Johnson-Cousins BVR Flat-Top Filters vs RGB Filters

2.81 Ani Shastry ... · view views · Share link
Hi all, I have recently been deliberating on filter choices for a new system I am putting together (it's a PlaneWave CDK20 that will be installed at Obstech). The system will predominantly be used for deep sky imaging. I was originally thinking about replicating what I have done previously on my other systems, and going with the typical broadband LRGB and narrowband HaOiiiSii filters. While the narrowband choice is reasonably straightforward (and special thanks to @John Hayes for his kind feedback on the 5nm vs 3nm Ha debate), I started looking at the transmission curves of the RGB filters and it got me thinking as to whether they were really the best choice. In particular, the flat-top Johnson-Cousins BVR filters that Chroma sells (and calls [sic] Bessell filters), that are typically used for aperture photometry, seem to be rather interesting even for deep sky imaging. And seeing that @Wei-Hao Wang (CDK20) and @Wolfgang Promper (AZ1500) have both imaged with J/C BVR filters and that their images look spectacular gives me more confidence. I would love more thoughts from the community on my observations below. Have we been sticking to RGB simply because of arcane / legacy reasons? Have other people specifically used the J/C flat-top filters before and have had positive or negative experiences? (Note a few other brands make Bessel filters, but they are not necessarily "flat-top"). Comparing the spectral bandpass, these are my observations (note that the black curve is the QE curve of IMX455/461 sensors, and the cyan line is the Luminance filter): + The BVR filters have slightly wider bandpasses in the red and blue regions. + There is very little overlap between each of the bands with BVR unlike RGB and the falloff is rather steep. + There isn't ~20nm band gap between green and red, which IIRC is there to filter out light light pollution from sodium-vapor lamps, and not really useful from a remote Bortle 1 site. - The only negative I can think of is that the luminance filter doesn't have as perfect of a match, but creating a superlum with L, B, V and R should mitigate that issue. And when considering overlap with narrowband filters for continuum subtraction, there is possibly a slight advantage to the BVR filters in that Oiii bandpass falls squarely within the V filter bandpass, while with RGB, both G and B oddly share the overlap in their falloff regions. Thanks, Ani Like

2.81

...

Hi all,

I have recently been deliberating on filter choices for a new system I am putting together (it's a PlaneWave CDK20 that will be installed at Obstech). The system will predominantly be used for deep sky imaging. I was originally thinking about replicating what I have done previously on my other systems, and going with the typical broadband LRGB and narrowband HaOiiiSii filters.

While the narrowband choice is reasonably straightforward (and special thanks to @John Hayes for his kind feedback on the 5nm vs 3nm Ha debate), I started looking at the transmission curves of the RGB filters and it got me thinking as to whether they were really the best choice. In particular, the flat-top Johnson-Cousins BVR filters that Chroma sells (and calls [sic] Bessell filters), that are typically used for aperture photometry, seem to be rather interesting even for deep sky imaging. And seeing that @Wei-Hao Wang (CDK20) and @Wolfgang Promper (AZ1500) have both imaged with J/C BVR filters and that their images look spectacular gives me more confidence.

I would love more thoughts from the community on my observations below. Have we been sticking to RGB simply because of arcane / legacy reasons? Have other people specifically used the J/C flat-top filters before and have had positive or negative experiences? (Note a few other brands make Bessel filters, but they are not necessarily "flat-top").

Comparing the spectral bandpass, these are my observations (note that the black curve is the QE curve of IMX455/461 sensors, and the cyan line is the Luminance filter):
+ The BVR filters have slightly wider bandpasses in the red and blue regions.
+ There is very little overlap between each of the bands with BVR unlike RGB and the falloff is rather steep.
+ There isn't ~20nm band gap between green and red, which IIRC is there to filter out light light pollution from sodium-vapor lamps, and not really useful from a remote Bortle 1 site.
- The only negative I can think of is that the luminance filter doesn't have as perfect of a match, but creating a superlum with L, B, V and R should mitigate that issue.

And when considering overlap with narrowband filters for continuum subtraction, there is possibly a slight advantage to the BVR filters in that Oiii bandpass falls squarely within the V filter bandpass, while with RGB, both G and B oddly share the overlap in their falloff regions.

