monochromatic refractor

This is an idea I've had for years - since I was young and wanted
a big refractor (150-200 mm) but could not afford on.

Most of the work (and expense) goes into controlling chromatic
abberation. What with all the doublet and triplet designs with
exotic lens elements.

How about a refractor meant for monochromatic viewing? You could
use a single piece of glass, and correct for geometric abberations
and not worry about CA (or use more than one element to correct
for geometric abberations). You'd need a set of filters that only
allow small slices of the visible spectrum through (monochromatic
filters)

SO all the viewing would be monochromatic, but it would be one way
to get a large and fast refractor without spending a fortune.

Imagine a 6-8" f/4 refractor for $500.00. It would be optically
very good.... just no color Also would be good for CCD imaging -
take three separate images (R, G, B) and then stack them for the
full color image.

-Eric B

wrote in message
oups.com...
This is an idea I've had for years - since I was young and wanted
a big refractor (150-200 mm) but could not afford on.

Most of the work (and expense) goes into controlling chromatic
abberation. What with all the doublet and triplet designs with
exotic lens elements.

snip


The biggest problem would be that blocking most wavelengths would mean
blocking most of the light, so you'd only be able to view very bright
objects.

Ed T.

On 14 Jul 2005 15:37:17 -0700, wrote:

This is an idea I've had for years - since I was young and wanted
a big refractor (150-200 mm) but could not afford on.

Most of the work (and expense) goes into controlling chromatic
abberation. What with all the doublet and triplet designs with
exotic lens elements.

How about a refractor meant for monochromatic viewing?

This works nicely for imaging, and lets you get as good of results with
an ordinary achromat as you can with an apo (by focusing separately for
the red, green, and blue filters). But for visual use, do you really
want to throw away half your light or more? You aren't going to see
much.

In any case, you wouldn't want a singlet because there will be too many
other aberrations. It is neither difficult nor expensive to make a well
corrected doublet. You can chop off a bit of one end or the other (with
a minus violet filter, for instance) and get some improvement on the
remaining CA. But if you want to get lots of light and low CA, you need
a triplet, or a doublet with exotic materials.

_________________________________________________

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

Ed T wrote:


The biggest problem would be that blocking most wavelengths would mean
blocking most of the light, so you'd only be able to view very bright
objects.

Ed T.

That's true, but when you factor in the perceptual biases of
human vision I wonder how significant this really is.

For example, scotopic vision is already nearly monochromatic.
The dark adapted eye is most sensitive to radiation at about
500 nm. Sensitivity drops off rapidly from this point - plus
or minus 50 nm from this peak (450-550 nm) the eye is only half
of sensitive, and 400 && 600 nm close to zero.

So, depending on what you're looking at, viewing may even be
improved by seeing in monochrome. I imagine certain emission nebula
may be helped too.

Be interesting to see how things look with a ~500 nm narrow pass
filter. Though I imagine these monochrome filters are pricey.


-Eric B

Chris L Peterson wrote:

This works nicely for imaging, and lets you get as good of results with
an ordinary achromat as you can with an apo (by focusing separately for
the red, green, and blue filters). But for visual use, do you really
want to throw away half your light or more? You aren't going to see
much.

In any case, you wouldn't want a singlet because there will be too many
other aberrations. It is neither difficult nor expensive to make a well
corrected doublet. You can chop off a bit of one end or the other (with
a minus violet filter, for instance) and get some improvement on the
remaining CA. But if you want to get lots of light and low CA, you need
a triplet, or a doublet with exotic materials.


See my response to Ed. For visual observing, human 'nightvision'
is already relatively monochromatic. Most of that 'wasted light'
the eye is not all that sensitive too. Filters that let light
through close to the eyes peak sensitivity may not appear all that dim.
From peak sensitivity at around 500 nm the eyes response drops off
rapidly.

Just an idea. I was wondering if anyone has any experince with this
kind of observing.

Also wondering how much a narrow pass filter around 500 nm would cost?

-Eric B

On 14 Jul 2005 16:29:49 -0700, wrote:

See my response to Ed. For visual observing, human 'nightvision'
is already relatively monochromatic. Most of that 'wasted light'
the eye is not all that sensitive too. Filters that let light
through close to the eyes peak sensitivity may not appear all that dim.
From peak sensitivity at around 500 nm the eyes response drops off
rapidly.

You seem to misunderstand scotopic vision. Yes, the peak is near 500nm,
but the rods have significant response on both sides. You can see the Ha
emission of nebulas with scotopic vision, even though it is deep red.

