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Hello everyone! Yesterday I made my first attempt at polar alignment with the Three Point Polar Alignment plugin using my NEQ 3-2 mount with motors (no go-to) and the ZWO462MC PRO camera with the ZWO 30/120 guide tube. With this equipment I had to use the manual method. I had previously configured the ASTAP application with the D50 catalogue, but during the process I had problems solving both the first 3 recognition images and the images it takes in a loop showing my alignment error. The problem is that not always ASTAP can solve the photo, trying it in a loop until I move a bit more de AR axis. This problem increases the time it takes to get the first three detection images, but I manage to get them by moving the AR axis a little more after several attempts. However, in the error correction phase this problem makes it impossible for me to correct this error as I have to move the sleeves around to get the image and this changes the error. I am including a screenshot in case anyone sees an error, and so you can judge the quality of the photo. I always try to show more than 30 stars with photos of about 10 seconds exposure. This test was taken from my balcony and there is a lot of light pollution, so maybe that is the problem...  I can give you any other information you need. Thank you very very much! |
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| Hello, I see form the image that the stars are out of focus or have a lot of halos and gradient. this is not very good for Astap tp work. Can you retro to get better focus ? Youbcan alsonuse your main camera and main scope with TPPA, there isna reason why are you using the Guide scope? |
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Timothy Prospero: Thanks a lot Timothy! Yes I think that is out of focus, but yesterday i couldn't get a better focus, I'll try to improve that next time (today I hope). I used the guide scope because I have planned to try the autoguide with PHD2, so I prepare the equipment for that. I'll try to do the TPPA with the main scope next time and then, if I am successful, move the camera to the guide scope to do the autoguide. Its better to do with the main scope? Sincerely, it makes more sense to do with that but i dont know if there are any reason more. Clear skies! |
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Assuming you plan to image with the main scope you should definitely polar align with the main scope, as the guide scope is never perfectly aligned with the main scope, and often can be quite far off. And of course you need to be in focus. |
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Linwood Ferguson: I agree that it is better to use the main scope, it will have better resolution. However, where the guide scope is pointing shouldn't make a difference from a polar alignment perspective, since all you are doing is finding the center of rotation and aligning it with the celestial pole. The center of the circle will be the same regardless where the guide scope is pointing. |
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Arun H:Linwood Ferguson: Hmmm... I think you are right. I had not thought that through completely, the plate solves are obviously off, but the rotation center is the same I think. Had not considered that. |
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Linwood Ferguson: Yes - and indeed this is the reason why equatorial mounts work at all! It doesn't matter where you point in DEC, the center of rotation is always the NCP as along as you are polar aligned. |
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Yeah, though where I get confused is that there is a field rotation that can get introduced guiding if the guide scope is unaligned, and my mind can't quite come to grips with whether that has any impact on that final geometry of repeated iterations of alt/az calculations. But I think it's either exactly right, or close enough for PA, at least as close as TPPA can be (it's not intended for high precision anyway). |
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Linwood Ferguson: This deviates from the OP's problem, but it is still worth mentioning. The apparent rate of movement of stars in the sky is affected by atmospheric refraction and is hence slightly different depending on where in the sky your scope is pointing. That's why your guide scope and main scope have to point to the same place (or close). If they are markedly far apart, corrections will be inaccurate. |
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I would disagree that TPPA isn’t meant to be precise. I find it very precise giving you the ability to get your alignment error down to less than 10 arc seconds if you care to. I believe it also takes into account atmospheric refraction in order to be more precise. For the purposes of guided imaging, it’s unnecessary to be that precise, and depending on the equipment you’re using, having some amount of polar alignment error can actually help your guiding. By causing your Dec corrections to be consistently from the same direction, a polar alignment error of an arc minute or so can help if you have backlash in the Dec axis of your mount. If on the other hand, you have a high precision mount with negligible backlash in the Dec axis, you can use TPPA to get the error down to where there is pretty much no drift in the Dec axis. CS, Bruce |
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I should add that in order to have accurate & repeatable results using TPPA, you absolutely need to be in focus. I found that running a focus run before using TPPA makes the difference between having a good result, and having a garbage result. Poor focus can also affect ASTAP when doing a slew and center when using NINA. Because of this, I always do an initial slew to my target, followed by a focus run, and then a slew and center in order to really nail the target. Otherwise, I have found that I can be significantly off target which necessitates starting all over again and that is exactly what you don’t want to do when you are doing any level of automated imaging. CS, Bruce |
