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Old 10-01-2020, 09:43 AM
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HeavyT is offline
Join Date: May 2019
Location: South Australia
Posts: 46
Focussing narrowband filters

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.
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