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Let's Photograph Comet 46P Wirtanen



 
 
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  #71  
Old January 1st 19, 08:15 AM posted to sci.astro.amateur
Paul Schlyter[_3_]
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Posts: 1,343
Default Let's Photograph Comet 46P Wirtanen

On Mon, 31 Dec 2018 09:26:42 -0700, Chris L Peterson
wrote:
Don't you see the beauty in performing that division?


Nope. It discards useful information. It's like characterizing a

aperture
telescope by focal ratio, and not providing the focal length and
aperture.


If all telescopes always had the same focal length it would be
practical to do that. Then the aperture could easily be calculated
from the focal ratio and the implicit focal length. Now, this is not
the case of course, but in the case of visual acuity tests the
Snellen chart is always at the same standard distance.

A patient with 6/6 vision and one with 3/3 vision may have
significantly different visual acuity; if you normalize them to 1.0,
you lose that distinction.


Did you ever see any visual acuity report giving the result 3/3? (is
that in feet or meters btw?)

The standard procedure for that would be to still use the standard
distance but to insert correction lenses in front of the patient's
eye. The strength of that lens must then of course be given in the
report.

In the rare case that the visual acuity test actually is performed at
a distance of 3, this is easily handled by adding a note to the
report. E.g. "acuity 1.0 at distance 3". If the "at distance" note is
absent, the standard distance will be implied.

Btw, how do American eye doctors give the strength of corrective
lenses, or of any lenses? In metric diopters like in Europe? Or do
they have their own diopter scale based on feet instead of meters? Or
do they just give the focal length in feet?
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  #72  
Old January 1st 19, 02:51 PM posted to sci.astro.amateur
Chris L Peterson
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Posts: 9,985
Default Let's Photograph Comet 46P Wirtanen

On Tue, 01 Jan 2019 09:15:14 +0100, Paul Schlyter
wrote:

On Mon, 31 Dec 2018 09:26:42 -0700, Chris L Peterson
wrote:
Don't you see the beauty in performing that division?


Nope. It discards useful information. It's like characterizing a

aperture
telescope by focal ratio, and not providing the focal length and
aperture.


If all telescopes always had the same focal length it would be
practical to do that. Then the aperture could easily be calculated
from the focal ratio and the implicit focal length. Now, this is not
the case of course, but in the case of visual acuity tests the
Snellen chart is always at the same standard distance.


That's what you're missing. The test is NOT always performed from the
same distance, and as a result, the distance is a meaningful part of
the test reporting.

A patient with 6/6 vision and one with 3/3 vision may have
significantly different visual acuity; if you normalize them to 1.0,
you lose that distinction.


Did you ever see any visual acuity report giving the result 3/3? (is
that in feet or meters btw?)


Yes. That's a common result in Europe (and it's meters).

Btw, how do American eye doctors give the strength of corrective
lenses, or of any lenses? In metric diopters like in Europe? Or do
they have their own diopter scale based on feet instead of meters? Or
do they just give the focal length in feet?


Diopters. Always. (By definition, a diopter has units of reciprocal
meters, so could always be considered "metric".)
  #73  
Old January 2nd 19, 08:41 AM posted to sci.astro.amateur
Paul Schlyter[_3_]
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Posts: 1,343
Default Let's Photograph Comet 46P Wirtanen

On Tue, 01 Jan 2019 07:51:15 -0700, Chris L Peterson
wrote:
If all telescopes always had the same focal length it would be
practical to do that. Then the aperture could easily be calculated
from the focal ratio and the implicit focal length. Now, this is

not
the case of course, but in the case of visual acuity tests the
Snellen chart is always at the same standard distance.


That's what you're missing. The test is NOT always performed from

the
same distance, and as a result, the distance is a meaningful part of
the test reporting.


OK I can buy that. But the test is usually done with the standard
distance. If done at a different distance, a differently sized
Snellen chart will be needed.

And when the standard distance is used, it can still be implied. If a
non-standard distance is used, that can be added as a note. "1.0 at
3m" contains just as much information as "3/3".

