Bernd_Eppinger

Hi, Antonio, Thank you very much. That drawing is perfect. Best regards, Bernd

Hi, group, I consider to update my Newtonian telescope by an ASA OK3 or OK3-Z focuser. I am aware that this will require drilling new holes in my OTA. But what I must anyway know in advance is the total height of the focuser. If it is too high, then adapting the telescope would be much more complicated than just drilling a few holes. Unfortunately, I was not able to find the required dimensions on ASAs site or anywhere else in the internet. So can anybody who has an OK3 or OK3-Z tell me the height from the OTA to the upper edge of the drawtube, without ASA's extension tube? Ideally the values for minimum and maximum focus position? Best regards, Bernd

Hi, Nigel, some comments: I have found some methods in Wikipedia to estimate the amount of required lightning protection and its effectiveness. Unfortunately only in German, as there seems to be no equivalent English entry. At least the Google-translation is mostly understandable, although Google's English is partly terrible. Here is the German Link: http://de.wikipedia.org/wiki/Blitzschutz Translation: http://translate.google.de/translate?sl=de&tl=en&js=n&prev=_t&hl=de&ie=UTF-8&eotf=1&u=http%3A%2F%2Fde.wikipedia.org%2Fwiki%2FBlitzschutz If you look at the chapter called "Rolling sphere method", you can see that even for the weakest lightning protection class IV, the maximum distance for which a lightning catcher can give protection from direct lightning is 60m, but only if your dome is 0cm high (unrealistic) and if the catcher is at least 60m high. For your 3.7m high dome, I would do the following math: Sphere with r=60m touching a vertical catcher at x=0 and the floor y=0 (dimensions in meters): (x-60)^2 + (y-60)^2 = 60^2; (Pythagoras) => x= 60 - sqrt(60^2 - (y-60)^2); (the other solution with opposite sign of the sqrt() makes no sense). For your dome, y=3.7m. This results in x=39.256m. Anything farther away than this will not protect your dome according to the weakest lightning protection class IV. And even if there's something in exactly this distance, it must be at least 60m high. Needless to say that the trees in 50m distance, your house in 100m distance and the church in 400m distance offer no protection of class IV. A similar calculation shows that a catcher on your dome (or the dome itself) will attract lightning from a radius of ca. 21m, if class IV is assumed and everything around your dome is flat (which is not the case: fence, mast). I don't know the law in France, but in Germany the law says you need lightning protection if a lightning strike could either easily happen or have severe consequences (http://de.wikipedia.org/wiki/Blitz#Baurecht_und_Blitzschutz). I think if a house burns down, that could be considered as a severe consequence. Anyhow, I am not aware of any modern house in Germany that doesn't have lightning protection. As I am no expert for lightning, I would recommend to check the following with your electrician. Besides, I don't know the details of your dome. Depending how the rotating dome is supported: if the bearing is massive and entirely electrically conducting (e.g. steel), I could imagine that a lightning conductor on the dome could be connected to the rotating part of the bearing, and that the fixed part of the bearing could be connected to the earthing system. If I had equipment for some 10000 EUR inside the dome, I would attach a grid of lightning conductors instead of a single one on the outside of the dome in the hope that they serve as a Faraday cage and reduce the field strength inside the dome in case of a direct or nearby lightning strike. How to build the earthing system: there are several links in the article, but I think you should discuss it with your electrician. Reading the article, I have come to the conclusion that the Earth connector of the mains supply on its own is not sufficient. However, I still think it could make sense to connect the mains Earth to your Earthing device in an attempt to reduce the voltage differences between floor and mains in case of lightning. I can't tell you if a conducting stake in the soil is sufficient. I assume it depends on its size and depth. Wikipedia says such devices ("Ground rods") may well be driven 9m or more into the soil ("nine feet" is a translation error by Google). Even if they are underground, I would think about adding a lightning arrestor where these cables enter you dome or your metal cages (if that exists for Ethernet), or at least ground the shield well. The electrostatic field of lightning can extend several meters below the Earth surface, because soil is not a perfect conductor. Or you may consider an optical fiber cable instead of CAT6, because it is non-conducting and doesn't conduct lightning to your electronics. Besides, as far as I know, transmitting high data rates (>1Gbps) over large distances (100m?) is easier when fiber is used instead of copper. Best regards, Bernd

