Galaxy imaging in CRGB (Clear filter) [Deep Sky] Acquisition techniques · Dan_I · ... · 23 · 1094 · 5

Hi,

is there anyone using a clear filter instead of a L filter for imaging broadband targets with a mono camera and reflective optics? 

I'm considering trying this to improve the SNR of my images, as the Wynne corrector of my Newtonian astrograph is well-corrected enough in the near IR according to his designer.

It would be also useful for galaxies near the galactic plane (obscured by galactic dust) and to detect distant quasars.

Thanks,

Dan

Joey Conenna:
The Airy disk (minimum spot size) increases with longer wavelength, so your ability to resolve fine detail will take a hit with that increase in SNR you'd get by picking up NIR signal.

Thanks Joey, that's a good point.

My scope has 250mm aperture. At 900nm Dawes limit is 0.9".

Isn't it safe to  assume that the lower resolving power would be hidden by the seeing ?

I've done some test imaging comparing going naked verses using a uv/ir cut. You can see the result here:

https://app.astrobin.com/i/0789ya

It's clear that you are throwing away a lot of energy by cutting off at 400 and 700nm.

Depending on what you are trying to achieve it's certainly an option that shouldn't be rejected out of hand. How useful it is on an all reflecting system will come down to how well your reducer is color corrected.

Joey Conenna:
The Airy disk (minimum spot size) increases with longer wavelength, so your ability to resolve fine detail will take a hit with that increase in SNR you'd get by picking up NIR signal.

Joey Conenna:
The Airy disk (minimum spot size) increases with longer wavelength, so your ability to resolve fine detail will take a hit with that increase in SNR you'd get by picking up NIR signal.

Thanks Joey, that's a good point.

My scope has 250mm aperture. At 900nm Dawes limit is 0.9".

Isn't it safe to  assume that the lower resolving power would be hidden by the seeing ?

Theres actually an offset to this, while the minimum size for the disk is increased, the operational values derived from seeing do not typically approach this value under normal conditions; the FWHM will be more or less averaged out due to the reduced seeing effects on the system in IR up until about 900nm. In many cases with average seeing conditions (1.5-2.2" for my location), the FWHM with IR is typically 1.2-1.6".

Under higher "visible light only" FWHM values, you will actually see decreased total FWHM's due to the averaging of the seeing when IR is included. IR seeing reduction is about .2-.3" most commonly.

This offset in FWHM minimum values only becomes an issue when seeing supports values lower than the minimum resolution provided by IR.

Shooting with IR itself, at least with my system, the minimum FWHM is about 1.15", Even on superb nights where in visible it would show 0.9", it cannot go below the aforementioned 1.15", but when imaging IR on average nights, I can collect data that can supplement good night's visible spectra data quite well due to the reduced seeing effect.

IR above 750nm is quite hit-or-miss to be honest, you begin to wander into larger wavelengths, structures in objects change (sometimes dramatically), and the transparency of the atmosphere becomes a roadblock itself.

The sweet spot for IR pass filters are found in the Bessel I or Sloan i' bands.

My suggestion is to actually shoot "luminance" using one of these IR filters, or any other reasonably good IR pass (There are issues with halos on most cheaper IR pass filters- it's extremely bad, Baader 685+ IR pass is about a baseline for where quality IR filters start), and both using it as L, and soft-adding it to the R channel in post processing.

I've found clear-glass imaging to be fun, albeit with an OSC, my stars end up slightly washed out color wise.

V:

Joey Conenna:
The Airy disk (minimum spot size) increases with longer wavelength, so your ability to resolve fine detail will take a hit with that increase in SNR you'd get by picking up NIR signal.

Joey Conenna:
The Airy disk (minimum spot size) increases with longer wavelength, so your ability to resolve fine detail will take a hit with that increase in SNR you'd get by picking up NIR signal.

Thanks Joey, that's a good point.

My scope has 250mm aperture. At 900nm Dawes limit is 0.9".

Isn't it safe to  assume that the lower resolving power would be hidden by the seeing ?

Theres actually an offset to this, while the minimum size for the disk is increased, the operational values derived from seeing do not typically approach this value under normal conditions; the FWHM will be more or less averaged out due to the reduced seeing effects on the system in IR up until about 900nm. In many cases with average seeing conditions (1.5-2.2" for my location), the FWHM with IR is typically 1.2-1.6".

