Wratten Filters for CA Correction in Achromats?

[John]

1 minute ago, Ian McCallum said:

I tried stopping the telescope down to about f19, but was still getting some colour fringing around Jupiter. It may have been CA or atmospheric dispersion.

I suspect it was atmospheric dispersion. An achromat of around 50-60mm diameter at F/19 should be colour free to all intents and purposes:

 

[vlaiv]

Just now, Ian McCallum said:

I tried stopping the telescope down to about f19, but was still getting some colour fringing around Jupiter. It may have been CA or atmospheric dispersion.

It should not be hard to distinguish between the two. Atmospheric dispersion is usually linear. One side is bluish while other side is red:

CA is the same all around (usually blue).

Depending on eyepieces used - you might also get some lateral CA - this happens with wide field eyepieces at the edge of the field and depends on eye placement

[Ian McCallum]

35 minutes ago, vlaiv said:

It should not be hard to distinguish between the two. Atmospheric dispersion is usually linear. One side is bluish while other side is red:

CA is the same all around (usually blue).

Depending on eyepieces used - you might also get some lateral CA - this happens with wide field eyepieces at the edge of the field and depends on eye placement

I used the full BST range of eyepieces from 25mm to 3.2mm.  I'm almost certain it was atmospheric dispersion. With an aperture of just 52mm, it was just too dim (even at low magnification) to be pleasant to observe. 

It's certainly dissuaded me from buying one of the ScopeTech range... 

[johninderby]

No noticeable CA on Jupiter with the ScopeTech 80mm f/15 but then it has very fine optics optics with the latest coatings. Even a stepped down cheap achro can’t match it as the optics aren’t as good..

Edited December 7, 2021 by johninderby

[Ian McCallum]

3 hours ago, johninderby said:

No noticeable CA on Jupiter with the ScopeTech 80mm f/15 but then it has very fine optics optics with the latest coatings. Even a stepped down cheap achro can’t match it as the optics aren’t as good..

I think it will be quite a while before I'd go for another telescope, as I need to let the bank balance recover.

[johninderby]

6 minutes ago, Ian McCallum said:

I think it will be quite a while before I'd go for another telescope, as I need to let the bank balance recover.

I know what you mean. Spending finished for a while although getting an RC ready to put on eBay so maybe a few quid coming into the hobby fund.

Edited December 7, 2021 by johninderby

[Muc]

16 hours ago, vlaiv said:

It should not be hard to distinguish between the two. Atmospheric dispersion is usually linear. One side is bluish while other side is red:

CA is the same all around (usually blue)

I never knew this practical  definition, many thanks, I always thought Blue and Red was CA. Good to know!

[Ian McCallum]

4 hours ago, Muc said:

I never knew this practical  definition, many thanks, I always thought Blue and Red was CA. Good to know!

I've had that with my 8" Dobsonian, in the past.  A filter like this is supposed to help...

https://www.firstlightoptics.com/zwo-accessories/zwo-125-atmospheric-dispersion-corrector-adc.html

[johninderby]

One thing to note with an ADC is that they are most helpful when your target is low down. The higher up the target is the less helpful it is.

[Ian McCallum]

1 minute ago, johninderby said:

One thing to note with an ADC is that they are most helpful when your target is low down. The higher up the target is the less helpful it is.

You'd think with the planets being up higher than earlier in the year, we'd have less atmospheric dispersion.

[johninderby]

Next year is looking better. Jupiter ar 40 degrees next August.

Edited December 8, 2021 by johninderby

[John]

25 minutes ago, Ian McCallum said:

You'd think with the planets being up higher than earlier in the year, we'd have less atmospheric dispersion.

I've found Jupiter better this year than last - hardly any AD. Saturn is lower so a little more affected by it.

