Problem with Star Halos: any advice gratefully received

Dear Astrobin friends,

I wonder if someone can help me with any advice.

Lately, I have been struggling with the increasing appearance of strong haloes around my stars. I am using a GSO RC8 + TS2inch corrector + ZWO 6200MM camera with 7-position filter wheel containing Optolong LRGBSHO filters.

I show below an example of wha I am struggling with - this is a stack of 15 x 300s B images on NGC1097 taken on a clear photometric night. You can make out just the faint right-angled jet (much more obvious in L) but it is substantially weaker that the halos, even around fattener stars.

Over the past few weeks, I have been trying to work out the likely cause, as the problem (appears) to get steadily worse. As might be expected from physics, the problem is worst in L, then B, G with R least (but still some) affected.

Variously I have attributed it to

1) high cloud - but it appears even in good conditions.

2) Dew on the sensor window? I always have the dew heater on.

3) Dew on other parts on the optical train? Filters/correctors? Is this common in an RC optical train? The halos don't seem to be correlated with dew point.

4) Dirty filters? I have cleaned them with isopropyl alcohol (and they looked cleaned anyway).

4a) Dirty corrector? See above.

5) Filters going bad? Sounds unlikely - since they are <1 year old - but would love to hear others experiences.

6) The Optolong filters are simply not up to the task? Do I really have to go Chroma?

7) My (lack of) skill at post- processing? The stretch below is standard STF autostretch in PI, perhaps I don't have a problem with my kit, and just need to get better at hiding the halos in post-processing.

Would be interested to get the expert view on whether I really do have a problem, and it the fix is obvious/trivial. I hope it is and I am just not seeing it.

[ On combining the RGB images, I have also seen an increased magenta halo around the central galaxy. I suspect thesis just due to the unequal stretches being applied to each channel and I would be taking more R data, given the 6200MM's QE curve. But I mention it also in case it is relevant. You can tell I am getting a little desperate.]

My hope is that someone can immediately diagnose this and say "you are doing X wrong". After two months of frustration I would love to know what X is.

Thanks in advance for your help

Brian

Hello Brian,
Well I'm far from an expert, especially since I don't use your filter set. I'm a OSC camera setup. However, I've read some on the Optolong filter sets and I don't think you have to worry much there, but maybe others will chime in for you there.
When it comes to halo's, 1st thing I think of is some sort of dew or moisture. I've seen that with my OSC setup at times. I've found that its not so much the sensor window but the secondary mirror as I use Newts for my imaging train.
At any rate, I'll be interested in what others say, as it will give me something to watch for myself.

Dale

Thanks Dave, Glad my issues can help others too.. part of the reason for sharing!

Would be interested to hear if others have problems with the 2ary dewing.

CS Brian

Hey, I’m following this too.
Nice pic, btw

Hi Brian,

I get halos on the brightest stars with my setup, which has no spider. So no spikes. My halos are complex though. I think the classical halo caused by internal reflections, is typically smooth in appearance, with a distinct ring edge. I do get that, but I also get striations due to the fact that my camera cables are routed through the optical path with a RASA. I also had many more distinct striations early this year because I got spider webs within the optical train. Your halos seem more of the striated style that fade toward the edge with no distinct edge on the perimeter. A bit like my spider web halos. But I doubt you would have missed that. In any case, if it is something like that, your setup should be easy to clean out, unlike mine. So be aware that there could be more than one issue causing this. It looks a bit like condensation to me when there is no rim on it. Be sure that you have not added anything to your setup, such as a dew shield that might be impinging on the light path. And try to clean your OTA innards just to be sure. My spider webs were from baby spiders that got in through the tiniest holes and as you would suspect were the finest silk that could be imagined. Only visible in the dark when illuminated with an LED headlamp. So check carefully for stuff such as this. A bright light aimed down and around the OPT during the dark may better highlight the culprit.

Hope you solve this soon!

Alan

Hi Brian,

I should say that if you do inspect the OTA with a bright point source light at night, don't get panicked by all that you see! You will see dust and stuff that you would never see during the day. It may look scary, but mostly that sort of stuff should not be an issue. If your problems are due to some cleaning issue, you should be concentrating your effort on looking for regular pattern dirt, which throws diffraction related noise (which is why one never wants to leave streaks on their optics, even if relatively clean) and conversely just the opposite, look for general haze that may not show up in daylight. For those that live in the fire prone western USA, smoke can leave such a haze. It can build up and create much the same issues as fogging. But haze such as this also acts as great nucleation for condensation and could well promote condensations under conditions where clean optics would not normally do so.