Hi, Nigel, Sorry, I was not aware of the drawings when I wrote my reply. As the cabinets are directly underneath the pier, and as you lay out your cables to the mount/telescope inside the pier, I think that electrically connecting the pier and the cabinets makes up a good shielding and hence a good protection against nearby lightning. Personally, I would additionally ground it. The cables from the weather-sensor that run into your computer cabinet may still need some form of extra lightning protection. As the dome is made from glass fibre, there seems to be no protection against direct lightning into the telescope / mount / pier. Or does the dome have a lightning conductor? As all cabling distances between power, computer and mount are short, I wouldn't expect ground loop problems there. Besides, the datasheet of the PSU100M 24V/20A (6EP1336-3BA10) says: "Potenzialtrennung primär/sekundär: Ja", so I would assume that the outputs are electrically isolated. As far as I can see, the data cables (USB & Ethernet) to the house are optical, so they neither contribute to ground loops, nor can they conduct lightning into your cabinets. Placing them underground is also a certain protection against direct lightning into the cables. Best regards, Bernd

Hi, Nigel, I think the following should be considered: Lightning: If your steel cabinets can be reached by lightning, then I would try to get an Earth-connection for them that is as good as possible. The ground conductor of the main electrical supply can be a good Earth connection, if the conductor diameter is big enough, especially because it is the same potential which your electronics uses. But I am not an expert for lightning prevention regulations and the required wire cross-sections, especially not in Britain, so you may better ask your local electrician. I have also made the experience what happens if a lightning flash comes down in 200m distance: in the case I have seen, all cables, especially loops, picked up the electromagnetic field caused by the lightning. The resulting currents evaporated copper conductors at various PCBs of my grandmother's telephone infrastructure. So my recommendation would be to lay-out cables that are connected at both ends close together to keep the area that can pick up the magnetic field small. Even better, but more complicated, would be to put these cables into a common grounded metal tube. Example: the USB- and the power-cable between the mount and the steel cabinets. In this case, it would make sense to connect the aluminium body of the mount (or the pier, if it has electrical contact) to one end of the tube. If your dome is made from metal and well grounded, and if your cabinets and cables (and the mount of course) are all inside the dome, then I think lightning should be less dangerous, except for the cables that enter the dome (e.g. mains supply, network/Internet). Radiated emissions: I haven't experienced problems due to radiated emissions with my DDM60 and my Laptop. If you fear them, it could help to connect your cabinets and the mount (pier) together and to ground them. Ideally, the cables between mount and cabinet should be as close as possible at the ground conductor, like for the lightning case. Ground Loops: To answer this, it would be necessary to know all your cabling. I don't think there is a general answer, I am sorry. But you can make your own thoughts. Problems with ground loops typically occur when there is a cable that carries some current in parallel with a cable that carries a sensitive signal, and if one of the conductors of the power cable (usually the ground conductor) is directly or indirectly connected to the sensitive signal cable at both ends. As an example, if your computer and your mount power supply both have a connection between ground and the protective conductor of the wall outlet, then there is a connection between the USB cable and the power cable via computer, power-cord, wall-outlet and mount supply. Inside the mount (at least for my DDM60), they are connected, too. Now if there is a voltage drop on the power cable (e.g. if it is long and thin), then there will be also a voltage difference between both ends of the USB cable. Although this will cause some current through the USB-cable's ground connector, this current will not be able to eliminate the voltage difference. The remaining voltage difference, if still big enough, can disturb the USB signal. If your house has an old-style cabling where zero-conductor and earth-conductor use the same wire, then you get the same effect between your computer and your mount power supply, this time causing an AC voltage difference between both USB ends. There are two different strategies against ground loops. You can break them by inserting something into the loop which blocks the loop current. If e.g. in the above example, the mount power supply is replaced by an isolated supply that doesn't have a connection between protective conductor and output, then the loop is interrupted and the USB cable will be able to force nearly the same ground potential at both sides. High frequency problems of ground loops can sometimes be solved by ferrite beads or common mode filters. The other strategy is to try to force equal potential between two sides of a signal cable by additionally connecting the grounds of both sides by a thick and low impedance conductor. This conductor should be laid out close to the signal cable to avoid creating a loop that picks up magnetic fields (or lightning, see above). I think that is what you are trying by grounding the pier. To be effective, there should also be a connection between the pier and the mount's ground (is perhaps anyway there, can be measured), and a direct connection between the cabinet and the computer ground. Best regards, Bernd

