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Focusing with the Sii Filter

Acquisition Deep Sky M78 NGC2068 Messier Objects
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Jerry Gerber avatar
Jerry Gerber avatar
i'm imaging M78 in SHO during the moonlight.  I am noticing that occasionally the Sesto Senso 2 focuser fails to focus when using this filter. I've also noticed that when my automated flats are taken, the Astronomik Sii filter requires a 9 second exposure, much longer than the Oiii and Ha filters (on NINA's auto-exposure setting).

Is this normal?  Is Sii much more opaque than other  filters?  All my filters are the same brand, Astronomik.  I'm assuming I can increase the focuser exposure time from 5 to 8 seconds, maybe that will help.


jerry
D. Jung avatar
D. Jung avatar
The wavelength spectrum is not constant, and the sensor response drops quite significantly towards 1micron. So it's quite normal to have different signal levels for different filters. 

If you have filters all from the same brand/series they should be parfocal and my recommendation would be to focus using the Ha filter and then switch to the SII filter.
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Alex Nicholas avatar
Alex Nicholas avatar
M78 has almost no OIII and no SII to speak of… and more often than not, adding Ha to it will only unnatrually blast the intensity of the red channel.

That asidek SII is almost always hard to focus through, as it is further into the infrared than Ha is, and as such, cuts out so much of a stars natrual light intensity. 

Upping the exposrue time to even 10~15 seconds may be required. Being Astronomik, I assume they are 6nm filters? My Astronomik 6nm SII requires 15s to platesolve or autofocus, for this reason, I would strongly recommend setting up filter offsets.
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Jerry Gerber avatar
Jerry Gerber avatar
Alex Nicholas:
M78 has almost no OIII and no SII to speak of... and more often than not, adding Ha to it will only unnatrually blast the intensity of the red channel.

That asidek SII is almost always hard to focus through, as it is further into the infrared than Ha is, and as such, cuts out so much of a stars natrual light intensity. 

Upping the exposrue time to even 10~15 seconds may be required. Being Astronomik, I assume they are 6nm filters? My Astronomik 6nm SII requires 15s to platesolve or autofocus, for this reason, I would strongly recommend setting up filter offsets.

That makes sense.  I upped the focus time with Sii to 7 seconds and immediately my HFR history dropped below 3.  If I have to increase it even more I will but probably won't  need to as I am in Bortle 1 skies and the seeing and transparency are both excellent tonight.

I do believe however that M78 does contain quite a bit of Oiii in the center of the FOV.  I did some test processing when I had just 1 night's worth of data and immediately saw a lot of blue/cyan.
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andrea tasselli avatar
andrea tasselli avatar
M78 is a reflection nebula and there is little to no OIII there, the blue you see should really be continuum emission in the blue range. I get the same with bright reflection nebulae as well.
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D. Jung avatar
D. Jung avatar
I guess you are not focusing on nebulosity, but on stars. I don't think there is such a massive variation over the field of view in star mass composition. Increasing the exposure should work fine if you don't have parfocal filters and have to focus with SII or OIII.
Alex Nicholas avatar
Alex Nicholas avatar
andrea tasselli:
M78 is a reflection nebula and there is little to no OIII there, the blue you see should really be continuum emission in the blue range. I get the same with bright reflection nebulae as well.

Thank you for saying that better than I could..

Yes, you can shoot Ha or OIII filters at targets that are not actually emitting  Ha or OIII, but still create an image, because the Ha bandwidth of 656.28nm is a part of the red spectrum.. This is why you can also take flats through a Ha or OIII filter using an LED flat panel. The flat panel does not contain ionised hydrogen that is emitting light, or doubly ionised oxygen that is emitting light. It simply emits light across the entire visible spectrum, which happens to include 495.9nm 500.7nm (which lie in the Blue/Green end of the spectrum, and is specifically the wavelength of OIII) and 656.28nm (which lies in the deep red end of the spectrum, and is specifically the wavelength of Ha)

So what you are seeing in those sub exposures, is blue green broadband light emission that is falling into the bandpass of your narrowband filters, but it is not light being EMITTED by an emission source... it is reflected light of the correct wavelength to pass your filters.

As per Andrea, it is t he continuum... 

Narrowband filters are largely misunderstood I think.

Their purpose is not 'extreme light pollution suppression'. Their purpose is to be used on targets whos gasseous particles are being excited to the point of ionisation.... This process happens in specific parts of the wave length, Hydrogen Alpha, Hydrogen Beta, Nitrogen, Sulphur, Oxygen, etc.... The point of narrowband filters is to ISOLATE specific emission line sources.  Yes, you can point an OIII filter at M45, and still see the blue whisps of the Pleadies, but they are not an emission source. The nebulosity there is dust that is reflecting the blue hue of nearby stars..

Only Emission Nebulae 'emit' light, hence the name. Reflection nebulae are not narrowband targets. Yes, you can make an image of them with narrowband filters, but that doesn't mean that the target is a hydrogen/sulphur/oxygen emitter, just that the wavelengths your filters allow through are part of the visible light spectrum.
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