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Strange stars from ED127

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#1 Masloff

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Posted 13 October 2015 - 10:45 AM

Well originally I posted this on the "Beginning and Intermediate Imaging" forum, but seems like it isn't the ideal place because nobody answered me.
The day of the lunar eclipse I took some data of M42 with my new Explore Scientific ED127 refractor simply as an experiment to see how it worked, the brightest stars have some kind of flare and chromatic aberration, that's normal or my scope have bad optics? Or it's because I wasn't in good focus? I'm pretty fresh in this, it's my first refractor and was my first time trying to do deep space astrophotography, so I really don't know what to expect, the mount wasn't well polar aligned, the seeing that night was "normal" around here. This is a single 13s ISO 3200 capture with my Canon 70D and a crop of the same.

M42 1.jpg

 

M42 2.jpg



#2 Masloff

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Posted 13 October 2015 - 10:52 AM

The images of the eclipse were good, at least to my eyes

 

Eclipse.jpg



#3 mattyk-usa

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Posted 13 October 2015 - 10:58 AM

Others will weight in I am sure, but that looks a little like moisture/dew on the objective, to me.



#4 Eddgie

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Posted 13 October 2015 - 11:00 AM

I have taken the liberty of copying your picture and I hope you do not mind.

The brightest star in the picture is showing some kind of diffraction artifact, or possible the lens centering screws are to tight.  You can see that there is a Maltese Cross shaped pattern around the star.

 

If the scope has radial centering screws, it is possible that they are a tiny bit tight, but more likely is that something is sticking into the light path.

 

With an out of focus star, see if you can see any indentations in the outside of the ring pattern.  If you can, use your finger in front of the objective to locate the angle and mark it. 

 

Then, take a flashlight an carefully inspect all of the inside of the tube, focuser, and any mounting couplers that you use in your configuration for anything that might be sticking into the light cone.

 

Some Infra red filters installed on some chips can also induce diffraction artifacts.  Many imagers remove the IR filter for astronomy because CCDs can actually capture IR that contributes to the image.

 

 

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  • Diffraction artifact.jpg


#5 Eddgie

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Posted 13 October 2015 - 11:05 AM

As mentioned, perhaps dew, but it looks highly symmetrical to me.   

 

I would also check to make sure that the focuser tube is collimated properly to the objective with a laser.   The small star to the lower right of the big one shows cut off.  This can be an artifact caused by the focuser tube being tilted.

 

You may want to check on a very bright star and just visually inspect the pattern to see if you can find any evidence of diffraction or focuser tube tilt.

 

If you have focuser tube tilt, when a very bright star is defocused about 20 waves, as you move the star to the edge of the field in N/S/E/W directions, you will see that the pattern starts to get "Cut off".  If he cutoff occurs about the same distance in every direction, the focuser is probably in alignment.  If on the other hand, you see that it starts to cut off much sooner in one direction than in the other, the focuser tube is likely tilted.



#6 Eddgie

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Posted 13 October 2015 - 11:07 AM

The more I look at the image, the more I ralize that allmost all of the stars in the upper right hand corner are being cut off.   Focuser is out of alignment, or your mechanical connection is not square or sagging.

 

I would identify this problem first.   A tilted or sagging focuser/adapter train/camera can give all kinds of weird diffraction artifacts.

 

Notice that stars do not look as sharply focused on that corner, and any tilt will cause the field to not be in focus on all four corners.

 

I would start then by looking for focuser collimation or sag in your imaging train.   Tilt can cause diffraction and defocus.


Edited by Eddgie, 13 October 2015 - 11:10 AM.


#7 mattyk-usa

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Posted 13 October 2015 - 11:17 AM

Eddgie, you're the professor here so I'll always defer to your position.  I will give my reasoning, in the hope that you can fix my thinking / assumptions where it's not correct...

 

I did posit the dew because of the extended blur on all stars.  I do see that symmetrical indentation that seems to indicate a diffraction effect, but it seems inverted from the "diamond star" I'd expect to see if it were just diffraction.  Also, the stars and the effect do seem to be symetrical, which is why I wan't thinking optical train sag.  



#8 Masloff

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Posted 13 October 2015 - 11:40 AM

The first time I used the telescope I took this, the stars didn't had any kind of strange artifact, they are only a little elongated because of bad polar align, so, seems like the scope didn't had any problem at that time, I doubt that something is sticking into the light path, I can't see anything inside the OTA neither the lens, for me sounds more probable that the focuser or something was tilted, it can be pinched optics too right? the night I took the M42 image we didn't had dew but the night was a little cold (about 10C, that's cold in South America)

 

CN1.jpg

 

CN2.jpg



#9 Masloff

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Posted 13 October 2015 - 12:02 PM

I also took this the night of the strange stars of M42, practically all the stars look fine (except some elongation) and only one have a noticeable flare 

 

CN5.jpg

 

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CN3.jpg



#10 GaryJCarter

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Posted 13 October 2015 - 12:09 PM

Look into the front of your objective. Do the spacers between the elements extend out into the light path? 



#11 Mark9473

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Posted 13 October 2015 - 12:11 PM

How high above the horizon was Orion at the time of imaging?



#12 Jon_Doh

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Posted 13 October 2015 - 12:16 PM

Could your lens possibly be out of collimation?



#13 Masloff

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Posted 13 October 2015 - 12:20 PM

How high above the horizon was Orion at the time of imaging?

it was about 30 degrees high

 

 

Could your lens possibly be out of collimation?

