Wratten Filters for CA Correction in Achromats?

[Louis D]

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.

Edited January 31, 2023 by Louis D

[Franklin]

8 hours ago, Louis D said:

At that point, I could clearly see red fringing, so it was back to light green filters to squash all fringing.

Have you tried the Wratten #11 yellow/green?

[Louis D]

3 hours ago, Franklin said:

Have you tried the Wratten #11 yellow/green?

No, the color makes me queasy.  I suppose I'll give one a try sometime.  I also intend to get a GSO or Meade (Japan) Yellow #8 just to see how it compares to the Yellow #12A.

The #11 Yellow-Green is a bit aggressive in the blue, but does let in some yellow and orange while squashing most red:

[Louis D]

It seems a rather simple effort of trial and error under the stars/planets to find the cutoffs for various achromat f-ratios at both spectrum ends to make them more usable by eliminating most of the unfocused light ruining sharpness.  Once these are cutoffs have been established, either dyed or interference filters could be made to pass only the well focused central region tailored to each focal ratio.  Sure, the color balance would be way off being a teal blue-green mixed with yellow-orange, but they could be sold as general purpose planetary and double star splitting filters for achieving the best sharpness while retaining as much of the spectrum as possible in fast achromats.

[Ratlet]

2 minutes ago, Louis D said:

It seems a rather simple effort of trial and error under the stars/planets to find the cutoffs for various achromat f-ratios at both spectrum ends to make them more usable by eliminating most of the unfocused light ruining sharpness.  Once these are cutoffs have been established, either dyed or interference filters could be made to pass only the well focused central region tailored to each focal ratio.  Sure, the color balance would be way off being a teal blue-green mixed with yellow-orange, but they could be sold as general purpose planetary and double star splitting filters for achieving the best sharpness while retaining as much of the spectrum as possible in fast achromats.

If astro-imaging has taught me anything it's that the colour balance isn't off, it's just a colour palette.

My concern about the colour cast of Jupiter on Mars when using the a cheap yellow and skyglow filter to boost contrast with my Newtonian lasted about a femtosecond compared to my absolute joy at the improvement in image quality.  Really interesting to read your observations as I just picked up an ST80 which I've yet to try in anger.

[Louis D]

I've had my ST80 since about 2000.  Since it was such a poor performer relative to my Dobs, I was very dismissive of it and relegated it to travel use as a spotting scope or white light solar scope (as during the 2017 eclipse).  I tried that Minus Violet filter shown in my post above and found it did so little to cut stray violet light that I abandoned experimenting with filters to improve the image in the ST80.  Now I've circled back to see if the right filters with just the right cutoff characteristics could make these achromats more palatable to reflector folks like myself.

@RatletI predict that someday in the next 5 years you'll pop for a 70mm to 100m ED or APO scope of some sort.  The difference in performance as compared to a similar f-ratio achromat is astounding.  My FPL-51 doublet AT72ED started me down this road around 2013.  In 2019, I upped my game and got a TS-Optics 90mm FPL-53 triplet APO after realizing what features mattered most to me.  Both are a lot of fun to use and hold their own against larger reflectors or compound scopes.  The best part?  No filtering is needed for clean, sharp images.

[Louis D]

On 31/01/2023 at 05:43, Franklin said:

Have you tried the Wratten #11 yellow/green?

I recently picked up a used Hirsch Light Green #11 which is more teal and less yellow than the Wratten standard.  I ran it through my spectrograph and found it passes a lot of violet and red.  I confirmed this on Venus, Jupiter, and Sirius tonight in my f/5 ST80 while imparting very little green color cast.  It helped a bit, but it was not a complete solution.

I then paired it with Hirsch Yellow #12A which lies midway between Yellow #8 and Yellow #12, and is very similar to a cheap yellow filter from China, just with a sharper cutoff and less attenuation as I showed in an earlier spectrogram post.  This cut out the majority of the residual violet as my spectrograph tests predicted while imparting a noticeable yellow cast.  However, spurious red remained.

I next tried my Meade Green interference filter and my cheap green filter from China.  Both snuffed out all the spurious violet and red, but with a strong green cast.  According to the spectrograph testing, much or all blue and yellow/orange/orange-red also gets cut.

Finally, I ran all the Roscolux green, yellow-green, and blue-green filters through the spectrograph and chose the ones with the best cutoffs at each end to test tonight.  Those three were R86 Pea Green, R89 Moss Green, and R389 Chroma Green.  Of the three, the R89 Moss Green had the highest transmission and passed the most spectrum.  All three cutoff all visible spurious violet and red, so I declared R89 Most Green the winner based on providing the lightest touch.

