Below is the English translation of a post from Mr. UTO's (Japan) blog.:
https://oozoraashiato.blog.fc2.com/blog-entry-2676.html (<origianl in Japanese)

Now that I finally had some time, I wrote about what I felt after trying out the IDAS DTD filter.

One of the features of the IDAS DTD filter is that it is designed taking into consideration, the ion tail wavelength of comets - in a similar way a competitive filter called "Comet Band Pass filter" is, but the major difference of the DTD filter is that it is designed to take the full advantage of the features of the latrest color sensors where the DTD passes through the near-infrared region.

Naturally, you can expect to capture comets' dust tails (continuous light from the reflected light of the sun) in that regard. And furthermore,
by utilizing the anti-light pollution characteristics of NIR, one can possibly capture nebulae and star clusters more clearly with the DTD than what one can expect without, even in dull skies.
The latest color sensors may also have near-infrared applications such as LiDAR. It has the characteristic of being transparent to infrared rays.

I would also like to take a look at what happens when astrophotography is performed with such a sensor.


M83　Epsilon 200 SVBONY SV705C IDAS DTD filter 2 minutes x 60 frames.　

Well, the most surprising results were obtained with M83 shots. There was a lot of water vapor existing in the air, and the path to the M83 was strongly pollued with city lights from the downtown. I thought it was a condierably tough condition for shooting where the city lights were reflecting on the vapor particles.


C/2022E3 ZTF comet Epsilon 200 SVBONY SV705C IDAS DTD filter 3 minutes x 8 frames.

I sometimes needed to increase the gain of the camera to 280, and under such a gain, the saturated section(s) became magenta. In other words, I am located in an area that is strongly light-polluted to that extent! If I remember correctly, the 3rd magnitude star of the Big Dipper could be barely observed in the sky at the time of the M83 shootings.

The degree of the light pollution was getting reduced after the midnight, but even so, I didn't expect I could have successfully captured the arm of the M83 so clearly under those inferror conditions! It was a surprise!!!

Also, please note that the HII region of the galaxy, which is difficult to capture with a color camera, is clearly shown as a red dot, just because the short wavelength band and near-infrared region are added to the properties of this nebula booster.

If we want to clearly capture the HII region of extrasolar galaxies, we also need to consider the red shift.
With a general narrow band filter with a FWHM of 10 nm or narrower, the H-alpha emission line shifts to the longer wavelength side even when shooting relatively nearby galaxies such as the Virgo cluster at a distance of about 40 million light years.This red shift makes such narrower band pass filter ineffective in capturing the H-alpha line..

Since the DTD filter has a wider bandwidth, amateur astrophotographers can capture almost any of the nearby galaxies with this filter and sucessfully be proud of the HII region capture results.

Although I was able to capture the faint arms of the galaxy even under the adverse conditions, I was left with an impression where depiction of M83's dark bands, which could have been more attractive, was a little weak due to NIR shootings. Also, it is easier to achieve color balancing with the DTD than the GNB filter (because the DTD added with the short-wavelength transmission characteristics), but I felt it was somewhat difficult to achieve in general with the DTD.

For suburban cities, I thought it might be a good idea to prepare images taken with an infrared cut filter as well and pick the best of both filters.

NGC1973-77　Epsilon 200 SVBONY SV705C IDAS DTD filter 2 minutes x 15 frames
* Imaging at an altitude of around 20 degrees.

Now, regarding possible ghosting that I worried about with the DTD filter, the shots of Betelgeuse that I used for focusing certainly had conspicuous circular ghosts.
However, I didn't feel any ghosting from this shot of the Running Man Nebula. I thought it was nothing to worry about. Yet, I think it will be difficult to suppress ghosting around Alnitak (Horsehead Nebula, Burning Jupiter Nebula) although I have not tried shooting it.
In addition, the characteristics of the Nebula Booster plus NIR transparency are effective even if it was shot towards the westward sky at a low altitude under bad conditions when capturing the ZTF comet.
The image taken seemingly appears clearly and beautifully.

