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EdgeHD 9.25 + .7 reducer: elongated stars in one corner only

Celestron EdgeHD 9.25" Equipment Tilt Coma
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Eyecon avatar
Eyecon avatar
Hello All,
After the 3rd replacement of the EdgeHD 9.25 OTA and .7x reducer, I'm finally getting decent results in general from my setup but I'm still struggling with corner star shapes, this time in one corner only. Collimation was verified through Skywave with a coma score of 10/10 and the bad corner moves around the frame when I rotate the camera. I know I had tilt in my camera according to Hocus Focus aberration inspector which I eventually corrected (through the Player One IMX571 APSC camera tilt plate) but I still get radially stretched stars in that one corner. The other weird thing is when I look at the corners during an autofocus run, the stars around the 3 normal corners don't seem to stretch tangentially to radially (extra to intra focal) as much as that one corner. 

What could be causing that? I hope it's not the scope or the reducer again, Celestron already replaced them 3 times and apparently were testing the scope + reducer each time on their bench before sending the replacements…previous scopes also had issues on-axis, this one at least does not!
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Bruce Donzanti avatar
Bruce Donzanti avatar
Do you get the same results without the reducer? I gave up with the reducer years ago on my C11" EdgeHD and just shoot at f/10.  Your issue(s) may have nothing to do with the reducer, but I would test that first because I had many issues in the corners when using it.
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Eyecon avatar
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Bruce Donzanti:
Do you get the same results without the reducer? I gave up with the reducer years ago on my C11" EdgeHD and just shoot at f/10.  Your issue(s) may have nothing to do with the reducer, but I would test that first because I had many issues in the corners when using it.

I appreciate the feedback, I just got the scope back from Celestron so only had the chance to test with the reducer…will try without it as well although my main use case for the edge hd is to match my 10” F6 newt setup so I can shoot the same target using two setups and reduce overall imaging time.
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Eyecon avatar
I tried the OTA without the reducer tonight and the stars were perfect, <1.8 arc sec FWHM and low overall eccentricity and round shapes across the entire APS-C field. Collimation (coma score on Skywave) was unchanged after removing the reducer.  

An interesting observation though: before I switched to the Player one IMX571 camera so I can adjust tilt, I was using my RisingCam IMX571 setup which does not have  a built in tilt plate. When I removed the reducer, I switched back to the RisingCam setup. Hocus Focus Aberration Inspector reported almost no tilt (<0.04deg) consistently through multiple autofocus runs.   With the reducer installed and identical setup otherwise in terms of the Rising Camera rig, the Moonlite focuser, the extension tubes…etc, I was getting around  0.8 degrees of tilt according to aberration inspector. With the Player one right, I had to really push the tilt plate quite a bit for Hocus focus to report lower tilt. And even with the player one, Hocus Focus showed high filed curvature despite being at the correct back focus distance (and I tried to shift that but a few mm's in and out to no effect).

 Is it possible that the reducer is introducing tilt due to the body of the reducer or otherwise tilted optics inside the reducer?  Could the reducer optics just be bad? Celestron said they bench tested this new reducer and OTA  combination before sending them out.  Tilt still  doesn't explain why only one corner shows noticeably stretched stars, so I'm starting to worry that the reducer is just bad.
Bruce Donzanti avatar
Bruce Donzanti avatar
It  certainly sounds like the reducer to me and I am not surprised.  I would send this info to Celestron but I am sure any reducer they send could cause the same problem.  Any part of the optical train can introduce tilt and in this case, as I suspected, is the reducer.  Also, don’t get all caught up in the numbers when checking for aberrations using HF.  It is a great tool but always use the eye test.
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Eyecon avatar
Eyecon avatar
Bruce Donzanti:
It  certainly sounds like the reducer to me and I am not surprised.  I would send this info to Celestron but I am sure any reducer they send could cause the same problem.  Any part of the optical train can introduce tilt and in this case, as I suspected, is the reducer.  Also, don’t get all caught up in the numbers when checking for aberrations using HF.  It is a great tool but always use the eye test.

Yes, I only use the HF numbers as for relative comparisons (worse or better outcome after adjustments) but I understand that they cannot be 100% accurate due to the non-linear nature of focus curves. I also noticed that the closer you get to perfect field flatness and zero tilt, the worse the model fit gets...not sure if this is an accurate observation though. 

One additional thing I forgot to mention: when I was getting stretched stars in that one corner, I shifted focus outwards to try and verify the tilt issue, figured if it was absolutely tilt I would get round stars if I move the focus to fix that specific corner and ruin all other corners. When I tried that, I got sort of roundish stars in that corner but they appeared a bit astigmatic and to my surprise the stars in the other 3 corners didn't really change by much. Not sure what this means.
V avatar
V avatar
I have the same issue with my 8HD's Reducer. It's purely the optics fault. The .7x reducer is not very good. I've measured a 30% increase in spot size, and dramatically worse field curvature at perfect backfocus. I've stopped using it, and simply run at F/10 with longer exposure times, the sharpness at F/10 is great.
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Eyecon avatar
Eyecon avatar
I just reinstalled the reducer to confirm that the bad star shapes were not a result of a problem with the setup. Unfortunately, after getting wonderfull stars edge to edge with the native FL setup, I can confirm the exact same issue with the top of my imaging frame and mainly a single corner on that top edge. I'll contact Celestron and see what they say since this is the 3rd OTA + reducer combo they have replaced for me and each time they say they bench tested them and end up sending me a replacement!