I won't lie, the R89 Moss Green image is green to yellow-green, but it is much less intensely green than either the cheap green or Meade green filters while still cutting out all spurious violet and red.

I feel vindicated that it would be possible to create a yellow-green interference filter passing all the light from about 470nm (dark teal blue) to about 610nm (orange-red) to cut out all visible spurious violet and red in fast achromats.  For now, the R89 Moss Green filter works well enough because having about a 50% transmission in the central passband is not an issue on these super bright objects that show loads of spurious violet and red in fast achromats.  With all three super bright objects, clarity was greatly improved by cutting out unfocused light at both ends of the spectrum.  An interference filter with ~95% transmission in this central passband would open up the list of viewable objects to dimmer ones.  Splitting tight doubles would become easier as well.

I composited the spectrograph results for the various filters and filter combinations described earlier:

A filter with this passband would agree well with the spectral frequencies that are well focused by achromats (the dotted green line) below.  From just below 500nm to just above 600nm is about ideal.

[Louis D]

I just got in from trying various filters for white light solar observing with my ST80 and Baader Solar Film.  My ST80 would not come to focus with my 1.25" Hercules solar wedge, so I had to use the BSF.

First, just adding an interference type IR blocking filter ahead of the diagonal helped sharpen up the image by filtering out unfocused far red light.  I kept it in place for all testing since it didn't shift color at all.  This effect would probably not be very noticeable at night with scotopic vision, but I might try it with Venus and the moon.

The best filters for getting sharp images in roughly the order of best to just so-so improvement:

1. Meade Green interference
2. Cheap Green
3. Meade Red interference
4. #12 Yellow
5. Svbony UHC
6. Zhumell OIII (that is right shifted to cover C2 lines from comets)
7. R89 Moss Green
8. #12A Light Yellow
9. Cheap Yellow

It became apparent that you want a narrowed (but not necessarily narrowest) passband so best focus can be more easily achieved by filtering unfocused spectrum.

The worst was a cheap magenta that made matters worse by cutting out the well focused green spectrum in the middle leaving the hard to simultaneously focus blue and red ends.  I could not bring the image to a combined best focus.

The orange, red, and blue cheap filters didn't help much either.  They didn't make matters worse, though.

My Lumicon OIII didn't help much despite the Zhumell OIII showing good results.

My collection of narrowband Optica b/c interference colored filters didn't deliver like I'd hoped.  The images were generally sharp in them, but it was quite difficult to hold the eye positioning because getting ever so slightly off axis caused blackouts.  Thus, it was difficult to draw any conclusions about them.  I thought the wider passband of the Meade filters showed details more easily and with the same sharpness.  Based on this, I'm not likely to buy a Baader Solar Continuum filter.

The Meade Blue was poor.  I think this is because it passed all unfocused violet.

Lighter filters like my #12A Light Yellow, Cheap Yellow, #11 Light Green, and R86 Pea Green didn't help much because they passed too much unfocused spectrum at either end.

On the other hand, the #12 Yellow was showing really good detail.  I wondered if pushing to the Cheap Orange would show further improvement; but no, it was actually worse for unknown reasons.  Apparently, there's a Goldilocks region for low pass filters right around the #12's cutoff which is very close to 490nm.

The Zhumell Urban Sky (Moon & Skyglow) filter failed to improve the image to any significant degree, even when combined with the #12A to block unfocused violet.  I didn't think to try it with the #12 Yellow because it was working so well by itself.

The R89 Moss Green caused barely any color shift to yellow-green while noticeably improving sharpness.  I should try combining it with the variable polarizer to cut some of the glare because it is such a light filter.  However, narrowing the passband to purely green with either the Meade or cheap green filters sharpened up the image further.

Overall, a cheap green or #56 Green would probably make for a good filter when white light solar observing in an achromat without a lot of cash outlay.  A #12 Yellow is another good, inexpensive option.

[Louis D]

I spent some time last night observing Venus, Jupiter, and Orion and vicinity with my ST80 and various filters.  Here are my observations:

