[Nass]

Yes, Darren, good work. Very dramatic . Not being funny though, what I'm after is an ND solution that'll give me really good strong colour. Accurate colour!

With welder's glass, are you able to get a faithful colour rendition or does the cast limit you to monochrome/near monochrome? I was under the impression that welder's glass renders very green? Have I been misinformed?

Also there's no EXIF data - what lens were you using & timings?

[Nass]

On another, related note, can I just say that I wouldn't read too much into those first testshots I posted. Just from tonights experimentation, if you rate this foil at 14 stops then the end result isn't that tinted at all. In fact all it takes is a blacks adjustment and a small shift magenta->green to get an accurate colour.

I'd post the evidence/examples but I think this is a bit of a moving ballpark so perhaps rather than boring everyone with endless experiment spammage I'll just post some examples when I have something concrete (likely to be a while, I'm on holiday soon). Unless people want to see the ongoing stuff?

[falconeye]

Unless people want to see the ongoing stuff?

I always love to see other people work

[darrenleow]

Yes, Darren, good work. Very dramatic . Not being funny though, what I'm after is an ND solution that'll give me really good strong colour. Accurate colour!

With welder's glass, are you able to get a faithful colour rendition or does the cast limit you to monochrome/near monochrome? I was under the impression that welder's glass renders very green? Have I been misinformed?

Also there's no EXIF data - what lens were you using & timings?

Sorry about the lack of EXIF data. These photos are linked from Facebook which strips all the data. I'll put the exposure timings here, all of them were shot with the 12-24mm lens.

1: 2 min, f/11
2: 1 min, f/13
3: 1.5min, f/8
4: 6 min, f/13

With regard to the color rendition, the green cast is easily eliminated as long as the viewfinder is blocked off from stray light, since it will show up as a magenta cast when one adjusts the WB to remove the green cast. I just decided to desaturate and play with the colors to add drama, looks like it worked!

Without viewfinder cover in broad daylight, hence the conversion to black and white


Here I remembered to cover the viewfinder. Colors look accurate enough for me, and for the price. I'm not sure why the bottom right corner is so blurred and distorted though.

[MattGunn]

I have searched the web for some info on the AstroSolar foil's spectral transmission. The curves are not yet on Baader's website. I have found one curve, which shows, that the foil has a slightly higher transmission in the blue part of the spectrum (about 2x the transmission in the visible part of the spectrum) and there is a transmission peak around 950nm, where it blocks much less effectively (30x its average blocking in the visual part). So, there is a slight IR excess, but on a very low level, if you look at it in absolute numbers (Gerhard Dangl's Astronomie Ausrüstung) That should not be enough IR to cause noticeable image degradation, as there is the IR filter in front of the sensor.

Ben

I am actually most curious about what Nass is trying to achieve

BTW, here is the spectral curve you mentioned:

((c) Gerhard Dangl)

Good quality short pass filters (IR block) will block around 99% of the IR light (I don't know the specs of the IR cut filters in Pentax DSLRs but I guess that their performance is around this) and so will pass around 1%. If the Baader film is passing aproximately 30 times as much light around 950nm as it does in the visible then the sensor will be recieving the IR around 950nm at aproximately 30% of the brightness of the visible wavelengths which could be significat enough to skwe the colour and could also account for the stray light issues. If this turns out to be the case then you might need an additional IR cut filter.

[Ben_Edict]

Good quality short pass filters (IR block) will block around 99% of the IR light (I don't know the specs of the IR cut filters in Pentax DSLRs but I guess that their performance is around this) and so will pass around 1%. If the Baader film is passing aproximately 30 times as much light around 950nm as it does in the visible then the sensor will be recieving the IR around 950nm at aproximately 30% of the brightness of the visible wavelengths which could be significat enough to skwe the colour and could also account for the stray light issues. If this turns out to be the case then you might need an additional IR cut filter.

Yes, that could be the case. I just don't know. I hope that Baader will release the "official" spectral transmission curve for their filter foil soon, as the curve, we are relying on currently, is measured by a user and we have no idea, how he did that.

A good filter to add to the foil would probably be the B+W UV+IR-Cut front filter, which is available in the usual screw-in sizes.

Ben

[falconeye]

the curve [...] is measured by a user and we have no idea, how he did that.

I trust him.

I mean, after all, a real planet was named after him, the 47494 Gerhardangl !

[MattGunn]

Accurate measurement of the transmision of a filter with an attenuation this high is likely to be difficult and will require a spectrometer with a high dynamic range. If someone wants to donate a small section (about 25mm square) then I will have a go at measuring its transmision with one of the spectrometers I look after at work but I'm not sure what signal to noise ratio I would be able to acheive without trying it.

[Nass]

Matt, I'm away on holiday tomorrow for a few days but when I return I'll send it over.

[Big G]

I use a B+W ten stop filter for very long exposures.

B+W 77mm 3.0/1000x (110) Neutral Density Filter (BW66729) - Warehouse Express

In bright sunlight, it will let me expose for about 1 minute at f/13, if I need longer I will stack a couple of 0.9 NDs on top.

In terms of colour cast, the B+W is very good with a slightly warm tone to it. If you don't like this, it's easily correct in PP.

[Nass]

I'm fairly ready to abandon this quest with Baader astrofoil and chalk it down to experience. Nice try but nil points

I think the critical flaw with this stuff is that it is highly reflective, and as such causes unavoidable light reflections which show up either as a white milky film and/or rings at low mm length. I particularly wanted to use this on sigma 10-20 to be able to stack a CPL but I've found that the B+W filter is everything I wanted this astrosolar stuff to be. These were my defining test shots inside (absence of the rings outside) if you'll excuse my messy kitchen

regular 50



--------

regular 50 with B+W



--------

regular 50 with Baader Astrosolar 3.8



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So, B+W - slightly warmer tone but easily corrected. Astrosolar 3.8 - milkyness, but correctable if it was a one in a million shot and I absolutely absolutely needed ND14 and only ND14.

[falconeye]

Astrosolar 3.8 - milkyness, but correctable

Thank You for the interesting experiment. I've learned from it.
The correction is easy though (black clipping at 36% + Gamma 1/1.5 correction).
So, when shooting the sun with a tele lens and Astrosolar 5, it may still be a good option.

I wasn't aware of the ND3 filter from B+W. I assume otherwise, the thread wouldn't have florished

BTW, Schneider says not to use the ND3 filter to look into the sun as it lets thru too much IR.

[Nass]

Yeah, always worth trying - it's fun to experiment.

Bottom line for me was that circular ring in the image that I always got below 15mm or so - ie in those tests above. If you're pointing anywhere near the sun (ie even 135 degrees away in the other direction) I'd still get it, regardless of the amount of blacking material and hood I surrounded the setup with. That was the real dealbreaker. In low light it's redeemable on anything below 15mm.

The astrofoil has interesting properties. When I look at the histogram produced by it, it is very sharply defined. The whole thing only covers 1/2 the width of the histogram, so there's a lot of stretching to do in there. This almost forces you to ETTR as far as you can but that adds issues of its own.

I'm sure that I'll end up using some of this stuff for the sun though, and it's in my bag as a backup. Fun fun fun

Last note for anyone trying this in the future, the best way to make a filter with this stuff is actually to use one of those cokin square filter metal mount things. It has the thread and is the perfect flanged shape to use with doublesided tape and black insulation tape.