 

[IB20]

I found bad seeing exacerbated both AD and CA at times, especially Jupiter. Didn’t have any problem with either when viewing Saturn. Venus on the other hand… 
 

[Louis D]

I just got in a #12 Yellow and tested it out tonight on Venus, Saturn, Jupiter, and Luna in my ST80 (80mm, f/5).  I had my AT72ED mounted on the other side of my DSV-2B mount for comparison.  I used matching Celestron Regal 8-24mm zooms in each to speed up matching views.  I also tried a Minus Violet pale yellow filter that is lighter than a #8 Yellow, a Zhumell Moon & Sky Glow, and a Meade interference green filter intended for imaging.

My recollections:

Unfiltered: Saturn looks best.  Least affected by unfocused violet and red, needs all the light it can get due to dimness.  Luna has a distinct violet cast, but is otherwise decent.  Venus and Jupiter look terrible.  I could make out Venus's crescent shape, but it was blurred.  Jupiter had no banding visible.

Minus Violet: No effect on Saturn.  Luna gets a bit less violet and a bit more yellow, slightly improving sharpness.  Venus and Jupiter showed no improvement.

#12 Yellow: Darkens Saturn too much without improving anything. Luna gets noticeably sharper, but strongly yellowed.  Venus sharpens up nicely.  Jupiter looks better, but unfocused red now intrudes excessively, blurring bands and other fine details.

M&SG: Darkens image, does nothing to remove violet.  Nothing looked better with it.  Saturn is way too dark to observe.

#12 Yellow and M&SG: Cuts a bit of the yellow cast on everything, but noticeably darkens bright objects.  I'm not sure it improve sharpness or contrast on any target since it doesn't cut unfocused red.

Green Interference: Saturn is sharpest, almost matching the AT72ED.  Luna is sharp, but not much different from #12 Yellow.  Venus is very sharp, cutting through atmospheric dispersion of red and blue.  Jupiter is nearly as sharp as the AT72ED.  Belts and a tiny moon (Io?) just off the edge are clearly visible with no unfocused violet or red.  The high transmission of this filter in a broad green band is a good match for the achromat's characteristics.

Overall, though, the AT72ED stomped all over the ST80 despite giving up 8mm of aperture.  The ED scope wasn't perfectly color free since there was a bit of a red/green rim on Luna, but it didn't seem to affect sharpness much.  However, if you're willing to heavily filter bright objects, the ST80 can produce sharp images.  It also works fine on dimmer objects like Saturn without filtration.

Edited December 9, 2021 by Louis D

[Louis D]

Here's the spectral response of the Meade green filter.  Despite not stacking it with the IR cut filter, it pretty effectively cut visible red:

By itself:

Stacked with IR cut:

I'm guessing my eye is not very sensitive to red light at 725nm and higher, but I can try stacking them sometime to see if there is a difference.  It would be nice if the green passband were shifted a bit further to the right like the Chroma version to better correspond to the eye's peak color sensitivity:

However, I only paid $30 for the set on clearance, not $1700 like the Chroma set; so I can't complain too loudly. 

[vlaiv]

1 minute ago, Louis D said:

I'm guessing my eye is not very sensitive to red light at 725nm and higher,

I don't think anyone's is to be fair.

Neither cones:

nor rods:

are sensitive to anything above 700nm

[Louis D]

That Meade green filter is so much brighter than my #58 green filter, and no wonder.  Look at the spectral response of the #58:

It peaks at just over 50% transmission in the 520nm range, and is much lower everywhere else.

It would be nice if someone made an interference filter passing the range from roughly 490nm to 630nm to filter out most of the poorly focused light in an achromat:

That Chroma green filter comes close if it was just a bit wider on the right side.

 

[vlaiv]

18 minutes ago, Louis D said:

It would be nice if someone made an interference filter passing the range from roughly 490nm to 630nm to filter out most of the poorly focused light in an achromat

https://www.teleskop-express.de/shop/product_info.php/info/p339_Astronomik-ASGruen1---Green-Interference-Filter--1-25-inch-filter-thread.html

Not quite steep cut off - but it does cover the range you mentioned.

