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Meade 127 Blue Halos cure


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Hi, guys,

For many years now I've enjoyed my Series 5000 Meade 127 refractor (f7.5, 950mm fl)  But, like most of its fellows and clones such as the Explore Scientific 127 (I'm pretty sure it's a clone, but will stand corrected!), it suffers from a tendency to produce blue halos on bright stars.  I'd been seriously toying with spending £££s on a 6" quality refractor, but thought it might be worth looking into the reason for the halos.  I experimented and I believe succeeded.  This image shows the before and after - I think you'll agree what I did has worked 🙂  Rather than go into all the gory details here, if you're interested please visit the appropriate page on my web site here.  Scroll to the lower half.  Entire image posted on SGL here.

Cheers,

Peter

 

Focus test full.jpg

Edited by petevasey
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9 hours ago, nfotis said:

Maybe a stupid question, but wouldn't using a monochrome sensor (and refocusing for each primary colour separately) achieve the same results?

N.F.

Please look again at the full description on my web site - you will see that indeed I am using a monochrome sensor with filters, and it is the Blue colour that is out of focus.  I first focus using the IR Luminance filter, then Red and Green are both in focus.  So the digital caliper modification allows me to reposition the focus for Blue without moving the telescope.

Cheers,

Peter

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15 minutes ago, petevasey said:

Please look again at the full description on my web site - you will see that indeed I am using a monochrome sensor with filters, and it is the Blue colour that is out of focus.  I first focus using the IR Luminance filter, then Red and Green are both in focus.  So the digital caliper modification allows me to reposition the focus for Blue without moving the telescope.

Cheers,

Peter

Problem is that you can't remove bloat from Luminance filter in this way as it captures all the wavelengths of light at the same time and some of it is bound to be out of focus as not all can be brought to a sharp focus at the same time.

Luminance sharpness is very important for overall sharpness of the image - much more than color sharpness, as most of detail information in the image is carried by luminance layer.

Here is little demonstration of the effect:

I've found this nice bird image online that we will use to demonstrate effect:

base.png.8a72eb825bc8b1f23fe5be161c752f16.png

I'm going to blur first luminance component, and then both chrominance components in Gimp separately by same level of blur. We can then compare the two to see differences.

lum_blurred.png.af9d2095419956dc14f91298a28f32f4.png

chrom_blurred.png.b9ee1ec8c0a03d3f90588a6a2e992624.png

It is not hard to tell the difference - chrominance blur is hardly noticeable, while luminance blur can't be missed.

To further emphasize this point:

whithout_color.jpg.98870c685275d99e0f44621a885f6c98.jpg

Can you actually see any difference in star bloat between your two sample images once color is removed from them?

Blue fringing is just aesthetic issue in images - real issue with unfocused light is blur that results from that. Blue fringing can be altered in post processing - there are even filters that remove chromatic aberration - or rather desaturate blue bloat from image automatically.

If you want to have real impact on this and improve your images - you can do following:

1. Use RGB only and skip luminance. This way refocusing will have noticeable effect on sharpness of the image

2. Use special Lum filters that remove far ends of spectrum that are most offending - like Astronomik L-3 luminance filter

3. Use aperture mask to stop down your aperture a bit. This cleans up chromatic aberration.

 

 

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Good point, and of course I'm aware that there will be some luminance bloat attributed to the out of focus Blue.  But of course the Blue is only part of the total spectrum captured by the Luminance filter, whereas the halos due to the individual Blue channel when the RGB channels combined are more noticeable.  I think you will have to agree that there is a substantial improvement between my two test images, even though still slightly imperfect.  And of course only really a problem where there are bright stars in the image.  If you compare my adjusted image which was used for the experiment with an earlier one, admittedly a different cluster, again I think you will agree that there is a substantial improvement in the final image.  Shown below.  Bear in mind that NGC6633 was imaged in Astronomical twilight so the background is a bit lighter.  Of course a moderate lottery win would go towards a William Optics 132, then problem permanently solved 😉

Cheers,

Peter

NGC6633.jpg

M38.jpg

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Don't get me wrong - I'm not saying that what you did is wrong - quite the opposite - it should be part of standard workflow - refocus on filter change.

Having motor focuser makes life much easier in regard to this, but what you've done is also time saver - to record offset for filter and then apply it on filter change (this is what software with motor focuser also does).

What I'm saying is that color issue is not that much of an issue - in sense that it can't be fixed. People shooting with OSC can't refocus for each color and they can employ simple trick to remove blue bloat in software.

Other than that - I've listed some of methods to deal with it and generally improve sharpness.

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That Gimp action is very impressive.  I can do a similar thing in Photoshop with Noel Carboni's Actions, below, (two applications of 'Reduce small violet halos').  But much more satisfying to prevent them happening in the first place!  BTW the image with the blue stars is Messier 38.

I have a motor focuser on my RC10, but find it much quicker to control it manually with a Bahtinov mask on a bright star than let the software struggle with a fainter star in the target field - often it would fail after several minutes - total waste of time!.  Fortunately the RC (carbon truss tube version) holds focus very well with temperature changes.  And all my telescopes live permanently in my Obsy, so are at ambient at the start of an imaging session.

Cheers,

Peter

Afteractions.jpg

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