Βρέθηκε - σπάνιος WITH 8" f/12 καθρέφτης

[ngc4565]

Ένας πολύ σπάνιος καθρέφτης έχει βρεθεί από τον κατασκευαστή καθρεφτών του 19-ου αιώνα "With". Αυτός ο "With" καθρέφτης είναι 8" f/12 και δίχνει απίστευτη λεπτομέρεια στους πλανήτες.

 

Έχω φορτώσει μια άριστη εικόνα Δία που τραβηκε με αυτόν τον καθρέφτη -

 

ο καθρέφτης θα δοθεί σε οπτικούς εμπειρογνώμονες για να αξιολογήσουν

την ακρίβειά του.

[ngc4565]

- Έχω προσθέσει περισσότερες πληροφορίες για την εικόνα του Δία με τον 8in f/12 G.H.WITH καθρέφτη.

 

Πηγαίνετε στην εικόνα για περισσότερες πληροφορίες -

 

Ο καθρέφτης θα δοθεί στον Carl Zambuto για αξιολόγηση -

[ngc4565]

Εδώ παιδιά τα αποτελέσματα της ανάλυσης που έγινε για τον "WITH" καθρέφτη από τον Carl Zambuto. Ο καθρέφτης είναι πραγματικά 8.5" f/10,5 και όχι 8" f/12 όπως προηγουμένως αναγγελθείς -

 

- η ποιότητα της εικόνας του Δία δίνεται περισσότερο στο "webcam + Stacking sofware" και όχι στον "καθρέφτη" -

 

 

==> It was a distinct privilege to evaluate a 134 year old telescope

mirror made by someone of renown from the 19th century. I believe the

finish date was 1871, the optician's name was Hereford(?) With. The

mirror is the property of Jim Phillips.

 

I did not measure the mirror, but rather shot some Ronchi images and

one knifedge null to show the edge.

 

The mirror is in fact an 8.5 inch F/10.5, (as opposed to an 8" F/12).

The focal length is 89.1 inches. The blank appears to be soda-lime

glass, at just about full thickness of 6:1. We found the glass to be

very sensitive to temperature changes.

 

It is of course very interesting to see what the state of the art was

in that timeframe. He was apparently the renown mirror maker of his

day, and he labelled it "absolute perfection". I found it to be a good

mirror, better in fact than many commercial mirrors I have evaluated

on the bench in the last 15 years.

 

My technical description is that the mirror is a rough sphere with a

somewhat rough microsurface and a narrow turned edge. That sounds bad,

but in fact is not as bad as it seems. We need to keep several things

in perspective. First, I understand the mirror was tested using the

star test. That will not show moderate surface roughness so easily,

and a narrow edge can escape as well. As far as correction, it does

have strong correction in what would be an outer zone, and a depressed

center. From center to edge it may be fully corrected, but its not

real smooth overall.

 

In the files section is a folder (With mirror) where four images are

found. One is a Ronchi image with four lines, another is a Ronchi

null, the third is a computer generated Ronchigram for the equivalent

of the mirror, and the fourth is a knifedge null showing the edge.

 

I suspect the surface is not thoroughly or uniformly polished, as

there are concentric blotches which are the coating peeling away.

There is too much of a non-random pattern to the concentric rings for

the polish to be uniform.

 

All that aside, this is a real find. I began to wonder what resources

Mr. With had at his disposal. I know he didn't call up United lens and

have the mirror shipped Federal Express. Let's go back to 1871. Where

did the glass come from, and who was fabricating (melting and casting)

mirror blanks? Did he have to cast it himself? How far did it need to

be shipped and how was it shipped? Where did polishing compounds have

to come from, and what grade and to what level did they exist? The

first gasoline car on the road didn't come along until 1893 so we know

there were no cars. Steam trucks did come along in the mid-1800's but

because of the roads which were made only for horse and carriage, they

usually only went from a factory to the nearest train station. Can you

image how he must have had to take delivery of a mirror blank,

abrasives and polish? Maybe he made his own abrasives, like Dobson

used to do. In any case, these would be pretty exotic materials for

that timeframe, no?

 

The correction on the mirror is probably better than 1/4 wave, and

having a reasonable surface it will show rather good images. Imagine

what it would be like to walk up to a privately owned 8.5" telescope

in those days, and see what that scope could show you! It was

certainly a very rare thing.

 

I feel the strength of the images taken with the mirror reside in the

web cam, the stacking software and the image processing software. Many

very good images are taken with mass-market Schmidt Cassegrains for

example, which are often little better than 1/4 wave, and they are not

known for really smooth surfaces and great inherent contrast. I would

put this mirror more or less in the same league, for image fidelity.

This mirror will actually hold its own against many name-brand mirrors

I have seen that were fabricated in the last 20 years. That being the

case, I think he did very well.

 

Obviously Mr With did good work with what he had. There were probably

few or none who would be considered a peer in those days. Russell

Porter was born the same year this mirror was finished, so the amateur

movement or the popularity of telescope making had not yet occurred.

 

You have a real piece of history, Jim. Thanks for letting me look at it.

 

CZ

[giorgosgr]

Και το συμπερασμα; Ενα κατοπτρο με λ/4 δινει μια χαρα ειδωλα! Αρκει να ειναι πραγματι λ/4. Μην πετατε λεφτα για λ/8 και παραπανω μια στις 100 νυχτες θα δειτε (εαν δειτε) την οποια διαφορα!!!

 

Optical Quality of Telescopes. "Sky and Telescope" March 1992

 

"Frankly, I am surprised by the results. Conventional wisdom among amateur astronomers suggests that a 1/ 10 wavefront mirror should easily reveal its superiority over a 1/4. wavefront mirror of the same design. This is not what I observed, and it suggests to that under typical backyard astronomy conditions a 1/4. wavefront optical system is perfectly adequate. On those rare nights of excellent seeing, maybe the 1/10 wavefront mirror would show its stuff, but even then I doubt the difference would be noticed by most observers, except in rigorous side-by-side comparisons on selected test objects. Clearly this exercise suggests that there are huge gains in visual performance as optical quality improves to 1/4. wavefront, and less conspicuous benefits with further improvement.

 

 

 

Προσωπικα νομιζω πως οποιοσδηποτε ερασιτεχνης με λιγη υπομονη μπορει να κατασκευασει ενα κατοπτρο 8" με επαρκως μεγαλο εστιακο λογο, f/7 ας πουμε και πανω, με ακριβεια λ/4.

Ακομα και το απλο τζαμι (soda lime plate glass) της σημερινης εποχης ειναι πολυ ανωτερο απο αυτο του 19 ου αιωνα.

 

Στο ιδιο αρθρο συνεχιζει:

 

The 1/4. wave mirrors intrinsic performance in this test was outstanding compared to that of many other telescopes I have used. To me this says two things. First, a good quality 6" f/8 Newtonian reflector- so seldom seen anymore -is a superb instrument for the casual or intermediate level observer. Second, if manufacturers advertise diffraction limited systems and deliver on their promise, everybody but the connoisseur of costly precision optics should be happy.

 

Ανευ σχολιων!

[aristarchus]

Τα πραγματα ομως αλλαζουνε οταν μιλαμε για φωτογραφηση.

[giorgosgr]

Αντωνη,

αφου στον Δια που ναι φωτογραφια υψηλης αναλυσης βγαζει μια χαρα ενα λ/4 δεν νομιζω να ναι προβλημα σε deep sky.