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Noob question #2

DIY optics reflector dob accessories
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#1 098jf234jw40mw3

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Posted 09 August 2020 - 03:25 AM

Is it possible to add a third mirror to a Newtonian? For example, at a 45° angle, located at the tube entrance? In this way, the body of the telescope could remain nearly fixed, flat at 0°, while the mirror is rotated to choose elevation (and of course pivot the whole scope, to move left/right).

Newtonian w/ third mirror?

 

If this would 'work', would it reduce image quality substantially? Would the extra mirror need to be a precision ground aluminized mirror?

 

Would this third mirror result in a final image that is right side up, instead of upside down and backwards?

 

Other consequences?



#2 sg6

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Posted 09 August 2020 - 04:35 AM

If you rotated tge mirror and left the main body still then you image would be elongated or shurnk in whatever direction - Alt and Az. If you are thinking along the terms of a diagonal they only operate as they are at 45o's anything else would result is a non-linear image.

 

What you seem to describe has been tried, basically didn't work, forget its name. A rather boring simple straight scope kind of beats everything. Lets face it Galileo made one 400 years ago, Newton had a reflector 300 years ago. In that time people will have tried just about all options.  We haven't made "different" refractors, we have made better glass for use in a refractor.

 

Both Galileo and Newton would recognise "their" scopes today. And could be disappointed we haven't progressed a great deal.

 

You can make a 3 mirror reflector, but the design is a lot different to the one you draw. It still operates as you point the axis of the scope at the target. The diagram simply attempts to deflect light into what is otherwise a fairly standard Newtonian. And as said the image would not be true owing to the reflection angles.



#3 jprideaux

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Posted 09 August 2020 - 04:49 AM

One variation of this was done at the “Mecca” of ATM.

https://stellafane.o...early/ptt1.html

And yes, notice the design here has the focus point going back through a hole in the first reflective mirror.

Yes, your flat mirror will need to not only be larger than your parabolic primary but also precision made and figured (to be flat) to the same quality as the primary or you will suffer image degredation. The extra mirror will flip the image one more time. Others more knowledgeable can give more details. I think it is the difficulty of the manufacture of a precision large flat mirror that prevents this type of scope from being done more.

Edited by jprideaux, 09 August 2020 - 05:05 AM.


#4 billdan

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Posted 09 August 2020 - 04:49 AM

Hi,

 

What you have drawn is similar to a Pfund telescope that is used for wheelchair access to the eyepiece.

 

Do a Google search on Pfund telescopes and you will see what I mean.


Edited by billdan, 09 August 2020 - 04:50 AM.


#5 jprideaux

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Posted 09 August 2020 - 05:03 AM

Here a link to another cloudy-night post talking about the pfund design.

https://www.cloudyni...und-telescopes/

I think you CAN do what you propose. The angle of tilt of the large flat will govern how big the flat needs to be. To cover a large range of tilt, you would need quite a big flat. Then once the light hits the primary, from that point on, everything has to be on-axis so a conventional Newtonian secondary would work as your diagram shows. The hard thing is the large flat.

Edited by jprideaux, 09 August 2020 - 05:45 AM.


#6 098jf234jw40mw3

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Posted 09 August 2020 - 03:46 PM

Here a link to another cloudy-night post talking about the pfund design.

https://www.cloudyni...und-telescopes/

I think you CAN do what you propose. The angle of tilt of the large flat will govern how big the flat needs to be. To cover a large range of tilt, you would need quite a big flat. Then once the light hits the primary, from that point on, everything has to be on-axis so a conventional Newtonian secondary would work as your diagram shows. The hard thing is the large flat.

Thanks! The telescope in the photographs does appear to be the mechanism I was imagining... although I meant to fix the mirror at 45° and simply rotate it along the long axis of the tube, to choose elevation. The entire tube would pivot left and right.

 

Right angle Newtonian 4
 
It's not clear to me why the flat would need to be a larger diameter than the tube? I don't know anything about optics...


