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Minus violet filter test

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#1 sg80

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Posted 05 April 2020 - 07:15 PM

It,s to windy and a cloudy forecast for viewing tonight. so I took my 80mm f11 scope out to see how much false color I could see during the day. I have 3 mv filters.I got 2 of them cheap.I have a Lumicon mv filter a Thousand Oaks mv filter and a Baader Fringe Killer.I used a 7.7mm Kson ortho 118 mag the target a tree and branches.without a filter the purple was easily seen never noticed it that much at night. the branches had purple shading around them.next up the Lumicon.it was reduced but still obvious without to much tint change. the Thousand Oaks did a little better a tiny bit less purple fringe but still obvious and the least tint change. The Baader Fringe Killer removed the most purple but added a pale yellow tint the fringe was still there but was harder to see.Why did I do this with all going.I have nothing better to do.  but I had fun.and since it does not look like. I will not get a apo anytime soon. I might be able to get a Celestron Omni 102 these may be of use since they have a little bit more false color.


Edited by sg80, 05 April 2020 - 08:17 PM.

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#2 markb

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Posted 05 April 2020 - 08:00 PM

I always found daytime testing like this to be surprisingly helpful. I look for violet fringes on power lines and tree limbs, I target threads on street signs and chimney caps, and resolve tiny writing on moderately distant targets, etc. 

 

Even the remarkably telling snap to focus test is easier to do in daytime than at night. It may not tell you what's wrong with the telescope, but it often will tip you off to a problem that might concern you enough to conduct further testing. My perfect ETX90, near perfect ETX90, and merely excellent ETX90 could all be easily distinguished in daytime focus snap and subtle view differences that were hard to put into words. The same can be said for high grade binoculars, some are just more eye pleasing and satisfying, I think is a mental response to combinations of sharpness across the central half as well as contrast and low CA. I recently found the photo testers attempt to quantify this as acutance. 

 

I'm sure others will disagree. For me it's easier to find a crisp day , high clouds no object, than a clear sky at night. I save night times for my most critical testing. It's usually warmer in winter and has less biting bugs in summer too!

 

 Your results are similar to mine. I found the Baader yellow tint to be far less bothersome at night.

 

I have 6 and 5 inch F5 Jeagers, so eliminating unfocused color is a valuable goal. One of my 5 in actually has defocused red, and one of the baader filters removes just enough red to make the scope much more fun to use. I have to check my notes on which particular filter it is, but the slight cut did show up in the Baader spectrograph charts.

 

Daytime testing totally sold me on the lp 495 Baader filter (originally suggest here on CN). If you can get one, new or used, it's absolutely worth a try!

 

I had toyed with the thought of trying the lumicon and thousand oaks if I found one used, but I don't think I'll bother now based on your results.


Edited by markb, 05 April 2020 - 09:52 PM.

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#3 drd715

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Posted 05 April 2020 - 08:28 PM

I also use a daylight test to evaluate the CA and resolution of a scope. I view black and white lettering (boat registration numbers) 4 inch at about 200 yards max power usuallyabot 300x. The amount of purple blur along the black/white edge and sharpness of the line is a good indicator as to the ability for that scope to show details in a astro view. Its amazing the amount of difference between scopes.

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#4 George Methvin

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Posted 06 April 2020 - 07:08 AM

I have the 120mm st f/5 scope and it seems to have fairly good optics, during daytime testing in which I do the same as you folks testing power lines trees and street sign I see very little CA at the lower powers its when I push the power up like using a 16mm eyepiece or higher I begin to notice the purple and even then its not that bad even when using my 4mm the CA is there but the image is still sharp.  I have found on good nights viewing the Moon I have no problem using the higher power eyepieces and still getting very sharp views but the CA is there and it does affect the contrast which I believe effect the sharpness.  After reading many article on CN about using minus violet filter to help with the CA  I got and started using a #8 yellow filter just to see if it would help any and it did make a difference in the image the CA was really reduced and the images had more contrast plus the views were sharper but at the cost of the images having a slight yellow tint to them there's always a trade off.  Now the yellow tint does not bother me at all and I don't even notice it when viewing the night skies but when viewing the Moon and brighter planets there is a yellow tint to be seen but I enjoy the better contrast and sharper views, so there's your trade off . Let me say this these fast scopes were not intended for planetary high power views but with a little help from a minus violet filter it can make your fast scope a little more enjoyable to use.  


