ASH-HELIX 250 Astrograph
250 mm F/3
large format CCD-optimized
< 4 micron spot size
almost no vignetting
After the ASH Lurie 200, an analog "wet film" astrocamera that has been developed for large format flat film, we now have a camera for the demanding CCD as well as the DSLR astrophotographer. Professional astrophotographers were enthusiastic about the ASH Lurie 200. The concept of a high speed focal ratio together with unprecedented sharpness over a large field of view is very attractive. More and more photographers asked us to make a CCD version of the ASH Lurie 200. Unfortunately this is not possible with the Lurie design since the focus lies within the camera. So to fulfill the demands of the CCD-photographers we had to design a completely new camera. The first task was to design an optical system that would satisfy the following requirements:
A perfectly flat field
Extremely high image quality so that the overall quality is limited by seeing effects - and tracking errors only and not the camera
High speed focal ratio
A large field of view to accommodate the new generation of large format CCD-camera’s
Neglectible vignetting and distortion
Outstanding color correction to fully exploit modern color-filters
To find an optical solution that meets these criteria, we contacted Harrie Rutten of CastoR Optical Design. Mr. Rutten is a well known-optical designer and co-author of “Telescope Optics, a comprehensive manual for amateur astronomers”.
Extensive calculations showed that a full aperture Houghton corrector in combination with a spherical mirror and a field corrector near the focus, would meet our requirements.
The optical system of the Helix 250 has a 250 mm (10”) aperture and a focal length of 750 mm giving a geometrical focal ratio of F/3.
Although it would be possible to bring the position of the focus outside the camera through a hole in the corrector, this is not an option for two reasons. First, this would limit the use of the camera to thin CCD-camera’s. Second, the heat generated by these camera’s would deteriorate the image quality. Instead we have designed a flat diagonal to bring the focus outside the tube, just like a Newtonian reflector.
To obtain a field free of vignetting, a heavily oversized primary mirror and large diagonal is used. The resulting obstruction is 45% linear. For a visual system this is would be too large but it is of absolutely no importance for an astrocamera. The effective focal ratio becomes F/3.4
One might wonder if a simple 250 mm F/3 Cassegrain or similar optical design would meet the requirements. The answer is no, this is not possible. On axis such a system might work but to obtain a fully illuminated field with small enough spot sizes is not possible.
Optmized design for high quality large format CCD and DSLR astrophotography
The Helix 250 is an advanced instrument with large and complex optics. It has been developed for the (semi)-professional astrophographers. The( almost) distortion free, large image field makes it ideal for not only common astrophotography, but also for novae, comet and asteroid detection. Its unprecedented optics will provide stunning images with even the largest possible CCD and DSLR-sensors.
Kaf-11000 CCD chip with 11 million pixels of
9 x 9 micron. The Helix 250 has been designed for the new generation large format CCD-camera's.
This resulted in a camera with a large flat field, unprecedented optical quality and a focal length that satisfies the Nyquist criterion.
The optical constraints for our new camera were very heavy and many calculations were necessary to get small enough spot-sizes across the field. Spot diagrams are used to analyze the image of a star in focus. They are used to measure the quality of the optical system. The smaller the spots, the sharper the image
Of course the questions arises of how small the spots must be. The exposure time for an astrocamera is long so the sharpness of the image will be determined by seeing (a). A second important factor, of course, is tracking (b). We also have the tolerances of the mechanical construction and optical imperfections of the camera (c).
If we assume a perfectly optical system, we can than calculate the effect of (a), (b) and (c) on the image quality:
We assume a very good seeing of only 2”,
a guiding error of 1” and an error of 2” due to mechanical and optical imperfections.
The theoretical deterioration of the image quality is then the square root of 2**2 + 1**1 + 2**2 equals 3 “.
At focus this corresponds to a spot size of 11 micron.
A more realistic estimate of seeing conditions of 4”,
a guiding error of 2” and a mechanical and optical error of 2”,
gives an total error of 4.9”.
This translates into 18 micron at focus.
If we take 6” for the seeing and a 4” tracking error,
the overall error adds up to 7.5” and 27.8 micron at focus.
The Helix 250 has been especially designed for modern large format CCD-camera’s. These camera’s have large sensors and small pixels.
As an example, we take a pixel size of 9x9 micron. According to the Nyquist theorem the optimum spot size must then be 18 micron. If we take the three cases, one can easily see that the seeing and tracking fully determine the optical quality of the image if we succeed in keeping the spot sizes very small.
The spot size of a diffraction limited optical system at F/3 for a wavelength of 500 nm is about 4 micron. It is not possible to get smaller spot sizes due to the wave character of light.
We are very proud to say that Harrie Rutten succeeded in designing the optical system in such a way that the camera is almost diffraction limited across the entire spectral range.
If we return to the three examples we can see the consequences. For simplicity we assume that the spot size is 5 micron for the optical system. To calculate the overall spot sizes we must add this 5 micron. This leads to 12, 18.7 and 28.3 micron for resp, (1), (2) and (3).
From this we may conclude that the Helix 250 is an extremely good camera to use with CCD-sensors with small pixels. Its focal length of 750 mm makes a perfect match between spot size and pixel size. For larger instruments seeing effects, guiding and mechanical errors will determine the image quality while smaller instruments will not be as sharp as the Helix 250.
Using 9 x 9 micron pixels lead to over sampling in larger instruments and under sampling in smaller instruments. With a focal length of 750 mm the Helix 250 matches the Nyquist theorem perfectly.
Focusing an astrocamera is extremely important but often very difficult. Especially for an an F/3 system where a shift of 9 micron results in a 3 micron “unsharpness". With the Helix 250 focusing is simple, accurate and easy to reproduce even after a rough ride in the back of your car. This can only be accomplished by a rigid system and a clever design as we will explain.
