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Two years and counting dealing with this image artifact.. help..

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#1 pbkoden

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Posted 17 March 2019 - 01:37 PM

Ever since I got my ONTC scope, I've had a persistent image quality issue. There is a donut/ring in the center of my image. I've posted previously about it here: https://www.cloudyni...ring-in-images/. I thought i had some progress, but it wasn't to be. Here is a sample of some of my calibrated and integrated data over the last 2 years with an automatic background extraction applied and screen transfer function:

 

 

IpStLFR.jpg

 

 

I've checked about everything I think I can including the following;

  • Taking flats through varying processes including LED panel, T-shirt, drywall ceiling - none make a difference
  • With and without an aperture mask in front of the main mirror - makes large star diffractions much cleaner, but I removed it just in case. no change
  • With and without dew shield to cut down on strange light - no change
  • Before and after recharging my camera desiccant plug - I checked it for frost/dew indoors after cooling and didn't see anything, but I recharge it anyways
  • Addressing light leaks around my focuser and primary mirror - no change
  • Hitting anything remotely shiny in the image path with flat black paint - no change
  • It affects all filters (LRGBHaSIIOIII), though very hard to see in narrowband
  • Secondary dew heater on high or low - no change

 

Things I'm starting to wonder:

  • I glued a split-ring secondary dew heater to the back of the secondary mirror with silicone when I got the scope. Did I warp the secondary mirror with the silicone?
  • Do I have a defective secondary or primary mirror?
  • Do I have a defective coma corrector? I got it at the same time as the scope.

I completely dismantled my scope yesterday and when putting it back together must not have gotten my secondary located perfectly. The center of my illumination shifted slightly in images taken last night. And I noticed that in those images the donut/ring shifted to match the center of illumination. You can also see it shifts a little in the above images as I tweak my scope and re-align mirrors. Another clue to work with.

 

Give me ideas before I just start replacing major components one at a time until its fixed.

 

Thanks!

Phil

 

 

 

 

 



#2 rgsalinger

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Posted 17 March 2019 - 02:09 PM

I think that this is a calibration error relating to the flats. To determine if it is, just look ONLY at the light images and do only a stretch. If they show the effect then I'm wrong. I've seen this myself a couple of times. I think it may have to do with the need for flat darks but I'm not sure. I seem to have tamed it by taking my flats with very short exposures. Again, I've seen this but only after calibration.

 

Rgrds-Ross



#3 pbkoden

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Posted 17 March 2019 - 02:40 PM

Yes, the images show the ring before and after any calibration. I posted the integrated images because it's more subtle before integration. After integration is when it really pops.

#4 Gipht

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Posted 17 March 2019 - 03:07 PM

Phil, do  you have access to a second camera or telescope to see if the aritifact is in the telescope  or the camera?  Does the artifact appear when you are not using the coma corrector?



#5 pbkoden

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Posted 17 March 2019 - 03:47 PM

I don't have access to another camera or scope, but could try imaging without a comma corrector. The camera and filters were used on my previous astrograph without showing this artifact, so I have reason to believe that it is not the camera.

#6 pfile

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Posted 17 March 2019 - 04:03 PM

did you try sky flats?

 

i still have this problem with my rig, but since it's not as pronounced as your problem i can usually get rid of it with DBE. i have come to suspect reflections off my flattener but have not tried removing it yet to verify.

 

one thing i had on my list of things to try was some kind of "night sky flat" but i'm worried that LP gradients would get me there. plus it's a royal pain getting enough signal and rejecting all the stars.

 

rob



#7 pbkoden

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Posted 17 March 2019 - 04:57 PM

Yeah, I can kill it with a scorched earth DBE approach, but there also goes any chances I have at some faint IFN or similar. This upcoming weekend looks clear. I'll try some data without my coma corrector and see how it goes.

 

I have not tried sky flats. I've done synthetic flats before which can help, but it's a ton or work to make them and the results can be hit or miss.



#8 calypsob

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Posted 18 March 2019 - 08:14 AM

is there a bright shiny gloss black spanner ring in your coma corrector?

 

Wait I remember this issue now after reading the other thread. I know its a ccd but can you share a histogram of one of your flats? Im curious how you are exposing them.

I typically try and get the peak just to the left of the middle, if dead center was 50% I put the peak between 46% and 47%. I used to get bad concentric rings with the samyang 135mm

but now my flats are as flat as a pancake. I realized that with a faster aperture, I needed alot more flats than I did at f5.6 and much much more than f8, at f8 I really had no vignetting.