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#5 Jon Isaacs

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Posted 06 April 2020 - 08:02 AM

I'm sure others will disagree. For me it's easier to find a crisp day , high clouds no object, than a clear sky at night. I save night times for my most critical testing. It's usually warmer in winter and has less biting bugs in summer too!

 

The problem I have during the day is that the seeing is rarely super steady so it limits the possible sharpness. 

 

I do look at phone poles and phone wires at a distance.  A dark pole or wire against a blue sky is an excellent target to show chromatic aberration at higher magnifications.

 

To see the chromatic aberration, the magnification must be sufficient that the exit pupil is about the size of your pupil. 

 

To George's scope. At low powers, achromats will show very nice images because your eye is masking the aperture.  If you're using a 25mm eyepiece in a 120mm F/5 scope, the exit pupil is 5mm.  If your pupil is 2mm, the scope is now a 120mm x 2/5mm  = 48mm aperture, the focal ratio is now 600mm/48 = 12.5

 

A 48mm F/12.5 is, by comparison, virtually color free.  Increase the magnification, the exit pupil decreases so the effective aperture increases.  At 60x, the exit pupil is 2mm and the full aperture is being used and the chromatic aberration becomes apparent.

 

Jon


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#6 George Methvin

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Posted 06 April 2020 - 01:31 PM

One other thing the #8 yellow filter is a cheap way to see if you will be happy with the yellow tint this filter imparts and if it does help your scope, this filter can be had for around $20.00.


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#7 j.gardavsky

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Posted 06 April 2020 - 02:25 PM

These are also my favourite tests on the 6" F/5 achro when the days are cold and cloudy.

 

All eyepieces I have ever had, have been tested, and about 1/2 has left my arsenal. Some eyepieces performance has been documented with the afocal photos.

 

The long pass filters help, I have 2 astronomy yellow filters, and a set of the yellow to orange filters from the microscopes accessory.

 

Best,

JG



#8 CollinofAlabama

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Posted 06 April 2020 - 03:19 PM

Don Pensack posts the famous Sidgwick-Conrady ratio chart here.  I always liked Gary Poliquin's article on the subject, as well.  In my experience, chromatic aberration above about 2.5 on the S-C scale becomes "unfilterable", despite what the chart says.  Yes, more radical filters like the Baader Fringe Killer and more powerful yellow filters can almost eliminate CA, they also make everything unnatural.  I found my old Fringe Killer converted Jupiter from his creamy Italian self into the Jolly Green Giant -- most unappealing.  Of course, everyone is different about these things, and some folks just want the purple fringe gone, and if that's all you care about, use one of those.  But for me, I can tolerate a small amount of CA.  So for me and my C102 GT (97mm clear aperture and 1016mm focal length for a SC ratio of 2.74), I utilize a pair of filters -- a two inch 81B equivalent and a 1.25" Baader Neodymium Moon & Skyglow.  The pair reduces CA to me to a very manageable level, with very good double star and planetary performance without making the view unnatural.  But I doubt the pair would work beyond an S-C ratio lower than 2.5  At that point you simply need to save up for an ED/APO.  Good luck



#9 jimandlaura26

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Posted 06 April 2020 - 03:39 PM

It,s to windy and a cloudy forecast for viewing tonight. so I took my 80mm f11 scope out to see how much false color I could see during the day. I have 3 mv filters.I got 2 of them cheap.I have a Lumicon mv filter a Thousand Oaks mv filter and a Baader Fringe Killer.I used a 7.7mm Kson ortho 118 mag the target a tree and branches.without a filter the purple was easily seen never noticed it that much at night. the branches had purple shading around them.next up the Lumicon.it was reduced but still obvious without to much tint change. the Thousand Oaks did a little better a tiny bit less purple fringe but still obvious and the least tint change. The Baader Fringe Killer removed the most purple but added a pale yellow tint the fringe was still there but was harder to see.Why did I do this with all going.I have nothing better to do.  but I had fun.and since it does not look like. I will not get a apo anytime soon. I might be able to get a Celestron Omni 102 these may be of use since they have a little bit more false color.

Given responses here, I would also suggest the Baader Semi-APO filter which does not add yellow color to achro image. Color change is very mild. Had some good successes here with my old Stellarvue Nighthawk. Good luck!



#10 Scott Beith

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Posted 06 April 2020 - 03:59 PM

I like dark shingles on the top of a roof backed by a blue sky.  The granularity of the shingles makes best focus easy and the contrast between the shingles and the sky will show any CA.


