Imaging luminance with a clear filter - Forums

Topic starter

Nov 17, 2023 · Edited: Nov 17, 2023

Hi,

my main interest in astrophotography is to get detailed images of galaxies.

In principle, with a monochrome camera one could use a clear flilter rather than a UV/IR cut filter for shooting the luminance data on broadband objects, if the optics are well-corrected at long wavelenghts.

Having a Newtonian astrograph, it seems a worthwile option to consider, especially since my backyard is heavily light-polluted and IR is less sensitive to light pollution.

In terms of resolution, the PSF would a bit larger but with my 8" mirror I should still be seeing-limited. Rayleigh's criterion gives 1" at µm while my seeing is usually in the 1.5"-2.5" range. And I will soon get a 10".

Are there downsides that should be aware of ? Maybe atmospheric dispersion could be an issue? Or chromatic aberration from my corrector (TS Wynne 2.5")?

Thanks,

Clear skies,

Dan

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Guillermo (Guy) Yanez

Nov 18, 2023

Hello Dan. If I get this correctly, you are considering shooting the luminance subs without cutting any wavelenght at all. That may or may not work for you, so the best advice would be to give it a try and let us know the results

. Galaxies do provide interesting data in the NIR bandwidth. That said, the deeper into IR you go, the larger the diffraction pattern for those wavelengths, so that is one thing you might want to consider. As you go further out in the infrared, the worse it will get.
Cuiv has this video where he shares his thoughts on the benefits of a dedicated NIR filter under light polluted skies. It might be interesting to check that option too. https://www.youtube.com/watch?v=vUX0KqfZ37U&ab_channel=Cuiv%2CTheLazyGeek

Cheers

Guy

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andrea tasselli

Frequent contributor

Nov 18, 2023

Atmospheric dispersion is definitely not one concern (in fact it reduces the longer the wavelength is) but color correction would be a most definite hurdle and all would depend on the subject (some are brighter beyond 670 nm than other so ymmv). Personally I don't think it adds much to anything once the cut-off is set at around 700nm after which the QE will start to drop precipitously past 800nm and being nearly nil at 1000nm (for typical CMOS sensors anyway). If anything it would blur things a bit but probably the luminance will be brighter. Unless you entirely image past 700nm I don't see any advantage at all w.r.t. light pollution. Obviously, adding refractive elements to the light path does engender additional risk of chromatic aberrations but again it all depends on the specific of your CC.

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dkamen

Nov 18, 2023

Another thing to consider is that the innards of your telescope may be dark/absorptive in visible but very reflective in infrared.

I have seen this with my TS Photon, when I was imaging without any filter it was impossible to make flats that would work. The moment I added a UV/IR cut filter, problem was solved.

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andrea tasselli

Frequent contributor

Nov 18, 2023

Another thing to consider is that the innards of your telescope may be dark/absorptive in visible but very reflective in infrared.

I have seen this with my TS Photon, when I was imaging without any filter it was impossible to make flats that would work. The moment I added a UV/IR cut filter, problem was solved.

That's very true.

Dan_I

Topic starter

Nov 19, 2023

That may or may not work for you, so the best advice would be to give it a try and let us know the results

Thanks Guillermo. The reason for asking before giving a try is that (1) I don't have a clear filter and (2) I don't have any free slot in my filter wheel...

andrea tasselli:
If anything it would blur things a bit but probably the luminance will be brighter. Unless you entirely image past 700nm I don't see any advantage at all w.r.t. light pollution. Obviously, adding refractive elements to the light path does engender additional risk of chromatic aberrations but again it all depends on the specific of your CC.

Thanks Andrea. Brighter luminance is what I'm after, but a blurred image is my main concern... Regarding the chromatic correction of the CC in the IR, I don't have such information but I may ask Massimo Riccardi who designed it.

Another thing to consider is that the innards of your telescope may be dark/absorptive in visible but very reflective in infrared.

I have seen this with my TS Photon, when I was imaging without any filter it was impossible to make flats that would work. The moment I added a UV/IR cut filter, problem was solved.

That's a good point thanks. The interior of the OTA is entirely lined with Protostar flocking material. It is less efficient in the near IR (about 15% reflexivity) but it may be OK nonetheless

https://www.fpi-protostar.com/img/flockreflect.gif

Eduardo Rigoldi Fernandes

Nov 19, 2023 · Edited: Nov 19, 2023

I changed the Optolong 36mm Luminance filter to the Baader Clear 36mm filter, upon the suggestion of @Paulo Cacella. The choice took into consideration the camera I use, the ZWO Asi 294mm, which is not a top-of-the-line camera from the brand itself, but has a Quantum Efficiency (Qe) that extends a bit more into the lower frequencies of IR than more modern CMOS cameras.

