Wratten Filters for CA Correction in Achromats?

[Louis D]

I tried out the same filters as in the April 12th posting above in the Astro Telescopes 6" f5.9 Sunday night on Venus and the moon.  Violet fringing is clearly stronger than in the ST80 while orange-red flaring is weaker.  This matches up well considering KUO tweaked the usual Fraunhofer doublet formula to favor red over blue as seen below:

Notice that KUO keeps the 546nm through 633nm lines very tight (under 0.5mm variation) center to edge versus 0.75mm in the Fraunhofer.  The down side is they allowed the blue (486nm/F) line to rapidly lose focus moving from center to edge (from 0.4mm to 1.1mm) while the Fraunhofer keeps it tight (about 0.5mm variation).

This is quite noticeable at the eyepiece.  Even with the Yellow #12A filter, blue fringing was quite obvious.  This filter was sufficient in the ST80 to cut violet-blue fringing, but not in the A-T 6".  However, the Yellow K2 perfectly cuts off all blue fringing.  At the other end, orange-red flaring was much less pronounced on Venus than in the ST80.  Filtering it out with the Cyan BG39 was not as impactful as with the ST80, especially on the moon where most folks would probably be content with the Yellow K2 by itself.

I tried the Baader Semi APO, and I can see why some folks like it on the moon with these scopes.  It suppresses violet fringing to a fairly tolerable level without greatly imparting a yellow color cast to the moon.  However, it was useless on Venus and left a lot of image sharpness on the table with the moon by allowing so much residual violet washing over dark features.

For maximum sharpness, I still couldn't beat the Meade Green interference filter.  However, the Green X1 and Yellow K2-Cyan BG39 combination both yielded nearly identical, less overwhelmingly green, images without violet, blue, or red fringing visible to my eye.  Thus, I felt that having a slightly less restrictive yellow-green filter is acceptable for best sharpness.

With this scope, it might be possible to allow more orange and red through than with the ST80 and still maintain a sharp image.  I know I struggled to see a significant improvement over using the Yellow K2 alone when stacking the Cyan BG39 with it on the moon.  The red fringing had very little detrimental effect on fine lunar details.  It was needed for Venus, but much less so with the moon.

I then tried the Meade red and blue interference filters on Venus.  I could get a sharp image focused with the red, but not with the blue.  Violet-blue fringing was quite obviously washing out the image sharpness.  KUO really destroyed just about all blue spectrum sharpness with their design choice.

I came away with a better understanding of achromatic doublet variations.  It was clear to me that fringe filtering needs tailored to each achromatic design for best results.

[neil phillips]

On 31/01/2023 at 03:08, Louis D said:

I've acquired a Hirsch #12A and a spectrograph in the last year and have done some testing of various yellow filters with the spectrograph, an artificial star (not a great one by any means, just a foil pinhole over a Cree LED tactical flashlight), and with my ST80.  First the composite image of several yellow filters in increasing violet/blue cutoff order.  The named ones with R## at the beginning are Roscolux filters that did a good job of sharply cutting-off violet/blue at a particular wavelength.

I didn't bother with the artificial star image of the Minus Violet filter because it wasn't handy when I was doing that particular test.  However, it should show plenty of violet.

