The Baader Minus Violet Filters

[Alkaid]

The Baader Achromatic Filters: Fringe Killer, Semi-Apo & Contrast Booster

 

Disclaimer – the author knows perfectly well that these can’t beat an Apo, so please don’t point this out to him – he knows 😉

 

Grab a coffee….sit back...and relax….this is meant to be a nice light-hearted review when messing about with achromatic telescopes and the Baader minus violet filters...

As I write this, it is the Coronovirus Lockdown in the UK. Unable to do very much or travel, I began to look for a little review project. I didn’t really want another telescope (really?) and having a couple of simple achromatic refractors I became curious about the Baader ‘trio’ of filters. From what I could gather, there had been quite a few reviews posted in previous years and what I did find was that about 50% of the written online reviews gave a verdict of ‘okay’ and the other half just said that the filters didn’t really do it for them and that they would just stick to either no filter or just spend more and buy an Apo (entirely justifiable). My curiosity got the better of me during these quiet times of boredom. I had for a little while thought about purchasing the Semi-Apo filter, thinking that it would certainly benefit both of my achro scopes. As you will see below, I was incorrect on that and I’m glad that I took the time to evaluate all three filters in both scopes. I then decided that this was what my ‘Lockdown review project’ would be and that I would share with others my own relaxed findings. As luck would have it, all three filters came up on ABS at the same time during the Coronovirus crisis….I bought them all together….all I needed now was:

 

A nice high contrast terrestrial test target.

A nice, bright sunny day.

A nice clear night with Moon.

 

When the time was right, I spent the full day and part of the night making examinations with the filters, examining their traits and ability to reduce C.A.

 

The Test Instruments:

I currently have two achromatic refractors. The first is the Skywatcher Evostar 90, a 90mm F10 instrument. My personal view is that it’s a reasonably good performer and a wonderful beginner’s scope for the Moon. At F10, the CA is very reasonable. The Evo-90 is a very lightweight tube and I have nothing really to complain about with regards to the optical figure – it is decent enough and will achieve 50x per inch on the Moon with a decent image. One issue is that the small lens means a darker image and smaller exit pupil than I would like at high power. CA is certainly evident, but not majorly bothersome for a relatively tolerant visual observer like me.

The second is a design most notable for it’s inherent CA , the little 102mm F5 Skywatcher Startravel. And you know what? I love this little scope, simply because I understand perfectly the limitations of an F5 Achromat. Used for it’s intended design purpose at low power, it works very well. At x20 on a terrestrial target, the image is bright, clear and sharp. The field of view is nice and wide. CA is there...but really hardly noticeable at this very low power. The little scope used like this makes a great spotter for the garden and wildlife. On the night sky, sweeping star-fields and finding bright DSO’s at low power is delightful. Widefield vistas are where this little scope is at. But up the power on subjects and the view is spoiled – blue hazy fringes appear on all high contrast edges and the CA smears over the entire view, robbing detail. Having taken delivery of the filters, I was particularly interested to see how the Baader trio would stand up against taming the colour in this F5 Achromat.

Two Achro's....

 

 

Diagonal:

The diagonal used was to be the same throughout testing - the particularly excellent TAL mirror diagonal.

 

Eyepieces:

The eyepieces used were Celestron’s Xcel 25mm, 9mm and 5mm. These are mid-quality eyepieces and will give the following powers in the two test instruments:

Evostar: x36, x100, x180

Startravel: x20, x55, x100

 

The Filters:

The Fringe Killer is marketed with a violet cut to the spectrum, removing the deeper purple hues to an image. It has a reasonable cut-off to the spectrum at 430nm, apparently cutting most of the violet hue. As a result, it imparts a yellow cast to the image, which terrestrially I did not find too bothersome, definitely different but you soon get used to it. On the Moon, the image also takes a yellowy shade. More on how I felt about that later...

 

The Semi-Apo is also marketed with a violet cut to the spectrum…..it also does not impart a strong colour cast to the image which will appeal to many. It has a slightly weaker cut-off point than the Fringe Killer at 420nm, I think (but could be wrong) that this is why it manages to retain it’s colour balance. This is the filter that many will just buy I would think, due to the name implying that it will improve the view in your achro no end. I almost fell into that bracket, and if I had not managed to get all three this is the one I would have just rolled with. We will see what it brings to the party….

