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I am using an iOptron CEM 120 with an EDGE 14 and ASI6200MM and an Innovations Foresight XM On Axis Guider (guide cam is an ASI0171MC). I've polar aligned with the iPolar camera and I'm perfectly level on a permanent pier with good RA and Dec balance. I'm shooting at F11 and notice trails when pointing *east* even on 30 second test shots (without guiding) - When guiding (PHD) with the ONAG the Dec is good but the RA is poor with occasional big jumps in no particular direction. BUT - the funny thing is that pointing *west* all is pretty stable.... Has anyone experienced something similar ? |
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| Sure it ain't the iPolar fault. Or polar alignment either as long you're about there. More likely you need to rebalance the load balancing to be RA heavy on both sides of pier. |
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Its just a camera/lens.Its what you and your software does with them to make an accurate polar alignment. Which software did you use to carry out the polar alignment.Ive never used the actual ipolar software so i cant comment on that if that is what your asking. What polar alignment error did you achieve at the end of your polar alignment process phd all configured correctly?(sometime when you change your setup you can easily carry over wrong setting) Sounds like a general guiding troubleshooting exercise to me... i use ipolars with phd/INDI .Very convenient having it there built into the mounts. |
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In practice I've found the ipolar accurate down to about 1-2 arcminutes based on PHD2's declination drift calculations from the logs. 2 arcminutes of PA error will give you about 0.3 arcseconds of drift at declination 0 in 30 seconds. This is 150% your image scale at 4000mm~ focal length. This is basically to be expected with unguided imaging at extreme focal lengths |
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In practice I've found the ipolar accurate down to about 1-2 arcminutes based on PHD2's declination drift calculations from the logs. If the OP is pointing "East" I doubt that it is likely that the scope is pointing at 0 Dec. |
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| Never mind the seeing is got to be equal or better than 0.3" to notice any drift. |
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I've been using the CEM120EC for several years, first with a PoleMaster but lately with N.I.N.A.'s plug-in for PA. I use PhD2. Here's my accumulated experience: 1. N.I.N.A. produces a more consistently accurate PA in about the same amount of effort and time. It's also great that visibility of Polaris is not required. 2. Once you think you have proper leveling and PA with the PoleMaster, slew to your target and do the PoleMaster PA again from that orientation, and you may be surprised how much it disagrees with the PA you just did with the scope nearer to its park/zero position, despite being in perfect balance. 3. The bubble level of the CEM120 is too small and poorly calibrated (in manufacturing tolerance of the printed circle to actual level). I found that adding two longer (~2.5"), larger linear bubble levels on E-W and N-S axes to mount base works much better, but check these first and be finicky in selecting known good ones. A trick to testing linear bubble level calibration is to level it on a surface then turn it around end to end, and see if it agrees with itself. If the level is not clearly marked as to which side is supposed to be up, try each of the four sides of the level to find the one that actually works best. Bear in mind that while GPS is accurate to tiny errors in angles and clocktime, the mount software's tracking calibrations assume a truly level mount, and eyeballing the bubble level(s) are by far the major source of error in the overall system design. 4. For whatever reason, my mount tracks much better with just a little imbalance in both RA and DEC axes. The DEC due mainly to wind gusts I believe. 5. In PhD2, experiment with your RA Threshold and Aggressiveness settings. I've had really good luck reducing both, so less error accumulates before a command is sent to the mount, but once sent, it is a small one (and more than one such gentle nudge is required ; 3 or 4 usually). I.e., Don't try to zero the error with one larger push. In general one should not see a corrective pulse in the opposite direction, but a smooth decline and nicely damped error reduction. 6. A perfectly focused guide camera can introduce more guiding jitter than letting it be a bit out of focus, so that PhD has a smoother Gaussian star intensity profile to examine. In a similar vein, for the atmosphere around here, exposure times around 5sec produce better results than 2 or 3 secs, to not excessively chase the seeing. The overall system response does not have the analytical nor mechanical bandwidth to correct for seeing anyway. Enable multi-star guiding. 7. If your target is low in Altitude, set the mount to King vs. Sidereal tracking, to correct for atmospheric refraction as the target rises. This reduces the number and frequency of guider corrections sent to the mount. 8. A too-slow CPU controlling your rig can introduce all manner of oddball effects that are hard to pin down until you realize you've asked too much of it. 9. Once your planning is done and your sequence is imaging, disconnect your rig PC from the internet until morning. I hope one of those tips helps. All that said I am very very pleased with the overall performance of the CEM120 vs. mounts I replaced with it! CS, Jim |
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| Excellent advice ! Many thanks ! I'll give it a whirl the next time out. |
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| I have been running a CEM 120 EC version for about 3/4 of a year now and I have had to replace some boards on it. My RA now is all crazy and won't search for zero position. Anyway, I do like the mount, but the tips from Jim sound great. I will say that I used the iPolar and found vastly different results when I jumped over to NINA to polar align. I felt that the NINA polar alignment worked better as well each time, so I always use that and just capped off my ipolar and do not use anymore. But I will second Jim's thread on making sure it's all level and slightly heavy on one side. I also find better results bumping aggressiveness way down on the CEM 120 in PhD. |
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Jim Lindelien: Hi Jim, Just wondering regarding point 4; which way do you find the imbalance to help? Counterweight heavy or scope heavy? and regarding point 5; what threshold levels do you think work best? (I've experimented from 60 up but have you gone lower than that maybe?) I've also tried a variety of time intervals and I've determined 3 to 5 sec to be best BUT I think it should be possible to go 10 seconds plus on this mount with the right parameters? Thanks in advance! Pete |
