Telescope Reviews: How fast is too fast?

There is in fact kind of an answer to this.

I am assuming that you are talking about performance at the center of the field. If not, then I mis-understood your question.

The book "Telescope Optics" shows ray tracing for a variety of different eyepiece designs at three different focal ratios: f/5, f/10, and f/15. I am simply conveying the information presented in this outstanding book. This is factual, not my opinion.

At f/15, virtually any eyepiece design is capable of putting the ray bundle into a tiny point inside a one arc minute circle (the smallest that can be easily resolved).

At f/10, the Hyugenian and Ramsden are BARELY hanging one, putting a theortical bundle outside (Hyugenian) or at (Ramsden) the one arc minute circle diameter. Past this speed, these two eyepiece start to fail quickly. At f/10, all of the other designs stay sub one arc minute (according to the traces).

At f/5, the Konigh and Plossls are close, but no longer confining the rays to a one arcminute circle (and for what it is worth, the Plossl has the smaller ray spread). The Abbe, Erfle, and Kellner are at about 2 arc minutes. The Ramsden is at about 5 arc minutes, and the Huygenian as at about 12 arc minutes.

I would say based on this that f/5 is too fast for all eyepieces to deliver the ultimate performance they are capable of.

F/6 would likley be about the fastest you could go before degradation starts to occur, however it will be TINY! Even the BEST observer would not see it. Still, it is there.

By f/5, it would look to me like even the best of these eyepieces (the Plossl and Abbe)might indeed start to loose the ability to collect the light cone accuratly.

Royce, the Mirror maker used to advocate not going faster than f/7 for a planetary scope, but recently he has concluded that f/6 is perhaps possible. The ray tracing I just used as the basis of this response seems to agree with that. Still, I would venture to guess that if it is acceptable at f/6, the vast majority of observers would not be able to discern a difference at f/5.5.

However, past f/5, and the effects may start to become noticable to very discerning astronomers even using Plossls (the best in this respect according to the ray tracing)

If you are talking about off axis, this is HIGHLY dependent on the eyepiece, and the over-riding abberations will likely come from the telescope itself, so it doesn't really matter so much. It is almost impossible to make a really fast telescope without encountering severe coma or astigmatism (or both). The eyepiece will exagerate this, but at this point, why would it matter?

Regards.

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Celestron C14, CGE (Big Al)
Astro-Physics 6" f/8 (Buffy)
Televue 101 (No name, but I call it my Widescreen HD Space TV)

The night sky is my mistress. She seduces me away from all other lovers.

Edited by Eddgie (11/10/09 12:32 PM)