Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Curved diffraction spikes in astrophotos

astrophotography
  • Please log in to reply
29 replies to this topic

#1 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 21 February 2017 - 10:19 PM

Hello,
I am very new to astrophotography and I had been taking some astrophotos for the last few months with my Orion 8" Newtonian Astrograph and Atlas EQ-G mount.  The camera I am using is Canon T5i non-modified.  When I take pictures that have bright stars in the field, I get diffraction spikes that are very unusual. They are not distinct crosses but cross with curved spikes.   I have attached some pictures along with this email.  Please look at them and let me know what could be wrong.
I am very confused.  I have checked the collimation and also the spider vanes and everything seems normal.  the effect is the same with and without coma corrector. Could anyone please suggest what they are and how I can get rid of them.  I am sure there is something fundamentally wrong I am doing or something I am overlooking.
Thank you,
Regards,
Chandra

 

M42.PNG

NGC2023.PNG



#2 Davester9

Davester9

    Apollo

  • *****
  • Posts: 1,316
  • Joined: 25 Jul 2015
  • Loc: San Tan Valley, Arizona

Posted 21 February 2017 - 10:32 PM

I believe your collimation is off a bit. It would appear your secondary is slightly tilted.



#3 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 21 February 2017 - 10:47 PM

Thanks.  But I checked my collimation using a manual and laser collimators and everything seem right on.  I also have a hard time getting all the circles centered perfectly.  Actually the secondary seem to be slightly off centered but I checked some other discussion forum and they said it was normal with fast newtonians.  My scope is 8" orion astrograph with f/3.9.



#4 Davester9

Davester9

    Apollo

  • *****
  • Posts: 1,316
  • Joined: 25 Jul 2015
  • Loc: San Tan Valley, Arizona

Posted 21 February 2017 - 10:53 PM

I've had one of those astrographs. Laser collimation won't tell you if the secondary is tilted. I had the same problem until I reseated the secondary 90° to the focus tube.



#5 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 21 February 2017 - 11:05 PM

Thanks Dave.  Could you please point me to a technique to that will help me re-seat the secondary 90 degrees to the focus tube at your convenience.



#6 Davester9

Davester9

    Apollo

  • *****
  • Posts: 1,316
  • Joined: 25 Jul 2015
  • Loc: San Tan Valley, Arizona

Posted 21 February 2017 - 11:26 PM

I set the tube on a table with the focus tube facing straight up. I checked it by placing a small level across the top of the focus tube. I anchored the OTA on each side with small wood wedges until it was stable and wouldn't roll.  I loosened the secondary and then used a small ruler on either side of it to verify that both edges of the mirror were the same distance from the inside of the OTA  where the focus tube protrudes. I double checked it a few times, repeating the process until it looked good. I carefully tightened the secondary back up, then collimated again with the laser.  Interestingly, when I put the laser collimator back in after adjusting the secondary, it showed the collimation was off even though I had just collimated it prior to the adjustment.  So I centered the red dot on the center circle of the primary with a small adjustment to the secondary.  Then adjusted the collimation set screws on the primary to center the laser on the bullseye on the laser itself.  Never had any problems after that.


  • cvnori likes this

#7 emflocater

emflocater

    Surveyor 1

  • *****
  • Posts: 1,559
  • Joined: 05 Jul 2011
  • Loc: Clarkson, N.Y.

Posted 21 February 2017 - 11:32 PM

Maybe the spider vanes are not perfectly straight but slightly bent at an angle.

Cheers

Don



#8 Pierre Lemay

Pierre Lemay

    Surveyor 1

  • *****
  • Posts: 1,522
  • Joined: 30 Jan 2008
  • Loc: Montréal, Canada

Posted 21 February 2017 - 11:35 PM

You have to be careful using a simple laser to collimate, especially an f/3.9. You need a barlowed laser (a laser with a barlow in front of it). The return beam will show a shadow of the center spot and it is this center spot that you will center on a paper target disc attached to the barlow (except for a small hole to let the laser beam through).

 

If you don't wish to use a Barlowed laser, you are better off using a Cheshire collimator. It's simpler but much more accurate than an unbarlowed laser. See here for more details on barlowed lasers.


  • Jon Isaacs, Jim Nelson, SteveG and 4 others like this

#9 Oberon

Oberon

    Soyuz

  • -----
  • Posts: 3,504
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 22 February 2017 - 01:17 AM

I've had one of those astrographs. Laser collimation won't tell you if the secondary is tilted. I had the same problem until I reseated the secondary 90° to the focus tube.

