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Low-cost colour filters to reduce chromatic aberration?

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#1 Ivor Redtorch

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Posted 04 August 2020 - 05:46 AM

Hi All

 

Hope you are doing well.

 

I have a f/5.5 refractor that shows spectacular chromatic aberration on bright objects like Venus (even with a Baader Contrast Booster Filter, that is described as a fringe killer). Would taking photos of such objects through RGB filters help, because I could then refocus for the blue?

 

If it would help, then do I need to buy expensive RGB filters or would the vastly cheaper generic ones do the job? I am wondering if the wider band pass of a cheaper blue filter would be narrow enough for my prismatic telescope not to split its components noticeably.

 

Prices do differ a bit, for example:

Astronomik Deep Sky RGB Filter Set is about 300 UKP, whereas a Pomya Eyepiece Lens Color Filter, 1.25" is about 10 UKP.

 

Well, perhaps not Pomya, as I have never heard of them, but I just wanted to show the extremes of the range (I know there's more expensive options than Astronomik but my photography skills aren't up to appreciating the advantages). For sure the 300+ filters give better results, and 13 UKP ones may be unusuable; however, for just combating CA, I wonder if 100s of pounds are necessary.

 

For my purposes Pomya might be fine, I don't know, and that's why I am asking those who do. I would only be using them for our moon, Venus, Jupiter, and Saturn, just the very bright objects.

 

We are entering difficult economic times, so now more than ever, it is wise to not spend more than is necessary to get the job done.

 

Thank you for any answers (even if just "Yes, No", which is my uninformed attempt at the answers).


Edited by Ivor Redtorch, 04 August 2020 - 05:46 AM.


#2 james7ca

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Posted 04 August 2020 - 06:54 AM

There is no absolute answer here because even with a set of RGB filters you are still going to be affected by chromatic aberration, just less so than if not using filters. I've used IR filters with my Celestron 102GT refractor (f/9.8 doublet) to do successful lunar imaging and I've done RGB imaging with a "low end" set of 1.25" LRGB filters from ZWO using telephoto lenses and a Celestron C90. But, the C90 being a Maksutov-Cassegrain is well corrected for color and I only mention that system for its use of the ZWO filters (a four filter set that can be purchased for about $90 U.S.).

 

So, a set of RGB filters will likely help, but whether you will find it a satisfactory result is kind of hard to say. That said, a simple doublet at f/5.5 is probably going to show a good amount of color and this will vary with the design and size of the objective. You'll also have more issues at the blue end of the spectrum, so even if red look okays the blue may not.

 

I think most people who image with achromats (at least for DSOs) do so using narrow band filters which have about one tenth or less of the bandpass you'd get with a broad band RGB filter.


Edited by james7ca, 04 August 2020 - 06:55 AM.


#3 Ed D

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Posted 04 August 2020 - 07:36 AM

I agree with what James said.  However, you don't mention what type of camera you have:  Color or mono, DSLR, ???  Also, do you already have a filter wheel, etc?  Have you already tried imaging and processing?  Knowing this can help us give you better ideas.

 

Ed D


Edited by Ed D, 04 August 2020 - 07:44 AM.


#4 james7ca

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Posted 04 August 2020 - 07:56 AM

...Color or mono, DSLR, ???...Knowing this can help us give you better ideas.

 

Ed D

Actually, I guess if the OP has a one-shot-color camera or DSLR then even without a set of RGB filters he could refocus for each color by trying to reduce the red or blue color fringing that he saw in the debayered color image. Then, with the proper software he could separate the RGB channels and take the best for each color and then recombine the individual colors to produce the final result. But, that would require a lot of work and I'm not sure how well you could optimize the focus for each color when you were looking at a full-color image.

 

Adding a separate filter to the camera might make the focus easier to determine, since the that would reduce the intensity of the contrasting colors (e.g. a red filter would reduce the apparent brightness of the green and blue channels). Still kind of a difficult process, but probably better than with no filter at all.

 

Also, you'd need the proper software to separate the pixels in the color Bayer pattern, not something that is likely to be had or used by a novice.


Edited by james7ca, 04 August 2020 - 07:59 AM.


#5 klaussius

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Posted 04 August 2020 - 08:06 AM

I've tried many things with my achromat and nothing really was worth the trouble.

 

Red and green get decent if focused independently, either by using a red/green filter or using the trick mentioned by james7ca (which is hard to pull off, let me tell you). But on my achromat blue would never get truly sharp. YMMV, but for me all that was a non-solution.

