123 replies to this topic

[Borodog]

I had an idea for how to easily correct CA in achromatic refractors, making them suitable for full color imaging with one-shot color cameras. This idea, which I call the synthetic blue channel method, has now been implemented in SharpCap 4.1, as of yesterday's release. You will find it in the right hand pane under Preprocessing (same place as darks, flats, planet disk stabilization, background subtraction, etc). It is suitable for planetary, lunar, and deep sky imaging, even solar if you are doing that with a OSC camera and a white light solar filter.

 

I've made a video explaining how it works and demonstrating it on several images, both planetary and deep sky. I didn't include lunar examples in the video, but it works spectacularly there as well (maybe the best of all, really).

 

https://www.youtube....h?v=Hc7Tjcq5WIg

 

To get the best out of this method you MUST image through some sort of fringe killer/minus violet/violet reduction/#8 pale yellow/495 long pass type filter. The filter must also have an IR cut (like the 495 long pass) or be stacked with an IR cut filter (for example the #8 pale yellow would have to back stacked as it has no IR cut). This is to prevent defocused blue and IR light from polluting the red and green channels as well as the blue, which is critical to the method.

 

My hope is that this lowers the bar to entry into the astronomical imaging and EAA hobbies.

 

Thanks, and clear skies.

Edited by Borodog, 30 January 2024 - 03:17 PM.

[gstrumol]

Congratulations Mike!! 

[Jim Waters]

Thanks for the info Borodog.

[steveincolo]

That’s a second extremely cool idea for you, Mike!  With some small enhancements, this could be used in EAA software that does multispectral RGB with mono cameras.  You’d only need to take G and B subs, a considerable time saving which is important for EAA.  And it would work with any scope, not just an achromat, maybe even better since no need for the minus violet filter.

Edited by steveincolo, 30 January 2024 - 05:15 PM.

[dcweaver]

Very cool. Love the innovation!

[Borodog]

That’s a second extremely cool idea for you, Mike!  With some small enhancements, this could be used in EAA software that does multispectral RGB with mono cameras.  You’d only need to take G and B subs, a considerable time saving which is important for EAA.  And it would work with any scope, not just an achromat, maybe even better since no need for the minus violet filter.

I presume you mean G and R. And yes, I jokingly suggested in one of the threads I posted in the imaging forums on the synthetic blue channel method that mono imagers could skip their blue filter. Funny, but nobody took me up on it. :O)

 

It would not actually be that hard to accomplish this now with a bit of scripting. You just need 2 instances of SharpCap running and a bit of scripting fu. The first instance uses the sequencer to switch the filters and take the R and G subs. A script watches the capture folder, and uses ImageMagick or something to combine the R & G subs with a synthetic blue channel into an RGB sub that it writes to a folder that is monitored by the folder camera in a second instance of SharpCap, which does the alignment and stacking. Voila. Color EAA with a mono camera.

 

And thank you both, Steve and DW.

Edited by Borodog, 30 January 2024 - 05:49 PM.

[steveincolo]

I presume you mean G and R. And yes, I jokingly suggested in one of the threads I posted in the imaging forums on the synthetic blue channel method that mono imagers could skip their blue filter. Funny, but nobody took me up on it. :O)

 

It would not actually be that hard to accomplish this now with a bit of scripting. You just need 2 instances of SharpCap running and a bit of scripting fu. The first instance uses the sequencer to switch the filters and take the R and G subs. A script watches the capture folder, and uses ImageMagick or something to combine the R & G subs with a synthetic blue channel into an RGB sub that it writes to a folder that is monitored by the folder camera in a second instance of SharpCap, which does the alignment and stacking. Voila. Color EAA with a mono camera.

 

And thank you both, Steve and DW.

I might actually try that, because I think my inexpensive ZWO RGB filters cause some blue bloat in stars.  If ImageMagick can do that, then this would work with any EAA application that can use a watched folder.  NINA could be used for the filter sequencing and imaging.