1. The IR filter did nothing to remove red fringing or sharpen the image.  It must have been the significant IR from the sun being filtered that sharpened my white light solar observing earlier in the day.
2. The sharpest views of Jupiter's bands and of Venus were with the Meade Green inteference filter.  The Cheap Green filter was a bit behind because of light attenuation, but was a good, less expensive option.  Both gave bright stars and star clusters a strong green color cast while allowing for pinpoint focus.  Both made the Orion Nebula invisible.
3. The #12 Yellow gave a sharper view of Jupiter's bands than the light #12A Light Yellow.  Both cut out all violet fringing.  The red fringing was more pronounced than with the #12A Light Yellow, perhaps because there was no blue left in the #12 to counteract it.
4. The #12A Light Yellow didn't really cause any noticeable color cast while cutting out all violet.  However, significant red fringing remained that ruined sharp focus.
5. The Cheap Yellow filter gave nearly identical results to the #12A except with a bit more scatter on Venus.
6. The #11 Light Green (Yellow-Green) filter didn't do a whole lot to block violet or red fringing.  On the violet end, it was similar to a #8 Yellow or Minus Violet filter.  On the red end, it did help to attenuate the red fringes a bit, but not completely.
7. Combining the #12A Light Yellow and #11 Light Green creates a near perfect way to cut violet and diminish red fringing on dimmer objects without introducing much of a color cast or losing much light.  When used on Orion, the nebula stood out a bit better and stars were more pinpoint with almost no color cast.  Venus and Jupiter still had way too much red fringing to be a useful combination on them.
8. The R89 Moss Green filter completely eliminated all violet and red fringing while maintaining a brighter image than the Cheap Green filter, but it was dimmer than the Meade Green interference filter.  Contrast on Jupiter was lower making the bands more difficult to make out for some reason.  Venus showed as a sharp disk.  I'll have to pair it with a variable polarizer to see if Jupiter was simply too bright.
9. The Orion Nebula stood out better with the Moss Green filter than with the #12A-#11 combination by darkening the sky more by attenuating some yellow and orange-red light and cutting all red light without cutting H-B or O-III bands.  The view was actually similar to the Svbony UHC but more pleasing because more stars were visible and more natural looking giving a better context.  The green cast was barely perceptible.  Star clusters around Orion were much more sharply focused by eliminating the unfocused violet and red light.  Bright stars were nearly pinpoint while having only a slight yellow-green cast.
10. The narrowband filters were really only useful on the Orion Nebula.  They didn't really do much useful for Jupiter or Venus.
11. The M&SG filter improved the visibility of the Orion Nebula slightly by cutting yellow and orange-red light which darkened the sky background of some light pollution.  It wasn't very useful on Venus or Jupiter without adding the #12A or #12 filters to cut violet fringing.  There was still plenty of red fringing ruining best focus.

[Ratlet]

I'm really enjoying this.  It's interesting to see empirical analysis of the filter combined with testing at the eyepiece.

Not sure how you manage to change filters so much without getting finger prints on them.  I struggle at the best of times (to the point I designed special filter cases).

[Louis D]

3 hours ago, Ratlet said:

I'm really enjoying this.  It's interesting to see empirical analysis of the filter combined with testing at the eyepiece.

Not sure how you manage to change filters so much without getting finger prints on them.  I struggle at the best of times (to the point I designed special filter cases).

Thanks for the feedback.  I'm an engineer who enjoys finding solutions to problems.  The problem here is how to get the best performance out of fast achromatic refractors.  There have been expensive solutions in the past (the Aries Chromacor), but I was intrigued by the possibility of simply improving rather than correcting the view.

I've been using my retired 14mm Pentax XL in a Parks GS 2x shorty Barlow for the testing at about 57x.  I place the object slightly off center to allow for more drift time on my undriven alt-az mount.  I then grab two filters, one in each hand, carefully by their edges (they're all in a shallow box nearby to avoid them disappearing into the lawn).  Next, I return to the eyepiece and move each eyepiece in and out of the exiting light cone using a variation on the blink comparator method to look for changes.  I sometimes move one, then the other, then both at the same time, into the light cone.  I even alternate which one is stationary while moving the other one in and out to observe what changes occur when stacking to better understand what each filter is doing.  This method only works with small objects like planets and stars since I can only see a small portion of the FOV while pulled back so far from the eyepiece to fit one, and sometimes two, filters in between my eye and the eyepiece.

I chose the Pentax XL because it has a smooth top with a rubber guard, so I wouldn't have snagging issues or eye lens scratching issues.  It also has no chromatic aberration issues of its own, so I wouldn't have to sort eyepiece chromatism from scope chromatism.  Lastly, since it was retired by my 14mm Morpheus, I wouldn't be too upset if something were to happen to it.

Grabbing the filters without putting fingerprints on them is difficult, but practice makes perfect.

Overall, I've been really surprised and enlightened by my discoveries.  I can't recall anyone else systematically comparing various filters on an achromat to narrow in on what works well and what doesn't.  There's been lots of work done in the area of figuring out which filters work best on which objects, but they always seem to assume the scope doesn't have strong chromatism of its own to sort through as well.

Often, folks write off fast achromats for planetary usage, but I've found that with the proper filtering, they can put up quite sharp and pleasing images if you can ignore the color cast.