Trifid Nebula M20　Epsilon 200 SVBONY SV705C IDAS DTD Filter 2 minutes x 36 frames.

Last but not least is the Trifid Nebula. I've taken many shots of this nebular at my house so far, but this is the first time I've been able to capture a blue reflection nebula as if it were enveloping the surroundings. I assume that the NIR transparent characteristics of the DTD are effective.
I hope you can feel from the shot that the stars got blurred due to a hight humidity. Yet, the ghosting of the bright stars does not bother me even in this nebula.

As for the dark band of the Trifid Nebula being an emission nebular, the detail taken is not inferior to that taken of extrasolar galaxies (seems a little weaker though...) Rather, the effect of this Nebula Booster came out where the red nebulae were captured very well, and the reflection nebulae were captured better than what I expected, thanks to the NIR effect.This shot is the best one among others that most effectively made use of the DTD filter.

By the way, with the latest near-infrared transparent color sensors in combination with the characteristics of such as the DTD and the GNB filters', color balancing can be attained by the use of visible light, and imaging is done by the use of NIR. So to speak, one can expect a "one-shot LRGB composite" effect with the use of such filters.

On the other hand, the near-infrared effect produces good images, but the saturation of stars and reflection nebulae becomes inevitably low, and enhancement in image processing is essential. Emission nebulae shots retain the same appearance and saturation as they are, so image processing is somewhat difficult. But with StellaImage (a Japanese software), it is possible to emphasize colors other than red with the Lab Color Enhancement function, so there is not much trouble.

Having said that, looking at this image of M20, I felt that the colors of the stars were not rich, except for the reflection nebulae.
This may be due to the filter's own characteristics, or may be due to shooting with high gain. Anyhow, I think it is certain that near-infrared transparent filters will lower the saturation of the objects in continuous light.

When using the GNB filter, I sometimes thought that it would be better to let the wavlengths of 650 nm or longer pass through for monochrome sensors, but of course, if you use such a filter with a color camera, only red can be clearly expressed as the on-chip color filter varies in characteristics from one to another. It is not hard to imagine that it can hardly obtain a decent color balance, then.

Naturally, it is extremely difficult to color balance galaxies with the GNB filter. Basically, if there are two colors in visible light, it is possible to express continuous light, but I had the impression that extrasolar galaxies are a little greener.
After all, it is easier to achieve color balance with a DTD filter that transmits three colors.

It takes about 2 minutes per image, so I think it's taken in a way similar to telescopic observation, but the image is brilliant. Eextending the exposure time per image would simplify image processing, but I didn't feel it was beneficial in terms of depiction as I use a bright optical system of F4.

Various factors such as the low dark noise characteristics of the SVBONY SV705C, the high sensitivity characteristics of the near-infrared rays of the IMX585 sensor, and the DTD filter making use of the NIR region have contributed to the great shots that were taken this time. The results are far better than what I had naturally expected a year or so ago before those gears actually came out!

As others have said, I think the time has come to do filter work with color cameras as well. Cameras and filters with such wonderful characteristics have come out.

In the metropolitan area, I think that the GNB filter is more beneficial due to the intensity of light pollution, but in the case of a local city, this DTD filter is easier to take color balancing (wider transmission equals to more photons), and thus recommendable.

The DTD filter seems quite useful even when shooting under bad conditions or at a remote location, so I think it's worth having one.
In addition, molecular clouds, in particular, glow even in the near-infrared, so I think it may be possible to expect good effects even when shooting under dark skies.

Also, I am looking to a large format sensor coming out with high near-infrared sensitivity. At present, the main applications are for industrial and automotive uses, so a 1/1.2 type of IMX585 is probably the largest we can expedct for the maker.
I wish it would come out in 4/3 format at least, but even now, if you combine the IMX585 with an RASA8, it is good enought in terms of angle of view. Tthe performance should be even better if it is combined with a short-focused, faster F-numbered optical system that is aberration-corrected up to the near-infrared.