Halos around bright stars

[Lanties]

Newbee questions :

My Optical setup :-  Esprit 100->Esprit 100 flattener -> ZWO OAG -> ZWO Filter Wheel -> ZWO 31mm filter set -> ZWO 1600MM-Cool. 

I have two questions based on the images below. I have asked this in the Zwo forum with one suggestion from there about flipping filters that did not work.

1. How do I get rid of the halos?

2. There appears to be a "glow" to the bottom right of the images. More apparent in the stacked Ha image right at the bottom. Again what can it be and how do I get rid of this?

[steppenwolf]

The glow is thermal noise from the amplifier circuit and calibrating your images with Dark frames will resolve this.

[vlaiv]

Hi,

Halos around bright stars in narrow band is usually due to reflection of light off of some components in optical train - filters being main culprit, but reducer can sometimes play a role (also optical window of CCD/CMOS if not coated properly, but sometimes it just happens).

Glow at the bottom of the image is AMP glow, and proper calibration (using dark frames) should get rid of it.

[AndyUK]

I'd suspect the filters too - I had something very similar once which I put down to light bouncing off the front of the filter, onto the back of the reducer, and then back towards the camera again...  and the double halo on the 2nd image could be caused by the light still being bright enough to do a second circuit...

 

 

[Lanties]

It is also my gut feeling that it could be the filters, after lots of reading and searching today. I did take a single 300 second image, over the weekend, with no filters and there was no halo. It was however not perfectly focused so I did not want to say that I could trust that specific image to be an indication of a filter problem or not. If only I could borrow a/some filter(s) to do tests before having to spend a lot more money. As I said in my introduction, a steep, and I forgot to mention, very expensive, learning curve.

[vlaiv]

From your description, everything being ZWO, so I guess everything is designed to fit together, will make this a bit harder than it really is, but here is suggestion:

Try different spacing to components. Interference filters and glass in general have such property that distance / thickness plays a part in constructive / destructive interference. So at that particular wavelength there might be constructive interference that causes reflected light to leave halos - if you shift something by a bit - it might be much less pronounced or go away completely.

If you have your flattener spot on - that is something you can't move (and you should not, since it has prescribed distance to sensor), but filters you can move (again depending on your setup) - as they sit between sensor and FF. Ideally filters should be as close to sensor as possible to avoid vignetting, but since you are using ASI1600 and 31mm filters - there is some "room" to adjust things. I also use 1600 but with 1.25" filters - and my filter drawer (very low profile) sits right next to sensor - I still get some vignetting, although not that bad.

[vlaiv]

Ah, another thing sprung to my mind. You have 31mm square unmounted filters? Made sure you put them right way around? Interference filters should be "pointed" to the right direction, they have front and back side. With mounted filters - camera / eyepiece facing side has thread (male) on it.

[Lanties]

^^^ Thank you for all the advise. I picked all ZWO to try and ensure all works well together and from a fitment point of view it certainly does. The setup is exactly as ZWO recommends. I can certainly try and move the filter wheel away from the camera and closer to the OAG. I have some spacers to play around with. I have the 31mm round unmounted filters. I actually flipped the NB filters this weekend just " in case"  but the results were slightly worse so I take it I had them in the right way originally.