Viewing by eye versus astrophotography
Peter Webb
When it comes to "seeing" detail, what improvement can you get by
photography over the human eye? The human eye is presumably better at
optical separation, but CCDs for minimum magnitude.
For example, I have heard that you can start seeing the spiral structure of
galaxies at about a 8 - 10" telescope aperture; if you are doing
astrophotography can this be seen with a lower aperture scope?
Chris L Peterson
When it comes to deep sky astronomical objects, the eye is vastly
inferior to modern electronic imagers, both in terms of resolution and
sensitivity. The main factor is sensitivity. While electronic detectors
have better QE than the eye (that is, they record a higher percentage of
photons), the main improvement comes from their ability to integrate
over long periods. The eye only "sees" a photon for about 100ms, so
outside that period there's no additive effect. A CCD or similar sensor
can accumulate photons for any length of time. This makes them, in
effect, many orders of magnitude more sensitive that the eye.
Also, the resolution of the eye is poor for dim objects. In fact, it is
very poor- even bad telescope optics are unlikely to impact the quality
of DSO viewing. In contrast, an electronic detector can be set up with
an objective of the proper focal length in order to achieve resolution
limited by the optics or the seeing, whichever is finer. This is only
possible visually with very bright objects (the Sun, Moon, and bright
planets), where high magnification can be used without losing too much
light.
Electronic detectors also achieve higher resolution than the eye or film
because of their independent pixels. This results in a nearly flat MTF.
While the eye and film have a resolution that varies with contrast,
electronic detectors do not, until the Nyquist sampling limit is
approached.
Electronic detectors have much greater dynamic range than either film or
the scotopic eye.
Of course, electronic detectors are capable of recording color in DSOs,
which is largely impossible visually.
One place where there can be an advantage to visual observation is with
the Moon and planets. That's because the short sampling time of the
eye/brain - a disadvantage for DSOs- works in our favor to help freeze
seeing effects. However, this advantage is increasingly overcome by the
use of lucky imaging techniques- stacking fast video frames based on
individual frame quality. This can produce equally good resolution to
the eye, with a substantial improvement in color and dynamic range.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
David Nakamoto
On this last point, about planets and the Moon, I'll concede that with
the Moon you'll see more with the unaided eye through most telescopes
than with imaging, but my own experience is that you see more with
stacking images from a video taken with good optics and good seeing than
with the eye through the same telescope, perhaps twice the resolution,
although I admit I've not measured it objectively yet. Basically, the
web or video camera with medium to high resolution and selectively
stacking images seem to push the resolution of what you can see closer
to the theoretical limits of the optics. It at least pushes them a bit
further than what the seeing might allow through the telescope with the
unaided eye.
Just my humble opinion.
--- Dave
Chris L Peterson
<david.n...@verizon.net> wrote:
>On this last point, about planets and the Moon, I'll concede that with
>the Moon you'll see more with the unaided eye through most telescopes
>than with imaging, but my own experience is that you see more with
>stacking images from a video taken with good optics and good seeing than
>with the eye through the same telescope, perhaps twice the resolution,
>although I admit I've not measured it objectively yet.
I agree. There might be some advantage to visual planetary astronomy
with poor seeing, or certain kinds of seeing. But even that is giving
way to video imaging, I think.
My point, really, was that planetary and lunar viewing is probably the
only case where visual observation and imaging are at least in the same
ballpark so far as comparisons go. With everything else, imaging is
unarguably far beyond visual, even where the imaging aperture is much
smaller than the visual.
David Nakamoto
some stars visible. 30 seconds later with a monochromatic imager, it
"resolved" into hundreds of stars. First light for that camera too.
--- Dave
William Hamblen
<webbf...@DIESPAMDIEoptusnet.com.au> wrote:
>For example, I have heard that you can start seeing the spiral structure of
>galaxies at about a 8 - 10" telescope aperture; if you are doing
>astrophotography can this be seen with a lower aperture scope?
You can photograph this with a telephoto lens.
Bud
Chris L Peterson
I imaged M51 once using a pinhole! Not beautiful, but somewhat more
structure, and far more contrast, than I've ever seen visually through a
telescope.
Paul Schlyter
Peter Webb <webbf...@DIESPAMDIEoptusnet.com.au> wrote:
>I know this question is almost meaningless ... but perhaps not completely.
>
>When it comes to "seeing" detail, what improvement can you get by
>photography over the human eye? The human eye is presumably better at
>optical separation, but CCDs for minimum magnitude.
The major advantage with photography is reproducibility. In a few
seconds (or a few hours, depending on what exposure time you use)
you get an image which later can be closely examined any number of
times by anyone. Visual observations o.t.o.h. can only be performed
"here-and-now" by the observer himself, one cannot later re-create the
same visual observation. True, you can draw, or describe, what you
see - but that's always a matter of interpretation which never can be
repeated later by someone else on the same visual observation.
Perhaps the best way to describe it is that visual observations are
"volatile".
The major advantage of visual observations is pleasure: it's always
more pleasant to see something live than to see a photograph or video
of it.
And that's why visual observations are so popular among amateur astronomers
(who observe for the joy of it) while at the same time it's virtually
never used by professionals (who are demanded to produce reproducible
scientific results).
>For example, I have heard that you can start seeing the spiral structure of
>galaxies at about a 8 - 10" telescope aperture; if you are doing
>astrophotography can this be seen with a lower aperture scope?
>
>
--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stjarnhimlen dot se
WWW: http://stjarnhimlen.se/
Chris L Peterson
>The major advantage with photography is reproducibility.
I would call that _a_ major advantage, not _the_ major advantage.
Advantages depend on intent, and reproducibility may not necessarily be
a goal.
>The major advantage of visual observations is pleasure: it's always
>more pleasant to see something live than to see a photograph or video
>of it.
Again, this depends on intent and personal taste. I've never seen any
object directly through a telescope (with the possible exception of
Saturn) that gave me anywhere near the satisfaction of seeing an image
appear on my screen, as the result of my own imaging effort. Without
imaging, I might not bother to own a telescope at all.
