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Celestron EdgeHD 11" Not Sharp

astrophotography cassegrain Celestron collimation equipment imaging reflector SCT
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#1 Bramlejd

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Posted 20 May 2019 - 01:56 PM

     For a few months now, I have been imaging with a Nikon d7000, EQ6r-Pro and a Tamron 150-600mm with pretty good results and decided to upgrade (just a little wink.gif  ).  The 11" was an ebay find, so it wasn't new.  I didn't have eye pieces, camera mount, or diagonal when I first got it, so I can't compare before and after, but the first thing I did was a thorough clean.  The corrector plate had what I can only assume to be tree sap on it, so it was difficult to clean, but I did do a thorough job.  There are a few specs of dust left on the primary, but for the most part, things look good!  

     I can't believe I did this, but I neglected to mark the corrector plate prior to removal.  I did notice the (what I assume was) factory mark along the edge, which I then aligned with the rivet at 3 o-clock.  The mark on the secondary matches this as well, so I assume I'm good.

     For my first attempt at collimation, I made a cardboard target with concentric circles with a bright flashlight behind a pinhole attached to the middle.  It has been raining since I got this a week ago, and I wanted to see what I could do with the "kitchen table" method.  Sighting in through the open eyepiece, then adjusting the secondary to center the return circle on the pinhole was straight forward enough, so I assume I am at least close.

     Now with it being cloudy and my living in an apartment, I have been trying to see what sort of clarity I could get through my window out to a Chevron sign about maybe 3/4 of a mile away.  I did try with the window open, re-collimated a number of times, and checked both through my 40mm eyepiece, and dSLR but the image is not crisp.  The words are readable and not terrible, but the support poles and edges of the sign are not sharp.  I'm used to general photography with "pixel peeping" and obsessing over clarity, but am getting nowhere near that with this scope.  Could there be something wrong, or are my expectations way to high?  Being the third owner of this scope, I'm worried there may be something wrong with it, but can see nothing obvious.  Is there anything I can check, or tests I can perform to determine if issues exist?

 

Long post...sorry.....thank you in advance for any help provided!



#2 pyrasanth

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Posted 20 May 2019 - 02:20 PM

The Celestron manual states that the only adjustable component is the collimation of the secondary mirror and the white paper states that the edge HD optics are aligned on a special jig. I know people do remove the corrector plate but you need to be comfortable with doing this and be certain you can put it back exactly in the same place to a high precision. I don't think it is as critical with non edge versions of the SCT.

 

Are you sure that the corrector plate on your newly acquired telescope is a real corrector plate & not just a piece of plain glass. Is the plate coated? There have been cases where a corrector plate has been broken and replaced with plain glass which is not going to work at all.

 

My 14" SCT when collimated well has razor sharp optics- I'm pretty sure if the conditions permitted I could see a gnat on an apple a mile away! so your expectations are not unreasonable.

 

I would send the instrument to Celestron as the edge optics are still built in the USA and not China so you should be able to get any issues corrected but if the corrector plate has been replaced with plain glass then it will be expensive to replace.


Edited by pyrasanth, 20 May 2019 - 02:24 PM.


#3 gezak22

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Posted 20 May 2019 - 02:25 PM

It could be many things: Poor collimation (the primary mirror is f/2, so very sensitive), a poor diagonal, thermal effects (heat rising from the ground between you and the Chevron sign [gas station = cars = exhausts]), maybe damage introduced as you removed tree sap from the corrector.

 

I would do a star test without a diagonal, after the scope has acclimated, preferably on a star that is not above the heat plume of a house.


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#4 ngc7319_20

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Posted 20 May 2019 - 03:33 PM

Hard to tell what is going on.  Maybe try your "window" test before dawn after the city has cooled off a bit.  Try a night where the inside and outside temperature are the same (if possible).

 

If you can find a tiny light source out the window somewhere -- something unresolved -- single small light bulb far away -- and look at the images from that, it will become more obvious what is happening.  Use a high power eyepiece like a 5 mm.  Look at the images inside, outside, and at best focus.  What do you see?  Can you photograph or draw it?  Is it just a boiling mess (atmospheric problem).  Or ellipses outside of focus (astigmatism),  Or comet shaped patterns (coma -- probably bad collimation).  A sharp dot plus big halo (spherical correction errors).

 

It is unlikely you will get the same sharpness you enjoy with the Tamron lens, just be cause the C11 aperture is so much larger, and requires much better atmospheric conditions.



