Observatory Saltsjöbaden and its astrograph
This is an article I wrote many years ago, here I have translated it to English. Lot of information I got from the retired astronomer Kerstin Lodén who had worked with this astrograph, sorry to say she had passed away after this article was finsihed. In that time the observatory belongs to Stockholm University and I did my astrophysics studies here.
My interest is obviously astronomy but besides that including: Technology, history, old buildings, photography, travel to mention a few. At Saltsjöbadens observatory there is an old observatory building that I've never been inside, namely the astrograph.
Observatory Saltjöbaden's astrograph building.
I talked to Nippe who was the chairman of the amateur astronomy club STAR about the astrograph, he would let me in this building. Here I got the chance to make a visit inside this building and get a view of how the astronomers did their photography work before the digital technology took over.
With this article I meet all my aforementioned interests, well let to be precisely travel becomes a little bit scanty.
The excursion to Saltsjöbaden:
We decided to visit the observatory in early August 2007. This is in early June and I have time to plan, investigate and collect info material before the visit to the astrograph. In early August, it was time and I took the train out to Saltsjöbaden, we meet at Saltsjöbaden railway station where Nippe was waiting with his car. This day there was heavy dark clouds and maybe rain later. The main reason for my visit to the astrograph is to take photos inside so it didn't matter.
We drive up the steep road to Karlsbaderberget (Karlsbader mountain) where the observatory is located, parking the car at the main building. It is now a construction site when parts of the old observatory is rebuilt to be a school house. We take us ahead building materials and other debris lying in our path towards the backside where astrograph building stands. It's obvious that it's been some years since I studied here. The paths are grassy and trees have grown large, hanging over the walkway like a ceiling.
Now it's not a long way to go and the beautiful building reveals itself. The building was erected in bricks, parts of the ceiling of the copper plate dome seems to have been replaced in recent times with a galvanized plate. Maybe just as well because Nippe was concerned about that some "copper thieves" took parts of the copper roof of an another observatory building.
Karlsbaderberget where the observatory is located is a natural beauty. Maybe a little too much natural beauty, the building will soon disappear in the jungle.
A great moment for me when Nippe unlocks the door to the astrograph.
I will first get to see in here. With the door wide open I get my first look into the most sacred. Thinking quiet as someone once said, "I see wonderful things". Then you first come in to the astrograph's "engine room", what we find there you can read about later in the text. Now we go back in time to find out how the observatory ended up out here in Saltsjöbaden.
Far back in time the observatory was placed at the Observatory Hill at Odenplan, where it had been since 1753. At 1920s they realized that light pollution was too bad to continue to operate here. A decision was taken to move out from central Stockholm to Saltsjöbaden, about 20 km southwest of Odenplan.
Knut and Alice Wallenberg Foundation contributed a great donation, which was an important part of the observatory to come. The inauguration of the new observatory in Saltsjöbadens took place in 1931. The hill where the observatory is located has the name Karlsbaderberget. A big advantage of Saltsjöbaden was of course the proximity to City of Stockholm for the sake of transport. With today demands, however, were never chosen a place like Saltsjöbaden to build an observatory on, not even with the light conditions that prevailed in the 1930s.
The instruments that were at the old observatory at Odenplan was outdated and only a few were moved over to the new observatory at Saltsjöbaden. The instruments were placed in meridians house, which today is called "Radio building", a building that stands today to the club STAR's disposal. The construction of the new observatory was started in the end of 1920s, among the major new instruments that were purchased and buildings erected was a one meter reflecting telescope, one double refractor and the astrograph. Long later in the 1960s they adding a one meter Schmidt telescope.
First entrance in the astrograph building, concrete pillar as the astrograph is anchored in glimpse in the dark.
Do you see such a sign in front of you, you can be sure that it is something expensive!
The astrograph in all its splendor, it's the big white tube at the bottom.
Over the main tube is the finder telescope and to the right one of the many counterweights as necessary to balance the instrument. Notice the beautiful domed ceiling in wood.
