Telescope used for review: Skywatcher 6" Achromat Refractor-f/8. Introduction: This is one nice little piece of equipment. It has a simple and elegant design. It is anodized aluminum with a flat black finish. This housing has 1.25" in-out barrels and can only be used with 1.25" eyepieces. It feels like it weighs about half a pound. There is also a really nice red engraved plate. Fortunately, it is a lot easier to use than it is to describe. As you can see in the photo, a portion of the filter protrudes from the housing. The filter's diameter is about 3". The wheel's edge that protrudes from the housing is numbered for use as a reference. The filter spins around using your finger at this location. Just rotate the wheel and watch the filter vary the renditions on a given object seen at the eyepiece. I simply rotated the filter wheel without regard to the numerical scale, as I was interested in all the possibilities that the VFS has. Since the wheel rotates so smoothly, it was easily to go back and forth while observing. The Variable Filter System, or VFS, is a unique form of interference filter. The filter's glass has coatings deposited on one side of its surface and is unlike died colored glass absorption filters. This method of filtration rejects most incoming light while allowing a very high transmission of light through three thin bandwidths or peaks to come through the eyepiece. What makes it unique is that all three bandwidths are "variable" and move in unison. Here is a scan showing the 2 extreme settings of one filter. You can see that the peaks are numbered to show the shift. Diagram provided by Al Misiuk, Sirius Optics. I don't consider the VFS as a "direct" replacement for some of the other LPR type of filters. I believe its main strength is, presently and primarily, a planetary contrast filter. The VFS is unique in it performance and use. Compared to the other LPR/Nebula filters, which I imagine would be used as a standard frame of reference, the VFS can get somewhat close. It overlaps some of the other filters' performance, but doesn't replace them. That said, VFS has similarities as compared to all the nebular filter types and much more. In my opinion it can encompass some of the benefits of the standard broadband and narrowband light pollution filters that I have been using for years with the exception of an OIII type of filter. In the OIII range, the VFS can get somewhat close, but it's just not the same. The OIII is very narrow regarding the bandwidth of light it transmits to an eyepiece. General Overview I believe the VFS exceeds the standard died colored glass absorption filters used for planetary observing. The VFS also contains within its adjustment range a setting that I have called a "RGB", or red, green and blue. At this setting, the filtered rendition of objects like the Moon and Jupiter appear almost neutral. For example with my 6" achromat refractor at this setting, Jupiter's disc shows a demonstrable gain in detail, but has the deep color saturation similar to what a 4" apochromat shows at about 150x magnification. This "rendition" appears like this in reflectors and SCTs. This is in no way similar to using a standard neutral density or polarizing filters. The VFS also has a broadband hydrogen alpha range where the eyepiece image shows stars and planets as a deep red. This range can also accentuate different details in emission nebulae. I'll describe this setting using M42 as an example later. In the sections regarding various observations I made with the VFS, I concentrated on what I felt were the "best views". It is very difficult to describe every setting the VFS could deliver simply because the VFS is "continuously" variable. That means that there are many small in between variations between what can be described as purple, blue, green, neutral, orange, and red. As an example, in the "blue" range I discerned a light blue, violet/blue, a deep purplish blue, a darker cerulean/cobalt type of blue, blue/green, a green/blue and aqua. At the eyepiece, the amount of different overall color tints are like that for any primary or secondary coloration in the VFS's range. For me, that makes the amount of filters available in the VFS's range somewhat uncountable. As you can see in the VFS's scan diagram, the peaks show a very high transmission with all the other wavelengths demonstrably squashed down. Additionally, and with the many tunable "in-between" filter settings notwithstanding, I feel the VFS has the unique ability to be fine-tuned it to an individual observer's eye. This makes it very useful indeed. Another unique aspect of the VFS is that was conceived and made entirely in the U.S.A. There are some restrictions that the filter imposes on an optical system. First thing was that none of my eyepieces could come to focus while using the VFS inserted in a 2" diagonal with my Skywatcher 6" refractor. I didn't try a 1.25" diagonal, as I don't have one. Consequently, all of my first observations were done "straight through". This can be remedied by using a Barlow inserted between the diagonal and the Variable Filter System + eyepiece. Using a Barlow worked in my refractor and Newtonian type scopes. There were no focus issues using the VFS with SCTs. *Note; Sirius Optics now offers the VFS with a built in 2x Barlow that addresses focusing issues with some but not all Newtonian and refractor telescopes. Observing Notes Using the VFS M42, M43, and NGCs 1973-1975-1977 "Running Man" Nebula 60x, 80x, 109x This filter's rotation really changes M42's appearance dramatically. You could start at the blue part of the spectrum and roll through to the red. Without describing every view one can get, I have to say that I was able to dial in 3 different, but very good views of these objects. Settings for M42 can really bring out the nebula as a whole with very black voids or, as I found out, bring out striking variations in M42's "wings". I got a very diaphanous and quasi-layered rendition of this part of the nebula. In the red, broadband