#7 098jf234jw40mw3

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Posted 09 August 2020 - 03:51 PM

Hi,

 

What you have drawn is similar to a Pfund telescope that is used for wheelchair access to the eyepiece.

 

Do a Google search on Pfund telescopes and you will see what I mean.

Thanks so much for the search term 'Pfund telescope'. It's very hard to google without knowing the name of what to search for.



#8 jprideaux

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Posted 09 August 2020 - 04:54 PM

I was merely referring to the dimension of the flat surface itself.    Your most recent drawing shows it correctly. 

If you just look at the area of the flat surface itself, one dimension will be the diameter of the tube and the other dimension (because of the 45 degree angle) will be the square-root of 2 times the tube diameter.  around 1.41 x diameter.  (shaped like an oval) for the minimal surface area to reflect a circle of light down the tube.  

 

You could play around with using a simple mirror from a home improvement store to test the concept but the quality of that mirror will give poor results.  Then try a bit better mirror like some binocular users use to reflect the sky up into their binoculars so they don't have too hold the binoculars up.  You can google search binocular mirror mounts.  Note, though, that binoculars are low power and the demands on the mirror would not be a great as a higher power telescope. 

 

I don't personally have any information on the next step up in mirror quality for flats.  Others might.



#9 098jf234jw40mw3

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Posted 09 August 2020 - 05:52 PM

I was merely referring to the dimension of the flat surface itself.    Your most recent drawing shows it correctly. 

If you just look at the area of the flat surface itself, one dimension will be the diameter of the tube and the other dimension (because of the 45 degree angle) will be the square-root of 2 times the tube diameter.  around 1.41 x diameter.  (shaped like an oval) for the minimal surface area to reflect a circle of light down the tube.  

 

You could play around with using a simple mirror from a home improvement store to test the concept but the quality of that mirror will give poor results.  Then try a bit better mirror like some binocular users use to reflect the sky up into their binoculars so they don't have too hold the binoculars up.  You can google search binocular mirror mounts.  Note, though, that binoculars are low power and the demands on the mirror would not be a great as a higher power telescope. 

 

I don't personally have any information on the next step up in mirror quality for flats.  Others might.

Ah, now I understand. The mirror has more surface area.

 

I guess the mirror should be as flat as possible, and the reflecting surface should be the front of the glass, and not the back?

 

Thanks for the binocular mirror search tip.

 

I suppose my question is more from curiosity than practical use.



#10 Mr. E. Figure

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Posted 11 August 2020 - 09:30 PM

I belive the Great Paris Exhibition Telescope would be an example of the principle described in the OP, albeit a refractor instead of a reflector.


Edited by Mr. E. Figure, 11 August 2020 - 09:31 PM.


#11 098jf234jw40mw3

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Posted 12 August 2020 - 01:36 AM

I belive the Great Paris Exhibition Telescope would be an example of the principle described in the OP, albeit a refractor instead of a reflector.

Wow! Thaaaaaaat's a telescope, lol.



#12 KBHornblower

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Posted 12 August 2020 - 10:18 AM



If you rotated tge mirror and left the main body still then you image would be elongated or shurnk in whatever direction - Alt and Az. If you are thinking along the terms of a diagonal they only operate as they are at 45o's anything else would result is a non-linear image.

 

What you seem to describe has been tried, basically didn't work, forget its name. A rather boring simple straight scope kind of beats everything. Lets face it Galileo made one 400 years ago, Newton had a reflector 300 years ago. In that time people will have tried just about all options.  We haven't made "different" refractors, we have made better glass for use in a refractor.

 

Both Galileo and Newton would recognise "their" scopes today. And could be disappointed we haven't progressed a great deal.

 

You can make a 3 mirror reflector, but the design is a lot different to the one you draw. It still operates as you point the axis of the scope at the target. The diagram simply attempts to deflect light into what is otherwise a fairly standard Newtonian. And as said the image would not be true owing to the reflection angles.

My bold.  I disagree.  Tilting the flat to angles other than 45o will not distort the shape of the image.  The worst that would happen would be loss of light if the sharply tilted flat is not large enough to cover the entire diameter of the primary mirror.


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