#5 photomagica

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Posted 20 May 2019 - 03:34 PM

It could be many things: Poor collimation (the primary mirror is f/2, so very sensitive), a poor diagonal, thermal effects (heat rising from the ground between you and the Chevron sign [gas station = cars = exhausts]), maybe damage introduced as you removed tree sap from the corrector.

 

I would do a star test without a diagonal, after the scope has acclimated, preferably on a star that is not above the heat plume of a house.

Yes - a very careful collimation on a star when the telescope is at thermal equilibrium is what you will really need to determine the current quality of the instrument. If you cannot collimate it to the point where you have star images that are reasonably round, concentric and symmetrical in and out of focus and show a diffraction pattern in-focus - then the telescope needs work.

 

Some searching around on Cloudy Nights will reveal how to collimate an SCT on a star.

 

Unless you are experienced and adept at tuning telescopes, the advice to return it to Celestron for a diagnosis and tune-up is really good advice.

 

From my own experience with a recent C-8 Evolution and other reports, current Celestron SCTs are very good quality with no bad samples that I have heard about - at least nothing that Celestron didn't correct under warrantee. Assuming the corrector is original, it should be possible to make the telescope perform in a very satisfactory manner.

 

Did you replace the corrector with the same face out as it came to you? If you can collimate the telescope on a star but it is still not sharp enough to deliver a good diffraction image, I'd try flipping the corrector over. Assuming nothing is wrong elsewhere in the telescope, you should be able to collimate to a fair to middling quality image even in the corrector is rotated wrong, provided it is the right way out.

 

BTW - unless you introduced really obvious coating/surface damage it is highly unlikely that your cleaning off the scum damaged the corrector. Don't worry about that. Just be patient until you can do a good star collimation and test. 

 

Best of luck - keep us posted on your progress.

Bill


Edited by photomagica, 20 May 2019 - 03:38 PM.


#6 Cpk133

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Posted 20 May 2019 - 06:12 PM

Just chill and wait till you can get it out and colimate with a star or buy yourself an artificial star.  I doubt your colimation is on based on the process you described.  


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#7 Jon Isaacs

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Posted 20 May 2019 - 06:45 PM

Just chill and wait till you can get it out and colimate with a star or buy yourself an artificial star.  I doubt your colimation is on based on the process you described.  

 

:waytogo:

 

"Now with it being cloudy and my living in an apartment, I have been trying to see what sort of clarity I could get through my window out to a Chevron sign about maybe 3/4 of a mile away.  I did try with the window open, re-collimated a number of times, and checked both through my 40mm eyepiece, and dSLR but the image is not crisp."

 

No big surprise there.. 70x out an open window at an object 3/4 of a mile... it would be surprising if it were crisp and sharp. 

 

Jon


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#8 Bramlejd

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Posted 20 May 2019 - 06:49 PM

     Thank you all for your helpful responses!  I've been reading and learning a lot from this forum and am very pleased with the kind and helpful responses I've been seeing here vs forums I've been a part of for other hobbies.

 

     I have attached a sample from my Nikon d7000 of the sign through the scope as well as from my iPhone 10s as a reference (not taken at the same time).  Only thing I did to the d7000 image is slide the temperature a bit so it wasn't so red from lack of IR filter.  There was probably a 20degF difference in temperature between indoors and outdoors, but I'm trying not to be the guy with a telescope pointing out into the apartment courtyard.  I am familiar with what heat may do outside from telephoto photography, but didn't see any atmospheric effects that seemed to correlate with the lack of sharpness. I also tried the cell tower next to it.  It's hard to see detail in it, especially when trying to look at the hanging cables etc... ChevronC11.JPG ChevroniPhone.JPG

 

     I did also try it at night both on the sign and at some lights at various distances with the same results.  What I see through a 40mm eyepiece and the dSLR appear to be very similar in terms of clarity.  Both are connected with Celestron hardware I purchased new.

 

     I wasn't able to remove the secondary mount from the corrector plate (didn't try very hard as I didn't really need to remove it) so the corrector is the same side in/out as I received it.  There are numbers etched in the edge as I've seen on images of others, and a hue to it when at certain angles, so I feel pretty good about it being original as well as having the coating intact.  Although I am the 3rd owner of this scope, I see no signs of defects, impacts, etc other than a small (maybe 2mm) light scratch in the corrector.  The etched number on the edge is right where the marker is that I put at 3 o-clock facing the scope.  I saw a posting somewhere with images suggesting that was the correct orientation. 

