minus violet filters

[raymo]

Probably an old chestnut, but does anyone have first hand experience of
the efficacy of these filters. They seem to be a bit expensive for me to buy
one and trust that it will significantly reduce the CA on my fast frac.
raymo

[glend]

I have a Baader Semi-APO filter for sale at the moment.

[multiweb]

Haven't heard or used the minus violet filters but I have a Baader Fringe Killer (http://www.baader-planetarium.de/sektion/gemeinsame_dateien/download_filter/fringe_killer_e.pdf) that I've used with Pentax lenses in the past and it did a great job at getting rid of those CA purple halos on bright stars.

[raymo]

Thanks Marc, I'll check and see what's left on the mortgage.
raymo

[doppler]

Here's what a LP blocking filter does with CA in a 120mm f5 achro. Still not cheap but I had one in my box of bits. A purpose made filter would have to be even better.

[raymo]

Yes, a big improvement Rick, from what I gather, a fringe killer doesn't
darken an image as much; in other words it's more selective. I'll have
to try my LP filter though, could be a good alternative.
cheers raymo

[doppler]

Working with a darker image sort of defeats the fast scope idea. I would also like one of those fringe killer semi apo filters in my kit but it's low on the need to get list. Some people have deep pockets, mine are turned inside out.

[Merlin66]

Might be worth doing a trial with a standard photographic -UV filter. These are cheap as chips and may be of acceptable quality....

[raymo]

Thanks everyone, I'll check out the suggestions.
raymo

[raymo]

Which actual filter did you use for those comparison images Rick?
raymo

[doppler]

It's an oldie, I got it in 1985. It's a Parks LPB Filter 28mm. It was about $100 at the time quite expensive. It has a green mirror reflection when viewed at an angle but slightly pink when looking through it. I should try some more experiments with it, last time I only got a few test shots due to bad weather.

[raymo]

Thanks Rick.
raymo

[janoskiss]

Just to put things in perspective a bit:

If a filter blocks 50% of the light that's like going up one f-stop. As far as amount of light per unit area in the focal plane this means: your f/4 would behave more like an f/5.6, an f/5 like an f/7, and more generally a factor of 1.4 increase in focal ratio.

If it has 70% transmission, that's just around half an f-stop: f/4 goes to f/4.7, f/5 to f/6, or a 1.2x increase in focal ratio. If you get a dramatic improvement in CA then it could be well worth it. Also if it's for DSLR photography, then with a good minus-violet the green channel, where often most of your luminance signal is, will suffer very little light loss.

Theoretically, the way to assess whether it's worth using a filter or not is to compare the filtered results with a stopped-down aperture and see which one gives the best image-quality vs photon-quantity compromise.

[raymo]

Thanks Steve, the next time the cloud gods play ball I'll give it a go.
raymo

[Don Pensack]

In the Baader line, 4 filters provide some violet filtration. Note: the higher the cutoff wavelength, the stronger is the minus violet filtration and the shorter the achromat that can be used.
[Aside: divide the f/ratio by the aperture in inches. If 3 or more, no filter is needed. If 2.5 to 3, a gentle filter (low cutoff) will work. If 2-2.5, use a strong minus-violet filter. And, if under 2, use the strongest minus violet filter and keep magnifications low, where CA is least objectionable]
Baader Moon & Sky Glow--~420nm (also cuts some other wavelengths)
Baader Semi-Apo--420nm (very little spectral change)
Baader Fringe Killer--430nm (blues not affected)
Baader Contrast Booster--485nm (this one kills most nebulae somewhat, so it's primarily a Moon/double star filter) (yields a "warmer", more yellow tone because all violet and some blues are gone)

Hope that helps.

[raymo]

Thanks Don, I was amazed at the difference in the two images of M42
that rick posted.
raymo