When I exposed at or above 50%, I felt that I simply had overpowered the signal in the bright centralized area and the edges were not getting exposed enough,

with a shorter exposure and a deeper integration the shadows and vignetting seemed to resolve much more accurately.

 

I started doing 50 flats at a time, still saw an unusually high level of noise in the corners when I stretched an image, then added 75 flats and finally 100. at 100 ther noise was balanced in a stretch and the vignetting was nowhere to be seen.  In fact there have been several occasions where I skipped DBE all together. With a ccd you may not need as many flats as I used with a dslr which is noisy in every single image at iso 1600.

 

If it helps any, your issue looks very similar to the non calibrated light integration at the end of chucks video

https://www.youtube....h?v=O6xR-OQnysE


Edited by calypsob, 18 March 2019 - 08:41 AM.

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#9 pbkoden

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Posted 18 March 2019 - 11:21 AM

Wait I remember this issue now after reading the other thread. I know its a ccd but can you share a histogram of one of your flats? Im curious how you are exposing them.

I typically try and get the peak just to the left of the middle, if dead center was 50% I put the peak between 46% and 47%. I used to get bad concentric rings with the samyang 135mm

but now my flats are as flat as a pancake. I realized that with a faster aperture, I needed alot more flats than I did at f5.6 and much much more than f8, at f8 I really had no vignetting.

When I exposed at or above 50%, I felt that I simply had overpowered the signal in the bright centralized area and the edges were not getting exposed enough,

with a shorter exposure and a deeper integration the shadows and vignetting seemed to resolve much more accurately.

 

I started doing 50 flats at a time, still saw an unusually high level of noise in the corners when I stretched an image, then added 75 flats and finally 100. at 100 ther noise was balanced in a stretch and the vignetting was nowhere to be seen.  In fact there have been several occasions where I skipped DBE all together. With a ccd you may not need as many flats as I used with a dslr which is noisy in every single image at iso 1600.

 

If it helps any, your issue looks very similar to the non calibrated light integration at the end of chucks video

https://www.youtube....h?v=O6xR-OQnysE

 

I've tried flats exposed to 35000 and 25000 ADU. 35000 is the QSI recommendation, so I've normally run with that. More recently I tried lower values after seeing what others were using. But no changes to the image results.

 

As far as number of flats, I normally do 30 of them. This is with the sensor cooled to -15°C, and there is virtually no noise at that temp with the short exposures.

 

I looked at the video. The largest difference I see is that I only ever have the one circle. I don't get concentric rings. Just the one circle in the middle.



#10 Jeff2011

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Posted 18 March 2019 - 12:17 PM

Was this scope bought new or used?  If used there is no telling what the previous owner did to it.  If bought new, you are probably out of warranty but it would not hurt to contact the vendor.

 

I am wondering if the size and or placement of the secondary mirror could cause that. My club has an 18 inch imaging newt that is used visually for outreach every Saturday night.  It has an oversized secondary mirror. If I place a 35 mm eyepiece in it, the center obstruction appears as a dark circle in the center of the eyepiece at twilight but is not noticeable when it gets dark.  I have tried imaging with that newt a couple of times but have not seen any issues with it that matches yours.



#11 Jon Rista

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Posted 18 March 2019 - 04:24 PM

Can you share a subset of your data? Flats, biases/flat darks, darks, and lights?

 

First, I think ABE is hurting you more than helping you. ABE alone can CAUSE these kinds of issues. I would try DBE only. That said, I think this could also be the result of internal reflection issues within your scope and imaging train, possible issues with filters if you are using any, or other internal scope/train issues. 

 

LP filters are often the cause of differences between flat field and light field. Filters often use interference effects to perform their filtration, and these effects can be highly dependent on the angle of incidence of the light. If your flats are from a flat fielder at the aperture, or off a wall or TV screen or something like that, the angles can be significantly different than from the sky/space. Pointing your scope at a flat blue sky, and exposing so that each/all channels fall within the middle of the ADU range could help, since light from a flat blue sky (and, note, do NOT use any kind of diffuser!) should have the same collimation as light pollution at night, and should avoid issues that result from different angles of light in the flats vs. lights. 


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#12 freestar8n

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Posted 18 March 2019 - 05:31 PM

Looks to me like reflection off a tube that needs to be flocked.  It's not that something is shiny - but it may be smooth and allowing a grazing reflection that isn't matched between the lights and the flats.

 

Frank



#13 happylimpet

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Posted 18 March 2019 - 05:45 PM

I must say those lights look familiar.

