Celestron Edge + Reducer - some experiences and experiments [Deep Sky] Acquisition techniques · GalacticRAVE · ... · 16 · 1186 · 6

GalacticRAVE 5.87
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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. Bildschirmfoto 2022-10-23 um 19.52.03.pngThe 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.

Bildschirmfoto 2022-10-23 um 19.52.38.png


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.  Bildschirmfoto 2022-10-23 um 19.53.12.png 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
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sunlover 10.46
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Exactly! I had similar results with Celestron reducer for EdgeHD11. So I also image without it. Best regards, Vitali
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jschella 0.00
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Also the same with the EdgeHD 800 and reducer. It's amazing that this has not been talked about that much until recently.  

Jason
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GalacticRAVE 5.87
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Jason: what is your workaround if I may ask? not using the reducer at all?
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jschella 0.00
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Sorry, I thought I had already responded to this, but I can find the response.

Right now, I am imaging at F/10.  Celestron was kind enough to let me exchange my reducer for another one. Just in case it another one works better.  I am now waiting for the new reducer to arrive.

I think, at a minimum, I will take a look and see if there is any aberration with the O3 filter.  Since the Ha and SII seem OK, and the O3 is OK, I can then at least image with the reducer for my NB and keep using F/10 for the LRGB.  This will help reduce the NB imaging times and speed things up a bit.

Cheers,

Jason
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HotSkyAstronomy 2.11
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In my experience (don't look at my AB, I used up all my free images and have to get a subscription soon it, doesnt show ANY of my new good images after I got my EQ6R.), I've found that the reducers have been hit or miss, my one for my edge 8 has the same blue fringin issue at the edge of the FOV of a full-frame camera, but not with the APS-C, but in my case, the red is also slightly fringed, albeit less than the blue channel.
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jschella 0.00
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For what it's worth, I did an experiment to see if by changing the backfocus distance I could reduce the blue aberration.  This was not successful.  I've attached the results for the L, R, and B filters. This shows a close up of stars in each of the corners except one (no star bright enough was there). And a number of different BF distances were used. NOTE (1) : that my optimum backfocus is ~108 mm (not 105.5).  NOTE (2) : seeing was not great that night.

You can see that the L and R were able to produce nice round stars in the corners. 

You can also see that the B filter produced not so nice stars in the corners. The shape was unaffected by changing BF distance.

It should be noted that I sent these images to Celestron and was told that they are with specs for the reducer (though they still agreed to replace mine).

Reducer_BLU.pngReducer_LUM.pngReducer_RED.png
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jschella 0.00
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In my experience (don't look at my AB, I used up all my free images and have to get a subscription soon it, doesnt show ANY of my new good images after I got my EQ6R.), I've found that the reducers have been hit or miss, my one for my edge 8 has the same blue fringin issue at the edge of the FOV of a full-frame camera, but not with the APS-C, but in my case, the red is also slightly fringed, albeit less than the blue channel.

***
I am using a QHY268M (APS-C). I did look at how big a FOV I could get without seeing the blue "fringing" and it ended up being smaller that my F/10 FOV.  If it had been bigger I would have stuck with the reducer and just cropped in. But I didn't want to lose any of my meager FOV.

Jason
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Linwood 5.76
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Jason Schella:
For what it's worth, I did an experiment to see if by changing the backfocus distance I could reduce the blue aberration. ...


Thank you for posting this.  I think you saved me a wasted evening. I've gotten acceptable results with the reducer but only by cropping, and the fact that stacking and registration cleans up a bit.  Generally I have just stopped using the focal reducer.
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GalacticRAVE 5.87
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note added to my thread: Eddgie at the cloudy nights forum pointed out that the flaring in blue is kind of expected if you look at the ray tracing result on page 22 of the EdgeHD white paper (tough this is still at 480nm, towards the green end of the B filter).
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GalacticRAVE 5.87
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Jason: consistent with my finding, I tried hocus focus in blue with a external focuser to see whether I can improve the results by varying back focus - with no success.
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rhedden 9.48
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I also was unable to get an APS-C sensor to work with my EdgeHD 1100 and the standard 0.7x reducer.  I had the blue flares, with pink flares on the inside, and the stars in the corners were not even round.  They were cross shaped and very ugly.  My reducer is old (2013) so it might be different from the ones being discussed here.  There is no sense in imaging at f/10 because the field of view is not much different than what I can get at f/7 with my old CCD camera and its smaller sensor, which does not have color fringe problems.  My APS-C camera now lives permanently on my refractor, and the EdgeHD is still imaging with the CCD camera and 0.7x reducer, just like it was in 2016.