Thanks,
Ani

26.84 John Hayes ... · view views · 3 likes · Share link
HI Ani, You've raised a very interesting point here. One thing that jumps out at me is that the bandpass for the BVR red filter will help a little to make up for the fall off in sensitivity of the IMX455 sensor at longer wavelengths. I also like the lower overlap between the B and G filters and your point about continuum subtraction is a good one. If it were me, I'd definitely choose the BVR filter set. And if you go in that direction, I'll be looking carefully at your results. Heck, I may have to change out the filters on my scope. Good luck with it! - John Like

26.84

John Hayes

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HI Ani,
You've raised a very interesting point here. One thing that jumps out at me is that the bandpass for the BVR red filter will help a little to make up for the fall off in sensitivity of the IMX455 sensor at longer wavelengths. I also like the lower overlap between the B and G filters and your point about continuum subtraction is a good one. If it were me, I'd definitely choose the BVR filter set. And if you go in that direction, I'll be looking carefully at your results. Heck, I may have to change out the filters on my scope.

Good luck with it!

- John

3.01 Mark McComiskey ... · view views · 1 like · Share link
I gave this some consideration a while back, and continue to do so, as I prepare to install a remote system in the US. However, I was specifically considering photometric filters with a large overlap. Something like this: https://www.chroma.com/products/sets/27105-classic-ubvri-set not flat topped, obviously, but substantial overlap. I think at one point I actually looked at a mix of flat topped and these to maximize transmission while still getting overlap. Vincent Perez discussed his preference for this kind of filter setup in one of his pixinsight articles: https://pixinsight.com/examples/NGC7331-CAHA/index.html I’ve not found much discussion on the merits of the overlapping approach, and I assume there is a loss in contrast. Interested to know if others have considered/tried this. Like

3.01

Mark McComiskey

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I gave this some consideration a while back, and continue to do so, as I prepare to install a remote system in the US.

However, I was specifically considering photometric filters with a large overlap. Something like this:

https://www.chroma.com/products/sets/27105-classic-ubvri-set

not flat topped, obviously, but substantial overlap. I think at one point I actually looked at a mix of flat topped and these to maximize transmission while still getting overlap.

Vincent Perez discussed his preference for this kind of filter setup in one of his pixinsight articles: https://pixinsight.com/examples/NGC7331-CAHA/index.html

I’ve not found much discussion on the merits of the overlapping approach, and I assume there is a loss in contrast. Interested to know if others have considered/tried this.

0.00 August Broe ... · view views · Share link
Hi Ani, Regarding the B and G filter overlap, it is actually quite intentional to have both the filter exactly overlap with the OIII emission line, as this will allow both channels to capture some sweet extra data. For this reason, RGB will have slightly worse color accuracy. Also, the overlap is often designed so that at least one of the filters, if not both, will capture the OIII signal at close to maximum transmission, so continuum subtraction is rarely an issue with RGB filters. The BVR filters do have an advantage over the RGB filters in the R and G overlap, as RGB filters are often made with a little gap in between, to combat sodium-vapor street lighting, which emits at 589nm. So J/C BVR will be able to squeeze out a bit more signal there, and also in the slightly broader B and R bandpasses. It's hard to say what will give the best results, as it will depend a lot on the target - I suspect RGB will have better SNR at a small cost of color accuracy for emission nebulae, while J/C BVR probably has ever so slightly better SNR on pure broadband targets. All in all, I don't think you will see any big difference between the two options, and most of it will probably be a matter of preference. Good luck - August Like

0.00

August Broe

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Hi Ani,
Regarding the B and G filter overlap, it is actually quite intentional to have both the filter exactly overlap with the OIII emission line, as this will allow both channels to capture some sweet extra data. For this reason, RGB will have slightly worse color accuracy. Also, the overlap is often designed so that at least one of the filters, if not both, will capture the OIII signal at close to maximum transmission, so continuum subtraction is rarely an issue with RGB filters.

The BVR filters do have an advantage over the RGB filters in the R and G overlap, as RGB filters are often made with a little gap in between, to combat sodium-vapor street lighting, which emits at 589nm. So J/C BVR will be able to squeeze out a bit more signal there, and also in the slightly broader B and R bandpasses.