• From the above, the Yellow #12A would appear to be a good contender along with the R312 Canary, so I tried them all out in my ST80.
• In my ST80, everything from Cheap Yellow down to Yellow #12 filtered out practically all visual violet when viewing Jupiter.  I'll have to try repeating the test on Venus sometime to be more equivalent to the artificial star.
• The minus violet did practically nothing to filter visible violet fringing.  It also added no noticeable yellow cast.
• The cheap yellow filter left a slight yellow cast to the image as predicted by the artificial star while leaving a bit of violet.  I think poor filter material led to scattering.
• The Hirsch #12A and R11 Light Straw actually looked the best.  Neither one imparted much of any yellow tone while at the same time squashing nearly all visible violet.  I was surprised at not seeing violet fringing in the latter despite the artificial star predicting it would be there.
• The R312 Canary imparted a noticeable yellow cast, as did R310 Daffodil.  Both did a good job of eliminating violet despite the artificial star's prediction of a bit of violet for the latter.
• The GSO Yellow #12 once again imparts a very strong yellow cast while completely squashing all violet and most of the blue.  At that point, I could clearly see red fringing, so it was back to light green filters to squash all fringing.
• Red fringing intruded in all views once violet was diminished or eliminated, so a true anti-fringing filter would need both violet and far red cut-offs.  A wide teal-yellow-green filter from deep blue to red-orange would probably work for this purpose.  I just don't know of any, even from Roscolux.
• It's too bad the Hirsch Yellow #12A is so hard to find used as it does a really good, and yet subtle, job of cutting out violet while leaving most blue.  The R11 Light Straw would seem a viable, and available, alternative once cut and fit into a filter holder.  A large sheet of it is cheap enough that it might be worth covering the entire objective to see how well that works.

I like the Baarder 495. But thats not very methodical i know. I only got it. because Neil English said it was good. Usual yellow cast though

[John]

I had a lot of fun playing around with Chromacor correctors and F/8 achromats a decade or so back. I wonder if a similar optical corrector could be produced today at a much lower cost, using modern glass types ?

The effect of having the wavelengths of light bought back to focus much closer to the same point, rather than filtering the straying ones out, was quite pronounced. The correction of spherical aberration was another important bonus.

Shame that they were such expensive and scarce devices. 

[Louis D]

My understanding from reading up on CN about the Chromacor is that Valery Deryuzhin at Aries Optical (in Ukraine) was utilizing some exotic surplussed Soviet glass of unusual dispersion properties that isn't normally produced and is very expensive to have a new melt poured.  Thus, we're not likely to see them made again anytime soon unless a Chinese glass producer surpluses some after making a similar optical production run for their government.  I'm not holding my breath waiting, though.

There's a bunch of Chromacor reviews here for anyone wanting to read up on them.

[Louis D]

7 hours ago, neil phillips said:

I like the Baarder 495. But thats not very methodical i know. I only got it. because Neil English said it was good. Usual yellow cast though

If they weren't so expensive for what they are, I would like to get a 1.25" version to compare to my 2" Yellow K2 filter that I bought for under $10:

From the Baader spectrogram, it appears very similar in that it just barely let's through the H-β line:

If you want to cut down on the yellow cast, just pair it with a Neodymium or Moon & Skyglow (Didymium) filter to cut out the pure yellow part of the spectrum.  That's basically what Baader did with their Contrast Booster filter:

You also lose a bit of the green portion of the spectrum as well, so I'm not particularly fond of this approach except when looking at certain planets like Jupiter that benefit from this.

Edited May 2, 2023 by Louis D

[John]

35 minutes ago, Louis D said:

My understanding from reading up on CN about the Chromacor is that Valery Deryuzhin at Aries Optical (in Ukraine) was utilizing some exotic surplussed Soviet glass of unusual dispersion properties that isn't normally produced and is very expensive to have a new melt poured.  Thus, we're not likely to see them made again anytime soon unless a Chinese glass producer surpluses some after making a similar optical production run for their government.  I'm not holding my breath waiting, though.

There's a bunch of Chromacor reviews here for anyone wanting to read up on them.

I was wondering if today there are glass types that are more readily available that could achieve similar results ?

Putting the wavelengths of light back where they belong (or very close to it) seems preferable to filtering them out to me. And SA correction is improved as well.

At one point Istar were working on a similar corrector to the Chromacor but they didn't pursue it favouring a move to ED glass in the objective design instead. 

This thread is about filtering though so I'll shup up about correctors I think 🙂

Edited May 2, 2023 by John

[Louis D]

2 hours ago, John said:

This thread is about filtering though so I'll shup up about correctors I think 

No, it's a legitimate train of thought if correctors could be produced in the price range of line filters.  Why filter if it costs about the same to correct?  The problem is, back 15+ years ago when the Chromacors were new, they were a $1000+ corrector for $300 to $600 achromats.  As lower cost ED and APO refractors started coming onto the market, the economics of it made no sense.

