- With shorter scopes (under f/5) for both refractors and reflectors it is difficult to control aberrations, and the scope may only be great for low power, and marginal for high power.
- If it's a reflector, the secondary obstruction is large at low focal ratios, this has undesirable consequences;
- Short eyepieces - under 10mm - impose significant difficulty making and polishing the surfaces to the required precision, and the precision required (ie tolerances) of the mechanical assembly are challenging.
A few thoughts:
- Short focal lengths make controlling aberrations more important. But it is definitely doable. A F/4 Newtonian with a Paracorr 2 to correct the coma can have a wide, coma free field, (greater than any 2 inch eyepiece available) and yet provide excellent views at high magnifications. The high magnification performance depends on the optical quality. Like wise, a Maksutov-Cassegrain has a very fast primary mirror (F/3 or so) but with a properly made meniscus corrector and secondary mirror, the aberrations can be controlled.
With refractors, it works the other way, you start out with a long focal length objective and then use a reducer/flattener to shorten the focal length. Scopes like the NP-series TeleVues and FSQ's series Takahashi's use dual EDs elements to achieve wide, flat fields at F/5.
- The size of the secondary depends on a number of factors. For example, my 22 inch F/4.4 came stock with a 3.5 inch secondary, 16%. Newtonians are the only designs capable of such small secondaries. My 25 inch F/5 had a 14% secondary. The secondary size is relatively unimportant as long as it is not too big.
- In terms of planetary performance, the eyepiece is the least concern, it basically depends on the size and quality of the optics in the telescope. Getting the image to the focal plane is the most important thing. Excellent eyepieces are available in nearly all focal lengths.
The conclusions I reached are:
1. Aperture only affects details but not the area you see.
2. At any given modification, say at 50x both scopes will have identical TFoVs.
Therefore as a total beginner I could retain TFoV even if a longer focal length scope is used by selecting EPs as appropriate to achieve a similar magnification.
It is more complicated than that:
1. Aperture does not affect the field of view. One way to think of this is to start with a large aperture telescope. If you are looking through it and someone places a disk with a hole in the center over the front of the scope, this reduces the aperture, the image will darken and less detail may be seen but the image itself will be unchanged, at least in terms of field of view.
2. At that same magnification, the TFoV depends also on the AFoV of the eyepiece. At 50x, a 50 degree eyepiece will provide a 1.0 TFoV, (50deg/50x = 1.0 deg) a 100 degree eyepiece will provide a 2.0 degree TFoV. It is both the AFoV of the eyepiece and the magnification that determine the TFoV.
In theory, you can achieve a similar TFoV by selecting a longer focal length scope and an longer focal length eyepiece of the same AFoV. But there is a limit because the 2 inch eyepiece format limits the possible field of view.
Consider two refractors, both with 2 inch focusers, one of the is a 100mm F/5, one of them is a 100mm F/10. The F/5 is capable of a very wide field of view. A 40mm eyepiece with a 65 degree field of view will provide a 5.2 degree TFoV at 12.5x.
To achieve that same TFoV, the 100mm F/10 would require an 80mm eyepiece with a 65 degree field of view. That is impossible with a 2 inch eyepiece because the widest field of view of an 80mm eyepiece that can fit in the barrel is about 32.5 degrees. This is because the field stop, the ring you see at the edge of the eyepiece, has a maximum possible diameter of 46mm, about 1.8 inches. To achieve an 80mm 65 degree AFoV eyepiece would require a 4 inch eyepiece and such eyepieces are not made.