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 😉

My comment was actually meant to be tongue-in-cheek, but I do think that everyone in this game should have a try of all scope designs to see what works for them personally. Sure, why not? When people speak of the crisp view, they are not wrong. Refractors give a really nice view of star fields, open clusters, double stars, planets and of course, the moon. They work well on the brighter objects.

Aesthetics and ergonomics are also factors...it’s really nice to sit down behind a refractor, very relaxing. Much better than standing up all the time with a newt and having to rotate it in the rings to get the eyepiece into a good position when moving to another part of the sky. Refractors just look better too. (Hope I haven’t started a war now). There was a guy on here that made me laugh when he said his wife had nicknames for his two scopes - the short fat 10” dob was named R2D2 and the elegant refractor was called Princess Leia.

The C8 is a good scope, ticking many boxes. It is easily manageable but still gives that nice big aperture. It can do almost all visual observing and when coupled with the focal reducer makes a good DSO hunter, particularly at a dark site. Without the FR it makes a good lunar and planetary scope, the long focal length gives good image scale. The C9.25 is the one I would go for though. Granted, it is less manageable but will outperform the C8. Everything I just mentioned above, the bigger scope will do a little better.

Excellent, I had not though of that and it will get him going at least. I'll get him to try it on a distant terrestrial object now.

Thanks, I do think that he's got muddled up and actually does not have enough out-focus.

I'll ring him again now, he said he's got 'good travel both sides' - which leads me to believe that focus should be perfectly achievable so that's why I'm a bit puzzled....back soon...and thanks.

Hi everyone, I'm posting for a mate of mine who is not on the forum. He's getting into Astro as a hobby and I recently sold him a Celestron 150mm F5 newt. He bought a 1.25" USB Astro camera from a retailer and is having issues with focus. He tells me that he is unable to focus on Venus - I have a feeling that there is not enough focus travel but with myself being a visual only guy, I haven't a clue what he needs to do. Does he need an extension tube? He has no issues focusing with a standard visual eyepiece. Thanks for looking - Steve

Good stuff! 130mm of aperture under a proper dark sky can be very, very good.

I appear to be incorrect on the smaller fracs then (I have not experienced a small ED one under a dark sky) so I'm pleased to hear that they do perform well in that case. Good to know!

Some of those smaller refractors mentioned have their roots in imaging, they have small high quality apertures which when combined with long exposures produce fantastic images. But visual is not where they are at, the aperture is too small for satisfying DSO hunts. If it's visual you want, then get the biggest aperture you can get in a small scope. What about the Heritage 130P with the fold up optics? It would sit nicely on the EQ3 and not take up much space at all when folded down. If you also happened to find yourself in the mood for just a quick look, then you can leave it on the little dob mount that comes with it. Only negative - no terrestrial viewing....when I am on holiday, I also like to look at distant objects on land or sea.....if you also like doing that then a refractor is the way to go....just get one with at least 100mm of aperture to also make DSO hunting enjoyable.

Excellent! It was the lack of YouTube comparisons that led me down this path too. I won’t do a vid myself but will watch yours with great interest should you get around to the project. I’ll put my own SGL review up soon. I am comparing the Semi Apo, Fringe Killer and Contrast Booster.

I promise to complete my little write up soonish, but will now probably need to await the next moon, probably from 1st quarter onwards so that I can take comparison pics through each filter. I have already found some interesting things with both the F5 and F10 fracs (they each prefer a different filter) but don’t want to spill all until I’ve got the images for you. Hope you guys will take a read when ready and don’t mind waiting....

I’ll be entirely honest, the Semi Apo was the one I would have just gone for, but my own finding whilst doing the review in progress in comparing all three Baader filters is that it was actually the worst. More on this soon (although all this is my own humble opinion!)

I bought all three Baader filters 2nd hand recently, to see which would benefit both my F5 ST102 and F10 Evostar 90. I needed a lockdown writing project! I’m halfway through writing a review complete with terrestrial and lunar shots, should be ready soon...

Used sct’s are the way to go, they come up often and at good prices. I love my C8 but could not justify a brand new one, just too expensive.

Mine is an older Starbright model (non XLT) with really good optics. It’s great- nice aperture with portability and has shown me a lot of detail on the moon that I could never hope to see with my previous smaller scopes. If you are a solar system observer then either the older one or the xlt will do the job due to the brightness of the target. If you prefer DSO’s then the XLT has a small gain in light gathering due to the coatings. Don’t get too hung up on it though, the difference is not overly huge. About 13%?? 🤔 If an old one comes up with good optics at a good price - buy it.

Yes it is a little variable at high power. But comfortable enough for over x200 and that’s great!

What power are you pushing? I’m at x222 with a 9mm. That’s as high as I can comfortably go.

Yes, it’s good tonight. Happy observing John!

Anyone looking at / imaging Gassendi right now? I’m looking at it through my C8. It’s wonderful. The seeing is ok, for split seconds I’m getting lots of detail with the internal rilles.

It's field curvature, nothing to worry about (for visual anyway) as present in many catadioptic designs. A focal reducer / field flattener will fix it for imaging, but not sure how you would get on with a Meade / Celestron one as these are made to suit F10 scopes. Hopefully some of the advanced imagers will know what to do...

Fantastic. Following this with great interest.

Yep. TAL100RS wasn't really an entry level scope either...

200P Dob. Very well regarded. Have not read a single negative review. 8" aperture will provide wealth of detail on Moon, also very good on Planets. Good all rounder on deep sky. Inexpensive (for a telescope). If I had my time again as a beginner, I'd have one of these. Only disadvantage - no tracking. But from what I gather, most Dob enthusiasts don't mind that. You can also de-mount it and put it on an EQ mount (HEQ5 as a minimum I would say) if you want tracking. You can also get tracking platforms (or even make one if you're handy).