Hiya,

Here is clop's (probably stupid) idea of the day, following another thread where someone was asking how we measure the diameter of stars.

I was playing with my laser pointer, like you do, and I shone it at a very shallow angle along my mantelpiece. Instead of a spot reflection there was a line reflection, several inches long.

I realised that I could

1) measure the length of the line reflection and the shallow angle and so work out the true width of the laser beam
2) measure the brightness of the beam along the line reflection and so work out the cross-sectional intensity distribution of the beam

Would it be possible to equip an orbiting satellite telescope (like Hubble) with a sensor tilted away from the line of sight, and then use it to directly measure the diameter of a star and its luminous cross-section? It seems like this would increase the resolution available. Maybe it would make planet-seeking easier?

clop

The light in a telescope is focussed at the sensors, and the sensors are arrayed in a focal plane more or less. They're also more efficient at steeper angles.

But I'm going back to bed, and think about this some more. I like it.

More efficient yes, but more useful?

Maybe we could design a lens or mirror which develops a non-orthongonal focal plane, so it focuses properly along the tilt.

clop

There are other techniques that are easier than making special optics that focus on a non-orthogonal plane:
- You could make your sensor so the cells are already so small they are getting diffraction limited resolution from the telescope
- You could put the sensor on a bed that moves half a pixel width in either directions, so that images take four times as long, but they have almost twice the dimensional resolution.

__________________
Forming opinions as we speak

In astronomy, one of the central problems is concentrating enough light in one spot so that an image is even detectable--in your system, you are spreading that light out.

But the technique can be useful--it's the same principle as the micrometer.

I thought you were going to bed

Yes but a nice bright star, -8 or brighter, we could smear and still detect easily.

clop

8 or brighter?

Still, the optics would have to be different enough that they probably induce losses of their own. What do you have in mind?

As you know, most stars do not have resolvable disks--which means there is nothing to spread out. I know that's what you are trying to do, but that "resolvable" idea is a limiting factor.

Ha ha, you got me, +8 or brighter!

Say we smeared the light from Arcturus across a sensor inclined at 89.5 degrees from orthogonal. Would the image smear out? If it does we can use this technique to measure Arcturus' diameter. If it doesn't then we incline the sensor some more, or maybe Arcturus is too pinpoint to work. But you know what I mean. If I shine my laser pointer straight at the wall it is very hard to measure the diameter of the beam. But if I shine it along a surface I can measure the diameter very accurately and very easily.

clop

This idea won't work for stars. Think for a moment about the effect of optics on starlight: they focus the light from a star at a point. If you tilt the sensor, you will change the distance of each part of the detector from the primary mirror, which means that the starlight will no longer be focused. Tilting the detector may tell you something about the properties of your optics, but not about the star.

There are many other ways to determine the apparent angular diameters of stars. One of my favorites _can_ be done with typical backyard telescopes (though only the proper uncommon circumstances): if you measure the light from a star very carefully and very rapidly -- at video rates (30 frames per second) or faster -- as an asteroid or the Moon passes in front of it, you can sometimes see diffraction effects which provide information on the star's angular size. This stellar occultation technique is one area in which amateurs are more active than professionals. It usually tells us more about the size of the asteroid than the star, but every now and then, it does provide valuable information on stellar sizes.

If you are interested in learning more on this technique, consider reading the archives of
the Yahoo IOTAoccultations site:

http://tech.groups.yahoo.com/group/IOTAoccultations/