Hi, Ralph, interesting, that must have changed between 5.1.1.1, which I am currently using, and 5.1.1.6. Because with 5.1.1.1, I always use the Homefind-button for manual Homefind (motors off), and it works. Hopefully Dr. Philipp Keller corrects it in a future version. Best regards, Bernd

Hi, Michael, > "If you want to help us you can copy these postings and post it here in a chronological way." I, too, thought about methods to keep the information of the previous forum. But if everybody copies postings on his own and independent from you, how can we guarantee that we get it chronological? In other words, if we try to help you this way, don't we cause more damage than doing good? An alternative posted elsewhere was to collect links in one thread, ideally with a short description. Taking this idea further, an administrator could then create an extra thread which contains these links in a sorted fashion. That may be easier than copying all important postings plus the relevant pre-postings and linked postings that are necessary to understand them. For this to work, an Administrator would have to be able to edit his posting even weeks later, when new links occur. And of course the old forum would have to continue to exist in future. What do you think about this idea? Best regards, Bernd

Hi, George, thank you for the update. So let's wait for ASA's reply. If ASA agrees, I would be interested in what they say. Best regards, Bernd

Hi, George, Thank you for creating the graph, that facilitates the evaluation. I see a handful "jumps" from high to low current. If they are fast (in the log, I think at least 2 of them might be fast), and if they are caused by friction and not by wind etc, then I think such jumps may cause an excursion of the DE axis that might be visible in the image. I don't know if the excursion is large enough to cause an intermittent slew to 30 and 31 comae berenices. At least the position error in the log is small. But it is possible that the excursion happens between the 6 sec. sampling interval, so that the log doesn't show it. It is also possible that when you made the "bad" photographs, there was more friction and a larger current drop when the friction was (suddenly?) released. In the first 2 logs, I find the following lines interesting (something like this occurs several times): 22:46:27.725: Axis 2 trying to correct OverCurrent Error 22:46:27.741: Axis 2 slewing to -4.78 22:46:32.171: Axis 2 slewing to 1.22 22:46:36.960: Axis 2 slewing to -4.78 That seems to show an intentional slew which Autoslew makes in DE in an attempt to get out of an over-current condition. Could something like this have resulted in the slew to 30 and 31 comae berenices? > "I don't really understand 'magnetic angle'. Once it has been established (manual homefind) why does it vary?" I am not sure about the precise definition which Dr. Philipp Keller uses for the "magnetic angle". My interpretation is, that in the Manual and in the User Interface, he actually means something like an offset between the magnetic field angle and the angular distance from the reference mark. (Note: probably multiplied by the number of the poles.) However, I have the impression that in the log-file, "magnetic angle" really means the absolute angle of the magnetic field vector. This would have to change when the geometric angle (e.g. of the RA axis) changes. Well, only my interpretation. I think Dr. Keller will know it better. But I have the impression that the magnetic angle might partly play a role in your problem. Did I understand it right that "I had to seset and start again" means you repeated the manual homefind? I get this impression from the log. If so, it is possible that you got a better magnetic angle the second time, resulting in a higher torque for the motor or less current consumption for the same torque. When I played with the Autoslew version which first had the manual homefind, I found that my magnetic angle was not very reproducible. I then reduced the magnetic angle find speed. This also seemed to influence the speed at which the reference marks are approached after the motor switches on during manual homefind. In my case, it improved the manual homefind precision drastically at the expense of a slower automatic homefind. However, I must admit that as a mobile user, I have a very special set-up, and I do the manual homefind without telescope and hence with bad balance. Best regards, Bernd

Hi, George, Were you able to watch the power consumption of your mount during the 10min exposure in your post from 12.05.2013 12:08AM? If so, did you perhaps see an abnormal increase in power consumption, which "suddenly" (i.e. within far less than 5 seconds) decreased back to normal? That might be an indication of a mechanical problem, in your case probably in the DE axis. There is a comfortable method of recording motor currents, encoder positions and position errors: Drive -> Servo-Setup, then Settings -> Log File Settings. Switch everything on and use an interval of e.g. 1 second. The log file is in Autoslew's profile-directory, subfolder ClServoLogs\. Don't forget to switch it off when you don't need it anymore. Best regards, Bernd