 

The Contrast Booster is marketed with the strongest violet cut to the spectrum at 495nm. It imparts an odd cast to the image which I can’t put a colour on, but will attempt to describe….it’s as if everything has turned a shade or two darker, with a much deeper hue. Again,  I’m not bothered too much about the hue and found that it was fine after some time at the eyepiece. Reports suggest however that the Moon takes on a brownish shade and as a mainly lunar observer I’m not sure that I can get on with that. We will find out...

 

From left to right......Semi-Apo, Fringe Killer & Contrast Booster:

 

Test Subjects:

The test subjects were examined visually at all powers. These subjects were:

A small wind vane on a building, white in colour against a dark background.

The top of a lamp-post.

The Moon.

 

Results:

First Visual Impressions Through Differing Powers

These initial statements are what were going though my mind as I played around with the scopes, eyepieces and filters on terrestrial targets. I chose the small windvane on a building first, then followed by the top of a lamppost to form my first impressions, as both have white bodies against a dark background.

 

F10 Evostar Statements – Terrestrial:

 

The Fringe Killer is reasonably effective with the F10 Evostar.

The Semi-Apo is not effective with the F10 Evostar. (Really??)

The Contrast Booster is fully effective with the F10 Evostar.

 

I was a little surprised by the filters’ use in the F10 scope. Remember how I was just going to buy the Semi Apo? Well, I’m glad that I didn’t. At 100x power, to my great surprise, it didn’t really do anything at all in the Evo to my eyes. The tone of the image darkened very slightly. But the CA was still most definitely there, not really muted….just darker...

 

The Fringe Killer did alright in the F10. It definitely reduced the CA by about 80-90%. At the same 100x power, it left a very small residual and tolerable amount. Quite a good show. The yellow image hue was there, but being a sunny day, after some time looking through it I stopped noticing the tone.

 

The Contrast Booster absolutely killed all CA stone dead. There was simply none to be seen at all. The image was quite dark though...maybe too dark. However after some time, I experienced the same effect as when using the Fringe Killer – I simply forgot about the colour cast and found myself examining a spider in a web, hanging underneath the top of the lamppost.

 

 

F5 Startravel Statements – Terrestrial:

 

The Fringe Killer is not effective with the F5 Startravel.

The Semi-Apo is not effective with the F5 Startravel.

The Contrast Booster is fully effective with the F5 Startravel.

 

This time using the Startravel, I examined the small wind-vane down the street from my house at x55 and x100. I knew that even at the mid power, the F5 scope would normally show plenty of colour. The Semi Apo once again did nothing. Compared to the F10, the larger CA in the F5 was absolutely apparent, just darkened.

 

The Fringe Killer also had a tough time. Although this filter worked quite well in the F10 scope, now in the F5 scope it was really struggling. The CA was absolutely still there and resembled the sameish image delivered by the Semi-Apo, albeit with the yellow hue.

 

I was pleased with the Contrast Booster. Once again, with it’s particularly strong violet cut-off it did give excellent results. CA was completely eliminated once again. I was expecting a tiny little bit, but there was absolutely none….a commendable result when used in the F5 scope.

 

 

Visual Impressions on the Moon

This got interesting. The Moon is my main subject for a telescope as I live in a lit up city.

 

F10 Evostar Statements – Lunar:

 

The Fringe Killer is reasonably effective with the F10 Evostar at low powers and very effective at high powers. (More on this...)

The Semi-Apo is not effective with the F10 Evostar.

The Contrast Booster is fully effective with the F10 Evostar, but encounters issues in a small telescope at high power. (More on this too…)

 

I started out – no filters - at x36. The image was crisp, but I could definitely see the slight blueing of the Lunar disc. I then added the Semi-Apo filter and although the image lost some of the blue, it seemed to do something else that I didn’t like. I switched back and forth, back and forth and then realised that the view with the filter had become a little dimmer but also lost a little contrast. Things were just not popping out at my eye as readily as they should. The story was the same at x100 and x180. I found it just slightly harder to detect detail AND there was still CA present. There are reports whereby users of this filter state that they prefer it as it imparts the least amount of colour cast – they are absolutely right about this, however for myself it simply took away something in the view and left it wanting. It went back into the box and I did not use it again with the F10 scope – this filter has been knocked out of the fight here.

