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Old 22-02-2018, 05:27 AM
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Such a great thread and excellent discussion - looks like quite a few myths are being busted
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  #42  
Old 22-02-2018, 07:19 AM
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Craig Stark's article on this topic ( was written in 2008):

http://www.stark-labs.com/help/blog/...ioAperture.php

As Craig notes: "... that once we’re well above the read noise, the effects I’ve mentioned here become weaker. "
I wonder how the new generation of ultra low read noise CMOS cameras, would change Craig's article if written today.

Last edited by glend; 22-02-2018 at 07:51 AM.
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Old 22-02-2018, 08:10 AM
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Hi Marc. I'd be a bit wary of the rain analogy - like all simplifications, it can be a little bit misleading.

As Erwin suggested, light definitely comes in from a range of angles. The light that has a wavefront parallel to the aperture is focused by the optics into an on-axis point. Anything that is focused off-axis comes in with a tilted wavefront - it comes at a different angle from a different part of the sky and is focused to a point away from the optical axis in the focal plane.

An image with a million pixels samples the light coming from a million different directions - that's how an image is formed. The pixel size and focal length determines how big an area of sky is looked at in each of those directions - for a given aperture and pixel size, the faster the scope, the bigger the solid angle of sky looked at by each pixel. So the faster scope will collect more light in each pixel (for a given aperture and pixel size). Thus, the amount of light getting to each pixel is determined by both the aperture size (how many photons can get through it from any direction) and also the area of sky sampled by each pixel (how many photons you start out with from any direction). The rain analogy completely misses the second bit and wrongly leads to the conclusion that aperture is all that matters - aperture is fundamentally important, but it is definitely not the whole story..
Ok, so you guys are talking about a lens/corrector in the front of the aperture to focus all ligh rays into the pipe. I thought we were comparing fast newts vs. RCs designs where the first optical element is located at the end of the tube.
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  #44  
Old 22-02-2018, 08:39 AM
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no, there is nothing in front of the aperture. The ray diagrams just use a simple lens as the optical element to illustrate how an image is formed. The same thing happens with more complex real optical designs, but it is harder to visualise. Will try to put together something.
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Old 22-02-2018, 08:50 AM
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no, there is nothing in front of the aperture. The ray diagrams just use a simple lens as the optical element to illustrate how an image is formed. The same thing happens with more complex real optical designs, but it is harder to visualise. Will try to put together something.
Like a fisheye lens exposed to all the sky yeah but I just can't visualise how rays that are not near parallel to a closed tube optical axis will hit the mirror at the end of it, with baffles as well, etc... The few that hit the primary sideways would they even bounce to the secondary or miss it altogether?
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  #46  
Old 22-02-2018, 08:56 AM
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When a lower f/ratio were better than a high one, regardless of the absolute aperture, as some claim, then we all use iPhones / Galaxies or other Android phones to do AP, as these are usually a 'bright' f/1.8... f/2/2. And for regular photography as well and the DSLR market would be dead and all reporter and professionals use lightweight phonecams.
And there were no EELT or JWST or other monster telescopes in use.

I can see it when I do AP with my Canon 70-300L (f/5.6) which has 55mm aperture and with my ED110. First is 300mm f/5.6 and the latter 600mm f/5.6.
See the difference that the latter shows more detail and fainter stars with the same f/5.6.
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  #47  
Old 22-02-2018, 09:01 AM
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Like a fisheye lens exposed to all the sky yeah but I just can't visualise how rays that are not near parallel to a closed tube optical axis will hit the mirror at the end of it, with baffles as well, etc... The few that hit the primary sideways would they even bounce to the secondary or miss it altogether?
It is about the angle of incident, and angle of reflection, the ray does not have to be perpendicular to the mirror ( these mirrors are not flat). You can demonstrate this with a ray diagram, or a ray string trace running from the edge of the mirror to the opening of the tube on the opposite side. With parabolic mirrors the angle of reflection will be towards the secondary.
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Old 22-02-2018, 09:06 AM
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[QUOTE=skysurfer;
When a lower f/ratio were better than a high one, regardless of the absolute aperture, as some claim,
[/QUOTE]

nobody is claiming that - aperture is fundamental, but it is not the whole picture