 

I've seen people get some decent results using light pollution filters (CLS; UHC), since they reduce the bandwidth considerably. But, again, not helpful enough in my case when I tried so YMMV.



#6 zxx

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Posted 04 August 2020 - 08:41 AM

My 127mm F 9.4 did ok with dim objects.  no filters. My F 5,  not so good

 

Capture m51 .png



#7 Ivor Redtorch

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Posted 04 August 2020 - 02:04 PM

Thank you for sharing experiences and advice, especially zxx for an example of what could be done, and james7ca for that in-depth answer.

 

Apologies Ed D for not giving all the details that might be relevant, I do have a filter wheel (1.25" and a 2" which is still on a slow boat from China). I have a Sony NEX F9 (a colour mirrorless DSLR) nice and light, and I have a 1.25" Bresser "Full HD" deep-sky astrocam (also colour), which doesn't really come with proper control software for linux systems - so still fiddling with that (good videos of chimney pots).

 

I have not tried much imaging, the reason for the post was because I am using the Summer, when the nights are short and not very dark, to get equipment sorted out. I do practice by processing other people's raw images that they upload for others to play with. I use mostly command line based implementations so I could batch process*; for example, numpy (the python package) to subtract darks etc, and imageMagick to stack. Also, as the clouds seem to clear so rarely now here, I wanted to have anything that might be useful already to hand, and the boats from China are getting slower. I took afocal images of Venus with a mobile phone and noted the blue fringes, and with Jupiter slowly coming around to where I can see it, I hope to give myself the best chance.

 

I also have all these filters, they have crept up on me, I can't remember buying some of these, but some are for visual, and I like visual too. I have a 3" f/5.5 and a 6" f/5, so they are both crazy fast, but they behave fairly well on DSOs, it's really the big bright objects that show the blue focusing beyond the RG. The latter has a 2" focuser. I have a some light pollution and the DGM NPB helps with visual. This time of year each OTA has a practically resident Neodymium filter.

 

1.25"

Baader Planetarium Contrast Booster Filter

Baader Neodymium Moon & Sky Glow with IR

Baader SII Narrow Band CCD filter

Altair Astro Narrowband 7nm H-Alpha CCD Filter
Altair AstroNarrowband 6.5nm OIII CCD Filter
Altair Astro CLS-CCD Filter with UVIR Block & AR Coating

 

2"

Astronomik CLS Light Pollution Filter

Altair Astro Narrowband 7nm H-Alpha CCD Filter
Baader OIII
DGM NPB
Baader Neodymium Moon & Sky Glow with IR

 

*highly unlikely the stars will appear for long enough for me to ever batch process, but it's a habit from my job.

 

Thank you again.

 

Ivor



#8 Ed D

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Posted 05 August 2020 - 06:50 AM

Ivor, thanks for posting your equipment info.    The Bresser camera has the Sony 290 sensor, same as my ASI290MC one shot color camera.  If you can fiddle with that one and get it to work to your satisfaction it would be a good planetary/lunar camera.  But, use the camera you are most comfortable with.

 

I have owned short focal ratio achromats.  When using my 102mm f/6.5 for visual I used a Wratten #8 Yellow color filter to good effect for blocking out the CA.  I also tried imaging Luna with the #8, as well as a Wratten #12 Medium Yellow filter.  With my camera software set to capture Mono the resulting images were good.  An inexpensive light or medium yellow color filter might be worth a try if wanting to keep expenses to a minimum for now.

 

Another use of filters I have done is to stack the yellow filter in front of my Baader Neodymium filter.  The yellow filter blocks wavelengths below ~475nm, the blue/UV region, and then the Neodymium filter blocks the IR and LP wavelengths.  You already have the Baader Neodymium filters, so a cheap yellow color filter wouldn't be much of an expense to experiment with.

 

If you find that you like lunar and planetary imaging you may want to consider investing in a 5" or 6" Mak, or maybe an 8" SCT at a future date.  For now, give the yellow color filter a try, and experiment on the moon - it's an easy target.

 

Ed


Edited by Ed D, 05 August 2020 - 07:09 AM.


#9 Ivor Redtorch

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Posted 05 August 2020 - 07:18 AM

Many thanks Ed D for those suggestions. I have a "moon filter", which happens to be just a medium yellow filter with 'moon' written on it, and I could try that first (with the Neodymium) and see the result.

