Telescope Clueless

I'd like to take some pictures of Saturn.

The current telescope I have is a 50mm with 700mm focal length.
It is an old Tasco falling apart, can barely keep it steady on the moon, let
a lone a star.

My current budget is under a $100.00

http://www.walmart.com/catalog/produ...uct_id=5223437

I was thinking of this one for $59.00 dollars and making an adapter for my
camera.



From what I'm gathering 50mm lens means how much light it will let in and
700mm focal length tells me how much it can magnify. Is there a mathematical
formula that tells how much a scope will magnify an object?

Thoughts & Suggestions WELCOMED!




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read this first: Telescope Buyers FAQ
http://home.inreach.com/starlord


--
There are those who believe that life here, began out there, far across the
universe, with tribes of humans, who may have been the forefathers of the
Egyptians, or the Toltecs, or the Mayans. Some believe that they may yet be
brothers of man, who even now fight to survive, somewhere beyond the
heavens.


The Lone Sidewalk Astronomer of Rosamond
Telescope Buyers FAQ
http://home.inreach.com/starlord
Sidewalk Astronomy
www.sidewalkastronomy.info
The Church of Eternity
http://home.inreach.com/starlord/church/Eternity.html


"7change.com" wrote in message
...
I'd like to take some pictures of Saturn.

The current telescope I have is a 50mm with 700mm focal length.
It is an old Tasco falling apart, can barely keep it steady on the moon,
let a lone a star.

My current budget is under a $100.00

http://www.walmart.com/catalog/produ...uct_id=5223437

I was thinking of this one for $59.00 dollars and making an adapter for my
camera.



From what I'm gathering 50mm lens means how much light it will let in and
700mm focal length tells me how much it can magnify. Is there a
mathematical formula that tells how much a scope will magnify an object?

Thoughts & Suggestions WELCOMED!




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Newsgroups
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"7change.com" wrote in message
...
I'd like to take some pictures of Saturn.

The current telescope I have is a 50mm with 700mm focal length.
It is an old Tasco falling apart, can barely keep it steady on the moon,
let a lone a star.

My current budget is under a $100.00

http://www.walmart.com/catalog/produ...uct_id=5223437

I was thinking of this one for $59.00 dollars and making an adapter for
my camera.



From what I'm gathering 50mm lens means how much light it will let in
and 700mm focal length tells me how much it can magnify. Is there a
mathematical formula that tells how much a scope will magnify an object?

Thoughts & Suggestions WELCOMED!
Forget using a 'camera'. The problem is that normal cameras are heavy
(hence need a good mount), and are large (Saturn is small). The best way
to work, is to use a 'webcam', which can grab a massive sequence of images
really quickly. You use the telescope as a 'lens' for this camera. The
second problem then is the focal length needed. To get Saturn at a
reasonable size, you need an effective focal length (even with the small
pixels of a webcam), something in the order of several metres. On a large
camera, this gets worse. Now you can lengthen the effective focal length
using a Barlow lens, but as you do so, the unit gets harder and harder to
aim at the planet (the effective magnification goes up). Seriously, the
sort of mount involved on the type of scope you are looking at, will
vibrate so much, that you won't e able to keep it aimed at the planet...
Now, to bring this into 'scale'. Saturn has an 'apparent diameter', that
varies between about 15, and just under 21 arc seconds (according to where
it is in it's orbit, relative to the Earth). Now this corresponds to about
the size of a 'dime', placed 100 yards from you.
When working 'visually', a scope has a 'magnification', which is simply
the ratio of it's focal length, to that of the eyepiece. So, to make
Saturn appear as large as the Moon (approximatly half a degree), would
require a magnification of about 180*, when Saturn is at it's largest.
This would then need an eyepiece with a focal length of 700/180=3.8mm.
Used with this much magnification, the sort of 'quality' of scope involved
here (and also size of scope - the diameter of a scope, sets a limit on
it's resolving power), will produce a really blurry image, the right size,
but will be almost impossible to keep pointed at the target.
When working with a camera, there is not a 'magnification' as such, but
instead an 'image scale'. This is because you can change the apparent
'magnification', by altering the size of the final image. The image scale,
is given by (a slight approximation):

Is = (206*Ps)/Fl

This gives the 'scale' in arc seconds (Is), produced by a camera sensor or
pixel 'Ps' uM across, using a focal length of 'Fl'. Now a typical webcam,
has small pixels. Often only 3uM across. So with the 700mm focal length of
the scope, this gives:

(206*3)/700 = 0.88

Now with Saturn at (say) 20 arc seconds across, this then gives an image,
just 20/0.88 = 22 pixels across. Not exactly large. The problem here is
that if you magnify something this small, it'll still only have 22 dots
across. Using a '*3' Barlow lens, increases the effective focal length t
about 2100mm, and the image scale to about 66 pixels, and is the sort of
point, where it starts to become possible to get a reasonable small image.
However at this scale, using a 640*480 webcam sensor, Saturn will move
right across the camera FOV, in just 40 seconds!. So without a tracking
motor, keeping the camera pointed at Saturn, becomes _hard_..
Realistically, save your money, and wait till you can afford a scope with
at least a tracking motor.

Best Wishes

Thanks for the advice, do you have a recommended web cam.
Saving sounds good.




"Roger Hamlett" wrote in message
...

"7change.com" wrote in message
...
I'd like to take some pictures of Saturn.

The current telescope I have is a 50mm with 700mm focal length.
It is an old Tasco falling apart, can barely keep it steady on the moon,
let a lone a star.

My current budget is under a $100.00

http://www.walmart.com/catalog/produ...uct_id=5223437

I was thinking of this one for $59.00 dollars and making an adapter for
my camera.



From what I'm gathering 50mm lens means how much light it will let in and
700mm focal length tells me how much it can magnify. Is there a
mathematical formula that tells how much a scope will magnify an object?

Thoughts & Suggestions WELCOMED!
Forget using a 'camera'. The problem is that normal cameras are heavy
(hence need a good mount), and are large (Saturn is small). The best way
to work, is to use a 'webcam', which can grab a massive sequence of images
really quickly. You use the telescope as a 'lens' for this camera. The
second problem then is the focal length needed. To get Saturn at a
reasonable size, you need an effective focal length (even with the small
pixels of a webcam), something in the order of several metres. On a large
camera, this gets worse. Now you can lengthen the effective focal length
using a Barlow lens, but as you do so, the unit gets harder and harder to
aim at the planet (the effective magnification goes up). Seriously, the
sort of mount involved on the type of scope you are looking at, will
vibrate so much, that you won't e able to keep it aimed at the planet...
Now, to bring this into 'scale'. Saturn has an 'apparent diameter', that
varies between about 15, and just under 21 arc seconds (according to where
it is in it's orbit, relative to the Earth). Now this corresponds to about
the size of a 'dime', placed 100 yards from you.
When working 'visually', a scope has a 'magnification', which is simply
the ratio of it's focal length, to that of the eyepiece. So, to make
Saturn appear as large as the Moon (approximatly half a degree), would
require a magnification of about 180*, when Saturn is at it's largest.
This would then need an eyepiece with a focal length of 700/180=3.8mm.
Used with this much magnification, the sort of 'quality' of scope involved
here (and also size of scope - the diameter of a scope, sets a limit on
it's resolving power), will produce a really blurry image, the right size,
but will be almost impossible to keep pointed at the target.
When working with a camera, there is not a 'magnification' as such, but
instead an 'image scale'. This is because you can change the apparent
'magnification', by altering the size of the final image. The image scale,
is given by (a slight approximation):

Is = (206*Ps)/Fl

This gives the 'scale' in arc seconds (Is), produced by a camera sensor or
pixel 'Ps' uM across, using a focal length of 'Fl'. Now a typical webcam,
has small pixels. Often only 3uM across. So with the 700mm focal length of
the scope, this gives:

(206*3)/700 = 0.88

Now with Saturn at (say) 20 arc seconds across, this then gives an image,
just 20/0.88 = 22 pixels across. Not exactly large. The problem here is
that if you magnify something this small, it'll still only have 22 dots
across. Using a '*3' Barlow lens, increases the effective focal length t
about 2100mm, and the image scale to about 66 pixels, and is the sort of
point, where it starts to become possible to get a reasonable small image.
However at this scale, using a 640*480 webcam sensor, Saturn will move
right across the camera FOV, in just 40 seconds!. So without a tracking
motor, keeping the camera pointed at Saturn, becomes _hard_..
Realistically, save your money, and wait till you can afford a scope with
at least a tracking motor.

Best Wishes




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