Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Why are refractors considered to be sharper than reflectors if resolution is a function of the aperture?

  • Please log in to reply
108 replies to this topic

#101 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 84,243
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 14 July 2019 - 06:27 AM

Of course I do cooling my 10 and 16 inch at least 1 or 2 hours and always check on collimation before look at stars. My 10 and 16 inch is better for deep sky objects than refractors. Not as sharper pinpoint stars at 3 mag or brighter stars. I don't use Paracorr or Coma Corrector. Low to medium power look great on deep sky objects. I can see NGC6207 by M13 easy in my 16 inch than my 5 inch refractor. I sold 10 inch Dobsonian and 5 inch refractor. For now I am happy with my 80ED, 100ED, 150ED, 180 MakCass and 16 inch Dobsonian.  Yes Illinois night might not like Calfornia. Perfect clear and no hazy night is rare in my area. Darker sky in upper Michigan but not long night because of tempeture drop so fast and need heat dew! Maybe I need Paracorr for my 16 inch and someday I might buy it!

A few thoughts:

 

- Are you actively cooling them with fans?  I find at least an hour is required with my 10 inch with a fan blowing on the mirror. 

 

- You compared your 5 inch refractor to a 16 inch Dob.  The Dob goes about 2.5 magnitudes deeper, the stars are about 2.5 magnitudes brighter.  To make an appropriate comparison, one would compare a magnitude 3.0 star in the Dob to a mag 0.5 star in the refractor.  And this doesn't account for the smaller Airy disk of the Newtonian.  My Dobs show pinpoint stars but they're not the same stars I am seeing in my refractors.  

 

- Larger scopes face greater obstacles.  My concern is not that your refractors provide prettier views, it's that 120x magnification in the 16 inch.  There's a lot to see at higher magnifications in a 16 inch, even when the seeing is not so great.  

 

How does M13 look in the 16 inch versus how it looks in the refractors?

 

Jon


  • Illinois, GeneT and airbleeder like this

#102 Illinois

Illinois

    Gemini

  • *****
  • Posts: 3,127
  • Joined: 18 Dec 2006
  • Loc: Dixon, IL. Bortle 2 land in Wis.

Posted 15 July 2019 - 06:08 AM

I don't compare my refractor and dobsonian. I like my 16 inch far better than my 10 inch that's why I sold my 10 inch. I like refractor for low power up to around 218 power for everything. M13 is better in my 16 inch than my refractor. I can see tiny galaxy by M13 in 16 inch that is not easy in my refractor.  For bright stars or very tight double that Refractor is sharper and pinpoint stars and my dobsonian have tiny coma that I might need parccor for reduce coma.  16 inch is best for fainter stars and faint deep sky objects. I like both my refractors and 16 inch. My 80ED is best for early morning as grab and go when I wake up and the sky is clear. 


  • Jon Isaacs and russell23 like this

#103 Suryaputhra

Suryaputhra

    Sputnik

  • -----
  • Posts: 30
  • Joined: 24 Jul 2019

Posted 01 August 2019 - 01:39 AM

Few extracts from other online available sources

 

"Refractor lenses do not focus all the colors at the same point but mirrors do. The visible light that you can see with your eyes spans a narrow allowance of light out there and light should be thought of in terms of wavelength. This has to do with lenses because when you slice an image into its colors before focusing the images, you can find that when red is in focus blue will also be out of focus. Blue and red are the some of the different types of light. When the blue is focused, the red will the come out of focus. This problem only comes about in refactor telescopes.

 

The reflector telescope provides a much sharper image because the microwave is just as in focus as the x-ray, this gives it its sharper image when you use it to look at a wide range of colors"

 

"Refactor lenses don't focus all colors on the same point. Mirrors do."

 

I have few couple of refactors and reflectors sizing from 4 inc to 12 inch what I have observed that if you know how to use a reflector at various weather conditions, light pollution and brightness of the object reflector gives sharper image. Off course collimation should be perfect.


Edited by Suryaputhra, 01 August 2019 - 01:41 AM.

  • Stephen Kennedy likes this

#104 gdjsky01

gdjsky01

    Ranger 4

  • *****
  • Posts: 377
  • Joined: 25 Nov 2006
  • Loc: Southern California

Posted 01 August 2019 - 02:21 AM

I am reminded when looking through 3 to 5 inch APOs, how EASY it is to see detail with an 18 or 22 inch reflector. Sure, you completely lose your dark adaption, but holy cow, the views!!! 

 

Now maybe that is because I use big dobs under good skies, like out near Palomar. And I know I have to vent, etc... excess heat. 

 

OTOH: Here I am tonight in SoCal, at home, under mag... I guess 2 maybe 3 skies. The gas giants look great!!!!! But seeing details between the NEB and SEB are almost impossible. Whereas I remember the view from 8000 feet AMSL in an 18.... 

