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INDI Library v1.9.3 Released (11 Nov 2021)

Bimonthly Stable INDI Library release introduces new drivers and fixes for existing ones. Some highlights:

Guiding failing when changing filter

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I noticed a problem a couple of nights ago, and worked out what it was last night.
I'm using a filter wheel, auto focuser and an OAG.

For the first time I had set a filter offset for some of my filters, and I (finally) realised that everytime the filter was changed, the focuser would move, which would cause the guiding to report large errors, which in turn would abort the subsequent capture.

There is an option to suspend guiding when running a full auto focus routine, but is there an equivalent to suspend when changing filter offset? Or some sort of filter change settle time?

Thanks

Alex
1 month 3 weeks ago #76573

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Has anyone else noticed this, it can't be just me surely.

Is there a way to introduce a pause between changing filter and starting the next exposure?
1 month 2 weeks ago #76738

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Don't OAGs usually pass their light via a prism unaffected by filter? That's how I remember my QSI538 camera work to avoid this exact issue.
Jasem Mutlaq
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1 month 2 weeks ago #76739

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Hi Jasem, thanks for the reply.

I don't think the issue is actually the changing of the filter, it's the associated change of focus (due to a filter offset) that seems to break the guiding.

Changing the filter from the EKOS EFW config page works fine, but changing filter from the Capture tab causes guiding to go crazy in both axis.

Alex
1 month 2 weeks ago #76740

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I have a similar set up, but all my filters are supposed to be "par-focal", and while the offsets are actually a little different from each other I don't lose guiding when re-focusing (I tend to use a low max-travel, not too high step size etc...).
It may also be that your OAG camera is at the limit of focus when your main imaging camera is focused, try checking that the guide camera is par-focal via the OAG to your main imaging camera?
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1 month 2 weeks ago #76742

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My filters are supposed to be par-focal too, but my scope is a SW 80DS Pro, which is a doublet and not great for chromatic aberation.

This was the first time I tried setting filter offsets, but either with or without an auto focus run, the guiding issue still happened either way. It seems that there's an issue with the guide cam taking images while the focusser is moving, just between filter offsets is enough to be a problem. It didn't need to actually run auto-focus, although if I do, I think it's OK as there's an option to suspend guiding while focussing, but that doesn't apply to focus offset changes triggered by filter changes.

Removing the offsets between filters fixes the problem from the Capture tab.


Alex
1 month 2 weeks ago #76743

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With any given filter when the main imaging camera is focused correctly, is the guide camera also focused, or is it sometimes out of focus with some of your filters?
I use one of these on my OAG www.firstlightoptics.com/zwo-accessories...helical-focuser.html
It helps me easily adjust the focus.
I set it so that guide camera is completely in focus when we're using a filter that has an offset that's nearest the average of all the filters.
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1 month 2 weeks ago #76744

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The OAG is in front of the filter wheel, the filter selected won't have any effect of the image in the guide cam.

The OAG is one of the ZWO ones where the focus is only set by sliding the guide cam up and down and locking it with a grub screw. So acurately setting the focus is really fiddly, but it won't be changed by changing the filter. As demonstrated by the fact that guiding isn't affected when changing the filter from the EKOS EFW tab, or when changing filter from the capture tab with no focus offset. It only fails when changing filter from the Capture tab with an offset set. That's why I assume it's related to the focusser moving.
1 month 2 weeks ago #76745

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You focus using your imaging camera, and that focus is affected by the filter. The focus on the guide camera will therefore change when you focus your imaging camera (your focuser is in front of both cameras). If you don't have the guide camera well focused then it will move focus with an offset. This isn't always a problem, and some say guiding is better when slightly out of focus, but if you are not somewhat par-focal then with some filters the focus on your guide camera could be too far to guide properly when focused with your imaging camera.
I agree it is difficult to get the imaging and guide cameras using that grub screw, which is why I use the helical-focuser that ZWO supply as an accessory, it makes getting your guide camera par-focal very easy.
As per my review on the link I gave:

"If you've tried to adjust OAG focusing in the past with an alan key in the dark you will know how annoyingly tedious it can be and the worst thing that can happen is the camera and OAG fitting can just fall off to the ground if you don't support it while adjusting.
With the Helical Focuser getting parfocal is much easier and faster:
1. Use a Tablet with ASICAP, connect an OTG to both the Guide and imaging CCD.
2. Focus the Imaging CCD with the main focuser.
3. Switch ASICAP to the Guide camera, and focus the guide with the helical focuser."
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1 month 2 weeks ago #76746

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Yes, that's true that of course both cameras move with the focusser.
The magnitude of the offset is very small though, I'd be surprised if it makes much difference to the guide camera directly.

I've definitely got to get one of those helical focussers, I didn't realise they made them as an add-on for the smaller ZWO OAG.

So what do I do? At the moment as far as I can tell the guide cam is focused as well as I can get it.
When the focus moves, guiding temporarily goes crazy, with large adjustments, that take a while to settle out again, meanwhile the imaging cam exposure that just started, is aborted due to the large guide error.
Once the (erroneous?) large guide pulse has settled out, guiding continues correctly and imaging restarts, but I've lost a significant amount of time.

A delay option, or an option to suspend guiding while moving for filter offsets would fix the problem, but unless I'm missing something there doesn't seem to be such an option. There is when auto focussing, but that doesn't work here.
1 month 2 weeks ago #76747

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A lot may depend on your focusing settings and this can depend on what focuser you are using and what focus motor you have attached to it.

For me, I think I use the Polynomial algorithm, and my intial step size / max step size and max travel are limited to quite low values, but big enough to notice a difference in HFR (by the time I am in focus and ready to start the scheduler I should not need to move the focus very much to correct focus for changes in temperature, sky position etc....)

I guess if some of these values are too big then the focuser could move way out of focus and back in again, and that, of course, is going to affect guiding.

"Steps" are different for different focusers and set ups, so it might make sense to check with others who are using the same type of equipment as you.
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1 month 2 weeks ago #76748

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So maybe guiding should be suspended also on "Filter focus offset change" ? Is that what you meant?
Jasem Mutlaq
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1 month 2 weeks ago #76749

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