 

Ive had VERY similar ones, and I have a very similar setup - 16" f4 and 12" f5 newtonians with a coma corrector.

 

I think its down to stray light entering the draw tube from unwanted angles. Fitting a dewshield certainly seemed to help me a lot, flocking may have done too. Also flocking upper parts of the secondary mirror holder. Anything that blocks stray light.

 

Get flocking, and make sure you have a dew shield on the front of the scope. I know you say you have, but maybe it wasnt long enough? Not itself well flocked?

 

One thing that did help me for sure was using flats made from lights. For example, at the end of the night, leaving the scope taking subs while not tracking, and making a flat using kappa-sigma rejection of all of those 'trailed' subs, to remove the stars. These worked much better than my standard flats, presumably because they better keep a record of the stray light entering the tube.

 

An example of mine, maybe not the best:

 

NGC3367SN-610-1_3-binbin.jpg

 

On this night I had moonlight shining into the scope which made it particularly bad. Do you have lights around?


Edited by happylimpet, 18 March 2019 - 05:54 PM.


#14 pbkoden

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Posted 18 March 2019 - 06:07 PM

Was this scope bought new or used?  If used there is no telling what the previous owner did to it.  If bought new, you are probably out of warranty but it would not hurt to contact the vendor.

 

I am wondering if the size and or placement of the secondary mirror could cause that. My club has an 18 inch imaging newt that is used visually for outreach every Saturday night.  It has an oversized secondary mirror. If I place a 35 mm eyepiece in it, the center obstruction appears as a dark circle in the center of the eyepiece at twilight but is not noticeable when it gets dark.  I have tried imaging with that newt a couple of times but have not seen any issues with it that matches yours.

 

Scope was new to me. And I hate to complain to the manufacturer when I don't have any basis yet on what the cause is. I can't say it's any part of the scope. Could be my coma corrector, camera, or mods I've made to the scope. Others on these forums have the same scope and have not had these issues.

 

 

Can you share a subset of your data? Flats, biases/flat darks, darks, and lights?

 

First, I think ABE is hurting you more than helping you. ABE alone can CAUSE these kinds of issues. I would try DBE only. That said, I think this could also be the result of internal reflection issues within your scope and imaging train, possible issues with filters if you are using any, or other internal scope/train issues. 

 

LP filters are often the cause of differences between flat field and light field. Filters often use interference effects to perform their filtration, and these effects can be highly dependent on the angle of incidence of the light. If your flats are from a flat fielder at the aperture, or off a wall or TV screen or something like that, the angles can be significantly different than from the sky/space. Pointing your scope at a flat blue sky, and exposing so that each/all channels fall within the middle of the ADU range could help, since light from a flat blue sky (and, note, do NOT use any kind of diffuser!) should have the same collimation as light pollution at night, and should avoid issues that result from different angles of light in the flats vs. lights. 

 

Here is some sample data including bias, dark, flat, light, and calibrated light (https://drive.google...9fg95jXSuFJF6On). The halo is very clear after calibration, or with a quick background extraction before calibration. Taken with a QSI683 @ -15C, Astrodon E-series luminance filter. It's a dim low-signal target, but one I'm currently working on.

 

I will try some sky flats this weekend (Friday is supposed to be clear) and see how they compare. I think I understand about the light collimation, and that makes sense. Though taking sky flats sounds like a general PITA.

 

 

I must say those lights look familiar.

 

Ive had VERY similar ones, and I have a very similar setup - 16" f4 and 12" f5 newtonians with a coma corrector.

 

I think its down to stray light entering the draw tube from unwanted angles. Fitting a dewshield certainly seemed to help me a lot, flocking may have done too. Also flocking upper parts of the secondary mirror holder. Anything that blocks stray light.

 

Get flocking, and make sure you have a dew shield on the front of the scope. I know you say you have, but maybe it wasnt long enough? Not itself well flocked?

 

One thing that did help me for sure was using flats made from lights. For example, at the end of the night, leaving the scope taking subs while not tracking, and making a flat using kappa-sigma rejection of all of those 'trailed' subs, to remove the stars. These worked much better than my standard flats, presumably because they better keep a record of the stray light entering the tube.

 

An example of mine, maybe not the best:

 

attachicon.gif NGC3367SN-610-1_3-binbin.jpg

 

On this night I had moonlight shining into the scope which made it particularly bad. Do you have lights around?

