What difference does focal length make?
J.T.King
like some clarification on...
Now I know WHAT focal length is.. at least I am pretty sure that it is
a measure of the ratio of the cone created by the largest objective
lense and the focal point (like the eyepiece or diagnol).
So my questions are these (all other things being equal):
1. Is "faster" focal length a smaller F number?
2. Why is a smaller F# better for Astrophotography?
3. How fast can focal length be if you want to take pictures of Is
1.25" or 2" deep sky objects?
4. How fast for planetary objects?
5. What is easier to view in: F6, F10, F12?
6. What focal length is better for eyeglasses?
7. What is the practical difference between otherwise identical scopes
with an F12 or an F10? a F10 or an F6?
8. Is a 1.25" or 2" eyepiece better with shorter or longer focal
lengths?
9. What else should I know about focal lengths?
Thanks! This is really plauging me, and I am sure it is confusing to
lots of people as well. Thanks for helping a beginner!
J.T.
J.T.King
>3. How fast can focal length be if you want to take pictures of Is
>1.25" or 2" deep sky objects?
This should read:
3. How fast should focal length be if you want to take pictures of
deep sky objects?
david_...@my-deja.com
J.T.King(askme!) (J.T.King) wrote:
> Ok, time for a newbie question that I am sure many beginners would
> like some clarification on...
>
> Now I know WHAT focal length is.. at least I am pretty sure that it is
> a measure of the ratio of the cone created by the largest objective
> lense and the focal point (like the eyepiece or diagnol).
Focal length is a measure of the optical path from the objective lens or
primary mirror to the focal point. Focal ratio (f/ratio) is the
focal length divided by aperture or diameter. See
http://www.adelaideoptical.com.au/glossary.htm#finder for more info.
> So my questions are these (all other things being equal):
>
> 1. Is "faster" focal length a smaller F number?
Yes.
> 2. Why is a smaller F# better for Astrophotography?
Shorter exposure times and larger possible photographic fields.
> 3. How fast can focal length be if you want to take pictures of Is
> 1.25" or 2" deep sky objects?
This depends on many factors.
> 4. How fast for planetary objects?
You need high "magnification" for the planets, hence a slow telescope
(high f/ratio) is an advantage. Eyepiece projection photography is
required. See Michael Covington's book on astrophotography.
> 5. What is easier to view in: F6, F10, F12?
It depends upon whether you want larger fields of view for a given focal
length ocular (eyepiece) or higher magnification. If your primary
interest is deep-sky observing an f/6 would likely be more desirable.
You can use longer f.l. eyepieces that will have greater eye relief with
a slow scope.
> 6. What focal length is better for eyeglasses?
See above. Today there are short f.l. eyepieces such as the Tele Vue
Radians and Pentax SMC XL's with 20mm of eye relief that can be used
with eyeglasses for high power work. A Barlow lens or Powermate can
also be utilized with a longer f.l. eyepiece to increase magnification.
> 7. What is the practical difference between otherwise identical scopes
> with an F12 or an F10? a F10 or an F6?
Higher magnification versus wider fields. Larger exit pupils (f/ratio
of eyepiece divided by the f/ratio of the scope) for a given eyepiece
with faster scopes. You'll have to choose your eyepiece focal lengths
according to the focal length of the scope.
In general the optics of a slow telescope will be superior and will be
easier to collimate. A slow telescope holds collimation better too.
> 8. Is a 1.25" or 2" eyepiece better with shorter or longer focal
> lengths?
True 2" eyepieces tend to be relatively long in focal length (say
greater 20mm) and can be used with any focal length telescope. However,
exceeding an exit pupil of 7mm or so results in "wasting" telescopic
aperture since all of the light emerging from the eye lens of the
eyepiece will not enter your pupil. This is not a problem with a
refractor because there is no secondary obstruction.
> 9. What else should I know about focal lengths?
>
> Thanks! This is really plauging me, and I am sure it is confusing to
> lots of people as well. Thanks for helping a beginner!
>
> J.T.
I suggest you get yourself a copy of Phil Harrington's _Star Ware_ or
visit some of the web sites devoted to telescope optics.
Dave Mitsky
Sent via Deja.com
http://www.deja.com/
gary...@my-deja.com
answer your questions. This is as much a test for me as an answer to
your questions. I hope someone will come in and correct me in the many
paces I am bound to be incorrect.
