TAL's new 8-inch refractor ..

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David Frydman
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Post by David Frydman »

A draw tube eyepiece holder can give fine adjustment if you gently turn it while slightly pulling it out or the other way.

The 120mm f/8.3 David Hinds O.G. scope was also used with a 3mm Clave eyepiece giving 330x, but was usually used at 250x for high magnification on planets and double stars.

David
adrian lord
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Post by adrian lord »

[quote="David Frydman"]Dear Adrian,


So when are you getting your 18 inch refractor?

ANSWER: never, when did I say I was getting one? I can barely lift the 6" with my dodgy back.

I think a cherry picker will make a good observing platform, and you can also use it to prune any obstructing trees.
Perhaps a large windbreak as well.







Regards, David[/i]
TMB/APM 152 apochromatic Refractor is my weapon of choice!
David Frydman
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Post by David Frydman »

Adrian,
I was just joking, sorry.
You mentioned that TMB do refractors up to 18 inches, and I suggested you might like the largest one.

I would also, except I am less able than you.

Regards, David
David Frydman
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Post by David Frydman »

Dear Adrian,

I am still fairly new to computers and had to look up what IIRC meant. I have to do this frequently as these abreviations are new to me.

I sent you a PM.

Regards, David
David Frydman
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Post by David Frydman »

Really sorry if anyone thought I was serious about using an 18 inch TMB refractor in the garden.

The telescope would be close to 30 foot long.
The mount would have to be about 18 to 20 feet high.
The mount would weigh I would guess several tons.
To view objects close to the horizon one would need to be perhaps 15 feet in the air.

If one won the eurolottery top prize, why not.
Build a dome to house it.
I suppose it would all cost about £300,000 at a guess.
If one had a suitable large field away from the lights.

If one had more modest aims it could be made into a folded refractor perhaps 12 foot long and this would need a much more modest dome.
This could be a reasonable project for someone with engineering skills.
It would take a lot of hard work, but could be completed at a more reasonable cost.

Regards, David
adrian lord
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Post by adrian lord »

I am also sorry for reacting a bit sensitively too....the trouble is with internet forums, like emails, the meaning can be ambiguous at times and the line between humour and sarcasm difficult to decipher, and I made the wrong judgement. In fact my first judgement was correct, then I read and re-read the whole thread a few times and.......

I always respect your wealth of knowledge, and dedication to the forum, you are one of the few that really keep it alive and are extremely helpful to others.

Clear Skies!!!

:D
TMB/APM 152 apochromatic Refractor is my weapon of choice!
brian livesey
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Post by brian livesey »

I can imagine you, David, having to climb over piles of stuff in your flat: binoculars, scopes, cameras, optical odds-and-ends and assorted memorabilia. Where do you keep it all? :lol:
brian
brian livesey
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Post by brian livesey »

There's a point where a refractor falls behind in performance compared to a reflector, due to loss of light caused by the increasing thickness of the objective.
I read somewhere that fully a third of the light entering the Yerkes 40-inch refractor, the world's biggest refractor, never reaches the eyepiece.
Would it be fair to say that the TAL 8-incher is about equivalent in light-gathering power to a ten-inch Newtonian?
brian
David Frydman
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Post by David Frydman »

It depends on the size of the Newtonian diagonal and whether the mirrors have enhanced coatings.
In a high class doublet refractor fully multicoated the transmission might be 98%. Say 1% reflection losses and 1% transmission losses. It could be the transmission losses are say 2%, giving 97% transmission.
With star images it depends how much is lost to secondary colour. Perhaps the Strehl amount.
But I would say the 8 inch Tal probably equals the 10inch Newtonian as you say Brian. Especially as the Newtonian will deteriorate rapidly if left out in the open through mirror tarnish.
The more important thing is resolution.
a 135mm long focus Merz refractor consistently outresolved my good Charles Frank 217mm f/6 Newtonian even if on occasion I was able to use 720x on the Newtonian.
Closer double stars were always seen with the smaller observatory refractor.
Fairly accurately a scope with a central obstruction performs at best as if the aperture was the main mirror size less the central obstruction.
For a 200mm system with a 50mm obstruction it resolves at best as a 150mm.
A 200mm with a 65mm obstruction performs as a 135mm, furthermore when the central obstruction is over 25% contrast suffers, and with photographic mirror lenses with 40 to 45% obstruction, contrast is very badly affected.
Then there is the problem of temperature effects which are much worse with Compound systems and fast Newtonians.
At best a long focus planetary Newtonian say f/9 or f/10 with a 20% or smaller obstruction can be excellent.

