Odd OIII filter gradient - Why?

[swag72]

I have noticed on the occasions that I have used my OIII filter that I get an odd gradient horizontally along the top and bottom of the image. When I take flats it is there too. I seem unable to get rid of it in flats as it just makes it worse.

I have recently taped up my manual filter wheel so that along the joins there is no possibility of light getting in. Yet this has made no difference. There is no evidence of a similar gradient in any of my other filters. The image attached has had no flats added - so as it comes out of the camera. A stack of 11x1800s.

Does anyone have any ideas where I can look now? I am thinking if it was light leakage I'd get it on all of my subs, not just the OIII.

[cfpendock]

Is it possible to rotate the filter to see if the gradient also rotates?

Chris

[swag72]

If I rotate the filter, the only way to do that is to unscrew it - If it's loose in the filter carousel could that not bring in further issues?

[martin_h]

Coukd you rotate the whole filterwheel 90deg to see if the band also rotates?

[swag72]

Yes I can do that - If it was a filter wheel thing, wouldn't I also be getting a similar gradient with the Ha filter?

[johnrt]

If you rotate the filter wheel and not the camera or scope & the patch moves then it must be the filter itself causing the issue. Is this a recent thing Sara?

[r3i]

HI Sara,

You say that you have taped up the joins on the wheel, but what about the bit that you rotate to select the filter?  I noticed light getting in there on mine so I make sure its covered up during imaging.

[swag72]

HI Sara,

You say that you have taped up the joins on the wheel, but what about the bit that you rotate to select the filter?  I noticed light getting in there on mine so I make sure its covered up during imaging.

I've taped up as far to the little rotating knob as I can - But, if it was a light ingress thing, don't you think it would show on all filters?

 

 

If you rotate the filter wheel and not the camera or scope & the patch moves then it must be the filter itself causing the issue. Is this a recent thing Sara?

It's always happened with just the OIII, but I thought it would be solved by the taping up of the filter wheel joins, so until now it's not been of bother. When I do flats you can see the gradient in there as well.

Looks like I'll rotate the filter and maybe take a flat this afternoon and see where the problem potentially lies.

[r3i]

I've taped up as far to the little rotating knob as I can - But, if it was a light ingress thing, don't you think it would show on all filters?

I don't know, I'm new to NB so have very little experience of their individual characteristics. It might be worth temporarily taping up the rotation knob whilst its set to OIII and taking some test shots which may (or may not) rule it out as the source of the gradient.

[swag72]

Filter wheel all tin foiled up ready for tonight! See how it goes

[Jessun]

Could it not be that OIII needs heavier stretching so that CCD 'anomalies' show up? Looks way too symmetrical to be light or even OAG reflections. Having said that, I don't even own a OIII filter.... Yet.

/Jesper

[swag72]

Here's the latest.

1) I have absolutely light leak protected the filter wheel with tin foil - No different, still get the gradient.

2) I have rotated the filter by 90 degrees and still get it in the same horizontal place.

I am wondering now whether it's my light panel - so I put a couple of pieces of paper over it - Longer exposures, but that should even out any light panel gradient discrepancies - Still got the gradient.

Any more ideas? I do want to get to the borrom of this.

[davew]

Hi Sara,

I'm late on this one ! This doesn't look like light leak to me. Far too even along the edges. It looks like reflection. The only thing I can come up with is the fact that Sony chips ( More than Kodak chips I think ) have their gold connectors along the long edges of the chip and aren't shielded.

Why this would only happen with the OIII I'm not sure. I've had a look at a few of your recent images and can see it on SH2-132 in the blue channel. I can't seem to see it on the blue channel on your CTB1 so it doesn't look like the blue filter does it.

I've had this myself in the past but not every time so never got to the bottom of it. I'll have a look round of course and see what I can come up with. Don't hold your breath

Dave.

[Oddsocks]

Only an idea...

Could this be back scatter illumination from inside the camera?

The ATIK460EX chip is oblong and looking at the camera window a larger area of rear PCB is exposed at the top and bottom of the chip compared to the sides.

Back scatter will be more prominent at certain wavelengths which may explain why it is more noticeable with the OIII compared to the Ha or L-RGB

The Tak FSQ85 has a very large illumination circle, much bigger than the camera chip so the problem with back scatter can be quite high.

Some camera manufacturers fit dark non-refective masks around the chip to suppress back scatter but there isn't one in the 460EX.

It would be relatively simple to cut a piece of black paper to mask the front window of the camera so that just the chip is visible and none of the chip mount or rear PCB is illuminated and try again.

If successful then a more permanent solution could be made by making a mask to fit inside the camera body.

William.

[davew]

Within 2 minutes a similar post......... SPOOKY

We must have been typing at the same time !

