Steven Sterner said:
This just crossed my mind : If you were to double stack two .63 reducers , would it yield a .3 reduction at level for visual use . I already know that a single reducer at .3 is for ccd only . Sort of the reverse idea behind using a 12mm Nagler type 4 with a 2x Powermate . I'll have to see if my neighbor has a .63 , give it a try ...I wonder if this has ever been tried ?
Steve S
As is true of barlow, I believe the actual power of a reducer depends upon the exact distances between the various components of the optical train. Stacking reducers would result in less magnification but the exact number is not necessarily. If the positions were proper, you could multiply them together, 0.63 x 0.63 = 0.4
I can think of several reasons why this is not advisable with an F/10 SCT:
- Turning an F/10 scope into an F/6.3 with a reducer/corrector is reasonable because it corrects for some of the aberrations and provides for a wider field of view with shorter focal length eyepieces. But turning an F/10 into an F/4 leaves you with all the difficulties associated with an F/4 scope, eye astigmatism etc.
- Turn an F/10 into an F/4 will not increase the maximum possible field of view because the field of view is limited by the internal workings of the scope, i.e. the rear baffle. The rear port is only so big and that is what causes vignetting in an SCT. It is like a field stop that is not at the focal plane. This limitation comes into play even at F/6.3.
An F/3 reducer is used with cameras that have a small chip size, this corresponds to a short focal length eyepiece or actually a small field stop. For example the chip used in an ST-237 is only about 7mm on the diagonal. Even with an F/3 reducer this only results in an effective chip size of about 23mm, much smaller than even the rear port on a C-5.
- The typical F/6.3 Celestron reducer is also a corrector, it cleans up some of the aberrations present in the SCT. Stacking them may cause over correction.
Bottom line: Stacking reducers would decrease the effective focal length of the telescope but it doesn't seem worthwhile.
Jon