Curious...just a suggestion, but it looks to me like your camera chip (or its internal IR filter) may be reflecting back to the mirror, and then back into the camera. If so, then the adjustment recommended above may have had the effect of deflecting the reflection off axis rather than collimating it correctly. In other words, the problem may arise when collimation is spot on, aka when the optical plane is truly parallel to the plane of the chip. Or something to that effect. Either way, slightly tilting the camera would test this theory nicely, but ultimately you don't want the camera tilted with respect to the focal plane.

Problem is that - if I'm right - the reflection is most likely to occur when the camera is correct and collimation is correct.  :gaah: 

Or maybe not. Maybe the reflection only occurs when collimation is wrong. Finger crossed!


Edited by Oberon, 22 February 2017 - 01:18 AM.

  • cvnori likes this

#10 Nils Olof Carlin

Nils Olof Carlin

    Vanguard

  • *****
  • In Memoriam
  • Posts: 2,227
  • Joined: 26 Jul 2004

Posted 22 February 2017 - 04:03 AM

The "spikes" are straight enough - no suspicion of the vanes. And stars appear sharp - no suspicion of miscollimation. Oberon's idea about reflections (#9) seems more reasonable to me, but the "tuft" of light emanates from the star which would be unlikely after long distance reflections (and what would focus such a reflection in a Nwtonian?). Diffraction about some object protruding into the light path might be possible - check for something like it. But the fact that the "tufts" are very much different around the brightest stars in your top picture suggests something else again. Something internal to your camera? The arched edges can hardly come from diffraction as such - internal vignetting somewhere?

 

Many things it (likely) cannot be. But what is it?


  • Daniel Mounsey, Jon Isaacs and happylimpet like this

#11 Oberon

Oberon

    Soyuz

  • -----
  • Posts: 3,504
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 22 February 2017 - 04:30 AM

It would help diagnosis if you would:-

a. send larger higher resolution images, and maybe some more examples.

b. take some photos of the inside of your telescope, clearly showing every thing in the optical path that might generate diffraction or reflections. Also through the focuser.

 

thx



#12 dan_h

dan_h

    Mercury-Atlas

  • *****
  • Posts: 2,996
  • Joined: 10 Dec 2007

Posted 22 February 2017 - 08:46 AM

If you magnify the images you can see that every star has a flaring light associated with it but no two are the same. To me, that would suggest that this is not the result of something in the optical path or a collimation problem. That only leaves the camera.

dan

#13 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,430
  • Joined: 01 Feb 2006

Posted 22 February 2017 - 09:17 AM

This problem comes up about every two years and it is almost always the same thing.

Somewhere in the system something is probably vignetting.   What you are seeing is probably the diffraction effect of the curved surface on the off axis light rays.   This is crystal clear because in the top image you can actually see the curve!!! If you look at the bright star on the left and the not quite as bright star near the right border, and you follow the arcs to where they would join, you will see that they would form a perfect circle!

 

diffraction ring.jpg

 

Chances are that somewhere you have a component with an inner diameter that is too small to pass the full light bundle.

 

Also possible that the secondary mirror is to small and this too can induce some diffraction if there is a metal can type holder.

 

Check your light path.     

 

You can troubleshoot it with math if you want.   What you need to do is measure the distance of the inner diameter of every component between the camera and the end of the focuser tube and then calculate the size of the off axis light cone at that distance ahead of the focal plane.  Any of them that fall below the diameter of the light bundle will cause the diffraction artifact you see.

 

Or, you can put in a sight tube and with your eye at the position of the focal plane, move all the way to either side of the sight tube to see if anything is blocking the edge of the primary mirror.  If you can't see the edge of the primary when your eye is at the same distance that the corner of your camera frame is, then whatever is blocking your view of the primary is the same thing that is vignetting the field..

 

Again, this comes around every now and again and the signature of an off axis vignetting source is amost always a circular diffraction artifact.

 

When it is this bright, it is almost always something close to the focal plane causing it. Filter wheels or other circular openings would be a possible candidate. 


Edited by Eddgie, 22 February 2017 - 09:38 AM.