J.T.King wrote:
> So my questions are these (all other things being equal):
>
> 1. Is "faster" focal length a smaller F number?
> 2. Why is a smaller F# better for Astrophotography?
Yes. This is a carryover from photography where smaller focal ratios
(f numbers) allow faster shutter speeds when all other things are
equal. This also holds true for astro-photography, where exposures are
often >30 minutes. Any errors of tracking with the mount will be
exacerbated in slower exposures. Faster/shorter focal ratios do not
affect visual astronomy in terms of light gathering.
> 3. How fast should focal length be if you want to take pictures of
deep sky objects?
I'll leave this to some astro-photographers.
> 4. How fast for planetary objects?
Planetary viewing is normally done with higher power and therefore
longer focal length telescopes allow high powers to be acheived with
the same eyepiece. In achromatic refracters, it is easier to make a
longer focal length lens in terms of absence of secondary colour.
Shorter FL lenses can be made with low colour, but they are more
expensive to produce. Some people believe that even with the expensive
apochromatic refractors, there are visual advantages (however slim) to
using a long (f10) FL for planetary viewing.
> 5. What is easier to view in: F6, F10, F12?
There is no difference in terms of "easier". The lower the focal
ratio, the wider the field of view with the same eyepiece and they can
take greater advantage from the 2" eyepieces for wide FOVs. Many people
find the wide,sweeping view through an f5 or an f6 refractor is hard to
beat. But many will concede that it is generally easier to achieve
higher power with a longer FL refractor of the same objective
diameter. This is less true as the quality and the cost of the
telescope approaches infinity.
With Newtonians, longer FL means a much bigger scope and
transportability can become a serious issue. on the other hand,
shorter FL (f5 and less)require precise collomation to get proper views
and they can be more difficult to collomate precisely, particularly
with beginners.
> 6. What focal length is better for eyeglasses?
Marginally, a telescope with a longer FL. This is based on the premise
that the longer FL telescope will achieve higher powers with longer fl
eyepieces. Longer fl eyepieces offer greater eye relief than the short
fl eyepieces and are therefore easier to use for people with eyeglass.
This is less true with the availability of eyepieces lines such as
Lanthanums, Radians, Pentax XLs etc. These eyepieces offer short fl
AND 20 mm of eye relief. They also command a premium price. They also
have many lenses in the makeup of the eyepiece, and purist prefer as
few lenses as possible between the eye and the object being viewed.
Orthoscopic eyepieces seem to be choice for people who wish to squeeze
the last bit of contrast and detail out of their system. These types of
eyepieces at short fl offer very little eye reliefe and are unusable
for people with glasses
> 7. What is the practical difference between otherwise identical scopes
> with an F12 or an F10? a F10 or an F6?
The shorter the focal ratio, the wider the field of view. Short
refractors are also easier to take with you on an airplane and shorter
newtonians are easier to put in the Chevette. Longer FL telescopes
will achieve higher power with longer fl eyepieces. Longer FL
refractorsare easier/less expensive to make "low colour". Longer FL
scopes are heavier and may require a more sturdy mount because of their
length. Longer FL newtonians may require a step or ladder system to get
your eye high enough off the ground to get to the eyepiece.
> 8. Is a 1.25" or 2" eyepiece better with shorter or longer focal
> lengths?
Generally speaking, a short FL telescopes will allow a wider field of
view over a long FL unit when using the same eyepiece. 2" eyepieces
can offer a wider FOV than a 1.25" eyepiece. Play with this calculator
http://www.atmsite.org/software/magfov.html
Gary Weber
Scot Mc Pherson
>
> 1. Is "faster" focal length a smaller F number?
Yes
> 2. Why is a smaller F# better for Astrophotography?
The smaller F/stop as its called in photography means the telescope will
take the same picture with a shorter exposure than a slower telescope. The
advantage is less chance for error due to imperfect tracking, etc.
> 3. How fast can focal length be if you want to take pictures of Is
> 1.25" or 2" deep sky objects?
I don't understand this question, sorry.
> 4. How fast for planetary objects?
