After decades of observation through Newts I have decided to try out the 'dark side' and buy a small refractor. I have a Bresser Messier AR102/f5.9, 600mm fl OTA on the way but I need some advice on that achromatic bug bear - color correction.
Not having to deal with this before, I am lost in a sea of options if I check out the net, what is a good filter solution for achros?
I have both the fringe killer and semi-apo filters from Baader Planetarium. I would recommend the semi-apo. It reduces CA on Sirus by about 80%. Your version of the AR102 is even faster than mine . The semi-apo does reduce light throughput very slightly where the fringe killer doesn't. However, the yellow tint introduced is much more pronounced in the fringe killer than the semi-apo. Its very slight in the semi-apo filter.
Thanks Adrian, I had looked at those two. What do you think of this one, the Contrast Booster - which seems to incorporate some of the Fringe Killer features along with the Moon and Skyglow:
Thanks Adrian, I had looked at those two. What do you think of this one, the Contrast Booster - which seems to incorporate some of the Fringe Killer features along with the Moon and Skyglow:
Hard to know which is a better choice by just reading, but I'll take your word on the Semi-APO.
I have no personal experience with that filter. I read good things about it. I think its down side was that it reduces light through put even more then semi-apo filter. Its more suited to larger achromats.
The Baader contrast booster is not a Moon and Skyglow + Fringe Killer, because that's exactly what the Semi-Apo filter is. The contrast booster only cuts IR with Moon and Skyglow. The Semi-Apo and Fringe killer cut the blue spectrum by 50% and as such would be better for achro refractors.
The Semi-Apo does seem to reduce noticeable CA, but as Adrian mentioned dims the view somewhat. It's good for bright objects, best left out for fainter ones. CA isn't noticeable on faint objects anyway.
If you want something cheap that works, use an ordinary yellow filter. The Semi-Apo ain't cheap, nor does it completely cure the CA.
As Kevin mentioned, the semi-apo is a combination of the Fringe-Killer and the Moon&Skyglow filters. See image: http://stargazerslounge.com/uploads/...1379628240.jpg
I think this was done to try and correct the yellow colour imbalance that the Fringe-Killer exhibits. A handy benefit of this is that it would help reduce the effect of light pollution from Sodium Vapour streetlights.
I can't comment from experience, however I did research these filters extensively to be used with my StellarVue SV70 doublet which exhibits severe purple/blue fringing.
In the end I decided I would have to buy a number of the filters to find one that worked to my taste and that would have cost more than the scope!
I then decided the best option was to wait for a better corrected doublet (or even splurge on a fluorite doublet or a good triplet) to pop up in the IceTrade section, but I've always just missed out on those that do come up for sale. Gotta be quick around here!
I went through a very long period of experimentation with Achro's and a vast array of different CA filters (not to mention many others). I personally did not like the Baader Semi-Apo filter as I found it imparted too much of a green hue in comparison to other filters.
Now, IMHO the absolutely best filter to compensate for CA (and one of the best general secrects in AA) isn't actually a filter which identifies itself as a CA filter but rather a LPR filter.
With the luxury of hindsight the filter I would recommend without a 2nd thought for compensating for CA is in fact the Astronomik CLS filter. It also does a great job at LPR.
I personally did not like the Baader Semi-Apo filter as I found it imparted too much of a green hue in comparison to other filters.
A green hue? Are you sure you had a Baader Semi-Apo filter? I don't see any added green hue on the Moon, Jupiter or any bright stars. Just a slight light yellow hue on the Moon and bright planets.
You could be correct as it is near a year since I had my last CA filter. I certainly had the Baader Semi-Apo and personally found the stacked combination of the moon and sky-glow filter with the fringe killer better than the single semi-apo (which is supposed to be these two combined anyhow). Whatever the case, I found the filter to significantly alter the general hue of the image through the ep as a product of controlling the CA. However, the Astronomik CLS diminishes the CA without imparting a huge hue in the process - hence why I thoroughly recommend it as the best even though controlling CA isn't its intended purpose. The Baader Contrast Booster is certainly the strongest CA filter I have encountered and if you are using something with terrible CA this is the one to use but it also gives the strongest hue..
Guys,
You all know you can't use a "filter" to turn an achromat into a full blown APO. It's impossible! If it were, these "wonder filters" would be selling like hot cakes.
All filters just reduce the light transmission at nominated wavelengths.
There is one filter, which I can assure you gives 100% APO performance and that's a very very narrow band OIII/HB/Ha filter!!
Doesn't let any other wavelengths through - that's a bit of a problem....
I use Edmund achromats in the Spectra-L200 spectroscope and have to accept that the "best" focus varies about 1.2mm from the Green/blue to the green/ red - such is life.
For "normal" astro work, the UV "halo" seems to be worse problem.
This can be removed, without really affecting the other wavelengths with a UV-IR filter (or the ol' UV blocker filters that Lumicon used to sell)
Depends if Glen wants it for visual or photo too. There's a couple of options photo wise. The achro should be fine for narrow band imaging with the filters Ken mentions. I've also discovered that RGB imaging through an achro reduces the overall appearance of CA, as long as each filter is focused separately. OSC is darn near impossible to do on bright stars, but like visual use fainter star fields seem fine.
For visual use, something like the Semi-Apo is probably the best bet. I don't see any colour cast with mine. Looks neutral to me.
Visual only guys. I don't expect any filter to deliver APO performance. Just looking for a consensus on good CA control, or good enough to not distract too much from the view.
To add another one. Baader long pass yellow.Cheaper than the others. Look up Agena Astro.Think it gets a lot of mentions in the CN thread. Semi-apo took too much light for me, the Fringe Killer turns it a bit yellow. Never used it as I'm happy enough with a Fringe Killer and a WO minus V (thanks Richard) actually the WO is better, more natural light.
Matt
After decades of observation through Newts I have decided to try out the 'dark side' and buy a small refractor. I have a Bresser Messier AR102/f5.9, 600mm fl OTA on the way but I need some advice on that achromatic bug bear - color correction.
Not having to deal with this before, I am lost in a sea of options if I check out the net, what is a good filter solution for achros?
Different "Minus Violet" filters have cutoffs at different places. The longer the wavelength of the cutoff, the more color tinting appears to happen because more of the violet and blue disappears.
In the Baader line, from weakest to strongest, with approx cutoff:
Moon & Skyglow 400nm (this one has some "notches elsewhere in the spectrum)
Semi-Apo 420nm
Fringe Killer 475nm
Contrast Booster 480nm
Note: 475-480nm is considered deep blue.
400-420nm is considered deep violet.
There are, of course, other "minus violet" filters, but these I am fairly familiar with.
It depends on the scope as to how much taming of the violet is necessary.
An f/6 achromat is likely to need a lot.
I'll attach a chart showing CA versus f/ratio in different apertures.
Thanks for that Don, it is very useful. So my 102mm/600mmFl F5.9 refractor is in the range of filterable, that's good news. When you say an f6 will "need alot" what exactly does that mean in terms of these cutoff numbers?
My Baader Contrast Booster filter arrived and it works very well. I didn't have a lot of CA but the Contrast Booster pretty much eliminates what I could detect around the limb of the moon and the sun through a Solar filter.
. The semi-apo does reduce light throughput very slightly where the fringe killer doesn't. However, the yellow tint introduced is much more pronounced in the fringe killer than the semi-apo. Its very slight in the semi-apo filter.