Thomas Womack
Chris L Peterson <c...@alumni.caltech.edu> wrote:
>On Sun, 05 Oct 2008 15:42:46 GMT, pau...@saaf.se (Paul Schlyter) wrote:
>
>>The major advantage with photography is reproducibility.
>
>I would call that _a_ major advantage, not _the_ major advantage.
>Advantages depend on intent, and reproducibility may not necessarily be
>a goal.
>
>>The major advantage of visual observations is pleasure: it's always
>>more pleasant to see something live than to see a photograph or video
>>of it.
>
>Again, this depends on intent and personal taste. I've never seen any
>object directly through a telescope (with the possible exception of
>Saturn) that gave me anywhere near the satisfaction of seeing an image
>appear on my screen, as the result of my own imaging effort. Without
>imaging, I might not bother to own a telescope at all.
I agree that, for planets, galaxies and nebulae, imaging is
unbeatable.
I've not seen images that capture the contrasty nature of a star
cluster seen through a large scope, basically because no screen
technology that I know of can do the transition between space-black
and star-white in the space of no more than the eye can see. Some
planetarium shows come close.
I have seen Omega Centauri through a 36" Dobsonian (yes, I know this
is something of a best case for visual observation), and that is much
more spectacular than any picture of a globular that I've encountered,
even from Hubble or VLT-adaptive-optics.
There's something of the same for the Moon, the blackness of the
shadows of the mountains versus the light bright enough to make you
screw up your eyes a bit (even in my pathetic 4") reflected from their
peaks.
On the other hand, I've also felt a reasonable sense of achievement
from taking some technically awful wide-field photos with a 70/2.8
lens and a DSLR and using them to rediscover Ceres. Not sure it's a
delight I can convey it words, but it did feel good to see that moving
dot ...
Tom
Paul Schlyter
>
>>The major advantage with photography is reproducibility.
>
>I would call that _a_ major advantage, not _the_ major advantage.
>Advantages depend on intent, and reproducibility may not necessarily be
>a goal.
Even if reproducibility is not a goal, it's hardly a disadvantage.
Unexpected things may always happen. And if you observed something
unusual, your observation will gain more credibility if you managed to
photograph it than if you merely saw it visually.
But of course, credibility may not be a goal.... :-)
>>The major advantage of visual observations is pleasure: it's always
>>more pleasant to see something live than to see a photograph or video
>>of it.
>
>Again, this depends on intent and personal taste. I've never seen any
>object directly through a telescope (with the possible exception of
>Saturn) that gave me anywhere near the satisfaction of seeing an image
>appear on my screen, as the result of my own imaging effort. Without
>imaging, I might not bother to own a telescope at all.
You're right! Your mileage may indeed vary. And an image (particularly
a live image) of sufficiently high quality may be indistinguishable from
a direct visual view.
>_________________________________________________
>
>Chris L Peterson
>Cloudbait Observatory
>http://www.cloudbait.com
Chris L Peterson
>Even if reproducibility is not a goal, it's hardly a disadvantage.
Of course not. I don't think I said anything to suggest that. I said
reproducibility is a major _advantage_.
>You're right! Your mileage may indeed vary. And an image (particularly
>a live image) of sufficiently high quality may be indistinguishable from
>a direct visual view.
Even a live image (which is typically the lowest quality image there is)
is very distinguishable from a direct visual view by virtue of being
vastly superior!
Peter Webb
"Chris L Peterson" <c...@alumni.caltech.edu> wrote in message
news:vkkie41h37asjsfe2...@4ax.com...
As you are still here, I might ask some even-more-stupid questions.
By way of background, I know a fair bit about theoretical astronomy, physics
(optics), and computers. I like gadgets and "systems engineering". I built a
6" reflector telescope when I was a teenager (ie ground the mirror by hand),
but have had no practical exposure to astronomy for (gulp) almost 40 years.
This suggests that if I wish to do some "observing", astrophotography would
better suit my skills and interests.
Now that telescopes are cheaper and I am richer, I want to buy a telescope
and get some practical experience.
On an about $2,000 budget, I can buy for example:
* A 12" - 14" Newtonian Dobsonian with a good collection of eye pieces, OR
* A 6" - 8" Newtonian on an EQ mount with motor drive and cheap DSLR.
From the remarks so far, I gather that I will be able to "see" more with a
6" and camera than with a 12" optically? (I am not particularly interested
in planetary observation).
Also, it has occurred to me that if I am doing prime focus imaging, then my
magnification is presumably determined by the focal length of the
telescope - I can't fudge this with eyepieces. I am aware of the impact of
f-stop on exposure times.
With a cheap Nikon DSLR, what is the approximate relationship between focal
length and magnification for a given focal length? For example, based upon a
60" focal length, my field of view on the the DSLR will correspond to what
approximate visual magnification?
Is it better to go for a long focal length reflector to get more
magnification, or a short focal length to reduce both exposure time and
magnification ?
Sorry about all the questions - but buying a 12" Dob for visual work or a 6"
EQ for astrophotography puts me on two very different courses, and I need to
decide which way I will be going.
Thanks
Peter Webb
Brian Tung
> The eye only "sees" a photon for about 100ms, so
> outside that period there's no additive effect.
I just had a crazy thought. (Moi?) Some drugs appear to have some
kind of "lag" effect, so that lights create longer trails (when the
head moves) in the visual field than normal. Is that all done in
the brain's "post-processing," so that no actual additional information
is included in those trails? Or is this a way to get the effects of
longer integration time, in the brain rather than the eye? (Unless
there's some retinal effect I don't know about.)
> Also, the resolution of the eye is poor for dim objects. In fact, it is
> very poor- even bad telescope optics are unlikely to impact the quality
> of DSO viewing.
Unless the DSOs are viewed at very high power. Optical quality shows
up at small true angular sizes; visual acuity shows up at small apparent
angular sizes. Since there is occasionally reason to observe DSOs at
large magnifications (perhaps those with structure at multiple levels of
granularity), this might rear its ugly head.