Surprisingly, I've found my brain performs a white balance on pale yellow and yellow-green images over time such that they seem color cast free.

There's also the fun of playing with my toys in a new way.  As a kid, I used to combine my toys in new and unusual ways such as racing my Hot Wheels cars down tracks through long Lincoln Log cabins built over the track.  I was amused by the change in sound as they went through the cabin and how they disappeared and reappeared.  So, I'm always looking for new ways to combine things in ways that haven't been done before.

[Ratlet]

1 hour ago, Louis D said:

Thanks for the feedback.  I'm an engineer who enjoys finding solutions to problems.  The problem here is how to get the best performance out of fast achromatic refractors.  There have been expensive solutions in the past (the Aries Chromacor), but I was intrigued by the possibility of simply improving rather than correcting the view.

I've been using my retired 14mm Pentax XL in a Parks GS 2x shorty Barlow for the testing at about 57x.  I place the object slightly off center to allow for more drift time on my undriven alt-az mount.  I then grab two filters, one in each hand, carefully by their edges (they're all in a shallow box nearby to avoid them disappearing into the lawn).  Next, I return to the eyepiece and move each eyepiece in and out of the exiting light cone using a variation on the blink comparator method to look for changes.  I sometimes move one, then the other, then both at the same time, into the light cone.  I even alternate which one is stationary while moving the other one in and out to observe what changes occur when stacking to better understand what each filter is doing.  This method only works with small objects like planets and stars since I can only see a small portion of the FOV while pulled back so far from the eyepiece to fit one, and sometimes two, filters in between my eye and the eyepiece.

I chose the Pentax XL because it has a smooth top with a rubber guard, so I wouldn't have snagging issues or eye lens scratching issues.  It also has no chromatic aberration issues of its own, so I wouldn't have to sort eyepiece chromatism from scope chromatism.  Lastly, since it was retired by my 14mm Morpheus, I wouldn't be too upset if something were to happen to it.

Grabbing the filters without putting fingerprints on them is difficult, but practice makes perfect.

Overall, I've been really surprised and enlightened by my discoveries.  I can't recall anyone else systematically comparing various filters on an achromat to narrow in on what works well and what doesn't.  There's been lots of work done in the area of figuring out which filters work best on which objects, but they always seem to assume the scope doesn't have strong chromatism of its own to sort through as well.

Often, folks write off fast achromats for planetary usage, but I've found that with the proper filtering, they can put up quite sharp and pleasing images if you can ignore the color cast.

Surprisingly, I've found my brain performs a white balance on pale yellow and yellow-green images over time such that they seem color cast free.

There's also the fun of playing with my toys in a new way.  As a kid, I used to combine my toys in new and unusual ways such as racing my Hot Wheels cars down tracks through long Lincoln Log cabins built over the track.  I was amused by the change in sound as they went through the cabin and how they disappeared and reappeared.  So, I'm always looking for new ways to combine things in ways that haven't been done before.

I had written off my Newtonian for planetary until you dropped the hot tip about using the m&sg filter with a cheap yellow.  I'm seriously considering just leaving it on my svbony zoom and altering my shadowfoam in the case.  Absolutely changed the game in terms of what I can see.  It's the difference between looking at a lightbulb and a photo.

Safe to say your engineering approach to this has me convinced!

 

[Louis D]

I picked up a Japanese made Meade #8 Yellow for comparison since they are so often recommended as a cheap minus-violet filter.  Through my spectrograph, it is nearly indistinguishable from my vintage minus-violet filter shown in my comparison images posted above.  It is indeed way off from Kodak's Wratten specification for a #8 Yellow and closer to a Wratten #4 Yellow.  The #12A Yellow is closer to the Wratten #8 Yellow specification.  It's too bad the closest modern equivalents are the cheap yellow filters coming out of China that show a bit of scatter.

Last night I had super clear and steady skies after a front moved through the day before.  I compared the various Yellow, Yellow-Green, and Green filters through my ST80 f/5 achromat.  I even combined a few of them with the M&SG filter.

Venus required the heaviest filtration:

• The #8 Yellow left noticeable violet flaring ruining the view.  No yellow cast could be detected.  Venus looked about the same with or without this filter.
• The #12A Yellow removed all violet fringing leaving a slight yellow hue, but the red fringing then overwhelmed the view.
• The #12 Yellow left a deep yellow color cast that when combined with the residual red fringing created an orange hue.
• The Hirsch #11 Light Green needed combined with the #12A to block enough violet to be usable.  It still leaked a fair amount red, but it was an improvement.
• The R89 Moss Green completely eliminated all violet and red fringing while still showing a bit of blue and yellow hues.  Venus was a sharply defined disk.  The brightness needed cut down a bit, though, for better contrast.
• The Cheap Green showed a purely green view that was darker and slightly sharper.
• The Meade Green (interference) showed hints of yellow shades and despite the brighter view due to high transmission still showed excellent contrast.

