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Red ghosting with CLS LP FIlter


Widefield

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Has anyone had any red colour problems when using an Astronomik EOS Clip CLS LP filter (non CCD) with a modded DSLR on an f6 refractor or similar?

My camera was recently modded (filter removal only - the original Canon IR filter is still in place). I have always had field curvature on the scope (GT81) especially with a reducer, less so with the dedicated flattener, but it has never shown any CA problems at all even in the corners.

Now however, with the CLS filter in place I get the sort of red ghosting shown below. It is clearly worse in the corners where curvature is worse, in some cases it is very near the center as well. For bright blue stars I get a blue to red effect and for orange stars it can have red almost detatched from the rest of the star :p

Without the filter there is a hint of the red to one side of the odd orange star so it is still there, but nowhere near as pronounced as with the filter in place.

I guess it could be that now there is more red getting through post mod it is maybe showing up CA in the scope that was not noticable before (and with the filter in and cutting out part of the spectrum it might appear worse still as there is a clear break in the spectrum to highlight it).

I would be very interested to hear if anyone else has had these sorts of issues or has an idea what it might be as at the moment I can't determine if its just inherent CA in the scope or if its an issue with the filter, my modded camera, a duff IR filter, misalignment or a combination of them all :hello2:

Any help much appreciated as its driving me mad! :)

Cheers,

Paul.

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I have just started using an Equinox 80 and have a modded 450D and CLS filter, I have a red Halo around the brightest star in the few pictures I've taken so far but nothing on the rest of the stars in the picture.

I would say you are right about getting more CA now that you've modded the camera

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Hi Sam,

On top of what I described I also have the same - a really strong red halo around the brightest star (in my case Vega - I just got in from taking more test shots and its there just the same as previous nights over the past week and a half).

I am hoping that the red halo is caused by high level cloud or some other atmospheric condition (the moon had a strong visual halo around it tonight and previous nights the transparency has been similar) but I am very concerned that the mod to the camera (mine is a 450D as well) has messed something up - red halos around blue stars is a new one on me, and its there with and without the CLS filter.

Does anyone know if we should be expecting a red halo around a blue star if the conditions are not great, or could this be something more serious to do with the camera mod?

Cheers,

Paul.

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Dont worry, its normal. My modded 1000d + Clip filter (CCD version) does exactly the same. The only two reasons I can think of is:

A) Its the optics, you will always get a red halo in all but the most expensive refractors (in which case you should not get them if using a newt).

:hello2: A modded camera is more sensitive to red, hence larger stars in the red channel.

Even so, both of the above can be fixed by splitting the channels, saving the files off, then running noels "make stars smaller" on the red channel master. Then re-combine for an RGB version minus (most) of the red halos.

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Thanks Uranium235, that has put my mind at rest there is nothing unusual going on and others are seeing the same thing. I will look into the post processing workflow you susggested.

What puzzles me is that, with the CLS filter in place I get the sort of 'smearing' of red off to one side of orange stars shown in the pictures, but without it it is much, much less noticable - its only if you really boost the red channel you see its there.

I assume the fact it is smeared rather than a symetric halo is down to a less that flat field or something out of line, but why there should be such a marked difference with and without the LP filter is less clear.

The current theory is

1) without the filter there is a continuous spectrum from blue to red so the error in red may be somewhat masked, whereas with the LP filter there is a big chunk in the Orange range missing, so the red error in the optics becomes much more obvious and apparently seperated from the rest of the star colour.

2) the CLS filter seems to have slightly higher transmission figures for Ha (97%) compared with around 92% for lower wavelengths. I am not sure how much difference that would make, but it is at least consistent with what I am seeing.

Either way it looks like it is an optics issue :hello2:

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Dont worry, its normal. My modded 1000d + Clip filter (CCD version) does exactly the same. The only two reasons I can think of is:

A) Its the optics, you will always get a red halo in all but the most expensive refractors (in which case you should not get them if using a newt).

B) A modded camera is more sensitive to red, hence larger stars in the red channel.

Even so, both of the above can be fixed by splitting the channels, saving the files off, then running noels "make stars smaller" on the red channel master. Then re-combine for an RGB version minus (most) of the red halos.

OK I give up - time to start asking stupid questions ;) How do you go about splitting the channels into different files from a colour DSLR file? I currently save them as RAW and then stack in DSS but I cant work out how/where to split them so I can treat them individually, other than by reducing the other channels to zero in DSS (or prior to stacking) and saving just the one colour at a time.

Would the approach you outlined also work for the non symetric 'smearing' I am seeing as well?

Cheers,

Paul.

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