Tests of 9 Different Chromatic Aberration Filters
by Gary Poliquin
First who am I? I am a 68 year old observer who has owned way too many telescopes of most types. I currently own 9 and do most of my observing from orange and yellow zones outside of Chicago.
We would all like to own an nice 5 or 6 inch APO but for many the $ are more than we can afford. So, for refractors, we are limited to Achromats. Of course in the 5" to 6" this means a really long tube to get rid of CA so we usually end up with something shorter and try to contend with the CA.
This brings us to 'minus violet' filters. I have spent some time acquiring 9 different filters suggested both here and by my fellow observers. My comparative tests of these 9 Chromaitic aberration (C/A) supression filters was done totally visually and the results are MY opinions and certainly not hard science.
The test were done from a orange Zone under very transparent skies on a 8 day old moon. I chose the moon as being so bright, CA should be at its worst.
A search of the Cloudy Nights archives (under refractor) turned up a chart entitled 'Equivalent Chromatic Aberration of Achromatic Refractors'. What this chart indicates is that if the scopes CA ratio (f/ratio divided by inches of aperature) = at least 1.3 the CA should be able to be filtered to some extent. My chosen scope was a Explore Scientific AR127 (5") f-6.5 with a CA ratio of 1.3. Since CA shows up more under higher magnification I used a 8mm Astro-Tech ED eyepiece for 103X.
Although these filters tested were in no special order they should be read in my test order as some tested later refer back to ones tested earlier.
Base line no filter:
Very apparent blue/violet ring around the moon especially on the fully illuminated smooth side.
Lumincon MV filter:
The specs on this filter indicate it has a 420NM cutoff. This means it should filter all violet but no blue. I fould this to be a very weak filter which made only a slight improvement to the CA. It added almost no yellow cast to the moon and no noticable dimming.
Kodak K1 photogtaphic pale yellow filter:
These are hard to find and are also were marketed as Nikon Y44, Tiffen #1 and the Heliopan #5. It is a very light yellow filter with a 440 to 450NM cutoff. This means it cuts into the blue spectrum about half way. This filter reduced the CA about 50% with the addition of only a slight yellow cast and only slightly dimmer.
#8 light yellow filter:
Very effectively kills about 90% of CA but adds a definite yellow cast with noticable dimming. Very cost effective but I think too much light loss for smaller (3" - 4") scopes.
81B photographic filter:
Before the advent of the modern MV filters this was the choice for CA suppression. It is still widely available in the 2" format. I found this filter to be very similar to the K-1 tested earlier.
Sirius Optics MV20 filter:
I found no specs on this filter and don't think it is still available except on the used market. It is a little better than the Lumincon on CA but not as good as the K1, However it adds NO discernable yellow cast or noticable dimming.
Celestron Minus Violet:
This is a strong filter that does as good as the #8 yellow but without adding the strong yellow cast. It does, however noticably dim the moon and I believe cuts out too much light for smaller Achros.
Orion V block:
With a 450NM cutoff it does a good job on CA supressing about 80% and does add a noticable slight yellow cast with mild image dimming.
Baader Semi Apo:
Does a very good job on CA suppression (80 to 90%) without adding any yellow cast and only slight image dimming.
Baader Fringe Killer:
I must admit I had some prejudice here because of all the great things I heard about this filter. Pre testing I expected this would be the clear 'winner'. Surprise it was not. It did do a great job on CA but added a definite yellow cast. I found it to be very similar to Orion's V block.
Again these are not scientific findings only my opinions.
- For for very mild CA I like the Lumicon and the Sirius MV 20 (if you can find one)
- For severe CA its the Baader Semi Apo
Hope this helps
Gary Poliquin (Oldtimer)