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Old 16-09-2018, 11:14 AM
boz_m (Alexander)
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Location: Sydney, Australia
Posts: 19
Differential flexure

I have a Celestron 8” EdgeHD and I am having trouble with my guiding. In PHD2 I get pretty good results (under 1 RMS error usually) but my subs move slightly frame to frame. Eg each 1 min sub moves consistently to the left. It does t rotate just slowly moves one direction.

The problem seems to be that my guide scope locks on fine but that doesn’t exactly align to the OTA. Is this differential flexure? I have heard long FL setups and SCTs can have this issue.

My guide scope has a pretty flimsy connection to the OTA and I can easily see movement if I give it a wiggle (not loose movement as the screws are tight, just not fully secure). If this is the issue would beefing up the connection fix the issue?

I want to avoid using an OAG. I tried it but Have issues then with the correct backfocus (I posted yesterday about this), interrupts the FOV too much and harder to get guide stars. Also my Guide camera has issues with spacing to get the same focus as the OTA. So if possible want to try to keep the guide scope rig
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Old 16-09-2018, 03:42 PM
kens (Ken)
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OAG is the best option but if you want to continue to use a guide scope then you really need to tighten up everything. We're talking micrometres of movement here so you'll never see it by eye. It could even just be things sagging rather than moving.
Also make sure you've locked your mirror.
A photo of your rig might help.
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Old 16-09-2018, 04:06 PM
boz_m (Alexander)
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Thanks ken. Yeah I just gave everything a good tighten and changed out the connection between the guide scope and the OTA for a more secure setup. Another thing that might have been an issue was the chords from the back of the guide camera so a fastened that also to the OTA so the chord is slack going to the guide camera. Will give it a whirl tonight and hopefully that has done the trick. Cheers
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Old 16-09-2018, 06:18 PM
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doppler (Rick)
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Hi Alexander, I am using an OAG and have the same drift in RA. In my case it's very slight and only noticeable when zoomed right in. I'm leaning towards PE, as the drift has become less noticeable since I did a belt mod to my mount and tightened the worm drive a bit. Here's a link to a discussion on the subject.
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