Occultation Disc for Imaging

Anyhone know how to make an occultation disc to block out the light of a bright object from swamping out an image to allow the detection of fainter objects?

Michael.

I think a Lyot-type coronagraph might be what you are describing.
I was pondering how this was done only a couple of weeks ago.

www.lyot.org/background/coronagraphy.html

The occulting disc is introduced at the focal plane then you have a lens arrangement to image the focal plane. The Lyot stop, apparently, cleans up some stray light from the occulted bright source but at the expense of reducing light from the faint source.

Not sure how critical a Lyot stop is for solar work (if that's what you are pursuing).

You'd have to have a good idea of the size on the focal plane of the bright object to make an occulting disc to match. On the whole it would not appear to be a trivial task.

On my quick trawl of the net there I saw one system (not for solar work) using a mercury blob that could be compressed to create differing size occulting discs.

journals.cambridge.org/download.php?file...0921968140bc4db80943

Good luck - whatever your plans.

Mark C.

Thanks Mark.

I was thinking more of imaging moons around planets (eg Mars).

That last link doesn't seem to work for me.

Last link isn't working for me either now - however the following is similar (also using a compressed blob of mercury as the occultation disc) - note that it was used to image moons of Neptune - image presented in the paper.

I haven't studied the time line to know who did what first but I believe that Bourget is an important player in this technique.

www.astrosurf.com/rondi/t60/corohg/bourget2001.pdf

Sorry about the earlier defective link.

Mark C.

Thanks Mark.
That's an interesting read.

Here's an article S&T did some time ago re visual observations of Mars' moons:
www.skyandtelescope.com/observing/object...n_2007_and_2008.html
I must see if this would work for imaging.

Thanks,
Michael.

Very good - it immediately rang a bell and the same technique is described in "Astronomy Hacks" O'Reilly (Robert Bruce Thompson & & Barbara Fritchman Thompson) pp 299-304. They seal off half of the field of view with tape and used a 9mm Plossl at 280X magnification with a 2x Barlow in a 10" f/5 Dobsonian (as stated) - claiming success with Deimos - but not Phobos at 2001 Mars close approach.

I would have thought that the tape would have to be very close to the focal plane for this to work - maybe it is.

Best of luck!

Mark C.