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Increasing etendue by collapsing a dobsonian

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#1 daslolo

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Posted 08 February 2020 - 09:01 AM

Some dobs can be collapse for transport, the sky watcher even has a collapse setting for binos.

 

 

 

 

Seems like a great way to reduce focal length to extend light gathering, what is your experience with that? And how do you move the focus point back to the eyepiece if it's even a good idea?

 

(me think the secondary will move too far in the focus beam of the primary and lose all benefit)

 



#2 Augustus

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Posted 08 February 2020 - 09:15 AM

That's not how focal length works.....


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#3 junomike

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Posted 08 February 2020 - 09:34 AM

The Focal Point of the Newt is "fixed" meaning it doesn't change.

Lowering the UTA (head) of a Dob would result in having to rack the Focuser out further (for single eyepiece use).

It does aid in reaching Focus for Binoviewers however the light cone is now clipped which decreases the Aperture and increases the Central Obstruction.

It's a nice trade off for some but using an OCS (usually comes with BV's) to reach focus is a better option.

 

The main benefit of a collapsible Dob is size for transportation as it's shorter to work with


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#4 TOMDEY

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Posted 08 February 2020 - 10:03 AM

Some dobs can be collapse for transport, the sky watcher even has a collapse setting for binos.

 

Seems like a great way to reduce focal length to extend light gathering, what is your experience with that? And how do you move the focus point back to the eyepiece if it's even a good idea?

 

(me think the secondary will move too far in the focus beam of the primary and lose all benefit)

That's not how étendue works. Unfortunately, very few people grasp/internalize the concept, even those who work in the field, and especially avocationals. I frequently mention Emmy Noether's Theorem, which is whence it all springs. It's, in essence, rigorous statement of the ~no free lunch~ invariant. The others' comments up there are also good and pertinent. PS: When you jigger the components of an already optimized configuration to accommodate e.g. a Binoviewer... you are almost always further compromising performance. That is to say, the radiance and luminance of a passive optical system can only go down, never up.    Tom


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#5 sg6

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Posted 08 February 2020 - 10:48 AM

Sometimes I actually worry about people and astronomy.

This reads the same as the person who cut 2" off the tube of their refractor to go from f/8 to f/5. Then asked on CN why they couldn't get it to focus.

 

Think 30-40 people read that before someone plucked up the courage to ask if they had really done what they said.

And yes they had.

 

Time to go eat, or at least have a coffee.


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#6 lonn

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Posted 08 February 2020 - 02:54 PM

Google "How does a Newtonian reflector telescope work". You have to know the basics first.
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#7 daslolo

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Posted 09 February 2020 - 12:29 AM

The Focal Point of the Newt is "fixed" meaning it doesn't change.

Lowering the UTA (head) of a Dob would result in having to rack the Focuser out further (for single eyepiece use).

It does aid in reaching Focus for Binoviewers however the light cone is now clipped which decreases the Aperture and increases the Central Obstruction.

It's a nice trade off for some but using an OCS (usually comes with BV's) to reach focus is a better option.

 

The main benefit of a collapsible Dob is size for transportation as it's shorter to work with

I don't understand why that is so drew the light path.

lF2FfHb.png

Based on this drawing it looks like:

  1. If I move the secondary towards the primary (by collapsing) the secondary will be too much inside the red light path so it will bounce back less light.
  2. the distance from the secondary mirror to the focal point (end of purple path) depends only on the size of the secondary and the angle of the red light path bounced from the primary mirror. that red angle comes only from the curvature of the primary mirror
  3. The solid angle drawn in light yellow look like it's a function of the distance(opening, primary mirror) and the aperture size.
  4. but if I remove the tube like in truss dobson and that awesome looking LSST, the solid angle is only a function of the mirror curvature and the distance(primary, secondary), the closer to the primary the secondary is and the wider solid angle it can capture

I read that 2 is partially wrong, the prime focus as it's called is only a function of the curvature of the primary, not the size of the secondary. why does the position of the prime focus on the eyepiece change?

