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Why Hubble space telescope has very small secondary mirror?

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#1 mvdmitri

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Posted 20 September 2021 - 10:49 AM

Hubble space telescope has two hyperbolic mirrors- primary 2400 mm and secondary just 300 mm. Why not 800-1000 mm like modern astrographs?

https://www.nasa.gov.../o173177366.pdf

https://www.ssl.berk...enReportHST.pdf

#2 NinePlanets

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Posted 20 September 2021 - 11:02 AM

Several years ago I was at a large regional star party where a NASA guy gave a talk about the Hubble Space Telescope.

 

I was surprised to learn that, yes, the telescope has a 94(?)-inch primary mirror but they only use 20 inches of its aperture at a time per sensor. In other words, the HST is only a 20-inch telescope, for all practical purposes.

 

At least that's what my wobbly memory has on the subject.


Edited by NinePlanets, 20 September 2021 - 11:03 AM.


#3 Tree-Line

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Posted 20 September 2021 - 11:03 AM

I did some googling, and it seems that the HST has an F-ratio of f/24, which is slower than many modern RC's. A longer f-ratio would use a smaller secondary, if my knowledge of optical systems is correct. I am far from an expert, however, so there may be another reason I'm unaware of. 


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#4 cuzimthedad

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Posted 20 September 2021 - 11:38 AM

Moving to the Science Forum



#5 ngc7319_20

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Posted 20 September 2021 - 12:49 PM

I did some googling, and it seems that the HST has an F-ratio of f/24, which is slower than many modern RC's. A longer f-ratio would use a smaller secondary, if my knowledge of optical systems is correct. I am far from an expert, however, so there may be another reason I'm unaware of. 

Yes, that is basically correct -- it is F/24 with a very small field of view, so the secondary mirror doesn't need to be very large.  

 

And yes, it uses all 2.4m of the aperture -- the comment about it being a "20-inch telescope for all practical purposes" is inaccurate, or at least misleading.


Edited by ngc7319_20, 20 September 2021 - 12:50 PM.

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#6 MitchAlsup

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Posted 20 September 2021 - 02:42 PM

Hubble space telescope has two hyperbolic mirrors- primary 2400 mm and secondary just 300 mm. Why not 800-1000 mm like modern astrographs?

Hubble was designed to be a "resolution" machine so that each pixel was congruent to an Airy disk. Wide Field Cameras has several Airy disks per pixel, High Resolution cameras have several pixels per Airy disk. This allows Hubble operators to chose wshich camera and optimize the image accordingly.

 

A long focal ratio helps in the resolution department.

 

Modern amateur astrographs, are wide field instruments, sometimes as fast as F/3 (Ricardi Hounders), and seldom slower than F/10 (older RC--modern RCs tend to be around F/8).

 

Hubble has all the time in the world to build up large mosaics of images to create wide field observations, free from the various distortions of the atmosphere, free from subtle variations of spectral throughput of the atmosphere, and capable of doing both astrometry, and photometry.


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#7 mconnelley

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Posted 21 September 2021 - 04:48 AM

Hello:

  As mentioned above, part of the driver for the long f/ratio was pixel sampling.  For an instrument like WFPC, the wide field part was under sampled, trading off resolution for more field of view.  The planetary camera part fully sampled the PSF.  The science cases that drove the design of HST didn't need terribly wide fields.  Ironically, I think it was the Hubble Deep Field that help push larger surveys with ever larger cameras.  

 

   Although the secondary mirror is small, the central obstruction is about twice as large as the secondary.  HST has a big baffle around the secondary.  Half of the time HST is observing in day time, and trying to see some of the faintest most distant things with the Sun shining on it.  When people ask about the impact of the central obstruction in a Cassegrain, think of HST, and how people seem quite happy with the images it makes.  

 

Cheers

Mike



#8 Keith Rivich

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Posted 21 September 2021 - 10:57 PM

I have always wondered how HST, or any space telescope for that matter, manages to get on an object. I understand the slewing and tracking but not the acquisition with such a narrow FOV.  Sky Safari Pro? smile.gif 

 

Anyone know how the go-to works?



#9 matt_astro_tx

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Posted 22 September 2021 - 07:13 AM

It uses star tracking cameras mounted in several places around the spacecraft to determine its position.  I would imagine its not unlike our multi-star alignment, platesolving, and PHD2 guiding.  It also uses reaction wheels to slew, track, and hold position, rather than thrusters that could foul the optics.

 

Here's a decent overview: https://www.nasa.gov...-control-system

 

"0.007 arcsecond pointing accuracy"


Edited by matt_astro_tx, 22 September 2021 - 07:22 AM.

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#10 ngc7319_20

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Posted 22 September 2021 - 01:41 PM

I have always wondered how HST, or any space telescope for that matter, manages to get on an object. I understand the slewing and tracking but not the acquisition with such a narrow FOV.  Sky Safari Pro? smile.gif

 

Anyone know how the go-to works?

 

It uses star tracking cameras mounted in several places around the spacecraft to determine its position.  I would imagine its not unlike our multi-star alignment, platesolving, and PHD2 guiding.  It also uses reaction wheels to slew, track, and hold position, rather than thrusters that could foul the optics.

 

Here's a decent overview: https://www.nasa.gov...-control-system

 

Besides the Star Trackers on the outside of the scope, there are also three Fine Guidance Sensors which are basically off-axis guide cameras looking through the main telescope.  These search for pre-selected guide stars near the target, and then lock-on to the stars and provide guiding during exposures.


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