 

The Fringe Killer was up next. At x36 the filter cast it’s yellow hue onto the image which was initially objectionable, however the CA was very much eliminated, just a slight touch of it on the Moon’s limb. At x100 and x180, I began to examine Copernicus and looked for detail within the crater for when comparing with the Contrast Booster. The three central peaks of Copernicus were easily on show and just below them on the plain, there is some rather rough ground, perhaps 10-20 km across with little craggy hills and valleys, the valleys appearing as tiny lines. The Fringe Killer showed the detail well at both x 100 and x180. I’ve not used the Evo-90 much recently and I started to enjoy the view so much that I forgot all about the yellow cast in the image. The contrast was good and things stood out. I then found something unexpected – the yellow cast actually helped the smaller aperture by brightening the image a little at high power. I discovered this when comparing with the Contrast Booster…

 

The Contrast Booster was now up...at x36 it cast it’s weird ‘deeper hue’ onto the Moon, also initially somewhat objectionable and as expected, there was absolutely no CA at all with excellent contrast. Time to up the power….returning to Copernicus at x100 the image was on par with the Fringe Killer, albeit a little darker but still bright enough. At this power, there was also zero CA on the limb (which the Fringe Killer could not match as it retained a very slight haze). But it was at high power at x180 where I gained a surprise….in this smaller aperture the Contrast Booster started to run out of light. The image was dim, I could still see the detail in the Copernicus floor, there was good contrast but the image was just….dimmer than when using the Fringe Killer. I spent an absolute age going back and forth between the Contrast Booster and Fringe Killer and realised that the yellowy tint of the Fringe Killer was actually brightening the view and although I could still see everything in both filters, the view in the Fringe Killer was a little brighter and therefore preferable to me in the little 90mm scope.

 

 

F5 Startravel Statements – Lunar:

 

The Fringe Killer is not effective with the F5 Startravel.

The Semi-Apo is not effective with the F5 Startravel.

The Contrast Booster is fully effective with the F5 Startravel.

 

Again, I started out at x20, no filter. The Lunar disc threw up quite a lot of bluey colour, but with a crisp image. I then inserted the Semi-Apo filter and after previous experience I was not surprised to see virtually no improvement in the view. Upping powers to x57 and then x100 gave the same lack-lustre image, with the CA not really reduced at all and due to the F5 scope being a harsher test, I’m not surprised. Back into the box with the Semi Apo…

 

The Fringe Killer was then inserted into the low power eyepiece, it gave a nice bright image, with the yellowy hue and better contrast than the Semi Apo but it was noticed that it was also struggling to control that troublesome F5 CA. Medium power at x57 and high power at x100 were quite good, but if you caught the Lunar limb in the view then the CA was most definitely still present. I think that as these powers are still quite ‘low’ in telescope terms, you would nearly always catch the limb in the FOV, therefore the residual CA would compromise your image. The Fringe Killer put up a good fight, but struggled in the F5 scope.

 

The Contrast Booster was next. It won hands-down. As before when I tried it on terrestrial targets, it showed zero CA. All powers used showed a pleasant view, albeit with the weird hue (browny??) but again after observing with the filter for some time I simply forgot about the hue. It’s a little bit like putting on sunglasses – after some time you forget you’re wearing them. At x100 it still put up an image bright enough to be pleasing, with excellent contrast. This was no doubt due to x100 still falling within a ‘medium power’ level and the 102mm aperture still sucking in plenty of light to enable the Contrast Booster to do it’s thing. I was able to observe the rough ground in Copernicus just like I did with the F10 refractor before, just at a lower power. I must admit to feeling rather pleased, because here now I could actually use the little F5 scope for a nice quick, decent look at Lunar, whereas before I had completely relegated the Startravel to a corner when the Moon was up.

If you have a simple achromat, you can try these filters for yourself, they often come up for sale. I found that they did improve some of the viewing for me, although I was disappointed with the Semi-Apo. It just didn’t live up to it’s name in my opinion. The other two were however worthwhile additions to my optical kit. I am also reading reports that the Contrast Booster is also a rather mean planetary filter for Mars & Jupiter. It will be tried in my C8 as soon as these planets make an evening appearance...

 

Note – the author did attempt some afocal digiscoping shots of the terrestrial targets and the Moon, but the results were so poor that he decided to omit them from the review. He is not an imager 😉

 

Edited May 31, 2020 by Alkaid

[John]

Very good report and comparison Steve

 

[Mark at Beaufort]

Steve that is a very good report on these filters. I enjoyed the read and your conclusions. I assume that someone with a 120mm or 150mm Achro, having gained more light, should consider the contrast booster above the others.

[Alkaid]

11 hours ago, John said:

Very good report and comparison Steve

 

Thanks John, I've had this in mind for some time.  Realised that I couldn't really review just the one.....I'd have to get all three!

[Alkaid]

2 hours ago, Mark at Beaufort said:

Steve that is a very good report on these filters. I enjoyed the read and your conclusions. I assume that someone with a 120mm or 150mm Achro, having gained more light, should consider the contrast booster above the others.