[Louis D]

I just received an affordable 600nm Shortpass filter.  It has a broad, high transmission passband for wavelengths shorter than 600nm and a sharp cutoff very close to 600nm.  Visually, it cuts out practically all red without noticeably dimming the view.  Below is the manufacturer provided transmission graph:

I paired it with my Yellow #12A, Yellow #12, Cheap Yellow, and Yellow K2 (#8) filters when viewing Venus through both the ST80 and ST150.  The best view was the combination with the Rokunar Yellow K2 cutting all violet/blue and orange/red fringing in both scopes.  The color was a pale green without much blue or yellow secondary cast.  The Meade Green was still showing the better image due to the narrower passband, but the intense green color was hard to get past.

I'll have to try out the various combinations on DSOs as skies and time allows.  I'd like to see if a high transmission combination (better than 80%) can be successfully used on brighter DSOs to sharpen star images without losing dimmer components at the same time.

Below is an update to my spectrograph image comparison showing the clearly higher transmission of the 600nm Shortpass filter and its combinations as compared to the Cyan BG39 filter and its combinations.  That, and the cutoff is tremendously sharper at orange-red than anything else I have.

The 600nm Shortpass filter is a work in progress because it is a square piece of glass that I hope to be able to cut 27mm and 46mm circular disks from for 1.25" and 2" filter housings, respectively.

[Louis D]

I've taken some images through the KUO 152mm with various filters and compared it against other scopes I own in another thread.  Here are links to the relevant posts:

 

[Mark2022]

On 02/05/2023 at 13:37, Louis D said:

If they weren't so expensive for what they are, I would like to get a 1.25" version to compare to my 2" Yellow K2 filter that I bought for under $10:

From the Baader spectrogram, it appears very similar in that it just barely let's through the H-β line:

If you want to cut down on the yellow cast, just pair it with a Neodymium or Moon & Skyglow (Didymium) filter to cut out the pure yellow part of the spectrum.  That's basically what Baader did with their Contrast Booster filter:

You also lose a bit of the green portion of the spectrum as well, so I'm not particularly fond of this approach except when looking at certain planets like Jupiter that benefit from this.

Louis, was the $10 price just a lucky one off or are they readily available in the States at that price? Can't  find them in the UK and to get from the US costs about £40 - 50.

[Louis D]

2 hours ago, Mark2022 said:

Louis, was the $10 price just a lucky one off or are they readily available in the States at that price? Can't  find them in the UK and to get from the US costs about £40 - 50.

I ordered some other filters from the ebay seller at the same time to experiment with to save on shipping costs.  The cost for the K2 filter came down to $15.70 with tax and shipping.

[Gemineyes]

I am arriving pretty late to this party!

I just got a TS152 RFT for myself as a Christmas gift, and am having a ball with this slick refractor! I have been playing with this quite a bit until just after new years, our weather here in NW Central Canada went south with an arctic front plaguing us for now... will do some more shortly. 

50+ years at the eyepiece, and have both APO and achromats. About 18-20 years ago, when the 80-100mm, f/6-f/9 achromats were much more common, I designed a novel (at the time, and IMO still a superior solution) filter for CA control. What makes this one unique is its method of color balancing. While Baader didn't do a bad job, theirs still either heavily tint an image, or kill WAY too much color information for planetary use. IMO. I designed one for Stellarvue, and they offered it as their XLCR-1 filter. It does a VERY good job as a CA filter, though more color-laden scopes still need a bit more to trim the residual, and this varies with scope design, observer tolerance, etc.