Your main point stands, of course--there's no contest, unless the target
is changing so fast that exposure imaging isn't possible.
--
Brian Tung <br...@aero.org>
NOTE: Below addresses changing soon...
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
My posts do not represent the views of either Aerospace of USC/ISI.
Brian Tung
> On an about $2,000 budget, I can buy for example:
>
> * A 12" - 14" Newtonian Dobsonian with a good collection of eye pieces, OR
> * A 6" - 8" Newtonian on an EQ mount with motor drive and cheap DSLR.
>
> From the remarks so far, I gather that I will be able to "see" more with a
> 6" and camera than with a 12" optically? (I am not particularly interested
> in planetary observation).
Yes, that's probably true. (Even with the planets, unless you live
somewhere with phenomenal seeing.)
> Also, it has occurred to me that if I am doing prime focus imaging, then my
> magnification is presumably determined by the focal length of the
> telescope - I can't fudge this with eyepieces. I am aware of the impact of
> f-stop on exposure times.
You can do eyepiece projection, though, to increase the effective focal
length of your telescope.
I'll let Chris Peterson chime in on the effect of focal ratio on
exposure times. I'll just say that there are some widely held myths on
this.
> With a cheap Nikon DSLR, what is the approximate relationship between focal
> length and magnification for a given focal length? For example, based upon a
> 60" focal length, my field of view on the the DSLR will correspond to what
> approximate visual magnification?
It doesn't work like that. For imaging, the parameter of interest is
image scale, measured in something like degrees per mm. The reason is
that the size of objects in the sky are angular, measured in degrees,
while the size of images on film or CCD are linear, measured in mm or
inches. The ratio of these two is the image scale.
On the other hand, when you use a telescope visually, the virtual image
you see has no fixed linear size. Instead, it also has an angular size,
which is generally much larger than the angular size of the original
object. The ratio between these two is the magnification.
Since one parameter has units of degrees per mm, and the other is
dimensionless, there's no meaningful formula to "translate" one to the
other.
> Is it better to go for a long focal length reflector to get more
> magnification, or a short focal length to reduce both exposure time and
> magnification ?
Depends on your target. (You probably guessed that.) For objects that
are small (like the planets), long focal length is useful, to increase
the image scale. Detail is king, you're trying to make sure that there's
a good match between the detail your scope can extract and the pixel
scale.
On the other hand, with large objects, you want to frame the entire
object, so wide field and short focal length is useful. Granted, you
could mosaic the whole thing, but unless you are extraordinarily
patient, you probably don't want to capture M31 by taking a mosaic of
74 images.
Chris L Peterson
wrote:
>I just had a crazy thought. (Moi?) Some drugs appear to have some
>kind of "lag" effect, so that lights create longer trails (when the
>head moves) in the visual field than normal. Is that all done in
>the brain's "post-processing," so that no actual additional information
>is included in those trails? Or is this a way to get the effects of
>longer integration time, in the brain rather than the eye?
That is crazy (in an interesting way <g>). The integration time of the
eye is a function of simple chemistry, involving the relaxation time of
the cis- to trans- species of opsins found in the rods and cones. I
doubt this is affected by many (if any) drugs. What you're describing is
probably a perceptual effect, at higher parts of the brain (above the
visual cortex). But it's interesting to consider whether this could
effectively increase sensitivity to dim objects.
> Also, the resolution of the eye is poor for dim objects. In fact, it is
>> very poor- even bad telescope optics are unlikely to impact the quality
>> of DSO viewing.
>
>Unless the DSOs are viewed at very high power.
Even so, I doubt optical quality is likely to matter (or seeing
conditions). The scotopic eye has a visual acuity of 20/200 or worse.
That means that resolution (to the extent it can be defined with vision)
goes from something like one or two arcminutes (for photopic vision) to
a half degree. And this is combined with the inherently poor spatial
resolution encountered for low contrast transitions in particular. I
think the optics would have to be pretty poor, even at high
magnification, to have much effect on the appearance of dim extended
objects. Of course, bad optics would be very obvious on any stars in the
field, and that's often an important consideration.
Chris L Peterson
<webbf...@DIESPAMDIEoptusnet.com.au> wrote:
>This suggests that if I wish to do some "observing", astrophotography would
>better suit my skills and interests.
It might. Imaging does hold a special fascination, I think, for
astronomers with an engineering bias.
>On an about $2,000 budget, I can buy for example:
>
>* A 12" - 14" Newtonian Dobsonian with a good collection of eye pieces, OR
>* A 6" - 8" Newtonian on an EQ mount with motor drive and cheap DSLR.
>
>From the remarks so far, I gather that I will be able to "see" more with a
>6" and camera than with a 12" optically? (I am not particularly interested
>in planetary observation).
I think that is certain, assuming that your main interest is nebulous
deep sky objects (as has been pointed out, star clusters are a DSO
exception in that many prefer their visual appearance).
A consideration when imaging is that while you can stack images to
achieve a high effective exposure time, there is a significant advantage
in terms of reducing noise to keep your subexposures as long as
possible. Under most people's sky conditions, that means at least a few
minutes. That's long enough that most mounts can't track accurately
without active guiding. So you should consider adding a small piggyback
scope and a DSI, webcam, or other inexpensive camera for guiding. This
will add a few hundred dollars to your project.
>Also, it has occurred to me that if I am doing prime focus imaging, then my
>magnification is presumably determined by the focal length of the
>telescope...
That's correct. You can calculate your image scale as
A = 206265 * d / F
where A is your scale in arcseconds per pixel, d is the size of your
pixels (for most DSLRs that will be around 6 um, or 0.006 mm), and F is
the focal length of your scope, in the same units as d.
Obviously, a shorter focal length will mean a lower resolution but a
wider field of view. You need to determine the balance between the two
that best suits your imaging goals. For high resolution, with the sort
of equipment you are considering, I'd aim for an image scale of 1-2
arcseconds per pixel. But you can go much larger in exchange for FOV and
still get excellent results.
