what lense to use

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Re: what lense to use

Postby The Man with the Corrugated Iron Roof » Sun May 15, 2016 9:36 pm

Quite honestly, I've heard about Maksutovs supporting more magnification per inch from more than one source but never heard the reason. Notice also that for refractors they must be APO to support high magnification or have a very long focal length. This is because achromats suffer from false colours. There's often false colour caused by Earth's atmosphere and loads of other factors. Achromats make it worse.
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Re: what lense to use

Postby dave.b » Fri May 20, 2016 8:01 pm

My understanding is that the longer the focal length of an achromatic refractor the less the chromatic aberration presents. Another brenefit, that probably explains this, is that the region of focus increases so no need for a fine focuser.
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Re: what lense to use

Postby Supercooper » Wed May 25, 2016 12:21 pm

Hi,

My Celestron Omni XLT120 is f8.35 and although it is an achromat there is only noticiable 'false colour' on Venus, Jupiter and the Moon. All other objects have excellent images and a very tiny amount of chromatic abberation.

As far as this goes the images in Newtonian reflectors are best for true colour but the relatively low resolution of these instruments ( see this thread above) means that you have to have approximately twice the diameter of the refractor to get the same detail.

Apochromatic three and five element APO telescopes come close to perfection but the cost is prohibitive at about ten times the price of a decent Achromatic Refractor.

IMHO

Barry :o)
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Re: what lense to use

Postby Supercooper » Thu May 26, 2016 12:05 pm

I found this beautifully written article on the Achromatic refractor. Well worth a read:

I hold a candle for achromatic refractors. I grew up with them, learned to see with them, used them faithfully through my twenties and thirties, and now in my early forties, I find them more appealing than ever. Indeed, there is something deeply compelling about well figured, durable, hard glass. That trusted combination of crown & flint served our telescopic forebears with distinction for nearly two centuries. From the balmy tropical climes of Brazil to the frigid wastes of the Russian Steppes, these telescopes have revealed all of the celestial treasures we amateurs keenly seek out today.

My experiences in the field lend me to believe that achromats are desirable in and of themselves. To my eye, they present uniquely beautiful images, faithful renditions of the cosmos that were actually experienced by our telescopic ancestors in ways that the modern apochromat user often fails to appreciate. Achromats are thus singularly qualified ‘time-machines’ that make a palpable connection with the telescopic heroes of the past in ways that are quite simply absent from the ‘garish beauty’ of the apochromatic image. Rare is the amateur who cannot learn a great deal from their colourful images.

But these sentiments are not just born from some sort of fatalistic (some have even said ‘delusional’) nostalgia. You see, despite the now ubiquitous appearance of those snazzy new refractors with low dispersion glass that have become the object of burning desire for so many contemporary amateurs, it pays to remember that not a single discovery has been achieved visually with them. Who can inform me of one object, an atmospheric feature on a distant planet perhaps, a lunar target, double star or nebulous patch maybe, that was not noted by our telescopic ancestors? The answer, of course, is no one can!

That fact alone alerts us to a noble truth. Those myriad dusty tomes from the rich lore of historical astronomical literature reveal that our love of colour pure observing in refractors is merely a cultural phenomenon and, thus can be unlearnt. We have become addicted to colour pure observing, intolerant of even modest amounts of secondary spectrum, and more believing of the bench test than the sky test. Like the Victorian obsession with sugar during the heyday of Empire, too many of us have developed an Über-sweet tooth and consequently find it difficult to imagine life without it. But there was life before sugar, and in many ways, we have been healthier for it. Stripped of much of its natural goodness, sugar is refined in a similar fashion to heroin and is almost certainly responsible for the alarming increase in the incidence of tooth decay in the 20th century, as well as contributing to the development of diabetes and ischaemic heart disease, two of the biggest killers in the contemporary western world.

