On this page, you can find
The Mark I system, which was based on a FAX-scanner 1-dimensional detector and a 50-mm camera lens, had a field of view of about 30 degrees wide and a pixel size of 56 arcseconds. Shown below is a tiny slice of one of the first scans of the Mark II system, which has the same sky coverage and pixel scale, taken by Tom Droege. The exposure time is 5 seconds per pixel. The entire slice, about 30 by 1.5 degrees. It's rather wide, so you might need to re-size your browser to see it all. I've taken the liberty of performing a rough version of flatfielding on this image, but it really hasn't been processed properly.
In the image above, North is to the left, East is down. The bright star just left of center is eta (=45) Persei, the slightly fainter star near center (with pair of fainter yet stars to its upper-right) is xi (=46) Persei, and the bright star at the upper edge, right of center, is zeta (=44) Persei.
You can click here for a closeup view of a small section of the image, just left of the center, which shows eta Persei on the left and xi Persei on the right. About 1/3 the way from eta to xi is a pair of stars, the lower of which is on the very edge of the chip; the upper star has mag V=6.3. About halfway from this star back to eta is a fainter star, with V=8.9. The faintest stars which can be glimpsed above the noise are about tenth magnitude.
See the TASS Software section for a program that converts Mark I images into FITS format.
Tom has made available another set of data files from the Mark II system. You can retrieve the original files via anonmous FTP from storm.fnal.gov. I have written another ANSI C program to convert the data from the native format (which is different from that of the previous data Tom released ... this new stuff is much simpler) into FITS. Here's the new source code to the translation program. (which as of 9/18/1995 writes correctly-justified numbers into the FITS headers).
The little picture of M31 above is a subsection of a larger frame. Here are the images themselves: I have cleaned them up a bit with very rudimentary bias-subraction and flatfielding. You can grab each of the four files, reduced by my simple-minded procedure, as GIF images by clicking below:
I am off and running to build the cameras. Yesterday I took a stack of drawings to my favorite shop and asked for a quote. If it comes in under $5000.00 I will just give them the job. If it is over that estimate, I have a few more places that I like, and will try them. The quote is for 30 sets of parts. The good news is that the shop was not busy. I lucked out again and shops are looking for work when I need it.
I have tried one of the $19.00 lenses and I can't tell the difference from one that cost $89.00. Beautiful, brand new camera lenses for $19! Nobody wants manual focus lenses anymore. We will give them a home. Since I will only be using about 0.2" by 0.3" in the center of a lens that was designed to produce a 35mm frame, it is hoped that the focus will be good enough. Comments on this assumption are welcome.
I have started to design the electronics. Compared to the Sony chip, the Kodak chip is a pain in the neck. Many different voltages and pulse levels are required. But I have a scheme that should allow using either the Sony or the Kodak chip with the same PC board.
I am not very organized, and except for a couple of you where I am in constant correspondence, I may not remember that you have shown interest. If you are really interested in running one of these things, make sure you stay in contact.
My goal is still to be running the first unit sometime in September.
Everything is being designed with fairly heavy aluminum plate. It will all be anodized black. I will use cap screws for assembly, so I hope it will look like it came out of a military R&D shop. I used to work in one.
The cameras are packaged in a metal box, 15" long by 5" wide by 4" high at the center section. The two side sections slope down the length to provide the +/- 15 degree in RA angle for the outside cameras. Sticking out of the top of the box are three aluminum tubes. These stick up 3 1/8" above the top. The center tube is flat with the box, the outside two are at a +/- 15 degree angle to the central tube. A flat plate screws to the end of the tubes to which a lens is glued. Lenses can be changed by unscrewing the plate. When the lenses are mounted, the whole assembly will be 15" long by 5" wide by about 12" tall. Should weigh only 10# or so.
Each camera tube is pivoted at the center. A screw arrangement allows rotating the tube about +/- 10 degrees with about a 1 milliradian adjustment capability. Clamps are designed to hold things in place once they are lined up.
The tube is mounted on a water cooled 3" by 3" block that is 1/2" thick. Three holes are drilled through the block which are threaded for 1/8 NPT fittings. I estimate that this will have a thermal resistance of 0.1 C per watt with modest cooling water flow.
The tube is designed to hold a fair vacuum. The primary purpose is to keep water out of the system. The tube is pinned and glued to the block. At the lens end, a step on the inside of the tube holds a window plate. An "O" ring seals the window plate which is held in place by the vacuum. The plate has an opening for a 1" diameter optical window.
A large TEC will be glued directly to the water cooled plate. A "T" shaped cold finger is pinned to the block for angular location. At the small end of the T, a second TEC is glued to the cold finger and to the CCD. The large TEC will be driven from a constant voltage supply and will provide most of the cooling. The second stage TEC will be driven from an operational amplifier in a control circuit that is driven from a DAC. We expect to be able to cool -40 C below the cooling water temperature, and to hold the temperature constant to 0.01. Only the center section will be controlled. The other two TECs will be connected in series. This will probably be OK, but I am still thinking about it. The basic idea is to avoid the 2-3 C drift that is expected as the assembly cools down through the night and the cooling water temperature wanders around.
A space is provided in the tube for a desiccant. A vacuum fitting and a hand vacuum pump will allow holding about -27 in hg vacuum.
As you may note from the mention of glue above, the device will be servicable with difficulty once it is assembled. The idea is to build the device cheaply enough so that the whole assembly can be thrown away on a major failure (like the CCD chip).
There are three printed circuit boards in the design.
The CCD chip will be soldered into a small thin board. This scheme allows making contact to the chip with very fine copper leads (the printed circuit traces). By supporting them on a substrate, one can easily make conductors that are 0.005" wide by 0.0005" thick by standard printed circuit techniques. This is roughly a #45 conductor. The epoxy fiberglass circuit board has about 1/600 of the thermal conductivity of copper, but there is a lot more of it. For the proposed design, the two conductivities will be similar, so thermal resistance will be similar to a #42 conductor. This printed circuit board provides a nice place for the pin interconnections an circuit bypass capacitors. This reduces to a minimum the number of leads that need to be brought from the CCD chip. Leads will be brought from the circuit board to a vacuum tight feedthrough connector in the water cooled block. A mating connector then allows very short leads to the printed circuit board in the 15" by 5" metal box.
Inside the metal box, and just below the camera tubes is a long printed circuit board that contains the analog measurement circuitry. A single 16 bit ADC will be multiplexed to read out the 3 KAF-0400 CCD chips. This board will also contain the level translation circuitry required for the Kodak chip and the temperature control circuits for the second stage TEC temperature control.
The electronics in the box, including the second stage TEC but not including the first stage TEC, will be powered from the supply in the controlling PC. The first stage TECs in a triplet will require a 12 volt, 8 amp unregulated power supply.
A 25 pin DB25 cable will connect the camera to the PC control card. This was picked because it is a readily available cable in any PC supply shop. I hate making cables.
The only electrical connections will be the cable between the camera box and the PC, and wiring from the bulk supply to binding posts on the camera box for the TEC.
A card in the PC will contain the digital control registers and a DAC adjustable voltage controlled oscillator. We expect to be able to select and control a horizontal line shift to one part in 2000. This should be sufficient to keep a star image within one pixel for both the KAF-0400 and the KAF-1600.
The digital control and interlock scheme for the data collection process will be the subject of another note. We hope all you programmers will read it carefully.
The objective of the Amateur Sky Survey is to design hardware which will allow amateurs to participate in a global sky survey. If the cost can be kept low enough, I will just "loan out" camera sets to those willing to operate them. The original goal was to search for moving/changing objects down to Mag 15. We are now trying to do a little better, as the present design may reach mag 16-17.
To save the cost of a tracking mount, the telescopes will be fixed. They will be operated in TDI (time delay integration) mode where the CCD colums are lined up East-West. The column cells are clocked so that the charge generated by a star is moved with the star as it traverses the column.
The basic element of this design is a cell which consists of a camera lens, a vacuum enclosure, a thermoelectric cooler and a CCD. Cells will be designed into triplets which will share electronics and a computer interface. The triplet is designed to take three measurements of the same piece of sky spaced by an hour in R.A. Using a KAF-0400 and a 135 mm camera lens, each triplet will cover 3 degrees of sky. The present plan is to cover the sky in lanes to reduce the cost. Possibly 3 degree coverage with a 3 degree space. Moving objects would typically take several days to cross a lane.
Tom Droege
Well, you all have beat me up enough. I went out in the backyard, and by cutting some limbs off a few low trees I can get to 0 declination looking up over the house. The problem is that I have 100+ feet mature Oak trees to the south of the house. The whole setup was carefully planned to be energy efficient. Most of the house windows are on the south side were they look into the trees in the summer and see the sun in the winter. Not an optimum setup for astronomy though. I drug the LX-200 out on the balcony off my bedroom where I would prefer to operate. The best I can do (using the LX-200 in transit mode) is about 12N. I would not want to try to operate below 15N as one of the trees might grow a leaf.
We should thus plan to start somewhere near 0 declination and worry about the other problems when we want more sky coverage. I am slowly coming to the conclusion that the best science is to be had by pointing all the cameras to the same strip of sky and measuring variable stars. This would appear to be immediately productive and thus satisfying. By getting many measurements a night we should be able to spot short period variable stars that are otherwise hard to find.
I am making good progress on the printed circuit board design and should be out for film near the end of the week. One of you has kindly offered to make the pc boards. I would be delighted to give you credit, but I hold such things confidential until I am told it is OK. Another person has ordered filters for the prototype and plans to order filters for production units. So you all can see we are beginning to pick up support. Not to undervalue all the good thinking about software design that has taken place here, and the analysis work done on the Mark II data.
We should have everything ready to send out the package for the printed circuits in a week. It will take about two weeks to debug and get the system running after we get the boards.
Meanwhile, my house is slowly filling up with electronic parts, mechanical parts, plastic tubing and fittings, lenses and other such stuff.
I plan to make a single unit first, and to put the V filter ordered in it. I expect I will run this unit about a month to make sure that it does not have any weak points. I will then ship this unit out to a volunteer that wants to develop real time data acquisition and display software. Best guess is 1 November to ship this unit out.
Meanwhile I will be building a triplet for me. This one should be ready by the time I ship the prototype. Then I will start working on the next triplet. Someone out there should have a nice Christmas present. My history at Fermilab is that "my" unit of stuff I design always ends up being given away to someone that has a greater need for it. I usually consider that a mark of a successful design.
I have thought a lot about how to do this - give away cameras. It is not easy to give things away. Normal human beings tend to get nasty when there is free stuff. Just watch people scramble for foul balls at a ball park, or for free trinkets at Mardi Gras. So I like to think of it more as selling to the highest bidder. In this case this means who does the most work or who offers to do the most work. I already am forming ideas as to who this might be. But it is very hard to predict the productive workers by who writes the best e-mail. I expect there will be quite a few mistakes made. That is OK. To help this process along, I will probably make a few single units first. Then upgrade them to triplets for the most productive operators. It is also probable that the best operators will not be the best software writers, data analyzers or data librarians.
I hope you all will help me do this. I would rather the group decided who got the first camera than me. But i will do it if the group does not do it for me. My personal bias is to ship cameras first to those who have done work of one sort or another. Next comes those that contribute materials to help in the camera construction.
Tom Droege
The whole process of puting cameras together has started.
This weekend I went down into the basement to try out my new "glue". This is the thermal conducting epoxy that I am using to bond the TEC. While the stuff is black, and probably has a filler, it is also very thin epoxy. I suspect they gain the most from just getting a very thin glue joint.
After assembly, I tested the cooler assembly.
Total Delta Delta t/ Power T - C watt 9.9 40 4.0 13.5 44 3.2 25.7 47 1.8 36.3 47 1.3
Delta t is relative to the cooling water temperature. This is a two stage cooler with a big block of aluminum between stage 1 and stage 2. Not the best way to do it, but for other reasons I need a space between the cold plate and the CCD.
These are pretty crude tests so far. It is not sealed up and dry, nor is it under vacuum. As a result of the moisture in the basement, everything grows a fur of ice crystals as soon as it gets below freezing. This does not make for good measurements as one should really take into account the heat load of freezing out the water. An added complication is that the ice crystal fur is an insulator, so one does not get a good measure of the heat load from the ccd.
But it is probably a worst case. I observed earlier that even the 27" Hg vacuum from the hand pump adds 5 C or so of cooling. Being dry would also remove the freeze out load.
But the above still tells a story. One is up against a rappidly rising curve of power vs cooling. There is a lot of territory to cover if you want to seak an optimum point. Each point takes about 10 minutes to settle. The plan is to run the main TEC at a fixed voltage and to run the second stage off a servo. So it looks like we can count on 40 C below ambiant. I figure we can always use 25 C cooling water. The way the whole thing is constructed, the lens and the case will tend to come to the cooling water temp. By running the "cooling" water above ambiant, this will heat the whole assembly, and prevent (I hope) dew problems. Unless they are burying a lot of bodies in Chicago, 25 C is usually above the night time temperature here.
50 e/sec for the KAF-0400 at 25 C, 6 C doubling temp (max) =.5 e/sec dark current at -15 C 9 micron pixels/135000 micron focal length= rad/pixel *24hr/2 Pihr/rad*3600sec/hr= .92 sec/pixel * 512 pixels= 469 sec/drift scan 469 sec/drift scan * .5 e /second = 234 electrons. SQRT = 15 e read out noise for KAF-0400 = 15 e (assume rms)
So at -15 C the nois due to the statistics of the dark electrons matches the read out noise. Not a bad place to operate. The dark current will probably be less as the typical doubling number is 5 C which would give us another factor of 1.5 after the square root. I also expect to be able to run at least 5 C cooler once I am in a dry vacuum. So another factor of 1.4 after the sq. root. Now the read out noise dominates the quadrature summation.
Further, I expect to be able to hold the temperature constant to 0.01 C, so we really can use the above estimate withoug worrying about changes in dark current over a long run.
All in all, it looks good enough.
Tom Droege
The Amateur Sky Survey is planned as a large area search for comets, nova and variable stars. The camera design is very simple and features drift scanning for low cost. With the 135 mm f/2.8 optics, we will search a three degree wide band of sky to an estimated mag 16. Running 8 hours a night each camera will cover 360 square degrees. Cameras are being designed as triplets so that three measurements will be taken spaced by 15 degrees and will cover 1000 square degrees a night. By spacing cameras around the world looking at the same sky strip, many measurements will be taken a day of the monitored sky. While many are sharing in the expense of production, it is possible that cameras can be provided at no cost to deserving locations that wish to share in this exciting project.
We are coming down the home stretch for the Mark III cameras. My house is full of mechanical parts. So far, everything fits together functionally if not beautifully. I have ordered 4 of the KAF-0400 chips which should arrive any day. The PC board design is finished, we are just trying to get all the paper work in good order so that we can send out for film and boards. Ron Wickersham is making the boards for the project at no cost.
We have assembled the thermal electric stack for one of the cameras and find that we can easily reach -40 C below ambient. Actually we should reach -50 C.
We now have a mail list. It is quite active and you can expect a half dozen messages a day. Mostly we are worrying about where and how to point the cameras when we get them built, and what to do with all the data. We welcome you to join in the fun and express your opinion on file format, file naming conventions, and all the other fun stuff that goes with handling a Gb or so of raw data a day.
To join in the fun send mail to:
tass-request@wwa.com
In the subject line put:
subscribe
You will get a confirmation.
See data from the Mark II camera on the page set up by Michael Richmond. He has also included a little descriptive material.
http://p674p06.isc.rit.edu/tass/tass.html
The following schedule is not overly optimistic. Actually, once we get the bugs worked out and are in real production, we could build two or three cameras a weekend. I just don't want to promise a higher rate.
Late September: PC Boards Arrive, start assembly and test.
Early October: First single camera tested - first light.
October: First data available. Start putting up data on the internet every clear night.
Early November: First triplet ready. Replace single with triplet and ship single unit to the volunteer writing the on line software. Start taking serious archive data.
Christmas: First triplet sent to a participant at a remote location in time to spot Santa and eight tiny reindeer.
After this, I plan to be able to ship one triplet a month until I run out of money, donations, or the will to do this crazy project.
Tom Droege
I have put up a "news" bulletin on the sci.astro groups and this has attracted a lot of new people. Welcome. You probably wonder how this effort is organized. Technically we have "laissez faire" management. This means we all talk a lot about what we might do. When there seems to be agreement on something, then a volunteer appears and says "I will do this thing."
An example of this at work is when Michael Richmond created a home page on the www. He just did it. We should all feel free to send him appropriate stuff to put there.
You may also wonder how it is financed. It is coming out of my pocket. I am trying to carry on a "gentleman scientist" tradition that goes way back in astronomy. I am not rich. It is just that I prefer to drive an old Toyota and do this work rather than drive a fancy car that I might otherwise afford. Several others have already made significant contributions. These just help us do more.
It is still a little early for any of you to try to do anything other than think about the possible problems. But soon I will have hardware, and then there will be real jobs to do.
The risk in this type of organization is that you will put a lot of work into something that no one wants to use. So it is your reponsibility to outline what you are undertaking to the group. Don't expect to ever get a consensus. There will always be non-workers who will say they have a better way of doing anything. The test will come when there is real data to process. Some will find their code is popular. Others will find their code goes unused.
I will certainly tend to support (i.e. with hardware) those who agree with my way of doing things. But I will also try to support those who have a different approach. In any case I do not feel that I "own" this group.
Tom Droege
One of our members got what might be a commercial query. I sent him the following policy statement. Of course this is just "my" policy statement, and as such is just my vote. Everyone is free to have their own version. It is quite acceptable to me to have list members who are commercial, amateur, academic, etc..
The general rule is that any tass member can speak for tass. Since there are no membership requirements this means that anyone can speak for tass. On the other hand, since no one controls tass, the only way to get tass to actually do anything is to persuade the list somehow that some of them should do the thing.
It is my intent to put the tass electronic designs in the public domain. I hope also that the software writers will choose to put the software into something like the GNU license.
Tom Droege