Sirius Optics Minus Violet Filters

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Review

Over the past year there has been some serious debate within the achromatic refractor community as to how to deal with false color.  One company has proposed an apochromatic lens system to completely correct an achromatic telescope.  Al Misiuk at Sirius Optics on the other hand decided to create a simple filter to remove excess violet from the image produced by a conventional achromatic refractor.

The filter arrived promptly five business days after it was shipped to me.  It came packaged extremely well and arrived upon my first inspection in perfect shape.  The construction of the body of the filter seems to be of high quality and generally is as good as my Orion color filters.  It also features threads for 1½ inch eyepieces as well as threads for stacking other filters.  The glass was also free from any noticeable scratches or flaws.

While this is not sold as a total solution to the false color problems created by a conventional achromatic system Al created this to be a quick and simple fix for the problem.  All of the tests I performed with this filter were on my 120mm f/8.3 Orion refractor.  It should also be noted that false color seems to affect people in different ways, I am one person who is really not bothered that much by it but I do notice it on my telescope.  I have also set up this test to get to the bottom of some rumors and claims that have been circulating the internet, mainly to find out if this finder helps focusing, acts as a semi-LPR filter, and aids in the splitting of doubles.

For this review I have carefully selected a few of the brighter stars that traditionally give refractors quite a bit of trouble.  I was also looking to include stars of various colors to see how removing one part of the spectrum would affect their overall color.  I also looked at a bright double as well as the moon.  The selected stars were Spica in Virgo and Arcturus in Bootes while the double I selected was Mizar in Ursa Major.  For comparison I looked at each object without the filter, with a light yellow (#8) filter, and with the minus violet filter.  I chose the #8 filter because in the past it has been my primary means of removing false color on this telescope for planetary viewing.  All of the tests were done with my generic 25mm and 10mm Plossl eyepieces and seeing was very good to excellent.

The Stars

The first subject of my experiment was the bright blue star Spica in Virgo.  At about mag. 1.2 this star is the bane of an achromatic refractor.  Without the filters the star shows as a very bright blue/white point with a well pronounced violet halo.  It should also be mentioned that most of the field of view also suffers from a bright bluish tint.  After placing the #8 filter into the eyepiece the halo and the direct false color are greatly reduced.  As the star is focused the final image becomes more of an emerald green as opposed to the correct blue/white tint.  I then reevaluated the scene without any filter before placing the minus violet (-V) filter on the eyepiece.  With the –V filter in place a great deal of the violet halo and direct color disappear, the surrounding view is also very nearly free of the bluish tint that haunts a conventional view.  The in focus view of the star is now a mixture of white and yellow/red with white being the primary color and the yellow/red peaking out at the edges.  Both inside and outside of focus the image is an aqua blue.

My next test was on the bright orange/yellow Arcturus in Bootes.  The main reason I chose to use this star in the test was to measure how the filter affects the visual color balance.  I wanted to see if the star could maintain its orange/yellow color with the filter in place.  This star is somewhere in the range of mag. .1 and is one of the brightest stars in the sky.  The filter free view shows an extremely bright star – so bright that it is in fact difficult to bring to exact focus with a conventional refractor.  While the direct false color was not as evident as it is on Spica there was a good deal of violet present in the glare of the star.  It should also be noted that the nearby mag. 6 – 7 star was not visible within the glare.  The entire field also exhibits a bright orange glow.  With the #8 filter in place almost all of the violet was removed from the direct glare of the star, but there was still a soft halo surrounding the star – almost worse than without the filter.  However the focus was sharp and the nearby star was visible well outside of the glare from Arcturus.  Lastly I screwed the –V filter into place.  Almost every trace of the violet glare had been removed as well as the slight violet halo.  The final color of the star was now red/orange instead of the orange/yellow.  The star was also much easier to bring to focus than without the filter as much of the glare had been removed, however the nearby star was not visible with the –V filter in place at 40x.

