Advice on when to set re-focussing in a nights sequence

[teoria_del_big_bang]

I am currently just getting into NINA advanced sequencer.
In my limited time so far imaging I have used parfocal filters, or at least I assumed parfocal as they are all from same manufacturer and all same thickness and have not refocused after filter changes but just re-focussed after so many images or after a certain temperature change. I can't say I have noticed a loss in focus on filter changes but probably not checked to the nth degree any change in focus just that it look okay to me by eye inspecting the images when zoomed in.
NINA now allows you to be more intelligent so I focus after a HFR change but wondering if that is enough?

I don't really want to be re-focussing every filter change so when I get some clear skies will take time to see if I need to use offsets to avoid this.
Does anyone find they need to use offsets or actually refocus after filter changes when using so-called par focal filters ???

Is using the HFR option enough, ?
In NINA there are also options to do so after temperature changes, after a preset time, after filter change, after a number of exposures but I would have thought using HFR would really then just do it when it was required, but then I think that means it actually has to go slightly out of focus before it takes any action so is it better just to do it every so often anyway ?

Sorry if I seem a bit clueless with this but it is obviously important not to waste images through bad focus yet not to waste potential imaging time focussing every few minutes as it does take time to re-focus.

A final thought I had was that when using LRGB could I allow it to maybe go slightly more out of focus during the RGB which will probably be binned 2x2 and then make sure it is precisely focussed before going back to the Luminance?
 

Steve

[knobby]

Good question, I've only recently set up an autofocuser and took the easy way out ... Refocus on a filter change 🙂

[teoria_del_big_bang]

40 minutes ago, knobby said:

Good question, I've only recently set up an autofocuser and took the easy way out ... Refocus on a filter change 🙂

I guess it is a question I could solve myself given lots of clear nights but if anything like last year that will not happen.

Is this the safe optio or have you seen you need to re-focus for different filters ?

Steve

[knobby]

I just chose the method as I couldn't see another way in APT, I set a plan up with about 20 minutes per filter.

When I look back at old captures, despite the zwo filters being 'parfocal' I think I can see a shift in sharpness on them .

I'm dabbling with NINA now so will look at the other options.

[AMcD]

With SGPro, I run the autofocus routine on the first filter I am using during the session and thereafter have SGP set to refocus for every 1.5-degree change in temperature and immediately after each filter change.  I have filter offsets entered for each filter so that the autofocus after filter change starts from roughly the right point for the new filter.  This seems to work pretty well. 

Not much time is lost to the autofocus routine when taking LRGB images as the autofocus routine for LRGB requires only nine 15 second exposures at Bin 1x1 plus download time.  It is, however, a different story with narrowband filters.  The autofocus routine can take up to five minutes with the Ha filter, even at Bin 2x2.

For LRGB I am using Antlia Pro LRGB filters. Whilst Antlia state that "The uniformity in substrate thickness delivers good parfocal performance across all Antlia filters" my understanding has always been it is a good practice to refocus between filters to account for any slight shift between them.

[Pompey Monkey]

Just remember that although your filters are parfocal,  it is very unlikely that the rest of your imaging train is too.

Any curved bit of glass has the potential to slightly change the focus at different wavelengths.

Yes, for the best results, you should refocus at every filter change.

In practice, due to atmospheric seeing and the sampling resolution of your equipment, you might not notice any difference if you just focus for lum every so often.

[teoria_del_big_bang]

15 hours ago, knobby said:

When I look back at old captures, despite the zwo filters being 'parfocal' I think I can see a shift in sharpness on them .

 

13 hours ago, AMcD said:

For LRGB I am using Antlia Pro LRGB filters. Whilst Antlia state that "The uniformity in substrate thickness delivers good parfocal performance across all Antlia filters" my understanding has always been it is a good practice to refocus between filters to account for any slight shift between them.

 

1 hour ago, Pompey Monkey said:

Any curved bit of glass has the potential to slightly change the focus at different wavelengths.

Yes, for the best results, you should refocus at every filter change.

Thanks for the info , looks like I really should focus on a filter change, or better still spend an hour or two next clear night and determine all my filter offsets then I can use those in NINA.