If 2 and 4 are correct then etendue only depends on the curvature of the mirror * surface of the mirror, which means that collapsing the dobs will only do a thing if the primary is also replaced

 

comments, corrections?


Edited by daslolo, 09 February 2020 - 12:50 AM.


#8 brentknight

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Posted 09 February 2020 - 01:12 AM

Focal length is determined by the curvature of the mirror and it terminates at the focal point.  Moving the secondary closer to the primary will cause the focal point to move farther away from the edge of the tube/truss/whatever.  You will also lose light that the too small secondary can no longer reflect back to the eyepiece.  You can get a larger secondary, but that won't change the focal point and it will also decrease the amount of light getting back to the eyepiece.

 

The yellow lines are something else you don't want.  This is called stray light and all it will do is illuminate the mirror - it won't come to focus.  You want to block that out, not let it in...


Edited by brentknight, 09 February 2020 - 01:23 AM.

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#9 daslolo

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Posted 10 February 2020 - 11:01 AM

The yellow lines are something else you don't want.  This is called stray light and all it will do is illuminate the mirror - it won't come to focus.  You want to block that out, not let it in...

I thought that was how the telescope gets a conical section of the sky but it seems to come from the pupil's aperture so then, the right way to trace a light path is backward : from the pupil's edges to the mirrors to the outside.

 

 

500px-Newtonian_telescope2.svg.png



#10 brentknight

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Posted 10 February 2020 - 11:58 AM

I am not an optical engineer, so I'm sure others could give you better explanations, but your green lines will never come to focus - as you have shown. Only parallel rays will return to the focal point. Other angles that hit the mirror will decrease contrast. They just light up the tube and the mirror... If you have a telescope, you can see this for yourself. Any light hitting the mirror not parallel to the optical axis of the primary will cause issues - shine a flashlight in there and you will see...
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#11 Eddgie

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Posted 10 February 2020 - 06:09 PM

daslolo,

When you move the top of a collapsable dob down, it does not change the focal lenght of the system.   

 

Suppose the focal plane of the telescope is 25mm from the top of the fully racked in focuser.

 

If you shorten the tube, the focal plane does not stay the same distance from the top of the focuser.  If you shorten the tube by 100mm, then now the focal plane will be 125mm from the top of the fully racked in focuser

 

As to why the telescope can loose aperture it is because in a Newtonain, the secondary mirror is usually only sized to fully illuminate about a 10mm circle.  For every multiple of the focal ratio that you move the secondary closer to the primary, the size of that circle will be made smaller.  For example, if the scope is f/5 and has a 10mm fully illuminated field, if you shorten the truss poles by 100mm, then the image cicle will collapse by 20mm, but since it is only 10mm to start with, when it goes past zero, the aperture is reduced.  

 

aperture loss.jpg

 

The lines representing the secondary are the same length. If they look different, that is an optical illusion. This descripes what happens though.  Shortening the tube simply moves the secondary mirror closer but the distance to the focal plane does not change, so the focal plane has to be moved further from the outside of the tube wall.

 

 


Edited by Eddgie, 10 February 2020 - 06:10 PM.

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#12 KLWalsh

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Posted 10 February 2020 - 07:22 PM

IF you don’t care about an image, and
IF the source is an illuminated region covering 2 pi steradians,
Then you might get more light through an imaginary circle at the output of the focus drawtube, depending on the luminance structure of illuminated region.
- But you’re not doing Astronomy, since these constraints don’t apply for astronomical telescopes.

#13 daslolo

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Posted 10 February 2020 - 07:26 PM

look what I found

https://ricktu288.gi...tics/simulator/

Attached File  newtonian.zip   596bytes   0 downloads

Annotation 2020-02-10 162122.jpg



#14 daslolo

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Posted 10 February 2020 - 07:36 PM

IF you don’t care about an image, and
IF the source is an illuminated region covering 2 pi steradians,
Then you might get more light through an imaginary circle at the output of the focus drawtube, depending on the luminance structure of illuminated region.
- But you’re not doing Astronomy, since these constraints don’t apply for astronomical telescopes.

