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Defective filter, or something else??

Antlia 3nm Narrowband H-alpha 2" Filters H-alpha
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David Ebbecke avatar
David Ebbecke avatar
Hoping to get some help from the group on an issue I had last night while imaging the California Nebula.

I installed a brand new Antlia 3nm 2" Ha filter in my filter wheel and quickly discovered there was something wrong with the images.  Perhaps to complicate matters, it was also the first night of imaging with the WO FLAT GT flattener:  https://williamoptics.com/products/flat-gt

I have the GT81 WIFD and wanted to start imaging at the native focal length of 478mm.  

Below are the images in question (single 300 sec image).  I also shot with LRGB and the images looked great - pinpoint round stars all the way to the edges.  It only seems to be affecting Ha.  Focus, backspacing, and guiding were all good.

Any ideas what this might be?  I know there have been reported issues with star halos on some Antlia filters, however, these images do not resemble those that I find online.


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Tony Gondola avatar
Tony Gondola avatar
It's out of focus. There is always a focus shift with filters like that.
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Oskari Nikkinen avatar
Oskari Nikkinen avatar
Out of focus, quite a bit so.
Dale Penkala avatar
Dale Penkala avatar
Agree with the above comments! Out of focus!
David Ebbecke avatar
David Ebbecke avatar
And those were my first thoughts as well as the images suggest, however, I've used the LRGB + Ha (different filter then this new one) and always maintained near perfect focus without adjusting between filters.  Could it be the focus sensitivity between filters is amplified using the flattener only vs. the .8X reducer/flattener that I've always used up until last night?
David Ebbecke avatar
David Ebbecke avatar
I get another clear night tonight before the clouds roll in.  I'll plan on reaching focus with the Ha filter in question and revisit the forum should the issue persist.

Appreciate the help!

David
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Tony Gondola avatar
Tony Gondola avatar
You're over thinking this. If you've spent any time at all looking through a telescope visually or at least manually focusing on a screen, you'd recognize what's happening right away. It's basic astronomy 101.
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Miguel T. avatar
Miguel T. avatar
Very simple, out of focus. Just refocus or configure a filter offset in NINA.

You haven't specified the model of your LRGB filters.
What you were expecting is parfocal filters of same thickness, meaning they all roughly focus at the same distance, and clearly they are not.

I don't think that perfectly parfocal filters even exist anyway. You should always refocus after switching filter, or have the offsets configured. As little the difference may be there will always be one.
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David Ebbecke avatar
David Ebbecke avatar
Gotcha, refocus!

Antlia 2" mounted 3nm NB and LRGB of the same brand.  These are advertised as parfocal and have always behaved as such until installing this new one yesterday.

I have never done visual astronomy hence my inability to recognize what appears to be the obvious.

Thanks again.
Dark Matters Astrophotography avatar
Dark Matters Astrophotography avatar
David Ebbecke:
Gotcha, refocus!

Antlia 2" mounted 3nm NB and LRGB of the same brand.  These are advertised as parfocal and have always behaved as such until installing this new one yesterday.

I have never done visual astronomy hence my inability to recognize what appears to be the obvious.

Thanks again.



Parfocal only means the filters themselves will not shift the focus point as you change between them. The optics will however which is why you need to either focus or use offsets when changing filters.
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David Ebbecke avatar
David Ebbecke avatar
Dark Matters Astrophotography:
David Ebbecke:
Gotcha, refocus!

Antlia 2" mounted 3nm NB and LRGB of the same brand.  These are advertised as parfocal and have always behaved as such until installing this new one yesterday.

I have never done visual astronomy hence my inability to recognize what appears to be the obvious.

Thanks again.



Parfocal only means the filters themselves will not shift the focus point as you change between them. The optics will however which is why you need to either focus or use offsets when changing filters.

Ahh, did not know this!  Thank you for the explanation.
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