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Old 05-02-2020, 03:12 PM
HeavyT
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Same histogram at 900 seconds, 11 orders of magnitude apart

Hi all

This summer I've dived into narrowband imaging with a mono astro cam (ASI1600MM pro). 100mm doublet at f7.6 (785mm fl or thereabouts). ZWO filters.

I've imaged the Pencil Nebula, have Thor's Helmet almost there, and have started on Eta Carina.

Last night I noticed my Carina Sii sub histogram in the same place as my Pencil Oiii histogram, both at 15 minute sub exposures.

*edit: I hasten to add: gain at 139/21 in all subs*

I believe Oiii let's more photons in than Sii, so that might explain it, also that the Sii signal in Carina might not be as bright as Ha or Oiii.

But given Carina is mag 3 and Pencil mag 14, I was expecting to achieve a healthy histogram with much shorter subs on Carina.

I'm using N.I.N.A. to run the sequence and I'm wondering if I've accidentally switched the filter naming or not entered correct settings to drive the histogram display...

Any thoughts welcome and appreciated!?

For the record: the stack of 25x900s looks basically noiseless and fairly well exposed in a pixinsight STF this morning. So it seems I got there, it just took longer than expected.

Clear skies all

Last edited by HeavyT; 05-02-2020 at 05:08 PM.
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Old 06-02-2020, 07:23 AM
HeavyT
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Might have started answering my own question last night: Oiii looking better exposed at 15 minutes. Received some information on Facebook that Eta Carina is not strong in Sii or Oiii, so perhaps that contributes. Still the strongest Sii signal I've ever captured though.
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Old 06-02-2020, 12:17 PM
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multiweb (Marc)
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This is Sii only widish fov. There's not as much as Ha, still a fair bit though but Oiii would be stronger than Sii in the center IMHO.
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Old 07-02-2020, 01:45 PM
HeavyT
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Quote:
Originally Posted by multiweb View Post
This is Sii only widish fov. There's not as much as Ha, still a fair bit though but Oiii would be stronger than Sii in the center IMHO.
Yep, Oiii definitely stronger across the centre regions, but when I processed the Sii and Oiii bicolour pixinsight said Sii was brighter, so I registered and linear fit against that one.

I might be able to gather Ha soon, and see what sort of exposure time is needed. I suspect it will be longer than the 4 minutes I've seen people use on astrobin at the same gain and focal ratio. Unless they've entered some specs incorrectly.

I've uploaded the SOO bicolour and the Sii stack here. Pretty pleased with the SNR in these. 15 minute subs have their advantages I suppose. Wind gusts kept my guiding around 1.5 arc second RMS so whilst the stars were round, I needed to do a bit of sharpening.
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