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| After locking the Alt/Az clutches, my polar alignment always changes, so it’s hard to get a really precise alignment. Is there any advice to avoid that? And is a 5’ alignment error good enough, considering that I will guide? |
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What method are you using for polar alignment and how are you measuring it later? 5' isn't ideal and will definitely increase the amount of guiding pulses sent to the mount. (more pulses, more adjustments, more chance to be off, etc) I generally use sharpcap's polar alignment and can get it to within 10" of error when aligned. BUT...if i repeat that process immediately it will show closer to 20-30". I'm not sure what thats about. I've also found that different method give you conflicting PA measurements. Sharpcap will say 10" or error, then if I use the iPolar, it'll show it's off. Throw in the NINA 3 pt alignment now and I have another method to tell me I'm off. I'm still waiting for someone to build in motorized auto polar alignment in a mount. (I heard Avalon has something??) |
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Rafael Sampaio: If I correctly understood your problem, it does happen when you make mechanical force to lock the clutches. After some trial and error on my CEM70 I manage to adjust to less than 1’ when mounted on a fixed pier. I took me some time to get used to prevent the small movement that is produced when tightening the screws that lock the mount aligned, so what I do is to adjust the polar axis assuming (predicting) a bit of movement when tightening everything. After a while it worked for me and now, I am in about 0.6’. However, this do not happen on my GEM45, it has a better system to tight and lock the clutches. If you refer to something different, I apologize and forget my comment. CS ! Claudio |
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Not sure what you mean by clutches. I use a Skywatcher HEQ5 mount and when I polar align, I lock the adjustment screws after each adjustment. On my mount, PA is very sensitive to screw tension. I can make minute adjustments by slightly reducing or increasing the tension on the screws. I use N.I.N.A.’s Three Point Polar Alignment plugin and I adjust to under 1’ total. Afterwards, I park the mount and repeat TPPA. If I’m still under 1’, I roll with it. Jim |
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Skywatcher (at least the EQ6-R did) has these clutch lever that allow you to tighten the mount down after PA. Ioptron doesn't have those. On the CEM70 you screw the mount into the tri-pier, then keep the ALT screws fairly tight when you do PA. (so that once you have PA aligned you can just stop rather then tighten additional screws and ruin PA) |
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Rafael Sampaio: This is a known issue on the 135 mounts. Locking the altitude clutch will change the alignment slightly. Best way I've found to counter this is to tighten them in stages. Light clutch engage, then adjust back, medium clutch, adjust then final clutch lock shouldn't do much change in PA. In general you want to aim for ~1 arcmin or less as your polar error. You can certainly get away with more (like 5') but you risk having less round stars as the guiding will have to make more adjustments to keep the mount tracking accurately since your mount isn't pointed correctly. |
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| on my ioptron mounts a slowly and iteratively tighten up the az/alt locking screws rather doing them tight one by one in one swoop.This prevents pressure being applied in one direction or another during the fastening process. |
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a slowly and iteratively tighten up the az/alt locking screws rather doing them tight one by one in one swoop Exactly, I do combine this approach to what I mention above, kind of "predicting" the slight movement when tightening the screws. I had some problems in my CEM70 if not tight firmly those screws. |
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| Try aiming a smaller error... I use sharpcap to polar align but other plate solving options work on the same principles. When it ask you to rotate the RA axis through about 90 degrees, use you GOTO system / hand controller instead of unlocking the RA clutch, then bring the mount to home again using you GOTO system not the clutch. |
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Cristian Rodriguez: Does this make a difference? I usually unlock the clutch and turn it manually |
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| It does make a difference to me. The less I touch the mount the better. |
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Cristian Rodriguez: ah, yes, fair point. It would be nice if Ioptron had a preset position for the 90 position on RA. (similar to goto Zero) That would make it simpler |
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| 5' of polar error is large. but...... if your RST is imbalanced, try polar alignment when scope is on top of the mount, and measure the error after moving the scope to a side. you might be surprised by how large the error is (depending on the amount of imbalance and your tripod strength). |
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Rohan:Rafael Sampaio: Yep, finish the PA fine adjustments with the locking knobs/lever 80% tight so movement is minimal when locking 100%. It very predictable motion. With Polemaster on 135E, I aim low and left, locking brings it up onto the target. But as the other users indicate, when I repeat the alignment it tells me I need more adjustment. Once through the process gives +/- 2' and the second iterations gives under 1' PA error (that I need for 2-3 min of unguided). |
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5' is a pretty large error. You'll get better results if you try to get a bit better aligned--particularly if you are using longer exposures and if you run all night. I'm a fan of the the QHY Pole Master. If I'm in a hurry, I can usually dial in the PA to within a minute or so. I used it on my 20" in Chile and achieved this level of alignment. Keep in mind that the scatter in this plot mostly reflects mechanical flexure in the system. |
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Nick Grundy: Sharpcap-style PA will produce more incorrect results the farther off you start, as well as with every photo taken during the adjustment phase. E.g. if you start at 10 degrees away from the pole, it may not even converge and if it does converge the solution is very likely to be incorrect (and NINA will warn you about that). Similarly, if you reach 20" arcsec error with a handful of adjustments and then spend the next ten minutes bringing it down to 10", it is very likely what you've reached is worse than the original 20". So two iterations are usually required to be confident about the result, and the second mustn't be too long. That said, 10-20 arcseconds is still a *very* tiny angle. Even the minuscule displacement caused when the scope moves to the zero position and back again is enough to explain it, let alone numerical errors. If you are guiding, as long as both your alignments report an error of 1 or 2 arcminutes or less, you are all right. Cheers, D. |
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| RST 135 Supplementary Manual V25 compr.pdf Page 12 puts numbers on the drift for a 5 min exposure based on PA error |
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Sharpcap-style PA will produce more incorrect results the farther off you start, as well as with every photo taken during the adjustment phase. E.g. if you start at 10 degrees away from the pole, it may not even converge and if it does converge the solution is very likely to be incorrect (and NINA will warn you about that). just to clarify, you are saying that the polar alignment gets better in full iterations? (i.e. start with the first assessment and adjustment, then start the process again from scratch, as that will produce a more accurate alignment when completed?) I've been using NINA's polar alignment a bit recently and it seems ok so far. Anyone done some comparisons with that? |