Thanks Mark, yes indeed - I think that with the larger aperture achro's the contrast booster would be a solid performer.  I'd like to try one with a 150mm F8 someday. 

[John]

The only filter of this type that I have used was a William Optics VR-1 minus violet filter which I tried with a Skywatcher 150mm F/8 achromat. The filter did reduce the violet halo surrounding the lunar limb by about 50% I reckon. The price for that was a pale lemon / yellow tint to the whole image.

In the end I decided to try other means of correcting the CA in these scopes.

 

[ScouseSpaceCadet]

I'm not surprised the contrast booster won out, as I've read similar reports.

However, I'm really surprised at the semi-apo evaluation, having spent almost a year using a Startravel 120mm utilizing a 2" Semi-Apo filter screwed into a 2" Skywatcher dielectric diagonal. Eyepieces were a combination of BST/Celestron XCel LX and an Aero ED 30mm.

It's hard to put a qualitive value on the experience, but I would say CA was reduced by anywhere from 50-70%, effectiveness reducing as magnification increased and with no unnatural colour cast added.

An an example would be a very obvious coloured halo on the moon became the smallest thin line, with no fringing at all through 25mm and 30mm eyepieces.

 

Edited June 1, 2020 by ScouseSpaceCadet

[Bobby1970]

9 minutes ago, ScouseSpaceCadet said:

I'm not surprised the contrast booster won out, as I've read similar reports.

However, I'm really surprised at the semi-apo evaluation, having spent almost a year using a Startravel 120mm utilizing a 2" Semi-Apo filter screwed into a 2" Skywatcher dielectric diagonal. Eyepieces were a combination of BST/Celestron XCel LX and an Aero ED 30mm.

It's hard to put a qualitive value on the experience, but I would say CA was reduced by anywhere from 50-70%, effectiveness reducing as magnification increased and with no unnatural colour cast added.

An an example would be a very obvious coloured halo on the moon became the smallest thin line, with no fringing at all through 25mm and 30mm eyepieces.

 

This was certainly my experience when using the semi apo filter also. I used mine with ST102 and ST120. 

Maybe this shows how some are bothered more than others by CA???

 

 

[ScouseSpaceCadet]

23 minutes ago, Bobby1970 said:

This was certainly my experience when using the semi apo filter also. I used mine with ST102 and ST120. 

Maybe this shows how some are bothered more than others by CA???

 

 

Strange innit? 🙄

[Alkaid]

Weirdly, for me the SA filter just ‘dimmed’ the CA. It did make it less noticeable but to me, absolutely consciously looking for it, it was still there and more so than with the other filters.  I think that is a good point though regarding sensitivity.

[ScouseSpaceCadet]

6 minutes ago, Alkaid said:

Weirdly, for me the SA filter just ‘dimmed’ the CA. It did make it less noticeable but to me, absolutely consciously looking for it, it was still there and more so than with the other filters.  I think that is a good point though regarding sensitivity.

I have Deuteranopia (aka red-green colour blind) so I don't know if that comes into play at all. Does it make me less sensitive to C.A. or do certain minus violet filters work better on us genetically challenged folks maybe?

[Alkaid]

That’s interesting, if you rack an Achro focuser in one direction (I think it’s inwards) you’ll get the green spectrum. If your condition means that you can’t see green, then you might not be seeing as much CA.  I had a TAL100RS that when in focus exhibited it’s false colour in the green spectrum rather than blue. 

[ScouseSpaceCadet]

45 minutes ago, Alkaid said:

That’s interesting, if you rack an Achro focuser in one direction (I think it’s inwards) you’ll get the green spectrum. If your condition means that you can’t see green, then you might not be seeing as much CA.  I had a TAL100RS that when in focus exhibited it’s false colour in the green spectrum rather than blue. 

I suppose we will never know for sure unless someone does research. I've searched before looking for other convos and articles about colour blindness and c.a. perception but it was fruitless. Just guessing, the Deuteranopia *probably* does have an effect.

Is @Bobby1970 colour blind I wonder?

 

Edited June 1, 2020 by ScouseSpaceCadet

[Bobby1970]

I'm definitely not colour blind. But have worn specs for almost 48 years. And my eyes have got worse over the last few years tbh. For me, CA was never a deal breaker. Yes a semi apo or full apo scope gives better views. More recently I have done very little visual observation. Perhaps some lunar. I'm more into imaging or EEVA nowadays. 

