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Wratten Filters for CA Correction in Achromats?


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I used #8 and it works very well - except the overall yellow tone to the image.

When imaging - it can be somewhat corrected with proper color calibration, but for visual it is noticeable.

Here is test that I conducted with 102mm F/5 achromat: Each row represents same image (one combination of factors) - in three different levels of stretch - high, medium, low. From top to bottom we have certain aperture (I used aperture masks) and next row is same aperture with wratten #8 filter. First is full aperture, then 80mm, 66mm and finally 50mm (I think) - but point is - look at photographic impact of wratten #8 in removing purple fringing (here blue as camera sees it as blue rather than purple). Even at full aperture filter reduces halo significantly (and remember - this is 4" F/5 achromat).

image.png.140487b726f8c81f76ba8a560d7d2edf.png

I've heard that 495nm long pass by Baader produces even better results.

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No 8 light yellow is the best of the wrattens IMO. Controls CA well, improves contrast and only imparts a very slight yellow hint. No12/No15 cut more but are very yellow. If it's for imaging through an acro the Baader 495 Longpass cuts all CA but again it's yellow, though for monochrome imaging it's the best. Then there are the Minus violet filters. William Optics VR-1 is basically the same as a wratten #8 and so is the Baader Fringe killer. There are two other MV type filters, both Baaders, they are the Semi-Apo and the Contrast Booster. They are in effect a combination of Minus Violet with the Skyglow Neodymium filter. The Semi-Apo is the milder of the two and I wouldn't recommend it with an achromat, better with an FPL-51 doublet. The contrast booster does exactly what it says on the tin but with quite a degree of light loss. It is a great filter for planetary contrast enhancement. But considering the costing of these things, for an achro I would just get a wratten #8, light yellow, for around a tenner.👍

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I was looking at the colour wheel and trying to work out what was the closest opposite colour to cancel out the purple fringing.  The closest I could see was Wratten #102.  Sorry that the image for the Wratten #102 being so small, but I felt the correct colour was somewhere between yellow and green.

color-wheel-primary-secondary-tertiary-colors-1024x806.png

31UkQ1KrOQL._AC_.jpg

Edited by Ian McCallum
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16 minutes ago, Ian McCallum said:

I was looking at the colour wheel and trying to work out what was the closest opposite colour to cancel out the purple fringing.  The closest I could see was Wratten #102.  Sorry that the image for the Wratten #102 being so small, but I felt the correct colour was somewhere between yellow and green.

It does not work quite like that.

You need to be looking at two different graphs.

First graph is similar to this one (but will be different depending on scope type):

secondary_spectrum_100f10.PNG

On the left right side - you have wavelength of light.

This graph represents focus shift versus wavelength. For example - if you have 550nm in focus (e line) - then h line will be about 4 mm defocused.

Above graph is for 4" f/10 Fraunhofer type doublet

Next graph that you want too look at is filter response curve:

image.png.9d098007722c11cf560086a062265054.png

and then compare those two graphs.

For example - in above image - Wratten #8 cuts all light below about 460nm and starts to pass significant amount of light only after 500nm or even a bit more

Getting back to first graph - this happens:

image.png.8d820343e4cf6e6e20e7bb98b03ab041.png

You effectively removed outlined part of the spectrum - most defocused bits really - what is left is either in focus or similarly defocused as r line slightly above 700nm.

 

 

Edited by vlaiv
Left/right is there a difference :D
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5 hours ago, Ian McCallum said:

I've ordered a Wratten #8 filter from @FLO, their own brand Astro Essentials in 1.25".

That will do the job. When used on the moon the filter will give a slight yellow cast, but all the fancy minus violet filters do that as well. At the end of the day there is only a small number of objects that you would want to use such a filter on anyway. Moon, Venus, Jupiter, Saturn, Mars and anything brighter than around mag2.

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1 hour ago, Franklin said:

That will do the job. When used on the moon the filter will give a slight yellow cast, but all the fancy minus violet filters do that as well. At the end of the day there is only a small number of objects that you would want to use such a filter on anyway. Moon, Venus, Jupiter, Saturn, Mars and anything brighter than around mag2.

And a lot cheaper too!😱

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I tried the Wratten #8 on Jupiter , this evening.  It did reduce the CA and also seemed to bring out the N and S equatorial belts, making them easier to see.  The yellowish colour wasn't unpleasant, actually quite nice.

Edited by Ian McCallum
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image.png.ba3c631b40312f6d80b443c910101f06.png

Wratten #8 is closer to 495 long pass in its characteristics.

Both start at 450nm and raise up to 500nm, but Baader hits 500nm at 95% transmission - with steeper curve and higher general transmission reaching 97-98%.

