Stupid refractor questions

8O I have a series of questions about refractors and comparisons to other types of scopes.

1. When a SCT or Mak indicates 20% obstruction is that obstruction of area (Secondary area/total area)?

How does this affect the resolving power. The formula for resolving seems to hint that this is just a function of diameter.

2. When trying to determine what size refractor does one need to compare to a Mak or SCT in light gathering, is it a simple matter of light gathering of Mak's mirror minus light lost to secondary. ie 10" = 25Pi light and 3" Obstruction = 2.25 Pi light so left is 22.75 Pi light equal to square root of 22.75 or 9.5 inches diameter.

If these two ideas are correct, the resolving power and light lost due to central obstruction is miniscule in terms of the overall overall package and all is left is loss of contrast.

Now for the kicker.

Why do we lose contrast?

hope you can help me wrap my mind around this one

TIA

Chester 8O

1: The obstruction is usually given as a percentage by diameter, not area. Certainly a 20% obstruction is by diameter (it would be 4% by area). Most SCT's and MCT's have obstructions around 30-35%, most newts have them around 15-25%.

The affect on resolution is complex. Do a search for MTF to find some more complete discussions, but basically the CO reduces contrast by putting some of the light from the airy disk into the diffraction rings. This mostly affects low-contrast detail like planetary surface features. A rough guide for the effect on that is that an obstructed aperture has equivalent resolution to an unobstructed aperture of D-d where D is the aperture and d is the obstruction diameter. This is a very crude estimate of the effect on low-contrast resolution, and not applicable to all things (for example, resolution of double stars is actually enhanced by the CO because the airy disk is made smaller by the light moved to the diffraction rings).

2: Yes, although you also need to take into account the reflectivity of the surfaces. A well coated refractor lens will pass 99% of the light that hits it (multiply by however many air-to-glass surfaces are in the lens). Most reflecting surfaces pass ~90% for standard coatings or ~96% for enhanced, or 99% for dielectric (usually only found on diagonals), so multiply thos numbers for each reflective surface (primary, secondary, and diagonal if there is one. Don't forget to multiply in the diagonal for the refractor, too).

3: Contrast. Part of it is the CO as mentioned above. The rest depends on the optical quality. A rough surface will scatter more light (refractor or reflector). Low-quality optics tend to be machine polished and relatively rough. So, cheap scopes (including cheap refractors) tend to have poor contrast. A very smooth surface doesn't scatter much light, so well-polished optics give better contrast. That is why certain brands (AP, TAK, TMB etc. for refractors, Zambuto, Pegasus, Royce, etc. for reflectors) have such great reputations for high-contrast optics, while others don't.

Jarad