I don't think that, I can't test it now because we are cloudy and I don't have my mount, but I remember defocusing stars that night and at least visually it was collimated 

 

 

Look into the front of your objective. Do the spacers between the elements extend out into the light path? 

I looked and I can't see nothing strange neither asymmetrical in the objetive



#14 Mark9473

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Posted 13 October 2015 - 12:54 PM

 

How high above the horizon was Orion at the time of imaging?

it was about 30 degrees high

 

 

Could be some atmospheric refraction at play then.



#15 Jon Isaacs

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Posted 13 October 2015 - 04:14 PM

 

 

How high above the horizon was Orion at the time of imaging?

it was about 30 degrees high

 

 

Could be some atmospheric refraction at play then.

 

 

In my experience atmospheric refraction is not uniform around the object, on an extended object, one side is purple, onecread or orange.

 

Jon



#16 Eddgie

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Posted 13 October 2015 - 06:17 PM

Eddgie, you're the professor here so I'll always defer to your position.  I will give my reasoning, in the hope that you can fix my thinking / assumptions where it's not correct...

 

 

 

Trust me, I am no professor... Just guessing.  It sure looks like some kind of diffraction artifact, and the way the stars look cut, I would still look at the focuser to see if it sagged or something.

 

The Pinion shaft on Crayfords can deflect under load and the tube can seesaw and pull away from the top rollers.  I have encountered this in reflectors and could see a huge change in collimation.

 

If you have not messed with collimation, and don't see anything sticking into the light path, then about the only thing that I can think of is deflection or decentering of some kind.

 

 

You can see that the stars appear to  be cut off on one side though, and that is consistent with a tilted or deflecting defocuser tube.

 

Do let us know when you find out. 



#17 Mark9473

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Posted 14 October 2015 - 10:50 AM

 

 

 

How high above the horizon was Orion at the time of imaging?

it was about 30 degrees high

 

 

Could be some atmospheric refraction at play then.

 

 

In my experience atmospheric refraction is not uniform around the object, on an extended object, one side is purple, onecread or orange.

 

Jon

 

 

I agree; it was just a thought upon seeing the red on only one side. The overall purple halo could be seeing related, I thought. But upon further scrutiny it appears the red arc is on the wrong side for it to be atmospheric refraction.



#18 Eddgie

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Posted 14 October 2015 - 05:57 PM

The fringing could also be an artifact of tilt in the system.  

 

This would be where I would be putting 100% of my efforts at this time.   If the focuser has tilted, if there is sag in the imaging train, of the the camera is not square to the focal plane, you can get all kind of artifacts.    

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#19 PowellAstro

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Posted 14 October 2015 - 06:38 PM

If you are using the diagonal, they have screws for collimation. They can be knocked out of alignment. I have had one such case.

#20 Illinois

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Posted 15 October 2015 - 06:43 AM

Maybe its from your camera and use your eyepiece to look at stars for test. Look at double-double stars in Lyra. Around 100X and see 4 pinpoint stars then its good telescope. Test on Venus to see if any color like purple at edge of Venus.



#21 Masloff

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Posted 15 October 2015 - 12:34 PM

I was looking today at the lens cell and clearly the lens cell have some tilt in relation to the dew shield, I don't know if the tilted thing is the dew shield or the lens cell but definitely they aren't parallel, I tried to create an artificial star and the airy disks are concentric but not perfectly round, i'm not pretty sure about that so I want to buy a chesire eyepiece and "real" artificial star, supposing that really I have a tilted lens cell, it can be collimated with the push-pull screws by me? I can't send the scope to Explore Scientific to do it, I am pretty far, I would be grateful if someone can handle me a good collimation guideline


Edited by Masloff, 15 October 2015 - 12:34 PM.


#22 Masloff

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Posted 15 October 2015 - 12:58 PM

I think that in this pic is visible the tilt

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  • image.jpg


#23 Eddgie

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Posted 15 October 2015 - 10:28 PM

Send a picture with the lens pointing at the camera.

 

Before you do the lens though, you should do the focuser tube and check it for tilt.  A refractor collimation should always start by checking that the focuser is perfectly square to the optical axis   

 

The best way to do this is to use a laser collimator inserted into the rear of the focuser tube (no diagonal).

 

If the focuser is off and you collimate using a Cheshire lens, then you will wind up tilting the lens to match the error in the focuser.

When imaging, two wrongs don't make a right.  If the focser is not perfectly aligned to the mechanical axis of the tube, that has to be fixed first.



#24 PowellAstro

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Posted 16 October 2015 - 12:10 AM

It can also be the lens centering is off and the lens was tilted to try and correct this centering error. If you have a set of V rollers, you can rotate the tube while reflecting a laser off the from lens to the wall. The lens is collimated when the laser reflection does not move while rotating the tube.

#25 Masloff

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Posted 16 October 2015 - 12:56 AM

I don't think the focuser is tilted but surely the lens cell is, looking closer I can see that the front ring of the lens cell is tilted in relation to the rest of the lens cell, I did some moments ago this star test with my DIY pinhole artificial star, there is that dark line going from the border to the center, I don't know what it is because nothing was in the light path, the rings aren't perfectly concentric and I can see that the lower right border of the disk is a little flattened

Attached Thumbnails

  • star test.jpg

Edited by Masloff, 16 October 2015 - 12:59 AM.



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