Focussing narrowband filters

[HeavyT]

Hi all

I'm just starting out using an astro cam and narrowband filters. I've noticed that the diffraction spikes returned from the Bahtinov mask are more like rows of dots than actual spikes. I can still make out "lines" and achieve focus I think.

However, I noticed that when I switched from Hydrogen to Oxygen, the focus didn't appear to need adjusting. I've read that you always need to adjust focus between different filters, so this puzzled me.

So my two questions are:

Should I be exposing the focus frames longer to achieve better diffraction spikes, and

Is it a problem that the focus doesn't change between filters? I'm wondering if perhaps the filters weren't changing in the wheel when I told them to - the other night my Hydrogen subs looked sharp but Oxygen did not, despite refocusing three times.

I have attached (had to crunch it down a lot) a stack of "focussed" 36 x 300s Ha subs, gain 139 offset 21. This is from a different night of imaging to the one I mentioned previously. Should it be sharper? I think it has had some stretching, denoise and possibly sharpening in Pixinsight but still looks fuzzy to me.

Caveat being, I was imaging through some bushfire smoke haze. I realize this probably accounts for most of the fuzziness in this stack, but I'm still keen to know whether Baht masks behave differently with narrowband filters. The HFR didn't stay below 3 for long, though was quite variable through the night.

Thanks all and clear skies.

[multiweb]

It's ok if your diffraction spikes look dotted as long as you can see the general pattern and center. Depending on the filters used and focal length and critical focus zone of your telescope the difference in focus between filters might not be big enough to notice visually. So no problem there.

[HeavyT]

Cheers mate, I'm at 765mm (900 x 0.85 reducer / flattener) using ZWO 36mm filters, 7nm.

Is it normal for Oxygen subs to look blurry even with a focused Bahtinov pattern?

[multiweb]

Yeah Oiii let's light in around the blues, turquoise colors. Blues are very likely to be scattered by the atmosphere. Ha is the high end of the deep red and that's always sharp and tight. So it's normal. You can work around it by shooting your Oii closer to the zenith or higher in the sky. Less air to go through.

[HeavyT]

Thanks again Marc