The instruments were purchased was considered to be of the highest quality, better could not be purchased! Sweden was recognized as very skilled in research of astronomy area already at the old observatory, now at the new with modern equipment it would be even better. The astrograph was ordered from the renowned German manufacturer Carl Zeiss, they belong even today the elite when it comes to optics design. You've probably seen their name on any modern digital camera's optics. The sum paid for instruments and dome were 137,750 Reichsmark, this was before World War II and the production took place during the years 1928 to 1929. I called up the Riksbank in Sweden to get an idea how much money this would meet today (in 2007). From the information service where I was told that 137,750 Reichsmark today would be equivalent to more than 0.3 million Euro, that money don't give you very much today. This was only for the astrograph, the other instruments with their buildings were probably significantly more expensive.
Luxury, a window, you will find it hardly in a modern observatory.
What is an astrograph?
Yes, one could answer simply, it's a camera. More than that it is not, but it's a camera for astronomical use and with their specific requirements. Astronomers used glass plates instead of film when they photographed not too long ago. Dimensions of a glass plate was big, often 160 x 160 mm but also even larger size could occur, this astrograph used several different sizes depending on what was to be photographed.
Field of view:
An astrograph is different from a telescope, one thing is that it has a larger corrected field of view. While Saltsjöbaden's double refractor with a focal length of 8 meters and mirror telescope of 18 meters compare to the astrograph's only 2 meters focal length. With the size of the glass plates used is obtained a field of view of about 4.5 x 4.5 degrees, this is counted as wide angle in the astronomy world, while in the normal photography world would be considered a telephoto (with respect to angle). As comparison, the moon takes up an angle of about 0.5 degrees and the Andromeda Galaxy about 3 degrees.
A typical simpler refractor telescope has a lens composed of only two lens elements, such lenses are often called doublet. An advantage of few lens elements is in addition to the economic light loss is low and the requirement antiglare lower due to fewer transitions between air / glass. Anti reflection treatment was not so well developed 75 years ago. The disadvantage of the simple two lens elements construction is that control of the lens refraction of light rays becomes low. The lens works only good when the lens f number is high. F-number represents the ratio of the focal length and lens diameter, are also known as opening ratio or brightness here.
The astrograph's glass plate holder, the square hole we see here is approximately 160 x 160 mm.
Double lens construction's limit for acceptable quality usually go at a ratio 1:12 or f/12 for the glass qualities that were to available at that time. For example, the double refractor's (as consists of two telescopes) which each have a double lens, where the relationship is 8000/600 = f/13. For an astrograph aims as mentioned earlier after a large field of view, the lens must have a flat focus field over the entire image area. In addition, they have low f number to reduce the exposure time and then have the ability to detect faint objects.
Here is the complicated construction with counterweights clear and the time scale to R.A. axis and again, admire the dome!
Some optical data Carl Zeiss have set for Saltsjöbaden's astrograph.
With this data, the opening ratio (brightness) is calculated to 2000 / 400 returns f/5. If the lens diameter "only" had been 200 mm it had gave f/10. The requirement exposure time increases with the square of opening ratio, the difference will be that a 30 minutes exposure at f/5 must be extended to 2 hours at f/10, in reality much worse. The difference between being viable or not.
To build a lens with the angle and big diameter lens in this dimension is very complicated and extremely expensive. This astrograph is a refractor construction, i.e. the use of lenses to refract the light. The lens is basically a triplet structure but where the outermost lens is split into two positive lens elements. The reason is that a single lens had given very sharp radii of curvature of the optical surfaces, with two lenses are the respective radii significantly less. As a bonus the optical-designer gets an another lens elements to use to the correction of light refraction through the lens system. A refractor in this the size of four lens elements is impressive. Now someone might say that his / her camera lens indeed has 12 lens elements, but it is quite another thing to mass produce small lenses to ordinary cameras, compared with this handmade mammoth. Modern cameras also usually have a zoom lens requiring more lens elements to handle the more complex optical design.