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#11 George Methvin

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Posted 06 April 2020 - 06:14 PM

Here another thing the 120mm St f/5 like most f/5 cheap refractor are made for rich field views which this scope does very well I did not get this scope for terrestrial viewing and yes it does have CA at higher powers. This scope was never intend for high power color free views of the moon or planets this scope is not and will never be a APO.  What these filter will do for this fast scope is enable it to get rid of some of the CA which increases the contrast and sharpen the views on the brighter objects. Minus violet filter are not for everyone it each person choice for me I don't mind the little tint of yellow for the better contrast and sharper views but then again that just me. I never got this scope for high power color free views for that I have my nice C-8.  I had thought about trying the lp 495 Baader filter read that it put a little more yellow tint to the image but gets rid of all the CA  not sure I am ready for to much yellow also the Baader Semi-APO filter sounds ok but don't want to put out $150.00 so I am happy with the result I am getting with the #8 yellow filter its sort a middle of the road plus its priced right. So clear skies to all you asto nuts out there and stay safe.


Edited by George Methvin, 06 April 2020 - 06:48 PM.

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#12 sg80

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Posted 06 April 2020 - 08:05 PM

I notice the purple fringe at night but nothing like I seen during my daytime test.so the filters do better when looking at celestial objects.


Edited by sg80, 06 April 2020 - 08:30 PM.


#13 George Methvin

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Posted 06 April 2020 - 08:18 PM

Yea that's true when scanning the skies and looking at M-42 I see no yellow tint at all in fact to me it seem the nebula in M-42 stands out more with the # 8 filter yet with no yellow tint to the stars.  I notice no yellow tint to my eyes when viewing open star cluster at higher powers and only the very brightest stars like Sirius show a little purple fringe around it but not bad at all for a f/5 scope.


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#14 AndresEsteban

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Posted 08 April 2020 - 02:38 PM

The problem I have during the day is that the seeing is rarely super steady so it limits the possible sharpness. 

 

I do look at phone poles and phone wires at a distance.  A dark pole or wire against a blue sky is an excellent target to show chromatic aberration at higher magnifications.

 

To see the chromatic aberration, the magnification must be sufficient that the exit pupil is about the size of your pupil. 

 

To George's scope. At low powers, achromats will show very nice images because your eye is masking the aperture.  If you're using a 25mm eyepiece in a 120mm F/5 scope, the exit pupil is 5mm.  If your pupil is 2mm, the scope is now a 120mm x 2/5mm  = 48mm aperture, the focal ratio is now 600mm/48 = 12.5

 

A 48mm F/12.5 is, by comparison, virtually color free.  Increase the magnification, the exit pupil decreases so the effective aperture increases.  At 60x, the exit pupil is 2mm and the full aperture is being used and the chromatic aberration becomes apparent.

 

Jon

Not taking into account night tests on stars and planets, an excellent test is to observe any kind of dark poles or small diameter tubes (i prefere them vertical)  against a blue sky at very high powers. I usually test refractors at powers between 2,5 D[mm] to 3.4 D[mm]. 
I use magnifications that are factors of D in millimeters, thus, I rate the scope according to how much colour appears beside the dark pole (mainly violet).

No false color at M = 2.5 D[mm] = a standard good scope for high powers
No false color at M = 2.75 D[mm] = very good scope for high powers
No false color at M = 3.0 D[mm] = excellent scope for high powers

No false color at M = 3.5 D[mm] = superb scope for high powers

 

Sure it will depend on the CA index, so long focus achromats should give excellent views. But it's not always the case because the objective may suffer from several types fo aberrations. Also, I could not ask a CA=3 scope to perform at high powers like a CA= 5.5 will do! That's obvious!

Anyway, following the scale I gave above, let's consider one of my vintage japanese achromats:a 60 mm (2.36 in) f/15.7 with a CA index = 6.42!!! (I love japanese vintage ones!)  These images were taken with a cell phone (LG K10), and the achro specified above using a 10mm Baader Classic Ortho (91x) and a Baader Q turret barlow 2.25x, thus giving (91x 2.25 = 204.75x ) or 205x. That means 205/60 = 3.4 D[mm]!!!! Antennas and poles are at the top of a building at 1.6 km (1 mile) way. Certainly a scope rated between excellent an superb!!!

No false color at M = 2.5 D[mm] = 190x => a standard good scope for high powers
No false color at M = 2.75 D[mm] = 209x => very good scope for high powers
No false color at M = 3.0 D[mm] = 228x => excellent scope for high powers
No false color at M = 3.5 D[mm] = 268x => superb scope for high powers


Building top at one mile away at 205x!!!

 

Building top at one mile away at 205x!!!

