[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: [APML] OT: M109, NGC 2403 and other collaboration images
Hi Jason,
>You're right, the Airy disc diameter increases as a function of f/#. But that still doesn't explain the larger aperture. A 32" f/2 scope and a 8" f/2 scope would have the same-sized star images assuming seeing is limiting the resolution of the images.
>
Neglecting aberrations and seeing, they have the same sized images in
_linear_ units on the focal plane (e.g 10 micron spots) because of the
same f-ratios. But the plate scale (arcseconds per pixel) of the 32" f2
is 4X greater than that of the 8" f2, so coupled with the fixed spot
size, this means that it has an _angular_ resolution which is 4X better.
This is because the plate scale depends linearly and only on focal
length, if the pixel/grain size is held constant. The focal length of
32" f2 = 64", 4 times longer than the 8" f2 = 16".
>But the larger aperture scope has a longer focal length, so you get a narrower field of view.
>
>
Yes, but as I've noted, what you lose in 4 times narrower FOV is
compensated for by the 4 times better angular resolution. In fact more
than compensated for, because better angular resolution is the Holy
Grail of imaging. It is one of the 3 reasons why the Hubble Space
Telescope was launched at a cost of $billions (the other two reasons
being: lower "sky" background in orbit, and avoiding the UV and IR
absorption bands of the atmosphere).
>I think this is why there are only a few locations on earth where large telescopes are used professionally...there really is no advantage to them (for imaging in particular) if seeing is the limiting factor.
>
Emm, that's not the real reason. But it does bring us to the next reason
why 32" is better than 8": Light collection! 4 times more aperture means
16 times (square of the aperture) more photons, giving you 4 times
(square-root of the photon signal) more signal-to-noise for any given
target.
If the telescope is still diffraction limited (not seeing limited), then
_point sources_ will gain directly from this (they will be 16 times
brighter per pixel). In addition, they gain even further S/N, because
they "sit on" a smaller angular area of sky, so their underlying noise
is actually _decreased_. That's the ultimate win-win: more star photon
signal and less sky photon noise.
With this 4x larger telescope, _extended sources_ will not have any
difference in their brightness-per-pixel (since the f-ratio is still f/2
and that determines the surface brightness), nor in their underlying sky
noise. But the fact that these extended sources cover 16 times more
pixels (remember the plate scale is 4 times finer in each dimension), at
higher angular resolution, is how their improved S/N is manifested...the
same surface brightness, but a larger image at 4x better angular
resolution of detail.
If the larger telescope were seeing limited, then the point sources lose
their brightness-per-pixel and lower sky-noise advantages, and their
situation becomes the same as that of the resolved sources - but that is
to say that they _still_ yield better S/N with larger aperture.
So why do astronomers build giant telescopes? Since they're
seeing-limited from the ground, image quality (angular resolution)
rarely has anything to do with it, other than what can be gained by
careful site selection based on seeing monitors (Mauna Kea, La Palma,
etc.). No, the reason for giant telescopes is simply to catch more
photons. They can get a spectrum of a distant quasar in a one hour
exposure instead of a one-week exposure. But Adaptive Optics schemes can
also turn them into seeing-beaters, in which case they would get the
improved angular resolution _and_ lower sky noise underlying point
sources as well.
Ray "who worked in the team which built the TRIFFID camera used to get
0.17 arcsec images in La Palma" Butler
>-Jason
>
>
>-----Original Message-----
>From: astro-photo-bounces@seds.org
>[mailto:astro-photo-bounces@seds.org]On Behalf Of Greg Hartke
>Sent: Thursday, March 03, 2005 7:56 AM
>To: Discussion of Film Astrophotography
>Subject: Re: [APML] OT: M109, NGC 2403 and other collaboration images
>
>
>Hi, Jason,
>
>It turns out that the diameter of an imaged point target target like a star
>increases with the focal ratio. For that reason (within practical limits),
>you get the smallest stars with the smallest focal ratio. The ideal imaging
>platform is thus a large aperture with a small to medium focal ratio.
>Unfortunately we just can't escape the fact that greater aperture frequently
>makes for better images.
>
>Regards,
>
>Greg Hartke
>Sykesville, MD USA
>
>----- Original Message -----
>From: "Lane, Jason R" <jason.lane@navy.mil>
>To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
>Sent: Thursday, March 03, 2005 10:46 AM
>Subject: RE: [APML] OT: M109, NGC 2403 and other collaboration images
>
>
>Hi Jason,
>
>I'm not clear on your definition of oversampling? Are you talking about
>oversampling seeing due to the pixel count of the FPA or grain of the film,
>or...? A longer focal length gives you a narrower field of view. At 32" and
>assuming f/8, that's a focal length of about 6500 mm. I could get the same
>thing on my LX90 with a 3.5x Barlow. Assuming I had a fancy enough camera
>to give reasonable enough exposure times at f/35 (!) and that seeing was
>such that the wider aperture isn't gaining anything, are you saying the
>images would be comparable?
>
>That's really just an advantage of the focal length, but not directly due to
>the wider aperture. People get different apertures simply because the
>aperture is a by-product of the different FOV's. In visual use, it's the
>other way around. They look for the biggest apertures they can afford. But
>for photography, what you really want is an f/0.5 system. <g> I think
>the basic question is this: Is aperture still just important for visual
>use, and is the speed of the system (a.k.a. f/#) still just important for
>photographic use?
>
>Cheers,
>-Jason
>
>_______________________________________________
>Astro-Photo mailing list
>Astro-Photo@seds.org
>http://seds.org/mailman/listinfo/astro-photo
>_______________________________________________
>Astro-Photo mailing list
>Astro-Photo@seds.org
>http://seds.org/mailman/listinfo/astro-photo
>
>
_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo