Viewing by eye versus astrophotography

In article ,
Chris L Peterson wrote:
On Mon, 6 Oct 2008 12:05:52 +1100, "Peter Webb"
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, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stjarnhimlen dot se
WWW: http://stjarnhimlen.se/

I agree and understand most of what you have written, except the following:


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,

Chris L Peterson wrote:
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

On Mon, 6 Oct 2008 16:08:41 +1100, "Peter Webb"
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
Cloudbait Observatory
http://www.cloudbait.com

On Mon, 6 Oct 2008 18:52:34 +1100, "Peter Webb"
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.
_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

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 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

Someone wrote:
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/?

Davoud

--
Don't re-elect the failures of the past eight years.

usenet *at* davidillig dawt com

"Davoud" wrote in message
...
Someone wrote:
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.


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 ?

Intentionally or otherwise, you implied that imaging is a great way to
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/Ju...1_ps_50pct.jpg

Dennis

William Hamblen wrote:
On Sun, 5 Oct 2008 00:49:22 +1000, "Peter Webb"
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

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:

http://www.picturetrail.com/sfx/album/view/13640785