Steve

[A320Flyer]

Steve,

I use focus offsets in NINA (used the Dark Customs plugin to set them up). But in the NINA options I don’t actually specify which filter is the autofocus filter! 
When doing NB, in my Sequence, I have:

Change filter to Ha (choose the filter for AF)

Autofocus

4x300 Ha

4x300 Sii

4x300 Oiii

Dither

Now loop the above until Dawn

With this loop, you get an AF every hour, and a dither in every 4th frame. The AF is done on the filter that I’m going to use for Lum, with the other filters using focus offsets. If your system is reasonably parfocal, you might not need offsets. 
 

If doing LRGB, I substitute the Lum filter for AF and adjust the number of exposures. 
 

My thinking on this is that I want best focus on the filter I’m going to use for the Lum in processing, but not so critical on the others (as I usually give them  a slight blur anyway).
 

HTH

Bill

 

[teoria_del_big_bang]

On 15/01/2023 at 08:48, A320Flyer said:

I use focus offsets in NINA (used the Dark Customs plugin to set them up). But in the NINA options I don’t actually specify which filter is the autofocus filter! 
When doing NB, in my Sequence, I have:

First clear night of the year last night and used the same plug in, very easy and didn't take too long.

Turns out all the filters (same manufacturer and thickness) were pretty similar anyway, the LRGB were all very close and the spread was only 32 pulses of the autofocuser which really is hardly nothing, the 3 NB filters also very close to each other with only 12 pulses between them which is probably why I could not see any noticeable problem not focusing on filter change.
I guess this will not always be the same for all filter collections.

Anyway better to have even these small offsets in and now just focus on HFR change.

Steve

[Adam J]

On 14/01/2023 at 14:17, teoria_del_big_bang said:

I am currently just getting into NINA advanced sequencer.
In my limited time so far imaging I have used parfocal filters, or at least I assumed parfocal as they are all from same manufacturer and all same thickness and have not refocused after filter changes but just re-focussed after so many images or after a certain temperature change. I can't say I have noticed a loss in focus on filter changes but probably not checked to the nth degree any change in focus just that it look okay to me by eye inspecting the images when zoomed in.
NINA now allows you to be more intelligent so I focus after a HFR change but wondering if that is enough?

I don't really want to be re-focussing every filter change so when I get some clear skies will take time to see if I need to use offsets to avoid this.
Does anyone find they need to use offsets or actually refocus after filter changes when using so-called par focal filters ???

Is using the HFR option enough, ?
In NINA there are also options to do so after temperature changes, after a preset time, after filter change, after a number of exposures but I would have thought using HFR would really then just do it when it was required, but then I think that means it actually has to go slightly out of focus before it takes any action so is it better just to do it every so often anyway ?

Sorry if I seem a bit clueless with this but it is obviously important not to waste images through bad focus yet not to waste potential imaging time focussing every few minutes as it does take time to re-focus.

A final thought I had was that when using LRGB could I allow it to maybe go slightly more out of focus during the RGB which will probably be binned 2x2 and then make sure it is precisely focussed before going back to the Luminance?
 

Steve

The problem with using the HFR option alone is that you can have situations when a couple of bad subs or high level cloud will cause a refocus when none is needed and indeed in some cases could make it worse.

Personally I always manually choose when to refocus but over the years have learnt the temperature shift that has to happen in order to require a refocus on my scopes. This is something that is easier if you are using a temperature probe somewhere in yoursetup.

I personally would never assume that my system (not filters alone) are parafocal in LRGB or SHO for that matter. This is because its about more than just the filters and I want the best focus possible in each channel. So I always set to refocus on filter change. It also makes sense to me to refocus after a flip as you have stopped imaging anyway to perform the flip and so I find it worth while. This is especially the case as you can get into a situation that the system is looking at different stars before the flip to after and so the base HFR changes post flip. It also negates any possibility of the focuser itself shifting during the flip.

Adam

[teoria_del_big_bang]

8 hours ago, Adam J said:

The problem with using the HFR option alone is that you can have situations when a couple of bad subs or high level cloud will cause a refocus when none is needed and indeed in some cases could make it worse.

Yes, I see that now, makes perfect sense 🙂
So, thinking about it now should I actually totally remove the re-focus on HFR, or just put it to a larger tolerance ?
From this point you raise I think I should remove the option all together and either re-focus every so often as a matter of course AND re-focus on a certain temperature change.
 

