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1 mirror + 1 camera... why not ?

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#1 luxo II

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Posted 30 June 2020 - 06:07 AM

I have a zwo camera on its way and for a while I have been thinking about the fact that it’s 78mm diameter. 40% of an 8” mirror, or ⅓ of a 10”.

So... why aren’t people building astrographs with just 1 mirror and a camera placed at the prime focus, and eliminate the newtonian secondary mirror, instead of persevering with the newtonians and all the collimation and flexure challenges.
I’d expect this to be a devastatingly simple thing to build, too.

Sure you will need a helical focuser, but these exist.

Or is this heretical ? untapped opportunity ?

Thoughts ?

#2 endlessky

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Posted 30 June 2020 - 06:24 AM

The RASA (Rowe-Ackermann Schmidt Astrograph) type telescopes work kind of like you suggested, although they have a corrector plate and some lenses before reaching the focus point of the camera. Also needed to be considered is that the camera will be in front of the light path stricking the primary mirror, so the smaller the primary mirror, the bigger (in percent) the obstruction will be for the same camera.

 

I don't know if this type of design could be done with a purely Newtonian type telescope. I suspect you would still need some lenses to correct for coma.



#3 luxo II

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Posted 30 June 2020 - 06:37 AM

Ok rule out the RASA.

I’m thinking more along the lines of a parabolic mirror with the possible addition of a corrector in front of the camera (coma and field flattener) - and it may or may not be necessary depending on f/ratio be sensor size.

Even at 8” f/6 this should slay the refractor brigade and quite possibly without a corrector.

#4 ToxMan

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Posted 30 June 2020 - 06:43 AM

If I am not mistaken, all of these catadioptrics are set up that way: https://catalina.lpl...bout/facilities



#5 vehnae

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Posted 30 June 2020 - 06:45 AM

A killer planetary setup would consist of a 1,25" barrel shaped camera and a 1,25" coma-correcting barlow stuffed into a helical focuser that is attached to a custom spider hub that would also have a tilt adjustment for collimation. The obstruction would be constant (46mm for ZWO helical focuser) regardless of aperture. Or skip the helical focuser altogether and focus with the main mirror that could be motorized more easily.

 

There's nothing wrong with this arrangement, but it is more constrained than a regular newtonian (no visual, no space for filter wheels, etc) which I think is the reason manufacturers are not keen to jump into that bandwagon.

 

  ++ Jari



#6 Tapio

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Posted 30 June 2020 - 06:49 AM

Well Unistellar eVscope is like that.

In addition to obstruction factor there is the consideration of camera producing heat and the minor point of cables.

 

But it can be done (and has been done). I remember british Martin Mobberley done it (but couldn't find it now).



#7 vehnae

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Posted 30 June 2020 - 06:50 AM

Many survey telescopes use prime focus because it allows for the widest possible field with fast f-ratios. The coma correctors in these systems are physically rather long and need very large front elements to avoid vignetting. The size of the secondary required to fold a f/2 beam to newtonian focus with a long and large corrector would be enormous! 

 

  ++ Jari



#8 luxo II

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Posted 30 June 2020 - 07:14 AM

What made me think of this was the number of fast newtonians that have big fat secondaries > than the camera diameter.

Better to remove the secondary and use prime focus.

I was thinking more like f/5... I don’t see the point in f/2.

Edited by luxo II, 30 June 2020 - 07:14 AM.


#9 Jon Isaacs

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Posted 30 June 2020 - 07:21 AM

Even at 8” f/6 this should slay the refractor brigade and quite possibly without a corrector.

 

At F/6, the coma free field is 4.75mm. In 8 inch, that corresponds to a field of view 0.22 degrees.  That's tiny for deep space and it's slow. Go to F/4 and faster where astro-graphs live and the coma free field is 1.4 mm which corresponds to a field of view of 0.10 degrees.  

 

Getting a wide, well corrected field, it's not easy.  That's why astrographs are expensive and use sophisticated correctors.  

 

And then there is still the questions of how do you focus?  How do you collimate.  What do you do about the camera and the heat from the camera.

 

Jon




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