Alan

Hi Brian,

I haven’t been doing a very deep analysis on the topic but maybe I can contribute a bit.

First of all, stars will show halos simply due to the fact that we stretch data. Here’s an example why. The first image shows a histogram transform (HT) curve (x is the input luminosity, y the transformed luminosity), where I‘ve set the middle point to 0.001. This looks similar to a very common HT curve used in PI.
The second image shows how the Airy-function (blue) is transformed (yellow) through this HT. (Reminder: the Airy-function describes the radial intensity from the diffraction on the aperture).

From this simplified example, we can see how physics and math alone produce enormous halos. Now add up seeing (Moffat distribution) and guiding errors (Gaussian) and it becomes worse.

Then there are reflections.
While all your points might be valid, you could distinguish dirt/dew from comparing bright and dim stars. If it’s dirt/dew, also faint signal is blurred while if it’s a reflection or stretch effect, bright objects are affected more than dimmer ones.
Reflections can occur on all optical interfaces. It might even be the coating of the flattener lenses.

Notice that reflections are leading to non-concentric halos, i.e. the center of the halo is radially (w.r.t. field center) shifted outwards w.r.t. the star position. This is more pronounced, the further the star is off-center.

To figure out where the halos are specifically from, I recommend following sequence where you capture images and analyze them.
1. Use RC only. No flattener, no filters.
2. Add flattener.
3. Add filters and vary distance between filter and camera.

I once had an issue with an OIII filter from optolong (there’s a thread here on AB) which had an IR leakage. I replaced the SHO set by a set from Baader, which performs well. OIII still shows halos but I consider that to be the nature of the process. As you may know from your own experience: even research scopes show halos.

Hope that helps a little.

Cheers,
Björn

Normally this stuff is due to the flattener (unless it's high clouds so you need to rule that out). While its optical power is small isn't zero. Never had this sort of halos on scopes without external flatteners.

Brian,
I completely agree that those halos are not normal and I’m not so sure that they are due to the filters. At least that’s not the first conclusion that I would jump to.

Since you are using a RC scope with a corrector, I would first want to look into the possibility of spherical aberration. A RC system all on it’s own will produce SA when the mirror spacing is not properly set. So, the first question is: How have you checked for proper mirror spacing?

The other potential source of SA is in the corrector. If your mirror spacing is correct, the very first thing to do is to pull the corrector out of the system and image a field of moderately bright stars to see how well the raw scope performs. If you get the same result, then it’s time to take a harder look at the filters. Filters that don’t block NIR can cause this kind of problem in a refractor, but not in a pure reflector. For that reason, I doubt that your problem is in the filters. I would personally put my money on the possibility that the correct is causing the problem.

Finally, I should also point out that another thing that could cause this sort of problem is very poor surface finish or dew on a surface one of the components. As long as you don’t have dew, it’s pretty unlikely that any component in your system has the kind of poor surface finish that would produce this kind of effect.

John

Imaging in west Texas (very dry climate) and upstate NY (relative humidity = 100% on most nights) with the same gear and filters has taught me a lot. I have halos here on humid nights, but much less so in my old subs from Texas. I do my best to keep dew off the corrector plate of my SCT and the objective of my refractor, but the halos are still there. I do think that scattering of light by moisture droplets in the atmosphere is a major contributor.

Imaging in west Texas (very dry climate) and upstate NY (relative humidity = 100% on most nights) with the same gear and filters has taught me a lot. I have halos here on humid nights, but much less so in my old subs from Texas. I do my best to keep dew off the corrector plate of my SCT and the objective of my refractor, but the halos are still there. I do think that scattering of light by moisture droplets in the atmosphere is a major contributor.

I do not doubt that high humidity can wreak havoc under conditions that cause condensation on surfaces or within the air column. However, humidity itself should have no impact on the visible wavelengths. IR, yes, for certain longer wavelengths. Water vapor is perfectly clear, lacking forms of condensation. Example is that Florida used to have some of the finest astronomical viewing. Dew point is critical, however. And at night, dew is not a rare occurrence. And sites of nucleation can promote that. That is why we can see dew on surfaces while still having perfectly clear skies. And why we use dew heaters!