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Like a fisheye lens exposed to all the sky yeah but I just can't visualise how rays that are not near parallel to a closed tube optical axis will hit the mirror at the end of it, with baffles as well, etc... The few that hit the primary sideways would they even bounce to the secondary or miss it altogether?
agreed, the angles are typically not large for a telescope (generally within about a degree of parallel to the axis). The point is though that they are definitely not parallel from different parts of the sky and the same mechanism as in JAs widefield example applies. Each point in an image is illuminated from an individual part of the sky and the ray bundle that forms it is not parallel to the ray bundles that form all other points in the image. Telescopes and fisheye lenses form images in essentially the same way - just over vastly different field angles.

The amount of light that gets into a pixel is determined by two things:
how wide each ray bundle is before it is focused by the optics (determined by the aperture) and
how much sky is included in each ray bundle (determined by the angular extent of the pixel).

Cheers Ray

Last edited by Shiraz; 22-02-2018 at 09:44 AM.
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Old 22-02-2018, 09:07 AM
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It is about the angle of incident, and angle of reflection, the ray does not have to be perpendicular to the mirror ( these mirrors are not flat). You can demonstrate this with a ray diagram, or a ray string trace running from the edge of the mirror to the opening of the tube on the opposite side. With parabolic mirrors the angle of reflection will be towards the secondary.
Jason should draw us one of his cool raytrace diagrams to illustrate.
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Old 22-02-2018, 09:52 AM
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Like a fisheye lens exposed to all the sky yeah but I just can't visualise how rays that are not near parallel to a closed tube optical axis will hit the mirror at the end of it, with baffles as well, etc... The few that hit the primary sideways would they even bounce to the secondary or miss it altogether?

100% on the money - That it the crux of it!
Only rays that are within the field of view of the telescope or lens will make their way through to the focal plane (unless limited by a field stop/iris). In the case of a telescope these rays are near parallel to the lens axis, typically no more than a few degrees down to fractions of a degree off-axis at higher focal lengths. For much lower focal lengths typical of camera lenses say 14 to 300mm, the field of view is larger and so the angle off-axis is much larger.

The diagram I drew earlier was a general case for a convex lens, but in essence holds true whether a lens or telescope. All that differs is the field of view.

For example for a 36mm x 24mm full frame, the horizontal field of view may be:

104 degrees FOV for 14 mm focal length
40 degrees FOV for 50 mm focal length
6.9 degrees FOV for 300 mm focal length
1.8 degrees FOV for 1150 mm focal length
1 degree FOV for 2000 mm focal length

Consider these field of views in relation to the Convex Lens ray diagram, in terms of the issue you raised of rays "near parallel to a closed tube optical axis".

In terms of the closed tube you mentioned it's internal diameter will (should !) always allow for the telescope's field of view, otherwise vignetting would result.

Best
JA

Last edited by JA; 22-02-2018 at 10:19 AM.
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  #51  
Old 22-02-2018, 09:52 AM
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Sooo... a C14 gathers more light than a 4" ?

I'd really like to hear from any Ceravolo dual FL owners on whether they see better S/N in the "fast" configuration.
As you know I have a C300. I also share another one at SRO. In my experience there is a significant SNR advantage in the f/4.9 configuration. This advantage comes at the cost of decreased resolution, of course, but provides a larger FOV.