Try a UHC or broadband skyblock filter sometime, and you'll see how much
the total brightness decreases when you throw away a lot of the
spectrum.


Also wondering how much a narrow pass filter around 500 nm would cost?

A UHC or OIII filter can be had for under $100.

_________________________________________________

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

Chris L Peterson wrote:

You seem to misunderstand scotopic vision. Yes, the peak is near 500nm,
but the rods have significant response on both sides. You can see the Ha
emission of nebulas with scotopic vision, even though it is deep red.


Right, right. Rod sensitiviy follows a tighter curve than cones
http://hyperphysics.phy-astr.gsu.edu...on/bright.html

Still you're correct. And when viewing things through a scope
(especially planets) there's enough light being gathered to engage
photopic vision.

Perhaps a doublet design which brings light from 450-550 nm into
focus. It would be better corrected than a doublet over the wider
visual range. Might work OK is used with a filter.

I was just wondering if anyone has ever tried anything like this
before.

It's possible that certain types of visual observing could be
enhanced in mono.

-Eric B

You would still want a doublet to correct for other aberrations. Consider
also spherochromatism. For photo work, you can get a good achro and refocus
for each color. For visual work, you are going to cut out way too much
light.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/

To reply, remove Delete and change period com to period net
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wrote in message
oups.com...
This is an idea I've had for years - since I was young and wanted
a big refractor (150-200 mm) but could not afford on.

Most of the work (and expense) goes into controlling chromatic
abberation. What with all the doublet and triplet designs with
exotic lens elements.

How about a refractor meant for monochromatic viewing? You could
use a single piece of glass, and correct for geometric abberations
and not worry about CA (or use more than one element to correct
for geometric abberations). You'd need a set of filters that only
allow small slices of the visible spectrum through (monochromatic
filters)

SO all the viewing would be monochromatic, but it would be one way
to get a large and fast refractor without spending a fortune.

Imagine a 6-8" f/4 refractor for $500.00. It would be optically
very good.... just no color Also would be good for CCD imaging -
take three separate images (R, G, B) and then stack them for the
full color image.

-Eric B

Total nonsense.

You could also have a water filter that filters out all the hydrogen and
oxygen.

By the way the cost of your scope would be ten times a good apo!

Back to the drawing board ....

Why dont you merge with Bill Vorce. Bill might have something he can rig

to your specifications, for $29.99. (Shipping will be $10,000.)

Jerry







wrote:

This is an idea I've had for years - since I was young and wanted
a big refractor (150-200 mm) but could not afford on.

Most of the work (and expense) goes into controlling chromatic
abberation. What with all the doublet and triplet designs with
exotic lens elements.

How about a refractor meant for monochromatic viewing? You could
use a single piece of glass, and correct for geometric abberations
and not worry about CA (or use more than one element to correct
for geometric abberations). You'd need a set of filters that only
allow small slices of the visible spectrum through (monochromatic
filters)

SO all the viewing would be monochromatic, but it would be one way
to get a large and fast refractor without spending a fortune.

Imagine a 6-8" f/4 refractor for $500.00. It would be optically
very good.... just no color Also would be good for CCD imaging -
take three separate images (R, G, B) and then stack them for the
full color image.

-Eric B

wrote:

Ed T wrote:

The biggest problem would be that blocking most wavelengths would mean
blocking most of the light, so you'd only be able to view very bright
objects.

It is perfectly OK for H-alpha prominence scopes though. No point in
having a fancy APO with extra glass surfaces for a bandwidth of 0.5A.

That's true, but when you factor in the perceptual biases of
human vision I wonder how significant this really is.

For example, scotopic vision is already nearly monochromatic.
The dark adapted eye is most sensitive to radiation at about
500 nm. Sensitivity drops off rapidly from this point - plus
or minus 50 nm from this peak (450-550 nm) the eye is only half
of sensitive, and 400 && 600 nm close to zero.

So, depending on what you're looking at, viewing may even be
improved by seeing in monochrome. I imagine certain emission nebula
may be helped too.

Be interesting to see how things look with a ~500 nm narrow pass
filter. Though I imagine these monochrome filters are pricey.

You don't have to imagine - most of the narrowband nebula filters offer
the opportunity to see exactly what it looks like (though with some you
will also need an H-alpha blocker too).

Dirty green stars do not look all that appealling and you lose at least
a couple of magnitudes worth of stellar continuum emission.

Regards,
Martin Brown