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Long focus 8” Newt

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#26 Garyth64

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Posted 24 March 2020 - 07:33 PM

For finding the correct secondary sizes, the online calculators can be useful, but how did they come up with those?

 

Someone had to use a formula or two.

 

Texereau's book has the formulas to find out the secondary sizes.  All you need to know it the diameter of the primary, it's f.l., how far inside the focus the secondary is placed, and the size of the image plane.

 

After plugging in your numbers into the formula a couple of times,  you may never have to visit one of the online calculators again.

 

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#27 clamchip

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Posted 24 March 2020 - 08:24 PM

I like this one:

http://www.loptics.c.../diagonals.html

 

Robert



#28 Marty0750

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Posted 24 March 2020 - 08:42 PM

Thanks Marty, I'm not very good with the various online calculators. Hence why I search search engines and the various astronomy forums to see if I can find the answers in plain English.

However compared to the other online Newtonian calcs, Stellafane is brilliant thanks.

David

David

 

The spot diagrams show virtually no difference between the different central obstruction sizes. This stands to reason as the size of the airy disk is exactly the same at f11 at all obstruction sizes

 

Spot diagram

 

All light is contained within the airy disk out to 0.5 field width. Therefore no coma, curvature of field nor astigmatism. Ie near perfect resolution across within a  0.5 degree field. Ie near perfect P-V at all your choices of C.O.

 

The question is how much contrast and brightness is affected by the size of obstruction (of your near perfect optics)

 

Increasing the size of the central obstruction not only dims the image  but reduces contrast Crucial consideration  for planetary observing!

 

The MTF (Modulation Transfer Function) diagrams show how contrast decreases as the C.O.increases. The graph lines droop leftward with increase C.O. size. The zero C.O. is more straight

 

MTF18%

MTF15%

MTF12%

MTF0%

 

Others here may be able to confirm if the difference is detectable in practice.

 

 

A curious caveat. Increasing the size of the C.O. increases resolution a bit because some of the light from the centre of the airy disk is transferredn to brightening in the airy ring(s) making the centre of the airy disk smaller and thus sharpening the image. This is similar to how "sharpening" works in image processing. It's just that the wave nature of light does it for you with C.O.Check this.

 

As far as I know planetary observers go for the smallest C.O. for best image contrast and brightness

 

Too small C.O. may present practical construction problems as the focal plane is much closer to the secondary with eyepiece possibly intruding into the tube too far and thus becoming and obstruction issue itself! 

 

Hope this helps

 

Marty


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#29 Dave1066

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Posted 26 March 2020 - 12:10 PM

David

 

The spot diagrams show virtually no difference between the different central obstruction sizes. This stands to reason as the size of the airy disk is exactly the same at f11 at all obstruction sizes

 

Spot diagram

 

All light is contained within the airy disk out to 0.5 field width. Therefore no coma, curvature of field nor astigmatism. Ie near perfect resolution across within a  0.5 degree field. Ie near perfect P-V at all your choices of C.O.

 

The question is how much contrast and brightness is affected by the size of obstruction (of your near perfect optics)

 

Increasing the size of the central obstruction not only dims the image  but reduces contrast Crucial consideration  for planetary observing!

 

The MTF (Modulation Transfer Function) diagrams show how contrast decreases as the C.O.increases. The graph lines droop leftward with increase C.O. size. The zero C.O. is more straight

 

MTF18%

MTF15%

MTF12%

MTF0%

 

Others here may be able to confirm if the difference is detectable in practice.

 

 

A curious caveat. Increasing the size of the C.O. increases resolution a bit because some of the light from the centre of the airy disk is transferredn to brightening in the airy ring(s) making the centre of the airy disk smaller and thus sharpening the image. This is similar to how "sharpening" works in image processing. It's just that the wave nature of light does it for you with C.O.Check this.

 

As far as I know planetary observers go for the smallest C.O. for best image contrast and brightness

 

Too small C.O. may present practical construction problems as the focal plane is much closer to the secondary with eyepiece possibly intruding into the tube too far and thus becoming and obstruction issue itself! 

 

Hope this helps

 

Marty

 

Thank Marty,

 

This post was most helpful in understanding design parameters of telescope design. I think I will aim for a 18% central obstruction at F11 in 6" aperture, that is equivalent to 100% Illuminated Diameter: 0.50 inch in  0.5 " ( 12.7 mm ). So 12.7 mm eyepieces and shorter focal length eyepieces will be fully illuminated. A 12.7mm eyepiece is 132x in a 6" F11. Which given my local seeing conditions, is a good place to start. Plus a 6" F11 with 18% CO is only 3% light loss compared to a 6 " clear aperture telescope comparing surface area.

 

David


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#30 dave brock

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Posted 26 March 2020 - 03:58 PM

And any gain in contrast by using a smaller secondary will be lost if said secondary has an edge issue.

#31 Garyth64

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Posted 26 March 2020 - 04:09 PM

Thank Marty,

 

This post was most helpful in understanding design parameters of telescope design. I think I will aim for a 18% central obstruction at F11 in 6" aperture, that is equivalent to 100% Illuminated Diameter: 0.50 inch in  0.5 " ( 12.7 mm ). So 12.7 mm eyepieces and shorter focal length eyepieces will be fully illuminated. A 12.7mm eyepiece is 132x in a 6" F11. Which given my local seeing conditions, is a good place to start. Plus a 6" F11 with 18% CO is only 3% light loss compared to a 6 " clear aperture telescope comparing surface area.

 

David

You may, or may not, be glad to know that using Texereau's formula agrees:

 

(6" - .5")  x 7"       +  .5"    =    1.08" for the secondary size.   

      66"

 

6"  x .18  = 1.08"

 

(It took longer to post this than it did to do the math.)


Edited by Garyth64, 26 March 2020 - 04:11 PM.

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