Dear friends,
spoiler alert: if you have a Celestron Edge + the Celestron LF reducer, and you are thoroughly happy with the results you are getting – think about not reading further: artefacts you recognize once you will see forever. No way to get back. So be warned.
The Celestron Edge series is a quite popular commonly used telescope, arguably, most of the long focal length photographs here on astrobin are done with his type of telescope. And I love my Edge 11 very much and am thoroughly happy with the performance.
However, I cannot say the same for the large field reducer (which come at 1250 Euro in Europe). Mechanically it is quite a solide piece of hardware, but its blue performance is wanting. As an example I show a minimal processed image of NGC 6946.
The data: 12x3min each in RGB. Run through standard WBPP in PixInsight, channel combined, then DBE, then PhotometricColorCalibration, noise reduced with NoiseXterminator, the mild stretch with arcsinh (to saturate colors), then masked stretch and a bit of HT. Whether conditions where ok, but not the best (no clouds, seeing about 2.5”). For my equipment, I have a ZWO 294mm with 31mm optolong filters. (and see the same effect with my EOS Ra)
On the first view it looks ok, but at closer inspection, in particular in the corners, the blue channel is misbehaving showing radially directed blueish flares around bright stars, while the red and green channels are flawless.
If I take the corrector out, the result is much better – no blue flares. Well, you may argue I got my backfocus wrong, or my collimation off, or I just got a lemon, or … or.. May well be, but I went to this beauful new search engine on astrobin, checked for the reducer and pixel peeped what others get – and those blue flares seem not to be uncommon, to say the least, including on award images. But none of the edge images suffer from this, when I checked unreduced Edge images.Also, when I go through various use groups (astrobin, cloudy nights), I found a considerable number of people complaining about the Edge reducer.
What to do?
First, I checked my narrow band images, and could find none of these effects in my Oiii data, so the reducer seems to be sufficiently well corrected at 500nm. Next thought: the optolong filters go deep into the blue, ie they are transmissive well below 400nm. So I added a Baader IR/UV cut filter (which cuts near 420nm, like the Astronomik L3) … but could not see any improvement. Finally, I found the Baader fringe killer. It cuts at 480nm, with a 50% transmission between 440 and 480 nm. And is actually seems to be (almost) a cure: Still a bit of a residual of blue flaring – but much much much better, as shown in the following image.
On the down side, one is losing about 1/3 of the light, so the flats, under otherwise identical conditions, took about 6 sec vs 4 sec. So my conclusion for the Edge aficionados are:
1) If you are on the market and thinking about buying the reducer – put your money rather in getting a larger sensor camera and bin.
2) If you need the FoV and do narrowband: the reducer works quite well
3) if you do RGB and don't need the FoV: use the unreduced Edge and bin (for me this is now my standard config for galaxy and PN work)
4) If you do RGB need the FoV: well, the reducer with the Baader fringe killer in the image train may be at least a work around (you actually may consider to use the fringe killer only to get good stars, as the flares only are visible at high contrast situations, like around bright stars).
Hope some of you may find these tests helpful. Or, even better, have a better solution. As far as I am concerned, I probably have to stars putting money aside to eventually get a FF camera ...
Matthias
spoiler alert: if you have a Celestron Edge + the Celestron LF reducer, and you are thoroughly happy with the results you are getting – think about not reading further: artefacts you recognize once you will see forever. No way to get back. So be warned.
The Celestron Edge series is a quite popular commonly used telescope, arguably, most of the long focal length photographs here on astrobin are done with his type of telescope. And I love my Edge 11 very much and am thoroughly happy with the performance.
However, I cannot say the same for the large field reducer (which come at 1250 Euro in Europe). Mechanically it is quite a solide piece of hardware, but its blue performance is wanting. As an example I show a minimal processed image of NGC 6946.
The data: 12x3min each in RGB. Run through standard WBPP in PixInsight, channel combined, then DBE, then PhotometricColorCalibration, noise reduced with NoiseXterminator, the mild stretch with arcsinh (to saturate colors), then masked stretch and a bit of HT. Whether conditions where ok, but not the best (no clouds, seeing about 2.5”). For my equipment, I have a ZWO 294mm with 31mm optolong filters. (and see the same effect with my EOS Ra) On the first view it looks ok, but at closer inspection, in particular in the corners, the blue channel is misbehaving showing radially directed blueish flares around bright stars, while the red and green channels are flawless.
If I take the corrector out, the result is much better – no blue flares. Well, you may argue I got my backfocus wrong, or my collimation off, or I just got a lemon, or … or.. May well be, but I went to this beauful new search engine on astrobin, checked for the reducer and pixel peeped what others get – and those blue flares seem not to be uncommon, to say the least, including on award images. But none of the edge images suffer from this, when I checked unreduced Edge images.Also, when I go through various use groups (astrobin, cloudy nights), I found a considerable number of people complaining about the Edge reducer.
What to do?
First, I checked my narrow band images, and could find none of these effects in my Oiii data, so the reducer seems to be sufficiently well corrected at 500nm. Next thought: the optolong filters go deep into the blue, ie they are transmissive well below 400nm. So I added a Baader IR/UV cut filter (which cuts near 420nm, like the Astronomik L3) … but could not see any improvement. Finally, I found the Baader fringe killer. It cuts at 480nm, with a 50% transmission between 440 and 480 nm. And is actually seems to be (almost) a cure: Still a bit of a residual of blue flaring – but much much much better, as shown in the following image.
On the down side, one is losing about 1/3 of the light, so the flats, under otherwise identical conditions, took about 6 sec vs 4 sec. So my conclusion for the Edge aficionados are: 1) If you are on the market and thinking about buying the reducer – put your money rather in getting a larger sensor camera and bin.
2) If you need the FoV and do narrowband: the reducer works quite well
3) if you do RGB and don't need the FoV: use the unreduced Edge and bin (for me this is now my standard config for galaxy and PN work)
4) If you do RGB need the FoV: well, the reducer with the Baader fringe killer in the image train may be at least a work around (you actually may consider to use the fringe killer only to get good stars, as the flares only are visible at high contrast situations, like around bright stars).
Hope some of you may find these tests helpful. Or, even better, have a better solution. As far as I am concerned, I probably have to stars putting money aside to eventually get a FF camera ...
Matthias