It's hard to say what will give the best results, as it will depend a lot on the target - I suspect RGB will have better SNR at a small cost of color accuracy for emission nebulae, while J/C BVR probably has ever so slightly better SNR on pure broadband targets.

All in all, I don't think you will see any big difference between the two options, and most of it will probably be a matter of preference.

Good luck smile

- August

26.84 John Hayes ... · view views · 2 likes · Share link
Mark, That's an interesting consideration but I personally wouldn't consider photometric filters with substantial overlap. Remember that the total signal is given by the signal spectrum x filter transmission x sensor responsivity integrated over the wavelength range so the first effect would be that it would add more variables to color calibration–especially when using SPCC. It also mixes up varying colors across the spectrum in a way that has little obvious benefit. Vincent's argument for overlap between the bandpass curves is really just an argument to be using more filters in the first place. Yes, some of the green signal overlaps into the blue but it's all mixed up so you don't know where it came from in the first place. If we want to get more accurate colors, we really should be using say six filters; not three. This article was clearly written before SPCC so I wonder how Vincent's thinking has evolved with the availability of better color calibration tools. John Like

26.84

John Hayes

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Mark,
That's an interesting consideration but I personally wouldn't consider photometric filters with substantial overlap. Remember that the total signal is given by the signal spectrum x filter transmission x sensor responsivity integrated over the wavelength range so the first effect would be that it would add more variables to color calibration–especially when using SPCC. It also mixes up varying colors across the spectrum in a way that has little obvious benefit. Vincent's argument for overlap between the bandpass curves is really just an argument to be using more filters in the first place. Yes, some of the green signal overlaps into the blue but it's all mixed up so you don't know where it came from in the first place. If we want to get more accurate colors, we really should be using say six filters; not three. This article was clearly written before SPCC so I wonder how Vincent's thinking has evolved with the availability of better color calibration tools.

John

3.01 Mark McComiskey ... · view views · Share link
Fair enough. I’ve hesitated to do go with the overlapping filters for quite some time. On the SPCC front, though, since one can input the specific transmission curves of the filters, wouldn’t the SPCC process handle all the complications? Im actually intrigued by the idea of using more filters to get more accurate colors. It seems like all our tools, though, are based on RGB. How would one even go about calibrating or using a fourth, fifth and sixth color filter? Beyond somewhat arbitrarily adding them to the RGB channels? Like

3.01

Mark McComiskey

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Fair enough. I’ve hesitated to do go with the overlapping filters for quite some time.

On the SPCC front, though, since one can input the specific transmission curves of the filters, wouldn’t the SPCC process handle all the complications?

Im actually intrigued by the idea of using more filters to get more accurate colors. It seems like all our tools, though, are based on RGB. How would one even go about calibrating or using a fourth, fifth and sixth color filter? Beyond somewhat arbitrarily adding them to the RGB channels?

2.81 Ani Shastry Topic starter ... · view views · 1 like · Share link
Hi all, Thank you again for your input, this has been a good discussion. I think I will indeed go ahead with the flat-top Johnson-Cousins BVR filters instead of the traditional RGB filters. I also have an email out to Chroma to see if they can make me a custom "luminance" filter that was a wider band-pass to match the BVR bandpass instead of the narrower RGB ones. I will report back what they find. I also downloaded the spectral data files for each of the BVR filters, but since PixInsight doesn't have a native way to directly import these and doesn't include these filters by default, I wrote a quick C# app convert them to the .csv file format that PixInsight expects. With this, I was able to import the spectral response graphs for use with the SpectrophotometricCalibration (SPCC) tool. Of course, I don't have any data to try if this works just yet, but @Wei-Hao Wang, if you have data that you are willing to share, I am happy to test it out. @Mark McComiskey, you'd have to map each of the 6 filters to their closest R/G/B equivalents. Probably something like matching the integrated area under the QE curve across the bandpass as close as possible to the R/G/B equivalents might work reasonably well. Many of the high-end renderers used to use spectra instead of just RGB triplets, so you'd get 7 colors to work with, which eventually would get tone mapped into RGB using the photometric response curves of the human eye. Ani Like

2.81

Ani Shastry

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Hi all,

Thank you again for your input, this has been a good discussion. I think I will indeed go ahead with the flat-top Johnson-Cousins BVR filters instead of the traditional RGB filters. I also have an email out to Chroma to see if they can make me a custom "luminance" filter that was a wider band-pass to match the BVR bandpass instead of the narrower RGB ones. I will report back what they find.