Wet ground, such as after a rainfall, can increase the concentration of water vapor close to the ground. This can promote the formation of ground fog. If one has a bright spot light, you can see the individual droplets within even a very faint ground fog. These can settle on surfaces if they are cold enough and lead to a wet optic. I do not know if there are remote sensors that can look for this and send alerts to the sleeping astonomer.

However, humidity itself should have no impact on the visible wavelengths.... Water vapor is perfectly clear

I respectfully disagree. It's scattering that matters, not absorbance or transmission (clarity). Because water droplets have a substantially higher refractive index than air, they can and will scatter all wavelengths of visible light, especially so when the water droplet diameters are about the same as the wavelength of light (few hundred nm). When there are a lot of tiny droplets, what you see is a cloud - the results of extensive light scattering. When there are a few droplets, you see haze, and possibly halos around stars.

I do think that scattering of light by moisture droplets in the atmosphere is a major contributor.

I think there shouldn’t be a question if condensation of water in higher atmospheric layers is an issue. We just need to look at the full moon. There are nights, when the moon shows with high contrast against the sky while on others days there’s a huge halo around it. Under such conditions, stars won’t behave differently. They‘ll show the very same behavior.

@Brian Boyle Are you using dew protection? At my place, I need the full equipment on my RC8: dew shield, secondary heater and a heater around the back of the OTA otherwise it will get bad.

Cheers,
Björn

Excellent example, Bjorn. The moon halo is exactly the same phenomenon to which I was referring.

However, humidity itself should have no impact on the visible wavelengths.... Water vapor is perfectly clear

I respectfully disagree. It's scattering that matters, not absorbance or transmission (clarity). Because water droplets have a substantially higher refractive index than air, they can and will scatter all wavelengths of visible light, especially so when the water droplet diameters are about the same as the wavelength of light (few hundred nm). When there are a lot of tiny droplets, what you see is a cloud - the results of extensive light scattering. When there are a few droplets, you see haze, and possibly halos around stars.

Water vapor is a gas and is perfectly clear and is completely transparent to visible light. Humidity is simply the measure of water vapor in the atmosphere. I'll let you do the research on that one. It is only upon the condensation of water vapor that particles of liquid water cause light scattering. You keep mentioning water droplets and equate them to humidity or water vapor. Those are all forms of liquid water, not water vapor and are not a part of the measure of humidity. Drizzle, fog, rain are all condensed water. Only difference is size. Rain can fall in extremely low humidity air. Alternatively, very humid air at air temperatures well above the dew point (or condensation point) will be clear. Clear as dry air. Unless there is other pollutants around. Clouds/fog are all composed of floating liquid water droplets or ice. Otherwise it would all be clear. Perfectly clear in visible light.

Your original point refers to condensed water droplets in cloud, fog dew, etc. and is well taken. Your experience with differences in a dry and humid environment must be due to some aspect of condensation. And it need not be a function of humidity, but rather your conditions relative to your local dew point.

I'll let you do the research on that one.

No need to get sarcastic. I apologize if you read my response as condescending.

All of my posts are about liquid-phase water droplets, which scatter light, not about water vapor. I mentioned 100% humidity because that is the saturation condition (dew point) at which liquid droplets begin to form with dropping temperatures, assuming suitable nuclei are available. None of this discussion has anything to do with the ability of water vapor (gas phase) to absorb visible light, which it does not.

Could Brian's halos be due to scattering of light by liquid water droplets or tiny ice crystals? I think so. The aforementioned condensation on mirrors is always in play as well.

No need to apologize at all! I was just being my picky old self in this thread. And for that, I should be the one to apologize. I wanted to be clear that humidity, in particular high humidity or relative humidity only imparts a potential threat, and by itself is not what causes the photo artifacts that we are discussing. I wouldn't have made it an issue if it weren't that so many people actually believe high water content air actually does impede visual light. Many do. Not necessarily on this site, but pretty much everywhere else. People often will look at the hazy horizon on those hot humid days and complain that it is the water vapor that is the culprit. If only that were true. Typically it is pollutants and at times water chemically reacted with pollutants that make smog.

And as I said in my last post, your point should be well taken. Only one of the many ideas posted here that Brian may want to consider.

Wow, everyone. Thanks for the great advice. I took out the corrector and saw no difference.

@John Hayes point about the mirror spacing is a good one - as I know that @Björn Arnold had the same issue with his RC8. However, I was still able to collimate and get good images over a full frame sensor (with the corrector, of course). Also the back focal distance (without corrector is as near as dammit to the advertised 254mm spec. Does this means that the mirror spacing is likely to be OK.

