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Guiding bad after dithering


MKR

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Hi,

Until recently I had no problems with guiding + dithering between the frames, but then I realised that I had been balancing the scope slightly wrong (doh...). I balanced it better for my next imaging session (just balanced, no east/west heavy). I calibrated PHD2 and off we go. The guiding seemed better than before, but after dithering, after 2nd frame, it totally lost it. It went haywire. As far as I remember my settling time is long - maybe 30s. Next frame was bad. It eventually started guiding within a reasonable error again. Then it dither, and lost it again. This time PHD2 said that it can't track anymore and gave up. I was using EKOS + PHD2. I switched off PHD2 and tried the same just with EKOS built-in guiding. Similar story.

What happened? Is anyone able to explain this? My mount is HEQ5 pro, scope 200p. I know it's a little bit on the heavy side for this mount, but it had been working ok-ish until now.

Thanks

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I am probably not the best person to help with this as many on SGL far more knowledge but they will probably need some more info such as the guide logs fro,m PHD2.

There was another thread on here very recently almost same issue i don't know if there is anything on there that may help
i-cant-seem-to-guide-anymore-phd2-not-giving-me-round-stars

Steve

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I don't *think* it starts the next image before it settles, however this settling (not the setting, but how long it really took to settle) took a lot of time, maybe 90s. It certainly didn't look scope being unstable after mount movement. I'm not sure how long the settling should last.

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I think this could be mechanical issue rather than mount.

If it takes long time to settle - it is a bit like needing a long time to dampen vibrations after touching the focuser.

I'd look at scope mount connection and how much weight is positioned at far end of the OTA. How big is your dove tail and where are your rings? What is your clamp like? Maybe look into upgrading clamp for a longer one?

What is the weight of your camera and other bits and do you have guide scope attached to the finder shoe or something like that?

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It's a standard HEQ5 + 200P setup from FLO:

skywatcher_explorer_200P_HEQ5_Pro.jpg

The camera is heavy, full frame DSLR. I use the same finder as above, but with an ASI224MC in. The weird thing is that I didn't have this problem before, but I was balancing my scope incorrectly - the rotation was wrong, so in one position it was good, however if you pointed the scope up it would fall. The problem started once I learned how to balance the scope properly. I don't know if this is a coincidence or not.

Also, probably unrelated and normal but thought I'll mention in case it's relevant: when the clutch is engaged I can still move the scope with some force. It doesn't move freely, but when someone tries to keep their balance by holding the scope, it can move (kids....). I hope that's normal.

Edited by MKR
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1 hour ago, MKR said:

Until recently I had no problems with guiding + dithering between the frames, but then I realised that I had been balancing the scope slightly wrong (doh...). I balanced it better for my next imaging session (just balanced, no east/west heavy).

I'm not sure what was wrong with your original balancing method? If there is any play/backlash in RA and you don't balance slightly East heavy, I think you will likely have some issues after dithering.

The guide log would be helpful so you could check if it an RA or DEC issue (or both!) Do you have PHD2 log viewer?

Edited by Tommohawk
added a comment re log viewer
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40 minutes ago, MKR said:

The problem started once I learned how to balance the scope properly. I don't know if this is a coincidence or not.

Large backlash can also act as elasticity in setup, and balance that is slightly off can compensate that by keeping gears "pressed" to one side so that they don't swing side to side (graph all over the place).

Perfect balance is only good if you have minimal backlash.

42 minutes ago, MKR said:

Also, probably unrelated and normal but thought I'll mention in case it's relevant: when the clutch is engaged I can still move the scope with some force. It doesn't move freely, but when someone tries to keep their balance by holding the scope, it can move (kids....). I hope that's normal.

That really should not happen. You should really need considerable force to move scope with clutches engaged (if it could be moved at all).

Mount probably needs a bit of mechanical tuning - both for backlash and in general.

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This is what I was doing wrong with the balancing before:

(5:30 shows a "balanced" scoped, when lifted up it falls. The fix is rotation, which I did for the first time recently).

I don't think I have the log, but I'll make sure to get one next time - waiting for a good weather again. I'll try do make it a bit east-heavy, I hadn't realised it was that important.

I'm slightly worried about the movement with clutch engaged. Perhaps I don't lock it hard enough? The mount is only a few months old..

Thank you all.

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OK I completely agree with that tutorial - nice job from Astronomyshed!

DEC balance issue is well made, RA is said to OK for visual when "perfectly" balanced, but the point about being East heavy for astrophotography is also made in closing.

RA probably causes more problems than DEC but is more easily solve just go slightly East heavy. (assuming the bearings are good this will sort even quite marked backlash)

DEC backlash is harder to sort because the corrections could be in either direction. 

I did a more detailed comment on this a few years back - see here.

Re the clutch, all it needs is to be able to stop things slipping in normal use. You absolutely don't want to be too tight as this can oval the bearing and spoil your guiding completely.

Edited by Tommohawk
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Just to add  comment regarding the HEQ5. Mine is fine for awhile then suddenly developed excessive backlash. Never really worked out why. Every so often I need to tweak the worm meshing slightly and it resolves until next time. Last time it happened I was imaging near the zenith and the guiding was appalling. Fortunately the cloud came and spoiled the session anyway!

I would also point out that a 200p on an heq5 for imaging is pushing the weight limits a bit.

Edited by Clarkey
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  • 2 weeks later...

i also have problems with dithering. I just dont understand why it messes up my guiding so bad.  It takes several minutes to get back to normal guiding. 

Since dithering is still important to me, i just manually dither: I use the coarse grid in PHD2 and choose one star and just move it randomly in one box of the grid every 10 minutes. This manual dithering costs me 30-45 sec max including settle time.

 

So why is the automatic dithering so bad. I use ATP to give the dither command and tried different dither step. Nothing helped.

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In most cases, guiding failure after dithering or after a meridian flip is caused by improper balancing and backlash. Use the guide assistant on both sides of the meridian. If the results, especially backlash, are significantly different you need to improve both balance and backlash. Try to get backlash below 1000 ms. PHD can handle what’s left. Slight imbalance and a small polar alignment error can help getting more consistent guiding, but the keywords here are slight and small.

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On 10/02/2022 at 07:49, wimvb said:

In most cases, guiding failure after dithering or after a meridian flip is caused by improper balancing and backlash. Use the guide assistant on both sides of the meridian. If the results, especially backlash, are significantly different you need to improve both balance and backlash. Try to get backlash below 1000 ms. PHD can handle what’s left. Slight imbalance and a small polar alignment error can help getting more consistent guiding, but the keywords here are slight and small.

I tried slight imbalance and it didnt improve the settle time. Maybe its backlash.

When i do manual dithering i always make the last small nudges in north and west direction. After that i start guiding and it only needs a few seconds to settle. I dont understand why the automatic dithering cant do it in the same way.

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