I also downloaded the spectral data files for each of the BVR filters, but since PixInsight doesn't have a native way to directly import these and doesn't include these filters by default, I wrote a quick C# app convert them to the .csv file format that PixInsight expects. With this, I was able to import the spectral response graphs for use with the SpectrophotometricCalibration (SPCC) tool. Of course, I don't have any data to try if this works just yet, but @Wei-Hao Wang, if you have data that you are willing to share, I am happy to test it out.

@Mark McComiskey, you'd have to map each of the 6 filters to their closest R/G/B equivalents. Probably something like matching the integrated area under the QE curve across the bandpass as close as possible to the R/G/B equivalents might work reasonably well. Many of the high-end renderers used to use spectra instead of just RGB triplets, so you'd get 7 colors to work with, which eventually would get tone mapped into RGB using the photometric response curves of the human eye.

Ani

26.84 John Hayes ... · view views · 2 likes · Share link
Ani, The CDK scopes have refractive field optics so be aware that the color correction may not extend very far outside of the normal Lum filter bandwidth limits. If the longitudinal color correction changes quite a bit in the red (for example), it won't matter so much when you shoot through the red filter because you can correct for it with a focus offset. However, it might lead to halos–either blue -or- red with the lum filter if you go too far outside the well corrected range of the optical system. So, you might want to check with PW just to get their take on using a lum filter with more bandwidth. It might be fine but it would be a bummer to have a special filter made and then discover that you get unacceptable halos with it. Having more bandwidth with the lum filter has some advantages but unless you are running a RC, be careful about the color correction. John Like

26.84

John Hayes

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Ani,
The CDK scopes have refractive field optics so be aware that the color correction may not extend very far outside of the normal Lum filter bandwidth limits. If the longitudinal color correction changes quite a bit in the red (for example), it won't matter so much when you shoot through the red filter because you can correct for it with a focus offset. However, it might lead to halos–either blue -or- red with the lum filter if you go too far outside the well corrected range of the optical system. So, you might want to check with PW just to get their take on using a lum filter with more bandwidth. It might be fine but it would be a bummer to have a special filter made and then discover that you get unacceptable halos with it. Having more bandwidth with the lum filter has some advantages but unless you are running a RC, be careful about the color correction.

John

3.01 Mark McComiskey ... · view views · Share link
So, as a thought experiment, if I have two filters that perfectly cover the space currently covered by the Green filter’s transmission curve and then map them both to green for processing, has anything been gained in terms of color precision or differentiation. I’m likely just being dense here, but what would be the advantages of having the extra filters? @John Hayes , I assume that same halo concern would apply to our flattener? Like

3.01

Mark McComiskey

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So, as a thought experiment, if I have two filters that perfectly cover the space currently covered by the Green filter’s transmission curve and then map them both to green for processing, has anything been gained in terms of color precision or differentiation. I’m likely just being dense here, but what would be the advantages of having the extra filters?

@John Hayes , I assume that same halo concern would apply to our flattener?

