Fast/slow Newtonians

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gooseholla
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Fast/slow Newtonians

Post by gooseholla »

Hello,

I am looking at possibly buying 2" eyepieces. However, i've been told that with a 'fast newt' they might start to show the spider. Now, what is a "Fast" or "slow" newtonian, and; will 2" eyepieces work with my Celestron 8 inch Newtonian (1000 F/5)?

Thanks
gary1968
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Post by gary1968 »

I would have thought that 2" EP's would be fine in a F5 scope. I have a F4 Newt and it shows the shadow of the secondary in a 2" 32mm ep....
I had a C8 F5 and never saw anything like that with that scope

Gary
56N / 3.5W
HEQ5 Pro. Skywatcher ED80, William Optics Field Flattener III, Canon 1000d - Imaging Rig. Guided with Orion 8x50 & QHY5.
Control Via Ascom, EQMod, Cartes du Ciel and PHD.
Everything will be OK in the end, if its not OK, its not the end..............
David Frydman
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Post by David Frydman »

Part of the reason the f/4 shows signs of the secondary with a 32mm eyepiece may be that the exit pupil is 8mm with that eyepiece.
With an f/5 it would have an exit pupil of 6.4mm with a 32mm eyepiece.
Few people have eye pupils of 8mm unless extremely dark, and if a fairly bright object is looked at the pupil will contract.

Regards, David
gary1968
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Post by gary1968 »

I think another reason David is that my particular F4 newt has an enlarged secondary so that it fully illuminates apc-s sensor cameras, it is an Orion 8" F4 Astrograph.
it doesn't bother me though as I very seldom look through that scope, coma is very obvious when used visually.

Gary
56N / 3.5W
HEQ5 Pro. Skywatcher ED80, William Optics Field Flattener III, Canon 1000d - Imaging Rig. Guided with Orion 8x50 & QHY5.
Control Via Ascom, EQMod, Cartes du Ciel and PHD.
Everything will be OK in the end, if its not OK, its not the end..............
David Frydman
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Post by David Frydman »

Dear Gary,
If you are using the 32mm eyepiece on the 200mmm f/4 scope, the magnification is 25x and the exit pupil 8mm.
If in fact your eye pupil is 6mm then you are effectively stopping the scope down to 150mm.
If as you say it has a large secondary obstruction it might be 75mm across, i.e. 37 1/2 %.
With the scope stopped down to 150mm this is then a 50% obstruction.
Not only might there be shadowing of the secondary and coma, but the contrast will also be poor.
The secondary obstruction ideally should be 25% or less but SCT are often 30% or so. Mirror camera lenses are often 40%, one of the reasons for poor contrast, although I love them.

The answer with your 200mm f/4 is to use a 25mm well corrected eyepiece that works well at f/4. The magnification at 32x will suit the scope and you eyes better.
I have an old 25mm Erfle that is very fine and in a 1.25 inch barrel.

Regards, David
brian livesey
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Post by brian livesey »

I'd always understood that Erfles are no good for short f. ratios.
brian
David Frydman
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Post by David Frydman »

Mind you if the 200mm scope is stopped to 150mm by your eye pupil the focal length is still 800mm so the scope is then f/5 about, so redressing some of the problem.

Regards, David.
David Frydman
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Post by David Frydman »

Hi Brian,
The famous and very good Soviet 8x30, 12x45 etc use a 5 element I think Erfle of 15.5mm focus. This is an exceptional eyepiece actually pinched from Zeiss.
It has about 68 degree field.
Nearly all binoculars are f/4, so clearly it works at f/4.
However it might also be a Bertele, i.e. Erfles may come in several flavours, so some may be computed for slower systems.

The only problem with most binocular eyepieces is pincushion distortion, but this is not usually significant for astronomy and applies to the outer field.

The 20mm and 25mm astro Erfles I have work well at f/5 as does the 32mm maybe 6 element 'Erfle' in a 2 inch barrel, a lovely eyepiece. All these from the 1970s or 1980s. Good eyepieces from this period are just as good as modern chinese eyepieces although U.S. Televue probably are a bit better.

Using binocular eyepieces is fine although really cheap chinese binoculars are often pretty awful optically all the way through.
If you have or buy a broken good binocular from a charity shop you get eyepieces for almost nothing.

The standard binocular 3 element eyepieces also clearly work at f/4.
They are often of 27mm or 18mm focus depending on the original binocular magnification. A 50mm binocular O.G. is around 200mm focus so a 7x50 has about 27mm. The O.Gs are often nearer 190mm focus.
a 10x50 perhaps 19mm rather than 18mm.
It is fairly easy to measure the O.Gs focal length once you have the front barrel or cell off the broken binocular.
The actual magnification of 7x and 10x might not be very exact.

Regards, David
David Frydman
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Post by David Frydman »

It seems that Erfles were made in 5 or 6 element versions, some designed by Erfle some by associates.
Berteles were in many varieties. I was not aware there is a 4 element Russian version and there are multi element wide angle versions.
Erfle was more a generic term in the 1950s and 1960s as a wide angle eyepiece.
There are enormous numbers of different eyepiece designs.
The initial Nagler was apparently made using only two types of glass whereas modern ones employ several different maybe exotic glasses although i think Televue has an eye on longevity and would not use glasses that might break down.

It is interesting to note that although older designs like Kelner might have employed just one glass type, modern versions of the same eyepiece might be made of better and different glasses to improve performance, and in general eyepieces employ more different or exotic glasses to improve performance.

There are also many slight variations in design, so it is impossible to be certain what an eyepiece actually is.
It is also known that in patents glass types were deliberately different to reality and curves also slightly different to protect the manufacturer from plagiarism.
Also tolerances might not be shown or be correct in patents.