The Double

While not the most challenging double in the sky I chose this one simply to test some of the rumors and to see how the filter reacts on dimmer stars.  This pair consists of a mag. 2.4 primary and 14” away a mag. 4.0 secondary.  Both of the stars are almost pure white with a very slight touch of blue (most likely caused by the refractor).  Mizar also exhibits a faint violet halo caused directly by the achromat.  With the –V filter in place at 100x the stars were easily split and seemed to focus just a touch easier.  The background sky seemed a touch darker and the split was also a bit more noticeable.  All color was removed from Mizar and both stars had a white/yellow appearance as opposed to the pure white/blue before.

While this was perhaps not the best test for this telescope I will be running more tests on the double star capabilities of this filter when I can get a good view of the Double-Double in Lyra.

The Moon

Sadly I am not able to test this filter on either Jupiter or Saturn from my home site.  From other reports the findings seem to be quite good and I really wanted to see for myself.  Perhaps I will be able to get to a good site with an open Western horizon in the near future.  But for now the moon would have to do.

I figured this would probably be the greatest test for this filter.  Although the amount of color my telescope picks up on the moon is not great it has always been the hardest to remove with other filters.  A normal view of the moon at 100x shows a thin blue line around most of the high contrast areas of the moon.  On the moon the major issue was the color change, while not as severe as a #8 filter the moon really picks up a golden hue.  However focusing did become easier as much of the blue fringe had been eliminated I also did notice a touch more contrast even though this may have been caused by the slight dimming of the moon by the filter.  Both inside and outside of focus there is still plenty of false color but it seems that in focus it is a bit easier to catch a color free view of the moon.  If you can stand the color shift this filter is quite useful for lunar observations.

The Myths and Facts

Online there have been a few myths about this filter that I had tried to at least check out during my tests.  While some of these myths may hold some truth for the most part it seems that this filter really only does its primary job with any type of certainty.  These results are only preliminary and I will be testing these other uses fairly robustly over the next month or so.

Myth #1:  The LPR Effect – While this is not as much a myth as a claim from Al at Sirius Optics that has been taken out of context.  In my tests I found that yes the background is darker but this does not necessarily mean that contrast was higher.  This filter would remove the glow from a Mercury light and a bit of the MH but these lights are not very common today unless you live near a stadium.  The most common lights are HPS or Sodium lights that burn yellow/orange and MH or Metal Halide lights that burn in the very light blue area of the spectrum that is really not covered by this filter.  In direct tests side by side with my Celestron LPR filter I found that the –V filter did not significantly increase contrast.  The end result is that if you want a good LPR filter get a LPR filter not a –V filter.

Myth #2:  This Filter Helps with the Splitting of Doubles – While I did not give any real tough doubles a shot this claim may hold some truth.  I found that Mizar was cleaner with the filter in place than without it.  But I also found this to be true when the #8 filter was in place so I see no real benefit between the –V filter and a simple color filter. 

Myth #3:  Focusing is Easier – Once again I did find that focusing was easier with the filter in place.  Both bright and dim stars as well as the moon seemed to find that magic point of focus with less effort than usual. However once again the #8 filter afforded me the same ease of focus and in addition allowed me to find the star near Arcturus that was not visible with the –V filter at 40x.  I would have to say though that the –V filter did excel at providing better focus on the moon and most likely the planets.

Negatives

The only real negative I found to this filter was the shift of color.  Blue stars become aqua and white stars become yellow/white.  The moon looks like a giant piece of gold but the planets (from other reports) do quite well.  In the overall scheme of things you must balance this against your dislike of the violet glow that accompanies an achromat.

Conclusion

While this filter at first sounds as if it is a miracle cure for false color even Al from Sirius Optics will tell you that it is not.  It does not turn your scope into a $2000 APO but it does offer a simple and affordable solution for cutting back the amount of false color you may see.  Sure there are the drawbacks such as a shift in the overall hue of the object that you are looking at for the most part this is fairly well controlled.  The bottom line is that if you are a conventional refractor owner and bothered by false color you will be quite happy with the results of this filter.  Even if false color is not as bothersome to you as it is to some you could probably still find a good place in your collection for the –V filter.

Submitted by Curt Irwin - irwincur@excite.com - Grand Rapids, MI

Review

This report cover the minus-violet filter from Sirius Optics. It costs $40 shipped. It is NOT the same as the older "minus-violet" once offered by many (and now making a comeback). That filter was designed for Black & White photo work. This uses dialectic coatings in the same manner as a light-pollution or nebula filter only the cutoff points are different. What it does is turn in excellent near-APO performance (roughly equal to an ED glass objective) from a conventional (color-filled) achromat. The review is in two parts.