Solar System object are unique in their proximity to us. Telescope aperture
isn't as critical for Sun, Moon and planet viewing, and neither is the
F/Ratio BUT slower F/Ratios can equate to better eye relief on some
eyepieces.
> 5. What is easier to view in: F6, F10, F12?
F/Ratio doesn't affect ease of viewing as much as it does astrophotography.
The laws that make a small focal ratio better for astrophotography don't
apply to viewing with your eye. The rules are different because your eye is
an optical device and not a planar piece of film.
> 6. What focal length is better for eyeglasses?
It depends on what your eyeglasses are for. If you are near or farsighted
you will probably find it best to simply remove your glasses and then focus
your telescope. If you have an astigmatism or something similar, you will
need to wear your glasses while looking through the telescope. Although
longer focal length give slightly better eye relief with some eyepieces, it
is your eyepiece selection that will have the most effect on how comfortable
your viewing will be while wearing glasses.
> 7. What is the practical difference between otherwise identical scopes
> with an F12 or an F10? a F10 or an F6?
This is kind of a loaded question, because Aperture and Focal Length are the
two primary specification of a telescope. It would be akin to asking " All
other things being equal, what is the difference between the gear ratios in
a VW Bug and a Pierce Tractor Truck" BUT all things being equal, since I am
asserting that normally aperture is the prime specification and focal length
being second only to aperture, the f ratio indirectly affects your
magnification...Magnification is really determined by focal length, but if
the aperture is equal, then you know we are using F ratio to figure focal
length.
The principle practical (in practice) difference between short and long
focal ratios will probably affect transportability of your telescope. If you
get a 7" F/15 Refractor, you are going to have a great deal of difficulty
getting it to your dark site, and maybe even have difficulty just getting it
outside without making a lot of noise.
Don't worry too much about F ratio unless you have a specific purpose in
mind for your telescope. The important things to consider in a telescope are
aperture, and optical quality.
If this is your first telescope, I would suggest getting a medium aperture,
medium focal length telescope (4-8 inches, f/6-10) most other telescopes are
designed or best suited to specific tasks.
> 8. Is a 1.25" or 2" eyepiece better with shorter or longer focal
> lengths?
2" eyepiece are general considered better for a number of reasons, principly
FOV. The 2" eyepieces have a greater diamter barrel and are thus not
physically FOV restricted for certain long focal length eyepieces.
BUT
1.25" is the most common size used at present, and extremely high quality
eyepieces can be had for less money than the 2"
> 9. What else should I know about focal lengths?
Focal length effects transportability and magnification...Longer Focal
Length magnify greater than shorter, but are MUCH MUCH more bulky.
--
Scot Mc Pherson
http://www.behomet.net
N27° 19' 56"
W82° 30' 39"
Scot Mc Pherson
an issue as f ratio...Faster ratios will allow you to take shorter
exposures...There are limitations to shorter focal ratios, they require
greater care in collimation and require more care in optical quality, so are
often more expensive than there higher f ratio counterparts.
For deepsky photography, aperture is the most important consideration, the
more the better (again up to a certain point)
--
Scot Mc Pherson
http://www.behomet.net
N27° 19' 56"
W82° 30' 39"
<J.T.King (askme!) (J.T.King)> wrote in message
news:3a8468be...@news.flashcom.net...
Sketcher
concerning the meaning of focal length as applied to a telescope.
I'll give one version of my own:
When you point a telescope at a star, light from that star is
collected by every point in the telescope's clear aperture. Let's
say the telescope's clear aperture is "CA." Now consider two light
rays that enter along one of the CA's diameters, one at each of the
end points of that diameter. Next, shift your attention to the
telescope's focal plane. Take our two light rays and consider the
angle between them 'just' before they reach the focal point. If those
rays were to continue in straight lines diverging from the focal
point, at some distance "F" from the focal point their separation
would be equal to the telescope's CA. That distance is the
telescope's focal length.
The physical distance between the CA and the focal point may be less
than, greater than, or equal to the telescope's focal length.
Now to answer the questions:
1. A "focal length" can be short or long, but it's never referred as
being "fast" or "slow." A "focal ratio" OTOH can be fast or slow, and
the number representing it is the same thing as an "f-number." Thus a
faster focal ratio translates into a smaller f-number.