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I also have the ASI462MC and, given its high sensitivity in the infrared, it needs a UV/IR (luminance) filter to achieve perfect focus. I use an Astronomik L2 1.25” with this camera for guiding. When I use the TPPA routine for aligning, I always use the main camera, through the main scope. |
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Alfredo Beltrán: Thanks Alfredo for your advice. I'll keep in my mind for when i decide to improve my equipment. Do you recommend that filter to do solar system photo too? |
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I should add that in order to have accurate & repeatable results using TPPA, you absolutely need to be in focus. I found that running a focus run before using TPPA makes the difference between having a good result, and having a garbage result. Poor focus can also affect ASTAP when doing a slew and center when using NINA. Because of this, I always do an initial slew to my target, followed by a focus run, and then a slew and center in order to really nail the target. Otherwise, I have found that I can be significantly off target which necessitates starting all over again and that is exactly what you don’t want to do when you are doing any level of automated imaging. I am very grateful for your answers Linwood, Arun and Bbraunccp! Its really helping me a lot. As I can read, I have to pay more attention to my focus, that I think that it could be betterr if I use the main scope because (in my case) have a better focus system integrated that the system that have de guide scope. I hope that this is the problem and not something else related to the configuration of NINA, ASTAP, etc (they really seem very intuitive but I didn't find any tutorial about the configuration of ASTAP). Again, thank you very much. Santiago |
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I would disagree that TPPA isn’t meant to be precise. I find it very precise giving you the ability to get your alignment error down to less than 10 arc seconds if you care to. I believe it also takes into account atmospheric refraction in order to be more precise. For the purposes of guided imaging, it’s unnecessary to be that precise, and depending on the equipment you’re using, having some amount of polar alignment error can actually help your guiding. By causing your Dec corrections to be consistently from the same direction, a polar alignment error of an arc minute or so can help if you have backlash in the Dec axis of your mount. If on the other hand, you have a high precision mount with negligible backlash in the Dec axis, you can use TPPA to get the error down to where there is pretty much no drift in the Dec axis. Or talk to the author, pretty sure Stefan has said it.  But there is really no such thing as completely accurate polar alignment. I recall using TSX's "Accurate polar alignment" option, and at the end it would give you about 5 (vague memory how many) different options for your choice: Refractive pole, actual pole, minimize DEC drift, minimize total drift, minimize rotation. There was a bit over an arc minute difference in these, and the presumption that each served a different purpose. I've often asked when you should use refracted pole vs actual pole, for example (which is available in TPPA). Someone told me "use refracted pole near the pole, use actual for the rest of the sky". I do not know if that's correct guidance or not, but I am not at all sure the proper answer is to always correct for refraction. I used to have a screen shot of TSX's choices, but cannot find it. Quite interesting for those in search of the one right answer. |
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Santiago Rodrigo: Yes, absolutely. And an ADC as well. See one of my results here:  |
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Linwood Ferguson:I would disagree that TPPA isn’t meant to be precise. I find it very precise giving you the ability to get your alignment error down to less than 10 arc seconds if you care to. I believe it also takes into account atmospheric refraction in order to be more precise. For the purposes of guided imaging, it’s unnecessary to be that precise, and depending on the equipment you’re using, having some amount of polar alignment error can actually help your guiding. By causing your Dec corrections to be consistently from the same direction, a polar alignment error of an arc minute or so can help if you have backlash in the Dec axis of your mount. If on the other hand, you have a high precision mount with negligible backlash in the Dec axis, you can use TPPA to get the error down to where there is pretty much no drift in the Dec axis. I agree with everything that you've said. I'm an MX+ owner, & I rarely do a T-Point model at this point, finding for myself that I have excellent results with PEC enabled & a good polar alignment. I realize that there are a number of accepted different options/methods for judging the accuracy of polar alignment. This is beyond the scope of relating a basic understanding of using available tools to get an appropriate/adequate polar alignment for a given equipment setup. I can say though that if I run TPPA, and adjust it so that there's about a 1 arc minute error, I will have dec drift in a particular direction if I turn off my guiding. If I run TPPA, and adjust it down to a total of close to 10 arc seconds total error, & turn off my guiding, I have no discernible dec drift over a few minutes time. Practically speaking, to me that's what a perfect polar alignment means. Since my MX+ has very little dec backlash, that's what I often go with. If I'm using my Atlas Pro mount, having some drift is best, so I shoot for an arc minute of error when polar aligning & that works well for that setup. CS, Bruce |
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Didn't intend to knock TPPA, and yeah, I generally try to get around a quarter arc minute myself. One thing though for many people to realize -- if you have a tripod, on ground or pavers, once you polar align things change - ground settles or swells, wood tripods expand with humidity, metal moves with cooling contraction.... those who image portably can have a "perfect" polar alignment at the beginning of the night and it will change considerably over the evening. I wish I had a real pier and permanent setup.... Anyway, obsession in PA for those starting out may not be fruitful. Indeed, for those with a lot of DEC backlash who guide benefit from some mis-alignment to reduce reversals.  |