 

Building top at one mile away at 205x!!!

 

Cellphone adapter

 

 

Usually all japanese vintage scopes with a CA index >5 (Conrady Criteria) are able to deliver sharp and perfect images with no violet color at magnifications around 2.75 to 3.5D[mm], depending of course on the CA index which must be above the Conrady criteria. 

Also consider that real eye images using our own eyes are way better than the ones with the cell phone, because in afocal mode there's always an interaction between the ocular and the cell phone lens. Sometimes this interaction gives good or excellent optical results and sometimes it ruins the image.

 As another example, my 76.2 mm f/16.4 vintage achromat usually works at 208x (6mm ep) with no false color at all, that means 2.73 D[mm]. A little bit lower max power due to the lower CA index. Still a planet and double killer!
 

76.2mm (3-in) f/16 Dean Beam (OKKK), vintage refractor

 


Clear skies for us all!
Andy

Edited by AndresEsteban, 08 April 2020 - 02:39 PM.

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#15 coopman

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Posted 11 April 2020 - 03:24 PM

Of everything that I tried, the least intrusive (while reducing the visible CA) was the #8 filter.  The Baader yellow filter made everything yellow, IMO.     


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#16 George Methvin

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Posted 11 April 2020 - 06:29 PM

I found something interesting today I was going through my astro junk box and found a 2inch Zhumell urban sky filter that I have got several years ago so I decide to try using both the #8 yellow filter along with the sky filter on my 120st f/5 scope. I found when using both filters together the yellow filter cut most of the CA but combined with the urban sky filter it got rid of the yellow tint in the image for a natural colored view a very nice surprise the only draw back is I think it may dim the image a very little if so not by much to my eyes.  I am going to use both filters on the scope over the next few weeks and see how I like it. Reading the info on the urban filter it said it removed light in the yellow range to get rid of sky glow from street lights.


Edited by George Methvin, 11 April 2020 - 07:19 PM.


#17 George Methvin

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Posted 11 April 2020 - 07:17 PM

I found this in a other topic here on CN the gentleman said " Yes.  Stack a Moon & Skyglow with the Fringe filter and it will reduce the CA and pretty much give you true color.  Stacking these filters will not give you as much light loss as using the Semi APO filter as that filter combines the Fringe with the equivalent of stacking two Moon and Skyglow filters".  So this may be what I am seeing with these two filters combined the #8 yellow gets rid of most of the CA and the Skyglow filters gets out the yellow to give more of a true color view. 


Edited by George Methvin, 11 April 2020 - 07:23 PM.

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#18 daquad

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Posted 12 April 2020 - 09:03 AM

Of everything that I tried, the least intrusive (while reducing the visible CA) was the #8 filter.  The Baader yellow filter made everything yellow, IMO.     

Makes sense.  The Baader 495 Long Pass, as its name implies, cuts off almost all the blue portion of the spectrum.  The # 8 Wratten light yellow cutoff is at 465 NM, so not as aggressive as the Baader.  The #12 Wratten yellow cutoff is ~500 nM and is closer to the Baader 495 Long Pass and will show a rather strong yellow image.

 

Dom Q.


Edited by daquad, 13 April 2020 - 08:51 AM.

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#19 markb

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Posted 12 April 2020 - 10:00 AM

The key to the Baader LP495 long pass is its extremely sharp cutoff.

The others, particularly the Written number filters, have a much shallower cut off curve. Even the other Baaders don't come close in violet reduction, and they are great in longer FL scopes.

Side by side, the LP495 gave superior Violet reduction with less dimming than anything I have tried. With an f5 6" 1969 Jaegers achromat differences are absolutely noticeable.

In an achromat, with or without filters, I can still pick up color when slightly defocused, a much easier and more sensitive test when judging fringing in focus.

That was the way I first noticed for certain how much red error was in one of my 5" f5s. I had been aware of a slight difference in color correction but found it hard to quantify. One of the Baader filters, I don't recall which offhand, does a better job than the LP495 only because of the CA differences, less violet, more red.

If I had an f7 or f8 refractor I'm sure the other filters would do an adequate job.

George, thanks for the tip on the differences between stacked Baader filters and the semi-APO. I had assumed they would give identical performance and never bothered to try the combo.