 

     The sky has been cloudy since receiving the scope, so I haven't had a chance to perform a star test or collimation yet (though I will).  I have no doubt at all that my collimation attempt is not perfect, but should be close at least.  It came with the "Bob's Knobs" so I even played with them a bit while under eyepiece and camera.  Other then of course shifting the image, I saw no noticeable change in clarity.

 

     I enjoy the DIY path (engineer) and would like to avoid the additional cost and down time of sending this to Celestron, but have also invested quite a bit of money in this.  Hopefully I can avoid that....

 

Thanks again,

Justin

 

     



#9 starcruiser

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Posted 20 May 2019 - 06:49 PM

Locate your local astronomy club if you haven't done so already. They will be more than happy to assist you in determining the issue with this 11" scope. Sending it back to Celestron should be your last option.



#10 Cpk133

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Posted 20 May 2019 - 07:14 PM

That's a nice view you have there.  Get an artificial star or do a search on making a diy one with a pin hole or small xmas tree ornament.  Then you'll be able to collimate at your convenience.  With the temp delta you describe, forget about an etched view, especially with 11" of aperture and all that focal length.  You picked a good spring to buy equipment.  It's been lousy just about everywhere.



#11 Bramlejd

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Posted 20 May 2019 - 08:39 PM

     Just took off the secondary to inspect it out of curiosity and found "repaired" written on the underside which worries me more about what could have happened to this and if it really was repaired.


Edited by Bramlejd, 20 May 2019 - 08:44 PM.


#12 starman876

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Posted 20 May 2019 - 08:42 PM

Pointing the scope out the window I doubt will work. Take it somewhere away out in the open and then look at distant things.  I have found the larger the scope the worse the view gets through an open window unless the temperature is the same inside and outside.   



#13 Nippon

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Posted 20 May 2019 - 09:17 PM

You are shooting through a lot of air as the saying goes. Even 7 to 10 thousand dollar Nikon and Canon telephotos will look bad shooting through that much distance on a typical sunny mid-day.


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#14 stevew

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Posted 20 May 2019 - 09:36 PM

     Just took off the secondary to inspect it out of curiosity and found "repaired" written on the underside which worries me more about what could have happened to this and if it really was repaired.

It could have come like that new from the factory. They sometimes find issues with certain components and send them back down the line for repair or adjustment before releasing the scope for sale.

Now if it said " needs repair" then I would be concerned.


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#15 ngc7319_20

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Posted 20 May 2019 - 11:37 PM

Wonder if you could set up an artificial star out there somewhere a couple hundred feet away -- on that wall, etc.  Or maybe better to just haul the whole thing to the nearest ball field at night.  At close distances there will be added spherical aberration, but you can still collimate, check for astigmatism, etc.   Even better would be to test on a real star, of course.


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#16 AxelB

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Posted 21 May 2019 - 01:21 AM

That test is non conclusive. There’s the open window thing and there’s roofs and there’s certainly heat radiating from asphalt around the gaz station.

You need to collimate from where there’s no heat sources, when the scope is fully at thermal equilibrium. Use a bright stat at least 45 degrees up in the sky. You also need a night of good seeing (no twinkling stars).

Edited by AxelB, 21 May 2019 - 01:23 AM.


#17 charlesyung

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Posted 21 May 2019 - 01:27 AM

     Thank you all for your helpful responses!  I've been reading and learning a lot from this forum and am very pleased with the kind and helpful responses I've been seeing here vs forums I've been a part of for other hobbies.

 

     I have attached a sample from my Nikon d7000 of the sign through the scope as well as from my iPhone 10s as a reference (not taken at the same time).  Only thing I did to the d7000 image is slide the temperature a bit so it wasn't so red from lack of IR filter.  There was probably a 20degF difference in temperature between indoors and outdoors, but I'm trying not to be the guy with a telescope pointing out into the apartment courtyard.  I am familiar with what heat may do outside from telephoto photography, but didn't see any atmospheric effects that seemed to correlate with the lack of sharpness. I also tried the cell tower next to it.  It's hard to see detail in it, especially when trying to look at the hanging cables etc... attachicon.gif ChevronC11.JPGattachicon.gif ChevroniPhone.JPG

 

     I did also try it at night both on the sign and at some lights at various distances with the same results.  What I see through a 40mm eyepiece and the dSLR appear to be very similar in terms of clarity.  Both are connected with Celestron hardware I purchased new.