Alright I can see you are smart and know things, so how about you

PUT the snark and those big words to rest and

LEVEL with me in everyday terms, preferably diagrams



#15 Augustus

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Posted 10 February 2020 - 07:43 PM

Alright I can see you are smart and know things, so how about you

PUT the snark and those big words to rest and

LEVEL with me in everyday terms, preferably diagrams

If you are so confident in your theory I advise you test it. You will quickly understand.


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#16 brentknight

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Posted 10 February 2020 - 08:33 PM

I honestly don't understand what your trying to prove or what your trying to do.  All of the diagrams and descriptions I have see so far demonstrate that it's not good design to move the secondary closer to the primary.  You can't get more light to the eyepiece than the full aperture of the primary (light coming in around the primary does you no good), and you can't shorten the focal length unless you change the figure on the primary.

 

Like Augustus said...if you have a telescope, you can test this very quickly.

 

BTW...your diagram above omits one critical element - the eyepiece.  Also, there is no way that the light rays will diverge that way inbound of the focal point.

I think some people here are trying to take you seriously, but it's starting to look like your just poking fun.


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#17 a__l

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Posted 11 February 2020 - 09:51 AM

That makes one sense, and I know the ATMs that did this.
If you have a long telescope, but you want to watch on your feet at the zenith (without a step).
Increase the size of the secondary. See how much cuts off your light cone at the level of the "tube wall" and suits this you.

Perhaps increase the diameter of the input hole "tube wall".


Edited by a__l, 11 February 2020 - 09:59 AM.


#18 Oberon

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Posted 11 February 2020 - 10:14 AM

I thought that was how the telescope gets a conical section of the sky but it seems to come from the pupil's aperture so then, the right way to trace a light path is backward : from the pupil's edges to the mirrors to the outside.

 

 

500px-Newtonian_telescope2.svg.png

This drawing is correct. Both green and purple are parallel, which is correct, and they both come to focus at two distinct points on the focal plane (a focal plane is made up of an infinite no. of focal points).

 

The focal plane appears at the focal length, and is determined by the curve of the mirror. This is why the focal length is fixed, and being fixed it means you cannot practically shorten the telescope substantially, you can only make small adjustments to optimise the distance between the secondary and the focal plane, generally done at the design stage when setting up for imaging v eyepieces, or for a short v long focuser.


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#19 Volvonium

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Posted 11 February 2020 - 03:05 PM

Your arm is one fixed length.  If you stick your arm straight out and bend your wrist so your hand is at 90 degrees, that is not unlike a typical reflector design, with the tips of your fingers being where you reach focus at the eyepiece.   If you bend your arm 90 degrees at the elbow, your arm length stays the same, but your fingers are pushed out further.

 

Unless modified by an optical corrector or focal reducer, a reflector's focal length will not change, since it is determined by the shape of the mirror.

 

This is a very simple explanation; once have more hands on time you'll understand.


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#20 a__l

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Posted 11 February 2020 - 05:57 PM

it means you cannot practically shorten the telescope substantially

As always, this is a question of terms. Always ask what is significant?
For example, 5-7 cm is significant or not?
For example, for a user who does not want to deal with an additional bench, this is significant!

 

What is usually SCT obstruction? Proceed from that it will not be worse.


Edited by a__l, 11 February 2020 - 06:15 PM.


#21 daslolo

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Posted 11 February 2020 - 08:47 PM

This drawing is correct. Both green and purple are parallel, which is correct, and they both come to focus at two distinct points on the focal plane (a focal plane is made up of an infinite no. of focal points).

 

The focal plane appears at the focal length, and is determined by the curve of the mirror. This is why the focal length is fixed, and being fixed it means you cannot practically shorten the telescope substantially, you can only make small adjustments to optimise the distance between the secondary and the focal plane, generally done at the design stage when setting up for imaging v eyepieces, or for a short v long focuser.

Got it! Thanks.

 

I was struggling with how much light gets in because all diagrams show the parallel as converging to only one POINT and not a PLANE. If I follow the light path backward from the plane then I do get a cone instead of a tube. Does that mean that, at fixed mirror curvature, the solid angle depends on the size of the focal plane? 