 

The sky itself is not an easy target in the UK, the North East Atlantic is a cloud machine, but the moon is as easy as it gets (second to chimney pots).

 

Ivor



#10 johrich

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Posted 05 August 2020 - 12:18 PM

You might want to check this out,  "Baader Yellow 495 nm Longpass Filter - 2"  According to the write-up on High Point Scientific  it "is a great choice for removing chromatic aberration from achromatic refractors.".   I actually have one and have been meaning to try it on my Orion 120ST achromatic refractor, but haven't gotten around to it, so I can't tell you how well it works.

 

 

johrich



#11 Cbaxter

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Posted 05 August 2020 - 04:24 PM

I agree with the suggestion from johrich. I learned astrophotography with an Orion 120ST achromat and found the 495nm longpass filter to be the best fringe reducer out there, and it's not too expensive either. One thing you will need to do though is white/color balance your image during post processing as the filter will yellow bias the images. Without the filter there were huge bright violet halos around most of the stars in my images. I tried many different fringe killers but none worked as well on the 120ST as the 495, and color balancing the final image was much easier than dealing with bloated stars with enormous violet halos. 

 

Regards. 



#12 Ivor Redtorch

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Posted 14 September 2020 - 09:57 AM

Thanks johrich

 

I will try the 495 nm filter, as it is relatively cheap and well recommended on here.

 

I might even do a with and without after photo, if I can stop fiddling with motors and controllers and look at the sky!



#13 TareqPhoto

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Posted 14 September 2020 - 10:05 AM

The question is, if you use your CA scope for red and green only and i brought Blue channel from another scope say ED or true APO or even reflector, will that help to solve the problem or blue will always be as CA even from mirrors combined with another channels from achromatic refr?



#14 Ivor Redtorch

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Posted 21 September 2020 - 11:49 AM

Hi TareqPhoto

 

Yes, this was what I was wondering when I started this thread (though, if I had an APO, I would probably just use that).

 

The CA comes from the RG and Blue focusing at different positions, so maybe I could get the blue in focus without the R and G (or in focus with less CA because I am dealing with a narrower range of wavelengths - and probably here, the better the filter quality, the better the result), but, as I would be focusing at a different position than the RG was focused at, wouldn't my blue image be a slightly different size? I also think the human eye is not so good at deciding if a blue image is in focus either.

 

Perhaps even if it were a slightly different size I could scale the blue layer in something like GIMP to match the RG; however, there might also be differences in distortion of the blue image too (because my lenses will be adding some distortion).

 

Consequently, the blue might never fit with the red and green, or it might fit well enough for the image to be better than taking an RBG image and using a fringe-killer filter.

 

As there are many variables I think I need to retire, move to a desert with plenty of clear nights, and test this.



#15 TareqPhoto

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Posted 21 September 2020 - 12:56 PM

Hi TareqPhoto

 

Yes, this was what I was wondering when I started this thread (though, if I had an APO, I would probably just use that).

 

The CA comes from the RG and Blue focusing at different positions, so maybe I could get the blue in focus without the R and G (or in focus with less CA because I am dealing with a narrower range of wavelengths - and probably here, the better the filter quality, the better the result), but, as I would be focusing at a different position than the RG was focused at, wouldn't my blue image be a slightly different size? I also think the human eye is not so good at deciding if a blue image is in focus either.

 

Perhaps even if it were a slightly different size I could scale the blue layer in something like GIMP to match the RG; however, there might also be differences in distortion of the blue image too (because my lenses will be adding some distortion).

 

Consequently, the blue might never fit with the red and green, or it might fit well enough for the image to be better than taking an RBG image and using a fringe-killer filter.

 

As there are many variables I think I need to retire, move to a desert with plenty of clear nights, and test this.

This will always be an issue then, so what about fluorite scopes, are they prone to CA as well?

 

Sound reflectors are still a better option for RGB over non corrected refractors, even if it is an APO it can still have slight percent of CA unless it is a high quality triplet maybe or high quality doublet, it is so complicated because it is all about budget really.



#16 klaussius

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Posted 21 September 2020 - 01:36 PM

Reflectors, Newtonians in particular, are a great way to combat CA on a budget. But they're usually bulkier and part of the cost just moves to the mount instead.

Fast newtonians require a bit of upkeep. Flocking, dealing with light leaks, collimation. None of that is a huge issue but all of that is particular to newtonians and so an added burden.

I still swear by them. They're hard to beat in bang for the buck.

But not as cheap as a cheap achro though. They still cost a penny.


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