#20 George Methvin

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Posted 12 April 2020 - 11:48 AM

Yea I was surprised at the results, this morning I got up and viewed the moon and really like the more true color views the filters give. The view were very sharp with all eyepieces 32mm and up with no trace of the yellow tint from the #8 filter plus very little CA when using the higher power eyepieces up to 4 mm so I am very happy with the result.  In comparison the views with out the filters were not bad but the moon was a washed with a slight purple haze at the higher powers and the blacks were not black but with the two filters in place the purple haze was all but gone and the image was sharp with good contrast even thought I notice there was a slight thin sharp purple fringe on the edge of the moon but the image of the moon was nicer to me with the truer color image.  A reminder this scope still is not a apo but with the filters it is a little more fun scope to use.  Hope this is of some use to others out there that own a fast f/5 refractor.


Edited by George Methvin, 12 April 2020 - 12:14 PM.

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#21 markb

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Posted 12 April 2020 - 12:55 PM

I think I'll try combining the Moon and Skyglow filter with the LP495.

I never thought about it before, but, George, your information makes it sound like a combo that needs a field test.

I cannot imagine what an equal quality APO f5 would cost! And all four 5 and 6 in Jaegers I have owned, all made in 1968 and 1969, Ronchi and star test stunningly well, aside from CA. My only APO is (are) the APM 100 binos, not inexpensive even at 4in and f5.75 IIRC.

Edited by markb, 12 April 2020 - 12:56 PM.


#22 George Methvin

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Posted 12 April 2020 - 01:20 PM

Good luck I see you can get a moon and skyglow filter online for around $24.00 so would not cost to much to try and see if it works for you. How I did it was to screw the #8 yellow filter into the skyglow and then screwed the skyglow onto the diagonal so the light first hits the #8 yellow filter to get rid of the CA and then the sklyglow to get rid of the yellow tint but it may work either way.  Still not sure how much light is lost using the two filters but I feel if any its not much so the best of luck to you and let me know how it works out.


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#23 markb

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Posted 12 April 2020 - 04:51 PM

From your earlier post, it appears the total light loss will be less than that of the semi APO, which puts it in the 'very tolerable' category.

The moon and skyglow, incidentally, did an awesome job on suburban Skies up until the destructive advent of LED lighting. My daughter was performing at a local high school in a relatively dark area (Bortle 8) with only low pressure sodium lighting outside. The lighting in the immediate area virtually disappeared upon holding the moon and skyglow up to my eye.

Oh well.

#24 daquad

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Posted 13 April 2020 - 08:34 AM

From your earlier post, it appears the total light loss will be less than that of the semi APO, which puts it in the 'very tolerable' category.

The moon and skyglow, incidentally, did an awesome job on suburban Skies up until the destructive advent of LED lighting. My daughter was performing at a local high school in a relatively dark area (Bortle 8) with only low pressure sodium lighting outside. The lighting in the immediate area virtually disappeared upon holding the moon and skyglow up to my eye.

Oh well.

Yeah, I have an old Lumicon light pollution filter that I used effectively to mitigate light pollution in my area.  Now that all the street lamps have been converted to LED's, the Lumicon is virtually worthless.

 

Dom Q.



#25 Jeff B

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Posted 13 April 2020 - 04:01 PM

I read this thread and decided to take some DPAC shots with and without my Baader Fringe Killer filter.  All images were taken inside of focus with a white light LED, which is really a combination of red, blue, and green LEDs.  For the individual color shots, I split out the individual red, green and blue channels from a single white image using photo software.  The scope was a 6" F6.5 achromat ATM OTA of mine.

 

First, the no surprise part.  The filter, mounted on the nose of my AP Maxbright diagonal, does indeed knock down the blue part of the spectrum considerably as you can see in the side-by-side color image.  No surprise there.

 

Now the surprise.

 

My sample fringe killer filter seems to have some optical power associated with it, specifically, a mild focal reduction.   You can see this in the other side-by-side set of "white light" shots.  The shot on the left was taken without the filter, the middle shot with the filter installed and the right shot with the filter carefully removed to duplicate the first shot on the left.  Nothing else was manipulated or changed except for carefully adding and removing the filter to the end of the diagonal.  You can of course also see the intense yellow tint between the ronchi grid bars which is more evidence that the filter removes the blue.  As the shot in the middle has more bars than the other two and all were taken inside of focus, I conclude, that the filter is adding a very mild focal reduction when placed on the end of the diagonal's nose piece.

 

An interesting and unexpected result.

 

Jeff 

Attached Thumbnails

  • 6 Inch F6.5 , RGB, Inside, Fringe Killer, white.jpg
  • 6 Inch F6.5 , White, Inside, with Fringe Killer.jpg

Edited by Jeff B, 13 April 2020 - 04:03 PM.

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