Odd OIII filter gradient - Why?

[Jessun]

Some 'flats' - just an aim at the ceiling indoors really so disregard that type of gradient. -5 degrees, 10 second for the NB, and 0.1 for the IR block.

Perhaps some interference after all, linked to a particular distance filter/CCD? It's going into quantum theory here with the double slit experiment...

[opticalpath]

Did you think about dropping a line to Don Goldman, Sara? Even if it's not an issue with the filter diretly, he has published stuff about reflections in relation to halos etc. and might have some ideas.

One thing that occurred to me was the characteristic of the ultra-narrow filters to have reduced transmission at very fast f-ratios. If there is some stray reflected light bouncing around, I suppose some could be coming in at moderate angles whilst other rays at more extreme trajectories .... depending on what part of the sensor they strike. Doesn't seem plausible though now I've said it!

Adrian

[swag72]

Atik are looking into it all Adrian. I'm sure they'll speak with Don if needed. They've got all of my test images thus far. I thought I'd pretty much eliminated the actual filter per se.

[Freddie]

This is going to be an interesting one as you say it has only happened since the introduction of the filter but have pretty much eliminated the actual filter. Must be some strange interaction with the various parts of the setup. 

[AndyUK]

(It looks as though I'm going to need to keep a close eye on this thread as I too have an ATIK 460 and as it happens the same filter wheel as Sara... and I'm in the process of trying to accumulate enough funds to start buying a set of Astrodon's )

[johnrt]

What is the situation with this now Sara?

[swag72]

I've heard nothing back from Atik as yet.

As an aside, I tried some Ha flats with the OIII data and it worked a treat.

[FLO]

I've heard nothing back from Atik as yet.

That doesn't sound like the Atik I know...

Have you determined where the problem lies? Camera, filter or filter-wheel?

Have you tried the camera with only the Astrodon filter in a nosepiece? That should provide some pointers.

I know we didn't sell you the camera, filter or filter wheel but as an Atik dealer I am as keen as everyone else to know the outcome 

ATB, 

Steve 

[swag72]

Hi Steve,

No idea where the problem lies at the moment.

1) Camera with no filter - fine, no gradient

2) Camera with filter in nosepiece and no light panel - no gradient

3) Camera with filter in nosepiece and light panel - no gradient

4) Camera into the filter wheel - gradient

Any ideas? I don't use any kind of nosepiece in the filter wheel.

[FLO]

1) Camera with no filter - fine, no gradient

2) Camera with filter in nosepiece and no light panel - no gradient

3) Camera with filter in nosepiece and light panel - no gradient

4) Camera into the filter wheel - gradient

Ask your supplier to discuss your findings with the manufacturer of the filter wheel. They might have experienced this before and have a solution. 

HTH, 

Steve 

[opticalpath]

Interesting the difference between the filter mounted in a nosepiece and in the filter wheel, Sara. Unless you're using different threaded adapters in the two setups - which conceivably could introduce some different reflections - I could only think of it being:

1) a light leak in the filter wheel and some stray light bouncing around inside, or
2) the fact that the filter is set at a different distance from the sensor when mounted one way rather than the other, which would change any reflections occurring between filter and sensor.

If an internal reflection is involved, the fact that it doesn't affect all 3nm filters equally might be explained by differing reflectivity of the surfaces at different wavelengths (q.v. Olly's post about 'non-reflective black' surfaces that actually reflect quite a lot of IR.)

Clutching at straws, though!

Adrian

[swag72]

I thought of stray light in the filter wheel Adrian, and so when I did one of the tests I wrapped the filter wheel up totally in tinfoil. There was no light getting in there!! But it still happened.

The camera is connected to the filter wheel via a threaded adapter. There's no nosepiece involved ( I know that was something mentioned earlier with regards a baffle).

The issue is still with the Atik developers - so hopefully they will be able to help in some way.

[opticalpath]

Hmm, a real mystery!  If the filter and the camera (minus filter wheel) give clean images, It does seem to point to the filter wheel.

 

Suppose it was internal stray light?  I.e. not leakage from outside, but to do with the incoming light that's rejected by the filter.  The rejected light is in effect reflected back off the filter.  It's also very much brighter than the small amount transmitted by the 3nm filter, so if even a tiny proportion of it could somehow bounce around inside the filter wheel casing and get around to the camera side of the filter ......  

 

Some peripheral incoming off-axis rays could even reflect off the filter mount or filter-wheel disk and faintly illuminate the interior of the filter wheel casing.    I'm just saying that in the extreme case where we have very faint transmitted light, very 'bright' rejected/ reflected light, long integration time and strong image stretching, that any internal leakage or spill of the rejected light in the filter wheel could become a visible feature that perhaps would not show up with less extreme filtering.

I hope Atik come up with some ideas.

 

Adrian

[zombi]

Until now i just read your great forum but now i am in troubles, i had to sign in - i also have similar gradient lines - did anybody get any feedback from Atik about it ?

