A barlow lens makes individual light cones narrower, but increases the off axis light beam angles strongly since it magnifies the image which leads to a divergent field of light. So with its 4x magnification the projected field increases 4 times also, being designed to project the advertised 90mm in perfect quality. Although no regular glass combination would allow to design a divergent bundle of light to emerge from the lens groups and still retain the full apochromatic properties of a precision objective lens - such as the APQ-lenses from Carl Zeiss were. It indeed does take two differently radied cemented groups of genuine Calciumfluorite (CaF2) - crystal with equally exotic partner glasses, to retain the full apochromatic properties. There is no other lens design existing having the same complexity.
However - the days of emulsion film are gone since long - and modern chips are much smaller still. For this reason the factor could be reduced even to 2,5x instead of 4x by moving the image plane much nearer towards the exiting lens surface than those ~150 mm required for the 4x factor. In this way, the perfectly flat and apochromatic field would be reduced to only cover a 25 mm image circle with perfection.
When aiming to 2x it can be that the lens surface has to be extremely near to the sensor, so any dry chamber or else could hinder this.
the performance is even much more impressive when it is used visually. Many people call it "worlds sharpest barlow lens" - we don´t disagree.
Also an auxillary optical system cannot correct the missing optical correction of the telescope optics (unless it is specifically designed for this purpose).
So, combining two flatteners will not produce good results - but you don't need another flattener.
Baader FFC – 2" / T-2 Fluorite Flatfield Converter
The CaF2 Fluorite-Flatfield-Converter can be combined with every optical system for photography and visual observations as best and most sophisticated barlow lens in the world
- apochromatic, fully multi-coated lenses (note: w. multicoated Fluorite-lens faces
- Resolution in lines is 10x higher than that of the best projection eyepieces
- 90 mm image circle for medium format cameras
For almost 20 years (as well as 20 years ago already) our customers had asked again and again: "Do you have a barlow lens or projection eyepiece – but with a flat field – which can be used with medium format (6x7 Pentax) cameras for imaging sun, moon and planets, but without the typical loss of sharpness?"
This bothered us back in those times , and finally one of the designers of the Zeiss APQ lenses calculated the ultimate projection system with a flat image field in 1997. To actually make this design become reality, extreme types of glasses were needed. In this case, two multicoated fluorit-lenses - at that time and until today - a nightmare to produce.
The result was an optical system with an unmatched sharpness, with a ten times higher resolution of line pairs than a Zeiss Abbe eyepiece. Even so the days of emulsion film cameras are long gone, this lens system has persisted to be "the sharpest barlow ever made".
By changing the distance to the camera sensor, you can vary the magnification between 3x and 8x. The optimum line resolution is set at 4x magnification, then you have the full sharpness over an image circle of 90 mm diameter. On the axis, the sharpness is only limited by your telescope – even at a magnification factor of 15x. We have tested this at an AstroPhysics lens telescope. Slowly slowly even the large diffraction limited field of 90 mm will become of importance again - with the advent of ultrafast and giant CMOS-cameras covering a field of 70 mm in diameter.
The FFC can be connected onto every T-2 thread and its 2" main body likewise fits into every 2" eyepiece clamp. With the help of our Astro T2™-System and the M 68-System you can connect it onto a whole world of telescopes.
For best results, you need to keep a certain distance to the original focus point. Please take a look at the PDFs in the Dowmload-section for technical information
Customer statement: The FFC on a Carl Zeiss APQ 130 - Januar 2004
When you are observing and photographing planets, the power and quality of each part in the chain is very important. The result is heavily influenced by design, material, polishing quality of the optical surfaces and the utmost in suppression of straylight. Even small optics can then deliver very good results.
About two years ago I had used the Fluorit-Flatfield-Converter by Baader for the first time ans was positively surprised. I could achieve a better contrast and resolution with the Baader FFC than with normal barlow lenses of good quality.
I use the projection system with up to 8x magnification for taking photos of the planets. This is a rather long setup, and the price seems high, too. But it is well worth the money if you take a look at the build-quality and the achievable results. During the Mars-opposition in 2003 I could take some images with the APQ 130 and the FFC. For such a relatively small telescope, I think they are really pretty good!
Dr. Harald Michaelis
How to use CaF2 optical systems. FFC-warning about possible loss of warranty:
The Baader-FFC contains lenses which are made of real Calziumfluorite-crystal. CaF2 has got an impressive refractivity which enables construction of the ultimate lens system that can deliver an incredible amount of sharpness across a very broad spectral range. No other barlow lens design can combine a similar sharpness with such a large image.
Unfortunately, such sophisticated optics are very sensitive to tension caused by temperature differences.
Under no circumstances must CaF2 be exposed to sudden or high changes in temperature. Do not use it for photographing the sun – the fast and high changes in temperature could destroy the Fluorit-lenses. Defects caused by thermal stress can be detected and measured here in house and are explicitly excluded from the warranty.
Also avoid fast changes in temperature during night-observation/imaging. Never use any Fluorite-lenses (especially not in the front lens of a telescope) at -20°C and then all of a sudden put it it into your living room at +20°C or more! In such a case, the resulting severe temperature stress just as well can be detected, when the lenses end up damaged.
Every CaF2 optic needs time to adapt to the surrounding temperature. To do so, for example bring the optical system into the outside in an insulated box and allow it to cool down inside that box over the time of an hour. Keep the box closed but outside. After the imaging session put the FFC back into the same box and allow it to gradually warm up inside that box while bringing it inside the house. Keep it there for at least one hour until you open the box and check that any moisture can dry out.
Please do not use the FFC, if you can not supply the environment and time, to treat this high-end optical system in the way it demands.
|Inner Connection (lens sided)||Thread, T-2 (M42 x 0,75)|
|Outer Connection (lens sided)||Barrel, 2" (50,8mm)|
|Outer Connection (eyepiece/-camera-sided)||Thread, T-2 (M42 x 0,75)|
|Magnification (x)||3x - 8x|
|AR-Coating||High transmission multi-coated (HT-MC)|