>I am aware of the impact of
>f-stop on exposure times.
It doesn't much matter for astronomical imaging. If your exposures are
reasonably long, the principal noise source is the statistical noise on
the signal itself. That is reduced by collecting more photons- which
means more time or more total aperture. In most cases, your best
strategy is to optimize the focal length for your desired resolution,
and then simply buy as much aperture as you can afford or you can
handle.
>With a cheap Nikon DSLR, what is the approximate relationship between focal
>length and magnification for a given focal length? For example, based upon a
>60" focal length, my field of view on the the DSLR will correspond to what
>approximate visual magnification?
There's no comparison possible between visual magnification and imaging
scale. In this case, your scale is
206265 * 0.006 mm / 1524 mm = 0.8"/pixel. That's probably oversampled,
so you might want to consider either a shorter focal length scope, or a
0.5x focal reducer. Your FOV will be about 1/4 degree at the 1524 mm
focal length.
If you have a choice, I'd recommend you use a Canon DSLR, not a Nikon.
Nikons have a serious flaw for astronomical imaging, in that they
internally truncate the histogram (they set an arbitrary black point
which is above the minimum intrinsic brightness), and this makes dark
subtraction, which is the primary image calibration tool, work rather
poorly. There are some cumbersome workarounds, but unless you have some
reason to go with the Nikon (like already owning lenses), I'd stay away.
>Is it better to go for a long focal length reflector to get more
>magnification, or a short focal length to reduce both exposure time and
>magnification ?
The answer depends on what you want to image. Keep in mind that your
mount's tracking ability, and atmospheric seeing, means that any
resolution gained beyond 1-2 arcseconds per pixel is likely to be empty.
Peter Webb
Guth would answer.
I'm sorry about the continued questions; whilst there is plenty of web
information on selecting and buying a telescope, I haven't found a site for
low-end astrophotography, and before spending the dough I want to know what
I should buy and why.
I have dropped a couple of questions in ... :
"Chris L Peterson" <c...@alumni.caltech.edu> wrote in message
news:m74je45idtl2ephbi...@4ax.com...
> On Mon, 6 Oct 2008 12:05:52 +1100, "Peter Webb"
> <webbf...@DIESPAMDIEoptusnet.com.au> wrote:
>
>>This suggests that if I wish to do some "observing", astrophotography
>>would
>>better suit my skills and interests.
>
> It might. Imaging does hold a special fascination, I think, for
> astronomers with an engineering bias.
>
>
>>On an about $2,000 budget, I can buy for example:
>>
>>* A 12" - 14" Newtonian Dobsonian with a good collection of eye pieces, OR
>>* A 6" - 8" Newtonian on an EQ mount with motor drive and cheap DSLR.
>>
>>From the remarks so far, I gather that I will be able to "see" more with a
>>6" and camera than with a 12" optically? (I am not particularly interested
>>in planetary observation).
>
> I think that is certain, assuming that your main interest is nebulous
> deep sky objects (as has been pointed out, star clusters are a DSO
> exception in that many prefer their visual appearance).
>
> A consideration when imaging is that while you can stack images to
> achieve a high effective exposure time, there is a significant advantage
> in terms of reducing noise to keep your subexposures as long as
> possible. Under most people's sky conditions, that means at least a few
> minutes. That's long enough that most mounts can't track accurately
> without active guiding. So you should consider adding a small piggyback
> scope and a DSI, webcam, or other inexpensive camera for guiding. This
> will add a few hundred dollars to your project.
>
Active guiding? Is that where the scope tracks on both axis, in case you
haven't pointed the scope correctly at the Pole ? How does a piggyback scope
or webcam help with the tracking? So you can make manual adjustments if its
not tracking properly? It seems to me that if the large scope isn't tracking
properly, then by the time you could see the error on a smaller scope it
would be too late ? ...
So, somewhere under 48" (f8) would be better. Also, might actually fit in my
car.
> If you have a choice, I'd recommend you use a Canon DSLR, not a Nikon.
> Nikons have a serious flaw for astronomical imaging, in that they
> internally truncate the histogram (they set an arbitrary black point
> which is above the minimum intrinsic brightness), and this makes dark
> subtraction, which is the primary image calibration tool, work rather
> poorly. There are some cumbersome workarounds, but unless you have some
> reason to go with the Nikon (like already owning lenses), I'd stay away.
>
>
No reason to buy Nikon, and it will be dedicated to astrophotography, so
thanks for the advice.
>>Is it better to go for a long focal length reflector to get more
>>magnification, or a short focal length to reduce both exposure time and
>>magnification ?
>
> The answer depends on what you want to image. Keep in mind that your
> mount's tracking ability, and atmospheric seeing, means that any
> resolution gained beyond 1-2 arcseconds per pixel is likely to be empty.
> _________________________________________________
>
> Chris L Peterson
> Cloudbait Observatory
> http://www.cloudbait.com
Again, thankyou both.
Paul Schlyter
You can fudge it with a Barlow lens though. With a Cassegrain telescope, the
secondary mirror works much like a Barlow lens.
>I am aware of the impact of f-stop on exposure times.
>
>With a cheap Nikon DSLR, what is the approximate relationship between focal
>length and magnification for a given focal length? For example, based upon a
>60" focal length, my field of view on the the DSLR will correspond to what
>approximate visual magnification?
First, you have no "magnification" in astrophotography. What you do have
is image scale, which can be expressed as e.g. arc seconds per pixel or,
on a printout, arc seconds per millimeter.
If you still want to express a "magnification" in astrophotography, you
must first decide on a focal length, f1, which you think corresponds to
a magnification of unity. Once you've decided on that, the "photographic
magnification" of your scope becomes:
F / f1
where F is the focal length of your scope. The tricky point is to
decide the value of f1, the focal length corresponding to unity
magnification. A reasonable choice for f1 is the focal length of
your "normal" (i.e. neither telephoto nor wide angle) lens to your
camera. Unfortunately, most DSLR cameras today come with a zoom lens
as the normal lens - and precisely where within the zoom range do you
place your unity magnification?
Also, the "magnification" of your scope will depend on which camera
you put on the scope - even if the camera has its lens removed. How
come? Well, if you put a full frame 24x36 mm camera, where the normal
lens has an fl of 50 mm (or perhaps 45 mm), your 60" f.l. scope will
produce a "magnification" of 60"/50mm = 30.48 so let's say x30 magnification.
But if your camera used a 45 mm normal lens, the "magnification" would
be 60"/45mm = 33.87 so let's say x34 magnification.
Most cheap DSLR cameras aren't full-frame 24x36 mm though, but rather
APS-C frame of 18x24 mm, and the "normal lens" won't have 45-50 mm f.l.
but rather 30-33 mm f.l. This will "increase the magnification" by a factor
of some 1.5 times, i.e. from x30-x34 to x45-x50 approximately.
Even though one can define "photographic magnification", it's still a
little fuzzy concept. The reason for this fuzzines is the absence of a
"standard camera" which defines unity magnification. In visual
observation we do have such a "standard camera": the human eye
>Is it better to go for a long focal length reflector to get more
>magnification, or a short focal length to reduce both exposure time and
>magnification ?
>
>Sorry about all the questions - but buying a 12" Dob for visual work or a 6"
>EQ for astrophotography puts me on two very different courses, and I need to
>decide which way I will be going.
>
>Thanks
>
>
>
>Peter Webb
>
Paul Schlyter
Chris L Peterson <c...@alumni.caltech.edu> wrote:
>On Mon, 6 Oct 2008 12:05:52 +1100, "Peter Webb"
><webbf...@DIESPAMDIEoptusnet.com.au> wrote:
>>With a cheap Nikon DSLR, what is the approximate relationship between focal
>>length and magnification for a given focal length? For example, based upon a
>>60" focal length, my field of view on the the DSLR will correspond to what
>>approximate visual magnification?
>
>There's no comparison possible between visual magnification and imaging
>scale.
It's quite possible to figure out a "magnification" corresponding to
an image scale. However, it is unusual to do so, and the definition
of this "magnification" remains fuzzy. What you need to do is to define
some "standard camera" which corresponds to unity magnification. In the
case of visual observations we do have such a "standard camera": the
human eye. So the visual magnification is easily defined as the image
scale on our retina through the telescope divided by the image scale on
our retina of a naked eye view.
Similarily a "photographic magnification" can be defined as the image
scale through the telescope, divided by the image scale through some
lens we've decided should define unity magnification (such as a 50 mm
lens on a traditional SLR camera, or a 35 mm lens on a modern cheap
DSLR using the APS-C image format).
Paul Schlyter
>It doesn't work like that. For imaging, the parameter of interest is
>image scale, measured in something like degrees per mm.
On electronic cameras it's more useful to relate the angular unit to
the number of pixels rather than to the linear size of the image detector.
Peter Webb
>
> Even though one can define "photographic magnification", it's still a
> little fuzzy concept. The reason for this fuzzines is the absence of a
> "standard camera" which defines unity magnification. In visual
> observation we do have such a "standard camera": the human eye
>
>
I think its a well defined concept, because I can define it quite simply and
reasonably unambiguously.
Consider a digital image. Imagine two stars on opposite sides of the frame.
Roughly, what eyepiece - what magnification - would I need to have a visual
field of view framed by these two stars? Now I know this has a little
"looseness" in it, because FoV varies between eyepieces of the same focal
length, but it should give an idea.
As I understand the argument regarding the chip size, the image generated by
my scope may be larger than 18 mm x 24 mm and hence this light is thrown
away - the camera has "zoomed" on the image. As the pixel density is greater
on smaller chips, this is akin to an optical zoom in that the full
resolution is available over a smaller part of the image. (Unless its
resolution limited, which it won't be unless its slower than f8 roughly).
This is all good.
Thanks ...
Pushing my luck very much,
Davoud
> > The eye only "sees" a photon for about 100ms, so
> > outside that period there's no additive effect.
Brian Tung wrote:
> I just had a crazy thought. (Moi?) Some drugs appear to have some
> kind of "lag" effect, so that lights create longer trails (when the
> head moves) in the visual field than normal. Is that all done in
> the brain's "post-processing," so that no actual additional information
> is included in those trails? Or is this a way to get the effects of
> longer integration time, in the brain rather than the eye? (Unless
> there's some retinal effect I don't know about.)
You would, of course, need to polar-align your head. Makes _my_ neck
hurt just to think about it.
Then, too, it's not hard to image the highly imaginative TV spots that
our drug-frenzied government would put out:
"This is M31 @.
"This is M31 on drugs # ."
Now, down to business. What drugs create the effect of light trails and
where can I get some for astrophysical research purposes?
Davoud
--
Don't re-elect the failures of the past eight years.
usenet *at* davidillig dawt com
Chris L Peterson
<webbf...@DIESPAMDIEoptusnet.com.au> wrote:
>Active guiding? Is that where the scope tracks on both axis, in case you
>haven't pointed the scope correctly at the Pole ? How does a piggyback scope
>or webcam help with the tracking? So you can make manual adjustments if its
>not tracking properly? It seems to me that if the large scope isn't tracking
>properly, then by the time you could see the error on a smaller scope it
>would be too late ? ...
When you autoguide (which is the only guiding method you want to
consider), a separate camera images a star near your target and uses it
as a position reference. A guiding program can calculate the position of
the star to a small fraction of a pixel width, so an error in position
can be detected and corrected before the object at the focal plane of
the imaging camera has moved significantly. It is common for the
guidescope to be light weight and short focal length, so as to provide a
wide FOV and to not add a lot of weight to the mount. It can be tricky
to make the system mechanically stable enough that the two scopes keep
the same pointing (with arcsecond accuracy) for a long time, but with a
DSLR your exposures aren't likely to be longer than five minutes or so,
and guided tracking for this long shouldn't be difficult.
Obviously, you'll want a mount that has both axes motorized, and has
inputs for a guider.
Chris L Peterson
<webbf...@DIESPAMDIEoptusnet.com.au> wrote:
>I think its a well defined concept, because I can define it quite simply and
>reasonably unambiguously.
Just keep in mind that any definition you use is your own. The idea of
magnification simply isn't used by imagers because it isn't useful. The
definition you've adopted is simply FOV (which as you note varies widely
between different eyepiece designs- a high magnification EP can easily
have a much greater FOV than a low magnification one). And the apparent
magnification of an image depends on how far away you view it.
It sometimes takes visual observers a while to get away from the concept
of magnification when they image, but they always get there <g>. All
that really matters is image scale (resolution) and field of view.
>(Unless its
>resolution limited, which it won't be unless its slower than f8 roughly).
Be careful with your terminology. My advice would be to forget that
there's such a thing as focal ratio. By itself, it tells you nothing
about resolution, exposure time, or most anything else of much value.
Resolution is determined by focal length, exposure time by aperture.
Those are the specifications you need to provide if you are discussing
the technical aspects of imaging.
Dennis Woos
> object directly through a telescope (with the possible exception of
> Saturn) that gave me anywhere near the satisfaction of seeing an image
> appear on my screen, as the result of my own imaging effort. Without
> imaging, I might not bother to own a telescope at all.
I feel exactly the opposite, especially when you include the satisfaction of
being able to locate interesting targets. However, I have to admit that as
the folks I observe with get older and their visual acuity lessens, visual
observing becomes more problematic. Most of our club members are somewhat
sceptical about seeing spiral arms (e.g. M51) in a 10" scope, and some folks
claim that I and a few others must be using our "averted imagination." If
someone has poor vision, then imaging must be a great way to enjoy
astronomy. Of course, I am not saying that you or any of the other fine
imagers have bad eyesight!
Dennis
Davoud
> > Again, this depends on intent and personal taste. I've never seen any
> > object directly through a telescope (with the possible exception of
> > Saturn) that gave me anywhere near the satisfaction of seeing an image
> > appear on my screen, as the result of my own imaging effort. Without
> > imaging, I might not bother to own a telescope at all.
Dennis Woos:
> I feel exactly the opposite, especially when you include the satisfaction of
> being able to locate interesting targets. However, I have to admit that as
> the folks I observe with get older and their visual acuity lessens, visual
> observing becomes more problematic. Most of our club members are somewhat
> sceptical about seeing spiral arms (e.g. M51) in a 10" scope, and some folks
> claim that I and a few others must be using our "averted imagination." If
> someone has poor vision, then imaging must be a great way to enjoy
> astronomy. Of course, I am not saying that you or any of the other fine
> imagers have bad eyesight!
I'm 64-years-old and a wannabe imager
<http://www.primordial-light.com>. My problem is that even when I was
24-years-old I couldn't see the fine structure in M1 or the fine
filaments in the nebulas in M45 or anything more than a gray donut in
M57, visually.
Intentionally or otherwise, you implied that imaging is a great way to
enjoy astronomy [only] if one has poor vision. There is more to it than
that, since a telescope with an eyepiece simply cannot show what is
shown on this page of mine
<http://www.primordial-light.com/deepsky.html>.
I wouldn't disparage visual deep-sky observing because it is as valid a
pursuit in the hobby as any other, I'm only saying that for me it has
always been true that each gray blob looks very much like the previous
gray blob and the next gray blob, even with a largish Dob (18" being
the largest I have observed with for any length of time). Whether
photographs are realistic in that they show how an object would
_really_ appear to the unaided eye if the observer was near enough and
the object was bright enough, especially in the rendering of the
object's visible spectrum, is irrelevant to me. Photos can show far
more of what is really there, and that's what I want. I think that, for
me, Rob Gendler's Orion Deep Field
<http://www.robgendlerastropics.com/OriondeepfieldL.html> epitomizes
the reason for doing wide-field, deep-sky photography. I believe it is
true that _no_ _one_ , pro or amateur, knew that Orion looks like that
until Gendler published his remarkable image.
As for "averted imagination" and objects that are definitely suited for
visual observation, have you seen Alan Friedman's site
<http://www.avertedimagination.com/>?
Peter Webb
"Davoud" <st...@sky.net> wrote in message
news:071020080029411742%st...@sky.net...
Brilliant photos.
I note some excellent photos were taken on a 85 mm aperture refractor.
Somebody in a telescope shop recommended a refractor - I never really
considered them - is there something special about refractors and
astrophotography ?
Dennis Woos
> enjoy astronomy [only] if one has poor vision.
Let me assure you that the "only" is a function of your averted?
imagination. In fact, my sons have dabbled in webcam imaging and I have been
supportive of their efforts. In fact, here is an image of Jupiter - homemade
5" reflector with homemade mirror made by my younger son when he was 12
years old:
http://www.woosfamily.net/~dennis/Jupiter_0030_391_ps_50pct.jpg
Dennis
Mark S. Holden
The darkness of your sky is also a factor here - in the "blue" light
pollution zone, M51 showed spiral structure and more detail in my TMB
115 than it does in a "red" light pollution zone in a TMB 229.
I do mostly visual observing, but when I have time, I do like to image,
and my deep sky images through the TMB 115 and my AP 140 always show
much more detail than I'd see visually with a monster dob. The problem
is I need to allow about 3 hours to aquire a good image, and then an
hour or two of processing to make it look nice. (folks who know what
they're doing may spend more or less time)
Here are a few of my images:
Dennis Woos
> pursuit in the hobby as any other, I'm only saying that for me it has
> always been true that each gray blob looks very much like the previous
> gray blob and the next gray blob, even with a largish Dob (18" being
> the largest I have observed with for any length of time).