hydrogen alpha setting, the stars and nebulosity shrank with what was left being bright. In this region of the filter's rotation, the nebula's extent looked somewhat like a view from a smaller scope. Interestingly this portion of the nebula presented here gave a look that was like thick "cloud-webs" with red stars. I found at this adjustment, just the brightest part of M42 was visible as off-white with red stars. This somewhat reminded me of what a passable black and white photo of M1 looks like in terms of texture. Strange. On a bright nebula like M42, seeing different renditions by rotating the VFS's filter gave me the impression of a "nebular anatomy lesson". I felt that I was simply peeling away different structures. That said, I think widely differing details could be photographically captured with clarity without over-exposure of the other wavelengths of light. I can't wait for the great emission nebulae in Sagittarius are well positioned for observing this summer. The "Running Man" Nebula had 2 really good sweet spots. One of these sweet spots looked similar to what you could get with a Broadband filter available from Orion Telescope & BinocuIar. In another sweet spot, I was able to see a broken, but apparently connecting strand of nebulosity between this object's extremities. I don't recall seeing this extension before in my 6" refractor. This object is a reflection nebula and appears blue in astro-images I guess the question is; were the views as good as the nebula filters that are currently available? Yes, even though there was one setting, on M42 that I thought really "nailed" it. This setting showed the greatest amount of nebulosity as it sailed right out the FOV at 60x away from M43. The void between M42 & M43 was jet black. I also felt that there was quite a bit of depth at this setting. Depth is something I look for in filters. At low magnifications, my impression was that scrolling in between the broadband, (green/blue stars), and narrowband, (yellow/green stars), "nebula" range would present only a few really good views of M42, M43 and the Running Man nebulae when observed all together. Since I didn't know exactly where I was in the filter's rotation, I can't say the one position brought out the best view regarding both the emission and reflection nebulae. Both were good and both were a bit different. I somehow think that there was a difference as to where the filter was adjusted in comparison to the views I mentioned above paragraphs. M78 60x, 80x, 109x This is a reflection nebula. M78 had a very small amount of contrast boost. I find M78, like M1, doesn't take to filtration in dramatic way in a 6" scope. M45 - The Pleiades 50x, 80x At what I felt to be the best setting, the view seems similar to Orion's Broadband LPR filter. This view presented stars with a blue/green, not green/blue, color cast. I make mention of the color cast because there are little variations in each small tweak of the filter wheel. For example, there is an area where stars turn a yellow/green and then a green/ yellow. This said, all the usual glowing nebulosities presented themselves. M76 - Little Dumbbell 60x, 150x, 171x I found a point in the filter wheel's rotation that gave me one of the better views. Maybe the best view I've ever had of this object through my 6" refractor. I saw the usual 2 lobes. In the past I always tried to struggle to see any peripheral nebulosity to no avail with the filters I have in my arsenal. With the VFS, I succeeded using the 2 higher magnifications listed above. Although I didn't mention this as of yet, this filter can bring stars to a very tight focus, even at high power. M46 & NGC 2438 60x, 80x, 109x, 150x, 171x This is the Open Cluster with the superimposed Planetary Nebula. The rotation of the filter wheel presented lots of different renditions of these adjacent objects. For example, I could see the cluster without the Planetary Nebula, or I could simply watch the PN grow in size with the background stars diminishing. At the higher magnifications, the PN started to present that "smoke ring in the sky" look. The light-green/blue to green/yellow area of the spectrum had the highest intensity for showing both objects best. Still, I could choose the level of "naturalness" of the filter's feel. This was beneficial since the object was in a light pollution dome at my observing site. Jupiter 200x, 240x, 272x, 300x The best, unforeseen advantage the filter has given is regarding Jupiter viewing. The amount of detail the VFS has showed is awesome. More importantly, I observed all the differing details at one setting and saw them consistently. The is not just a little gain in contrast since I've been easily and clearly seeing features that people usually "interpret" or need photo-images to verify. Additionally, the filter can take any magnification I threw at it. In my achromat refractor, the range of benefit takes place on a portion of the filter's full range. I've found that reflectors and SCTS, with their absence of chromatic aberration, there was benefit anywhere on the filter's range of adjustment. For example, there is a blue range just before the LPR/green range in the filter's rotation. In my achromat, Jupiter is a monochrome blue to blue/gray. In a 12" reflector, the blue is an intense cerulean/cobalt blue with Jupiter's bands turning red. This was a very rich and detailed image. This one went deep inside Jupiter's equatorial bands. That said, the "yellowest" green setting showed all manner of Jovian details simultaneously with my 6" refractor. Having excellent seeing conditions while using the VFS had brought out a myriad of Jupiter's lower contrast details. I found that the "yellowest" green rendition in the VFS's adjustment range to work best for my Jupiter observations. The observation I'll describe was during a Europa transit. With the GRS was at Central Meridian, (CM), and at 200x & 240x, it seemed to have 2 different kinds of outlines. The outlines showed as a darker underlining plus a lighter outlining that