Jupiter required a noticeable step down in filtration:

• The #8 Yellow left barely noticeable violet flaring.  A barely noticeable yellow cast could be detected.  Adding the M&SG filter cut most of the yellow cast, leaving an odd color cast (pale magenta?)
• The #12A Yellow removed all violet fringing leaving a noticeable, but subdued, yellow hue.  The red fringing was all but unnoticeable unlike with Venus.  Adding the M&SG didn't cut the yellow color cast much at all.
• The #12 Yellow left a deep yellow color cast that overwhelmed the view.  The belts were a bit sharper, though.
• The Hirsch #11 Light Green did a decent job.  Moving it in and out of the light cone, it did cut some of the unfocused red while imparting a light yellow-green hue that was unobtrusive.
• The R89 Moss Green completely eliminated all violet and red fringing while imparting a blue-green-yellow color cast.  It was probably overkill for Jupiter, though.
• The Cheap Green showed a purely green view that was darker and slightly sharper.  It seemed like overkill.
• The Meade Green (interference) showed excellent contrast, but with a strong green-yellow color cast.

The Moon, being a greatly extended, but very bright object, was somewhere between Jupiter and Venus in filtration needs.

• The #8 Yellow left noticeable violet flaring and a violet wash all gray areas.  A barely noticeable yellow cast could be detected.  Adding the M&SG filter cut most of the yellow cast, leaving an odd color cast (pale magenta?)
• The #12A Yellow removed all violet fringing leaving a noticeable, but subdued, yellow hue.  The red fringing was unnoticeable unlike with Venus.  Adding the M&SG created an odd color cast (pale magenta?) on the gray areas.
• The #12 Yellow left a deep yellow color cast that overwhelmed the view revealing greenish tints to shadows.
• The Hirsch #11 Light Green needed combined with the #12A to block enough violet to be usable.  It was a slight improvement over the #12A alone.
• The R89 Moss Green completely eliminated all violet and red fringing while imparting a strong blue-green-yellow color cast.  Details were nice and sharp without fringing.
• The Cheap Green showed a purely green view that was darker, but not any sharper.  It seemed like overkill.
• The Meade Green (interference) showed excellent contrast, but with a strong green-yellow color cast.
• The Meade Red (interference) showed excellent contrast once refocused for red.  It left a strong red-orange color cast.
• The Meade Blue (interference) revealed strong violet blurring on high contrast details.  It needed combined with the #12A to create a teal-blue filter that showed excellent contrast once refocused for blue.
• I could see using the above Meade color separation filters in a filter wheel with the #12A on the front of the diagonal as a pre-filter to cut violet for the blue filter for all bright objects.  The #12A has no effect on the green and red filters (verified with blinking it in and out).  That way, the observer could get sharp views in teal-blue, green-yellow, and red-orange after refocusing for each filter's passband.  It's just too bad the human mind can't combine RGB channels sequentially in time.  I suppose if the filters and focus could be changed rapidly enough it might work.  Alternatively, use three achromats filtered in RGB and use relay lenses to send the outputs to a combiner prism and view the result.  3-CCD video cameras do the reverse, so I know it can be done.

Orion nebula and star clusters:

• The #8 Yellow barely helped with bright star fringing.  Violet was subdued but still obvious, as was red flaring.
• The  #12A Yellow was a better choice for violet flaring, but left red flaring on bright stars.
• The Hirsch #11 Light Green was just about the best choice, possibly combined with the #12A Yellow to eliminate the residual violet flaring.  Without the #12A, violet and red flaring was still slightly visible, but not too intrusive.  Overall, probably the best choice.  The nebula was still visible, but contrast wasn't really improved any.
• The R89 Moss Green gave sharp views with no flaring, but was a bit dark.  The faintest component of the Trapezium was all but eliminated.  However, the nebula stood out a bit better by squashing some light pollution.
• The green filters eliminated flaring (good) and the nebula (bad), so not appropriate for this view.

It became clear to me that there is no one filter that is best for all objects in an achromat.  You want to apply the lightest touch possible to get to a sharp view.

The #8 Yellow is barely useful.  I think it was useful only on Jupiter, but not much else.

The #12A Yellow filter, being halfway between the modern #8 and #12 filters, desperately needs reintroduced as the Goldilocks of minus-violet filters.  It cuts all violet fringing on all objects without cutting too heavily into the useful blue spectrum, thereby leaving a light yellow cast instead of a heavy yellow cast as with the #12 Yellow.  Until then, either pick up a cheap yellow filter from China or cut your own from Rosco's R11 Light Straw material.

