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Last night I did a rough focus and polar aligned to within .7 arc seconds on both axis. I then slewed to Arcturus to achieve precise focus using a tri bahtinov mask. Next I slewed to a guide star near the meridian on the same side of the meridian as my intended target (M13), calibration went smoothly. After acquiring the target I took a test shot and started imaging on autorun. Everything OK until after around 40 subs. Guiding went from under .3 to .5 arc seconds on the DEC axis until suddenly the error shot up to over 3 arc minutes of error! Sometimes this also happened on RA axis as well. Sky conditions were superb, no wind, no water vapor, Bortle 2, same as night before when guiding was stable all night. After that guiding continued to go from good to terrible for the remainder of the session. I tried adjusting aggression but it didn't help. Guiding with the ZWO ASI290 mini and the Skywatcher Evoguide 50ED. Might someone know what could cause this to happen? Guiding usually goes well for hours... Thanks, Jerry |
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| something snagging, high clouds, binding in the mount, flexure, could be several things |
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| As has happened to me on some occasions, one or more tripod leg moves and there goes good polar alignment. Could be a snag that got released, a night critter bumping into your mount, etc. |
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I will get a pop every now and then but for the most part my guiding is very stable and between 0.5"-0.7" RMS total. A few things I have found that affect my guiding and PA are: 1) I always do a focus before I do a PA even if I have not moved my setup from the previous night. I always do my pre-PA focus with either no filter (I am running a filter drawer as I have a OSC camera and for my object I will use a narrow band filter) or I use the L filter on my filter wheel. I have an EAF so I do an automatic focus but you should be able to do the same with a mask. I have found that any other filter, even RGB, can mess up my PA. When I do my PA with my AA+ I always shoot for a solution that is less than 1' and I try for less than 30". 2) Like you I use and ASI290MM mini cam for a guide cam. My guide scope is a 32mm Williams Optic guide scope. My guiding exposure is always at 2 seconds. I find I need to refocus my guide setup and recalibrate Phd2 probably every few weeks if I am out a lot or every time if I have not used my equipment much in the past few weeks. 3) My mount is a SW AM5 mount with no clutches. I place my OTA as best as I can find in the CG of that assembly even though with SW transmissions it's suppose to not matter. I find the CG very easy using a long Philips head screwdriver. I place it under the full OTA assembly which is on a Vixen dovetail plate and just roll the screwdriver left and right to find the center balance point. I mark it with a pencil and then when I place the assembly on my mount, I put my mark at dead center. You don't mention your mount but I assume you have it balanced well. 4) I don't use the automatic recalibration feature of Phd2 during a session. I tried that once after a meridian flip and the result was a disaster. For me possibly, I may have had come clouds come in just at the time it was trying to do that but that points out the problem of doing that. If I had just left it alone my mount probably would have tracked through the short cloud cover and recovered. As it was it just went bananas and all I got for the rest of the nights images were streaks. You did mention your target was M13 near the meridian. Did you do a meridian flip? Did it start failing after or before the flip? It would help if we knew your complete setup. |
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Here's Here's what I'm working with: ZWO AM5 mount Askar 130PHQ refractor ASI2600MC camera ASI290 mini guide camera Skywatcher Evoguide 50ED guidescope ASIAIR Plus with latest firmware and 2.1 app Here's what's not causing the problem: Tripod moved or bumped Clouds Poor focus with both guide and imaging cameras Here's what might be the issue: Cable snag or cable motion messing with a secure connection Some mechanical issue with something not tightened and causing motion ??? |
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As a newbie, I've been only capturing images since January, so I am by no means an expert in this but I'll give you my experience. In the past 6+ months, lI have had guiding that has been phenomenal and guiding that has been very terrible, all within the same target capture. I've sent messages and log info from the Air back to ZWO for analysis and they have responded fairly quickly. I had issues with the ZWO EAF and did the same and the techs provided me with good info to help solve the problems. So if you haven't checked with then, I would suggest you do so. The second thing I would check after a PA is that you send the mount to its home position. I have found that if I don't do that, the mount often looses track of where it is. But if the guiding at that point is good, then it usually will get back on track. Third, make sure the guiding is not affected by the focus. If focus is not good, it screws up the guiding, as I have found myself. I usually do a rough focus on a star after going to the home position following PA and then manually focus with the air to get into the ballpark before AF takes over. I've had conversations with ZWO techs and the mount techs and they both say if the subs look good, then don't worry too much about the guiding variations during capture. They claim focus is actually more important for some reason. I don't necessarily agree, but I've gotten great subs even when the guiding numbers varied dramatically and poor subs when the focus was off. That's all I got... |
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| What do your AA+ and Phd2 logs say? There is a LOT of good info in both of them. Looks like all of us have covered the usual suspects. Perhaps time to contact ZWO and ask for support. |
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| Backlash could be the issue and the balance of the scope in a specific orientation is causing the backlash to swing. |
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I always do my pre-PA focus with either no filter Hello, can you explain more why it’s preferred to do PA without filter? Does it really make a difference? Thanks |
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I always do my pre-PA focus with either no filter I can only speak for my setup but yes is does. The two mounts I have tested this on are my AM5 and EQ6R-Pro. I have seen as much as over 1' deviation of doing a PA with or without a filter. How I test this is I do a PA without a filter and then do one with a filter. Look at the difference between the two and the associated pointing error. If I do a PA without a filter or just my L filter, and then direct my AM5 to point at an object there is a difference in the pointing error. If I do a PA with a filter the pointing error is larger to the same object. My guiding tends to be worse also, which is always related in part to your initial PA. I did not take an exhaustive number of data points. That is actually something I would like to do down the road but I've not done it now. I just saw some differences and then focused on what gave me better results. |
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I always do my pre-PA focus with either no filter Filters reduce the amount of light. If you're having trouble plate solving when doing PA you could try not using a filter. That being said, I've had almost no trouble doing PA with even a NB filter such as the L-Ultimate. Bortle 1,2,3 or 4 skies help as well. Also lower focal length makes PA and plate solving easier, at least that's my experience. |
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Phil Hoppes:I always do my pre-PA focus with either no filter Thanks for the help. I will try PA with no filter next time I image. Cheers, |
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One real simple, but basic thing can happen (and has happened to me). One of the set screws that holds my guide scope in the mounting rings loosened. When that happened, I completely lost accuracy in my guiding. If you have this sort of setup, I would make sure my set screws are tight. |
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| Download the logs from the ASI Air and look at them in PHD2Viewer. Maybe post it here so we can see it as well. It won't be fair to say more without knowing what the logs show. |