 

Here’s a question for you: what difference does it make, if any, that OSC cameras have some bleed over between the channels?  Here’s a graph for the ZWO ASI2600MC, but I think it’s pretty typical.  Mono RGB filters have sharper cutpoints, and maybe that would have an effect too.

Attached Thumbnails

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Edited by steveincolo, 30 January 2024 - 06:18 PM.

[dcweaver]

You could look in the Jocular OSC Python routine(s) to see if the de-Bayer process compensates for bleed over. I looked at it once, and it appeared that during de-Bayering, each channel would get reconstructed to fill in missing values for a complete array of separate R, G, and B data. Then each channel was combined through some blending math to get the final color representation.

 

Jocular might even make a good a testbed to see if anything gets artificially inflated between the mono process and the OSC process. If you saw something, you could apply scale factor based on surrounding pixels levels in the bleed over colors, that brought things in line with what a mono camera gives.

[alphatripleplus]

One interesting  potential use of this tool with (some) APOs that may exhibit blue fringing:

 

If you look at the spectral response of  OSC cameras (like the 2600MC shown above), you'll see that between 400-450nm, the B channel is the only channel with any noticeable sensitivity in that narrow spectral range - the R and G channels have less than 10% relative response from 400-450nm.

 

So if you have an APO that has a bit of blue fringing, it's likely that replacing the actual blue channel with a synthetic blue should remove the blue fringing. As a UV cut filter only cuts below about 400nm, it won't help with potential blue fringing from 400-450nm. 

Edited by alphatripleplus, 30 January 2024 - 08:17 PM.

[steveincolo]

One interesting  potential use of this tool with (some) APOs that may exhibit blue fringing:

 

If you look at the spectral response of  OSC cameras (like the 2600MC shown above), you'll see that between 400-450nm, the B channel is the only channel with any noticeable sensitivity in that narrow spectral range - the R and G channels have less than 10% relative response from 400-450nm.

 

So if you have an APO that has a bit of blue fringing, it's likely that replacing the actual blue channel with a synthetic blue should remove the blue fringing. As a UV cut filter only cuts below about 400nm, it won't help with potential blue fringing from 400-450nm. 

That’s exactly what I was thinking for my AT130EDT.  There’s some blue fringing, which might also be due to the inexpensive ZWO filters I’m using.

[alphatripleplus]

That’s exactly what I was thinking for my AT130EDT.  There’s some blue fringing, which might also be due to the inexpensive ZWO filters I’m using.

The nice thing is that it should be very easy with the latest SharpCap to test if it kills those blue fringes.

[Borodog]

I might actually try that, because I think my inexpensive ZWO RGB filters cause some blue bloat in stars.  If ImageMagick can do that, then this would work with any EAA application that can use a watched folder.  NINA could be used for the filter sequencing and imaging.

 

Here’s a question for you: what difference does it make, if any, that OSC cameras have some bleed over between the channels?  Here’s a graph for the ZWO ASI2600MC, but I think it’s pretty typical.  Mono RGB filters have sharper cutpoints, and maybe that would have an effect too.

Depending on the scope, it may not be the filters. My AT70ED, which is nominally an "APO" definitely shows a little bit of CA. I have been using a William Optics VR-1 violet reduction filter, which tames it to an acceptable level. I have noticed in testing the synthetic blue channel method on those images, however, that it is still there, if only a tiny bit, and the method completely eliminates it, leaving the colors in the rest of the image essentially untouched.

 

Regarding the bleed between channels, I am no expert. But it does mimic the way that the color receptors in our eyes work, and in that sense, it is the mono filters that are unnatural. It seems like mono filters will produce more color separation and hence more vivid colors. But it also seems that the sharp cutoffs of mono filters could potentially cause problems precisely because they don't mimic the eye's response. For example, look at these transmission curves:

 

https://www.astronom...mik/lrgb_en.pdf

 

Any wavelength above 640nm becomes the same monochromatic shade of red, precisely because there is no blue or green response. Same thing for wavelengths below 460nm, monochromatic shade of blue. There's a short range of monochromatic green. How important is this? Probably not very, but it means, for example, that H-alpha and Sii are the exact same shade of red. 