Wratten #8 reaches only 60% at 500nm and peaks at 525nm.

Wratten #12 starts beyond 500nm and reaches peak at 550nm.

As cutoff filter - 495 is the best as it has the steepest curve.

@Ian McCallum

What scope do you have and how much are you bothered by CA? Have you considered stopping the scope down a bit together with wratten #8 filter?

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4 minutes ago, vlaiv said:

image.png.ba3c631b40312f6d80b443c910101f06.png

Wratten #8 is closer to 495 long pass in its characteristics.

Both start at 450nm and raise up to 500nm, but Baader hits 500nm at 95% transmission - with steeper curve and higher general transmission reaching 97-98%.

Wratten #8 reaches only 60% at 500nm and peaks at 525nm.

Wratten #12 starts beyond 500nm and reaches peak at 550nm.

As cutoff filter - 495 is the best as it has the steepest curve.

@Ian McCallum

What scope do you have and how much are you bothered by CA? Have you considered stopping the scope down a bit together with wratten #8 filter?

Where would a Wratten #9 fit in there?  I'm using the Sky-Watcher Evostar 120 Achro.  I've only managed one night with the #8 and was impressed. I've still to stop down the telescope.  I found the CA was pretty obvious, after coming from a SW 200P Dob...

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1 minute ago, Ian McCallum said:

Where would a Wratten #9 fit in there?

According to this:

image.png.ad72f2ec9afe3ff4fd9f3f90b8f5e1d6.png

it would not be very good - it would start too "early" at 470nm - about same place as #8 but would have gentle slope - ending the same as #12.

Evostar 120 is rather nice scope for this sort of thing. You can use its 5" (or rather 4.72") of aperture on deep sky and then switch it to be 4" F/10 for planets. Just make 100mm aperture mask for it. You can also have 80mm F/12.5 if you want - by making 80mm aperture mask. With about 70mm - you won't need filter as you'll be hitting Conrady standard (F/ratio divided with aperture in inches >=5 - minimal/no visible CA), but you'll be limited to say x100-x150 magnification.

3d printed ones will likely be the best, but I've made them out of PVC pipe plugs as well.

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7 minutes ago, vlaiv said:

According to this:

image.png.ad72f2ec9afe3ff4fd9f3f90b8f5e1d6.png

it would not be very good - it would start too "early" at 470nm - about same place as #8 but would have gentle slope - ending the same as #12.

Evostar 120 is rather nice scope for this sort of thing. You can use its 5" (or rather 4.72") of aperture on deep sky and then switch it to be 4" F/10 for planets. Just make 100mm aperture mask for it. You can also have 80mm F/12.5 if you want - by making 80mm aperture mask. With about 70mm - you won't need filter as you'll be hitting Conrady standard (F/ratio divided with aperture in inches >=5 - minimal/no visible CA), but you'll be limited to say x100-x150 magnification.

3d printed ones will likely be the best, but I've made them out of PVC pipe plugs as well.

I'd like to add some more Wratten filters to my collection, but for different things (not CA) - mostly to enhance planetary views, etc.

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1 hour ago, vlaiv said:

Wratten #8 is closer to 495 long pass in its characteristics.

Both start at 450nm and raise up to 500nm, but Baader hits 500nm at 95% transmission - with steeper curve and higher general transmission reaching 97-98%.

Wratten #8 reaches only 60% at 500nm and peaks at 525nm.

Wratten #12 starts beyond 500nm and reaches peak at 550nm.

As cutoff filter - 495 is the best as it has the steepest curve.

I've got a #8 pale yellow filter labelled as minus violet similar to the Meade #8 below, and it doesn't do nearly enough on an ST80 to improve contrast that's lost to stray violet light, so I've got a GSO #12 on order for $15 to see if it improves the view.  I'm sure a Baader 495 Long Pass would be superior, but I'm not willing to spend $53 on an experiment involving what appears to be a non-interference coating filter.  I'll also try stacking it with a generic M&SG filter I've got to see what effect that has as well.

This graph indicates yellow color filters to be a bit of gamble as to what you'll actually get, so I'm going to try going more saturated this time:

spacer.png

If the #12 yellow doesn't cut it, I could always try a #15 dark yellow.

Edited by Louis D
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All the wratten yellow filters cut CA in varying degrees, with #8 being the mildest and #15 the most aggressive. The Baader 495 longpass is closest to the #12 but has a higher transmission. The wratten #11 (yellow/green or light green, as it's sometimes labelled) helps to improve contrast on lunar with an achromatic as well.

When out with my 80mm f/11.4 achro I use a 2" Lumicon MV which tends to just stay in the diagonal nosepiece. At higher powers for planetary/lunar the Baader Contrast Booster works well too.

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