3.01 Mark McComiskey ... · view views · Share link
@Ani Shastry , I meant to ask: 3nm or 5? Like

15.16 Wei-Hao Wang ... · view views · Share link
Mark McComiskey: However, I was specifically considering photometric filters with a large overlap. Something like this: https://www.chroma.com/products/sets/27105-classic-ubvri-set not flat topped, obviously, but substantial overlap. I think at one point I actually looked at a mix of flat topped and these to maximize transmission while still getting overlap. No. Don't, unless you have to (for scientific research). The traditional Johnson UBVRI filters are dyed glasses. The dye absorbs the wavelength of light that we don't want, to create the bandpass. Nowadays, the filters that mimic UBVRI filters are interferometric filters. They rely on layers of dielectric material coated on glass to reflect away the light that we don't want. Now, the filters in your link have bell-shaped transmission curves, rather than square-shaped curves (like the ones shown Ani's first post). This means that for a lot of wavelength ranges, the same wavelength of light can go through and can also get reflected away. A large portion of the reflected light can go back to any optical elements in front of your filter (like flattener, reducer, etc), gets reflection back (a small fraction, of course), reaches the filter again, and then a good fraction of this light goes through the filter. This creates terrible reflection halos on moderately bright stars, or even not-so-bright stars under long integration. Filters with square transmission curves doesn't suffer from this because light either 100% (almost) goes through or 100% (almost) reflected away. There is very very little light that can be reflected, come back, and then go through the filter. I used such standard, bell-shaped UBVRI filters at the beginning, because our telescope was intended for scientific research and education, and I learned the halo lesson a hard way. Later I switched to Chroma's modified UBVRI filters with square transmission curves. This dramatically reduces the reflection halos. Of course, because the passband is no longer identical to the the traditional UBVRI filters, some corrections are required for accurate photometry. But given how bad the traditional ones are, this is an acceptable compromise. So, unless you love donuts everywhere in your image, don't use the UBVRI filters with bell-shaped transmission curves. Like

15.16

Wei-Hao Wang

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Mark McComiskey:
However, I was specifically considering photometric filters with a large overlap. Something like this:

https://www.chroma.com/products/sets/27105-classic-ubvri-set

not flat topped, obviously, but substantial overlap. I think at one point I actually looked at a mix of flat topped and these to maximize transmission while still getting overlap.

No. Don't, unless you have to (for scientific research).

The traditional Johnson UBVRI filters are dyed glasses. The dye absorbs the wavelength of light that we don't want, to create the bandpass. Nowadays, the filters that mimic UBVRI filters are interferometric filters. They rely on layers of dielectric material coated on glass to reflect away the light that we don't want.

Now, the filters in your link have bell-shaped transmission curves, rather than square-shaped curves (like the ones shown Ani's first post). This means that for a lot of wavelength ranges, the same wavelength of light can go through and can also get reflected away. A large portion of the reflected light can go back to any optical elements in front of your filter (like flattener, reducer, etc), gets reflection back (a small fraction, of course), reaches the filter again, and then a good fraction of this light goes through the filter. This creates terrible reflection halos on moderately bright stars, or even not-so-bright stars under long integration. Filters with square transmission curves doesn't suffer from this because light either 100% (almost) goes through or 100% (almost) reflected away. There is very very little light that can be reflected, come back, and then go through the filter.

I used such standard, bell-shaped UBVRI filters at the beginning, because our telescope was intended for scientific research and education, and I learned the halo lesson a hard way. Later I switched to Chroma's modified UBVRI filters with square transmission curves. This dramatically reduces the reflection halos. Of course, because the passband is no longer identical to the the traditional UBVRI filters, some corrections are required for accurate photometry. But given how bad the traditional ones are, this is an acceptable compromise.

So, unless you love donuts everywhere in your image, don't use the UBVRI filters with bell-shaped transmission curves.

3.01 Mark McComiskey ... · view views · Share link
Good to know! Thank you for the information. Like

2.81 Ani Shastry Topic starter ... · view views · Share link
John Hayes: Ani, The CDK scopes have refractive field optics so be aware that the color correction may not extend very far outside of the normal Lum filter bandwidth limits. If the longitudinal color correction changes quite a bit in the red (for example), it won't matter so much when you shoot through the red filter because you can correct for it with a focus offset. However, it might lead to halos--either blue -or- red with the lum filter if you go too far outside the well corrected range of the optical system. So, you might want to check with PW just to get their take on using a lum filter with more bandwidth. It might be fine but it would be a bummer to have a special filter made and then discover that you get unacceptable halos with it. Having more bandwidth with the lum filter has some advantages but unless you are running a RC, be careful about the color correction. John @John Hayes, thanks for bringing this up, this is a good point to verify with PlaneWave indeed. I did shoot an email to Bill Dean asking him to see if there would be issues with the refractive corrective optics on the CDK20. Now that being said, don't people sometimes use "Clear" filters with these telescopes instead of "Luminance" filters where the bandpass is between 300nm and 1200nm without issues? Mark McComiskey: @Ani Shastry , I meant to ask: 3nm or 5? @Mark McComiskey, 5nm is my thinking at the moment (haven't hit the order button yet). I have these on my CDK24 and I have been rather happy with them. John did say the additional contrast may not be worth the price in the context of if I already had the 5nm filters to re-use. I need to buy new filters, but even in the context of that, I still think 5nm may be the way to go (why pay a bit more for something that precludes the ability to capture Nii signals at the slight loss of contrast). Like