[Borodog]

One interesting  potential use of this tool with (some) APOs that may exhibit blue fringing:

 

If you look at the spectral response of  OSC cameras (like the 2600MC shown above), you'll see that between 400-450nm, the B channel is the only channel with any noticeable sensitivity in that narrow spectral range - the R and G channels have less than 10% relative response from 400-450nm.

 

So if you have an APO that has a bit of blue fringing, it's likely that replacing the actual blue channel with a synthetic blue should remove the blue fringing. As a UV cut filter only cuts below about 400nm, it won't help with potential blue fringing from 400-450nm. 

The question is whether or not replacing the blue channel with a synthetic one reveals a faint red fringe that had been masked by the blue. If that's the case, you'll still want to image through some sort of mild CA filter to get the most benefit. As I mentioned above, this is definitely the case with my AT70ED.

[MartinMeredith]

Bravo Borodog! I've implemented the basic idea in Jocular for mono + filters, and can flip between RGB and RGsynB to quickly see any differences.

 

Since these are not live captures I've linked to my gallery.

 

First impressions are that it maintains (NGC 2169) and in some cases improves (NGC 2017) star colours, so it is certainly a potential time-saver for those of us doing mono + filters. B is generally a bit of a pain too as sometimes it can be hard to find enough stars to align on, so doing without B will help there too.

 

It was particularly fun to see Sigma Orionis, one of the bluest clusters.

 

The only place so far that I'm seeing a big difference is in bright nebulosity like M42 or M8, and bright planetaries eg M27. In both cases the purply-reds become golden-oranges (rather nice actually). These cases would perhaps benefit from a different weighting of R and G in creating the B. [in edit] I've just read far enough into your thread in the deep sky imaging forum where you get something similar; I'll try your suggested formula. 

 

I should say that these are with a Newt, so no CA to correct.

Edited by MartinMeredith, 31 January 2024 - 07:59 AM.

[alphatripleplus]

The question is whether or not replacing the blue channel with a synthetic one reveals a faint red fringe that had been masked by the blue. If that's the case, you'll still want to image through some sort of mild CA filter to get the most benefit. As I mentioned above, this is definitely the case with my AT70ED.

Yes, it is certainly possible that a red fringe would be revealed, particularly with a doublet like the 70ED. However, I think it's still worth trying the synthetic replacement for scopes with better colour correction than the 70ED, but which still have a little blue fringing, to see if there is any resulting residual red fringing.

Edited by alphatripleplus, 31 January 2024 - 08:50 AM.

[steveincolo]

Bravo Borodog! I've implemented the basic idea in Jocular for mono + filters, and can flip between RGB and RGsynB to quickly see any differences.

 

Since these are not live captures I've linked to my gallery.

 

First impressions are that it maintains (NGC 2169) and in some cases improves (NGC 2017) star colours, so it is certainly a potential time-saver for those of us doing mono + filters. B is generally a bit of a pain too as sometimes it can be hard to find enough stars to align on, so doing without B will help there too.

 

It was particularly fun to see Sigma Orionis, one of the bluest clusters.

 

The only place so far that I'm seeing a big difference is in bright nebulosity like M42 or M8, and bright planetaries eg M27. In both cases the purply-reds become golden-oranges (rather nice actually). These cases would perhaps benefit from a different weighting of R and G in creating the B. [in edit] I've just read far enough into your thread in the deep sky imaging forum where you get something similar; I'll try your suggested formula. 

 

I should say that these are with a Newt, so no CA to correct.

Great examples!  One thought would be to use reference stars with known colors and set the constants for a least squares fit or something like that.

[steveincolo]

Mike, I see that your great idea wasn’t necessarily met with the reception you hoped for in the other forums, but this forum seems to me the perfect place.  Wanting to do EAA with a fast achromat is pretty common, while achromats are a bit foreign to those forums.  

