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now I have to decide whether in the fully illuminated field to put 0 (visual only) then fitting a 31mm minor axis secondary with a 14,8% (practically a refractor) or whether to do it photographically by putting 6mm with a 36mm secondary and obstruction from 17,3%. In the first case I would do a Dobsonian, in the second I would equip it with a bar and rings.
In that case I think it goes to taste.
For example, I would not like zero CPL, just because maybe it could make me appreciable the decrease in brightness at the edge compared to the center of wide-field eyepieces from 82 ° up.Furthermore, I would think that the difference in definition between the two obstructions is not so appreciable. So I'd stay on the second case.
keep in mind that I would only use it for the planetarium, then ortho con 42-45 degrees of campo.Ora we think about it well.
Andrea
Interesting discussion…
I think that the cpl should be designed also taking into account the resulting correct field which, for a newton f9 is definitely higher than i 6 mm.
Atmos provides some great simulation tools for light spots, which allow to evaluate the extension of the corrected field on the focal plane.
An optical project always has to deal with a series of compromises, in this case, an extended focal plane is dispensed with in favor of less obstruction.
Whether it is the right thing to do can only be determined by the user of the tool… I personally would have chosen a greater obstruction to fully exploit the wide corrected and flat field of an f9 dish.I agree with Giulio, in this case I don't think the difference between the two obstructions is perceptible.
Unfortunately Atmos I know how to use very little, just enter the data for the newton simulation. I understand what you mean, but keep in mind that I am not looking for the photographic tool for large fields but, given the peculiarity of the mirror, a tool 99% visual and 1% whim shot with asi 120MM or MC. As for the parabolic, I don't think that with a 1.4mm deflection the mirror can be parabolic, but I don't know about mirror processing (even if I have 4 calcium sodium discs 203mm thick 20mm doing nothing that inspire me a lot but I wouldn't know where to start…..)
As you can see, un 200F9 (parabolic) gives a correct range of 15 mm focal plane, where the spot size is smaller than the Airy disk ( white circle ).
to set up a 15mm CPL I would need a 43mm secondary with an obstruction close to 21%. we are at one and a half times what I had in mind :D
you could look into reducing the fire draw, maybe with a low profile focuser, at which point you would go back to lower values for the obstruction.
In any case the mirror must be parabolic, with the spherical mirror ( even if f9 ) on wide fields at low magnifications one might have the sensation of an enjoyable image, but in high resolution all the details would be lost as the focal point would not be determinable. Spherical aberration on the optical axis, albeit of small value in an f9, it is still present and makes the stellar size in the focal plane larger than the Airy disk, effectively nullifying the achievement of the resolving power of the optics even in perfect seeing conditions.
Hi Massimo,
but it sure will be, I repeat, I'm not a great parable expert (although I would like to work myself a mirror but I really don't know where to start) mine was more of a curiosity, if for an f9 the arrow is about 1,4mm, it is possible to parabolize it? if it is to be very easy and quick to do I guess.Andrea
Yes Andrew, it is possible to parabolize an f9 and it is certainly a fairly fast operation, at least in terms of workmanship.
The difficulty is in fact in the precision and reliability of the measurements given the slight deviation of the parabola from the relative sphere for such a long focal length.…although I would like to work myself a mirror but I really don't know where to start…
Well, you are in the right place !
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