And what determines the size of the focal plane?

 

BTW...your diagram above omits one critical element - the eyepiece.  Also, there is no way that the light rays will diverge that way inbound of the focal point.

 

Glad you brought that up because I don't understand how eyepieces work. When I change eyepieces the image goes from bright to dark and the image looks zoomed in. Do they clip a portion of the focal plane then expand it to pupil size?

 

If you are so confident in your theory I advise you test it. You will quickly understand.

 Read the thread, you'll see that I gave up on the idea of shortening the tube back with the diagram showing that mirror curvature defines focal length. Now it's about understanding how the optic work. You build your own telescope, I could use your knowledge.



#22 Augustus

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Posted 11 February 2020 - 08:49 PM

You build your own telescope, I could use your knowledge.

I think everyone here has done a good enough job at that.



#23 KLWalsh

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Posted 11 February 2020 - 09:35 PM

Alright I can see you are smart and know things, so how about you
PUT the snark and those big words to rest and
LEVEL with me in everyday terms, preferably diagrams


Honestly, no snark was meant. I was just using the terms for lighting. The term ‘entendue’ is not widely known, so I thought that if you knew that term, you were familiar with the subject.

Two pi steradians is the angular region enclosed by a hemisphere.
If the hemisphere is uniformly illuminated, then the brightness of the cone of light, where the eyepiece goes, won’t change no matter how the mirror spacing is.
If the hemisphere is not uniformly illuminated, then you might gain brightness as the mirror distance changes.
But only 1 specific distance between the mirrors will give you an image at the eyepiece position, when the object being viewed is at infinity - as astronomical objects are.
If the mirrors aren’t at that specific distance, then your optical system is simply a ‘light bucket’. You’re catching photons, but you’re not forming an image where the eyepiece goes.
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#24 Eddgie

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Posted 11 February 2020 - 09:38 PM

 

And what determines the size of the focal plane?

 

 

There is a formula somewhere that tells you what the theoretical size of the focal plane is, but in practice it is the design of the telescope that determines the size of the user accessible focal plane. 

 

Earlier, I mentioned that a typical Newtonian designed for visual use will typically have a 10mm fully illuminated circle.  Inside this circle, the user gets access to the full aperture of the telescope and anything inside that circle is seen at 100% brightness. 

 

Outside of that circle, you still see a field, but it is very slightly vignetted, meaning you are not using the full aperture of the telescope, but unless the illumination falls off sharply the difference in brightness is difficult to see.

 

What defines the maximum size of the field (the most field the user can access) is the field stop size of the eyepiece and the point where the illumination falloff becomes objectionable.

 

For example, you could use a 56mm Plossl with an f/5 telescope, but the illumination falloff would probably be intrusive,  Some people would say that you can't use a 56mm eyepiece in an f/5 telescope becuase the exit pupil would be to big, but that does not stop you from using a 56mm eyepiece to get a wider field of view.  The aggressive illumination falloff near the edge of the field though would probably give the field an unpleasant look. 

 

Things that influence how much illumination falloff there is will be things like the speed of the system, the presence of a baffle like in SCTs and MCTs, the diameter and length  of the focuser tube in a refractor, and the size and diameter of the focuser tube in a reflector. 

So, there is a theoretical number for the maximum field of an instrument, but in practice, you have to use the amount of field that the observer could actually utilize and this is typically less than the formula value. 


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#25 Jon Isaacs

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Posted 12 February 2020 - 01:26 PM

Glad you brought that up because I don't understand how eyepieces work. When I change eyepieces the image goes from bright to dark and the image looks zoomed in. Do they clip a portion of the focal plane then expand it to pupil size?

 

 

Eyepieces are basically magnifying glasses, they bring your eye closer to the object, in this case, the focal plane.

 

The closer you get, the larger the image you see but the light is more spread out sonits dimmer.

 

The eyepiece has what's called a field stop, it's a metal ring at the focal plane of the eyepiece that determines how much of the focal plane you will. When you look through the eyepiece, you see the field stop as a black ring at the edge of the field.

 

Jon


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