 

I have some flocking and have hit random parts of the scope. I also hit virtually everything with flat black paint earlier this week. A full flocking of the tube could be on my list if needed. Adding the dew shield didn't make a difference in the artifact, but it does add to the wind sail effect, so I don't want to use it if I don't have to.

 

But yeah, the sky flats definitely sounds like a possible solution.

 

I do not have any local lights currently, the scope is in the middle of 10-acre square with trees all around except where my house is. No exterior house lights on, and many of the images are taken with no lights in the house on at all. But some of the images were taken with a lot of local light from my old house. The amount of local light doesn't make much of a difference. But the brighter the overall light (more light pollution, partial moon) does make the ring show up more.

 

Thanks!

Phil


Edited by pbkoden, 18 March 2019 - 06:09 PM.


#15 Jon Rista

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Posted 18 March 2019 - 07:42 PM

 

Here is some sample data including bias, dark, flat, light, and calibrated light (https://drive.google...9fg95jXSuFJF6On). The halo is very clear after calibration, or with a quick background extraction before calibration. Taken with a QSI683 @ -15C, Astrodon E-series luminance filter. It's a dim low-signal target, but one I'm currently working on.

 

I will try some sky flats this weekend (Friday is supposed to be clear) and see how they compare. I think I understand about the light collimation, and that makes sense. Though taking sky flats sounds like a general PITA.

 

 

Thanks for the data.

 

Regarding sky flats. I mean against a DAYTIME empty blue sky. Not nighttime flats with stars. Daytime blue sky flats are super, super easy. No diffusion of the aperture, just leave it open. Make sure the sky is clear and it is an empty blue sky. Such flats allow very short exposures, which if you have a mechanical shutter may be an issue. In that case, find a time of day (evening or morning) when the sky is not too bright, but still bright enough for flats and empty. Otherwise, the same general rules apply for how to acquire flats, such as median level, sub counts, total electron counts, etc. 



#16 ChrisFC

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Posted 19 March 2019 - 06:39 PM

Might not be the same as your problem but I had a similar donut shaped artefact on images from my'lesser' Newtonian. Turns out it was light entering the back of the scope around the primary. I had been imaging in my backyard and there was some light coming from the house towards the back of the scope. Put a bit of kitchen foil over the back and it cleaned up the artefact.
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#17 pbkoden

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Posted 19 March 2019 - 07:21 PM

Okay, got some sky flats in as the sun was going down. Set the camera to minimum exposure length and started taking images as soon as the sky ADU reached ~45,000. Every flat taken dropped 1,000 ADU as the sky was darkening. I was only able to get 20 flats in the 25k to 35k sweet spot. This is obviously not going to work long term as I can only capture flats for one filter in the window I have at dusk.

 

But anyways, the sky flats did not fix my issue. Below is an integrated calibration from the other night. I haven't touched the camera/focuser since those lights, so this should be an ideal matchup. I did an ABE, limiting the function degree to 1, so it does not add any artifacts of its own. The circle is still visible in the image and will get worse with more integration.

 

 

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  • Ring2.jpg


#18 pbkoden

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Posted 19 March 2019 - 07:27 PM

Might not be the same as your problem but I had a similar donut shaped artefact on images from my'lesser' Newtonian. Turns out it was light entering the back of the scope around the primary. I had been imaging in my backyard and there was some light coming from the house towards the back of the scope. Put a bit of kitchen foil over the back and it cleaned up the artefact.

 

When I got the scope I added a circular panel at the back of my scope for my primary cooling fans. It's a decently tight fit and blocks the majority of light that would get through, but there is a gap. I could try and add some felt fringe to the edge to make a better seal against the sides of the tube.

 

 

q7q3aa5.jpg



#19 happylimpet

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Posted 20 March 2019 - 04:00 AM

I honestly think you need to focus on stopping light entering the drawtube from the inside. Youve established that its primarily a problem with the lights and not the flats (though I expect it is also present to some extent in the flats, hidden by the dominant vignetting etc).

 

The thing which strongly points towards this for me, is the left/right asymmetry of the effect, as with mine. If it was light entering the bottom of the scope I wouldnt expect this asymmetry. But of course when light enters the drawtube it does so very much from the side, hence left/right asymmetry.

 

I know that that is what caused a very similar effect on my images. The worst I ever saw it was with moonlight shining into the top of the scope, and the thing which reduced it hugely was putting a dew shield up (actually asymmetric, just on the 'far side of the scope, to block skylight entering the drawtube).

 

I wonder if your dew shield did reduce it, just not to zero? Have you done a quick test with/without? Perhaps it reduced it by 75% but it was still visible. 