One last thought on aperture... let's take the example of a 300mm aperture scope with 50% obstruction and a 50mm f/4 lens. The scope sucks down photons at 432 times the rate of the lens. If we assume that aperture is all important, then we come to the conclusion that all those DSLR astro images must be fake, or perhaps were taken over a period of several weeks
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Old 22-02-2018, 09:57 AM
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As you know I have a C300. I also share another one at SRO. In my experience there is a significant SNR advantage in the f/4.9 configuration. This advantage comes at the cost of decreased resolution, of course, but provides a larger FOV.

One last thought on aperture... let's take the example of a 300mm aperture scope with 50% obstruction and a 50mm f/4 lens. The scope sucks down photons at 432 times the rate of the lens. If we assume that aperture is all important, then we come to the conclusion that all those DSLR astro images must be fake, or perhaps were taken over a period of several weeks
My C11/Hyperstar combo is about the same FL as my FSQ106N. I've also noticed it is a *little* faster that the FSQ so it seems to support your argument.

I wonder if it has better resolution too? Or is it the placebo effect?
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Old 22-02-2018, 10:20 AM
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I wonder if it has better resolution too? Or is it the placebo effect?
Red wine & cheese effect, Marc

More seriously, the larger aperture will give you a theoretical Airy disk size more than two times smaller than the FSQ, so this effect is quite likely real.
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Old 22-02-2018, 10:29 AM
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Red wine & cheese effect, Marc
Pfweeh... I already feel better.

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More seriously, the larger aperture will give you a theoretical Airy disk size more than two times smaller than the FSQ, so this effect is quite likely real.
That's it. I'll put my FSQ for sale then. I enjoy too much lifting my 26kg "big john" over my head trying to slide it and aim for that thin rail profile on the dovetail.
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Old 22-02-2018, 10:39 AM
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That's it. I'll put my FSQ for sale then. I enjoy too much lifting my 26kg "big john" over my head trying to slide it and aim for that thin rail profile on the dovetail.
Clearly surplus to requirements
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Old 22-02-2018, 12:36 PM
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As per Erwin and Ray's comments on the field of view (acceptance angle, solid angle of view or however named), it is like considering a ray diagram for a convex lens with an object at infinity with on-axis parallel light rays(BLUE) and off-axis parallel (GREEN and RED) rays from infinity. The field of view being formed by the angle between the GREEN and RED rays as shown.

This is shown in the attached ray diagram, with the 3D effect forming a cone, or more correctly a truncated cone at the lens surface.

Best
JA
Great diagram. Certainly helps toward visualising an optic's field of view.

Then I pondered the "magnifying glass" concept for a bit...it was sort of a Eureka moment for me ...and considered the flux being converged by an optic.

With a BIG magnifying glass, and a sunny day, you can burn most things
with a concentrated image of the sun.

But a (very) long focal length lens gives you a nice big solar image, but no smoke.

Similarly, a tiny short focal length lens gives you a tiny bright spot, but again no smoke.

The simple physics is not enough concentrated flux in either case.

But this only applies to extended sources (eg Sun/Moon/Nebulae) that you can concentrate.

With point sources..eg stars.... aperture wins every time.
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Old 22-02-2018, 01:35 PM
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also applies to stars Peter. Faster optics produce smaller more intense focal plane spots from unresolved objects like stars - works just the same as with extended sources.
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Old 22-02-2018, 01:43 PM
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Actually a mirror/optics maker should be able to put this one to bed. There are a few here who figure and test optics. Stefan Buda, Bratislav, Mark Sutching. Maybe they can chime in.
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Old 22-02-2018, 01:53 PM
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But a (very) long focal length lens gives you a nice big solar image, but no smoke.
That depends on the physical aperture and FL has nothing to do with this.
From the Sun we get 1000W/m2 so a lens of given aperture will concentrate the same amount of energy, regardless of its diameter. So smoke will appear.

My 40cm Dob instantly ignites a newspaper when pointed at the Sun and no eyepiece in it.
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Old 22-02-2018, 01:59 PM
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Actually a mirror/optics maker should be able to put this one to bed.
which bit of it
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