I have also found over the past 10 years that the stars are much smaller when I shoot HOO narrowband - and I don't think it's the well depth of the camera or exposure length.  I suspect the different wavelengths of light do not come to focus at the same back-focal distance with the reducer in place, so broadband subs are not truly focused.  I noticed my Astrodon E-series filters appear not to be parfocal with the EdgeHD and reducer.  I need to re-focus whenever I switch to blue, but green and red are about parfocal.   Hmmmm.   I am no expert in optics, so take it with a grain of  salt.  I can always get lower FWHM values with HOO subs compared to RGB, no matter what exposure length is used, and there are no color fringes with HOO.
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jhayes_tucson 22.30
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What you are seeing is an optical aberration called transverse color.  I don't know how transverse color is affected by the back spacing and I can't tell from the way that you posted your data.  What I can tell you is that I've had two reducers for my C14 Edge and neither one performed very well--no mater how I set it up.  The tolerances are very tight for this kind of component and I don't think that Celestron is up to the task of meeting the tolerances in regular production.  To be fair, I also have a reduce for my Astro-Physics GTX130 that is equally poor, which is an indicator of how hard it is to make a good reducer.  My feeling is that you are almost always WAY better off getting a camera with a bigger sensor than trying to add a reducer.  I've seen a few reducers that have worked really well but it a crap shoot with terrible odds of getting a good one.  Most folks who buy a reducer end up paying for it AND then buying a camera with a bigger sensor.  So, skip the middle step and just go to the bigger sensor in the first place--it's cheaper and better.

John
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jsg 8.77
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Thank you for sharing your experience.   I have the Celestron 8" Edge HD scope which is brand new and sitting in a case, I haven't been able to test it out yet due to constant rain. 

I've been thinking of getting the reducer, but you're definitely making me re-think this.   I'm new to astrophotography so, forgive me if I don't understand some concepts.   I have an ASI2600MC camera which has an APS-C format sensor.  If I image at f10 without the reducer is there anything I can do to reduce exposure time or get a larger FOV other than get a reducer? 

I think I can use bin 2, or 4 using ASIAIR Plus; will that help?   Also, since I am in Bortle 7 skies, I assume that to really make use of an f10 imaging system with a 2032mm focal length, I will need to travel to some dark sky sites.   Yes?

I've seen some excellent images here on Astrobin using the Edge 8 and the .7 Celestron reducer.  Now I am wondering whether the reducer will help me get shoot objects that are too small for my 100mm refractor...

Thanks,
Jerry
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jhayes_tucson 22.30
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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.

Yes...that's exactly right.  I've been saying this same thing for years!   You are still way ahead even if you don't bin.  The reducer is all about field size and it has nothing to do with signal.

John
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GalacticRAVE 5.87
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@Jerry Gerber as far as the Edge 8 is concerned you probably maxed out for the FoV with the 2600. For the 9.25, 11 and 14, you could use a 6200 instead, which would not be that much more expensive than the reducer + 2600 (except in Mono, when then filters also add to the cost). As @John Hayes repeatedly mentioned (I recommend his talk at the AIC), signal wise there is no difference between reducer and no reducer but 1.4x larger pixels (if everything else remains the same).
Actually you probably will have higher SNR b/c fewer optical elements (reflections etc).  So also no difference at what Bortle you are measuring. Binning does the same as larger pixels except some read noise issues when you really get to very short exposures/very dark skies. Regarding your setup, with bin 2, and Edge8 and a 2600 you basically have a pixel scale of 0.77“ - which is a very suitable sampling for most backyard type situations (well, if you take it to Paranal or Mauna Kea, you need to reconsider). So unless you are desperate for the FoV or do predominantly narrowband IMO there are better investments than the reducer.

you don‘t see the problems for most images unless you pixel peep (than I see it for most of them), and in my experinence BlurXTerminator also helps with the artefacts (but does not completely correct them). Also the various tools to remove flares in photoshop etc can be of help. But these are fixes, not solutions.
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Linwood 5.76
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Jerry Gerber:
If I image at f10 without the reducer is there anything I can do to reduce exposure time or get a larger FOV other than get a reducer? 

I think I can use bin 2, or 4 using ASIAIR Plus; will that help?   Also, since I am in Bortle 7 skies, I assume that to really make use of an f10 imaging system with a 2032mm focal length, I will need to travel to some dark sky sites.   Yes?

Binning does not change field of view, only resolution.

A full frame sensor increases field of view, and at least on a C11 it will nicely cover a FF sensor on both the F10 and F7 configuration, though as mentioned I get poor corner stars with the reducer.

Bear in mind that going from F10 to F7 only improves exposure time on diffuse objects, it does little to help things like globular clusters.  Most DSO work is diffuse objects so that's usually fine, just mentioning it.  Point sources are all about aperture, diffuse objects about focal ratio (in terms of exposure). 

Another key reason for a reducer is to better match your focal length (or more precisely your image scale) to your seeing.  There's not a lot of point at imaging at (say) 0.3"/pix when your seeing is 2.5" due to atmosphere, a 0.7x reducer doubles your scale giving you a wider field with no effective loss in resolution (emphasis on effective -- you loose resolution but its resolution that does not really contain data). 

A full frame camera is not that expensive if you do color, but doing MONO it means full frame filters, and if doing narrowband mono getting good quality narrow band filters (think Chroma) is really expensive.
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