There needs to be an interference filter with a similar passband to the R89 Moss Green filter to improve transmission.  The blue end needs to cut on at around 570nm to cleanly get H-B emissions at 586nm and cut off at around 610nm to eliminate red flaring while leaving the yellow and orange-red spectrum that focuses fairly closely to green.  This filter could act as a jack of all trades to sharpen up achromat views of bright planets, the moon, and bright stars and star clusters while still allowing nebula to be seen in context.

[Louis D]

I went ahead and added the Meade Yellow #8 spectrograph image to the rest of the minus violet spectrograph images above.  The Yellow #8 cuts just the tiniest bit more violet than the Minus Violet.  I have no idea why they strayed so far from the Wratten Yellow #8 spec, which is much closer to the Cheap Yellow or Yellow #12A below.  I've got a vintage 48mm Yellow #8 (also known as Y, K2, Y48, or 022 by other filter specs) camera filter on the way to use on 2" eyepieces.  I'll run it through the spectrograph once I have it to see where it falls on the "spectrum" of minus violet (yellow) filters.  I'm really curious because it looks way more yellow than the Meade #8 Yellow in its photos.

[Ags]

I use a Wratten #8 when viewing planets at lower elevations - it significantly reduces atmospheric dispersion.

[Louis D]

45 minutes ago, Ags said:

I use a Wratten #8 when viewing planets at lower elevations - it significantly reduces atmospheric dispersion.

When viewing Venus at low elevations, I found my Meade Green interference filter worked best in my achromat by squashing all spurious red and blue, both from the scope and the atmosphere.  I then tried it in my 72ED, and it did the same for atmospheric smearing, leaving a nice and sharp view of the planet in brilliant green.

[Louis D]

I bought a few 48mm vintage camera lens color filters to try since they can be directly threaded onto 2" eyepieces.

First up is a Rokunar Yellow K2 - #8 which appears to be very close to the original Wratten spec for a Yellow #8 with a 19% cut-on around 480nm.  In fact, the Hirsch Yellow #12A appears to be very similar to the Wratten #4 spec with a 42% cut-on around 470nm.  The Meade Yellow #8 seems paler than even the Wratten #3 spec which specifies a 38% cut-on at 460nm.

I've run the Rokunar Yellow K2 through my spectrograph and found it to be about midway between my (GSO) Yellow #12 and Hirsch Yellow #12A.  On Venus, Jupiter, and the moon, it performs more similarly to the #12A than to the #12.  It cuts all violet while imparting only a slightly deeper yellow cast to the image than the #12A.  The #12 imparts a noticeably yellower cast to the image than either of the other two.

Here's the lineup of my various yellow filters as seen through the spectrograph:

Next up is a Rokunar Green X1 -#11 which appears to be very close to the original Wratten spec for a Yellow-Green #11 with a 26% cut-on around 480nm, a peak of 50% or better from 500nm to 550nm, and a 19% cut-off around 610nm.  This is almost exactly where achromats are best corrected.  It is almost identical to the R89 Moss Green in appearance, spectrographic analysis, and in use on Venus, Jupiter, and the moon.

Here's the lineup of my various green filters that I've found useful with a fast achromat as seen through the spectrograph:

Edited March 26, 2023 by Louis D

[Louis D]

This thread got me interested in revisiting fast achromats a bit more, so I recently picked up a used Astro Telescopes 6" f5.9 first generation KUO achromat for a good price.  It's basically the same optically as the current Starwave 152mm F5.9 V3 and TS-Optics RFT 6" f/5.9.  My first impression is how much heavier it is than a 6" f/5 GSO Newt (about 10 pounds heavier).  I mounted the two on either side of my DSV-2B mount.  Surprisingly, it didn't have any motion issues with the two once well balanced.

Going back and forth, I would have to recommend the Newt as the better all around scope for most folks.  It's lighter, cheaper, color free, and the eyepiece is in an easier to view from position (at least for me being a Dob guy).  That being said, it's pretty special viewing bright objects with no spider diffraction spikes in the achro frac.  However, you have to filter the view in the frac to get a sharp view which brings this post back to the subject of this thread.

I also picked up a Baader Semi APO filter for a good price recently.  All I can say is, it cuts maybe 50% of visible violet fringing while trying to maintain a neutral color balance by cutting yellow and orange spectrum using Neodymium glass.  It cuts violet by using violet reflective coatings instead of yellow glass.  As such, this filter is probably more appropriate for daytime use where such things matter more.  I'm just glad I didn't pay full price for it.  Pairing it with either the Hirsch Yellow #12A or Rokunar Yellow K2 eliminated all residual violet fringing at the cost of a bit more yellow cast.