went almost all the way around it. The BA oval was south of and just preceding the GRS by a little bit. Together both features looked like a squashed figure 8. The BA Oval looked like an obvious light colored hole. Though low in contrast, it was easily observable with the VFS and almost disappeared without it. The GRS was also followed by 2 or 3 white ovals sandwiched between it and Europa's shadow. In the NEB there was a long diagonal rift sloping northwards to the following side. This rift was punctuated by slightly larger white ovals, the largest being at the northern side. This rift was outlined by darker "striping" on both sides. There were 2 dark spots, the darker one inside the preceding side. Associated with this feature in the NEBs were 2 prominent festoons, one following the other. Festoons plainly looked like a row of chimneys with delicate smoke arcing out of them. There were a couple of other minor festoons, but these 2 were the most noticeable. Additionally, this was the first time in a while that I was able to clearly follow Europa across the SEB. As the GRS neared the preceding limb, I noticed the STEBs slope southward. At this time, a white oval feature was just following the CM here in the STRZ. This aforementioned oval was also visible on top of the light colored zone. The reason I mention a list of all this stuff is that they were all plainly "seen" at the "yellowest" green filter setting. My wife, who is not an active observer, also saw all this stuff. :) Sirius One night's surprise is that I split Sirius again for the 2nd time. I used a Variable Filter set to the broadband hydrogen alpha, (red), range plus a MV-1 in the eyepiece. At 428x, Sirius was red with an Airy disc that had only 4 or 5 diffraction rings. The Pup was inside the diffraction rings and appeared powder blue with this filter arrangement. The stars appeared the same way at 480x and 500x. Unfortunately and unlike the first time, nobody else was around to confirm split. Conclusions As I said in the beginning, this filter can dramatically change an object's appearance. This can range from high visibility to complete invisibility. I think that some people might find this "hide and seek" to be fun. I personally found the filter to be most engaging when attempting to fine tune the better views that the VFS is capable of delivering. My experience with various interference filters has made me sensitive to what I call filter artifacts. The Variable Filter System had no such artifacts present anywhere in the wheel's rotation. Any strange and wild looking views can be seen if using an achromat refractor with chromatic aberration. This is evidenced by some "dissonance" at just a couple areas in the VFS's range of rotation. An achromat with chromatic aberration just can't reconcile the focus of some filter peaks at certain adjusted settings. The two settings that come to mind is a "purple" setting and one of the "red" settings where a blue halo intruded so much that I could not achieve any type of sharp focus. As I said previously, this is not an issue with telescope designs where chromatic aberration is absent. Very small rotations allowed me to zero in on the best views. Usually there were at least 2 or 3 so-called best views. Another interesting thing happened was when I was observing with other people. When I recommended adjusting the filter to get the best view to achieve the brightest image with largest extent of nebulosity of say M42, nearly everyone had the filter adjusted to a slightly different area in the VFS's rotation. For some, the difference in where the VFS was set was very different than where I had it adjusted for myself. As with any other filter I've gotten from Sirius Optics, my initial experiences were that they didn't do what I thought they would. In regard to the VFS, it took a little time to get used to the idea that I could wildly change the presentation of any object. That said, I really tried to "get over it" and tune the filter to what I thought was the best possible views. Once I got a feel for it, I found that I could get more than one "best" view. Then I would have to decide on a 1st, 2nd, & possibly 3rd best view. I know that doesn't sound simple, but I guess that's the point of a continuously variable system. In regard to my 6" achromat, I found it best to have the filter's setting somewhere in the predominantly "green" zone to start out with. This makes initial finding and seeing most non-stellar objects easier. Having said this, I almost wished there were an unaffected or clear portion of the wheel to make finding and/or centering objects easier. I believe that the VFS's main strengths and benefits are for planetary observing. The details that the VFS presents of Jupiter's disc had been spectacular. The most detailed rendition was achieved using the "yellowest" green rendition. Though this may be a psychedelic rendition, it calmed down with extended viewing as my eye got used to it. A simple Wratten yellow/green type of filter doesn't even come close to presenting this much detail and do it consistently. There are blue and "neutral" renditions that are also very good for picking out low contrast details inside the equatorial belts. Once again, telescopes without chromatic aberration get the benefit of the filter's entire range when viewing planets. I intentionally left out Saturn observations. Saturnian details are relatively difficult to "filter" out. I guess this has to do with its spectral content. I still consider Sirius Optic's Minus Violet filter, or MV-1, best for the rings. That said, the VFS works to a degree for details on Saturn's disc. Other than what is said in the next paragraph, these benefits are similar to what could be seen with a standard color filter set. I really wish I had the VFS during the last Mars opposition. After four months of using the VFS, I have to admit that it really has blown me away. After about the first three weeks of using it on Jupiter, I was simply not getting enough sleep, it was that good.