Single lens objective refractor

OK, here is another one of my harebrained ideas that is probably
flawed in some obvious way that you fine people will point out to me.
But here goes.

It seems to me, with the advent of digital cameras and computer
graphics, you could build a single objective lens refractor and fix
chromatic aberration with PhotoShop. You could take three pictures.
The first focused in the blue part of the spectrum, the second in the
mid frequencies, and the last in the red zone.

Extract the cyan and black from the first, yellow and black from the
second and red and black from the third. Than, using PhotoShop
carefully resize the blue and red images to the same size as the
yellow. Finally using something like Registar stack them together.

Another dumb idea brought to you by,
James King

There are many aberrations other than chromatic. Read up on the subject.
Actually, I once did make a 3" refractor with a single lens objective, and as
long as you stayed about 6X-10X, it was fantastic!!
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

"Wfoley2"
There are many aberrations other than chromatic. Read up on the subject.
Actually, I once did make a 3" refractor with a single lens objective, and
as
long as you stayed about 6X-10X, it was fantastic!!
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

Many Moons ago (1940s-50s) you often saw ads in Popular Mechanics and
the like for 600X, (ha! ha!), telescope kits for the huge sum of around
$3.00. These were just a Plano convex objective, also with single element
lenses to make eyepieces out of. The diameters of these varied, some as
large as 3 inches. If you tried to use them at full aperture as I recall
the images were lousy, but if you stopped them down to say 1-1/2" you would
get an acceptable image. Acceptable that is until you compared it to a good
achromatic. Colour filters for correction probably won't work all that well
as they are rather broad band, but it would be an interesting thing to try.
Ed
An ATM'er from the days when money was scarce! A 6" reflector was the norm
and a 12" was a dream machine!

The 3" I cobbled together was from the days when there were still wooden
spools, which could be carefully sawn to make 2 mounts for simple 2-element
eyepieces. I think this was probably about 1946 or so. The eyepiece lenses
were from old box cameras (which could probably go today on E-Bay for enough to
buy a Nagler) and negative lenses taken from cheap opera glasses. I have never
had any scope that showed M42 more beautifully. But, then, seeing was much
better in 1946 than now.
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

"Wfoley2"
.. But, then, seeing was much
better in 1946 than now.
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

You sure have that right! For street lighting we had a bare light bulb
at each block intersection, not a mercury or sodium vapour lamp scattering
light all over hell at four house intervals. Energy conservation, my BUTT!
:-( The only really dark place now is the far side of the Moon during the
lunar night!
Ed

On 23 Feb 2004 11:09:37 -0800, (Lurking Luser) wrote:

OK, here is another one of my harebrained ideas that is probably
flawed in some obvious way that you fine people will point out to me.
But here goes.

It seems to me, with the advent of digital cameras and computer
graphics, you could build a single objective lens refractor and fix
chromatic aberration with PhotoShop. You could take three pictures.
The first focused in the blue part of the spectrum, the second in the
mid frequencies, and the last in the red zone.

One problem is that color sensors have crappy filters in front of them. A result
of this is that lots of red makes it through the blue and green, etc (or similar
for CMY). This is one reason that color sensors don't do a very good job with
most astronomical targets. For terrestrial imaging, there is lots of image
processing going on to reconstruct something like accurate color. But anything
that makes it through the wrong filter in your case is just going to contribute
to blurring, which isn't really fixable.

People using conventional three filter imaging with B&W sensors or film get very
good results without needing an apochromat by doing just what you suggest- but
their filters are much higher quality. Normally, you use an achromat since the
more complex design minimizes other aberrations as well. A single element
objective that is well enough corrected for good imaging would need to be
aspheric, and would probably end up costing more and performing worse than a
basic achromat.

_________________________________________________

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

On Mon, 23 Feb 2004 20:43:11 GMT, Chris L Peterson
wrote:

On 23 Feb 2004 11:09:37 -0800, (Lurking Luser) wrote:

OK, here is another one of my harebrained ideas that is probably
flawed in some obvious way that you fine people will point out to me.
But here goes.

It seems to me, with the advent of digital cameras and computer
graphics, you could build a single objective lens refractor and fix
chromatic aberration with PhotoShop. You could take three pictures.
The first focused in the blue part of the spectrum, the second in the
mid frequencies, and the last in the red zone.

One problem is that color sensors have crappy filters in front of them. A result
of this is that lots of red makes it through the blue and green, etc (or similar
for CMY). This is one reason that color sensors don't do a very good job with
most astronomical targets. For terrestrial imaging, there is lots of image
processing going on to reconstruct something like accurate color. But anything
that makes it through the wrong filter in your case is just going to contribute
to blurring, which isn't really fixable.

People using conventional three filter imaging with B&W sensors or film get very
good results without needing an apochromat by doing just what you suggest- but
their filters are much higher quality. Normally, you use an achromat since the
more complex design minimizes other aberrations as well. A single element
objective that is well enough corrected for good imaging would need to be
aspheric, and would probably end up costing more and performing worse than a
basic achromat.

_______________________________________________ __

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

Wouldn't a single element lens need to be refocused for each RGB image
and thus be of a difference image scale eg blue image 'smaller' than
red image? CYM wouldn't work as each filter transmits two primary
colours - each coming to a different focus.

On Tue, 24 Feb 2004 20:28:28 GMT, (Maurice Gavin) wrote:

Wouldn't a single element lens need to be refocused for each RGB image
and thus be of a difference image scale eg blue image 'smaller' than
red image?

Yes, but the original poster proposed doing just that. You can fix the image
scale during post processing.


CYM wouldn't work as each filter transmits two primary
colours - each coming to a different focus.

CYM wouldn't work as well because the filter bandwidths are wider, that is
correct. Everybody who uses this technique with tricolor imaging and achromats
is using RGB filter sets.

_________________________________________________

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

In article ,
(Lurking Luser) wrote:

It seems to me, with the advent of digital cameras and computer
graphics, you could build a single objective lens refractor and fix
chromatic aberration with PhotoShop. You could take three pictures.
The first focused in the blue part of the spectrum, the second in the
mid frequencies, and the last in the red zone.

Remember to filter each exposure, even then ( especially then? )
calibration is going to be bitchy or worse.

If you want to try single-piece-of-glass stuff why not figure a
lens to around F:30 or F:45? Most aberations become trivial at
longer F ratios and focal lengths.

Dark skies,

tom

PS Note that a 15cm F:30 lens has a focal length of 4.5 meters;
20cm at F:45 = 9 meters. You'll have to talk real fast to get
this past the spousal unit.

--
We have discovered a therapy ( NOT a cure )
for the common cold. Play tuba for an hour.

Just take a picture in color and you will get radial lines for the stars
with a short focal length refractor. I'm the owner of the 200" refractor
(4.25" diameter single lens refractor) and the color isn't too radical at
that focal length but since the objective is so small, the Airy Disc will be
of a significant size for a digital camera when running at prime focus.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!