 

     I wasn't able to remove the secondary mount from the corrector plate (didn't try very hard as I didn't really need to remove it) so the corrector is the same side in/out as I received it.  There are numbers etched in the edge as I've seen on images of others, and a hue to it when at certain angles, so I feel pretty good about it being original as well as having the coating intact.  Although I am the 3rd owner of this scope, I see no signs of defects, impacts, etc other than a small (maybe 2mm) light scratch in the corrector.  The etched number on the edge is right where the marker is that I put at 3 o-clock facing the scope.  I saw a posting somewhere with images suggesting that was the correct orientation. 

 

     The sky has been cloudy since receiving the scope, so I haven't had a chance to perform a star test or collimation yet (though I will).  I have no doubt at all that my collimation attempt is not perfect, but should be close at least.  It came with the "Bob's Knobs" so I even played with them a bit while under eyepiece and camera.  Other then of course shifting the image, I saw no noticeable change in clarity.

 

     I enjoy the DIY path (engineer) and would like to avoid the additional cost and down time of sending this to Celestron, but have also invested quite a bit of money in this.  Hopefully I can avoid that....

 

Thanks again,

Justin

I do own C4, C8 and C11, image does appear very blurry for a C11 scope. How do you perform the collimation test? I found that I need to use a pinhole light source as artificial star to perform good collimation test at home. My setup for my C11 collimation test was done at basement with 40-50' of distance between C11 and the pinhole light source. However, I need to use 2 diagonal to have enough back focus distance to create a readable airy disc. Otherwise, I'll find a clear night, turn-off the apartment light and pint the C11 to a star at the sky, above the chevron sign, for a star test inside your apartment.

 

There are multiple reason that will cause the image blurry, my C11 scope is current fighting with the Astigmatism optical error. 



#18 Mitrovarr

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Posted 21 May 2019 - 01:53 AM

Be careful judging a large aperture scope in the daytime. Large scopes just look awful in the day in general because the seeing is universally terrible.


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#19 RogeZ

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Posted 21 May 2019 - 07:08 PM

Im not sure for a C11, but for a C14 the distance needed to focus that artificial star is almost 300’. I think 50 is severely short.

#20 Bramlejd

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Posted 21 May 2019 - 08:42 PM

Thought I'd share an update....Read a post on making sure the primary and secondary are concentric by looking down the tube with the secondary removed.  Thought I could see it being out a bit but it was hard to tell so I came up with a little trick.  Drew concentric lines on a clear sheet of plastic and taped it over the end, then looked down and made sure all circles from the plastic, primary, and secondary holder were concentric.  I then adjusted the set screws a little with the outer ring holding screws loosened.  Went back and fourth a few times to get it, but I did find that I did have to adjust it...a couple millimeters would be my guess.  I read somewhere that the holder can have some slop in the corrector plate, so while I wasn't able to remove it, it could have been fastened down out of center by one of the previous owners.  Still being indoors looking out through cloudy light with an admittedly flawed collimation, I'm not going to call it until I can actually get out there having done a proper collimation, but there does appear to be an improvement in the sharpness of the image...if only just a little.



#21 CHASLX200

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Posted 22 May 2019 - 06:10 AM

Land objects won't tell the true story. What looked ok in a 2005 C8 i had on land was nothing like how bad it was really was on stars at high powers.  These scopes vary so much from a rare insane, sharp SCT to complete mush dogs.   You need a cooled scope on a steady nite , check collimation and go from there.



#22 Swanny

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Posted 22 May 2019 - 09:25 AM

I think sending to Celestron for recolmination is around $350. I think that will get you closer to where you want to get quickest.

#23 gfstallin

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Posted 23 May 2019 - 11:45 PM

Thought I'd share an update....Read a post on making sure the primary and secondary are concentric by looking down the tube with the secondary removed.  Thought I could see it being out a bit but it was hard to tell so I came up with a little trick.  Drew concentric lines on a clear sheet of plastic and taped it over the end, then looked down and made sure all circles from the plastic, primary, and secondary holder were concentric.  I then adjusted the set screws a little with the outer ring holding screws loosened.  Went back and fourth a few times to get it, but I did find that I did have to adjust it...a couple millimeters would be my guess.  I read somewhere that the holder can have some slop in the corrector plate, so while I wasn't able to remove it, it could have been fastened down out of center by one of the previous owners.  Still being indoors looking out through cloudy light with an admittedly flawed collimation, I'm not going to call it until I can actually get out there having done a proper collimation, but there does appear to be an improvement in the sharpness of the image...if only just a little.

