Please comment (about f-number and chromatic aberrations)?

Question

According to Conrady's standard a 70mm achromatic refractor must have f/14 - many are close to this.But a 125mm aims fot f/25 and 150mm - f/30!!!
The big achromats on the market are f/10 - f/8 and they cost hell a lot of money and if that above is right they should perform way worse at the same magnification and you would be better off with the mid ones and is that the case???Sure not!
If chromatic aberation is caused by rays falling at steeper angle and thus get more refracted then if the f-ratio is kept same it's fine.The only possible explanation I see comes from the fact that a bigger f/10 refractor will give more magnification with same eyepiece than the smaller refractor so if the smaller one is given such an eyepice (to reach that mag.) It will suffer the same chromatic aberration at f/10 but because of diffraction and other optics limitations it will be out of the question to detect it in so deteriorated image.

And vice versa - in the future when optics are so precise that with a 70mm achromat one will be able to reach 300-400x it won't be the diffraction but the chromatic aberration that would be a problem because that refractor would need f/20 or above even though it is 70mm.
Whoever solves my problem - 10!

Answer 1

First of all, read my article which discusses this problem:
http://www.gaherty.ca/rogers/achrefract.htm

J. B. Sidgwick in his Amateur Astronomer's Handbook derives a less stringent requirement for chromatic aberration than Conrady, but the required focal RATIO is still a function of aperture, meaning that the focal LENGTH is a function of the square of aperture. Also, remember that there is no free lunch! Alvan Clark and Zeiss and Unitron used to make very fine achromatic refractors, and they were always around f/15. The glass used today for achromats is no different from what was available back then. How is it possible to have 150mm f/8 refractors today? By allowing them to have horrendous amounts of chromatic aberration! Our modern "lifestyle" demands short focal ratios, so Synta makes them and people buy them, but the colour error is terrible. There are still one or two companies which make classic f/15 refractors, such as Antares (105mm) and D & G (5" to 11" aperture), but few people buy them because they're definitely not portable and require a massive mount in a permanent observatory. If you think a Synta 150mm f/8 costs a lot of money, look at D & G's prices:
http://www.dgoptical.com/refractor.htm

F/15 achromats really only work well up to 3" aperture. Above that, you need to go to f/20 for a 4", f/25 for a 5" etc., and nobody makes these...the compromise is f/15. I've used a couple of largish achromats 9-12", and the chromatic aberration is bearable, but I once had a look at Jupiter through the Clark 36" at Lick Observatory: colour galore!

There are no magic telescopes. Largish achromats must either have long focal ratios or use exotic glass; both are expensive. I've ended up getting rid of all my achromats, and now have mostly reflectors (Newtonian or Cassegrain) and a couple of small Orion ED refractors to satisfy my "refractor hunger." The Orion 100mm ED is the best combination of optical quality and price I've found yet. Stay away from the fast achromats; they won't show you why refractors can be almost magical. The 100ED comes close.

Answer 2

An achromat, by definition, has two lens elements of different dispersions, generally crown glass and flint glass. The Conrady standard applies to them.

But it's entirely possible to make a three-element objective, which is much superior and throws the Conrady standard out the window. These are called "apochromatic" objectives, and they are indeed much more expensive than achromatics -- but they're generally worth it in terms of image. The third element allows much more of the visible spectrum to focus at the same distance.

Answer 3

It is an old debate, reflector or refractor. I would think that it was settled, given that almost all new professional telescopes are reflectors. But as has been pointed out above, the new 3 element apochromatic refractors have made a difference, at least for small telescopes. But they are SO expensive!

Answer 4

The focal length of a particular type of lense in blue light is x times as long as the focal length of a lense in red light. This x is very close to 1 but the small difference is what causes the color fringing, as you probably know.

As aperture increases and focal length stays the same (the F-number getting smaller), the size of the out-of-focus blue light increases at the point where the red light is in focus. This of course explains why lower focal ratios produce more aberration.

Let's say we hold the f-ratio constant and increase both aperture and focal length. As the focal length increases, the difference between blue focal length and red focal length gets bigger. This is because fb = fr * x, so if you double fr, the difference between fb and fr will also double. As such, the point where blue light is in focus will be further from the point at which the red light is in focus in this longer focal length sitatution, again increasing the size of the out-of-focus disk of blue light at the point where red is in focus.

I hope this helps you understand the situation.

Answer 5

i have no clue of what ur talking about