by Tommaso Massimo Stella


INTRODUCTION


Since few months
Optolong filters has made available the new L-Extreme double narrow band filter intended mainly for acquisition with color sensors.
After the enormous success of the L-Enhance filter, the well-known manufacturer of astronomical filters has decided to focus again on products that allow you to do astrophotography even from very polluted places and, above all, without resorting to monochrome cameras.
Let's quickly see the characteristics of the new filter:
-Double narrow band pass
-7nm for OIII (24nm for L-Enhance which includes OIII and Hb)
-7nm for Ha (10nm for L-Enhance)






From the characteristics it is easy to guess that the filter is suitable for capturing images of emission nebulae therefore not recommended for galaxies, clusters and reflection nebulae.


Before moving on to the direct comparison, let me thank Optolong Filters in the person of Stella Song who went out of her way to let me have a definitive version of the filter so that I can test its characteristics as per project.


I chose the East Veil nebula (NGC 6992) as the subject because it has areas where OIII is predominant compared to H-alpha and vice versa. In some areas the two ionized gases overlap so the filter's ability to contrast and highlight the filaments is put to the test.
The images we will see are the stack (without any processing) of two frames of 300 seconds (250 gain) calibrated with dark but not flat (we will see why) acquired with the following equipment:
-Apochromatic triplet TS FPL53 102/714 not reduced but flattened.
-ZWO ASI 294mc Pro cooled to -5 C (Bortle 5 sky) and 0 C (Bortle 9 sky)

Stacked by Deep Sky Stacker and stretched by  PixInsight 1.8

The acquisitions from Bortle 5 sky were made in Maruggio (TA) while those from Bortle 9 in Taranto. The two places are a few hundred meters from the sea.

The evaluation method is purely visual (even if I tried to obtain equivalent stretches).


BORTLE 5 SKY


From Bortle 5 sky, in the following image, you can see a slight reddish gradient (bottom) on the frames captured with L-Extreme while those captured with L-Enhance look cleaner.
The stretch was performed with STF (channels not connected) on the AutosaveExtreme and the parameters reported exactly identical on AutosaveEnhance.
At first glance, two things are noticed:
-minor presence of stars on L-Extreme
-OIII more pronounced on L-Extreme






Going into the details, a greater "presence" of the OIII zones is confirmed with the L-Extreme filter having obtained better contrast and lower saturation of the stars as shown in the following image






Since the difference in tone between the two images is evident, STF has been manually adjusted (by manipulating the parameters of stretch, edit icon) to make them uniform as best as possible.
As can be seen, the OIII remains much more marked in the AutosaveExtreme.





By examining the differences between the areas where H-alpha is more present, we may have the illusion that the 'red' is less deep in the shot taken with L-Extreme but by observing better some areas with a prevalence of hydrogen, we notices a higher reddish saturation (albeit minimal).
The initial sensation is due to the strong presence of OIII overlapping the H-alpha.





BORTLE 9 SKY


Moving on to the acquisitions from the Bortle 9 sky, more or less the same differences in behavior between the two filters are confirmed but unfortunately the reddish gradient of the L-Extreme becomes very present.
The stretch was performed with STF (channels not connected) on the AutosaveExtreme and the parameters reported exactly identical on AutosaveEnhance.
Also in this case we note:
-minor presence of stars on L-Extreme
-OIII more pronounced on L-Extreme






Normalizing the stretch confirms the greater presence of OIII in the images taken with L-Extreme but, under very polluted skies, the difference is really minimal.
The stars shrink in much the same way.






A surprise instead comes from the acquisition of the H-alpha which seems to benefit most from the narrow bandwidth of the L-Extreme.
In the following image, the higher red saturation is more evident than in the frame taken with L-Enhance (right).





BORTLE 5 vs BORTLE 9 SKY


Let us now compare the behavior of the two filters directly under different skies.
On the left we have the detail of a frame acquired under the Bortle 9 sky and on the right that of one acquired under the Bortle 5 sky.
Let's first analyze the performance of the L-Enhance filter.





As expected, the image on the right is more contrasted. Both OIII and H-alpha are better. The darkest sky always wins, even with narrowband filters.
However, the difference is not dramatic.

Let's now evaluate the behavior of the L-Extreme under different skies.






Once again the L-Extreme filter returns a sharper and more contrasted OIII but in the comparison between skies with different light pollution there is a greater difference between Bortle 9 and Bortle 5 compared to that returned by L-Enhance.


GRADIENTS


As anticipated, the L-Extreme filter has problems of greater gradients than those of L-Enhance but, luckily for us, we also find them in the Flat fields so they are eliminated during the calibration phase.
The following image shows on the left a flat acquired with L-Extreme and on the right the same flat made with L-Enhance (by simply changing the filter in the 2 "filter holder, without removing the optical train).

Optics: TS APO FPL53 102/714 reduced and flattened with 3" TSRED379






The frames in the figure are Master Flat with 21 subframes each acquired with the appropriate Sharpcap tool.
In the case of the L-Extreme, after having calibrated an image with these flats, there is a difficult to eliminate overcorrection that does not depend on the ADUs and/or how the sub frames are acquired.
I did several tests with various light sources and different stacking software (DSS and PixInsight), the result is the same.
Intrigued by the shape of the gradient, I tried to rotate the filter inside the filter drawer without noticing any difference in the final result.
For completeness, other tests were performed with different optics (a Takumar 200mm lens and a Newton 200 f5) always obtaining the same defect.
Only the unreduced triplet 102/714 returned a different flat in the position of the gradient (bottom right and not top) as shown in the following image.






CLOSING THOUGHTS


Did I like the L-Extreme filter? YES
Is it a big step up from L-Enhance? NOT ALWAYS

The contrast on the OIII, as expected, is considerably better than that obtained by the L-Enhance but it is not enough, in my opinion, to decide to upgrade.
It is certainly recommended for those who do not have narrow band filters for color cameras because the yield is at least equal to that of L-Enhance (under very polluted skies) on H-alpha and certainly better as regards the OIII but I would also recommend it to those looking to shoot very difficult objects from low polluted skies.
The only negative aspect to consider is the annoying gradient that is difficult to completely eliminate only with calibration (in postprocessing you can do it), a defect that worsens proportionally to light pollution.

PRO
-Response in the OIII is excellent in any situation
-Response in the H-alpha is equal to or greater than that of the L-Enhance
-You can also photograph from Bortle 9 skies obtaining an unimaginable contrast
-Excellent even from very dark skies

VERSUS
- Gradients difficult to completely eliminate with simple flat calibration





I will close the test with the final image of the Velo Est, the result of 7 hours of shooting under the sky Bortle 9 and 2 hours under the sky Bortle 5 shooting exclusively with Optolong L-Extreme filter





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