Advice is all over the map here. The artificial star idea is certainly valid. You can make one or buy one. The cheapest available are around $20 from Hubble Optics. Depending on the size of the star you use, you do not have to be 300 feet or 300 miles from the artificial star to achieve accurate collimation. Now, you do have to be at around 300 feet or so (there is a formula for it) to star test your optics for various aberrations. Closer than a certain distance, and spherical aberration presents itself. That will not be an issue for collimation. To collimate your optics, you only need to be far enough from the artificial star so that you cannot resolve it. For the 50 micron artificial star I have, this distance for me was about 100 feet (~33 meters). It could have been shorter, but probably not by much. This allowed me to see an airy disk of the light point created by the artificial star. For a C8 and C9.25, this was all I have ever needed. The C11 might present another challenge... 

 

With my specific C11 unit, collimation on an artificial star does not lead to perfect collimation when looking at a real star. The C8 and C9.25, at least the units I have, there is no discernible change in collimation when looking at an artificial star in a horizontal position and looking at a real star at an apparent elevation of 40 degrees or more. Collimation holds regardless of the angle of the OTA (horizontal/flat or nearly vertical and looking near zenith). My C11 presents mirror flop when angled at the sky. What happens is, the mirror tilts ever so slightly once the OTA is no longer pointed at a target that is on or near the ground, like an artificial star. Once the OTA is "lifted" at a angle, looking at objects in the sky, the angle of the mirror changes (the forward tilt)/relaxes. This changes collimation. This has to do with the weight of the mirror in the C11. Some people have experienced this with smaller OTAs such as the C8, but that has not been the case for me. 

 

Anyway, your telescope can be perfectly collimated when looking at a star, but be a significant amount out of collimation when looking at the horizon - or vice versa. Because of this, the only thing an artificial star allows me to do with the C11 is practice collimating it during the daytime or a cloudy night. Practice should not be dismissed. This will allow you to know which collimation screws you should adjust in order to collimate properly on a real star. Of course, you can skip this step and go straight to a real star. I needed practice, but not everyone is the same in this regard. I'm not the bravest or most confident soul when it comes to mechanical things. 

 

I've found this graphic on this page to be quite useful for knowing which collimation screw to adjust:  

https://asterism.org...ain-telescopes/

 

Of course, you can figure this out on your own at the eyepiece within a few minutes.

 

As a word of warning, your first collimation experience might take some time. Well, it did for me (see: aforementioned confidence in mechanical skills). Still, once you get the hang of it, either with an artificial star or a real one, you'll be able to do it in five minutes or less for a telescope that is already near acceptable collimation. Tighten down the screws properly and collimation generally holds really well. I've never had to collimate my C9.25 in the 4 years I've owned it. My C11 and C8 have not required collimation in the last 9 months, and unless I drop one of them, I doubt they'll require any adjustments in the near future (year or more). Still, I do check quickly each session. One other thing to note, when I write "tighten" the collimation, they should not be so tight that you have trouble turning them back in the opposite direction if you need to. One description I've read is that you shouldn't have to grip the screwdriver with enough force to break an egg when turning the screw. I don't know how helpful that is as I don't purposely eat eggs or crack them, but it might be useful for you. Eventually, you get a feel for "too tight." The collimation screws on my C8 and C11 are roughly of even tightness. I don't have to struggle to turn them, but they certainly wouldn't turn of their own accord with vibrations typical of careful transport (i.e. not dropping the OTA down a flight of steps). 

 

George


Edited by gfstallin, 23 May 2019 - 11:51 PM.

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#24 ngc7319_20

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Posted 24 May 2019 - 01:36 AM

 

With my specific C11 unit, collimation on an artificial star does not lead to perfect collimation when looking at a real star. The C8 and C9.25, at least the units I have, there is no discernible change in collimation when looking at an artificial star in a horizontal position and looking at a real star at an apparent elevation of 40 degrees or more. Collimation holds regardless of the angle of the OTA (horizontal/flat or nearly vertical and looking near zenith). My C11 presents mirror flop when angled at the sky. What happens is, the mirror tilts ever so slightly once the OTA is no longer pointed at a target that is on or near the ground, like an artificial star. Once the OTA is "lifted" at a angle, looking at objects in the sky, the angle of the mirror changes (the forward tilt)/relaxes. This changes collimation. This has to do with the weight of the mirror in the C11. Some people have experienced this with smaller OTAs such as the C8, but that has not been the case for me. 

 

Yes, even if the primary flops a bit with elevation, the artificial star should get you close on collimation.  Later you can give it a little tweak on a real star.  Indeed the experience is useful.  You can learn which way to turn the screws, and roughly by how much.


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