These leaves humidity and dust. The optical path looks pretty clean to me, but thanks to @Alan Brunelle for the spider tip (Alan, I thought over the 2ary spider at first!). I do worry a little about my 1ary begins a bit dusty, but - from experience working with a much larger mirror - I don't think it is at a level that could cause this effect. But a wash might be needed.

On humidity, this is initially what I put it down to. But I don't see such halos on images taken contemporaneously with a camera lens.

As @Björn Arnold says, it might just be the wings of the stellar profile I am seeing on stretch. When the good weather lets up, I will do some more investigating - guided by all the ideas here. In the meantime, I have a new rig to play with and more auroral activity forecast.

CS Brian

The only way to figure it out is definitely a systematic test.

Once I also had dew on the filter (I don’t use a flattened on my RC

so it’s possible. After looking at your image several times, it reminds of dew that I had on the secondary. I‘ll check if I still find these subs.

Maybe one point: your image is integrated. It’s helpful to use a single exposure only to assess imaging quality.

Regarding dirt: my guess would be that you‘d see the dirt with the naked eye if it causes problems. When I had noticed a slight degradation of the image quality during a session and I looked into the OTA with my red flashlight, I could immediately see the culprit (dew).

If you do your test sequence, I recommend a fairly bright open cluster since you have stars in many different brightnesses.

Best,
Björn

Brian,
Have you carefully star tested the system?

John

Many thank John and Bjorn. I have star tested the system about 3 months ago, looking at the non-flatness of the focal plane and assessing the efficacy of the field-flattener. Everything looked good then - consistent with the lack of halos I was getting around single subs back then.

Single subs also show the effect, indeed it was where I first noticed it. This includes short 0.1s images of bright stars for focus tests.

Looking through my optical train, the "dirtiest" object is, in fact, my secondary. This would be consistent with Bjorn's observation above.

I guess this could have happened with heavy dews couple with dusty atmosphere…? But, if it is the case, it is a helpful lesson to check the optical train carefully, including the bits you can easily see.

Might be time for a clean?

Brian Boyle:
Many thank John and Bjorn. I have star tested the system about 3 months ago, looking at the non-flatness of the focal plane and assessing the efficacy of the field-flattener. Everything looked good then - consistent with the lack of halos I was getting around single subs back then.

Single subs also show the effect, indeed it was where I first noticed it. This includes short 0.1s images of bright stars for focus tests.

Looking through my optical train, the "dirtiest" object is, in fact, my secondary. This would be consistent with Bjorn's observation above.

I guess this could have happened with heavy dews couple with dusty atmosphere...? But, if it is the case, it is a helpful lesson to check the optical train carefully, including the bits you can easily see.

Might be time for a clean?

JMHO here is all but, it would have to be quite dirty but, it couldn’t hurt Brian, just be carful so you don’t scratch the secondary. If you decide to do it and please give us an update on that because it will be interesting to see if that makes a difference or not.

Dale

Brian,
I have to agree with Dale. The secondary would have to be absolutely filthy to produce the kind of halos that you are seeing. If the system has very little SA and the mirrors are relatively clean, then the halos are being caused by either condensation somewhere in the system (like the camera window) or by clouds or fog. The only time that I've seen halos like that are when it gets cloudy or foggy--right before they close the observatory roof. I can't imagine how your filters might produce those kind of halos.

John

Given the information in your reply, it jogged a memory of a thread (here I think), about someone needing to extend the baffle on their secondary to solve some issue. It worked, but I do not think that it was a halo that was the issue. I think it solved an internal reflection issue, such as John has mentioned. All it took was some thin black cardboard and tape to fix, so it would be an easy thing to try. I am no expert on the RC design, so I am now in the realm of throwing stones at this problem. You will solve this!

Alan

Hi Brian,

Before you start a cleaning, I am suggesting following test:
Take the flattener out. Attach your camera without any filters and don't cool your camera (rules out dew on cam window from cooling). Image a bright star close to zenith on a clear night.

That should show you if the main OTA produces clean images.

Björn

EDIT
PS: Since I still had my dew shield from the SCT, I am always using it on my RC. First it helps a bit with dew (at least a bit but heating is necessary). It helps with possible stray light issues (neighbors) and hopefully it slows the build up of dirt on the optical surfaces….
or maybe it's just a placebo. Don't have a reference for testing.

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