Sirius Optics Minus Violet Filters
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
Myths and Facts
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.
#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.
#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.
#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.
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.
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 - email@example.com - Grand Rapids, MI
report cover the minus-violet filter from Sirius Optics. It
filter arrived from Al and I expected it would be a couple of
I had last laugh as more was clear than cloudy and it got
was very low and as I said, seeing was poor. I first
back to Jupiter. I tried blinking the filter in and out
improvement was dramatic. The purple smear along the edge was
I was focusing and moving back and forth, I noticed a
Al mentioned, there is also a slight LPR (light pollution reduction) effect with
clouds claimed Saturn before I had a chance to see it. There
one point I felt like I was going from the scope to a
point I hadn't anticipated is that some of the biggest
and wind were both coming on so I went to Castor. As I
the 8 and 11 easily split it with or without the filter.
is short with very little actual observing actually
Part Two - Intermission...
is part two of the minus-violet filter from Al's group.
was a gap in the clouds where I could observe Saturn so I
the first thing I noticed was how much easier it is to
Division was present without the filter but very sharp
the filter I could see some detail on the planet and as I
did get my wife to look through and her comments may offer
had trouble finding best focus on Saturn (having just come
the best comment was one she saved for last. She thought
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 - firstname.lastname@example.org - USA