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| I agree with others on focus, use primary scope. But also would like to add that you need to make sure your focal length is correct. Been there, done the wrong focal length. |
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Hello, as far as I know, there is something very important in plate solving with NINA, the focal length and the camera parameters have to be the good ones to work properly. I do not know which parameters you gave to NINA, the main camera on the guide gamera, the imaging telescope focal length of the guide scope focal length but if NINA ASTAP do not know the right parameters for plate solving this is a very known reason for ASTAP to fail. I know it because I forgot once after adding a powermate in the light path and no way to plate solve ;-) By the way, poor focus is also a good reason for astap to fail. You can start NINA by targeting any region of the sky and look at the HFR till you get the minimal value (the same process as autofocus but manually) if you take your time to pause and let one capture with steady focus, and use the zoom to see the HFR numbers, it works well. Clear skies |
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Indeed, I would say plate solving failures reported on NINA's discord mostly come to either improper setup of ASTAP (wrong or no database) or wrong focal length. This can be especially true in a few cases:
One thing people can do is take a good clear image with lots of stars and save it, and run ASTAP manually, hit the sigma button and go to the pixel math 2 tab and it shows the effective focal length. You should use that in NINA, it is "right" even if you think it is not what it should be. This is really true for all non-blind plate solvers not just ASTAP (and you do not want a blind solver really for NINA, since they take a long time). Also, ASTAP changes somewhat rapidly, and it is a good idea to keep it at least fairly current: https://www.hnsky.org/astap.htm |
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Thank you all! I think we just created a very interesting discussion on this topic, again, thank you very much. Finally, I tried yesterday to do the TPPA process again , and it worked! The problem was the focus, I found that I could improve it when I really thought I was seeing the best focus that the tube and camera would allow me to get. I did it with the guide tube because I don't have a long enough extender to be able to achieve focus with the main telescope, so I was forced to try again with the guide tube which, although it has a worse focusing system, allowed me to achieve it. All the other issues that have been discussed in this thread are very interesting and have helped me a lot to understand the operation of these programs and understand their settings, you can be proud of everything you know about this precious discipline. I wish you the best skies! Best regards, Santiago |
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Santiago Rodrigo: Hey! Ahmad here, and I totally feel your frustrations with polar alignment. I've dabbled with similar setups in the past and faced some similar challenges. The ZWO462MC PRO is a solid camera, and the NEQ 3-2 is a robust mount. However, polar alignment can be a tricky process, especially in light-polluted areas. Based on your description, a few things come to mind: Light Pollution: This can be a significant factor when you're trying to get a clear image for alignment. Even with a 10-second exposure, the light pollution might be washing out some of the fainter stars that ASTAP needs to solve the image. Image Clarity: It might be worth checking if the images you're capturing are clear and not blurred due to any focus or movement issues. A blurred or slightly out-of-focus image might give ASTAP a hard time solving. ASTAP Settings: Ensure you've set the correct parameters in ASTAP for your equipment and observing conditions. A misconfigured parameter might result in failed image solving. Star Saturation: Overexposing the brighter stars might create issues. Try to ensure that the stars aren't overexposed in your images. Mount Movement: Any movement, even slight, can affect the alignment process. Ensure the mount is stable, and there's no play or backlash causing the movement. Looking at a screenshot would definitely help, but I'm afraid I can't directly view images through this platform. However, I'd suggest comparing your images to solved ones available online or in ASTAP forums to gauge if there's a significant difference in quality. Polar alignment can indeed be a time-consuming process initially, but with practice, it gets more manageable. I hope you find these suggestions helpful, and wishing you clear skies ahead! |
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Santiago Rodrigo: Hey Ahmad! I appreciate your reply, it's very helpful and complete. I'll save it for my next trip when I hope to improve my polar alignment by getting away from the light pollution. I think this is the factor that is affecting me the most when I try to polar alignment. Anyway, thanks to the answers of the users in this thread, I managed to get to the second step and ASTAP solved the plates in all the pictures I took. However, when I managed to go to the second step in TPPA (the step in which the error is corrected according to the indications that it shows you) it happens that if I don't touch any screw, the error increases in each photo that is taken, and it is difficult for me to reduce the error as it is not a fixed value. I understand that this means that the program has not done its job properly (because of some mistake of mine), but I can't understand what could be the error that generates this problem. Has anyone encountered this problem and can give me some hints on how to solve it? Thank you very much Santiago |
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Hi Santiago, just check twice that your mount is tracking at the good sidereal rate. I remember once with NINA having encountered this strange feeling that the error on TPPA was increasing and it was partially true: my mount was not tracking so NINA assuming it was tracking the polar alignement error was increasing. Frédéric |