[Mark at Beaufort]

I have really enjoyed reading this thread especially as I bought a SvBony 90mm f5/5 Achro a few months back. I already owned a Baader Neodymium which I screw into the front of the diagonal. Reading the above comment about the Green Interference filter would an IR/UV cut filter together with a suitable wratten filter provide a similar result?

 

[vlaiv]

39 minutes ago, Mark at Beaufort said:

I have really enjoyed reading this thread especially as I bought a SvBony 90mm f5/5 Achro a few months back. I already owned a Baader Neodymium which I screw into the front of the diagonal. Reading the above comment about the Green Interference filter would an IR/UV cut filter together with a suitable wratten filter provide a similar result?

 

No. IR/UV cut really does nothing for visual (except protect your eyes if you are looking at something high in UV). We don't see that part of spectrum anyway so cutting it off makes no difference to us.

What you can do is try #56 and #58 - greens, on their own

#56 will let more light in and be less aggressive in removing unwanted light, while #58 will in essence be the same as interference as far as what it passes - except it will be much darker because it passes less than 50% of light in that range compared to interference that passes more than 90% of light.

 

[Mark at Beaufort]

Thanks @vlaivI had a feeling you would know so thanks for the extra advice as well.

[Franklin]

Baader Contrast Booster works very well in a fast achro for cutting CA but it does dim the image a bit as well.

[Louis D]

I'm going to try adding a light blue filter I have on hand (it might be an 82A, I'll have to check) to the #12 yellow to see if I can block some of the spurious red without losing the yellow/orange part of the spectrum as with the green interference filter and without dimming the overall view too much.

I've been researching interference shortpass filters that cut-off at 600nm to combine with the #12 yellow to broaden the passband of the green interference filter while eliminating the unfocused red light.  None are particularly cheap and might be a bit small (1 inch) for 1.25" usage in stock sizes.

The #12 yellow already acts like a pretty good longpass filter with approximately a 500nm cut-off.  Unfortunately, there doesn't appear to be a Wratten equivalent for blocking red wavelengths.  I'm not sure what color the eye would perceive all spectral colors combined except for red.  I'm thinking it's sort of a teal blue-green.  None of the Wratten spectral graphs look like a horizontally reflected yellow filter.  I'm guessing it's an undesirable color, so no one ever thought to make a filter in that color.

[Louis D]

Doing more filter research, it turns out my Minus Violet (MV) filter might have either a 420nm or 455nm cutoff and might also be referred to as a Wratten #4 Yellow.  Either way, it's not particularly useful visually with fast achromats.

There's also a Minus Red (MR) filter, or a #44A Light Blue-Green/Cyan.  However, it isn't really a true analog of the #12 Yellow (Minus-Blue) filter at the other end of the spectrum since it doesn't have a level pass band up to about 600nm.  It's more of an arch that cuts off spectrum on either side of a peak around 480nm and fully cuts off by 560nm, well before spectral red.

[Louis D]

I've been messing around with some cheap Chinese color filters I picked up off of ebay for cheap (the 6 filter set), and combining their light green (similar to a #56) with a GSO #12 deep yellow should block all of the blue (the yellow does this) and red light (the green does this) while transmitting 76% of the green and much of the yellow and some of the orange wavelengths.  The Meade green interference filter passes about 94% of the green and most yellow while blocking most orange and red wavelengths and allowing a bit of blue.  Adding the #12 yellow should block the last of the blue, but it might be superfluous for achromat refractor usage.  I could not distinguish any significant difference in hue while holding the #56+#12 combo to one eye and the Meade green to the other eye.  I'll have to try out the two filter variations on bright targets at night in the ST80 sometime as a verification step.

BTW, that cheap filter set comes with an excellent magenta filter that equally passes about 78% of the blue and red wavelengths while passing only 44% of the green.  I'll be trying it out as a Mars filter at the next opposition.  It's way cheaper than the Brandon Magenta filter.