Use of Sloan SDDS Photometric Filters for Distinguishing Two NIR Bands

[Tom Lewis]

Seems one could use astronomical filters for false color terrestrial NIR photography. I see there is the Sloan Digital Sky Survey (SDSS) photometric system. I believe these are offered by Alpine Astronomical, Antlia, Asahi Spectra, Baader, and probably others. For some of the brands, the highest wavelength filter is bandpass, and for others it is lowpass.

Two of the filters in this photometric system sit in the NIR. One has mean wavelength of 748nm and another's is 893nm. I believe the cutoff wavelengths are around 688-850nm and 812nm (lowpass). If these could be detected separately, and then each assigned to a false color, then maybe the viewer could distinguish between the content in these two NIR bands.

astronomytechnologytoday.com/2021/12/08/baader-sloan-sdss...

articles.adsabs.harvard.edu/pdf/1996AJ....111.1748F

I noticed you can get 2" Sloan SDSS filters that are designed for telescope eyepieces. Here is an example of an adapter that probably could be used to mount these to a 49mm photographic lens thread  https://www.bhphotovideo.com/c/product/1580708-REG/sensei_sdrpa_4948_pro_49_48mm_aluminum_step_down.html.  Depending on the lens, there might be some vignetting.  

 

Even the 2" Sloan filters are not cheep.  Are there other filters available for these bands that are less expensive because they don't conform to the Sloan system standard?

I'm wondering if anyone out there has tried this with either Sloan filters or other filters? 

And I'm wondering if anyone knows whether there is likely any differentiation in the content for common subjects here on earth between these two NIR bands?

[Andy Perrin]

We have had many (many many...) threads on what the board calls TriColour images, not just in infrared but also in ultraviolet. I believe one of the best of these are Stefano's thread here, where he uses different wavelength LEDs instead of using bandpass filters:

https://www.ultravioletphotography.com/content/index.php?/topic/4650-how-to-make-a-tricolour-image-my-method/

 

Another great one is Bernard's work in UV using bandpass filters from Omega Optical:

https://www.ultravioletphotography.com/content/index.php?/topic/3500-fullfalse-colour-uv-photography/&fromsearch=1

 

There are many bandpass filters for sale online from various manufacturers, with no need to restrict things to astronomical manufacturers. In particular, Thorlabs has an excellent selection of small filters (25mm):

IR: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=16235

Vis through UV: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=1860

These can often be found for less money on eBay, but I've been very pleased with them.

 

Edmund also sells bandpass filters but they are very expensive. 

 

Most recently I have become a fan of the MidOpt bandpass filters:

https://midopt.com/filters/

(And again, one would be a fool not to check eBay for them first, before buying from their official resellers.)

[Andy Perrin]

Quote

And I'm wondering if anyone knows whether there is likely any differentiation in the content for common subjects here on earth between these two NIR bands?

(The bands are 748nm and 893nm center wavelengths.) For those two wavelengths, there is significant differentiation for water content in particular, and we have had a lot of fun with the "dark water" phenomenon, which has been thoroughly explored. It is maximum at 976nm but noticable by 893nm. 

 

My post back here of it at 980nm: https://www.ultravioletphotography.com/content/index.php?/topic/3505-dark-water-and-the-nir-absorption-peak/

 

Eka's pictures at 950nm: https://www.ultravioletphotography.com/content/index.php?/topic/4166-censored-nude-and-water-950-nm/#comment-39603

 

Fandyus's pictures at 980nm and a new cheap filter: https://www.ultravioletphotography.com/content/index.php?/topic/4732-black-water-with-a-silicon-sensor-for-739/#comment-47600

(There were more posts than this, but I'm not trying to be exhaustive.)

 

 

[Tom Lewis]

Thank you, @Andy Perrin.  I need to absorb. 
 

Tom

[Ming]

12 hours ago, Tom Lewis said:

I noticed you can get 2" Sloan SDSS filters that are designed for telescope eyepieces. Here is an example of an adapter that probably could be used to mount these to a 49mm photographic lens thread  https://www.bhphotovideo.com/c/product/1580708-REG/sensei_sdrpa_4948_pro_49_48mm_aluminum_step_down.html.  Depending on the lens, there might be some vignetting.

 

Tom, welcome to the forum.  You can remount 2" astronomy filters in 52mm filter rings.  See the discussion at https://www.ultravioletphotography.com/content/index.php?/topic/3473-cannot-understand-how-to-adapt-the-baaderu-to-my-camera/

I did that with a BaaderU and couldn't be happier.

 

 

[Tom Lewis]

Thanks, @Ming.  I probably have some old 52mm filters sitting around from which I could extract the ring.  When you remounted your astronomy filter, did it reduce vignetting?  

[Ming]

The remounted BaaderU works fine on EF 40mm f/2.8, but it does cause vignetting on EF 28mm f/2.8.  However, both lenses have severe UV chromatic aberration toward the periphery.  Vignetting is a lesser concern.

 

[ulf]

These filters are dichroic and designed for a rather narrow beam angle with the light coming in a direction normal to the filter surface, the way it does from a telescope.

There will be differences in blocking characteristics and transmission in the corners if you try to use them on a wide angle lens. That is the case for all dichroic filters, including the BaaderU.  

[Tom Lewis]

Ah, thank you @Ming and @ulf.  
 

If I did this, I would consider using Midwest BP Series filters like @bvf did in this post.

 

I am now coveting the BP695, BP735, BP810, and BP845.

[Andy Perrin]

Yes, I strong recommend using a medium telephoto lens, like 70mm or longer, with dichroic filters. That’s just how things work out. 

[Tom Lewis]

56 minutes ago, Andy Perrin said:

Yes, I strong recommend using a medium telephoto lens, like 70mm or longer, with dichroic filters. That’s just how things work out. 

GNK

[Andy Perrin]

26 minutes ago, Tom Lewis said:

GNK

What does that mean?

[Tom Lewis]

27 minutes ago, Andy Perrin said:

What does that mean?

Good to know.

[Andy Perrin]

15 minutes ago, Tom Lewis said:

Good to know.

Did you misspell the abbreviation? Seems like it should be GTK then, not GNK!

[Tom Lewis]

37 minutes ago, Andy Perrin said:

Did you misspell the abbreviation? Seems like it should be GTK then, not GNK!

Oh my, I must be getting old!

[Ming]

Thank you, @ulf.

 

[Andrea B.]

MaxMax has a nice set of 3 IR bandpass filters, beautifully made. But I've never been able to see all that much difference between the photos made with them. At least at ground level for typical landscapes. Aerially, maybe more differences are apparent?

LINK

 

Here was my feeble attempt using the Maxmax color bandpass filters to make a tricolour image. I should really try again. These were kinda lame-o. LINKIE

 

Foliage and leaves tend to be equi-reflective in IR from 700-1000 nm. Although, some foliage/leaf types may be more or less reflective than others. (Example:  Pine/spruce needles are not as IR-reflective as leaves of deciduous trees.)

The sky tends to appear darker as IR wavelength increases, but in a tricolour image the sky will usually become some uniform false colour. Referring to MaxMax's examples, it is the man-made stuff - chairs, patios, houses - which provide some interesting tricolour possibilities.

 

****

 

Do note that it is important to verify that IR astro filters do not pass any visible (or possibly uv) light. Sometimes "light leaks" which are important to photography are not considered a problem depending on what deep space object is being viewed. (Yes, I know I need an example here.)

[Tom Lewis]

Thanks, @Andrea B..  I especially appreciate the info about the how the variation in spectral reflectance for foliage and leaves can be limited.

 

Those filters are expensive!