Hi, George, > "Strangely there is a 10.92 magnitude star over on the right that has a beautiful straight line showing above and below it." Another miracle. I think it is likely that the trail is really caused by this star (I think TYC1990-2169-1), because the trail passes exactly through the star, and because I think I can see where the trail stops near the upper edge of the image. That would have been the maximum excursion of the overshoot. The only explanation I have why colour could make a difference: HIP62778, according to StarryNight, has an extreme B-V colour index of 1.46, a surface temperature of 3567K and is 7.53mag bright. It seems to be a red giant. TYC1990-2169-1 has a B-V colour index of 0.73, a surface temperature of 5630K (i.e. almost sun-like) and a brightness of 10.84 mag. I could imagine that the extreme red colour and perhaps infra-red of HIP62778 falls into an area where your refractor is not so well corrected and spreads the central part of the light over more pixels. This would make the area around the star trail brighter and the trail itself a bit darker and more difficult to detect. But I still find it hard to believe that this effect could make a difference of 3.31mag. Best regards, Bernd

Hello, George, > "Also if the star was saturated then RBI is less likely." I agree. I would even say this excludes RBI. I cannot see how RBI could create a ghost image with more than some 100 electrons per pixel, while saturation for the KAF8300 is at 25500 electrons. > "the DEC axis seems to have some mechanical stiffness, and there maybe some tight spots" I remember there were discussions in the Yahoo ASA forum about position errors. Some of the problems seemed to be caused by mechanical problems, especially for older DDM60 mounts. See here: http://tech.groups.yahoo.com/group/ASA_AstroSystemeAustria/message/7459. The post also says that ASA can repair it. If we assume that your DDM60 really has a point where it "hangs" in the DE axis, then I think we could construct an explanation for your effect. Assume the DE axis is really stopped at a tight spot during an exposure. Then the controller will increase the DE motor current in an attempt to overcome the tight spot. If it overcomes the friction without a position error, then the DE-axis will suddenly start moving quickly, because it is accelerated with high torque. The controller will try to counteract this, but it will have a nonzero reaction time which depends on the PID setting. Therefore, the telescope would make an overshoot in north- or south- direction and then come back to the original position. In your case, it would probably be the north direction. The whole thing would look similar to the first derivate of the step response which can be seen in the tuning dialog. If your refractor is not too heavy and the PID settings are good, it can happen within a second altogether. One consequence of this explanation: At least with my PID settings, I remember a slight overshoot when the mount reaches the required position. Hence I would expect to see a trail on HIP62778 in south- and in north direction. > "the scope is a refractor so there should not be any diffraction spikes." This is interesting. I think I can see some weak "diffraction spikes" to north and south in your image posted on May 9th 04:32PM at HIP62778. If they can't be diffraction spikes, then they may be the trails we are looking for. Best regards, Bernd

Hi, George, > "The guiding was without any sudden jumps. The previous image was also without any jumps." I assume this implies that there were also no sudden jumps between the two exposures, except for the expected dithering. If so, it would make RBI as an explanation really unlikely. > "What about this as a possibility? [...] Autoslew makes a rapid Northward movement and returns, in less than 5 seconds. [...]" I think that could work as an explanation. Not only would the stars around NGC4725 leave dimmer trails, but these trails would also be partly masked by the telescope spikes which become visible due to the much longer exposure time of these stars. Concerning the brightnesses: according to StarryNight, the brightest star to be expected in your exposure is HIP62778 with a brightness of 7.53mag. The darker of the 2 suspected ghost stars is 30 Comae Berenices, allegedly with magnitude 5.75. Assuming those numbers are approximately correct, the intensity ratio between the two would be a factor of 5.15. This together with the bigger size and the spikes of HIP62778 could explain why its trail is not as well visible. Your theory would also lead to the same shape of the star-trails that is visible in your images. I also think that a DDM mount could be fast enough to do such a slew forth and back in 5 seconds or less. My only problem is: I have never seen Autoslew doing this. What could have caused it? Could you do one last test? Stack your two images that show the anomaly so that the stars are aligned, magnify the part around HIP62778 to pixel scale 1:1, and stretch it so that the sky background becomes a noisy gray. If your theory is right, then this star should have a very faint trail downwards, which might look just like a more prominent telescope spike. Perhaps it becomes visible after stacking. With some guessing, I think I might already see something like this in your post from 09.05.2013 04:32 PM, but it is not clear. Best regards, Bernd