Old Clothes & Porridge

Now, don’t get me wrong. I am not suggesting that ED refractors are bad for your health. But some of their users create a misleading impression, which exaggerates their powers. You won’t see any more with an apochromat compared with a good, traditional achromat. With patience and practice, your eyes will easily bridge the gap. Nor will you necessarily become a better observer in possessing the former. Expensive, fancy glass will not turn you into an E.E Barnard or a Reverend Dawes. Those Righteous Dudes were made of altogether different stuff to the majority of us.

Neither was the apochromat developed in response to some sort of mass outcry from the amateur community. As I have shown elsewhere, refractors with better colour corrected glass prescriptions were first produced in the late 19th century for photographic purposes and not as a result of the petitions from an army of visual aesthetes. And it is in this application that their sweetness shines through for sure.

Me? I’ve never had much of a sweet tooth, despite experiencing the views through a wide range of doublet and triplet apochromats from 60mm to 150mm. And with these thoughts in mind, I decided to return to old clothes and porridge, as it were, a way of life that would ‘cut my sugar intake’ to see where it would lead me. And that sent me harkening back to the land of the Czars.

I have spoken before of my admiration for Russian achromatic optics. Over the years, I have owned and used no less than five 4-inch f/10 Tal 100R telescopes and have been duly impressed by their consistently high quality. Assembled in the Novosibirsk Instrument Making Plant, Siberia, I have yet to encounter a lemon among them! Furthermore, after traversing the length and breadth of the country, and having being used to death by previous owners in the fickle British climate, they invariably delivered very satisfying views of Sol, Luna and bright planets with astonishing efficiency. And, as for double stars, these 4-inchers proved magnificent!

But there was another Tal refractor I had my eye on for many years. Larger than its 4” sibling, it sported a 5” (125mm) doublet objective with a focal length of 1125mm (f/9) in a uniquely tapered tube. Called the Tal 125R, this telescope was as rare as hens’ teeth in the UK and considered by many to be a modern classic. Owning one proved to be a pipe dream for me, as when ever a sample came up for sale, it was sold on to someone else within minutes or hours. But early in 2012, a unique opportunity presented itself.

Man bearing gifts

A fellow amateur astronomer and keen planetary observer based in the UK, described his experiences with a Tal 125R in a leading online astronomy forum. To cut a long story short, the encounter proved trenchantly disappointing for him. Looking at Jupiter and the Moon, the images were only useable at low power. Shocked by his descriptions, I felt there was something terribly wrong with the instrument. Warts and all though, I crossed the Rubicon and decided to contact him directly, with an offer.

Last year, I acquired a fine old Meade 127ED with great, colour-pure optics. I liked that ‘scope very much and indeed described my experiences with it in a CN review. But as good as that scope was, it didn’t set my mind racing quite as much as that ill-conditioned 5” Russian glass. I suggested we barter the instruments in a straight swap and to my glee, he agreed. So, in early February, my most excellent ED refractor went south and his big crown-flint travelled north of Hadrian’s Wall, to the heart of central Scotland.

To read more go to: http://www.cloudynights.com/page/articl ... omat-r2687

Cheers, Barry
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Re: what lense to use

Postby The Man with the Corrugated Iron Roof » Thu Aug 25, 2016 10:19 pm

Thanks for the theory. I'm guessing that by "refractor" you mean APO or ED. The more common (and cheaper) Achromat is less likely to reveal as much detail. On the other hand, I have seen it written by someone I trust that long focal length achromats are pretty much as good as APOs. I've never tried one, so cannot verify.

What I've managed with my 127mm Maksutov has been discovered from years of experience. They really are great performers.
How can I be one with the universe when we don't know what 96% of it is.

My website: http://www.philippughastronomer.com/

My blog: http://sungazer127mak.blogspot.com/

Photos: https://www.flickr.com/photos/philippughastronomer/
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Re: what lens to use

Postby Supercooper » Wed Sep 07, 2016 10:39 pm

No no no...

Achromatic is all anyone needs. And f8 - f10 is fine, no need for 'long focal lengths'.

