Ha in the Witch Head?

[Rodd]

I have heard tell that there is Ha around the Witch Head Nebula--but not IN the nebula itself.  This is reportedly nice for adding character to the background, which was my plan to start.  I captured an hour of data just to see how much Ha signal there was and if it would be something that interests me.  It was actually very difficult to frame the target while it is invisible on a sensor barely large enough to hold it--or part of it.  After integrating the hours worth of subs, I was a bit shocked to see the Witch head--clear as day.  Not the outlines of a dark patch before an Ha background--but the nebula itself before a darker background.  How could this be?  So I proceeded to capture 4 more hours of data for a total of 5 hours.   Based on this result, I think it safe to say that there is indeed Ha in the Witch head Nebula.  I do not know what surprises me more--that there is in fact a decent amount of Ha in the Witch Head--which is a reflection nebula, or the fact that the Witch Head looks strikingly like a big eared elf! 

FSQ 106 with .6x reducer and ASI 1600  60 300 sec 3nm Ha subs.  The data has been aggressively stretched to accentuate the nebula

 

 

[Rodd]

Is this not surprising?

[wimvb]

13 minutes ago, Rodd said:

Is this not surprising?

Remember that Ha filters don't discriminate between Ha and white light at the same wavelength. A broadband target such as the witch head will also emit light at Ha wavelength. What you would really need to do is broadband subtraction: Ha - k•Red, where k is a normalisation parameter.

[Rodd]

1 hour ago, wimvb said:

Remember that Ha filters don't discriminate between Ha and white light at the same wavelength. A broadband target such as the witch head will also emit light at Ha wavelength. What you would really need to do is broadband subtraction: Ha - k•Red, where k is a normalisation parameter.

But ha filters do pick up ha emissions and not broadband. That is why we can shoot ha for galaxies. I recently [removed word] 5 hours of ha for the. Cone and did not pick up anything of the reflection nebula that is close by. Also the cave had a reflection nebula that I don’t pick up in Ha 

[Rodd]

Besides--this is way too much Ha to account for by just saying it is broadband leakage.  Based on comments I have read from some pretty reliable people on this forum, the prospect if finding Ha within the Witch Head was very remote at best.   Maybe those comments were made some time ago and opinions have changed.  

[newbie alert]

18 hours ago, Rodd said:

But ha filters do pick up ha emissions and not broadband. That is why we can shoot ha for galaxies. I recently [removed word] 5 hours of ha for the. Cone and did not pick up anything of the reflection nebula that is close by. Also the cave had a reflection nebula that I don’t pick up in Ha 

Point your scope at the moon with a ha filter... It's not emitting any hydrogen alpha  and the light we see is reflected sunlight... One explanation is that the witch head is using reflected light from the star Rigel so it could be why that's showing up in the image...

The true ha part is in the coned hat area way out of shot... Olly had an image a few years back

First time I shot this target I was out in a bottle 4 area and 6 min  DSLR subs wasn't showing anything, wasn't until I stacked and stretched it showed the outline... Moving on a few years I was just packing up for the night at home( bottle 5) and done a single 5 min sub with my atik 460... It was there in a single sub.. although this seems to be my bogey target as everytime I plan to shoot it, something happens, like it's a full moon, or clouds come from nowhere and destroy my evenings, only to become clear again the next full moon...arh!!

Sorry Rodd, this seems like I'm questioning what you wrote, I'm not...

Edited December 17, 2022 by newbie alert
Added info

[ollypenrice]

My Ha for this region did follow the outline of the which as Rodd's does but I also found a band of Ha roughly perpendicular to the Witch, crossing it roughly mid way.

https://www.astrobin.com/383965/

I've since pushed the signal a bit harder than in this rendition thanks to StarX.

Olly

[newbie alert]

1 hour ago, ollypenrice said:

My Ha for this region did follow the outline of the which as Rodd's does but I also found a band of Ha roughly perpendicular to the Witch, crossing it roughly mid way.

https://www.astrobin.com/383965/

I've since pushed the signal a bit harder than in this rendition thanks to StarX.

Olly

Nice but at 6.071 arcsec/pixel.. some have self combusted 

[Rodd]

2 hours ago, ollypenrice said:

My Ha for this region did follow the outline of the which as Rodd's does but I also found a band of Ha roughly perpendicular to the Witch, crossing it roughly mid way.

https://www.astrobin.com/383965/

I've since pushed the signal a bit harder than in this rendition thanks to StarX.

Olly

This ha seems to be in the background. Anyway you could post just the Ha?

[wimvb]

23 hours ago, Rodd said:

But ha filters do pick up ha emissions and not broadband. That is why we can shoot ha for galaxies

To isolate true Ha from broadband red, Vicent Periz recommends red continuum subtraction. If this is not used for galaxy images, the core, which consists of old red srars but no Ha clouds, will also become red. This is actually a common mistake in galaxy image processing.

[Rodd]

21 minutes ago, wimvb said:

To isolate true Ha from broadband red, Vicent Periz recommends red continuum subtraction. If this is not used for galaxy images, the core, which consists of old red srars but no Ha clouds, will also become red. This is actually a common mistake in galaxy image processing.

Yes--but this method does not work well for nebulae, at least for me. 

[Rodd]

23 hours ago, Rodd said:

Remember that Ha filters don't discriminate between Ha and white light at the same wavelength.

BTW--if the light is the same wavelength, then it isn't white light--its Ha, as light is defined by its frequency/wavelength. 

There is only one way to really tell here, and that is to compare Ha images.  It seems to me that there is a great deal more Ha in my nebula than in other images I have seen--where the Ha is almost exclusively in the background.

Edited December 17, 2022 by Rodd

[wimvb]

42 minutes ago, Rodd said:

BTW--if the light is the same wavelength, then it isn't white light--its Ha, as light is defined by its frequency/wavelength. 

White light contains all wavelengths, including 656.28 nm (which is the Ha emission spectral line). Ha emission is defined by its source. Light from other sources can still have a wavelength (among others) of 656.28 nm. Ha filters pick up this colour/wavelength regardless of its source. That's why we don't need a Hydrogen discharge lamp to make Ha flats, but we can use ordinary white light sources (led panel). In fact, the bandwidth of any Ha filter determines the contrast and image quality you get. A 7 nm filter will pick up Ha plus red at wavelengths between 652.78 and 659.78 nm. A 3 nm filter will pick up Ha and red light with wavelengths between 654.78 and 657.78 nm. Less red light, therefore better contrast. But they all pick up some light that is not created by Ha emission.

[Rodd]

1 hour ago, wimvb said:

White light contains all wavelengths, including 656.28 nm (which is the Ha emission spectral line). Ha emission is defined by its source. Light from other sources can still have a wavelength (among others) of 656.28 nm. Ha filters pick up this colour/wavelength regardless of its source. That's why we don't need a Hydrogen discharge lamp to make Ha flats, but we can use ordinary white light sources (led panel). In fact, the bandwidth of any Ha filter determines the contrast and image quality you get. A 7 nm filter will pick up Ha plus red at wavelengths between 652.78 and 659.78 nm. A 3 nm filter will pick up Ha and red light with wavelengths between 654.78 and 657.78 nm. Less red light, therefore better contrast. But they all pick up some light that is not created by Ha emission.

 The point of this thread is to question whether the Ha picked up in my stack is normal for this nebula.  Based on color images I have seen, it is not.  But that could be due to processing.  I need to see an Ha stack of the Witch Head for comparison--which I have never seen.  In all images I have seen--Ha is only in the background