ZWO ASI 294MM:

Optolong filter in the white line:

Clear filter:

The replacement of the L filter with the C filter was recent, but there is a noticeable detection of small stars that do not appear in the L filter but are detected in the C filter. Initially, the only difficulty I noticed was with the IR from the remote observatory's WiFi cameras where my setup is installed. We had to turn off the IR.

The weather has not been cooperative, but soon I will conduct tests with the L and C filters simultaneously, on the same object, on the same night, and with the same setup, to demonstrate the difference. I am happy with the exchange.

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Dan_I

Topic starter

Nov 19, 2023 · Edited: Nov 19, 2023

Eduardo Rigoldi Fernandes:
I changed the Optolong 36mm Luminance filter to the Baader Clear 36mm filter, upon the suggestion of @Paulo Cacella. The choice took into consideration the camera I use, the ZWO Asi 294mm, which is not a top-of-the-line camera from the brand itself, but has a Quantum Efficiency (Qe) that extends a bit more into the lower frequencies of IR than more modern CMOS cameras.

Very interesting post Eduardo thanks! Looking forward to seeing the results of your comparison.

I presume you also image with a Newtonian right ?

My ASI183mm camera is not as good as the 294 in the deep red/near IR but it is not that bad.

33% absolute QE at 750nm for the IMX183 vs. 37% for the IMX294: http://www.astrosurf.com/buil/asi294mm/

Freestar8n

Nov 19, 2023

I use an Astrodon C filter with EdgeHD11 and see no impact of chromatic aberration. It lets more signal in and has no disadvantages I'm aware of. It may not work well for refractors - but seems fine for EdgeHD.

It's a particularly good match if you use photometric filters such as Sloan or Johnson-Cousins for RGB, since they also extend into the IR a bit.

Frank

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Eduardo Rigoldi Fernandes

Nov 19, 2023

Eduardo Rigoldi Fernandes:
I changed the Optolong 36mm Luminance filter to the Baader Clear 36mm filter, upon the suggestion of @Paulo Cacella. The choice took into consideration the camera I use, the ZWO Asi 294mm, which is not a top-of-the-line camera from the brand itself, but has a Quantum Efficiency (Qe) that extends a bit more into the lower frequencies of IR than more modern CMOS cameras.

Very interesting post Eduardo thanks! Looking forward to seeing the results of your comparison.

I presume you also image with a Newtonian right ?

My ASI183mm camera is not as good as the 294 in the deep red/near IR but it is not that bad.

33% absolute QE at 750nm for the IMX183 vs. 37% for the IMX294: http://www.astrosurf.com/buil/asi294mm/

Hello Dan!

Note that the IMX294 sensor is not the one used by the ASI 294MM; instead, it is used by the ASI 294MC (color).

The Asi 294MM uses the IMX492 sensor, and its Quantum Efficiency (Qe) at 750 nm is 60%, according to the ZWO graph.

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Alex Varakin

Jan 4, 2024

I have done imaging without any filter with my RCs and it was good. The only reason I am not doing it now is that I use OAG and guider does not have consistent focus between filters and no filter.

Alan Brunelle

Jan 4, 2024 · Edited: Jan 4, 2024

I have done some NIR work over the last couple years. If you have a fully reflective system, then it can achieve focus of all wavelengths of light at the same focal plane. That does not mean that the wavelengths can focus equally well. IR images will be less sharp than vis. If your camera has decent efficiency around 900-1000nm or higher, there is a strong signal from most stars, with redder stars becoming brighter and blue stars get dimmer. But the added signal will "look" defocused compared to just the vis image. It's not because of a chromatic aberration as seen in a refractor, but just a function of longer wavelengths not being able to focus as sharply.

Emission nebulae disappear in NIR specific imagery. So those wavelengths will contribute nothing to many common targets. NIR images of galaxies can be interesting, but lose sharpness and detail. I don't see NIR signal contributing positive details to an otherwise vis image. Likely just reduce detail.

If your astrograph has ANY refractive components, be it corrective plate, comma corrector or field flattener/reducer, you will be adding refractive aberration to the focus issues. This includes using a clear "filter" since the steep light cone of an astrograph will be prone to refraction through a clear glass. I have no idea of why you would use a clear filter vs no filter in a purely reflective system.

IF you have light pollution issues, it would be the correct thing to use a light pollution filter instead and cut NIR and UV. You also make no mention of UV which presents still other issues.

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