[Alfian]

Thank you for a very interesting "road test" of these filters, and some interesting comments too. I tried a Fringe Killer in a 80mm F.7.5 achromat. I thought I could see some very subtle improvement in discerning finer luna features, within reason for an F7.5 80mm, the yellowish tint was something I couldn't live with. I've seen good comments about the contrast booster, some people are very positive, and have been tempted to give a try on the ST120 and the Tal 100R although the latter is pretty nice as it is.

[Alkaid]

Hi everyone,

Had the F5 102 out on Jupiter last night.  Knowing that this would be a tough, perhaps futile test, popped in the Contrast Booster at x100. The planet had a nice tone to it which looked normal to me, fawns and browns etc. After some time and patience whilst the scope acclimatised and detail had to be teased out due to atmospheric dispersion blurring in and out, I picked out the GRS, plus a few other distinct zones.
 

Not bad at all for a little F5 achromat with the planet so low down.  I didn’t bother trying it without the filter, from previous experience the image would have been a mess under these conditions.

Then to Saturn, very small but clear, with a few moons in the vicinity.

Then our own Moon, the brown hue of the CB filter being immediately noticeable, but the image was good.  As it was almost full moon I did not linger, just a quick look.

Liking the CB filter in this scope...

Cheers Steve

[steveex2003]

What a great read!

in a given (blind) situation I fully believe I would have spent/thrown my money at the APO, then the Fringe Killer and possibly something else and even something else again before the Contrast Booster for CA cut.

Edited August 6, 2020 by steveex2003

[Alfian]

Some time ago I bought a Baader fringe killer and it worked pretty well on reducing CA on the luna limb but found the yellow cast distracting. I've just bought a s/h Baader  semi-apo filter to play with so it will be interesting to see how it works out. In all honesty the CA on the Tal100 and the Vixen is not at all bad anyway but the ST120 and the little Optic Star 90  will be a useful test. Will report back when weather plays ball.

[Alkaid]

21 hours ago, Alfian said:

Some time ago I bought a Baader fringe killer and it worked pretty well on reducing CA on the luna limb but found the yellow cast distracting. I've just bought a s/h Baader  semi-apo filter to play with so it will be interesting to see how it works out. In all honesty the CA on the Tal100 and the Vixen is not at all bad anyway but the ST120 and the little Optic Star 90  will be a useful test. Will report back when weather plays ball.

Hi Alfian,

That’s great, I’d be really interested to hear your experience with the S/A filter.  For me, it ‘dimmed’ the halo to a darker shade, went from ‘blue’ to ‘navy blue’, still apparent.  But this is interesting, maybe it’s just me as we’re all different and have differing tolerances.

Please would you take the time and effort to advise your findings with all of your Achro’s? That would give a great comparison from another perspective, I’d love to read this if you have the time.

Cheers Steve

[Alfian]

1 hour ago, Alkaid said:

Hi Alfian,

That’s great, I’d be really interested to hear your experience with the S/A filter.  For me, it ‘dimmed’ the halo to a darker shade, went from ‘blue’ to ‘navy blue’, still apparent.  But this is interesting, maybe it’s just me as we’re all different and have differing tolerances.

Please would you take the time and effort to advise your findings with all of your Achro’s? That would give a great comparison from another perspective, I’d love to read this if you have the time.

Cheers Steve

Will do, although with the usual weather and other distractions it may take a little time, but I'll do my best.

[Alkaid]

54 minutes ago, Alfian said:

Will do, although with the usual weather and other distractions it may take a little time, but I'll do my best.

You know, with the lighter nights, my review took a couple of months ha ha.  Please take as much time as you like (you might want to wait for the darker nights).  Absolutely no rush and only if you are able and feel comfortable with it.  ATB Steve

[John]

As well as CA, a lot of these achromats have SA - spherical aberration. That is what gets in the way of their performance at high powers IMHO.

 

[Alkaid]

1 hour ago, John said:

As well as CA, a lot of these achromats have SA - spherical aberration. That is what gets in the way of their performance at high powers IMHO.

 

So true John, yes.  All aberrations drop of with focal ratio...I’d like to try one of those F15 fracs (think johninderby bought a 90mm one recently, looking forward to his report on that one.)

[John]

12 minutes ago, Alkaid said:

So true John, yes.  All aberrations drop of with focal ratio...I’d like to try one of those F15 fracs (think johninderby bought a 90mm one recently, looking forward to his report on that one.)

I saw quite a bit of SA in the 150 F/8's that I had. When I corrected it using a device called a chromacor the effect was quite startling.