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Tracking starts of fine, but then... heater strip?

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#1 PeteS_MA

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Posted 19 September 2019 - 08:07 AM

I have an Orion Atlas and just last night a new issue popped up. The mount was at about 50% weight capacity, I was using PHD2 for guiding and taking 10 minute exposures of the Pelican. The first sub was fine, but the next two showed increasing trailing and those subs were unusable. I recalibrated on the target and got another good sub, but then trails again on the next one. I slewed to the equator and recalibrated again and ran guide assistant, then slewed back to the target and got three more good subs before it was time to flip. I rebalanced the mount so that it was still east-heavy and starting imaging again, but the same problem came back and I had to slew back to the equator (staying east-pointing-west) to calibrate again before I could get solid subs again. By the numbers, tracking overall was very good almost all night, some of the best I've ever had, with less than 1" RMS error reported by PHD2. The guide scope is attached using an ADM dovetail that I've used nearly a dozen times now with no problems. The rig overall hasn't changed much, except that last night I added a heater strip to the guide scope for the first time. I saw that the trail directions were not aligned along RA or DEC axes but were consistent across the bad subs. Has anyone ever seen this before? Do I just need to recalibrate PHD2 at the equator at the start of every session? I just can't figure out why the subs are showing trails while the guide scope and PHD2 seem to the think that everything is going great.


Edited by PeteS_MA, 19 September 2019 - 08:08 AM.


#2 rgsalinger

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Posted 19 September 2019 - 09:01 AM

Without more detail and seeing your subs, it's hard to tell what's going on. You need to recalibrate at the equator only when the guiding optical train has been changed in any way. So, yes, each night if you set up anew, you'd best do an guider calibration at the equator. It sounds like a balance problem, but in DEC balance, not RA. Try slightly unbalancing the DEC axis and see if that fixes it.

 

Since the heater was a change, you should remove in and see if the problem goes away. I mean it's possible that if the heater was on full tilt you might get some odd distorting in the guide scope objective lens. I run my heaters (when I run them at all) all night long at low levels. That seems to avoid having to deal with dew up to the point where I just give up on close the roof.

 

Rgrds-Ross



#3 PeteS_MA

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Posted 19 September 2019 - 10:23 AM

I was thinking the heater cable was tugging on the guide scope assembly at different angles depending on the orientation of the mount, but even that doesn't seem right. And running without a heater isn't an option in Massachusetts right now, it gets dewy within an hour of sunset this time of year.




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