 

However tonight, you think I am gonna haul out a 100+ pound scope in the middle of LA? Hell no! I hauled out a StellarVue SV90T and a 3-6mm Zoom Nagler. Because seeing Saturn tonight is way better than waiting till I can get the 22 out.

 

Different Scopes for different circumstances. "To observe or not to observe? For me? Not much of a question!"


Edited by gdjsky01, 01 August 2019 - 02:23 AM.


#105 John Fitzgerald

John Fitzgerald

    In Focus

  • *****
  • Posts: 7,871
  • Joined: 04 Jan 2004
  • Loc: ROR Obs. near Pettigrew, Arkansas

Posted 01 August 2019 - 10:58 AM

Perhaps the mirror in "Illinois" 16 inch mass market reflector is not well corrected.  I have heard of Lightbridge mirrors being way less than diffraction limited.  Simply a possibility to throw out there, to explain the soft views.



#106 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 26,161
  • Joined: 01 Feb 2006

Posted 03 August 2019 - 06:12 PM

While there has been a lot of talk about reflectors vs refractors, I have not seen a good answer to your question. though one person put his finger square on it.

 

You asked about resolution and resolution is a function of two things and pretty much only two things.  

First, there is angular resolution and linear resolution.  Angular resolution is used for point sources, but linear resolution i is generally used to describe performance on extended targets (Planets) 

While aperture sets the angular resolution, it is focal ratio and focal ratio alone that sets the linear resolution. The linear resolution of at telescope is set by its focal ratio and focal ratio alone.  Any two f/10 telescopes will resolve the exact same number of lines pair at the focal plane (180 line pair per millimeter for f/10, 360 line pair for f/5, and so on). Now this is independent of design, quality, or obstruction.   Linear resolving power is totally a function of focal ratio. ( I know people will not believe it or will not understand it, but this is in fact the way it works.  This is well described in Suiter's  Star Testing book and the Telescope Optics books. 

 

Now the bigger question is what happens to those lines when we view them in a telescope, and for that you need to have a way to conceptualize contrast transfer.  This is the most fundamental attribute of contrast loss"  In the presence of contrast loss, a white line on a black background will appear wider and darker than it really is.  A black line on a white background will appear narrower and lighter than it really is.  

 

So, what causes contrast loss?

 

Basically, four things cause contrast loss:

 

  1. The size of the aperture..  A smaller aperture looses more contrast than a larger one due to more diffraction
  2. The qaulity of the optics.. A less well made scope looses more contrast than a wall made optic
  3. The presence of an obstruction will reduce contrast as opposied to an unobstructed scope, and this depends solely on the size of the obstruction, which is a function of the size vs the aperture
  4. Chromatism..  A scope with more chromatism will have less contrast transfer than a scope with less chromatic aberration. 

 

Of these, the most significant is the size of the aperture.   A decently made 10" scope with a 33% secondary obstruction will still preserve more contrast than a well made scope with a 4" aperture because the 4" aperture is always limited by the diffraction caused by the aperture itself.

 

If the obstruction is kept small, even a 6" reflector can maintaining more contrast than a 5" Apo.

 

They way that contrast is measured is how much the black and white lines become darker and wider (for white lines) or narrower and brighter (for black lines).

 

Different things affect the frequency (line spacing) differently. For example, fine scale roughness will lower contrast for all frequencies pretty much the same, while a very large central obstruction will lower contrast of the lines with a width of two to four times the width of the Airy Disk of a star.

 

So, diffracion of the aperture itself is actually one of the largest inhibitors of contrast transfer, and the smaller the aperture (all other things being equal) the more contrast is lost.

 

So, we have three different components; Angular resolution is mostly a function of aperture, linear resolution is a function of focal ratio, and contrast transfer is the effect on the lines at the focal plane due to  aperture, quality, chromatism, and obstruction in the system.   This is expressed in how much different width lines would loose contrast at the focal plane. 

A perfect scope would show the lines loosing contrast steadily as the got narrower.

 

An obstruction could cause wider lines to loose more contrast but very narrow lines to have better contrast. 

The smaller an aperture the more contrast the lines would loose due to diffraction alone. 

 

As someone said, contrast is the quality that makes an image look bold and sharp.  Angular resolution is what determines the size of the Airy Disk.  Obstruction, choromatism, and quality are what take the energy out of the white lines and spreads it into the black lines, making them less dark. 
 


  • drmark67 likes this

#107 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 84,243
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 03 August 2019 - 08:21 PM

You asked about resolution and resolution is a function of two things and pretty much only two things. 