 

When you look into the top of the scope, can you see the bottom lens surface of the CC? Obviously if you can that will be  major problem (I could without my dewshield on the 16") but even if not it probably only takes one reflection off the inside of the drawtube to throw light into the CC from one side.

 

Also try flocking the inside of the drawtube where possible, particularly the bottom end well inside the scope. I think this also helped me.


Edited by happylimpet, 20 March 2019 - 04:04 AM.


#20 MDWingsFan

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Posted 20 March 2019 - 08:55 AM

One idea...  I had similar issues with my 6" Newt.  In my case I used a dark room, and a diffused light.  I set my scope up in the room, and used a dim diffused light while dark to take a set of light frames.  After each exposure I checked the image to see if my leaks were moving or stable, and where the light appeared brightest.  This allowed me to narrow my leaks down to the tube/mirror seal, and the mirror/rear interface.  For my issues the use of that controlled setting made the search much easier, and I didn't waste under skies time debugging...  Just my $0.02.


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#21 pbkoden

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Posted 20 March 2019 - 11:31 AM

When you look into the top of the scope, can you see the bottom lens surface of the CC? Obviously if you can that will be  major problem (I could without my dewshield on the 16") but even if not it probably only takes one reflection off the inside of the drawtube to throw light into the CC from one side.

 

You gave me an idea here. My coma corrector sticks into the telescope tube by about 3/16"-1/4" when I'm in focus. Not enough to occlude the primary mirror, but enough to probably catch stray light. I may need to move my primary mirror up towards the focuser and get a longer drawtube for my focuser to get the tip of the coma corrector out of visibility to the sky.


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#22 happylimpet

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Posted 20 March 2019 - 12:16 PM

You gave me an idea here. My coma corrector sticks into the telescope tube by about 3/16"-1/4" when I'm in focus. Not enough to occlude the primary mirror, but enough to probably catch stray light. I may need to move my primary mirror up towards the focuser and get a longer drawtube for my focuser to get the tip of the coma corrector out of visibility to the sky.

Cool, yeah that would help. I think you're getting there with this! Or put a 'parfocalising ring' or similar around the CC so it sits higher in the drawtube, which can then be placed deeper into the focuser acting as a shield. Something like that.



#23 pbkoden

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Posted 20 March 2019 - 06:11 PM

Okay, maybe a smoking gun? Here is an image of the inside of the tube when I am at focus showing the coma corrector protruding into the tube. Heck, you can even see reflections like crazy off of it in this angle. I can move my mirror to the next set of mounting holes, roughly 1.5" up the tube. This should allow me to slide the coma corrector up into the focuser drawtube and out of the path of light intrusion. I've already ordered a 1" drawtube extension from Moonlite to allow me to reach focus at the new position. I'll let you know how it goes.

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#24 freestar8n

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Posted 21 March 2019 - 02:33 AM

The very sharp edges and repeatability of the ring in your images suggested to me an evil tube somewhere that needed flocking.  I hope you have found it.

 

Other methods of taking lights may help if somehow they end up reproducing the glare in the same way as in the lights.  But I think the real solution is to find the cause and hope it isn't within some sealed element that you can't flock easily.

 

Even a tube that looks very dull and flat black can do this sort of thing if the light hits it at a grazing angle.

 

Frank



#25 happylimpet

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Posted 21 March 2019 - 05:02 AM

Okay, maybe a smoking gun? Here is an image of the inside of the tube when I am at focus showing the coma corrector protruding into the tube. Heck, you can even see reflections like crazy off of it in this angle. I can move my mirror to the next set of mounting holes, roughly 1.5" up the tube. This should allow me to slide the coma corrector up into the focuser drawtube and out of the path of light intrusion. I've already ordered a 1" drawtube extension from Moonlite to allow me to reach focus at the new position. I'll let you know how it goes.

There you go. I always said it was the same as my problem! Hehe. But very glad you're getting there.

 

Good strategy. Once you've recessed the CC an inch or two, much less direct light will shine onto the CC, but you'l still have potential problems with light ricocheting off the inside of the drawtube (all those shiny surfaces you pointed out) , so flock the hell out of that, use a dewshield at least on the far side of the tube to block skylight shining into the drawtube, maybe put some flocking around the top of the spider assembly (can block quite a lot of light from the far top side of the tube and itself is in shadow so dark)...etc etc.....

 

This explains why the dewshield didnt stop it initially, though Im sure it reduced it.




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