Much more cost effective are my two recent purchases detailed in my previous post: the Rokunar Yellow K2/#8 and Rokunar Green X1/#11.  Both are high quality Japanese made photography filters in 48mm mounts, so they can be used as 2" astro filters.  On bright objects (above magnitude 2), the Yellow K2 cut all violet while introducing just a slight yellow cast and adding very little to no additional scatter.  Sirius looked more pleasing without spikes in the achro than it did with spikes in the Newt.  I did not attempt to see the Pup in either, though.  However, there was residual red fringing which made Sirius sort of orange depending on how I refocused with the filter.  Thus, in went the Rokunar Green X1 filter.  All violet and red fringing was gone with just a slight green cast added to the star.  It yielded a distinctly sharper view.  Basically as sharp as the Newt, but with less scatter because of a lack of central obstruction and spider vanes in the frac.  The 50% transmission of this filter was not an issue on such a bright object.  If anything, it made it easier to view.

Overall, I'm very pleased with these two vintage photography filters.  I'd love for GSO to bring them back in 1.25" size.  Right now, their Yellow #8 is more like a Wratten Yellow #3/#4 and their Yellow-Green #11 is more like a Hoya Green X0 which has no Wratten equivalent that I can find:

 

The Hoya Green X0 (GSO #11) passes way too much orange and red to be useful at blocking residual red fringing once violet fringing has been blocked.  That, and it's not quite aggressive enough at blocking violet fringing.  It's very similar to my Hirsch Light Green #11, which I've found to be ineffective at completely blocking either violet or red fringing on bright objects.

The Rokunar Green X1 was definitely sharper and caused less scatter than my home-brew R89 Moss Green film mounted in a 1.25" Meade #8 Yellow filter or 2" Skylight 1A filter.  I'm glad I was able to locate an optical glass equivalent to the R89 film.

The Rokunar Yellow K2 was just slightly more aggressive than my Hirsch Yellow #12A at cutting violet/blue wavelengths.  I would consider either acceptable for cutting violet fringing on bright objects.  Both are way better than either a modern GSO Yellow #8 (too weak) or GSO Yellow #12 (too strong).  The cheap Chinese made Yellow filters come closest, but with more scatter.

The biggest surprise of the night was that violet fringing was all but invisible at magnitude 2 and dimmer.  Thus, the Orion belt stars and Collinder 70 looked just as good in the frac as in the Newt.  The same goes for the Orion nebula and nearby open clusters.  It's hard to say if they looked better in the frac to justify the weight, cost, and color fringing issues on brighter objects.  I just enjoyed discovering that large, fast KUO achromats are almost indistinguishable at moderate powers from a Newtonian.  I'll keep the 6" f/5.9 achro for a while to see if it grows on me.

[alacant]

On 20/11/2021 at 14:05, Ian McCallum said:

 

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Edited March 27, 2023 by alacant
borrar

[alacant]

On 20/11/2021 at 14:05, Ian McCallum said:

Wratten 11 or 102 might be good to reduce the purple fringing

Hi

We hardly ever look through telescopes but reserve that right once a year or so to point our violet fringe special -an  ES ar152 f5- toward Sagittarius. We find that if you use a series of filters, one after another, you'll notice any colour they impart. Stick to one (a #8 is good) and after a while, you don't notice.
The purest stars we've seen is with a Bessel V.

Cheers and HTH

Edited March 28, 2023 by alacant

[Louis D]

3 hours ago, alacant said:

The purest stars we've seen is with a Bessel V.

Agreed.  They're basically the same as the Green X1 filter just with higher transmission which doesn't really matter on bright stars, planets, the moon, and the sun:

Bessel V curve:

Or Johnson-Cousins V curve (the green one):

Hoya (Rokunar) Green X1:

I looked for an affordable V filter, but couldn't find one.  I'm happy with the X1 for now.

[alacant]

14 hours ago, Louis D said:

Green X1 filter just with higher transmission

Ah, OK. That may explain why when looking at star fields in particular, the Bessel makes the others -apart from perhaps the yellow 8- seem dull by comparison.

Also, based purely on words overheard at the eyepìece, the susceptibility to being sensitive to purple fringing seems to be age dependent. Older observers may well benefit financially, filter-wise!

Cheers
 

Edited March 29, 2023 by alacant

[Louis D]

7 hours ago, alacant said:

Also, based purely on words overheard at the eyepìece, the susceptibility to being sensitive to purple fringing seems to be age dependent. Older observers may well benefit financially, filter-wise!