My filter arrived from Al and I expected it would be a couple of days before I could use it. It was raining when I came home but when I took the dog out before bed, I noticed it had cleared up. can go back together) but I was out the door in time to see the clouds coming in. Ever notice how the wind in the trees sounds like someone faintly calling "Sucker!"

But I had last laugh as more was clear than cloudy and it got better. Unfortunately, seeing was not good and clouds came back, so the report is brief. Almost all observing was done using Plossl was also used at times. Clouds got me before I could try an Orthoscopic. It was cold and windy, which made vibration a cold attic and was therefore in somewhat thermal eq. If it clears, I may go out again later for a look at the moon.

Jupiter was very low and as I said, seeing was poor. I first focused without the filter and saw the familiar purple glow. Let me add that outside of losing detail, I am really annoyed by the glow, more so than most others. I suppose this is due to growing up on reflectors and a SCT. When I first purchased the scope, I was disappointed with how much color there was. Reading reviews had led me to expect less. Again, it seems to be a personal matter as to how disturbing the purple glow is. I suspect those who are most bothered will most appreciate this filter.

Anyway, back to Jupiter. I tried blinking the filter in and out by hand, as is done with a narrow-pass filter to find a planetary. It looked fairly dramatic so I threaded the filter in.

The improvement was dramatic. The purple smear along the edge was gone. The seeing still produced a slight boiling along the edge, but it cleared, I was now limited by seeing. Without the filter the two equatorial belts were easily seen, but with the filter I could see a bit more and easier. I think a newcomer to the planet would have really appreciated the filter as things were clearer.

As I was focusing and moving back and forth, I noticed a difference I hadn't anticipated. The telescope is easier to focus with the filter in place. Even on Jupiter, I could focus without having to concentrate on the moons. Later, I tried this on a number of stars of varying brightness. Without the filter, there is still a slight range where focus is almost equal. But with the filter, there is almost an audible "click" as it comes to focus. Any movement in either direction is now obviously out of focus. I hadn't thought of this before, but it makes sense now that I have seen it. Again, I suspect that this will be a personal matter, with some observers noticing a big difference here and others noticing less improvement. It would be interesting to be able to test each observer's sensitivity at different wavelengths and see if this is a part of it or if it is just personal taste and preference for color free viewing.

As Al mentioned, there is also a slight LPR (light pollution reduction) effect with the filter. The sky is definitely darker, though the effect is not as pronounced as the Orion wide band filter I have.

The clouds claimed Saturn before I had a chance to see it. There was some haze and light clouds moving around so I tried a few brighter stars. Procyon, Castor (more on that), Pollux, and Arcturus were available and gave a range of colors. Again, the effect was impressive. In the past, even when I knew it was focused, there was always a nagging desire to focus the blue or check for fog. That was gone. Holding the filter and blinking (difficult with low eye relief) produced a big difference. The LPR effect was nice, giving a slightly darker sky. Blinking it by hand gave a sense of actually changing telescopes. However, I am not use to blinking and at one point was confused when the glow was almost as big with the filter as without. Then I realized I had breathed on the filter and fogged it. This might help some to relate. The effect of the filter was as great as unfogging the objective (I live less than two miles from Puget Sound; Lots of water = lots of fogging)

At one point I felt like I was going from the scope to a photograph. This is partially an artifact of the blinking technique. But the biggest cause for me was that suddenly it looked like a reasonably crisp photo where stars are points and not purple smears.

Another point I hadn't anticipated is that some of the biggest changes were not in the very bright stars, but those somewhat fainter. It wasn't until I went back without the filter that I realized I could not get the images as sharp without the filter. Even when I was not consciously disappointed with the purple smear, it had still been there, degrading the image. I had not expected this and was reminded that I like tight little points of light for my stars.

Clouds and wind were both coming on so I went to Castor. As I mentioned, I did get to plan out the night's targets and was operating by memory, trying to beat the clouds. Castor was the only bright blue double I could think of.