Regarding the very fine Astrophysics triplets with ex NASA flint glass of extreme characteristics and the scopes claimed to t be 1/56 RMS surface errors.
Yes they are fantastic and regarded as the ultimate.
But how has the glass stood up to 30 years of weathering.
These scopes may have deteriorated if the glass is that exotic.

So a complex subject.
In addition a centrally obstructed system can sometimes outperform if the double star is exactly the right separation as the Airy disc pattern sometimes allows the double stars to fall in the gaps of the Airy disc pattern.

If I was fit it would be nice to have the 8 inch Tal or a 9inch Astrophysics Starfire in the backgarden.
But it won't happen.
When you win the lottery perhaps you will enjoy one of these.

Regards, David
Lady Isabella
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Post by Lady Isabella »

Hi David

Having used a scope with the exotic short-flint glass side by side with a modern Triplet refractor,
I personally would take the more modern scope.
I have not heard off any problems so far with the NASA glass and there are people on the CloudyNight
forum who are still using the scopes with no problems.

Your 120mm f/8.3 David Hinds lens sounds interesting, while I remember David making mirrors
I was un-aware of his lenses. The f/8.3 ratio sounds typical of Chinese made Achromat lenses.
I only say this as I have a Synta made 120mm f/8.3, and it is excellent for an Achromat,
easily taking powers up to 280x on Mars and the moon. Back in the 80s I was using Japanese
made Achromats which were good, however my cheap Chinese scope is every bit as good as
the scopes I had back then. A couple of weeks ago I had the lens out of the cell, it had
a date of manufacturing on the side along with quite a few figures and Chinese writing.

Last week I stripped down a 150mm f/8 Synta lens, and again all the calculations and figures
appeared on the edge of the lens written in pencil. This scope is also excellent on the planets
in spite of the C.A. that is visible. With the lens stopped down the eyepiece views are
just as good as the best Achromats I have used including Zeiss scopes that I used.

I bought several used Synta refractors where the owner's was not pleased by the performance
and hence were selling the scope. However on closer inspection it was found the lens had
been shipped out of collimation, after just 5 minutes work the views were noticeably more sharp
on the moons and planets.
David Frydman
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Post by David Frydman »

Thanks Isabella,
Somebody had 3 David Hinds 120mm O.Gs and a Fraunhofer 105mm f/16 I think, and i bought them all very cheaply.
I had all made up into fine telescopes. By getting four made the price was much less than a one off.
I was warned that coating them could lead to breakage and was at my own risk.
I had two of the 120mms single blue coated and they were fine.
They found various homes. The fraunhofer becoming a solar telescope.
The 1000 mm focal length I think arose because the tools David Hinds had produced that focus.
I don't know how many O.Gs David Hinds made. At least four.
I think he said about 6,000 mirrors when I asked, but I may recall wrongly, so am open to correction on this. I think he said only one had been perfect without afterwork.

I was thinking that the Tal special flint might only take a single coat, so the total transmission might be 95% possibly 96%.

I think some scopes now are made with special glasses part way to ED but may be cheaper than full ED.

