Central obstruction of HST

Hello:
There's a lot of back and forth about how large of a central obstruction is too much, how much it decreases contrast, etc. My comment here is unlikely to change much, but I hope to provide a little perspective. I think we can all agree that the Hubble Space Telescope is a good telescope and provides high contrast images. It has a fairly small secondary mirror at around 12", but it's central obstruction is about 33% (around 31") from what I've been able to gather from Faint Object Camera and NICMOS documentation available on line. Since HST is in space, having a smaller central obstruction would actually help it, unlike most ground based telescopes that are seeing limited. I can only assume that there is a need for such a large secondary mirror baffle due to the bright environment that HST operates in. Now I don't feel so bad that the cassegrain I put together has a 32% obstruction. If a 33% central obstruction is good enough for HST, it's good enough for me!

I suppose my point is that there are aspects of making a telescope more important than getting the central obstruction as small as possible. Beyond a certain point, shrinking the secondary doesn't gain much in light of seeing effects while it limits the illumination of the field of view (and in cassegrains, a small secondary makes the baffleing more difficult). Of course, this doesn't mean that I'll run out and get a 6" secondary for the 17.5" dob I'm working on, although having a 3.5" fully illuminated field could be interesting.

Cheers
Mike Connelley

Interesting post! But there is a difference
between imaging and visual observing that is being ignored
here. An imaging telescope can recover to some extent
from low contrast by exposing longer (to reduce noise)
and then enhancing the image in post-processing. This
is what is routinely done on HST. Its also what amateurs
do, for example, when imaging planets with webcams.

With visual observing, what you see at the eyepiece
is what you get. If low contrast prevents you from
seeing some planetary feature, there is no recovering
from it. You can't see it, and thats that. You can't
expose 50% longer (or whatever number) to fix the low
contrast.

I agree that in poor seeing, the seeing will limit
the contrast, and not the secondary size. But on those
rare nights of excellent seeing, the secondary will
indeed limit what you can see.

The baffles on HST are relatively large. This
is because it operates in broad daylight while attempting
to detect some of the faintest objects in the universe.
Good baffling was incredibly important. HST would be
an even better telescope if you could somehow have a
smaller obstruction and avoid stray light -- there is
some range of very faint objects and low contrast
features that cannot be detected in a reasonable
exposure time, due to the large baffles. But the
designers did the best they could with the constraints
they had to work with (fit in Shuttle, operate in
full sun, etc.). Its a good telescope, but it is still
limited by the large obstruction.

>>>I think we can all agree that the Hubble Space Telescope is a good telescope and provides high contrast images. It has a fairly small secondary mirror at around 12", but it's central obstruction is about 33% (around 31") from what I've been able to gather from Faint Object Camera and NICMOS documentation available on line.
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My understanding is that the central obstruction issue is mostly related to visual astronomy where image processing/data reduction techniques can't be used.

It is also my understanding that one of the advantages of the Ritchey-Chretien is that while the images may not be visually pleasing, they are free from coma and symetric so that they can be analyzed more accurately, an advantage in imaging when the spot size maybe larger due to film or digital limitations.

From what I understand, most Ritchey-Chretien's have central obstructions up around 50%.

Jon