2. A smaller f-number is *not* always better for astrophotography.
It will result in shorter exposure times, but sometimes a larger image
scale is desirable. For that, a longer focal length must be used, and
unless one switches to a different telescope, that translates to a
larger f-number
3. For deepsky astrophotography it's generally best to use the
shortest focal length that will provide sufficient image scale for the
object being photographed. I would strongly advise you to consult a
good book on astrophotography for a detailed treatment of this topic.
It cannot be adequately addressed in just a few words.
4. For planetary photography it's desirable to compute a focal length
(image scale) that best matches your telescope's resolution, your
seeing conditions, and the grain size of the film. Again, it would,
IMO, be best if you were to consult an appropriate book on this topic.
5. You need to be more specific than: "What is easier to view in:
F6, F10, F12?" I don't know what you're asking.
6. For an eyeglass wearer the eyepiece's eye relief is the primary
concern. Certain relatively expensive eyepieces will provide the
necessary eye relief with a short focal length telescope, while less
expensive eyepieces can do the same with a longer focal length
telescope.
7. The across-the-board difference between equal aperture scopes of
different f-ratios is focusing tolerance. The shorter f-ratio scopes
will be more "sensitive" to focus. Other differences will depend upon
the specific design and manufacturing details of the individual
telescopes.
8. For a telescope with a short enough focal length, there would be
little or no advantage to using 2" eyepieces. Once you get beyond f/5
or so the 2" eyepieces become more advantageous for wide-field work.
Regardless of f-ratio, at least some 1.25" eyepieces should prove
useful for most amateur telescopes. Many of the people who have been
in this hobby for a long enough time tend to use eyepieces of both
sizes.
9. If possible, visit a public library and read up on the subject.
Raw answers, even from me ;-), may prove to be of little value until
you've achieved a thorough understanding of the underlying concepts.
Sketcher
To sketch is to see.
http://www.mcn.net/~sketcher/index.html
Alan French
Keep in mind that there is a difference between the world of photography and
the world of astronomy.
When talking about faster lenses photographers are talking about lenses that
have more aperture and collect more light, allowing a shorter exposure.
When astrophotographers talk about faster lenses they are talking about
giving up image scale to concentrate the light on a smaller portion of the
film to get a shorter exposure.
Clear skies, Alan
<gary...@my-deja.com> wrote in message news:95u6cr$jg1$1...@nnrp1.deja.com...
Mike Ruskai
>Ok, time for a newbie question that I am sure many beginners would
>like some clarification on...
>
>Now I know WHAT focal length is.. at least I am pretty sure that it is
>a measure of the ratio of the cone created by the largest objective
>lense and the focal point (like the eyepiece or diagnol).
The ratio of the focal length to the aperture size is focal ratio. The focal
length itself is merely the distance from the middle of the objective to the
focal plane. The focal length is determined by the shape of the objective.
>So my questions are these (all other things being equal):
>
>1. Is "faster" focal [ratio] a smaller F number?
A smaller focal ratio is indeed "faster", and is the "f" number (not "F",
which is reserved for focal length, not ratio).
>2. Why is a smaller [f/#] better for Astrophotography?
That begs the question. Is it better? For deep-sky wide-field
astrophotography only, yes. This is because it takes less time for the same
quantity of light to reach the recording surface.
For extended object astrophotography, a "faster" scope will simply require
more additional lenses to make the image large enough for the recording
surface, which will absorb more light, and end up being slower than the same
optical setup with a larger ("slower") focal ratio.
>3. How fast can focal [ratio] be if you want to take pictures of Is
>1.25" or 2" deep sky objects?
I'm going to assume that the above sentence should read as follows:
"How fast can focal [ratio] be if you want to take pictures of deep sky
objects?"
That depends on the angular size of the object being photographed. The size
of the image at the focal plane is determined solely by focal length (not
ratio). How much the focal length needs to be increased for a reasonable
image size ultimately determines the effective focal ratio.
If a focal length of X is required to make the image large enough to be
detectable on the film or CCD, then a faster scope may require a
tele-extender or Barlow lens to reach that effective focal length. A slower
scope may already be at that focal length, and hence be faster than the
"faster" scope, by virtue of not requiring additional lenses (which absorb
more light).