Try to forget perfect images and getting a mortgage to own an APO - Waste of time. A complete falacy that an APO will show you more detail. Just a more colour correct image. A good quality Achromatic telescope can be yours for less than a couple of hundred quid, and will provide diffraction limited views for the next century at least. Diffraction limited is the same for all appertures, whether a single lens or five element ED or APO.

If you want to hear it from an expert other than me have a look at the blog of a 40+ observer who willingly swapped his APO for an Achromatic Tal 125R and reckons he's come out waaaaaay on top of the deal... > http://www.cloudynights.com/page/articl ... omat-r2687

Cheers, Barry
Attachments
Helios150f8 (6Small).JPG
Not an APO - Thank goodness!
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Re: what lense to use

Postby dave.b » Sun Dec 25, 2016 11:14 pm

A good quality achromatic doublet of around f7 or greater is fine for visual observing as the image processing your eye will achieve is truly marvelous. But a camera is not as clever or as forgiving. Moreover astro imaging falls broadly into two camps, wide field deep sky, and planetary. Deep sky favours low f ratio scopes and the lower the f ratio the greater the chromatic aberration, even with a doublet design. And this means loss of contrast. B&W LRGB imaging can work around this to some degree by a colour channel alignment processing step, but the luminance channel will by softer due to the lower contrast. Planetary imaging requires a high f ratio, which has the effect of reducing the chromatic aberration and increasing the size of the focus point. That's why it is deep sky imagers who seek triplet APO designs.
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Re: what lense to use

Postby david48 » Fri Jan 06, 2017 6:52 pm

Sathyasheelan wrote:I pointed out that when you use the Barlow lens you also have to use an eyepiece in the Barlow lens or it won't focus.


Actually, that's not quite true. A Barlow lens is essentially, a concave or "negative" lens, which diverts light beams instead of concentrating them to a focus.

But that doesn't mean that a concave lens can't produce an image, by itself, when used in a telescope.

In fact the first telescopes in history, back in the 17th century, used concave lenses as eyepieces. Such an eyepiece was used by Galileo in 1610. It enabled him to focus an image of the lunar mountains and craters. And also - crucially in the history of astronomy - the 4 main satellites of the planet Jupiter. Plus other images, such as the phases of Venus. And a kind of indication of Saturn's rings (though Galileo's small-diameter, low-quality object-glass didn't enable him to fully resolve them).

Galileo's combination of Convex Object Glass + Concave Eyepiece is still used today, in the form of "Field Glasses" or "Opera Glasses". I have some excellent examples in my collection. Especially the Russian 2.5 X 24 and 4 X 30 glasses. These give bright high-definition images. Though the field of view is small - which is the main disadvantage of a "negative" concave eyepiece.

So it would be possible to use a Barlow lens, by itself, in a telescope. If you focus it carefully.

However, you'd get the following results:

1. Low magnification
2. Very restricted field of view - like peering through a keyhole!

Therefore don't try to use the Barlow by itself. Use it with a proper eyepiece.
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Re: what lense to use

Postby Aratus » Sat Jan 07, 2017 1:52 pm

While it is true to say that a Galileo type telescope used a concave lens for an eyepiece, the system works because such an eyepiece intercepts the light rays before they focus. The telescope would have to have a short tube and a focussing unit with a large range for it to work. Most refractors have the focus point inside the tube which would make it difficult to place the barlow where it could reveal an image.

Having said that I once came across a demonstration telescope which was designed to use either concave and convex eyepieces. It was essentially 2 tubes, one inside each other and the eyepiece could be moved to a position before or after the focus point as desired. The difference between the 2 kinds was very obvious.

To be fair, Galileo simply copied the modified Dutch terrestrial telescope which were designed to see upright images of military units. Ironically the original telescopes used concave eyepieces but were discarded as useless because they gave an inverted image. Fortuneately Kepler revisited the issue and concluded that convex eyepieces and long focal lengths would give better astronomical telescopes. Galileo, on the other hand, ignored Kepler's findings and stuck with his 'key hole' telescopes.
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