Also there are large numbers of prism designs mainly used in binoculars, some highly complex and difficult to make and perfectly align.

I was not aware what the predominant design of binocular 3 element eyepieces was. Thanks Brian.
Again there are probably many variations.

What really interests me is that the Koehler eyepiece using perhaps 11 elements has a 120 degree field used mainly for military binoculars.
I have heard of old binoculars with 90 and 100 degree fields perhaps of German design and also the original 110 degree eyepiece designed perhaps in the 1960s by Al Nagler, but was not aware of a 120 degree eyepiece.

It seems that Orthoscopics are still valued because of the lack of distortion.
I still use them. In fact I use several older eyepieces of various designs.
The beauty of the really wide field eyepiece is that it is easier to find and follow objects on tripods and mounts that are undriven.
I must admit I was amazed when using an early Nagler perhaps the 7mm that Jupiter and its moons looked the same near the field edge stop to the central view.

The modern pre eminence of Plossls may be a passing thing and in the future a different design might be the most widely used.

regards, David
Lady Isabella
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Post by Lady Isabella »

David Frydman wrote:What really interests me is that the Koehler eyepiece using perhaps 11 elements has a 120 degree field used mainly for military binoculars.
I have heard of old binoculars with 90 and 100 degree fields perhaps of German design and also the original 110 degree eyepiece designed perhaps in the 1960s by Al Nagler, but was not aware of a 120 degree eyepiece.
Hi David

I understand that the very wide eyepieces were designed for use in Submarine periscopes.
I know that during WW11, A. Tronnier while working for the Schneider company was designing and
making 110 degree multi-element eyepieces for German military binoculars.

After the War, Tronnier emigrated to the USA and found work at the Farrand Opical Company,
he continued to develop and design very wide and complex eyepieces for American military binoculars.
In 1957, Al Nagler started working at Farrand, where he came into contact with designers such as Tronnier.
David Frydman
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Post by David Frydman »

Thanks Isabella,
I was aware of most of this but had forgotten the details.
I spent many days and weeks at the Imperial War Museum I think in south London where I was allowed to go through many wartime documents and reports.
They have so much stuff there it would take several lifetimes to go through it all.
I think I read that the 110 degree eyepieces had a lot of distortion of various types.
The pity is that nowadays wide field binoculars are not made.
I have the full range of Russian Chronos wide angle binoculars and although they work and look good in camoflage the mechanics are sloppy. They are probably now difficult to find in the U.K.
Other than that there are few current wide angle binoculars I can think of. I mean at least 7.5 degree fields in 10x50.
If all these ultra wide binoculars were available 50 or more years ago why are there no 9 or 10 degree 10x50s now.
I think there are Dekarems with 11.2 degrees in 8x40 and 13 degree 7x binoculars in vintage models.
The problem is bird watchers need a 6.5 degree field and their standard binocular is 8x42 and a high power is 10x42, so fields of 65 degree are now the extra wide angle.
But astronomers lose out as much wider fields are desirable to pick out meteors, artificial satellites and to find comets.
But the chinese multiple binocular types just don't seem to include these.

From about 1978 to 1988 my main binocular was the Minolta Standard 10x50 with 7.8 degree field. I used this daily but it eventually went out of collimation and I was not able to get it recollimated because of my mobility problems.
I spent at least 15 years searching for a replacement without success. I had seen them on ebay but don't have access to this method of purchase and have been warned that what you see often turns out badly.
Two weeks ago I found a replacement 10x50 Minolta Standard. It is in fine condition.
Surprisingly although my eyes now are not usually happy with 5mm exit pupils these feel fine.
I measured the field angle accurately and it is 7.6 degree with these.
Maybe somebody with eyes able to get closer to the eye lens could get 7.8 degrees or maybe if you move your eyes around you can get this.
But my quest is over.
I still have not found a replacement for my superb selected 20x60 Soviet binocular. I have found several but they are all vastly inferior in ultimate resolution and quality of optics. But anyway they may be a bit heavy for me nowadays.

Regards, David
brian livesey
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Post by brian livesey »

My Russian TENTO's are good optically, but the mechanics are spartan ( a style known as "Soviet Realism"? :lol: ). They don't have the aesthetic lines of, say, Japanese binoculars, but I wouldn't part with them.
TENTOS are probably identical to David's KRONOS glasses.
brian
David Frydman
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Post by David Frydman »

Brian,
The Kronos may include the Russian copy of the Soviet 20x60, but of inferior quality after 1990 about.

The Kronos or Chronos I am talking about are 75 degree apparent fields in 6x30, 7x35, 8x40 and 10x50. In various finishes and possibly various coating options.
As with many binoculars the back part is the same with different length barrels of increasing aperture and increasing focal length.
This method of producing a range of specifications for the least cost is used extensively in binocular manufacture.
You could do it yourself.
If the back end is high quality you could add much longer larger aperture barrels.
This was done for the Moonwatch project looking for Sputniks etc. an American elbow scope perhaps it was about 8x50, I have one somewhere, and there were large numbers made had I think 5 inch aperture long barrels added. These were used by teams of amateurs to track early satellites. Perhaps they were paid for their work?

I do not know of any current 75 degree field binoculars.
There is the 85 degree about Bushnell superwide? but the quality is poor. I think it uses combination mirrors and prisms. The resolution is not good enough, at least the one I tested, for astro serious use.
The Kronos are sloppy mechanically and tiring to use as the eyepieces rock and may change focus if you hold them up to the sky or push your head against them. Optically they are O.K.

If anybody knows of any current manufacture binoculars with 75 degree plus fields please let me know.
I think there are Fuji and Nikon versions but vastly expensive and heavy. I mean sub £200 binoculars.

Regards, David
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