[Borodog]

Bravo Borodog! I've implemented the basic idea in Jocular for mono + filters, and can flip between RGB and RGsynB to quickly see any differences.

Since these are not live captures I've linked to my gallery.

First impressions are that it maintains (NGC 2169) and in some cases improves (NGC 2017) star colours, so it is certainly a potential time-saver for those of us doing mono + filters. B is generally a bit of a pain too as sometimes it can be hard to find enough stars to align on, so doing without B will help there too.

It was particularly fun to see Sigma Orionis, one of the bluest clusters.

The only place so far that I'm seeing a big difference is in bright nebulosity like M42 or M8, and bright planetaries eg M27. In both cases the purply-reds become golden-oranges (rather nice actually). These cases would perhaps benefit from a different weighting of R and G in creating the B. [in edit] I've just read far enough into your thread in the deep sky imaging forum where you get something similar; I'll try your suggested formula.

I should say that these are with a Newt, so no CA to correct.

Did you watch the video I linked? The method to use with bright Ha emission nebulae is not 2G-R, its G + (G-R), where the second term (G-R) black clips when R > G.

[Borodog]

Also, please, anyone that tries this, either the SharpCap tool or the method in another tool, please post images.

[Borodog]

Correction: The Baader 495 long pass filter does NOT have an IR cutoff, so it would also have to be stacked with one.

[Far_Southern_Skies]

Mike, congratulations on an outstanding idea. And of course, a big thank you to Dr Glover for the implementation.

 

The results are astonishing.

 

Some background. Last year I experimented with my Vixen NA140SS to see if it was suitable for EAA - a 140mm refractor with a flat field at f3.9 (with reducer) sounds attractive. The NA140 has a second pair of correcting lens, (similar in design to the Petzval design) that creates a flat field and significantly controls CA. The results were not good enough and I put the idea on hold and kept the scope as my main observing refractor.

 

I have put some images through the Test Camera tool and applied the CA reduction tool. All I can say is wow !!!!!.

 

All of the original images displaying CA were taken in April 2023. Seeing conditions hovered between 5 and 6 FWHM. My observatory is under Bortle 4 skies.

 

My first comparison is for the Carina Nebula. Note, the originals were recorded without any filters, other than an IR cut filter.

 

NGC 3372_2023-04-25_NA140SS_ASI585_SC_40X15.0s_600s_Gain252

 

 

With Deep Sky 1 Ca Reduction: I have not made any adjustments.

 

 

I will post some further comparisons shortly.

Edited by Far_Southern_Skies, 01 February 2024 - 04:45 AM.

[Far_Southern_Skies]

Comparisons continued from previous post:

 

Jewel Box Cluster_NGC 4755_2023-04-25_NA140SS_ASI585_SC_65X4.0s_260s_Gain252. Original with no filters, other than IR cut.

 

 

Revised image with Solar System CA Reduction. No other processing.

 

[Far_Southern_Skies]

Comparisons continued from previous post:

 

Omega Centauri_NGC 5139_2023-04-25_NA140SS_AS585_SC_75X4.0s_300s_Gain400. No filters, other than IR cut.

 

 

Revised image with Solar System CA Reduction. No other processing.

 

 

 

 

[Far_Southern_Skies]

Comparisons continued from previous post:

 

Hamburger Galaxy_NGC 5128_2023-04-25_NA140SS_ASI585_SC_38X8.0s_304s_Gain400

 

 

Revised image with Solar System CA Reduction. No other processing.

 

Edited by Far_Southern_Skies, 01 February 2024 - 04:50 AM.

[Far_Southern_Skies]

Comparisons continued from previous post:

 

NGC 4945_2023-04-25_NA140SS_ASI585_SC_38X8.0s_304s_Gain400

 

 

Revised image with Solar System CA Reduction. No other processing.

 

Edited by Far_Southern_Skies, 01 February 2024 - 04:50 AM.