RGB and chromatic aberration

[Merlin66]

2 minutes ago, cesco said:

why they are prone to diffuse internal reflection towards the green and blu part of the spectrum creating halos and star bloataing.

Within the context of AP imaging this is not the case....

[cesco]

16 minutes ago, cesco said:

In the section IMAGINE- TIPS TRICKS AND TECHNIQUES   -   FILTERS FOR DSO IMAGING, I read: Baader produce a significantly cheaper LRGB filter set. These are absorptive rather than interference filters. They do let through slightly less light than the more expensive interference filters and but I suspect the differences aren’t huge. One advantage of absorptive filters is that they generate far fewer internal reflections. This can reduce the risk of large, unnatural looking star halos which you do occasionally see with interference filters. These Baader RGB filters aren’t IR blocking so you need an additional IR blocking filter on the nose piece of your filter wheel.

In addition, the topic is further discussed in:               www.sk-advanced.com/category/chapter-7-interference-filters

I am not so sure as well, by now. At the end of all, I used ITF filters just once, and I know I relied on parfocality, so I focused on R and left the position untouched for G and B. My next issue is tryng again with proper focusing, as it is now clear to me that filter parfocality does not cure my achromatic objective residual chromatic aberration.

[vlaiv]

3 hours ago, cesco said:

I am not so sure as well, by now. At the end of all, I used ITF filters just once, and I know I relied on parfocality, so I focused on R and left the position untouched for G and B. My next issue is tryng again with proper focusing, as it is now clear to me that filter parfocality does not cure my achromatic objective residual chromatic aberration.

Ok, so ITF filters will produce strange halos in certain cases due to reflections, but this has nothing to do with blue halos that are from chromatic aberration. It is usually due to multiple reflections inside system (light passes thru ITF, reflects on something - like protective glass covering the imaging chip - reflects again from back side of ITF and ends up on imaging chip. These kind of reflections are usually much larger (order of magnitude) then star image. If star image is couple of pixels across, this kind of halo will be 30-40 pixels across. It is still formed because unfocused light - but one that traveled additional length (bounced back and forth one time).

Also you can't really expect parfocality of ITF filters to sort out halos from CA. They make sure that you don't need to refocus on filter change on systems that have single good focus position (reflectors, apo triplets, etc). Achromats don't have single good focus position (the curve previously posted) so with parfocality you are making sure that you will get CA with achromat :D. So again you should refocus for each filter do minimize this.

[cesco]

3 hours ago, vlaiv said:

Ok, so ITF filters will produce strange halos in certain cases due to reflections, but this has nothing to do with blue halos that are from chromatic aberration. It is usually due to multiple reflections inside system (light passes thru ITF, reflects on something - like protective glass covering the imaging chip - reflects again from back side of ITF and ends up on imaging chip. These kind of reflections are usually much larger (order of magnitude) then star image. If star image is couple of pixels across, this kind of halo will be 30-40 pixels across. It is still formed because unfocused light - but one that traveled additional length (bounced back and forth one time).

Also you can't really expect parfocality of ITF filters to sort out halos from CA. They make sure that you don't need to refocus on filter change on systems that have single good focus position (reflectors, apo triplets, etc). Achromats don't have single good focus position (the curve previously posted) so with parfocality you are making sure that you will get CA with achromat :D. So again you should refocus for each filter do minimize this.

Yes, the first thing I'll do will be taking better care focusing the three channels, then I will try processing as you suggested. Last chance will be going back to broad band absorptive filters...,

[cesco]

On 26/1/2017 at 17:52, cesco said:

Yes, the first thing I'll do will be taking better care focusing the three channels, then I will try processing as you suggested. Last chance will be going back to broad band absorptive filters...,

After prolonged bad weather I caught a less than decent night, with altitude wind and whispy clouds speeding along. Nonetheless it was decent enough to allow a try to careful focusing RGB filters. I was able to bring to the same FWHM minimum value R and G filters, and B only to a somewhat higher FWHM value.  I shot 2-min and 5-min exposures. R and G star images resuted identical size, and exactly superimposable, while B star images were slightly larger and presented a slight blu halo only visible after magnification. All star images were nearly perfect circles, and after color combining yielded bright white circles. I can confirm that careful focusing is needed with these parfocal filters after each change, though a boring task, and that the objective chromatic aberration renders these filters parfocality useless.

M81. 2-min exposures, 12 B, 15 G, 10 R. Baader Neodymium filter always on. Baader RGB dielectric filters on Baader manual filter slit. Atik 428EX on Meade 7" ED f/9 achromatic refractor. Skywatcher AZ EQ6 mount. Atik Titan guiding camera on 50/700 guiding scope. Capture software: Artemis; guiding software: PHD2. B/W images were stacked with DSS, colours and RGB stacking, Registax6.