2.81

Ani Shastry

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John Hayes:
Ani,
The CDK scopes have refractive field optics so be aware that the color correction may not extend very far outside of the normal Lum filter bandwidth limits. If the longitudinal color correction changes quite a bit in the red (for example), it won't matter so much when you shoot through the red filter because you can correct for it with a focus offset. However, it might lead to halos--either blue -or- red with the lum filter if you go too far outside the well corrected range of the optical system. So, you might want to check with PW just to get their take on using a lum filter with more bandwidth. It might be fine but it would be a bummer to have a special filter made and then discover that you get unacceptable halos with it. Having more bandwidth with the lum filter has some advantages but unless you are running a RC, be careful about the color correction.

John

@John Hayes, thanks for bringing this up, this is a good point to verify with PlaneWave indeed. I did shoot an email to Bill Dean asking him to see if there would be issues with the refractive corrective optics on the CDK20. Now that being said, don't people sometimes use "Clear" filters with these telescopes instead of "Luminance" filters where the bandpass is between 300nm and 1200nm without issues?

Mark McComiskey:
@Ani Shastry , I meant to
ask: 3nm or 5?

@Mark McComiskey, 5nm is my thinking at the moment (haven't hit the order button yet). I have these on my CDK24 and I have been rather happy with them. John did say the additional contrast may not be worth the price in the context of if I already had the 5nm filters to re-use. I need to buy new filters, but even in the context of that, I still think 5nm may be the way to go (why pay a bit more for something that precludes the ability to capture Nii signals at the slight loss of contrast).

26.84 John Hayes ... · view views · 1 like · Share link
Ani Shastry: John Hayes: Ani, The CDK scopes have refractive field optics so be aware that the color correction may not extend very far outside of the normal Lum filter bandwidth limits. If the longitudinal color correction changes quite a bit in the red (for example), it won't matter so much when you shoot through the red filter because you can correct for it with a focus offset. However, it might lead to halos--either blue -or- red with the lum filter if you go too far outside the well corrected range of the optical system. So, you might want to check with PW just to get their take on using a lum filter with more bandwidth. It might be fine but it would be a bummer to have a special filter made and then discover that you get unacceptable halos with it. Having more bandwidth with the lum filter has some advantages but unless you are running a RC, be careful about the color correction. John @John Hayes, thanks for bringing this up, this is a good point to verify with PlaneWave indeed. I did shoot an email to Bill Dean asking him to see if there would be issues with the refractive corrective optics on the CDK20. Now that being said, don't people sometimes use "Clear" filters with these telescopes instead of "Luminance" filters where the bandpass is between 300nm and 1200nm without issues? Mark McComiskey: @Ani Shastry , I meant to ask: 3nm or 5? @Mark McComiskey, 5nm is my thinking at the moment (haven't hit the order button yet). I have these on my CDK24 and I have been rather happy with them. John did say the additional contrast may not be worth the price in the context of if I already had the 5nm filters to re-use. I need to buy new filters, but even in the context of that, I still think 5nm may be the way to go (why pay a bit more for something that precludes the ability to capture Nii signals at the slight loss of contrast). Ani, I don't have any direct experience with clear filters in a CDK so I can't say much about it. Come to think of it, I do actually have the prescription for the CDK20. It was published in "Telescopes, Astrographs and Eyepieces. Unfortunately, I haven't spent any time looking at the color correction. What I can say is that the field optics have very low optical power and that implies that they might not have much effect on the color correction of the scope; but, that's just a guess. John Like

26.84

John Hayes

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Ani Shastry:
John Hayes:
Ani,
The CDK scopes have refractive field optics so be aware that the color correction may not extend very far outside of the normal Lum filter bandwidth limits. If the longitudinal color correction changes quite a bit in the red (for example), it won't matter so much when you shoot through the red filter because you can correct for it with a focus offset. However, it might lead to halos--either blue -or- red with the lum filter if you go too far outside the well corrected range of the optical system. So, you might want to check with PW just to get their take on using a lum filter with more bandwidth. It might be fine but it would be a bummer to have a special filter made and then discover that you get unacceptable halos with it. Having more bandwidth with the lum filter has some advantages but unless you are running a RC, be careful about the color correction.

