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Filtering an achromat for DSLR imaging


parallaxerr

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I'm in the process of putting an imaging rig together, using my ST120 achromat as a starting point. Having seen some great Ha narrowband results with achromats, I've got a Baader 7nm Ha filter to go on my modded Nikon.

However, I would also like to have a go at colour imaging so have been reading lots about reducing the dreaded achro blue/violet fringing. I have imaged previously with the ST120 and a Baader semi-apo filter, but the results still showed significant fringing.

So, I have read a lot over on CN about using a wratten #12 yellow filter, which has the aggressive cut off below 495nm where the blues and violets live. Several posts show some excellent results after the hue has been corrected to account for the yellow cast. One thing I noticed though, is that there are several references to using the filter stacked with a UV/IR cut filter to prevent out of focus IR fringing.

Q. At what wavelength would IR fringing be a problem? My camera is being modded with a 700nm short pass filter so maybe it won't be an issue?

I then looked at the transmission lines for various (Baader) filters and noticed that the contrast booster effectively replicates a stacked #12 and UV/IR cut, notwithstanding a slight extension into the reds (which my camera should cut at 700nm) and with sodium blocking to boot.

Q. Would the contrast booster be a viable alternative to stacking the other two filters? The only difference would be the additional passing of approximately 680-700nm.

I'd be interested to hear any thoughts on this, perhaps someone has tried the same?

Note: I'm not expecting miracles and am fully aware of the limitations of achros, an apo is on the (ever lengthening) wishlist :icon_biggrin:

Cheers

Yellow.jpg

UVIR Cut.jpg

Contrast Booster.jpg

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The yellow-filters are rather a "Poor Man's Version" of the specialty filters from Baader designed to cut the dreaded "blue-tinge" from achromats such as the Baader Contrast-Booster  and the Baader Semi-Apo Filter., and the Baader Fringe-Filter. All of these do about the same thing, but most people have a personal preference among these. But one thing is a constant:

They all impart a yellow-tinge!

Rather like the Baader Solar-Continuum Filter (which imparts a strong green colour to the Solar-Disc) - some folks swear just a regular Green-Filter works just as well.

I have both! :D

Dave - the Filter-Nut

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Hi Dave,

The yellow tinge is not too much of a concern as post processing "should" enable re-balancing of the colours. Looking at the transmission lines, I'm starting to think there's no need to spend out on the more expensive option as the yellow cuts the UV and the camera cuts IR.

I think the fringe killer and semi-apo are more expensive as they're designed to try to keep the colour balance a little more natural, certainly my semi-apo still lets a lot of the blue through, OK for vis but no good for imaging.

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I did some experimenting with an achro on Lunar what I did to get rid of fringing was to shoot a full colour sub then one with a blue filter with the focus adjusted, I then split the channels in PS and added in the new clean blue and combined back to RGB, the only slight issue with this method was that the blue channel was a slightly different size so had to be tweaked.

Alan

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3 minutes ago, parallaxerr said:

Sounds good Alan, I've read of similar approaches using a full set of RGB filters. As you say, the differing image sclaes need tweaking but the results are good.

It works to a degree but If you dont mind a mono image you can just delete the blue channel and convert the others to b/w. 

Alan

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I find this extract from the contrast booster blurb odd, considering there's such a small difference in the IR cut of both filters...

"Combined with the Baader UV-IR Cut filter, the Contrast-Booster makes the ideal filter set for digital imaging."

Maybe this band between 680-700nm is where IR bloat occurs?

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A little further research:

It looks like IR bloat occurs beyond 700nm so my camera with the 700nm short pass should be OK. The UV/IR cut filters are aimed at full spectrum cameras that don't have this feature.

The contrast booster cuts UV at the same wavelength as the Yellow, but having looked at the propoer transmission graph on Baaders website, the contrast booster cuts a lot of other wavelengths too.

So, the yellow filter combined with the 700nm short pass in my camera "should" address both UV/violet & IR fringing.

Bargain.

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31 minutes ago, happy-kat said:

I'm going to try this with my lens that has loss of CA. I think you have startools.

Link here

Yeah you're right Kat, I have star tools. I remember discussing removal of CA previously in the No EQ-DSO thread. I hope to minimise it with filters first then process the rest out. Sounds easy enough, famous last words!

There was a user in the same thread who used to advocate the use of a UV cut filter, I'll have to have a re-read.

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2 hours ago, happy-kat said:

I'm going to try this with my lens that has loss of CA. I think you have startools.

Link here

Hi. Works well with a wratten 8 and a uv-ir cut. Use StarTools' LRGB module to load only the blue.

Autodev

Decon to say 3.2

Restore to wiped and deconvoluted 

Save the new blue

Now load the r and g from the original, but this time use your new blue. Tiny stars limited now by (for me at least) the size of the red.

HTH

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I spent a few hours tonight reworking some old data from my achro that I knew had bad fringing. These two images are from the same original data but I used the techniques in alacants thread to reduce the halos and violet fringing. I couldn't reduce a few of the stars as they were just too saturated and I didn't make any effort to draw detail out, it was just an exercise in bloat control. Anyway, here's the before & after results...

I hope with a little filtering at the point of acquisition that acceptable results can be had, then I can perhaps add some Ha in for more luminance and reds.

15025437_10154697384486639_3830039733074220669_o.jpg

Fringe Killed.jpg

Fringe Killed.tiff

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