In the RFT, the semi-Apo Baader is not bad, but due to the combined curve, it "flattens" the subtle tints on planets like Jupiter. The Fringe Killer gets the CA out, but at the cost of intensity of image. So, as expected, always a tradeoff. My 2", SV-prototype XLCR I still have, and used it on this scope (and my ST120 f/5, which also works well with it). It did need a touch more, to trim the mauve on the deep red CA side, so I added the Astronomik L-3 (and also tried with the Baader UV/IR-CMOS cut) both pass ~420-680nm, I think my samples of the Baader is closer to 670-675nm red cutoff, and so is the L-3. This is needed as the SV curve sloped past 700nm, cutting around 720nm. Without the red cut, more mauve is present. 

I will post some shots of this combination, and my scope with CW rail, on the SV-M2C mount on TR3 tripod with Berlebach  3kg CW system shortly. I will also post another "RFT" type potential filter system I am playing with on this scope for wider field sky sweeps: It is an Optlong L-Pro with Baader FK filter. Passes all relevant critical bands: H-beta, O-III, SWAN C2 cometary, H-a, and is pretty well color balanced with just a hint of pale bluish hue. But also does pretty well (better than Semi-APO) on Jupiter and Saturn, though the XCLR filter allows more peach tint of Saturn to show.  

The FK/L_Pro combination is a bit more useful than the SA filter, IMO, as the curves of the L-Pro are in slightly different areas and allow more orange color information to be retained. Also, passing the green more offers some advantages also. Still, some work to be done. I am also trying a Hoya-Y1 (Y44 equivalent) I got on eBay and has yet to arrive (winter storms here anyway so no loss yet! lol), and with the L-Pro may also be interesting to play with. Stay tuned on that. 

SCOPE Setup this "Boxing week" after Christmas photos attached. Also note the L-PRO curve: with the FK killing to ~455nm, and a bit off the red end, this acts as a good color-balanced stack for general DSO/cometary!

Finally: IF I can get my SV XCLR curve to be augmented for more colorful scopes, would a version available to the general astronomy populous with achromats be of interest? If it took say 95% CA out and color balanced for planetary, but did not kill too much light (my current version is like a ND0.1 filter, or 78-79%T).

Something worth having? CS all and happy 2024! 

Darren

 

 

 

 

[Louis D]

Check my recent post to the TS 152 thread:

I've been chasing the same goal of getting rid of spurious color in fast achromats.  I've come to the conclusion I'd rather live with a bit of yellow-green tint and give up H-alpha for non-nebula observing.  As such, a nice high transmission, sharp cut-off 470nm long pass filter gets rid of all the violet fringing while leaving almost all blue.  This is fine for DSO observing.  However, for solar system observing, adding a high transmission, sharp cut-off 625nm short pass is a must to cut red fringing.  The color cast goes a bit more yellow-cyan than yellow-green with it.  It's certainly possible to then stack whatever nebula, pollution, or Neodymium filters suit your tastes.

3 hours ago, Gemineyes said:

I designed one for Stellarvue, and they offered it as their XLCR-1 filter. It does a VERY good job as a CA filter, though more color-laden scopes still need a bit more to trim the residual, and this varies with scope design, observer tolerance, etc.

Can you post the XLCR-1 curve as a point of reference?

[Louis D]

I may have found the Holy Grail of dyed filters for blocking both violet and red fringing in fast achromats.  I knew the color would have to be somewhere in the yellow-green part of the "filter spectrum", but all the ones I had tried up to now were not blocking violet (and quite often red) sufficiently, and some were just too strong a green color resulting in low transmission of all wavelengths.

I have been experimenting with older 48mm camera filters from the photographic age of filtering during image capture because they're cheap and come in a huge variety of color variations.  I bought a 4000K LED lighting panel to approximate balanced sunlight to photograph them so you all can see what I see when holding them up to a light.  Its spectrum is rather weak in the blue/teal blue region because it uses a violet LED to excite yellow-orange phosphors.  I manually color balanced my camera on the blank light before taking images.  I then normalize all images to have a white background with RGB values equaling ~248 for all because that results in an image brightness on the screen approximating what I see with my eyes.