John

@John Hayes, thanks for bringing this up, this is a good point to verify with PlaneWave indeed. I did shoot an email to Bill Dean asking him to see if there would be issues with the refractive corrective optics on the CDK20. Now that being said, don't people sometimes use "Clear" filters with these telescopes instead of "Luminance" filters where the bandpass is between 300nm and 1200nm without issues?
Mark McComiskey:
@Ani Shastry , I meant to
ask: 3nm or 5?

@Mark McComiskey, 5nm is my thinking at the moment (haven't hit the order button yet). I have these on my CDK24 and I have been rather happy with them. John did say the additional contrast may not be worth the price in the context of if I already had the 5nm filters to re-use. I need to buy new filters, but even in the context of that, I still think 5nm may be the way to go (why pay a bit more for something that precludes the ability to capture Nii signals at the slight loss of contrast).

Ani,
I don't have any direct experience with clear filters in a CDK so I can't say much about it. Come to think of it, I do actually have the prescription for the CDK20. It was published in "Telescopes, Astrographs and Eyepieces. Unfortunately, I haven't spent any time looking at the color correction. What I can say is that the field optics have very low optical power and that implies that they might not have much effect on the color correction of the scope; but, that's just a guess.

John

2.81 Ani Shastry Topic starter ... · view views · Share link
Thanks for the additional thoughts @John Hayes. Circling back on this, PlaneWave came back and said that there should be no issues with the refractive optical elements from the additional bandpass of a wider luminance filter. Also, Chroma came back and said that they have a CT550/bp300 filter (which I assume means central-tendency / central-transmission of 550nm and a bandpass of 300nm) which should closely match the requirements of acting as a luminance filter for the J/C BVR filters. They provided me with the spectral response data of that filter, and here's what it looks like when I import that into PixInsight: the max transmission is around 95% instead of 98-99% of the regular luminance filter, but that should be able to be overcome using additional exposure time (or perhaps that shouldn't make a difference at all given the 3-4% delta). Depending on how expensive this is compared to a regular luminance filter, it might be a viable (or even a better) alternative. Like

2.81

Ani Shastry

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Thanks for the additional thoughts @John Hayes.

Circling back on this, PlaneWave came back and said that there should be no issues with the refractive optical elements from the additional bandpass of a wider luminance filter. Also, Chroma came back and said that they have a CT550/bp300 filter (which I assume means central-tendency / central-transmission of 550nm and a bandpass of 300nm) which should closely match the requirements of acting as a luminance filter for the J/C BVR filters. They provided me with the spectral response data of that filter, and here's what it looks like when I import that into PixInsight: the max transmission is around 95% instead of 98-99% of the regular luminance filter, but that should be able to be overcome using additional exposure time (or perhaps that shouldn't make a difference at all given the 3-4% delta). Depending on how expensive this is compared to a regular luminance filter, it might be a viable (or even a better) alternative.

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	Image of the day	Top pick	Top pick nominations	Total submitted
Deep sky
Solar system
Extremely wide field
Star trails
Northern lights
Noctilucent clouds
Landscape
Artificial satellite

	Image of the day	Top pick	Top pick nominations	Total submitted
Backyard
Traveller
Own remote observatory
Amateur hosting facility
Public amaeteur data
Professional, scientific grade data
Mix of multiple sources
Other
Unknown

AstroBin

AstroBin

Image Index:

Contribution Index (beta):

Johnson-Cousins BVR Flat-Top Filters vs RGB Filters [Deep Sky] Acquisition techniques · Ani Shastry · ... · 14 · 456 · 4

AstroBin

AstroBin

Image Index:

Contribution Index (beta):

Johnson-Cousins BVR Flat-Top Filters vs RGB Filters [Deep Sky] Acquisition techniques · Ani Shastry · ... · 14 · 456 · 4

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