Both the 8 and 11 easily split it with or without the filter. However, without the filter, it was like seeing two approaching headlights in a fog. They were discernable, but the glow was bothersome. With the filter, there was clearly dark sky between them. Again, the change in focus was noticed. Even with the vibration from manual focus, with the filter, it all but "clicked" when I reached focus.

This is short with very little actual observing actually accomplished. I will post more as I observe again. The summary is there is still some false color and there is some yellowing. I think this will again be a matter of personal preference. I would opt for more filter (ie, less blue and more yellowing). I suspect the double star observers would be split (pun not intended) with those going for close separations preffering a stronger filter and those going for colors opting for less filtering. The filter made a big improvement on Castor. It sounds like the chromacor does more, but for the first time, I am very satisfied with this scope. I really like the way it clicks in on the focus and now enjoy each image it throws up. Now if I could get Jupiter to reverse course…

Part Two - Intermission...

Hello again,

Here is part two of the minus-violet filter from Al's group. Again, observing is done through a 150mm Synta refractor under fairly light-polluted skies. Unfortunately the sky did not cooperate and I only was able to observe for a short while.

There was a gap in the clouds where I could observe Saturn so I tried some different tactics from last night. Observing was done with a 6mm UO Orthoscopic. To the best of my knowledge, this is a true Abbe design. I figured this eyepiece was pushing the limit on seeing with Saturn down so low, but I deliberately wanted to see how the filter would respond under these conditions. I also asked my wife to observe and will share her impressions as well.

Again, the first thing I noticed was how much easier it is to focus with the filter when looking at a planet. Either Titan or a star of the correct brightness was present (I haven't checked to see where Titan should have been) and I could check focus with the star. For stars or planets, there is now a definite "focused" point where the image snaps into focus.

Cassini's Division was present without the filter but very sharp with it. As with last night, the filter brought a sharp edge to the planet that revealed the effects of seeing. Without the filter, I could still tell the seeing was bad but I was obviously operating under the double handicap of seeing and chromatic aberration. After looking with the filter and then removing it, I kept wanting to refocus the image even when focus was the best I could get. After observing with the filter, it always seems out of focus when the filter is out.

With the filter I could see some detail on the planet and as I mentioned, Cassini's division was sharper than I have ever seen through this scope, even under much better seeing conditions. Without the filter I would not have even looked through the 6mm for more than a second and would have gone down to an 8 or 11mm eyepiece. With the filter, the 6mm held up well and if the clouds had held off, I would have tried a 5mm next. Unfortunately I was not able to do so. (I had to drive my wife back to her office---Hey, someone in the family has to make money to buy telescopes  :-) By the time I got back, the clouds had closed in completely.

I did get my wife to look through and her comments may offer better insight than my own. First, she is not an experienced observer. She has had maybe four or five 20-second glances through this scope so far and has been disappointed with most of her views. The only view she liked was the moon through colored filters (which would have had a similar effect). At the same time, she is an experienced artist with a great eye for color. She once attended a workshop taught by Bet Borgeson (do a web search and notice her prices) and Bet offered her a job doing backgrounds, layering color on color to produce the results she wanted. So, while she is not experienced at the planets, she has a far better eye for color than I do.

She had trouble finding best focus on Saturn (having just come Saturn). She thought the filter was in and commented that it didn't look like the filter did much. I then put the filter in. She now focused quickly and was amazed (her word "amazing!") Her description was that with the filter, she could see a subtle range of colors on Saturn that were completely washed out when the filter was not in. She also commented that "that split in the rings" (Cassini) was seen without the filter but with the filter it was sharp. I thought back to a review by Ed Ting (I think it was the Questar 7 review), that said when even the public notices the quality difference…

Perhaps the best comment was one she saved for last. She thought for a while and then said, "The difference is that if you call me outside to see the first view, I am going back inside as soon as I look. But I will stay out and get cold with you to see the view with the filter." For my wife, that is huge.

It should be noted that Chuck does not have any affiliation with the people at Sirius Optics.  This review is his personal and unbiased experience with the equipment.

Submitted by Chuck Taylor - chucktaylor3@qwest.net - USA