Halo around bright stars in Ha

[billyharris72]

Hi all:

A clear night tonight so I had a go at imaging the Flame and Horsehead Nebulae in hydrogen alpha. Stacked around 40 120 second shots from my ASI1600, using an old Nikon 200m lens at F4. The shot almost came out okay, except for very prominent halos around the brightest stars. I just wanted to ask if anyone else has had problems like this and if so, whether there is a solution. First thought is maybe the lens is not up to the job, but I'm pretty sure it doesn't do this when I shoot in LRGB. Photo below for reference.

Thanks,

Billy.

[SyedT]

18 minutes ago, billyharris72 said:

Hi all:

A clear night tonight so I had a go at imaging the Flame and Horsehead Nebulae in hydrogen alpha. Stacked around 40 120 second shots from my ASI1600, using an old Nikon 200m lens at F4. The shot almost came out okay, except for very prominent halos around the brightest stars. I just wanted to ask if anyone else has had problems like this and if so, whether there is a solution. First thought is maybe the lens is not up to the job, but I'm pretty sure it doesn't do this when I shoot in LRGB. Photo below for reference.

Thanks,

Billy.

 

Mine does the same thing, it's due to the brightness of those particular stars:

I use an ASI 1600MM-C on a WO GT81 scope with Baader Ha 2" filters. There's a method to remove them in Pixinsight, if you use it to process your images: http://trappedphotons.com/blog/?p=616

[vlaiv]

I think it is filter problem - filters are causing reflections.

You may think that filters you used don't cause reflections, and you are probably right but you need to check the manufacturing specs for filters and what is minimum F/ratio that they could be used at.

F/4 is pretty fast and all interference filters are targeted for specific wavelength - best work at 90 degrees angle of incidence. With steeper angle properties of filters change - both reflective and transmission properties. It is quite possible that this particular filter has trouble with F/4 and that some of the light hitting it reflects multiple times in filter substrate and bounces back as unfocused star.

Telltale sign of this is that halo is not symmetric around stars that are away from optical axis. In the image Alnitak has halo almost centered on the star while Alnilam has it offset.

Other option is that there is double reflection going on in optical train. First reflection is from sensor cover glass and then it bounces of filter again. It would still be dependent on filters ability to handle F/4.

[Charic]

58 minutes ago, billyharris72 said:

Horsehead Nebulae in hydrogen alpha.

Is it just me or is your image reversed? thats the first thing I noticed!

[ollypenrice]

1 hour ago, SyedT said:

Mine does the same thing, it's due to the brightness of those particular stars:

I use an ASI 1600MM-C on a WO GT81 scope with Baader Ha 2" filters. There's a method to remove them in Pixinsight, if you use it to process your images: http://trappedphotons.com/blog/?p=616

Those stars are equally bright for anyone who images them but not everyone gets these halo effects. I certainly don't get them, so they are created by the optical train. It could be the filters on their own, or the relationship between the filters and other glass, like the chip window - creating reflections. What filters are these?

Olly

[SyedT]

22 minutes ago, ollypenrice said:

Those stars are equally bright for anyone who images them but not everyone gets these halo effects. I certainly don't get them, so they are created by the optical train. It could be the filters on their own, or the relationship between the filters and other glass, like the chip window - creating reflections. What filters are these?

Olly

I initially thought they must be the optical train as well. I created a thread about it as I was getting a very significant halo effect on Alnitak and one or two other stars with my Astronomik Type 2c G & B filters (not R for some reason), and someone posted that they were getting the same effect with their Baader LRGB filters. I've attached the G and R files respectively. The image I  posted before was taken with a Baader Ha 7nm 2" filter. It's a shame, as I was otherwise pleased with both sets of filters! My imaging train consists of the 1600MM-C -> SXUSB filter wheel -> Baader Varilock 20-29 -> WO 0.8x FF/FR -> WO GT81.

[ollypenrice]

I've seen exactly the same effect on Alnitak a long time ago when imaging with an early TV Genesis. Alnitak is one of the most troublesome stars in the sky so I wouldn't worry over-much if you get artefacts on it. The fact that it varies between filters could point the finger at the filters but, equally, at other possible sources like reflections.

Olly

[Tommohawk]

Ive only dabbled with my ASI1600MM +Ha filter (31mm) but I think I saw the same thing on bright stars when used with my 300mm camera lens. I also wondered initially if the problem may be with the lens. I hope to test this by replacing the F4 lens with an F4 newt without CC - never enough clear skies for testing though!

That said I think Vlaiv has it right. The optical window on the ASI1600 has an AR coat but this is not optimised for Ha wavelength  - so any oblique incident light from a fast system may suffer multiple reflections

There's a nice bit on reflections here.

[Tom OD]

 

13 hours ago, vlaiv said:

I think it is filter problem - filters are causing reflections.

You may think that filters you used don't cause reflections, and you are probably right but you need to check the manufacturing specs for filters and what is minimum F/ratio that they could be used at.

F/4 is pretty fast and all interference filters are targeted for specific wavelength - best work at 90 degrees angle of incidence. With steeper angle properties of filters change - both reflective and transmission properties. It is quite possible that this particular filter has trouble with F/4 and that some of the light hitting it reflects multiple times in filter substrate and bounces back as unfocused star.

Telltale sign of this is that halo is not symmetric around stars that are away from optical axis. In the image Alnitak has halo almost centered on the star while Alnilam has it offset.

Other option is that there is double reflection going on in optical train. First reflection is from sensor cover glass and then it bounces of filter again. It would still be dependent on filters ability to handle F/4.

You can rule out the Fast F ratios by masking the aperture, which will make it a longer F/Ratio.

I do remember Baader having a coating issue on their O3 filters, but they replaced those for people. My money is on internal reflections from the bright stars. The off axis halos seem to suggest that too.

Tom

[alexbb]

I also have reflection halos experiences, they're even brighter on OIII compared to Ha. Moreover, the Panasonic MN34230 sensor does not have AR coating itself, only the protective windows is AR coated and this leads to hideous (in my opinion) microlensing diffraction reflections around the brightest stars, but that is a different issue.

My bet is on the filters. What Ha filter did you use?

Check this out:

https://www.cloudynights.com/topic/598035-what-do-you-get-for-your-money-with-astrodon-filters-seriously/#entry8206073

[billyharris72]

Thanks all for the excellent advice on this. I wasn't sure, but the idea of internal reflections certainly makes sense. I think I'll start by trying the same camera setup on another optical train (130 P-DS, still quite fast but at least different) and see how I get on with that.

18 hours ago, vlaiv said:

You may think that filters you used don't cause reflections, and you are probably right but you need to check the manufacturing specs for filters and what is minimum F/ratio that they could be used at.

Thanks for this - I did not realize that this was a limitation, so one to check.

4 hours ago, moise212 said:

My bet is on the filters. What Ha filter did you use?

I used the ZWO 31mm Ha filter. No idea if these are regarded as good, bad or indifferent.

Thanks,

Billy.

[ollypenrice]

Tom's is a good idea. If an aperture mask worked you could shoot a few subs with it in place and substitute the afflicted stars with clean ones in Ps layers. In principle, anyway! I haven't tried it.

Olly

[Tommohawk]

Billy - one other thing to consider.

Obviously 25mm and 36mm mounted filters can only go one way round. But the 31mm unmounted filters have a right and wrong way round - the coatings are different on each side. It's likely this would make some kind of difference. The info on which way round to position the filter is on the ZWO website, but doesnt ship with the filters. Not sure if you are aware of this?