Doubt regarding the positioning of starshade in new world's mission of Nasa

Question

The new worlds mission of NASA has an occulter with the telescope, Why did they place the occulter at a distance instead of attaching it to the telescope such that there is a hole as big as the size of the exoplanet we are examining (keeping in the mind the distance) so that the starshade only blocks the starlight while the hole recieves the exoplanet light. Is there a specific reason for tilting the starshade instead of keeping the starshade and telescope in a straight line. I hope my question is understandable, if you require a diagram than do tell me.

Answer 1

We can't image the exoplanet with normal scopes. That also means you can't point a hole at it with any sort of precision. You'd either point at nothing or at the star itself almost every time. We know where the star is, so blocking the light from that is simpler.

The telescope needs to have a large area for light gathering. If the hole is too close to the telescope, then the shadow on the image is no longer precise. Some parts will not see the exoplanet through the hole and some parts will see the star. It won't be able to mask the portions you want.

Is there a specific reason for tilting the starshade instead of keeping the starshade and telescope in a straight line.

They will be in a straight line. But besides the shade blocking the light from the star being imaged, the shade must not be lit by our sun at the same time or it would overwhelm the instrument. By tilting the shade, the side facing the telescope can be kept dark.

The New Worlds website at the University of Colorado has a section on the benefits of an external occulter vs a similar structure within the instrument:

Why use an occulter instead of a coronagraph?

There are a number of positive aspects of using an external occulter instead of an internal coronagraph. The attributes are as follows:

Light

1. An external occulter substantially reduces the amount of light entering the telescope. In turn, this reduces the background light scattered into the focal plane, thus enhancing the planet detection limit.

2. Since the occulter is not built into the telescope, scattered light is further reduced without additional optical surfaces and apertures. There are fewer internal obstructions within the telescope that could result in scattered light.

Tolerances

1. When using an external occulter, the polishing tolerances of the primary mirror and supporting optics of the telescope can be less stringent. This provides higher contrast and fainter detection limits than are capable with a coronagraph.

2. More relaxed metrology (measurement) requirements imply a lower required level of technology development. In other words, New Worlds Observer is technologically ready!

Signal

1. Fewer optical surfaces also translate into less signal loss. This is especially important when searching for extremely faint planets.