Under higher "visible light only" FWHM values, you will actually see decreased total FWHM's due to the averaging of the seeing when IR is included. IR seeing reduction is about .2-.3" most commonly.

This offset in FWHM minimum values only becomes an issue when seeing supports values lower than the minimum resolution provided by IR.

Shooting with IR itself, at least with my system, the minimum FWHM is about 1.15", Even on superb nights where in visible it would show 0.9", it cannot go below the aforementioned 1.15", but when imaging IR on average nights, I can collect data that can supplement good night's visible spectra data quite well due to the reduced seeing effect.

IR above 750nm is quite hit-or-miss to be honest, you begin to wander into larger wavelengths, structures in objects change (sometimes dramatically), and the transparency of the atmosphere becomes a roadblock itself.

The sweet spot for IR pass filters are found in the Bessel I or Sloan i' bands.

My suggestion is to actually shoot "luminance" using one of these IR filters, or any other reasonably good IR pass (There are issues with halos on most cheaper IR pass filters- it's extremely bad, Baader 685+ IR pass is about a baseline for where quality IR filters start), and both using it as L, and soft-adding it to the R channel in post processing.

I've found clear-glass imaging to be fun, albeit with an OSC, my stars end up slightly washed out color wise.

Thanks for your message and for sharing your experience. It conforts my feeling that under normal seeing conditions there would be no resolution penalty.

Tony Gondola:
I've done some test imaging comparing going naked verses using a uv/ir cut. You can see the result here:

https://app.astrobin.com/i/0789ya

It's clear that you are throwing away a lot of energy by cutting off at 400 and 700nm.

Depending on what you are trying to achieve it's certainly an option that shouldn't be rejected out of hand. How useful it is on an all reflecting system will come down to how well your reducer is color corrected.

I’m getting a 404 error when I try your link.

Tony Gondola:

Tony Gondola:
I've done some test imaging comparing going naked verses using a uv/ir cut. You can see the result here:

https://app.astrobin.com/i/0789ya

It's clear that you are throwing away a lot of energy by cutting off at 400 and 700nm.

Depending on what you are trying to achieve it's certainly an option that shouldn't be rejected out of hand. How useful it is on an all reflecting system will come down to how well your reducer is color corrected.

I’m getting a 404 error when I try your link.

Sorry, I had just removed it from my gallery.
No filter is on the left. The difference was obvious even during capture. The 585 does have pretty good sensitivity beyond 700nm so results will very depending on the system and sensor.

Very interesting and encouraging!

I have finally ordered a clear filter from Baader (same thickness as my ZWO filters). I hope to make tests soon and I'll report the results here. 

I will try also imaging in CGB, i.e. using a synthetic R+IR layer, obtained as  C- (G+B) , for the red channel. Maybe not for the stars, but at least for the galaxies. It could be potentially very useful to bring colors to very faint distant galaxies in the background.

Clear skies,

Dan

You could later on get a Bessel or SDSS I band filter, and use that for the IR, as the IR does have differences in signal and resolution on average nights. Using it as L, or as an addition to R can increase the detail in any target you shoot as long as the FWHM for the IR is lower or equal to the FWHM for RGB. Some nights you may experience RGB Below the IR limits.

It will be interesting to compare L and C of the same integrations taken in the same night back to back, and LRGB and CRGB again with same integration.

Update : I got my best 60sec luminance sub ever with this scope using the clear filter :  median 1.36" FWHM according to FWHMEccentricity.  It is not a scientific proof, but for me it validates the fact that letting the IR light hitting the sensor does not impair resolution.

Tony Gondola:
Glad it worked out for you. It certainly works for me. There aren't a lot of free lunches in this hobby but this little change might well be one of them. The graph below pretty much tells the story.

Thanks Tony. If you add the light pollution spectrum in the mix, you get free lunch and free diner as well 

Interesting discussion!  I wonder if one way of making the best of the extra near IR signal while minimising colour distortion would be to create an HDR Composition of combined  luminance with the UV/IR filtered image used to describe the core and inner parts and the clear filter image luminance merged so as  to describe the halo and fainter outer arms where it adds most to the signal - and there is little colour anyway?