Viewing by eye versus astrophotography

In article ,
Chris L Peterson wrote:
On Sun, 05 Oct 2008 15:42:46 GMT, (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.

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 pleasu 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


--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stjarnhimlen dot se
WWW: http://stjarnhimlen.se/

On Sun, 05 Oct 2008 22:44:16 GMT, (Paul Schlyter) wrote:

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!
_________________________________________________

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

"Chris L Peterson" wrote in message
...
On Sun, 05 Oct 2008 22:44:16 GMT, (Paul Schlyter) wrote:

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!
_________________________________________________


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

Chris L Peterson wrote:
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
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.

Peter Webb wrote:
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.

--
Brian Tung
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.

On Sun, 5 Oct 2008 19:54:17 -0700 (PDT), (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?

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
Cloudbait Observatory
http://www.cloudbait.com

On Mon, 6 Oct 2008 12:05:52 +1100, "Peter Webb"
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.
_________________________________________________

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

Thanks to both you and Mr Tung for responding. I was worried that only Brad
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" wrote in message
...
On Mon, 6 Oct 2008 12:05:52 +1100, "Peter Webb"
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 ? ...


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.


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.

In article , Brian Tung wrote:

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.




--
----------------------------------------------------------------
Paul Schlyter, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stjarnhimlen dot se
WWW: http://stjarnhimlen.se/

In article ,
Peter Webb wrote:

"Chris L Peterson" wrote in message
.. .
On Sun, 05 Oct 2008 22:44:16 GMT, (Paul Schlyter) wrote:

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!
_________________________________________________


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.

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, Grev Turegatan 40, SE-114 38 Stockholm, SWEDEN
e-mail: pausch at stjarnhimlen dot se
WWW: http://stjarnhimlen.se/