One "gray blob" looking like another is a good point, and I think it raises
the issue of being a good visual observer. Again, please do not assume that
I am saying that you are not - I have never observed with you! However, I do
find that most of the folks I observe with do not observe well. They do not
research what they can see (thanks to the imagers!) and they do not spend
sufficient time at the eyepiece to see as much as there is to be seen.
Visual observing is a skill, and I think that a lot of folks never even
realize that let alone acquire some level of proficiency. Of course,
everyone takes their observing as they like it, and that is great. However,
I do believe that a little bit of knowledge and skill would help a lot of
folks get more out of visual observing.
Dennis
Davoud
> > I wouldn't disparage visual deep-sky observing because it is as valid a
> > pursuit in the hobby as any other, I'm only saying that for me it has
> > always been true that each gray blob looks very much like the previous
> > gray blob and the next gray blob, even with a largish Dob (18" being
> > the largest I have observed with for any length of time).
Dennis Woos:
> One "gray blob" looking like another is a good point, and I think it raises
> the issue of being a good visual observer. Again, please do not assume that
> I am saying that you are not - I have never observed with you! However, I do
> find that most of the folks I observe with do not observe well. They do not
> research what they can see (thanks to the imagers!) and they do not spend
> sufficient time at the eyepiece to see as much as there is to be seen.
> Visual observing is a skill, and I think that a lot of folks never even
> realize that let alone acquire some level of proficiency. Of course,
> everyone takes their observing as they like it, and that is great. However,
> I do believe that a little bit of knowledge and skill would help a lot of
> folks get more out of visual observing.
As one who was strictly a visual observer for many years (except for
photographing bright Solar-System objects, comets, and eclipses) I
agree entirely.
Davoud
> ...In fact, here is an image of Jupiter - homemade
> years old:
>
> http://www.woosfamily.net/~dennis/Jupiter_0030_391_ps_50pct.jpg
Well, you can tell your son that that's a good image by any measure.
Tom Polakis
> Just keep in mind that any definition you use is your own. The idea of
>
> It sometimes takes visual observers a while to get away from the concept
> of magnification when they image, but they always get there <g>. All
Chris,
Very good inputs to this thread, as usual. I adhered to your point of
view regarding image scale until I was pressed by my wife about what
magnification the image was on the display. I told her that it
depends on how close you are to the monitor, which was not a
satisfying answer! Then I reconsidered that people typically sit
about 1 1/2 feet from their monitors, and that it is helpful to
compare the size of the image to a visual telescopic view. I came up
with this estimate for my system:
I appreciate all aspects of imaging, but I come down with Paul
Schlyter's view about take pleasure of a visual view. No matter how
"superior" images are, for me, they are like listening to the stereo
versus seeing the music live. Images on a monitor never put a shiver
down my spine; they never transport me to the object. But that's just
my view on the subject. Astronomy is a great hobby thanks to the many
ways it can be enjoyed.
Tom
Chris L Peterson
<webbf...@DIESPAMDIEoptusnet.com.au> wrote:
>I note some excellent photos were taken on a 85 mm aperture refractor.
>Somebody in a telescope shop recommended a refractor - I never really
>considered them - is there something special about refractors and
>astrophotography ?
No, they are simply one type of telescope design, and every design has
its advantages and disadvantages. The advantage of a refractor is that
it tends to be the most structurally solid type, exhibiting little
flexure and seldom requiring collimation. Refractors usually have more
optical surfaces, so they often have higher order corrections allowing
smaller spot sizes farther from the optical axis- useful if you have a
large sensor. On the other hand, they often show chromatic aberration,
which is a problem for single-shot color or unfiltered imaging. Also,
they never provide a lot of aperture, and what aperture they do provide
is expensive in comparison with reflective designs. Aperture is what
determines total exposure time, so if you have a small scope, you'll
need longer imaging sessions. Of course, the smaller aperture also means
a smaller, lighter scope, which can be used with a smaller, less
expensive mount.
Like most issues of astronomical equipment, it's all about tradeoffs.
Davoud
> > ...a telescope with an eyepiece simply cannot show what is
Peter Webb:
> Brilliant photos.
Thanks for that kind remark. I've seen better and I'm trying to do
better. I'm not one to blame others for my own shortcomings, but poor
skies in the mid-Atlantic region of the U.S. are killing me.
> I note some excellent photos were taken on a 85 mm aperture refractor.
> Somebody in a telescope shop recommended a refractor - I never really
> considered them - is there something special about refractors and
> astrophotography ?
Some of my photos were made with a 76mm refractor.
You are no doubt familiar with the Mac-PC flame wars that erupt from
time to time on the Web--Mac users are Commies, PC users are Nazis, or
vice-versa, and so on.
Those wars are minor scuffles compared to the Reflector-Refractor wars
that have devastated entire worlds. I don't participate in those wars
because I don't have sufficient knowledge of the advantages of the two
systems to argue the matter, even if I were inclined to do so.
Instead, my deep-sky imaging 'scopes are a a 180mm modified Newtonian
reflector _and_ a 106mm refractor. I choose which 'scope to use based
almost entirely upon whim. It is true, however, that the layout of my
tiny observatory makes it more convenient for me to use my astronomical
CCD camera on the refractor and my modified DSLR on the reflector. I'm
not locked into that configuration, however. I decide which camera to
use on which 'scope based on my non-scientific, or at best,
quasi-scientific, assessment as to which camera is more suitable for a
certain target. I often decide based on the results I have seen from
others' work. I am not a researcher; my objective is to produce pretty
pictures.
It's worth noting that, all other things being equal, refractors are a
bit easier to use than most reflectors. A fast astrograph such as my
180mm f/2.8 Newtonian is difficult to focus. I have fixed that problem
with a RoboFocus and a speed reducer. I stole that idea from this web
page <http://tinyurl.com/epsilonrobofocus>.
