Strong halos with CLS CCD

[Jannis]

Just got my new Polemaster and CLS CCd filter from Bernard and tested it today.

Having great experience with narrowband earlier i had hoped to use the CLS for the L channel on the same lens and camera - but no such luck.

Here's how it looks wide open at F/3.5:

And at F/8:

Also for compasison is a 7nm Ha, also taken wide open at F/3.5:

Bernard said something about it might help to add another filter to filter out the part of the spectrum causing the halos, but other then that it's either stopping down the lens a lot, or getting a new lens to replace it. But thought i could ask you guys if it's possible to deal with this in any other way? I've started to love this setup as it's full manual, very compact, solid, and nice FOV, so was hoping i wouldn't have to change it - but i guess if it's all down to the optics it's not much one can do..

[Uranium235]

If its worth anything, I use the CCD CLS (clip filter) with an f1.8 lens and dont get halos that bad. I dont usually shoot at 1.8 but its also fine at f3.5 and 4. Leads me to suspect it might be the lens rear element here (ie: does not have anti-reflective properties).

[Jannis]

I forgot to mention, I'm using 2" filters and the filter is attached in the front of the lens using a 49->48mm step-down ring.

It's promising to hear you use it without these halos, i hope to soon try it on my more modern Canon 50mm 1.4 lens and see if it's any difference.
Might be like Bernard said, that these lenses from the 80's simply were never intended for this use and these wavelengths, and therefor and have both focus and reflection issues.

Any decent quality lens made for digital sensors should more or less solve this issue then?
 

[Uranium235]

Ahhh so its front mounted... Even more odd that its causing halos. The reflective surface must be the front element then, perhaps the coatings from the 80's just arent as good as modern ones. If you have another (newer) lens to try it on, give it a shot.

[Jannis]

I will give it a try on a newer canon lens and see how it goes. Forecast promises clouds for a good while though, so might take some time...

[alexbb]

You could also try it during the day to check some sharp edges with a light background to see if it's more a chromatic aberration issue or on a very bright light to see if it's flaring due to internal reflections.

Your halos look well separated from (or contrasty to) the central area and also concentric. I think the CLS blocks some wavelength ranges and that's why the halos have that contrast even though the chromatic aberration would spread more even. I see no other reflections of the brightest stars.

But it's late and cloudy here too so better read it just as a thought. I've no reason to stay awake.

[Xplode]

Did you test without the filter too?

If you're interested in trying another lens i have an Asahi SMC Takumar 135mm f/3.5 lens here.

[Jannis]

I have not tested it without any filters. 
I don't have IR/UV blocking filter in 2" size, so i'm unable to test it with the QHY5L, but i will be able to test the lens with the CLS filter on an unmodded DSLR to see if the halos are. I will also try the filter on another lens as soon as i have the chance to see how it works there. Will update on the result.

If you live anywhere close to Fredrikstad it would be very interesting to test your 135mm lens for comparison.

[Xplode]

Could you check for sure that you have the CLS CCD filter?

The visual version doesn't block IR and your halos definitely look like IR bloat

[Jannis]

It does indeed say CLS CCD on both the filter and the box, it arrived sealed in the Astronomik box. Is it any way to visually see the difference on the CCD and none-CCD version (in case they did a mistake at the factory and put in the wrong glass)? 

[sharkmelley]

It looks like a old lens and it probably wasn't designed to focus H-alpha and near IR in the same focal plane as visible light.  This effect is known as chromatic aberration and is often referred to by astro-imagers as "IR bloat".  Unfortunately, in cases like this the only solution is to stop the lens down so much that it becomes too slow for most astrophotography.

I had a lens like that and with a colour camera the "haloes" around the stars were red making it look like a severe case of pox or measles:

 

Lenses back in those days had a special mark for focusing IR light.  That would have been a big clue if I had thought enough about it! I ended up junking the lens.

BTW, that image would have been the Witches Broom using a very old Nikon 300mm F4.5 lens on a full spectrum Canon 300D.  It used an old style CLS filter - not the CLS CCD.  The effect might have been slightly less severe with a CLS CCD filter.

Mark

[Adam J]

On 08/11/2016 at 13:41, Jannis said:

It does indeed say CLS CCD on both the filter and the box, it arrived sealed in the Astronomik box. Is it any way to visually see the difference on the CCD and none-CCD version (in case they did a mistake at the factory and put in the wrong glass)? 

You say you have a 2 inch UV/IR filter? If you want to make sure that it is the CLS CCD and not just a CLS resulting in these issues just stack the two filters. If the UV/IR solves the problem then you have a CLS not CLS CCD. 

I originally got the non-CCD version myself and use it stacked with the UV/IR block just like you and I dont have any Halos like that on it. Maybe on a very very bright star such as the one between the horses head and flame nebula....the name escapes me, but even then not as bad as you have on much much dimmer stars. 

I think your issue really is that the front of that lens is not anti reflection coated.

Only other thing to try would be to reverse the filter...I am told that they dont work both ways around....could have been mounted incorrectly. 

Oh and if you have no coated optical window in front of the QHY5 then you will also get reflections from the surface of the CCD.