Quote:
Originally Posted by N1
What a difference 2.5" can make  |
Well as a matter of fact it makes a huge difference. The difference is 2.7" not 2.5" BTW.
I have explained why this is so on many previous occasions, but for Richard's benefit I will spell it all out again.
Note the Secondary Mirror sizes of the 2 scopes.
250mm TAK Mewlon (9.8" clear aperture)= 72mm (2.83") Secondary.
317.5mm/F5 Zambuto/Teeter (12.5" clear aperture) = 53.3mm (2.1") Secondary
So the 250 Mewlon has a 72mm secondary for a 28.9% Central Obstruction and the 12.5" Teeter has a 53.3mm secondary for a 16.8% Central Obstruction (CO)
When you deduct the Central Obstruction and calculate the effective clear aperture of each telescope the 250 Mewlon has an effective clear aperture of 9.38" and the 12.5" Teeter has an effective clear aperture of 12.32". This equates to an effective increase in the diameter of the telescope of 31.3% and an effective increase in light collecting surface area of 72.5%. This is significant. The human eye can detect a 5% change in light intensity / brightness. The 12.5" Teeter will throw up a significantly brighter image. In addition the 12.5% will have a theoretical resolution (seeing limited) of about .45". The 250 Mewlon will have a theoretical resolution of about .6".
That's a 72.5% increase in light gathering power and a 25% increase in resolving power.
Lets introduce some additional factors. The 28.9% Central Obstruction of the 250 Mewlon will have a noticeable effect on the Modulation Transfer Function Curves (MTF). If you look at the two graphs below you will see that a telescope with a CO of 20% or less has a similar MTF curve to a perfect unobstructed telescope. Whereas the telescope with a 32.5% CO has a similar MTF curve to a clear aperture telescope with a 1/4 wave of spherical aberration. Consequently, even though a 250 Mewlon may have almost perfect optics, the best it can hope for is to perform like a 1/4 wave telescope, due to its large central obstruction.
How does this affect image quality? Here are some links to some simulations of the effect on image quality of different aberrations and aperture by world renowned photographer Damien Peach. When you look at these simulations keep in mind that we are not comparing a 25cm telescope against a 30cm telescope but against a 31.7cm telescope.
http://www.damianpeach.com/simulation.htm
Lets introduce some more factors. The 250mm Mewlon has 2 additional air to glass surfaces compared to the 12.5" Teeter. The corrector and the star diagonal, which further reduces light gathering ability and introduces additional diffraction effects.
It's a no brainer as a visual instrument. If you want to take pictures, that's an entirely different argument.
If you don't believe me just ask your wife if she would notice something which increased by 31.3% in diameter.
Cheers,
John B