Experimenting with various combinations of these filters and with unmounted industrial filters mounted in empty 48mm frames, I tried various combinations in both by 80mm f/5 and 152 f/5.9 achromatic refractors by holding them between the eyepiece and my eye.  As I've stated over in the TS-152 F5.9 thread, having sharp cutoffs at 470nm for violet and 625nm for red is just about perfect to eliminate all perceptible unfocused violet and red while leaving as much of the spectrum for visual work as possible.  The result is a light yellow-green image tint for this and similar filters.

Below is an image I captured showing some of the contenders, annotated with what each is.

Long story short, the very new to me Vivitar Light Green #11 (X1) from Japan in the center, dating from somewhere in the 70s or possibly 80s, is just about perfect for blocking violet and red color fringing on bright solar system objects in fast achromats.  The LP470+SP625 combination above it has slightly higher transmission and sharper cutoffs at each end, but yields just about the same visual color tint and intensity as well as suppression of color fringing.  I also verified their spectra are quite similar on the blue end while the Vivitar tails off on the red end rather than having a sharp cut off like the industrial combo.  It looks like the latter is more yellow than yellow-green, but the SP625 is an interference type filter that reflects red and doesn't photograph well with this reflectively backed LED light panel.  I had to elevate it 3 inches off the surface to get any blue-green tint at all.  The Hirsch Light Yellow #12A is there for reference in that it has almost exactly the same passband (yellow tint and saturation) as the industrial LP470, just with a more gradual cut off.  It demonstrates that there is a slight blue-green tinge to the LP470+SP625 combination, yet it is slightly stronger than shown when used in a telescope and viewed with eyes rather than a camera sensor.

For background, I already had the following filters also shown above:

• Hirsch Light Green #11 - Doesn't cut violet or red very well at all.  Very disappointing filter for this purpose.
• Panagor Yellow-Green X0 - Slightly better at cutting both ends of the spectrum than the Hirsch.
• Rokunar Green X1 (#11) - Best green I've found, cuts all violet and red fringing while passing the rest of the spectrum, but has maybe 60% transmission and a strong green tint.
• Meade Green #56 - Doesn't cut violet very well, good otherwise if stacked with a light yellow filter such as the Hirsch.
• Cheap Green - Very close to pure chromatic green with decent transmission compared to most green astronomy filters.  Cuts all violet, but leaves H-beta line.  Strongly attenuates yellow, orange, and red, so some signal is lost in those bands.

I swapped back and forth between these and other filters, narrowing down my favorites.  The Vivitar Light Green was a surprise winner.  I thought it would be the same as either the Panagor X0 or Rokunar X1, but it isn't.  It's somewhere in between as you can see above.  Truly a Goldilocks of Yellow-Green filters for fast achromat users.  I could not discern a significant enough difference between it and the industrial LP470-SP625 combination on the moon and Jupiter to recommend the much more expensive latter combination on bright objects.  The fact that I got the Vivitar as part of a $30 lot of 12 used filters is all the more remarkable.  I had bought the lot to scavenge 48mm frames, not paying much attention to the Vivitar thinking it would just be a duplicate yellow-green filter.

I'll try some other Light (Yellow) Green X1/#11 filters such as those from Hoya and Tiffen if I come across them for a good price in good condition.

I'll also look into buying some astronomy targeted 1.25" yellow-green filters such as vintage Meade 4000 and GSO relabels to see how they stack up to the above lineup.  I need to find a good 1.25" yellow-green filter for the ST80 because it has a 1.25" focuser.  The Hirsch Light Yellow #12A stacked with an industrial 27mm SP650 mounted in a 1.25" frame is pretty decent, but still leaks some red fringing.  I haven't been able to locate a 27mm unmounted SP625 so far.  When the Hirsch Light Yellow is stacked with a Hirsch Light Blue #82B (really a cyan color), it cuts all red fringing, but yields a strong yellow-cyan color as seen in the image in this post.  I need to update that picture sometime with images through some of the newer yellow-green filters and combinations.

For reference, the Vivitar Light Green #11 (X1) shows up in 48mm size on ebay in the US quite often for under $15 shipped.  Make sure to get the Japan version (labeled on the filter as such).  Later versions made elsewhere might not be the same tint.  I've seen this with the Meade Yellow #8 filters.  Some are lighter and some are more saturated, depending on the vintage.

