Thanks 昇仔 for detailed experiment on explaining cause of vignetting. It is clear that the 4th lens element blocks part of the light. From the design, I don’t know whether the 3rd element also blocks part of the light. Anyway, blocking does matter and it causes the vignetting.
The mechanism of dark band is still unclear to me, maybe I am not knowledgeable to get the gist of why blocking part of the light in the middle of the light cone will cause dark band. The cross in Newtonian, is a kind of blocking, will cause bright diffraction spikes and how come the blocking in FSQ will case dark band not bright spikes or bright band?
Just a thought, the phenomenon of asymmetry light spot around the focus may not be understood in term of light cone. If we think in term of light cone, we think in term of geometrical optics. It is symmetrical around the focal point anyway. However, the wave nature of light is omitted. Think about the geometrical optical cannot explain the finite size of light spot. Geometrical optical tells you that a point, no area, can be formed, which is not true.
Maybe an experimental approach can be used. We can try to put an olive shape mask and place it in the middle of the light cone in ordinary design either doublet or triplet deign. In this case, we can put the olive mask in front of the reducer. So the blocking is in the middle of the light cone and see if it produces:
1. Dark band in doublet or triplet design.
2. Asymmetric spot around focus in doublet or triplet design.
** I cannot do it today because all my telescopes are in school lab now.
They second thing I don't understand is that why Taka don't make the aperture of the fourth lens larger to reduce the vignetting
This is an easy one. As I mentioned in my previous post that large rear lens are needed for less vignetting.
Secondly, Wah’s explanation on large rear lens are needed for less vignetting is correct. If the ray diagram provided by Tak is correct, it explained already. However, if a smaller rear lens is used, some light will be blacked. But the point is that the so-called image circle provided by a telescope maker depends on the tolerance. If the tolerance is big, the image circle is large. So without the tolereance, the size of the image circle quoted by the telescope maker may not reliably reflect the vignetting performance. As far as I know top class reducer/flatteners are very big in size. They need 3” or even 3.5” focuser to hold them.
Big size reducer/flatteners provide vignetting free images and at the same time, they are very expensive. Only really top class telescope makers provide this option for their perfectionist customers. See the Astrophysics flatteners below:
http://www.astro-physics.com/index.htm? ... /photo_acc
The so-called 2.7” flattener costs US$525 to US$700. From the description, it stated that it supports Pentax 6x7 Camera. However, there is another so-called 4” flattener and it costs US$2100!!!
It also supports Pentax 6x7 Camera. BUT in the description it stated clearly that
“This optical marvel was originally designed to take full advantage of the medium format Pentax 6x7 camera, and will provide pinpoint stars all the way to the edge of its field with no vignetting!” [愛的轟炸]
In addition the telescope must be upgraded to a 4” focuser.
So the reason why Tak doesn’t use a bigger 4th lens is simple, the cost. But the trade-off is 60% luminosity of the image circle. Just like the cheaper Astrophysics 2.7" flattener which is so-called Pentax 6x7 supporting. As far as I know for truly support Pentax 6x7 reducers, 88mm image circle, all are above HK$10,000!
There is no free lunch...