If the image is large enough already, the faster scope will be at an
advantage. Maybe. Faster reflectors have larger secondary obstructions,
which may kill as much or more light as a focal compressor (reduces effective
focal length, hence lowering focal ratio) on a faster scope (which has a
smaller secondary obstruction). This doesn't apply to refractors, of course,
or off-axis reflectors (which don't have an obstruction, but are also quite
rare).
Ignoring light transmission issues, scopes with larger focal ratios will have
a more aberration-free image. So, the inherently faster scope will then
really only be best for small-sized deep-sky objects, where the field edges
aren't terribly important (that's where aberrations will be more apparent -
though a quality scope may make them insensible without careful inspection).
>4. How fast for planetary objects?
Planetary photography requires magnification. The longer the focal length
(larger focal ratio), the closer to the ideal magnified size you'll be - and
the less additional hardware you'll need to put on for a decent image size.
>5. What is easier to view in: [f/6, f/10, f/12]?
Magnification is the telescope's focal length divided by the focal length of
the eyepiece. So, faster scopes require smaller eyepieces for higher
magnifications than slower scopes. Given the same magnification with
identical quality optics, the image will be the same (using different
eyepieces, of course, for each focal ratio).
As I said, the higher the focal ratio, the less apparent aberrations will be
(which will only be at the field edges in any case, with a quality
instrument). So, in practice, the longer focal ratio scope will produce a
better image at a given magnification.
That's ignoring the fact that at higher magnifications, the slower scopes
will require more additional hardware again to get there, since there's a
practical lower limit to the focal length of an eyepiece.
>6. What focal [ratio] is better for eyeglasses?
Non-issue. Eye relief in the eyepiece is what's important there. Though
since a higher focal ratio scope gets higher magnifications with larger focal
length eyepieces, and the latter tend to have more eye relief than smaller
focal length eyepieces, the larger focal ratio scope can easily prove less
difficult to use with eyeglasses.
>7. What is the practical difference between otherwise identical scopes
>with an [f/12] or an [f/10]? a [f/10] or an [f/6]?
Slower version has larger obstruction, if a reflector, and more aberration at
the field edges (whether reflector or refractor).
>8. Is a 1.25" or 2" eyepiece better with shorter or longer focal
>lengths?
2" eyepiece is only useful for low-magnification wide-field views.
>9. What else should I know about focal [ratios]?
>
>Thanks! This is really plauging me, and I am sure it is confusing to
>lots of people as well. Thanks for helping a beginner!
Aside from optical characteristics, it affects the size of the optical tube
assembly in non-Cassegrain reflectors and refractors. The tube in those is
about the same size as the focal length. In Cassegrain reflectors (classical
Cassegrain, Schmidt-Cassegrain, Maksutove-Cassegrain), the light is bounced
back and force, so the tube length is only a fraction of the focal length (in
fact, decreasing focal length usually has no effect on the tube length, but
just makes the secondary larger).
--
- Mike
Remove 'spambegone.net' and reverse to send e-mail.
Edmund Wellington
david_...@my-deja.com wrote:
>snip<
> Larger exit pupils (f/ratio of eyepiece divided by the f/ratio of the
--
++++++++++++++++++++++++++++++++++++++++++++
Edmund R. Wellington;
Wooding Hill Observatory.
++++++++++++++++++++++++++++++++++++++++++++
Bill Nelson
: Now I know WHAT focal length is.. at least I am pretty sure that it is
: a measure of the ratio of the cone created by the largest objective
: lense and the focal point (like the eyepiece or diagnol).
No, you are thinking of focal ratio. Focal length is the distance that
light focuses away from the surface of the primary.
The f ratio is the ratio between the focal length of the primary and
the diameter of the primary. So if the focal length is 72 inches and
the diameter is 16", then it is an f4.5 primary.
: So my questions are these (all other things being equal):
: 1. Is "faster" focal length a smaller F number?
That is focal ratio - more commonly called the f ratio or f number of
the objective. The lower the number, the faster the mirror/lens.
: 2. Why is a smaller F# better for Astrophotography?
It isn't, necessarily. A fast system allows shorter exposures.
On the other hand, aberrations and collimation are more of a problem
with the faster systems.
If photographing planets - what you want is focal length. This is
also the case for nebular objects.
To improve resolution and light gathering, you want a larger diameter
primary objective.