It is interesting and encouraging that the Chinese O.G s are hand finished and of high quality with pencil markings. That is how it should be. OVL scopes seem very good.
I have bought scopes where the O.Gs had been disassembled for cleaning and an element put back the wrong way round. The image went from bad to very good.
It is the millions of awful chinese binoculars that bother me. It is a free for all.
By forgetting quality control vast sums are saved. The customer is the quality controller and usually does not have a clue if the binocular is good or bad.
This is like British Leyland in the 1970s.
Quality control was in the hands of the buyer. If the Maxi's engine fell out or my Austin 1800 was starved of oil when going round corners with the sideways engine it was the customer who suffered.
The Finnish agent did a fabulous job of rebuilding the engine, but I was the quality controller.
No wonder British manufacturing almost sunk without trace until the Japanese took over our factories.

I don't have much experience of ED or Fluorites but the few I have tested are very good.
I think if they are well coated and sealed against moisture they are fine, although early Canon 300mm f/2.8 fluorite lenses may be very suspect.

Regards, David
David Frydman
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Post by David Frydman »

I had a Ross 4 inch triplet, perhaps contact triplet collimater lens in a cell.
Uncoated and a rod passing through the edge of the three components right at the edge where a tiny hemisphere has been cut out of each element for the rod.
I think as a telescope I had to turn the whole cell round and test it back to front.
It is perhaps f/15.
There is no trace of false colour whatsoever to my eyes and it is a very fine lens.
Ross evidently were capable of the highest quality work.
It is in a brass cell and I would think possibly 1930s ??

Regards, David
Lady Isabella
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Post by Lady Isabella »

David Frydman wrote:It is interesting and encouraging that the Chinese O.G s are hand finished and of high quality with pencil markings. That is how it should be. OVL scopes seem very good.
David
Yes, I was rather surprised by the level of attention and detail that someone at the Synta factory
had taken with both lenses. I had fully been expecting Synta to simply grind and bang the lenses out,
particularly when you consider the price that the scopes sell for. Maybe I was lucky and got some good ones,
however reading other reviews it would appear that other experienced observers have also been
surprised and pleased by the performance of these scopes. My 150mm scope shows some stunning
sharp detail on the planets.

I have also seen some reviews where the scopes have had noticeable problems with bad under
and over correction, So the quality control could be a bit hit and miss?
David Frydman
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Post by David Frydman »

I have probably tested about 8 OVL or Celestron short focus refractors and all have been good although I don't have any facilities now for testing at high power. These have been 80mm and 102mm f/5.
Also the Acuter 80mm spotter is excellent.

So at least these have at least been hand checked and adjusted.

I am sure some are not so good considering the price.

As you say some may simply be out of collimation.

My original 123mm f/5 Jaegers O.G. scope was very good at 145x in its Jaegers cell. It was cemented and coated and sold as 5 1/8 inch diameter.
About 1975?
At 210x with 3mm Clave it did show signs off being slightly out of collimation, but the scope was wonderful, and probably still is.
It had a 3 inch focusser and took giant WW2 Kodak coated eyepieces unfortunately some examples had thorium, some did not. It was also great with a Barr and Stroud uncoated 3/4 inch eyepiece in 2inch barrel and superb with 8mm RKE eyepiece.
The Swift 4mm, actually 4.3mm unfortunately also contained thorium.

I used the scope extensively every clear night for years on an original Broadhurst Clarkson mahogany tripod from the Starboy 3 inch refractor, with ten inch spacing holes and brass altaz top.

Nowadays one can buy the equivalent OVL scope cheaply by tomorrow's delivery, when it took me a year to order the Jaeger O.G. from New Jersey and have the scope made to my specification, with integral handle and sighting holes in the handle and giant push pull focusser.

Regards, David
brian livesey
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Post by brian livesey »

Regarding lack of quality control on some optical products, this can surely only go on for so long.
The companies that neglect quality control will ultimately lose out their more stringent competitors.
Regarding light loss by the great Yerkes refractor, it might be that the objective has since been given modern coatings to greatly reduce the loss. Even so, it's still a hunk of glass for the light to penetrate.
brian
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