraction effects. Refractors and cat's do not suffer from this. Image effects are very important when doing high resolution studies. It's no accident optic designers go out of their way to limit them.

 

[turbo]

Diffraction is the name of the game. Mirror supports degrade images due to diffraction effects. Refractors and cat's do not suffer from this. Image effects are very important when doing high resolution studies. It's no accident optic designers go out of their way to limit them.

It's not only the supports (vanes, etc), the central obstruction of cats' secondaries degrade the images, too, and the larger the obstruction, the worse the degradation. Some commercial producers of these scopes express the % obstruction in terms of "secondary area/primary area" instead of "secondary diameter/primary diameter" (the traditional expression) to fool the less-wary buyers with the unrealistically tiny numbers. I won't name the lying weasels, but here is a link:

http://www.celestron.com/tb-trms.htm

Here is a link to the real story:

http://www.laughton.com/paul/rfo/obs/obs.html

Check out the image comparing the effects of wave error and central obstruction on image quality. A 'scope with 1/16th wave optics and a 33% central obstruction yields an image only about as good as a 'scope with 1/4 wave optics and NO obstruction. If you buy a C8 from the company that I will not name, you will get an 8" scope with a 34.3% obstruction (2.75"/8"), and you will be exceedingly lucky if you get 1/4 wave optics. Where does that leave your image quality?