Quote:
Originally Posted by Shiraz
.... try imaging with a Barlow in the optics - this will change only the focal length (not the aperture), but you will certainly see a major drop in star brightness. ...
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I have tried that homework in the past, with my MTO, which after necessary modifications (removing focus stop) behaves as a system with variable barlow (it is possible to have a mag factor around .5 ~ 2 depending on sensor distance), and there was no difference observed (between F4 and F5) that I should've been concerned with too much. Of course, when star image (Airy's disk et al) is larger that pixel size, then you are right in theory, but additional Barlow is not a right way to determine this fact because there is additional element(s) in the optical train (not in the case of my MTO that I described earlier).
Also, you have to consider the fact that star images on CCD (CMOS) sensors are saturated very quickly - so their size will not be the measure of resolution of details..
So, for the non-saturated stars, the quick calculation shows that the
airy's disk diameter for Theta (angle of first minimum) is:
Sin(Theta) =~ 1.22*lamda/Aperture_diameter
or
Theta =~ 1.22 *lambda*(Fno/FL)
The physical size (radius) of the Airy's first minima on the sensor is:
h = FL * Tan(Theta) =~ FL * 1.22*lambda * (Fno/FL)
h = 1.22 * lambda* Fno
For 8", F4 (800mm FL)and yellow light (Sodium line, 589nm) this radius is:
2.8um, or the diameter of the first minima is 5.4um
For 8", F5 (1000mm FL) this diameter is 7.2um.
The size of the pixel on my Canon 400D is 5.7um.
So, the theoretical image size for both F4 and F5 8" mirror is comparable with the pixel size, meaning slightly better resolution with F5, but only just.. not significant at all.
I think Lewis should consider price and FOV as the most significant/important parameter for his decision (I would..)