Edited January 20 by Louis D

[Louis D]

I picked up a Lumicon #11 Yellow Green in a 1.25" to complement the Vivitar Light Green #11 in 2" size.  It has an almost identical cutoff in violet to the Vivitar, but with higher transmission of blue wavelengths.  In that respect, it's a better minus-violet filter.  However, at the other end in the red, it has somewhat higher transmission in the red wavelengths than the Vivitar, so less useful as an all around violet and far-red cut filter.  This was confirmed visually through my ST80 on Jupiter and the moon.  It's barely better than using the Hirsch #12A Light Yellow or LP470 by themselves, which cut no red at all.

I took a group shot below with it and some other yellow-green, yellow, and green filters.  As you can see, it has just about the same yellow density as the Hirsch #12A and LP470 with just a hint of green to cut some red.

I find it interesting that I now have four #11 Green/Light Green/Yellow-Green filters, and none of them match.  The Rokunar X1 version is actually closest to the Kodak Wratten spec.  The rest are closer to the Hoya X0 spec in that they transmit more blue-violet, have higher transmission overall, and transmit plenty of red.  There is no Wratten # equivalent to X0 that I know of.

[Mr Spock]

If you want to use an achromat for solar the Baader Continuum offers close to perfect correction (with a suitable Herschel wedge of course). My 120mm f8.3 is about as sharp as it gets for solar 👍

[Louis D]

I'll have to try again with my b/c Optica Green line filter.  It passes about a 10nm wide band centered roughly on 525nm.  I've also got violet, blue, yellow, orange, and red versions to try as well.

[Jim L]

On 19/01/2024 at 16:02, Louis D said:

…

For reference, the Vivitar Light Green #11 (X1) shows up in 48mm size on ebay in the US quite often for under $15 shipped.  Make sure to get the Japan version (labeled on the filter as such).  Later versions made elsewhere might not be the same tint.  I've seen this with the Meade Yellow #8 filters.  Some are lighter and some are more saturated, depending on the vintage.

Louis, thanks for your very interesting experimentation using filters for fast achromats. Can you tell me whether the 48mm camera filters you’ve been testing thread into 2” Astro gear (eyepieces, diagonals)?

[Louis D]

1 hour ago, Jim L said:

Louis, thanks for your very interesting experimentation using filters for fast achromats. Can you tell me whether the 48mm camera filters you’ve been testing thread into 2” Astro gear (eyepieces, diagonals)?

They thread in perfectly and their outer diameter is such that they are no wider than the eyepiece insertion barrel.  It's a wonder to me why 2" astronomy filters are priced so high relative to their 48mm photography dopplegangers.

On a different note from blocking spurious violet and red in achromats, look for 48mm warming (81 and 85 series) and fluorescent lighting color correcting (FL series) filters.  Being gentle red/orange/brown filters, they help with bringing out some darker features of Mars while not taking away too much light.  They tend to be pretty cheap on ebay as old photo stores close out their new-old filter stocks.  That, and some come from estate sales in large lots for super cheap.

[Jim L]

30 minutes ago, Louis D said:

They thread in perfectly and their outer diameter is such that they are no wider than the eyepiece insertion barrel.  It's a wonder to me why 2" astronomy filters are priced so high relative to their 48mm photography dopplegangers.

On a different note from blocking spurious violet and red in achromats, look for 48mm warming (81 and 85 series) and fluorescent lighting color correcting (FL series) filters.  Being gentle red/orange/brown filters, they help with bringing out some darker features of Mars while not taking away too much light.  They tend to be pretty cheap on ebay as old photo stores close out their new-old filter stocks.  That, and some come from estate sales in large lots for super cheap.

Thank you, Louis.  I just purchased a 48mm NOS Vivitar Light Green #11 (X1) for the princely sum of $13.87 shipped, and I’ll be on the lookout for the various other filters you’ve recommended.