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To get an idea of the astrograph's optical performance, we can compare with my own camera, digital SLR camera manufactured by Canon, model 350D and a planned 200 mm f/5 lens. The sensor in this camera corresponding glass plate (film) has a dimensions 22x15mm comparing with this astrograph's 160x160mm. With Canon camera mounted on astrograph's lenses were each picture area just one hundredth of the visual field area astrograph takes into a single image! One can also imagine that the Canon camera mount a lens with 1/10 of the focal length of astrograph. Since the sensor in the Canon camera close to a tenth of the film (the side), the same angle obtained. A lens with a 200mm focal length and data brightness f/5 is nothing extremely objective. Why now not so, it would have been much cheaper? The answer lies in a lens theoretical resolution determined by objective lens diameter. It sets a maximum limit the resolution no matter how good the optical construction are. According to Carl Zeiss data, astrograph resolution 0.3 '' ( '' means arc seconds and 1'' is 1/3600 part of one degree), which is close to or equal to the theoretical resolution (the there are many different methods to calculate this on, among other things, affects the wavelength of light, commonly referred to wavelength 550 nm, yellow light). A lens at 200mm focal length and f/5 have a lens diameter of 40 mm and is thus at best a resolution of 3''. It shall also be mentioned, the seeing of atmosphere is almost never better than 2" in longtime exposure photos.
It is not just to cover as big image area as possible as glass plates are out of this impressive size. Limitations of contemporary photographic emulsions made it advantageous to work in large format. A glass plate's resolution can be estimated at 50 lp / mm, but can be considerably higher, lp stands for line pairs, a black line followed of a white line. A primitive attempt to convert this to today's popular expressions "megapixel" is to figure as it corresponds to 100 pixels per millimeter, which gives 100x160x100x160 = 256000000, thus 256Mp, not bad, and this was 75 years ago!
My Canon's CMOS sensor has 8 million individual sensors (pixels), however, interact more individual sensors to form a "color sensor point". Feels pretty paltry in this comparison! Atmospheric seeing puts unfortunately a practical limit for how high the total angular resolution can be. My own estimate of the resolution obtained is that with the exposure times required with film in the 1930s it was not so much better than 2'' in practice, normally much worse. Later I will try to get a photographic plate taken with this astrograph. Imagine what exciting it would be to take a similar picture with current technology and compare!
On the astrograph is also mounted a finder telescope, also this of refractor construction with a focal length of 2000 mm. Lens diameter is more manageable 200 mm. More on this finder telescope function later.
This astrograph is old and was taken out of service during the latter part of the 1950s. I thought here was no active astronomer today who can tell me how all the parts of the work was done at this astrograph. What if I had interviewed been someone who worked at that astrograph. Imagine my surprise when I came in contact with the now retired astronomer Kerstin Lodén, she has written down a lot of information to this article about how it was to work with the astrograph in that time.
Here initiates Kerstin Lodén by talking about herself as an astronomer:
"I am an associate professor of astronomy, but began early as student at the observatory in Saltsjöbaden. It was in mainly when I worked with the astrograph. Later I became what was then called amanuensis in 1957. During this time I was spent also two years at the sun observatory in Anacapri (predecessor to La Palma Station). After this I was left in Saltsjöbaden to 1993 with a break for stays in South Africa and the United States."
In the following paragraphs of tracking and glass plates are some parts more generally described. My understanding is however, that in some parts it should be in pretty good overview how it was to use this particular astrograph also, Kerstin supplements with information.
Tracking and motor operation:
When you doing astrophotography and takes images of faint objects (not the moon, the sun and planets), it requires very long exposure time particular in that days insensitive glass plates.
Kerstin Lodén says:
"Exposure times due course, on the bright stars the program was. To get down to B = 14 m, which was about as far down to the faint stars that telescope allowed, exposed at most 20 minutes. Though must also exposing the default stars in each panel (more on this later). For spectral plates was the longest exposure time I used 60 minutes. It could therefore be quite a few plates in one night."
To make long term exposures possible the camera (astrograph) have to be mounted on a swivel device so the rotation of the earth can be counteracted.
It is this small motor that driving the telescope of several tons! To function correctly it requires that all equipment are carefully balanced. The force transmitted from motor to telescope is through the black vertical axis in the background.
Here is where the driving motor shaft come up, underneath and in the middle of the image. It drives the worm gear (you see part of it) to R.A. (Right Ascension) axis. It does one revolution in 23 hours and 56 minutes (but the scale is 24h) to counter compensate the Earth's rotation. It is a "star day" taking this time, 4 minutes shorter than our normal "sun day". Has to do with the Earth's motion around the sun.
The counterweight on Dec. axis. All these counterweights must be perfect balanced if the motor drive control shall have its precision maintained.
This precision mechanics have also been designed and built by Carl Zeiss. The engine propelling the assembly can be synchronized by a central clock. Such construction is at a constant rate, however, if you seeks high resolution images is not even this enough. Metal construction (assembly), which holding the astrograph is sensitive to temperature variations, they deform and thus give "pointing" camera error when the night's chill affect the assembly. Yet another thing complicates the whole, the atmosphere refracts light different for different elevation angles. Taking exposures of several hours (which is not made here) it will object to be photographed go from low to high elevation angles and down again and thus affect the engine speed necessary rotational compensation.
Even the second axis, Dec. (Declination), need to be fine-tuned during the exposure. This shaft has no motor, the adjustment is made entirely by hand. Dec. axis is perpendicular to R.A. axis. Here is the exchange and lever of this riveting, it is mounted on the tube.
Object tracking, finder telescope:
In the 1930s, there were no computers or electronic cameras, the speed of the motor was refined manually. It is precisely for this aforementioned finder telescope was used to. Kerstin Lodén told me that this astrograph were controlled entirely by hand.
To mention just some of what the astronomer had to put up with. The observations made in the dark, thus, in winter and at night. Local heating of the air providing atmospheric concern and thus blurred images, that means you do not have the observatory heated in the areas where the astrograph stands. In this cool now had astronomer for maybe up to one hour to sit and stare in the finder telescope and precision control the drive's motor, constantly vigilant upcoming deviations.
To help in this the finder telescope's had a crosshair where you tried to keep a reference star centered. Professor Gösta Gahm at Stockholm Observatory have told me about observations on other instruments in the 1960s how he used an electric heated flight suit!
On top on the astrograph sits a finder telescope mounted.
It is at this upper telescope tube astronomer sat and peered in to precision control the electric R.A. motor manually and sometimes small Declination corrections. The astrograph must always be precisely targeted to a reference star, otherwise, blurred stars!
Here we see three of the counterweights to R.A. axis and a complex system of shafts and struts.
Here is the floor from the bottom of the triangle frames are attached to the lifting devices. What we look in the middle is the cast foundation to the astrograph. The sign informs that max 6 people may stay on this floor.
For a long time to follow the reference star you need to sit at a comfortable height. The astrograph's height where the photo plate and finder telescope are varies greatly with which angle upwards it faces. It has been solved so that the dome floor is built as a lift.
The instrument is attached to the a concrete foundation through this elevator floor so that vibrations from the floor is not passed over to the instrument when observer and other staff walk on this floor. Here are some pictures that describes this competitive construction. A similar construction also the great double refractor has.
The engine and the special transmission. On the engine output shaft can also be see an electro mechanical brake.
The construction is very strange, a motor driving a gearbox where there are three axes that exit. The axis go to respective legs that hold the platform above (floor), it drilled a hole straight through the concrete foundation for one axis. From each of these three axes based chain that operates a telescopic device in turn lifts the platform. To reduce the load is also a counterweight connected to a wire system.
The counterweight with its wire and pulley.
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Dome shutters pulled to the side of a rail when they shall be opened.
Dome of the astrograph building has an important function, to protect against the weather elements. It must also being rotatable because the opening should always be in the middle where the astrograph is pointing. Compared to modern domes this an orgy of wooden handy craft.
With this dial you operate the dome shutters. To the left of the other side of the opening (out of picture) is another knob for turning dome.
"The astronomer alone had to keep track of even that the dome was in the right position with its opening. The gap must be all the time standing in the middle of the lens, and the lens moves slowly throughout the time so it came to be vigilant on this. Imagine the embarrassing situation that could happen to expose the inside of the dome!"
Glass plates (film):
An astrograph gives no high-quality images unless the whole chain is optimized. A very important link in this chain is the glass plates. Glass plates photosensitive surface, the emulsion is about chemistry in the highest degree. To get the maximum performance astronomers had to prepare the glass plates, exactly how much of the herein disclosed technology utilized here at the observatory in Saltsjöbaden I don't know.
Kerstin Lodén tells:
"The glass plates were bought finished (Eastman Kodak or Agfa). The format was 16 x 16 cm for the so-called direct plates. It was in the 1940's and 50's in the B (blue-sensitive), or V (visual, ie more yellow sensitive). Of the astrograph's view of field of 4.5 degrees, we utilized a field of 1.5 x 1.5 degrees to the direct plates we used. There was also a different size, 18 x 24 cm used for spectral. At such observation was added a prism lens to the telescope in front of the lens. This way we had spectra, while not detailed but many stars at once. The prism was put on the daytime and had to sit at a time so you had to plan your observations then. In some instances we screened of the lens opening from 40 cm to 20 cm."
A very important feature of the glass plates, or rather, photo emulsion that it is coated with is its sensitivity to detect the photons (light).
Nippe explains that this certainly is not a simple lens hood, it is a dew mantle!
An English expression to make photo plates more photosensitive.
Several techniques used:
Gas atmosphere, the glass plate placed in a sealed chamber and exposed for a gaseous atmosphere in few hours to days. Nitrogen and hydrogen were common gases, hydrogen is highly explosive!
Chilling, glass plate was cooled down during exposure.
After glass plates had undergone these processes could storage time may be as short as a couple of hours. Then in other words, they must be exposed directly, in some extreme cases "gassed" during ongoing exposure. The first two methods performed before the exposure while cooling was performed during actual exposure. Emulsion sensitivity could be increased by a factor of 2 to 25 times with these methods.
The English expression reciprocity, a very negative characteristic, namely, that the sensitivity decreases with exposure length. Variation may be so large that only a 1/20 of the sensitivity obtained at an exposure of one hour relatively one minute. One way to reduce this was too cooling the film / glass plate. This was done practically so that on the camera was placed a piece "Dry ice", dry ice is carbon dioxide in solid. When dry ice "melts" it cools the glass plate. Carbon dioxide melts at -55oC and that is the temperature of the glass plate it cooled to.
Printing and storage:
The latter developing process was obviously also a very important step in the process to develop a picture. The handling of the film and glass plates required great care, they were very sensitive to dust, scratches, damaged by moisture and inappropriate temperatures.
What research was done with this astrograph?
Kerstin Lodén tells:
"One important area was to determine the position of various stars, it was used as a basis for statistical calculations. Measurement of light intensities and spectral properties was another important area. For the astrograph there was an important accessory, a prism. The prism was low refracting and covered the entire front lens, this could perhaps made up to a thousand parallel spectra taken of stars in a single exposure."
Kerstin Lodén tells:
"However, that usually it was more in hundreds of spectra at the observations made outside the Milky Way densest areas. With this accessories the astrograph became a spectrograph. Spectrogram of a star was a modern method in 1930 to determine which class the star belonged to."
In the basement we also found a wooden box with an exciting accessory. There was a heavy metal frame with metal wires stretched in, dimension was such that it seemed be mounted over the 40 cm lens. What function was neither I nor Nippe to figure out.
Kerstin Lodén says:
"The mysterious wooden box, I remember, on closer reflection. Perhaps it was already there with the delivery of the astrograph? To the best I know it has at least not been used."
Who has worked on this astrograph?
Kerstin Lodén says:
"This instrument was mainly used for determination of the distribution of stars in the Milky Way. The astrograph was involved from the beginning of the observatory existence. Bertil Lindblad, the first director, had developed a method of so called short spectra, i.e. not very detailed ones, determine stellar absolute magnitudes. By also measuring the same stellar apparent magnitudes we could get stars distances.
There was a method which was suitable for statistical use:
In addition, Gösta Gahm on Astronomical Department told that Bertil Lindblad was probably engaged in the purchase of the astrograph. Another astronomer, Per-Olof Lindblad's research work was to determine the Milky Way's structure, Per-Olof is otherwise son of Bertil.
At last Kerstin Lodén tells about the exciting work at the astrograph:
"The telescope was pretty handy, not so slowly turning as the big double refractor. But the dome and the knob to rotate the the dome was tough when it became cold and severe cold, occasionally put a stop to my observations. As to the photographic plates. In order to determining magnitudes must also be the same pre photographed plate having a sequence of stars whose magnitudes were known, reference stars.
In the processing done later in a photometer at daytime compared new stars with reference stars. Such a sequence was the area around the North Star where some astronomers had previously determined magnitudes for a number of stars of different brightness. We must therefore when you had finished the exposure of its interesting field turning the telescope up against Polaris, and - on the same plate - exposing this area. After one usually short exposure moved the telescope slightly in declination and exposed again to yield two images of Polaris Stars. This is to be to distinguish the stars to: You then got double images of reference stars. North Star self that is so bright was of course a large light circle that could not be used as a reference.
I remember the first time I did this I discovered that the North Star is double. On each plate had thus three exposures. It could happen in rich star field that the stars interfered with each other. On spectral plates needed no photographed default stars. There, the spectra of stars small rectangles. It sometimes happen that some stellar spectra fell on each other more or less. Then it was necessary to turn the prism 90 degrees, in the daytime ofcourse, and it took a new plate with another dispersions direction.
Spectral plates, which had a larger size, was mounted in cartridges that was very heavy and cumbersome and was difficult to handle in the dark. Day after when woke up everything would be developed. It was important that during the observation time had noticed all the plates (what was up and down plate!) and that it had been wrote down correctly in the observation book over all photographed fields, exposure times and any cloud disruption. On good nights, several instruments was running and it was crowded in dark rooms after the session. The result of reductions of all observations in this program, covered many fields appropriately distributed across the sky, eventually led to an understanding of how the number of faint stars are distributed if you go up or down from the Milky Way's plane. They then got a confirmation on the assumption that our galaxy is a disc where the stars is concentrated in certain areas. A similar programs in the southern sky were carried out by J.M. Ramberg and by me on Boyd Observatory South Africa in the 1950s. There were instruments that was suitable. After a while more confirmations collects on how our galaxy looks, it was found by radio observations of the spiral arms for example. And you could perform observations smoother with photoelectric photometry."
This dial seems to have had an important function, we managed however, not clear out what. Kerstin think it had with the focus to do, perhaps some of our readers know?
How it ended?
Astrographs of refractor type came more and more replaced by the "fast" Schmidt telescopes. The same here, in the mid-1960s it was purchased to Saltsjöbaden just such a telescope. A Schmidt telescope is essentially a mirror telescope and has only one lens element which having a weak correction. Schmidt construction reduces the problem that refractor has with its lens design, namely that the lenses refract different wavelengths (colors) in different ways. Surely one of the major problems with this astrograph I told you about. The Schmidt telescope did not last long, light conditions that existed here made it unusable. Already after some years it was realized that the observatory must once again move to a darker place, this time to La Palma out in the Atlantic Ocean.
Kerstin Lodén says (astrograph):
"In the latter part of the 1900s it was used as far I know as a instrument of views, the finder telescope was suitable for it."
A last look up at the astrograph before we close the doors and leave the building. An exciting day!
Thanks Kerstin Lodén associate professor of astronomy and Professor Gösta Gahm for all material and stories obtained for making this article possible. Also thank to Nils-Erik (Nippe) for his efforts together with members of STAR to preserve this amazing instrument.
Here I have found an article that Kerstin and her husband Lars Olof had written about the solar eclipse 1954 in Sweden:
I was sadly told that Kerstin is not here with us any longer, she passed away at the age of 80 in 2009. We are infinitely grateful that we can have some of your memories of your work with the astrograph, thanks Kerstin! I hope you have found a place up there among the stars.
I only had contact with Kerstin by telephone and email, but our future plan was that we should have meet each other at Stockholm University. There we should had tried to find a glass plate in the archives from her work with the astrograph and scanned it and compare it with todays technology.
(click on the image and you get a high resolution image in a new window)
A panorama of the astrograph.
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Observatory Saltsjöbaden's astrograph
Saltsjöbaden's astrograph (video):
This impressive astrograph was built of Carl Zeiss Jena in the 1930s. If you haven't been there and live in Stockholm, Sweden, visit it! When video has started you can if you want switch to full-screen and HD resolution.
A look inside a observatory building