8 hours ago, Adam J said:

over the years have learnt the temperature shift that has to happen in order to require a refocus on my scopes. This is something that is easier if you are using a temperature probe somewhere in yoursetup.

I do have a temperature probe so maybe I need now to take the time out to learn, like you have, what amount of temperature change does what to the focus. The Sesto Senso focusser has a temperature probe and you can enable a temperature compensation but so far never used this, I also have a temperature probe on my Pegasus PPBA I can use in Nina to refocus on a certain change in temperature.

8 hours ago, Adam J said:

It also makes sense to me to refocus after a flip as you have stopped imaging anyway to perform the flip and so I find it worth while. This is especially the case as you can get into a situation that the system is looking at different stars before the flip to after and so the base HFR changes post flip. It also negates any possibility of the focuser itself shifting during the flip.

Yes totally agree and I would always refocus after the flip.

Thanks Adam for all the good advice 🙂 

Steve 

[jjosefsen]

I would look into setting up offsets and use those when changing filters, it cuts back a lot on the time spent refocusing.
There is an addon by DarkArchon that adds a wizard functionality to calculate the offsets by doing a series of autoruns.

Even though my filters are parfocal, there is some change between the filters, so I think it is worth it.

In regards to the HFR change, it is a running average pr. filter that you can customize when setting up your sequence.
I believe I use something like 8% change over 5 images, and it does not trigger if a thin layer of clouds drifts in and affects one image.
Should you get "false positives" you could tune those parameters to be less effected by it, if you so chose.

[teoria_del_big_bang]

1 hour ago, jjosefsen said:

I would look into setting up offsets and use those when changing filters, it cuts back a lot on the time spent refocusing.
There is an addon by DarkArchon that adds a wizard functionality to calculate the offsets by doing a series of autoruns.

Yes I used this to set my offsets and so easy 🙂 

Steve

[teoria_del_big_bang]

21 hours ago, Adam J said:

It also makes sense to me to refocus after a flip as you have stopped imaging anyway to perform the flip and so I find it worth while. This is especially the case as you can get into a situation that the system is looking at different stars before the flip to after and so the base HFR changes post flip. It also negates any possibility of the focuser itself shifting during the flip.

Just looking at this, how do you set NINA to refocus after a flip, mine has just flipped but did everything perfectly, recentered and all that but no refocus.

Do you have to use the plug in DIY Meridian Flip ?

Steve

[Adam J]

1 minute ago, teoria_del_big_bang said:

Just looking at this, how do you set NINA to refocus after a flip, mine has just flipped but did everything perfectly, recentered and all that but no refocus.

Do you have to use the plug in DIY Meridian Flip ?

Steve

Huum I thought it was one of the standard options I'll check for you later. 

[Adam J]

2 hours ago, teoria_del_big_bang said:

Just looking at this, how do you set NINA to refocus after a flip, mine has just flipped but did everything perfectly, recentered and all that but no refocus.

Do you have to use the plug in DIY Meridian Flip ?

Steve

So it's actually in the auto meridian flip settings within Nina not the sequencer bit. 

https://nighttime-imaging.eu/docs/master/site/advanced/meridianflip/

Adam

[mintakax]

On 15/01/2023 at 01:48, A320Flyer said:

Steve,

I use focus offsets in NINA (used the Dark Customs plugin to set them up). But in the NINA options I don’t actually specify which filter is the autofocus filter! 
When doing NB, in my Sequence, I have:

Change filter to Ha (choose the filter for AF)

Autofocus

4x300 Ha

4x300 Sii

4x300 Oiii

Dither

Now loop the above until Dawn

With this loop, you get an AF every hour, and a dither in every 4th frame. The AF is done on the filter that I’m going to use for Lum, with the other filters using focus offsets. If your system is reasonably parfocal, you might not need offsets. 
 

If doing LRGB, I substitute the Lum filter for AF and adjust the number of exposures. 
 

My thinking on this is that I want best focus on the filter I’m going to use for the Lum in processing, but not so critical on the others (as I usually give them  a slight blur anyway).
 

HTH

Bill

 

I'm just starting with NINA and can't locate the Dark Customs plugins in the available plugins list. Are they no longer available?