[Louis D]

Off topic a bit again, but here's a group shot of the warming filters on the LED panel:

The CC050R is a Color Correcting Red filter of strength 50, where the density ranges from 05 to 50, with 05 being the faintest and 50 being the densest.  The #81B is a 52mm I got as part of a larger lot, so not very useful except to hold above an eyepiece.  I'm most partial to the CC050R, FL-B and #85B so far due to their more saturated hues.  However, they're all quite subtle compared to traditional astronomy colored filters.  I never cared for the #21 Orange or #25 Red filters on Mars.  They're too dark and in your face.  These warming filters are all similar to the very expensive #85 Salmon Brandon filter, but in all sorts of hue and saturation variations.

I'll compare them more at the next Mars opposition and report back at that time.  I think I paid less than $70 shipped for all seven filters across several different lots.

[Louis D]

I picked up a BP500 Green-Blue (cyan) filter and tried it out last night in the KUO 152mm.  It's very similar to the Hirsch #82B Light Blue (light cyan), just with a bit higher transmission in the middle and with a bit longer tail-off in the red.  It paired nicely with the LP470 Light Yellow to create a pale yellow-blue-green filter to eliminate pretty much all false color at both ends of the spectrum, at least to my old eyes.  I compared it against the new Lumicon #11 Yellow-Green, Vivitar Light Green #11 (more of an X0), and Hirsch #12A Light Yellow stacked with the Hirsch #82B.

On the moon, they all worked pretty well.  I'd probably be content with any of them over no filtering.  It's remarkable how much crispness each of them added to the image.

On the Orion Nebula, the LP470/BP500 stack had the highest transmission with the Hirsch combo close behind.  It was no surprise that the Lumicon and Vivitar filters dimmed the nebula a bit because they start tailing off before 500nm in the green/blue/violet end of the spectrum.  You don't really need much if any filtering on such dim objects anyway, but I was specifically looking for transmission characteristics.

On Orion's Belt/Collinder 70, there is a bit of violet and red fringing still visible on the bright stars, but it is quite tame compared to the filterless view.  The yellow-green-blue color cast is barely noticeable after a few seconds.  I guess my brain was just factoring it out.

Lastly, the Newport BG39 cyan glass is way dimmer than either the BP500 or Hirsch #82B with a much sharper cutoff around 600nm instead of 625nm or even a bit further right.  I'm considering retiring from my main filter wallet it as I see no practical use for it.

By the way, all of the LP, SP, and BP filters were made by MidOpt in the Chicago metropolitan area.

[Louis D]

One other observation I made accidentally while observing the moon was thinking I had the LP470 in my fingers when in fact I had the BP500.  I was mystified why I was seeing only about 50% of the violet disappearing.  I then realized I had the wrong filter.  However, I had a much greater epiphany in retrospect.  Why was I seeing any violet reduction when the BP500 blocks no violet at all, just orange and red?

I went back to the BP500 to try and figure it out by blinking it in and out of the light path from the eyepiece to my eye.  I noticed when adding the LP470 that it was attenuating red and orange fringing quite well, but when I removed the LP470 from the stack by blinking it in and out, the red fringing turned to purple/violet!  I then tried the really weak Meade #8 Light Yellow by itself.  It cut the far violet fringing, but none of the near-blue violet fringing.  I then added the BP500 to the stack blinking it in and out.  Suddenly, the amount of residual purple/violet fringe dropped way down to near acceptable levels.  Near-blue violet fringing I had been seeing with the weak yellow #8 filter was actually red fringing combining with blue that was fairly well focused creating the impression of near-blue violet fringing via purple fringing.  It's just that my eye can't discriminate violet fringing from purple fringing.

Now I'm thinking a weaker yellow may be sufficient if a strong enough cyan is applied at the same time to avoid red fringing creating the impression of near-blue violet fringing.

I'll have to try more experimentation with smaller objects such as the bright planets and stars before drawing any stronger conclusions.  I just thought it weird that red fringing could manifest itself as violet fringing on an extended object like the moon.