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Size of Region that is Abberation-Free for Various Telescope Designs

ATM Reflector Optics
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#1 ephemeralephemeris

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Posted 03 August 2021 - 05:39 AM

Hi all,

 

I'm just at the threshold of amateur telescope making (my copy of Texereau's "How to Make a Telescope" is currently in the mail), and although I am some way off yet from building the reflector telescope that I really want (I see many iterations and smaller scopes in my future), I am already thinking about that final design. I came across the topic linked below in the ATM forum that discusses how well different optical telescope designs compare in correcting off axis abberations in astronomical images. My question is related to that topic but is somewhat flipped around. I am primarily interested in spectroscopy of point sources, so a telescope design that provides image correction over a wide field is not strictly necessary; in addition, a minimisation of the number of optical elements would beneficial by increasing the throughput of the system. With this in mind, is it known, or possible to calculate, the size of the on-axis region that is free of image aberrations for a given telescope design? I understand that this is likely a complex problem with multiple factors at play, but if this is documented, or if there are known ways that can be used to work this out I'd be very interested in knowing more.

 

Thanks for reading!

 

Tom 

 

https://www.cloudyni...atic-gregorian/



#2 ccaissie

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Posted 03 August 2021 - 07:23 AM

Wow, that's the #1 topic for a lot of ATM'ers and optical designers.  

 

The off-axis aberrations and size of useful field are illustrated in several ways for telescope designs.

Spot diagrams are the most common graphical method.  The spot formed by the optics is compared to the airy disk which is the theoretical smallest area of converging rays.

 

Spot diagrams can show the theoretical quality of the image across the field

 

https://www.cloudyni...ptical-quality/



#3 davidc135

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Posted 03 August 2021 - 08:27 AM

I know very little about spectroscopy but can you say what the characteristics of a near optimal system would be, given your particular application, practical and space restraints etc. Eg aperture, speed, focal length etc.

 

David



#4 MitchAlsup

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Posted 03 August 2021 - 10:49 AM

With this in mind, is it known, or possible to calculate, the size of the on-axis region that is free of image aberrations for a given telescope design? I understand that this is likely a complex problem with multiple factors at play, but if this is documented, or if there are known ways that can be used to work this out I'd be very interested in knowing more.

The Sidel method is the general way of doing this.

 

Given a list of surfaces, thicknesses, and glass types, it produces 5 terms that define the image quality. These terms can then be expressed (with a bit more math) into blur spot sizes. The terms represent {defocus, spherical, coma, astig, Petzval curvature}

 

This was developed in the 1800s, but the modern way is to use free optical software and punch in the data. {OSLO and ATMOS}



#5 ccaissie

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Posted 03 August 2021 - 12:02 PM

If, as I understand spectroscopy, you are taking a very small area...probably a star, and using that intense light of mixed frequencies to create a spectrum.  You must have some way of expanding the point to a line...usually a slit, and then through a refractive or reflective grating to spread out the different frequencies.  

 

So you'll need lots of light in a very small spot, devoid of color altering factors....Just sayin'....sounds like a Newtonian reflector.

A friend of mine who developed the FlipFlat, took spectra of stars when completing his M.S. in astrophysics.  .I lent him the scope, a home made 6" f/4 Newtonian....so you're not talking huge equipment.   

 

I'm not sure how refractors do in this pursuit.  My friend may have also used his Televue 101, which has fine color correction.

 

If you pick a line shaped glint of the sun, like off the edge of some chrome Chevy trim, or the glint on the leg of your shiny tripod, it creates a very narrow line, and when viewing it through a grating, or through a prism, or off the surface of a CD disk, you will see the spectrum of the sun, and many of the Fraunhofer lines.  This should start you on your way to finding ways to get a spectrum from other sources, capturing it and manipulating it.

 

https://skyandtelesc...r-spectroscopy/


Edited by ccaissie, 03 August 2021 - 12:07 PM.



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