First, there is angular resolution and linear resolution.  Angular resolution is used for point sources, but linear resolution i is generally used to describe performance on extended targets (Planets)

 

While aperture sets the angular resolution, it is focal ratio and focal ratio alone that sets the linear resolution. The linear resolution of at telescope is set by its focal ratio and focal ratio alone.  Any two f/10 telescopes will resolve the exact same number of lines pair at the focal plane (180 line pair per millimeter for f/10, 360 line pair for f/5, and so on). Now this is independent of design, quality, or obstruction.   Linear resolving power is totally a function of focal ratio. ( I know people will not believe it or will not understand it, but this is in fact the way it works.  This is well described in Suiter's  Star Testing book and the Telescope Optics books.

 

Something to ponder:

 

Angular resolution and linear resolution are really the same thing, one is measured in angular units, one is measured in linear units.  At the eyepiece, what we see is angles, we don't see linear measures.  

 

One can transform linear resolution into the angular resolution of the lines.  This turns out to be only dependent on the aperture.

 

In terms of line pairs, that would be LP = 1800/(Fr x mm)  To convert that distance between line pairs, the actual resolving power, it is inverted.

 

r = (Fr x mm)/1800    = (FL/D) x mm/1800

 

To covert that to angular resolution, one divides by the focal length of the telescope:

 

r = (FL/D) x mm)/(1800 x FL) = (1/D)*mm/1800) radians where D (Diameter) and FL (Focal Length) are both measured in milimeters.  

 

Resolution is normally measured in arc-seconds.  To convert from radians to arc-seconds:  1 radian = 2.06 x 105 arcseconds. 

 

r = (1*mm/D) x 2.06 x 105/1800 arc-seconds  = 114 mm/D arcseconds.  

 

In terms of inches, this is r = 4.84 in/D arc-seconds.  

 

The angular resolution of a line pair is inversely proportional to the aperture and independent of the focal ratio.  

 

Jon



#108 Keith Rivich

Keith Rivich

    Vanguard

  • *****
  • Posts: 2,139
  • Joined: 17 Jun 2011
  • Loc: Cypress, Tx

Posted 03 August 2019 - 10:35 PM

I have pretty much unlimited access to an 11" f/15 refractor. A doublet. The scope performs better then my 12 1/2" but can't compete with my 18 or 25". Many times I have set my 18" up on the same night I have the 11" and can do direct comparisons. Maybe if the scope was a triplet...nah. 11"s is still 11"s. All things being equal aperture rules.



#109 ausastronomer

ausastronomer

    Vanguard

  • *****
  • Posts: 2,258
  • Joined: 30 Jun 2003
  • Loc: Shoalhaven Heads NSW (Australia)

Posted 04 August 2019 - 11:23 PM

While there has been a lot of talk about reflectors vs refractors, I have not seen a good answer to your question. though one person put his finger square on it.

 

So, what causes contrast loss?

 

Basically, four things cause contrast loss:

 

  1. The size of the aperture..  A smaller aperture looses more contrast than a larger one due to more diffraction
  2. The qaulity of the optics.. A less well made scope looses more contrast than a wall made optic
  3. The presence of an obstruction will reduce contrast as opposied to an unobstructed scope, and this depends solely on the size of the obstruction, which is a function of the size vs the aperture
  4. Chromatism..  A scope with more chromatism will have less contrast transfer than a scope with less chromatic aberration. 

 

Of these, the most significant is the size of the aperture.   A decently made 10" scope with a 33% secondary obstruction will still preserve more contrast than a well made scope with a 4" aperture because the 4" aperture is always limited by the diffraction caused by the aperture itself.

 

If the obstruction is kept small, even a 6" reflector can maintaining more contrast than a 5" Apo.

 

The other thing which people don't take account of is the fact that it takes a bit of care and know how to get a medium to large aperture Newtonian to perform at it's best and deliver it's best quality images.  Many people who own Newtonians have no clue how to collimate and set them up properly and don't understand the effects of thermal equilibrium and proper cooling, with fans to cool the mirror and also to remove the boundary layer above it.  The other thing they don't understand is how to properly baffle a Newtonian.  I have a 10"/F5 tubed Newtonian that has annular knife edge baffles starting from above the focuser drawtube all the way down the tube to level with the top of the primary mirror.  The tube is also fully flocked.  It also has a central obstruction well under 20%.  I can guarantee you it has contrast as good as any refractor going around.

 

In addition to the set up issues, many Newtonians have pretty poor optics and the owners don't even realise this.

 

Because of the above, countless people look through Newtonians owned / set up by others, which are not performing at their best and representative of what a Newtonian of that aperture can do.

 

Refractors on the other hand you just plonk them down and 20 minutes later they are cooled and ready to deliver their best.

 

My Newtonians are always meticulously set up and once properly cooled give images "almost the equal" of any refractor of the same aperture, for infinitely less dollars.  Compared to smaller aperture refractors, "they eat them"

 

Cheers




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics