I would call them speed bumps, but speed and this process are oxymoronic. From the last post out of Steve’s optical shop you may recall that our interferometry images are analyzed by the Open Fringe software and produce a 3-D image of the surface that is corrected for the desired curve. That means that when we get our perfect parabolic shape, the image will be flat. The interferograms are so sensitive, and the software is so flexible, that we will never see a flat surface, but when we say “flat”, we mean flat-within-a-reasonable-limit. Here’s what we had on June 5.
This looks more like the foothills of the Sierras than the wide, flat Central Valley tomato fields. And at this stage of our work, you can clearly see that the mirror is not symmetrical (!).
No Symmetry = Astigmatism.
Among other things. But never mind, we know we’re not at our destination, so we should just enjoy the scenery on our way there. We’ll take a few snapshots to remind us of the fun we’ve had on this trip and use them for slideshows, later, to bore the neighbors and relatives.
The 3-D surface is compelling enough to get my attention, but even more interesting in some ways is a graph that shows how a vertical profile of the mirror appears in sixteen different directions. Picture a sheet of something very flat (a plane) passing vertically through the mirror from north to south, as you stand above it. Imagine it making the left half of the mirror disappear and then put your eye at the mirror surface level to look at the revealed edge of this intersection of plane and mirror. And then imagine that your eye can see variations in height of a few wavelengths of red (laser) light, each one about 635 nanometers. Now repeat that process sixteen times in sixteen different, evenly spaced directions.
This is what we see on that ride. In the center of the mirror, the lines are closer together, which tells us that the surface heights are similar as we take that vertical cut through the mirror in the sixteen different directions. As we get further out toward the edge of the mirror, we see the effect of the Sierra foothills in the 3-D surface image. In some directions the hills are high, much higher than the central valley. In other directions, the hills are really lower than the central hill (an alluvial plain?).
It doesn’t take much to imagine that this would not produce a wonderful image at the eyepiece.
But this is just sightseeing and we have more to our story of what we did on our summer vacation. Steve, Larry and Mark have been hitting glass all summer. Since that day in early June, they have been pushing the smaller tools (the smallest is 12 inches) on the higher hills and using the bigger tools (the biggest is 26 inches) to occasionally smooth out the roughness created by the local figuring work. They work a routine of glass pushing and figure testing with the interferometer, with same day analysis and reporting. There have been fourteen of these sessions since June 5, and there is progress.
You can see that by August 5, 2013, the change is dramatic. The central hill has been the subject of small tool polishing, as have the many hills around the outer zone of the mirror. That work has resulted in significant smoothing of the surface. The profile lines show the story even more clearly – the variations of height across the different directions on the mirror surface are gone. The same curve and shape is present across the entire mirror. This means that we are successfully removing astigmatism from the figure. Steve says we still must be vigilant to keep it out of the figure as we continue toward our destination. Below you can see the 3-D surface rendering from Open Fringe, to compare to the June 5 version, as well as a synthesized foucault shadowgram, if you are used to seeing a mirror figure in that form. This synthesized mirror surface is also a product of the Open Fringe software. Note that the spike in the upper right of the 3-D image is just an artifact of the masking tape we place on the mirror at that spot to ensure we have images registered correctly in the interferograms.
Our quest continues on this optical road trip. We will continue to see the mirror surface flatten until we come out into the Valley of the Best Possible Figure and park the RV for a short break before we begin the next leg of the journey.
NEXT TIME: Yes, I know we have designed the mount for a curved secondary mirror, but wouldn’t a 1-meter folded Newtonian be wonderful?!
The perfect parabolic figure on a mirror as imaged by an interferogram and analyzed by Open Fringe would be a perfectly flat surface. This image shows the progress we have made in previous sessions that has focused on the central hill – now it’s a central ripple. We are leaving that mountain now to focus on the hilly outlands that comprise our astigmatic figure.
To orient yourself in comparison to previous images, you need to stand on your head. Or turn the page upside down. We made a change to the interferometer that results in the image being captured directly through it, instead of reflected off of the beamsplitter surface, which introduces a reflection. The effect in our igram images is that top is at the bottom and there is no left to right reversal. We think this will be less confusing, but time will tell!
In addition to the instrument changes, Steve and Mark have found that they are able to better stabilize the fringes on the camera focal plane by taking a feed of the image in the viewfinder and sending it to a computer monitor. The bigger image makes it easier to see when all is in focus and the fringes are sharper across the face of the mirror. In this run, we got the best quality igrams ever, as judged by the smaller number of unwrap errors.
A brief explanation is in order. The Open Fringe software implements algorithms that trace the fringes from edge to edge. The software has been designed so that it identifies errors in the fringe tracing to give the operator some sense of the quality of the igram – after all, you will be making some possibly momentous decisions about removing glass based on this analysis. The 40-inch F/3.6 mirror we are making presents particular problems because there are so many fringes. Since each fringe represents the difference of a wavelength of light in the height of the surface above some well-chosen plane, and because an F/3.6 mirror is a bowl instead of an F/8 plate, if you take my meaning, there are many more wavelengths of light between the center and the edge on such a fast mirror. More fringes puts them closer together and harder to separate on the camera’s image surface, hence more unwrap errors.
We’re striving for unwrap errors that number less than 1,000. With these latest igrams, we reached 2,500, down from 9,000 in earlier sessions. Once Open Fringe does all of this algorithmic tracing, it then averages and visualizes the surface. In order to make it easier for the eye to detect the errors in the figure that this work is all about, it removes the parabola mathematically. If the surface were perfect, this “artificial null” would make the analyzed image perfectly flat – a plane.
On Monday, Larry, Steve and Mark used the 12 inch tool to work for 30 minutes each on the outer zone high spots. We are pursuing a strategy of local correction instead of wholesale re-spherizing and re-figuring. In the Open Fringe analysis images we are moving toward flat.
Another figure of merit we are interested in improving is the astigmatism that is apparent from the igrams we take in each session (we used eight on this one). This session produced a landmark in the analysis you are looking at because this is the first time that all of the individual igrams have been even visible on this graph. You can see them clustered in the upper right and lower left quadrants. Our next goal will be to move them to the center and to make the number of wavelengths of light distant from that center smaller and smaller. That will tell us that the mirror is flat in all directions from the center – no astigmatism. And that, my children, will tell us it’s time to aluminize.
Seven months and twenty-two days ago I posted the beginning of our interferometry odyssey. We are hardly at the end, but it seems time to send out an update(!). The near continuous work on the mirror and the tests of its surface with the Bath interferometer are well described by the title of this post. When doing something for the first time, you should frequently get your head up and look around to see if you can find evidence that you are on the right track. You should also expect that you will make simple errors – it takes practice to avoid those.
From September until December our intrepid team worked on learning the art of making interferograms and, from our learning, changing configurations of the instrument and the way we made the interferograms themselves. During that time we were working in Mark’s shop where we could comfortably set up the apparatus at the Radius of Curvature (ROC), which is twice the focal length of the primary mirror. We experimented with lasers (settling on a 4.5 mW lab laser with circular beam), diverging lenses (focal lengths of 40mm, 15mm and 10mm – sticking with the 10mm), positions of the components (converged finally on components crowded as close to each other as we could get them) and camera lenses and cameras. Mark’s 20 Megapixel sensor with 50 mm lens has given the best results so far.
An interferogram is not much use without the software to analyze it. We are using Open Fringe, written by Dale Eason, and supported on the Yahoo interferometry group. During that time Dale made changes unrelated to our work that updated the software from version 12 to version 14. He was adding features and fixing bugs, and we were learning how complex this game is and why the software is so complex to deal with it. The software requires you to take a raw interferogram image through a process that sets configurations for the analysis, establishes an ellipse that corresponds to the edge of the mirror, standardizes its size and resolution, computes a fast fourier transform (FFT) of the fringes and then computes the surface of the mirror from that data. With the surface characterized, you can adjust how it is displayed and how it is analyzed and then save different kinds of data points and reports to share. Along the way, we learned many ways to get unhelpful results from the software and then corrected what we had learned with the support of the interferometry community.
In early January the team decided to move the mirror back to Steve’s shop. By this time we were becoming confident enough in our testing that we trusted what we were seeing in the test reports. I wouldn’t say that we had solved all of the problems that would cause us to get somewhat different results from test to test. But we did find enough consistency to be willing to go back to work on the mirror. The star tests we did over the summer indicated astigmatism in the figure, and the tests were showing the same result – only with the needed detail about where it was.
We had also already begun to understand the value of taking interferograms (igrams, for short) at different rotations of the big mirror. Test stands can introduce astigmatism of their own as the mirror, a big piece of glass that you think of as a solid, immutable object, bends under its own weight. Starting with this phase we began to take igrams of eight different rotations, with four igrams at each rotation point. They were, as close as we could get them, at 45 degree angles to each other. Since each igram itself has some variation due to air currents in the room and vibration of the apparatus, we averaged the four together at each rotation point, and then averaged them across all the rotations, after correcting the igrams to have the same rotation orientation. During this phase we also learned how to better standardize our analysis process so that we could compare session to session without fudging. Standardizing made it clear that we had been fast and loose with our understanding of image reversals in the instrument – all of the raw images were reversed left to right due to the star diagonal effect of the beam splitter.
We started in January thinking that we would work on the surface of the mirror in Steve’s shop, and then transport the mirror back to Mark’s shop to test. Not ideal, but attempts to get to the ROC in Steve’s shop were frustrated by that distance, about 24 feet, just a little bigger than the usual 20 foot by 20 foot shop dimension. It wasn’t feasible to do the tests as we had done the Foucault tests, wheeling the mirror on its test stand down to the end of the driveway, and setting up the light source and razor in the shop. The Bath interferometer is much more sensitive to wind currents and temperature gradients. Dismayed by the need to make this commute with the mirror in the coming months, we did the experiment of setting up the mirror on its test stand in the darkness of the shop, and then opening the door of the shop that led to the house, and setting up the apparatus at the ROC inside Steve’s kitchen. We were concerned at first that even the temperature differences and convection currents between the house and the shop would cause us problems, but after a few sessions this became a routine and workable setup. We owe a big debt to Steve’s family for accommodating this arrangement!
The conventional wisdom of the mirror-making community is that when you have astigmatism in a mirror surface, you use the big tool to return to a sphere (and, by the way, wipe out any figuring you have done so far). It’s a reboot. By unanimous vote, none of us were willing to do that. Instead, we wanted to try and make corrections more locally to see if we could remove the astigmatism and other defects that way. You can see in the January test results that, in addition to the astigmatism that shows itself in the 50% radius as variations in depth, we have a central mound that needed work. We decided to use the small tool and to gently work the surface to avoid going too far, since you can always take a little more off, but it’s hard to put glass back.
Once or twice a week we worked the surface and tested. We analyzed the igram images and decided to continue the same way. The drop box was filling with igrams and processed images and reports and we weren’t making progress. It was so puzzling we began to re-examine our process for using the software and the test results. We did a hot sponge test. The software can easily show the surface inverted, and it is difficult to tell if you are looking at the real surface or the inversion of the surface, and thus be doing bad things with your mirror work. By heating a part of the mirror with a hot sponge and taking igrams in that state, you can easily see the hump in the glass caused by that thermal expansion. In frustration, we appealed to the community. The Open Fringe author, Dale Eason, kindly analyzed a session of igrams and gave us many tips, confirmations and corrections to what we were doing. With that data in hand we embarked on two sessions of mirror work that finally resulted in changes in the central protrusion.
We are getting better now in the way we do the iterations in testing and working on the mirror and are steadily improving in our ability to get consistent surface analysis from the igrams. The April report shows that the hill is much reduced in size. If the mirror surface were a figure of revolution, then the set of profiles to the right (think of a knife edge cutting through the mirror at 16 different orientations, each separated by 22.5 degrees) would coincide with each other. We know that this image does have test stand astigmatism removed, and that some of the astigmatism is caused by what are known as unwrap errors. If you look at the October igram, you can see the fringes very clearly in the center of the disk. Toward the edges, though, it just looks fuzzy. With sufficient zooming you would see there are very tiny fringes still there, right to the edge. When there is insufficient resolution in the image, the software can lose the fringe as it analyzes it, causing an unwrap error.
Our next step is to reconfigure the Bath interferometer. Community members have suggested that a more common configuration exchanges the position of the laser and the camera. This should allow us to both remove the left-right inversion and to move just outside of ROC to get larger fringes at the edges, smaller fringes in the center. We’ve also ordered a 6 mm focal length diverging lens that should give us a bigger igram at the same distance from the apparatus. We are experimenting with the resolution of the images being analyzed, too, again trying to reduce the number of unwrap errors, this time by affecting the way they are laid down in pixels in the image. Once we have settled on the test process with these adjustments, we will continue trying to make local corrections to remove the astigmatism. Our back up plan is, of course, to spherize if we can’t get the astig to drop below acceptable levels that way.
As a team, we have certainly been getting our patience bone exercised with this work! The secret to getting the telescope to the point that we can install it in the observatory will be continuing to look for how we are doing it wrong, however subtly, and correcting our errors.
Mark, Steve and George met at the Hillestad workshop on August 21 to make the first try at getting the Bath interferometer that Steve built working. From the last post you know that one of the big challenges was not having enough space to do the test without setting the mirror or the interferometer up in Steve’s driveway. Out there it was subject to the breezes of Nature, and while pleasant, they make it hard to get an interferogram out of the apparatus.
The telescope was positioned at one wall of the workshop, dropped down to zero altitude, with the secondary spider (the temporary pyramid one that we used for star testing at prime focus) removed. At the other end of the workshop we set up the Mark II version of the Bath interferometer, as Steve began to call it that night.
This image is from the front of the instrument, with the photographer’s back to the telescope mirror. Leftmost is the aluminum lens mount containing at 9mm F/1.0 biconvex lens. The purpose of that lens is to diverge the beam of the laser that passes straight through the beam splitter, illuminating the full diameter of the 40-inch mirror. The center of the photograph shows the flat mirror, set at 45 degrees to the laser beam. It reflects the part of the laser beam that is reflected off of the beam splitter face at about 45 degrees from the original path, and bounces that undiverged beam down to the 40-inch mirror. The diverged beam from the lens is reflected from the entire surface of the big mirror back to the small flat, where it is reflected back into the beam splitter and on through it to the image plane of the interferometer. And the undiverged beam off of the flat is reflected by the center of the big mirror back through the lens where it is diverged and reflected off of the beam splitter to the image plane. The beam splitter is just behind the diverging lens in this photo.
In this photo from the side and above, you can see the layout, including the laser (a salvaged laser pointer module with battery supply). The image plane is in the upper right and that’s where we set up the camera to photograph the interferograms. Steve’s experience with this apparatus paid off pretty quickly – he was able to adjust the beam, beam splitter, lens and mirror through a step-wise process that got them all lined up, and Mark was then able to position his camera to take an image of the interferogram.
You can see in this gallery the first three interferograms we recorded. These are not yet diagnostic interferograms – they are just showing that we can get one (one step at a time!). A useful interferogram will have concentric rings, each ring indicating an additional wavelength of deviation from the central height of the mirror. Our interferograms are showing some misalignments that we can work out, including the images with the two bright red dots – those are images of the incident undiverged beams that can be eliminated by rotating the beam splitter slightly, and adjusting the other components to keep everything lined up.
Alignment with the Mark II apparatus posed some problems because of the nature of its construction from spare parts, so we decided, with this success, to build the Mark III. If you click on this link (Bath Interferometer v0.3) you will open up a PDF of the design of the Mark III using off-the-shelf optical lab parts from ThorLabs. (You’ll need to use an Adobe Reader to get the 3D features – once it opens, click on the image and then you will be able to click and drag on the image to rotate it and zoom in to see how it all fits together). The parts just arrived, and this is an image of a test assembly.
Next step is to construct the Mark III from these parts, and that will involve some low tech machining of the optical rail and a part to hold the beam splitter. Once we have it assembled, we should be able to do our first tests with the new apparatus the week of 9/10 and attempt our first diagnostic interferogram inside the darkened Hillestad lab.
This will be the last star test for a while. This was the best diagnostic image we’ve taken and it was done with the Orion Starshooter Autoguider – the hand-me-down from the CCD project now that Steve Smith has replaced it with a Lodestar autoguider for better computer control and eventual remote operation. We used Stark Labs PHD (a pre-release version) to save this 1 s integration image. The squarish shadow on the top is an 8.5″ sheet of black construction paper meant to make sure we know where on the mirror the image is oriented. That’s the “top” in the alt-az mounting, and currently the mirror is rotated so that the E zone is at the top. ‘E’ comes from the letter names for the sixteen zones we’ve marked out on the edge of the mirror.
The image we captured is from inside focus, almost 12mm inside focus. This is an image of Antares which was quite low in the sky. In addition to altitude impacts on the image, we were battling with clouds throughout the session. You can see a very not-round part of the image shape and we think that is caused by some form of astigmatism of the mirror’s surface. These two images, inside and outside of focus, but more like 6mm on either side, and both at 1s exposures, confirm that high level diagnosis.
From these images we’ve learned enough to know that we need to make adjustments to the mirror figure before we coat it. In order to take that next step, we need to know how to change the shape of the surface, and the star test is not diagnostic in that sense – it tells us that we have astigmatism, and that it has a particular orientation on the surface, but not where the high or low spots are that we need to work.
That information may come from our next task, to get Steve Follett’s Bath interferometer working. He and Mark had tried this before but the conditions for using that measuring technique were not ideal in Steve’s shop. The interferometer must be placed at the radius of curvature (ROC) which is twice the focal length of the parabolic mirror, or 7,328mm from the mirror surface, or about 24 feet away. In Steve’s shop that always meant the mirror was outside on the driveway and the test is sensitive to air currents. In Mark’s shop, there is room in the enclosed shop to set the full test up. The test ran into some other issues having to do with the illumination of the mirror with the laser, but we will need to try it again and start troubleshooting until we get it working. We’ve described our situation to the Yahoo Interferometry group and have had encouraging comments that we should be able to use this method to test our mirror.
If you’d like to read up on what the Bath interferometer test consists of, you can check out this well-written description on the web:
Steve Follett’s version is a right-angle device. The basic idea is that a laser passes through a beam splitter that divides the beam’s intensity in two and allows one half to pass straight through while the other is reflected at 90 degrees. One of those beams is diverged so that it expands and reaches the entire mirror surface, which then reflects that diverged beam back to a focus near the diverging lens where it is re-collimated by the same lens. The second beam is reflected straight back to the image plane of the interferometer where the reference beam and the test beam are allowed to interfere. The difference in path length causes fringes to appear that indicate changes in distance across the surface of the mirror compared to the test surface. The fringes are photographed and then fed into an open source fringe analyzer to produce images of the shape of the mirror. The arrangement is straightforward, but the devil is in the details of getting the optical elements correctly lined up so that intensities match and so that we get good fringes and not too many or too few. We have the experience of the Interferometry group to guide us, but part of what we will be doing will be a first – mainly getting this to work for an f/3.6 mirror of this diameter.
The interferometry setup sessions will begin this week. I’ll post pictures of the setup so you can compare to the StarryRidge versions in the link, and eventually we should be able to post our interferograms and the analysis that comes out of them.
And, in case you were wondering, until we get this test working and see our first analysis, we won’t know how hard it will be to get the mirror figure into the correct shape. That will determine the answer to the “when” question, as in “When will this mirror be coated?”.
Our experience with the focal point during Star Test – Chapter 1 resulted in making the pyramid spider (patent pending) a bit taller. You can see it here in this image as Larry rebuilt it, complete with the motofocuser. All but Larry arrived on Monday night, July 23, for the next round of testing – vacations do take precedence. This time we found we were a little too long, but that we could adjust the legs of the pyramid a half an inch closer and that was enough to give us some range of motion around the focal plane. We were aided by a convenient Moon to focus on, this time – can’t miss that target!
Before we started getting focused and pointed though, our experience with collimation taught us to do our rough and next level of fine collimation before we start doing video. Dickson’s collimating eyepiece was used to get lined up. We also tested, without the secondary this time, the deviation from collimation that we got when we changed the altitude of the OTA. At the focal plane, we saw the spot move from centered to about one-third inch off of center when moving from 30 degrees altitude to 60 degrees altitude. We attributed that to the pyramid spider, but we’ll want to do some more testing
We started on some low targets that we lined up by putting a visual eyepiece in place at prime focus, setting up the finder/refractor and then swapping the video eyepiece in. We were able to line up on Spica that way, and then, under SiTech control, move to Saturn, not far away. We did find Saturn (not in the field of the primary, but certainly in the refractor) and imaged it for a while, and even thought we were capturing the video, but my pilot error revealed we were just ‘ready’ to record and not recording.
Dickson brought along his illuminated reticle and using that on the refractor/finder saved the night. He was able to use a lower power eyepiece to find a star (Arcturus was our first target) and then to switch the illuminated reticle to more precisely center the object in the field, and that allowed us to find the star in the primary. It still required some search, but I think we’ll get better at this, and once we have a permanent mount in place, we should be able to depend on the SiTech drive to get us to our desired locations in the sky.
The star tests had no batwings (see Chapter 1) and inside focus, we could see the brightened edge indicating a turned edge that we already knew was there, and some spikes in one direction that we attributed to a light breeze blowing through the scope. The oval shape was not ideal, but the real surprise came inside focus where we got a drum shape with a roundish disk that had flat or slightly indented sides.
This is astigmatism, no doubt about it, but we knew that we had lost two of the ‘button’ supports on the primary mirror cell out of the 27 required, and they were lined up with the distortion, so we decided our next step was to fix the primary mirror cell at a work party the next Saturday. In the meantime, we cleaned up the video of our tests of Arcturus and Vega (nice and high and dark this time) and sent Star Test – Chapter 2 off to the Free-ATM experts list for evaluation. The experts concurred about some kind of astigmatism, but Mel Bartels also noted some “strong undercorrection particularly in the central zones with a modest turned edge and possibly modestly overcorrected midzones.”
On Saturday, July 28, Mark, Dickson, Steve and I gathered and removed the cell and mirror from the telescope, separated the mirror from the cell, and repaired the missing ‘button’ supports on the mirror cell. We needed to let the epoxy set so we left the scope stored with the cell in place but the mirror out and will do the next star test on Saturday, August 4. While we had the cell out of the scope, we examined all of its ability to move under the weight of the mirror and found some improvements to make with small adjustments and re-working how the collimation bolts were attached to the main support triangles of the cell. In this picture you can see Steve and Dickson masking the mirror’s edge with about one-half inch of masking tape to “remove” the turned edge.
At this point we are not making any predictions about how the next star tests will look – we’re just going to wait and see, and be prepared to take the primary back to Steve’s shop for some minor adjustments, if needed. But we might get surprised by a perfect star test, who knows?
We naively made our first attempts to star test the primary mirror with the secondary mirror in place. Perhaps we were just full of hope rather than naive – you’ll have to be the judge. Shortly after we established that we did, indeed, have an optical system, we gathered to try the telescope out on a bright star to see how it was performing.
Small telescope users will be familiar with the star test. You focus the telescope on a bright star, and then move the system a bit in and out of focus. We use it with our scopes to check collimation – the shadow of the secondary mirror should be centered in the diffraction ring, and the ring itself should be nice and round. Telescope makers know that the star test will tell you if the mirror’s surface has the right shape, and that it can be a fairly sensitive indicator of turned edges and astigmatism, in addition to collimation of the system.
Our first attempts gave us what we started calling batwings (thanks, Larry!). It was certainly clear that we would not get an immediate thumbs up on the primary so we could send it off to get coated. We guessed that the secondary could have been deformed by our temporary mounting method (double sided foam tape) or the curve of the secondary mirror itself. We decided we needed to test at prime focus.
The focal length of the primary mirror is 3,664 mm, putting it beyond the secondary cage by about 300 mm or more. We thought about diagonals but decided to create a kind of pyramid spider to hold a temporary eyepiece holder. Larry crafted the addition and got it installed on the secondary cage, and provided a motorized focuser, to boot. During the afternoon, he and Mark measured and re-measured and decided that the first pyramid sides were too long, so they cut them down to the desired size.
The full team of Dickson Yeager, Steve Follett, Mark Hillestad and Larry McCune gathered that evening and started by doing some rough collimation and then tried to get a focus. Nothing terrestrial was visible from Mark’s driveway that was far enough away to reach focus, so we waited for twilight. With an eyepiece in the focuser, it became clear that we needed more distance to reach focus. We found two extension tubes and the combined tubes allowed an eyepiece to be adjusted into the focal plane at prime. The irony wasn’t missed by any of us, of course. With the extension tubes in place, we checked and found that we could see about the inner two-thirds of the mirror – the rest was vignetted. Not great, but still worth testing.
Our earlier star tests were purely visual. If they had shown us a perfect optical system, we would probably have left it at that. Knowing that we needed the diagnostics that the star test would give us, we decided to try and record the test on video. VMOA owns a Meade Electronic Eyepiece that has an 8mm square imaging sensor and electronics to produce a signal on an RCA cable suitable for recording. We later calculated that with that size image at prime focus we had an image size of about 7 arcminutes. That was the cause of all the time we then spent trying to find things.
Even though we have a 2″ refractor piggybacked on the mounting to use as a finder (and it with a tiny finder of its own), it was impossible to locate an object in such a small field. We got flat-out lucky to find Vega as twilight was beginning – searching the area we happened to see a very out of focus Vega move across the video monitor and were able to walk back to it and focus on it. And it wasn’t inspiring. The batwings were back, extended in one direction inside focus, and outside focus in the other direction.
We were certainly out of collimation and we began the slow task of getting it into collimation. That procedure consisted of Steve adjusting the collimation bolts on the back of the primary mirror, while I watched the video monitor and adjusted the pointing of the telescope with the guide speed adjustments. I should say that we did lock onto Vega in The Sky and the mount was tracking very well throughout the evening. That said, every touch of the collimation bolt threatened us with loss of the star, so we moved slowly – a little collimation improvement, a little re-centering. Finally, after about 15 minutes of this, we moved the collimation enough to lose Vega from the field, and were then unable to recover it.
We knew that it would be difficult to find an object given this experience and our collimation was not quite there yet. So we looked for a star that was low in altitude. That allowed us to replace the video eyepiece with a visual one and to climb a ladder and stick our head and body in front of the primary to help us find Antares. Once we had it locked for tracking and centered, we swapped the video eyepiece back in and we were back in business, sort of. As you would expect, low altitude makes for not very good seeing. We did continue to make progress with collimation, but in analyzing the images later, we found that we were able to improve our image size at focus only from 32 arcseconds to 16 arcseconds. Barely passable seeing should be about 6 arcseconds, but we were just looking through too much atmosphere, and we were not going to find anything much higher in altitude without a head at prime focus. And then we collimated Antares right out of the field. We satisfied our curiosity by imaging Mars for a while and then called it a night.
You can see the whole video (7m 24s) here: Star Test Video – Chapter 1. You may need to download it to view it – QuickTime and iTunes should both show it, as will any player that will play an MP4 format video. We played this video with commentary for the docent team at our last meeting and made plans for Star Testing – Chapter 2.
Creating something new is messy. I’ve talked in this blog many times about the iterative nature of our design for Project 40 and all of the moments when we have run into a problem that has caused us to backup, figure what to do next, try a few different things, and then settle on a solution that gets on to the next bump. Each time we learn something new about the technology, and not a little about ourselves. This week, it was the turn of the optical design to give us a lesson.
Last week we were jubilant about seeing the telescope mount come together for the first time. There is nothing like seeing an idea that has inhabited our imaginations take form in the space before us. Last Saturday we gathered for the first star test of the uncoated primary mirror. As I reported earlier, we purchased the flat diagonal mirror and we just got the coated secondary back, so we now had all of the optical components nearing completion for the first time. Assembling them into the telescope, designed to hold them all, and star testing the primary before we coat it, was such an important milestone, we were literally trembling to see it. The hope was that we would see an image, slightly out of focus, that would confirm for us that the primary was as good as we think it is, and with no astigmatism.
The first setup of any telescope under construction can be a painstaking process. Steve Follett is our lead and moved us through that process, adjusting the primary, the secondary and the flat to get things centered and on line. He brought the design specs with him, for reference, and we went to place the secondary mirror at the recommended distance from the primary and discovered an error. The calculations we had done nearly two years ago to specify the primary-secondary distance had gotten missed in our communications and the secondary curve was chosen at a point five inches further away than the mount design was built. We’ d built in some adjustment, but not that much, and we could not find focus.
Now, Cassegrain focus is an interesting beast. Our Naysmyth design is simply a Cassegrain with the light path folded to an eyepiece off to the side instead of passing through a hole in the primary. Many of you have experience with a Cassegrain in the form of the Schmidt-Cassegrains owned by many amateurs, and know that the focuser is a knob on the back of the primary that moves the primary mirror a very tiny amount. That works because the distance between the primary and the secondary mirror in a Cassegrain very sensitively controls the location of the focal plane. For some commercial Cassegrains, a change of 1 mm in that distance can change the location of the focal plane by 10 mm – a 10x mechanical gain. We knew all this and began to try and figure out where the focus was happening by looking for something like focus at ridiculous distances from the eyepiece.
All of our attempts to find the focus on that Saturday night failed. I have to say, Larry, Steve, Mark, Dickson and I were heartbroken. We certainly hadn’t given up, but the thought that we had gotten the calculations on the optics that wrong was really confusing. The optical system was behaving as if the light cone was not a cone at all, but a collimated beam that would not be brought to focus without a telescope in the focuser tube. By the time we shut down that night, we decided to go home and re-do the calculations to make sure we understood what we thought we understood, and to figure out the best way to extend the location where the secondary is mounted, without making the tube too long for the observatory. In short, our plans for Project 40 were in serious jeopardy. There was already talk of making a new secondary mirror if need be.
Sunday was spent going over web sites, ray tracing program results, calculations and emails back and forth for how best to extend the secondary cage mounting point. By the end of the day on Sunday, I had produced a calculation that showed our secondary could not possibly work (it turned out to be wrong), while Larry worked out an extension of the secondary cage.
On Monday, Larry and Mark put the new secondary cage extension on the telescope. The original design of the telescope mount called for the secondary to be mounted at 108″ from the primary, with some adjustment higher and lower than that. The curve on the secondary was put on the glass assuming a 113″ distance. For this design, we’d calculated that the distance multiplier would be about 15x. A 5″ difference in the secondary-primary distance would have pushed the focal plane 75″ away from the eyepiece, if those calculations were correct. The new secondary extension allowed us to move the secondary up to 119″ away from the primary.
Mark reported, though, that even with the extension, he and Larry could not bring trees 400 yards away into focus. Did we need to move the secondary farther away? Were the trees too close to focus the telescope? This really didn’t make any sense – we had to be doing something wrong. We planned to get together again on Tuesday evening to continue troubleshooting.
Before sunset on Tuesday, Mark, Larry and Steve rolled the telescope out of Mark’s garage. They pointed the telescope at the Sun and used that bright star test source to search for the focus, and found it – 8′ outside of the focuser with no eyepiece in place. Before the test could be repeated with a new calculated focal distance (96″/15 means we move the secondary 6.4″ further away) the Sun went down. That evening, we tried to focus on trees and stars and were once again frustrated – no sensible focal plane. The good news for that night was that we could find the focus. Standing eight feet away from the telescope, holding an eyepiece, was not going to be workable for the public, though. We decided that the Sun was needed to locate this thing. We also decided to ask for help, so I posted our quandary to the AltAzimuth Yahoo group that specializes in 1-meter and larger telescopes.
Just after noon on Wednesday, Steve and Mark met and tried again. They were once again able to find the focus point of the telescope by pointing at the Sun, and, projecting the focal plane on a white background, could see sunspots. By making small movements and finding the next focal plane location, they found that the actual multiplier for this optical system was more like 30x. And they finally achieved focus at the desired location on the focuser with the secondary at 115-11/16″ from the primary. Relief, followed by joy! In an instant, the world made sense again. By that time, we’d gotten a response from Dave Rowe on the AltAzimuth board confirming our calculations were in the ballpark and with advice on finding focus.
We gathered again on Wednesday evening, this time with a much-changed mood in the air. It took most of the evening to get the scope lined up with temporary finders attached to the mounting frame so that we could find a star in the eyepiece. We finally did find Vega, but the seeing was so poor that we were unable to definitively call the primary mirror ready-to-coat. More testing will follow until we are certain enough to move forward. But the satisfaction of understanding something we didn’t understand before, and of finding the place where Nature, not our calculations, said our focal plane must be, created a glow that I know will last me for a long time. This won’t be our last challenge. But it will be a memorable one.
Mark Hillestad and Larry McCune were quite busy today.