I've heard of that as well.  Older eyes tend to filter violet light.

From a quick internet search on the topic:

Apparently, age 70 is about the starting point.  The eye's lens starts to yellow, filtering violet and some blue light.  It can also mess up distinguishing green from yellow-green.  Cataract surgery that replaces the natural lens can fix this issue as a side benefit.

Edited March 29, 2023 by Louis D

[Louis D]

Well, I finally found an affordable, high quality cyan BG39 (minus red) glass filter.  It's a 1" (25.4mm) diameter bare disk, so I mounted it in a spare Meade UV/IR blocking filter mount with a just less than 1" clear aperture due to the retaining ring.  I had to leave the original UV/IR glass in place to keep it from falling out the front.  It's specified bandpass from the manufacturer (Newport) is below:

At best, it has an 80% transmission at 500nm (teal or blue-green).

Here's how it looks by itself and in combination with other filters through the spectrograph:

As you can see, it does a fine job of completely blocking orange-red and red light starting right around 600nm as specified.

I tried it out by itself and in combination with various other filters and in comparison to various other filters on Venus last night in my ST80:

• By itself, the Cyan BG39 filter transmits quite a bit of violet, but it does an excellent job of blocking red.  I saw simply a violet fringed blue-green disk with no orange or red flaring.  I'd say it was sharper than with any Yellow filter thanks to that.
• The best combination was with the Yellow K2 (#8) filter.  All violet and blue fringing was blocked along with the orange-red flaring leaving a nice and sharp disk.  The color cast was a pale blue-green.
• Next best was with the Hirsch Yellow #12A filter which left barely perceptible violet-blue fringing.  The color cast was a a very slightly more blue blue-green disk than with the Yellow K2 filter.
• The combination with the Hirsch Light Green #11 yielded a slightly darker image than with either yellow filter above.  It still had lots of violet-blue fringing, but no red-orange flaring.  The combination color cast tended toward blue-green while the Green #11 by itself tends toward yellow-green.
• The Yellow #12 combination was rather dark in comparison and overly yellow-green.  I wasn't a fan of it despite it blocking practically all poorly focused blue and red.  I would rather use the Meade Green interference filter to achieve that goal.
• In comparison, the Rokunar Green X1 (#11) yielded almost identical results to the Yellow K2 or Yellow #12A, just with a yellow-green instead of a blue-green color cast.  The best performing BG39/Yellow combinations both have sharper cutoffs and slightly higher combined transmission especially in teal/blue-green, but on Venus, it made little to no difference.  I preferred the Green X1 simply for the simplicity of a single layer filter.
• The Hirsch Light Green #11 by itself simply does too little to block either violet or red to be of much use.
• The various yellow filters do a good job of blocking violet, but the resultant orange-red flaring is probably just as bad as the violet fringing.  The violet fringing is well away from the disk while the orange-red appears as flares practically on top of the disk.  It's easy to ignore the violet corona, but it is impossible to ignore the orange-red flares distorting the disk image.

To try to break the Green X1 vs. Cyan-Yellow K2/12A tie, I tried the Green X1 and Cyan/Yellow combinations on the Orion nebula and the Trapezium.  Alas, I couldn't tell a difference on such dim objects.  No filtering was best by far for them.  I simply couldn't see any fringing on them without filters, so why use them?

On Sirius, I didn't find the color fringing all that objectionable.  I tried the various filter combinations, but still preferred the unfiltered view.

I've come to the conclusion that the poorly focused red and blue/violet light of fast achromats is really only a problem for bright solar system objects.  Unfortunately, that's what most beginners first point these scopes at and are less than impressed by the resultant views.  I'm sure at higher powers, it would also affect the ability to resolve globular clusters and close double stars, but most folks don't ever use these scopes for those purposes.

I'm more convinced than ever that a high quality 470nm to 610nm (or 480nm to 600nm if the cutoffs can be very sharp) bandpass interference filter of high transmission would be ideal to market as a planetary filter companion for fast achromats.  Now, we just have to convince some retailer to come to market with one.

Edited April 12, 2023 by Louis D

[vlaiv]

57 minutes ago, Louis D said:

I'm more convinced than ever that a high quality 470nm to 610nm (or 480nm to 600nm if the cutoffs can be very sharp) bandpass interference filter of high transmission would be ideal to market as a planetary filter companion for fast achromats.  Now, we just have to convince some retailer to come to market with one.

There are already filters like that on the market (or very similar).

How about green from RGB?

Might be a tad restrictive - but this is Baader offering - I've seen some other filters being less restrictive (but maybe not so sharp edges).

Astronomik for example:

maybe for start try to see how these two fare?