Probably the most obvious tell-tale as to whether a photo was made with
a Newtonian or a refractor is the diffraction spikes that are visible
on bright stars on Newtonian images (and on images from
Ritchey-Chretien designs such as the Hubble). Some people love the
spikes and others hate 'em. SAA participant Anthony Ayiomamitis
simulates the metal vanes of the secondary-mirror support that cause
these diffraction spikes by fixing threads to the front of his
refractors, and he does it to beautiful effect, IMO. See, for example,
his photo of M45 at <http://www.perseus.gr/Astro-DSO-M45.htm>. Those
stars "should not" have diffraction spikes because Mr. Ayiomamitis used
a refractor. As I noted, some would reject them because they are
artificial. I like 'em; beauty remains in the eye of the beholder. I
haven't seen many complaints about the (genuine) diffraction spikes on
the Hubble images, either.
In other words, I don't know if there is anything special about
refractors and astrophotography.
Chris L Peterson
>Those wars are minor scuffles compared to the Reflector-Refractor wars
>that have devastated entire worlds...
Happily, like the fabled Goto-Star Hop wars, these seem to have
disappeared into the past. I think most people have caught on to the
idea that there are some questions that can't be answered with simple
good/bad responses.
Paul Schlyter
Tom Polakis <polaki...@cox.net> wrote:
...........
>I appreciate all aspects of imaging, but I come down with Paul
>Schlyter's view about take pleasure of a visual view. No matter how
>"superior" images are, for me, they are like listening to the stereo
>versus seeing the music live. Images on a monitor never put a shiver
>down my spine; they never transport me to the object. But that's just
>my view on the subject. Astronomy is a great hobby thanks to the many
>ways it can be enjoyed.
Your comparison with sound recording was good! And the comparison can
be brought further: just like astronomical imaging can show us things
we wouldn't be able to see otherwise (such as UV, IR, X-ray, radio images,
or just images of very faint objects), sound recording too can make us
hear things we wouldn't hear otherwise (e.g. the sonar echoes of bats,
or sounds too weak to be audible to us). However, the pleasure of
actually seeing or hearing something we *can* see and hear is much
bigger than viewing an image or listening to a recording, no matter how
good the image/recording is.
One related side note: young people today are more and more unwilling
to pay for CD records, they prefer to download their music for free from
the Net. But this may not be just simple greed, for young people of today
spend more money on attending live gigs than earlier generations. Perhaps
preference of live over recorded is an important factor here?
Paul Schlyter
>A fast astrograph such as my 180mm f/2.8 Newtonian
A Newtonian isn't an astrograph... http://en.wikipedia.org/wiki/Astrograph
>Probably the most obvious tell-tale as to whether a photo was made with
>a Newtonian or a refractor is the diffraction spikes that are visible
>on bright stars on Newtonian images (and on images from
>Ritchey-Chretien designs such as the Hubble). Some people love the
>spikes and others hate 'em. SAA participant Anthony Ayiomamitis
>simulates the metal vanes of the secondary-mirror support that cause
>these diffraction spikes by fixing threads to the front of his
>refractors, and he does it to beautiful effect, IMO. See, for example,
>his photo of M45 at <http://www.perseus.gr/Astro-DSO-M45.htm>. Those
>stars "should not" have diffraction spikes because Mr. Ayiomamitis used
>a refractor. As I noted, some would reject them because they are
>artificial. I like 'em; beauty remains in the eye of the beholder. I
>haven't seen many complaints about the (genuine) diffraction spikes on
>the Hubble images, either.
The creation of those diffcation spikes is interesting, and clearly
shows that being realistic isn't the same as being beautiful.
In modern sound recording/editing software there's a comparable
phenomenon: one common digital filter emulates the distorsion of those
old vacuum tubes !!!! Why would anyone want to deliberately add
distorsion to their own recordings? Well, they like the sound.....
Dennis Woos
> phenomenon: one common digital filter emulates the distorsion of those
> old vacuum tubes !!!! Why would anyone want to deliberately add
> distorsion to their own recordings? Well, they like the sound.....
My younger son has built and uses two vacuum tube guitar amplifiers, and has
no interest in messing around with modern solid-state equipment emulating
vacuum tube sound/distortion. He has taught me that the "instrument" is not
only the electric guitar, but also includes the amplifier and the speakers.
This is not obvious (at least to me), as amplifiers and speakers are
purchased separately and are connected to the guitar with cables, etc.
However, in a more limited way all musical instruments are made up of
sub-instruments that can be mixed and matched (piano actions, flute head
joints, bassoon bocals, violin bows, etc.) In fact, one can argue that the
musical instrument includes the venue and the listener's ears and aural
processing. Compare this to the idea of a telescope including the sky
conditions and the observers's eyesight and visual processing. A good
musician plays the whole instrument, not just the part under their fingers.
Dennis
Mark S. Holden
>
> I note some excellent photos were taken on a 85 mm aperture refractor.
> Somebody in a telescope shop recommended a refractor - I never really
> considered them - is there something special about refractors and
> astrophotography ?
>
>
Newtonians give the most bang for the buck, Apo refractors give the most
bang for the aperture, and require less maintenance, but they also cost
the most.
The lack of a central obstruction helps improve contrast. You can get a
feel for it by holding your thumb up at arms length and looking at
something in the distance. You can see "behind" your thumb but your
thumb is also there to add confusion.
Maintenance on a reflector includes periodic collimation, and infrequent
re-silvering of the mirrors.
The largest "portable" refractor I've seen is a TMB 229 owned by a
friend. The views are glorious, but you really need at least two people
to set it up, and three or four is better. A couple friends have 10"
newtonians they can easily transport and set up by themselves.
Chris L Peterson
wrote:
>The lack of a central obstruction helps improve contrast.
However, this is pretty much a non-issue if you're using the scope for
imaging. Most of the premiere imaging instruments in the world (and out
of it, since this includes the Hubble) have very large central
obstructions. Furthermore, when used visually, the contrast effects of
the central obstruction are only relevant to lunar and planetary
observing. The eye's visual acuity is too low when viewing dim objects
to come close to being affected by the effects of the CO on the systems
MTF.