This is example from my Atik 383, EFW2 and SII filter:

I have similar lines on Ha and OIII.

[swag72]

Sadly, I never got to the bottom of this. Atik initially seemed interested in my fits files and I provided them with many different permutations - with reducer on, with different filters, filter carousel taped up in certain areas etc. The last I heard in October 2013 was that they were going to get a 3nm Astrodon filter to do their own tests.......... I never heard any more.

Perhaps ping them an email and see what they say - It may jog their memory or make them take up the baton again.

I wish I could help more than that, but sadly not. Do let us know what you decide to do.

Do you have access to another make of camera? I ask as I often wondered if it was the glass that Atik used in their nosepiece. I have no access to another imager out here in Spain, but this was something I always wanted to try.

[FLO]

Hi Sara, I was chatting with your supplier (of the Astrodon filters) this morning. He wasn't aware you were having difficulties but he is reasonably confident he knows the answer so please contact him  

HTH, 

Steve

[swag72]

Hi Sara, I was chatting with your supplier (of the Astrodon filters) this morning. He wasn't aware you were having difficulties but he is reasonably confident he knows the answer so please contact him

HTH, 

Steve

Thanks Steve, I'll drop him a line!

[gingergeek]

Sara, keep us posted on the result of your comms as you're not the only one with the "issue".

[Sp@ce_d]

Sara, keep us posted on the result of your comms as you're not the only one with the "issue".

Yes, been holding off investing in these filters with that in the back of my mind.

Sent from my iPhone using Tapatalk

[dmahon]

I must admit, increasingly I find that the only need I have for flats is for vignetting and for dust spots on the CCD itself. Basically, just using one set of flats (eg green) seems to work fine across all filters - and even if they are several months old.

[swag72]

I contacted Ian and he wasn't aware of any of the lead up to this - ie: the break down of the imaging train that I sent to Atik etc. He suggested that it could have been due to the moonlight. Sadly, this wasn't the cause as the gradient was on the flats prior to any lights ever being taken..........  so back to square one.

[opticalpath]

This must be hugely frustrating, Sara.  Trying to look at it afresh, going back to your earlier experiments - I thought the results were quite telling. 

Setting aside how other filters behave and thinking only about OIII wavelength light for the moment .....

- You got no gradient without the filter, or with different filters - so the problem is not intrinsic to the camera sensor alone.

- You got no gradient with the filter mounted in a nosepiece - so the problem is not intrinsic to the filter alone - i.e. there IS a configuration with this filter that works OK and does not produce the gradient.

- You only see the gradient when the filter is in the filter wheel, so the problem must be caused by the filter wheel itself, or by virtue of where it positions the filter (differently) in relation to the sensor.

If you're at the tearing-hair-out stage, and you're certain that the gradient was absent with the filter in the nosepiece, I would try a couple of things, in effect to try to make the filter wheel configuration look as close as you can to the nosepiece configuration that you know works.

1. Check  the separation distance between filter and sensor when the filter was mounted in the nosepiece and the gradient was absent.  How does that compare with the separation distance when the filter is mounted instead in the filter wheel?  If they're different add a threaded spacer to make them about the same and try again ..... to eliminate the possibility that the problem is appearing only at a particular spacing. 

2. If you have some flocking material, make a short tube of it and place it in the draw-tube, pushing it right into the filter wheel so it completely surrounds the OIII filter.  This isn't to stop external light leaking in - you already checked for that - but to prevent any incoming starlight that is rejected by the filter (i.e.reflected off the front-facing surface of the filter) from bouncing around inside the filter wheel casing, and also to stop any incoming light from getting past the filter wheel disk somehow. 

In other words, try to make the filter wheel mounted filter look as though it's in a closed tube like the nosepiece, with the same separation, and see if it behaves any differently.

Adrian

[lensman57]

Hi Sara,

I have been following this thread with interest for some time as I have some issues ( totally different from yours ) with Baader Oiii CCD filter. Just as a side note I thought to mention that it maybe worth checking the orthogonality of the Oiii filter in the filter wheel to the sensor, it may just be the reason.

Regards,

A.G

[ChrisLX200]

Is the direction you look through the filter different when it's screwed into the carousel and when screwed directly into the nosepiece? If so, try temporarily taping it in place on the nosepiece the other way around to check. The filter is built up from multiple layers deposited on glass so the order in which they interact with light may be affected.

ChrisH

[swag72]

Thanks all for your continued thoughts on this. I have learnt to accept it and process the out the problem although it would be better if it wasn't there in the first place!

@Adrain - A most detailed reply, thanks. In my head though I'd discounted the filter wheel as the others that have posted with similar problems have different wheels. Apart from the filters themselves, THE common factor is the Atik. But when I break down my imaging train again (if I can find something that will work as flocking material) I'll give your ideas a go.

@AG - What camera are you using when you are getting the gradient issue? The filters have all been screwed up and tested that they are flat in the carousel, I don't think it would be orthogonality with just the one filter?

@Chris - An interesting idea, I'll be sure to check that out.