Dave

[Oddsocks]

Dave,

Just what I thought too...

I had been pondering this problem all day, I had seen this banding before with an old SXH9 camera, Astronomik OIII filter and an old TMB80/600 but couldn't find any images that I might have kept.

Since then I've changed scopes, cameras and filters several times but not seen the banding again.

Only thing I could think of was a larger image circle size over illuminating the inside of the camera with the old TMB80/600 that is not happening now with any of my current scopes.

William.

[davew]

William,

It's good to have someone to bounce ideas around and you've reminded me of when it's happened. Using a reducer on my refractor and only with OIII. So,

Sara. I know it's a pain but while you are creating a mask with your blunt Blue Peter scissors try this. Take a flat with the reducer on and aim for around 20000 ADU. Unscrew the reducer, put the camera and filter wheel back on and take another flat again around 20000 ADU. Put them side by side and compare.

If they look just the same then at least we know it's not reducer induced.

Dave.

[swag72]

OK, so I have ruledout the light panel as I just took a flat against my white obs wall and while I couldn't get anywhere near the 20k, the horizontal banding was aready clar to see.

If I try the masking idea and back scater first, before I take the whole imaging train apart! Can I just conform what I need to do,

I need to cut a piece of black per with a hole in it that is the size of the chip (or various straight cuts) and out it  the front window of the camera so that only the chip is showing through? Is that right?

I also tried an SII flat last night and I got a bit of a similar banding as well as another odd diagonal gradient that was there thoughout regardless of filter rotation. And, when I look very clodely at my Ha, there's a little something there too.

[FLO]

I can see how you might think it is reflectance but that is unlikely.

It might be power supply or some other interference.

I don't think it is something you can fix Sara so if it is one of ours please drop me a line so we can arrange collection. Otherwise you should discuss this with your supplier. 

HTH, 

Steve

[swag72]

If it's a power supply thing Steve, wouldn't I be getting the SAME gradient across all the filters? As it is, I am getting different ones. Also, it's only just started with the 3nm narrowband filters, with the wider bandwidth I wasn't getting any of this. The power supply has stayed the same throughout as has the cable configuration.

[FLO]

If it's a power supply thing Steve, wouldn't I be getting the SAME gradient across all the filters? As it is, I am getting different ones. Also, it's only just started with the 3nm narrowband filters, with the wider bandwidth I wasn't getting any of this. The power supply has stayed the same throughout as has the cable configuration.

I am guessing power or interference. I don't think it is reflectance Sara, you will need to return your camera so we can put it through testing. We don't charge for collections (even from Spain) and will make it as quick and painless as possible

If isn't one of ours then contact your supplier. 

HTH

[davew]

Ah well. Blew our ideas out of the water

If it's a known problem then it looks like a return to base.

Good luck,

Dave.

[FLO]

I might be wrong but reflections don't normally look that neat. I think it is electrical in nature but I haven't seen it before which suggests it is power or interference. 

We won't know for sure until Sara returns it to her supplier (I know now it isn't one of ours). 

[Oddsocks]

It could be power or a chip defect but I have seen exactly the same artefacts with OIII filters on a different camera and as with Sara the artefact was not present with Ha or L-RGB.

Reflection artefacts can be an issue when you have two optically flat partially reflective surfaces exactly parallel with each other, same as when you stand between two mirrors in a fun fair and see infinite reflections of your self bouncing into infinity. In this case I'm thinking it could be reflections of the CCD and chip socket bouncing back from the OIII filter.

It would not hurt to try a black paper mask on the outside of the camera window, if the artefact does not change then at least that would eliminate the reflection or back scatter idea with no risk to the camera and Sara would have to disassemble the image chain anyway to return the camera to the supplier for testing.

Sara's image looks quite different and dramatic when inverted and stretched a little.

William.

 

[FLO]

I'll be happy to be proved wrong

Some time ago a similar conversation discussed how an Atik EFW2 filter wheel fitted with a 1.25" filter disk and a 2" nosepiece, with a full 2" opening, might suffer reflection problems. Some were concerned that light entering the wheel illuminated part of the two filters either side of the one in use. I don't think anyone actually experienced a problem with reflections but it could, in theory, happen. Atik noticed the concern so produced a new 2" nosepiece (for the EFW2 fitted with a 1.25" filter disk) with a reduced aperture at the rear so light only fell on the filter in use. Atik's own testing concluded the problem was theoretical, it was unlikely to happen in the real world, but customers were concerned so they created the new nosepiece to reassure them. Perhaps this is a similar situation. I don't think anybody has reported reflections from the edge of Sony sensors (Atik aren't the only manufacturer using a CCD window larger than the CCD) but if there is enough concern it might lead to new windows with apertures cut to fit. I shall watch this thread with interest.