The no - standard distance might contain fractional meters btw. Would
you then want ti write it as, say, 3.56/3.56?


Diopters. Always. (By definition, a diopter has units of reciprocal
meters, so could always be considered "metric".)


Among amateur astronomers there's a curious mixture of units.
Telescope aperture is often given in inches, while focal lengths is
often given in millimeters.
  #74  
Old February 16th 19, 03:17 PM posted to sci.astro.amateur
corvastro
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Posts: 19
Default Let's Photograph Comet 46P Wirtanen

On Sunday, December 23, 2018 at 4:30:39 PM UTC-8, hleopold wrote:
On Dec 22, 2018, Davoud wrote
(in article ):

hleopold:
...Always a joy to see your photos, David, I look forward to more in the future.
I mostly read here, hardly ever comment. Not to mention I am jealous of your
set-up, though not of your location, as bad as our seeing is here you have
it worse.


Your words are kinder than I deserve! I have complained here about
Maryland sky conditions from time to time, but not too bitterly, I
hope. I try to take it with equanimity; I get what I get and I try to
make the best I can of it.


As, for the vast majority of us, we all must.

Same for my vision. I am 74 years old and I have macular degeneration.
My corrected vision in my left eye is 20/25, i.e., very good. But over
the past few months I have noted a deterioration in my right eye to the
point where I cannot read with my left eye covered. My ophthalmologist
referred me to an optometrist who specializes in low vision. My right
eye can't be "fixed," but a new pair of glasses, coming in about two
weeks, will help.


Age eventually gets us all, I am only 67, in 3 weeks to be 68, and my eyes
just ain’t what they used to be. Of course in my case I have to go back to
when I was 12 when my eyes went from practically perfect to very near-sighted
and very strong astigmatism in both eyes. Correctable but slowly getting
harder to focus as I age.

Good that I'm not a visual observer; I can still
focus on a laptop display, but faint fuzzies don't work for me in an
eyepiece. I use a Bahtinov mask on my Tak 106
https://www.flickr.com/photos/primeval/30364292358.


Way back in the late 70s early 80s I did, for a while, try photography with
my scopes, but otherwise I have been strictly a visual observer. I can’t
believe the great stuff that we have these days. I am not really a fan of
digital photography in many ways, I loved shooting film, and that was how I
did astro photos back then. Used a home-brew camera and cut my 35mm film to
single frame. My camera was much like one that was described in Sky and
Telescope back about that time, starting with a short extension tube and
built up the body out of flat sheets of black opaque plastic glued together
to hold the film in place plus a dark slide to take the shot. If was fun, but
fairly frustrating at the same time. At that time I did most of my
photography using B&W film, so I was able to do my own processing. Not really
an easy thing to do on a Navy ship. About once a week I would take over the
head for our dept. berthing area to do processing. Even at night the light
leaks were ridiculous, I used a lot of black tape every time. And hoping like
everything that no one needed to use it for the few minutes while
transferring the exposed film to the tank. Yeah, I did used a changing bag
but you probably know how that works.

These days, though, I just mostly like to look at things through the scope.

--
Harry F. Leopold

The Prints of Darkness (remove gene to email)


You could have used one of the various methods that I used in my youth which allowed me to use a standard 35mm camera - so I didn't have to cut up individual frames of film.

1) Afocal - focus telescope for visual use, mount camera focused at infinity, then do fine focus with telescope eyepiece.

2) Projection - I experimented a bit with positive projection. That involves just mounting the camera without a lens, then focusing the telescope eyepiece further out to move the primary image to the camera focal plane.

3) Direct Objective - what I finally chose, but it took a lot of work as my telescope was a reflector. I think it would have been even more difficult with a refractor. I had to move the primary mirror forward a bit to make the image plane accessible, use a larger secondary to capture the whole light cone and a larger diameter eyepiece holder(2") to keep the lens tube from blocking some of the cone. This produced by far the brightest and sharpest images. The primary disadvantage was that I needed to use a short extension tube for my eyepieces when doing visual work. But since I had installed a 2" lens holder, I needed an adapter anyway so my 1 1/4" eyepieces would fit..
 




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