Hi, George, My explanation obviously assumes that the faulty images are the first images in a series. If this is not the case, then I would have to assume that Autoslew suddenly jumps to 30 and 31 comae berenices and back, with the shutter open, and without storing that image to disk (otherwise you would have seen it). I agree that this is very unlikely. What a pity; the direction and distance would have so well matched. > "I still find this hard to accept because I use autoguiding, and set the scope guiding before any images are taken." A delay of several seconds before taking the next image makes the RBI effect a bit weaker depending on the chip temperature, but it does not completely remove it. At least not, when the chip is cooled. Here is a link to a paper by Richard Crisp that explains RBI: http://www.narrowbandimaging.com/images/RBI_presentation_crisp_7249-22_with_speaker_comments_paper.pdf I have once read a more detailed one, but I can't find it at the moment. > "but why were no stars in the ngc4725 area trailed?" Because the telescope was not moving when you imaged this area. My theory, in a bit more detail, goes as follows: You pointed the telescope at 30 and 31 comae berenices with the shutter of your camera open. This is a very weak point, because this seems not to have happened on May 8th. You slewed towards NGC4725 with the shutter still open. That creates an image of north-pointing star-trails on the sensor. Areas with high infra-red contents, and according to some papers, also very saturated areas of this image, create trapped electrons below the photosensitive shift register of the CCD. You close the shutter, and the camera flushes the sensor (i.e. reads out an image and possibly discards it). This removes all photo-electrons from the shift register. However, those that are trapped below the shift register structure are not removed. After setting up dithering and waiting for the mount to stabilize, you make an exposure of NGC4724. As the mount is stable, you get no trailed stars from the NGC4725 area. But the trapped electrons from the previous exposure with the two bright trailed stars slowly leak into the photosensitive shift register of your CCD. After 10 minutes, when you finish the exposure, a considerable amount of trapped electrons has leaked into the shift register. When the CCD is read out, these electrons create a "ghost image" from the previous exposure. Well, only a theory and with at least one very weak point, but I have nothing better at the moment. Best regards, Bernd.

Hi, George, It seems easier to say what it is likely not. Using your celestial coordinates and the geographic coordinates of Stonehenge (which seems to be in Wiltshire), and interpreting your time either as British Summer Time or as UTC, I was not able to find any object in StarryNight Pro that could match your trailing stars of 2013-05-05. Although this doesn't 100% rule out a celestial object or a sattelite, it means at least that I can't say which object it is. Also, I can't imagine any CCD defect that creates trails that are not exactly vertical or horizontal. An RBI effect during just 10s readout of such strength can also be ruled out, because you cooled your camera. If it is an RBI effect, then it needs more time (see below). Reflections of Earthbound objects in your optics could cause parallel trails, but not with a bright "star" and a weaker "tail" like yours. Reflections of a brighter star outside the image area are also unlikely, because the star that creates it would have the same movement as the telescope and the other stars, so it wouldn't cause a trail. Shooting stars - also unlikely. Why always 2 and always almost exactly at the same positions in the sky? A theoretical possibility: you imaged two very bright stars. Then, with the shutter open, you move the telescope away almost exactly in South direction. Immediately after that, you take an image at the new telescope position. In this case, you get the trailing stars as an RBI effect (i.e. a "ghost" image of your previous exposure). The 10 minutes exposure time should be sufficient to get most trapped RBI electrons into your image. In the resulting image, the two star trails would be absolutely parallel. Also, if you do the same thing twice, you get the same distance of the two stars and their trails each time. Slightly North of your imaged area, I can see the bright stars "31 comae berenices" and "30 comae berenices". They are almost exactly north of your imaged area, which would fit to the direction of your trails. Their angular distance is 32'02", which seems approximately similar to the distances of your trailing stars. And they are almost exactly on an east-west line, with the right (western) star a bit more north, like your trailing image. Is it possible that you had the telescope pointing to these 2 stars and the camera shutter open, when you started to slew to the imaged area? That would have allowed the trails to appear as an RBI image on your next exposure. Best regards, Bernd