Question: Why not use an internal coronagraph?

Flying a separate occulting craft thousands of kilometres away and holding it in the right position for long enough may seem difficult and expensive. However, internal coronagraphs (which have been flown aboard spacecraft before) have their own suite of problems as well.

Below is an attempt to compare the advantages and disadvantages of internal coronagraphs with external occulters. Clear pluses and minuses are simply noted, while more complex assessments are highlighted with a few words.

		Internal ("Classical") Coronagraph			External ("Paleolithic") Occulter
		Pros:	Cons:		Pros:	Cons:
Internally Scattered Light.			Technically challenging for higher star light suppression.		None! No scattered light!
Control of occulting spot position?			Has been an issue in previous coronagraphs.		Control is the name of the game!
Usable with any telescope science instrument?			No.		Yes.
Place target anywhere in image plane?			No.		Yes.
Variable spot size (target optimized)?			No.		Yes.
Optimum PSF redistribution for planet search?		Some designs can optimize.	All used to date are not optimally shaped.		Better than previous classical coronagraphs.
Occulting Spot Shape		Can use arbitrary shape.	Design fixes shape.		Some shape change capability with occulter tilts.	Can't make occulter into some shapes.
Light Suppression factor		Some designs with higher theoretical contrast, but not demonstrated.			Better than previous generation of classical coronagraphs.	Highest suppression requires distant, large, slower occulters.
Exposure length limits.		Detector/background limited.				Limited near higher gravity gradients.
Separate Spacecraft?		Unnecessary.				At least telescope + occulter.
Operations		Well understood & simple.				More complex. Infeasible near Earth.
Cost		Coronagraph within telescope.	Microengineering costly for higher performance		Competitive for smaller occulters.	Usually always more for larger occulters.
Observation rate limits		Just slew the telescope around.				Must move hundreds to thousands of kilometres.
Lifetime		Life potentially unlimited.				Limited by s/c fuel and operations.