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Newtonian- glass secondary support?

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#26 kalasinman

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Posted 07 August 2014 - 07:55 PM

@Mladen, interesting video. Thank you-Jack



#27 kalasinman

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Posted 08 August 2014 - 06:07 AM

Interesting?

http://www.astronomy...com/spider.html



#28 izar187

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Posted 10 August 2014 - 04:24 PM

Another issue with windowed secondary support is thermal effects.

Those of any closed tube reflector.

 

To counter them will require at the least a dew shield for the top end.

As well as most likely top and bottom end vented, variable speed, vibration free, active cooling for the rest of the ota.

 

This is some more of the story of why there are not more commercially produced windowed newts.

By the time you make one, the window may as well be a corrector plate. 



#29 kalasinman

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Posted 10 August 2014 - 09:04 PM

There seem to be 2 types of reports posted here. The more common, based on application of various theories. The other based on actual ownership and operation of a windowed Newtonian.

After considering the issue of tube currents and vents/fans etc. I was wondering why it would be effective to install a fan in order to overcome tube currents. Is one type of air flow better than another? I can see, based upon the following video, that a fan my be effective in initial cool down, but during use is detrimental. I hope you find it interesting, as I did.

 

http://www.youtube.com/watch?v=UbH4AS-0ViE

 

--Jack



#30 izar187

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Posted 11 August 2014 - 01:47 PM

There seem to be 2 types of reports posted here. The more common, based on application of various theories. The other based on actual ownership and operation of a windowed Newtonian.

After considering the issue of tube currents and vents/fans etc. I was wondering why it would be effective to install a fan in order to overcome tube currents. Is one type of air flow better than another? I can see, based upon the following video, that a fan my be effective in initial cool down, but during use is detrimental. I hope you find it interesting, as I did.

 

http://www.youtube.com/watch?v=UbH4AS-0ViE

 

--Jack

The windowed newts I have observed with were both small, 4" or so, and dew magnets.

It was necessary to keep the window dew shielded, or covered up whenever not actually looking thru and ep.

They cooled down noticeably slower then similar sized open newts.

 

The variables involved for fans on open newts are significant.

Mirror thickness, mirror diameter, glass type, cell design, tube diameter, tube materials' thermal characteristics.... all effect the outcome.

Part of why there's a healthy debate on it.

 

In practice, adjustable speed, vibration free air flow in open newts, makes a significant difference at the eyepiece, during observing.

Planetary details sharpen up.

Tiny dso's reveal themselves as the stars in their ep field sharpen up.

Difficult doubles become a bit easier.

All because the optical  thermal issues in the newt ota have become improved.

 

I have not used any mak-newts, and do not know if they incorporate active cooling.

But many are vented, top and bottom.

http://www.google.co...w=1549&bih=808 

 

Upon closer examination of some of the images in the link provided, it appears some do have side mounted blowers.


Edited by izar187, 11 August 2014 - 01:54 PM.


#31 kalasinman

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Posted 11 August 2014 - 08:29 PM

@izar187, very interesting. Agreed about "healthy" debate. Besides the list of construction variables, there are operational and environmental variables.

So far it seems like smaller and/or thinner mirrors, cool down time and degree may , if manipulated properly, may negate tube currents. I understand if deltaT is zero, there will be no currents, right?--Jack



#32 izar187

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Posted 13 August 2014 - 04:00 AM

 I understand if deltaT is zero, there will be no currents, right?--Jack

In theory, yes.

Unless ones hand is on the tube wall.

Or the night time temp continues to go down.

Or a breeze picks up.

 

But much of the real business of active cooling takes place at the surface of the primary.

Which is why many truss and strut tubes newts have blowers.

It's not for tube currents, nor just cooling the primary down.

 

Anyway, closing the end of a reflector with a window will add significant complexity to fixing the seeing inside the tube and at the mirrors' surface.

Setting the scope out for a few hours to cool in advance, will yield all the condensation issues one gets with a corrector plate, but without any of the correction.    

 

For an 8", to eliminate spider diffraction spikes, my vote would be a single curved secondary support, rather than an optical window.  



#33 kalasinman

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Posted 13 August 2014 - 04:50 AM

@izar187, the stated purpose of a corrector plate is to correct spherical aberration caused by the use of the cheaper spherical mirror. I will be using a Hyperbolic mirror.

If you are folding coma into that assumption, I will use a coma corrector at the focus tube.

as far as diffraction effects, I find the following interesting, especially the last 2 images.  http://www.astronomy...com/spider.html

I am sure our humidity will provide dew to challenge me, but we don't have the extent of cool down temp differences that other places may have. Temperatures are reasonably stable most of the year and usually vary day to night 6-7 c or so. Most of the time, I will be imaging from an upstairs balcony and leave the instrument set up there for long periods.

I have a long time to work on this before actually building anything, as the goal is to first modify an instrument as a test bed, then incorporate what I've learned in the building of a new instrument. All relevant comments are considered and appreciated, as I am learning by undertaking this project.--Jack



#34 izar187

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Posted 14 August 2014 - 10:53 AM

@izar187, the stated purpose of a corrector plate is to correct spherical aberration caused by the use of the cheaper spherical mirror. I will be using a Hyperbolic mirror.

If you are folding coma into that assumption, I will use a coma corrector at the focus tube.

as far as diffraction effects, I find the following interesting, especially the last 2 images.  http://www.astronomy...com/spider.html

I am sure our humidity will provide dew to challenge me, but we don't have the extent of cool down temp differences that other places may have. Temperatures are reasonably stable most of the year and usually vary day to night 6-7 c or so. Most of the time, I will be imaging from an upstairs balcony and leave the instrument set up there for long periods.

I have a long time to work on this before actually building anything, as the goal is to first modify an instrument as a test bed, then incorporate what I've learned in the building of a new instrument. All relevant comments are considered and appreciated, as I am learning by undertaking this project.--Jack

Understood sir.

 

Only pointing out that windowing a newt negates one of the best advantages of this type of reflecting telescope.

The open tube. 



#35 kalasinman

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Posted 15 August 2014 - 01:55 AM

@izar187, obviously it does close the tube. There are other advantages, such as increased contrast due to reduced diffraction. There is no free lunch.--Jack








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