Maximum stability is obtained by placing the focus as close to the tube as possible using no moving parts. This also resulted in a smaller diagonal. The distance from the mounting plate to the focus is 60 mm. This is necessary for the optical quality. A spacing adapter is mounted on the CCD-camera with or without filter wheel or on the film cassette. These adapters guarantee the 60 mm distance to the focal plane.
2= modified 6x6 roll film cassette
3= large format CCD camera
4= any other CCD-camera
To focus the camera, only very small displacements are needed. Just like the ASH Lurie 200, focusing is done by turning the micrometer screw at the back of the camera. The micrometer moves the primary mirror. Focusing is completely backlash free and there is no image-shift. The position accuracy is better than 0,01 mm. Instead of a micrometer a small stepper motor can be mounted that is compliant with modern motor-focusing programs.
As can be seen in the above diagram, there is an extremely small difference in focus position for violet and red. If the camera has been focused in green light and this position is fixed, all spot sizes will be smaller than 10 micron for colors between 397 nm and 673 nm. Although this result is already outstanding, it can even be better! Diffraction limited can be achieved by focusing for one particular wavelength. This means that diffraction limited pictures can be taken for H-alpha light! Focus differences are only a few hundreds of a mm and can easily be calibrated with the micrometer or stepper motor.
Modern CCD sensors are as large as or even larger than small film. Of course a high quality instrument like the Helix 250 must have a constant optical quality across the field of 43 mm. Only then can we say that we have a very high quality rich-field camera.
As can be seen in the diagrams, our optical designer has once again succeeded. In practice there will be no quality difference between the corners and the optical axis. Beside that, the focus is perfectly flat. The maximum deviation is only 0.06 mm from 400 nm to 700 nm.
The flat field makes the Helix 250 also very suitable for film. With a modified 6x6 roll film cassette, images with a 52 mm diameter can be taken! Of course, a SLR camera can also be used.
The use of modern RGB filters is used by many photographers to obtain a well balanced color image. These filters normally work in the spectral range from 400 to 700 nm. (see the picture of SBIG).
A lot of efforts have been put in the optical design to minimize the size of the spot for the RGB-filter curves. And we are very proud of the result!
As can be seen in the spot diagram below, the spots of the Helix 250 are all smaller than 10 micron for wavelengths between 397 nm and 673 nm. Although this is already an outstanding result, it can even be better!
Even smaller spot-diagrams can be obtained by refocusing. Refocusing means that for each color-channel (RGB, H-alpha) the focus is slightly adjusted. This results in almost diffraction limited image quality. In the diagram below we show the spot diagrams for 656 nm (H-alpha). As you can see the spots are smaller than 3 micron!
In the table below more diagrams can be found for those who are interested. There are two series of diagrams. One for a field of 42 mm (CCD) and one for a field of 50 mm (roll film).
Vignetting & distortion
Vignetting leads to light drop-off near the corners of the field. Every astrophotographer knows this effect. In the Helix 250 the primary mirror is larger than the entrance pupil of 250 mm. This means that vignetting only starts at 12.5 mm off-axis. At a distance of 21 mm (corner of small film) vignetting is only 4%.
The Helix 250 has almost no distortion. At 10 mm off-axis the distortion is only 0,004% which corresponds to 0,4 micron. At 15 mm the distortion is 0,01 mm which corresponds to 1,5 micron. At the corners of a small film negative distortion is 0,02% and 4 micron. This means that even in the corners distortion is only visible at a sub-pixel scale. This makes the Helix 250 very useful as an astrograph to determine the position of super novae, planetoids etc.
To maintain the optical and mechanical stability, much attention must be paid to the mechanical design. The optical elements must be exactly collimated in our small factory and then permanently locked in the right position so that there is no need to re-adjust or re-collimate.
Furthermore, the focusing system must be accurate and reproducible even during temperature changes. Only then is a astrocamera easy to use in the field. The following diagrams shows the drawing of the camera.
The camera consists of a 4 mm thick Phenol-resin tube and a number of molded and precision machined aluminum rings. CCD- and photo camera's are mounted by means of an adapter and a quick/release bayonet on the camera.
The full aperture corrector consists of two lenses and is mounted is a rigid cell. Each lens has its own cell to minimize internal stress. Three invar rods connect the corrector cell to the primary mirror cell. The thermal expansion of these invar rods is almost zero so the focal plane does not vary with temperature.
The Helix 250 is almost diffraction limited. To fully exploit the optical excellence, a small fan is built into the camera to eliminate temperature differences between the inside of the tube and the open air. Tube currents and mirror seeing can else deteriorate the image quality. The fan has a micro filter to prevent dust from entering the camera.
The primary mirror is made of Astro Sital which has an expansion coefficient of almost zero. Thermal stability is very important for an astrocamera and has been a major design issues for the Helix 250. The result is an extremely stable camera which needs no refocusing what so ever.
A micrometer screw at the back of the camera changes the position of the focus. A position accuracy better than 0,01 mm can be achieved. An auto focus stepper motor system is available which can be used with auto-focus programs. A housing protects the focusing mechanism unit.
HELIX 250 SPECIFICATIONS
Ease of use:
250 F3 Houghton in Newton
configuration with field corrector
F3 nominal, F3.4 effective
265mm Astro Sital
< 10 micron for all colors over the
entire FOV diffraction limited after
color channel focusing
special resin tube and aluminum castings.
not necessary because 3 invar rods
are built inside the optical tube.
moving the primary mirror by a
micrometer (0,01 mm) or motor focus
2 component paint
Special spacing adapter and
quick-release bayonet available
for most CCD-camera's
no, because the system has no spider vanes
no need for collimation or adustment
light weight dew shield with quick release bayonet