: 3. How fast can focal length be if you want to take pictures of Is
: 1.25" or 2" deep sky objects?
: 4. How fast for planetary objects?
I don't understand what you are asking here.
: 5. What is easier to view in: F6, F10, F12?
The F12 would be easiest to align, and would give the highest magnification
for a given diameter primary and given focal length eyepiece.
The F6 would be the hardest to align. But for a given diameter primary and
given focal length eyepiece, the view would be brightest.(lower magnification)
: 6. What focal length is better for eyeglasses?
Focal length of the telescope, and f ratio as well, are pretty much
immaterial. It is hard to use glasses with short focal length eyepieces.
The reason for this is that they have shorter eye relief.
But if you do not have astigmatism, you should be able to view without
the glasses.
: 7. What is the practical difference between otherwise identical scopes
: with an F12 or an F10? a F10 or an F6?
An f12 (and probably an f10) scope would probably be spherical, so less
expensive than an f6 - which had better be parabolic.
For a given diameter scope, the f12 would have an optical tube that is
approximately twice as long as the f6. For a given eyepiece, the f12
magnification would be twice that of the f6.
With the f12 scope, even the very cheap Kellner and Ramsden eyepieces
work fine. For the f6, you really need eyepieces with better correction.
: 8. Is a 1.25" or 2" eyepiece better with shorter or longer focal
: lengths?
Immaterial. The larger eyepiece will give a larger field of view,
assuming that the 1.25" cuts off part of the light cone - which may
not be the case with a high f ratio scope.
: 9. What else should I know about focal lengths?
I would suggest going to a few of the web sites, and read the
tutorials on scopes - where there are discussions about diameter,
focal length, f ratios, aberrations etc.
They will do a far better job of explaining the subject than I have
managed here.
Bill Nelson (bi...@peak.org)
david_...@my-deja.com
Edmund,
You're quite right. What I meant to say was the focal length of the
eyepiece divided by the f/ratio of the telescope.
Exit pupil is also equal to aperture in millimeters divided by
magnification.
Brian Tung
> : 4. How fast for planetary objects?
>
> I don't understand what you are asking here.
I think he means what are typical focal ratios (or lengths) for the
planets.
The deal with planets (as compared with DSOs) is that they are itty
bitty little things. Image scale is directly related to focal length,
not ratio. A focal length of 2000 mm creates an image twice as large
as a focal length of 1000 mm, regardless of aperture (so long as the
object actually appears on the film/CCD!).
It turns out that it takes a fantastically long focal length (by the
ordinary standards) to take pictures of the planets. A picture of
the moon on 35 mm film at 2000 mm just about fits. Jupiter, on the
other hand, is about 50 times smaller, so on a 4x6 print, you would
get a Jupiter just 2 mm across. Not very impressive. In order to get
any appreciable size and therefore detail out of your image, you need
a focal length closer to 10,000 mm, at the very least; better would be
20,000 mm.
Now, that is 20 m, or about 66 feet. Most telescopes in amateur hands
do not have focal lengths like that. Instead, if you are going to
take an image using your telescope as the lens, you will have to
employ a technique called eyepiece projection. In this configuration,
you have the telescope, then they eyepiece, then your camera (without
its usual lens). Depending on how far back your camera is from the
eyepiece, this amplifies the effective focal length of your telescope.
Example. You have a telescope with a focal length of 2000 mm. You
do eyepiece projection with a 6 mm eyepiece, with a film plane to
eyepiece separation of 60 mm. Then the effective focal length of
this combination is 2000 mm times (60/6)-1, or 18,000 mm. This would
create an image of Jupiter about three-quarters of an inch across on
a 4x6 print of 35 mm film.
Aperture is not at all worthless for taking pictures of planets, it
just doesn't affect the size of the image. What you get from aperture
is shorter exposures for the same brightness, better resolution (in
principle), and perhaps worse seeing effects.
Anyone really interested in the topic would do well to read Michael
Covington's book, Astrophotography for the Amateur. (Or something
like that. What can I say, I've only skimmed through it, but it sure
looks good.)
Brian Tung <br...@isi.edu>
Astronomy Corner at http://astro.isi.edu/
C5+ Home Page at http://astro.isi.edu/c5plus/