Are there any good .6x-.7x reducers that I can use with the EdgeHD 9.25 + a Moonlite focuser? I only need the reducer to somewhat match my 10" F6 setup since I have two IMX571 cameras with identical filter sets. One of the main reasons I got the EdgeHD is to be able to half the amount of time it takes to shoot a single target by using both the SCT and the NEWT in a tandem setup on two separate mounts. I also started to get into planetary images and it's also nice to have the native 2350mm FL for smaller DSOs.
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Bruce Donzanti avatar
Bruce Donzanti avatar
Sorry to hear that but expected as we discussed.  The only one out there is by Optec - a 0.62x reducer for C9.25 and C11 and C8.  I tried it years ago and it is ok.  That might be your best bet, actually, your only bet.  Starizonia tried to develop a 0.4x reducer for years but gave up.
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Bruce Donzanti:
Sorry to hear that but expected as we discussed.  The only one out there is by Optec - a 0.62x reducer for C9.25 and C11 and C8.  I tried it years ago and it is ok.  That might be your best bet, actually, your only bet.  Starizonia tried to develop a 0.4x reducer for years but gave up.

I also just read about the Astrophysics CCDT67 in a cloudnights post describing similar issues as ours. Apparently this is a very good and much cheaper reducer but I'm not sure I like the idea of having things attached through a 2" eyepiece holder rather than being threaded and I'm also uncertain how to achieve the appropriate BF distance for both the OTA and the reducer.
Jim Lindelien avatar
Jim Lindelien avatar
It's been my understanding that a reducer such as the Optec that was designed for non-HD will not perform well on HD Celestrons. Won't it add curvature to the already flat field? Does anyone have hands-on experience? Thanks.
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Bruce Donzanti avatar
Bruce Donzanti avatar
It's been my understanding that a reducer such as the Optec that was designed for non-HD will not perform well on HD Celestrons. Won't it add curvature to the already flat field? Does anyone have hands-on experience? Thanks.

No- it is designed for the Edge series and I have used it.  With that said, it isn't perfect.
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Eyecon avatar
Official response from Celestron: 

Your star image shows good collimation, but no reducer will have perfect stars corner to corner.  They all show some amount of astigmatism off-axis.  The best way to correct this is to use BlurXterminator in post processing to correct the star shapes.  That reducer appears to have some slight tilt toward the upper left, but replacing the reducer again will not yield perfect stars in the corners.  That is a perfectly normal  limitation of our reducer design which was released when the most popular cameras had 9 micron pixels vs the 3.76 micron pixels popular now.
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Bruce Donzanti avatar
Bruce Donzanti avatar
LOL-  of course.  I am glad they admitted it and yes, BlurXterminator will get you near ideal.   This will essentially be the same issue with most reducers made for the larger pixel size.  I just went through this with the new 0.73X FSQ85 EDX Takahashi reducer made for full frame cameras.   I could never get it to work in all 4 corners and had to use BlurXterminator.  However, even with that, I had to slightly crop. This is why I no longer use reducers with the newer, smaller pixel cameras.  Perhaps you should try BlurXterminator  if you have it and see if it gives you a satisfactory result.
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Eyecon avatar
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BXT definitely works but just feels wrong to use given the price point of the EdgeHD optics 🤣

I could have saved a lot of money and just gotten a C9.25 instead and just used BXT to get good edge performance.
V avatar
V avatar
BXT definitely works but just feels wrong to use given the price point of the EdgeHD optics 🤣

I could have saved a lot of money and just gotten a C9.25 instead and just used BXT to get good edge performance.

Just shoot at F/10 at this point, your FWHM's will drop dramatically as will your disappointment in the field performance. Plus, less glass.
Eyecon avatar
Eyecon avatar
After 6 months of back and forth with Celestron support, a few hundred dollars of test equipment and software, and probably over 80 hours of dedicated bench testing and trial and error in my observatory,  I finally solved the single corner distortion issue I've been having with the EdgeHD 9.25 reducer!

The problem turned out to be a mechanical one and had nothing to do with tilt or the reducer itself. For whatever reason, tightening the primary mirror locks beyond a certain point seems to induce stress on the primary mirror and causes a specific deformation that produces stretched star shapes in the area close to where the locking rods (or maybe just one of the rods) are attached to the primary mirror mounting mechanism.

I only found out because I noticed an increase in the astigmatism coefficient with the mirror locks engaged and when running the native FL images through Innovation Foresight's Skywave wavefront analyzer (which I use to collimate my reflectors...highly recommend this amazing software!!). I had no reason to suspect the locks to be the culprit because at the native FL, the corners all looked good, and the astigmatism introduced by tightening the primary mirror locks was certainly not visually discernible in the images across the entire APS-C field. It seems the corner problem was always there and the reducer was just expanding the FOV enough for that bad corner to enter the FOV of the camera.  Below are a couple of screen shots showing the impacted corner with the locks engaged and disengaged:




Mirror Lock Engaged:

  

Mirror Lock Disengaged:




Hoping this can help anyone having the same issue.
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Brian Diaz avatar
Brian Diaz avatar
HI


the same issues with my  celestron 9.25 edge HD  with reducer 0.7x celestron

say thank you to the reducer bad quality 

Cs

Brian
Jonathan Simmons avatar
Jonathan Simmons avatar
I'm on my second reducer from celestron and I have the same issues with corner stars.  Not going to bother sending back for another reducer.  I have zero issues when not using the extra glass on my edge 9.25 and love it at the native fl.  I'm chalking it up to a purchasing mistake and will likely just shoot at f10 going forward, I've spent far too much time trying to figure out any tilt issue which I don't think actually exists to the extent the reducer would indicate.  I also leave my mirror locks unlocked for the autofocuser to do its job overnight.