  • Jim Nelson, Asbytec, Oberon and 4 others like this

#14 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,430
  • Joined: 01 Feb 2006

Posted 22 February 2017 - 09:25 AM

And one other way to check and troubleshoot and you can just image a defocused star at the center of the field and at various distances from center.   If there is a vignetting source, you will see a point where a curved bite starts appearing out of the defocus pattern that gets worse the further you move from the center of the field.    Now in an visual scope this is due to the fact that the secondary mirror is undersized, but this will not usually cause a diffraction artifact.   I your case though, the diffraction artifact points to a restriction in the outer rays of the light cone.   This would almost always be caused by some component between the front of the focuser tube and the focal plane in an imaging scope (which should have a big enough mirror to fully illuminate your chip).

 

Possible that it is a reflection from an improperly baffled component but my money is on vignetting.  


Edited by Eddgie, 22 February 2017 - 09:44 AM.

  • Ernest_SPB, Oberon and cvnori like this

#15 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 22 February 2017 - 10:05 PM

Hello All,

 

This is very good and I really appreciate all the responses.  I am so glad to be a part of this community.  I have read all the posts and I am still trying to understand some of the terms used here like vignetting, etc.  I went ahead and took a look at the OTA and found a few problems pointed out in some posts and here are my first observations:

 

1.  The OTA is horribly out of collimation (although some of the stars were distinct points in the images).  I am sure I checked it using my laser pointer and the cheshire before the session and I am somewhat baffled when this could have happened.
2.  When I first got the OTA I immediately introduced two pieces of foreign material that include a aluminum and a Teflon ring in the secondary mirror assembly. This is between the housing and the mirror to prevent the wear due to repeat collimations.  I can see some part of the two pieces when seen at an angle from the focuser hole.  Additionally I also added Bobs Knobs for easy collimations.  I dont know if the added rings would cause any diffractions but I will remove them in my next session and see what happens.

 

I think I have a lot to fix before my next observation.  I shall keep updated.

 

 

 

Attached Thumbnails

  • pic-1.GIF
  • pic-2.GIF


#16 Oberon

Oberon

    Soyuz

  • -----
  • Posts: 3,504
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 22 February 2017 - 10:19 PM

No, those rings won't have any detrimental effect. The primary mirror cannot see them, they are well hidden behind the secondary. So don't worry about them, because whatever is causing your problem it is affecting light coming from the star via the primary mirror.

Good photos btw. Send some more plz.



#17 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,430
  • Joined: 01 Feb 2006

Posted 22 February 2017 - 10:46 PM

Yes, these are not the issue.   You need to be looking at everything behind the focuser first,

 

Also, I was patient and explained how to diagnose the problem several ways.   You should try some of them. 


  • Oberon likes this

#18 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 23 February 2017 - 10:03 AM

Eddgie,

Your troubleshooting tips are very important.  Thank you very much and I shall get on to them one by one.  Firstly I am trying to eliminate all the items that do not cause the issue at hand.  in the path of the light I have the following components in the order:

  1. OTA Shell, spider vanes with secondary mirror support block
  2. Primary Mirror
  3. Secondary Mirror (the 8" Astrograph comes with a larger mirror; 70mm minor-axis)
  4. Crayford focuser tube
  5. medium profile (length) adapter tube threaded into focuser tube.  Note that it does not use all the provided threads when bottomed and I also see some part of the exposed threads have lost its black coating exposing shiny metal.  I dont know if this will be a big concern.
  6. Coma corrector with an add on ring for securing camera T-ring
  7. T-ring for Canon T5i
  8. Canon T5i Body w/CMOS sensor.

In the past I took pictures w/o coma corrector (that includes ring) and I had the similar diffraction rings.  So I can eliminate the 6.  So only thing left between the end of focuser tube and the camera is the T-ring, the focuser tube, the medium profile adapter tube.  Focuser tube is part of the crayord assembly and I do not think it is the cause.  So I am left with T-Ring and the medium profile adapter.

 

Now I can follow the method in your 1st post to see where the real problem is.

 

Thanks

Chandra


Edited by cvnori, 23 February 2017 - 02:41 PM.


#19 Oberon

Oberon

    Soyuz

  • -----
  • Posts: 3,504
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 23 February 2017 - 05:42 PM

Is your adaptor tube 2" or 1.25"?



#20 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 23 February 2017 - 10:44 PM

Hi Oberon,

The adapter I have is a 2" tube.  I think Eddgie's post is making some sense but I do not understand it fully.  I am tying to do the math method he described but without a sketch it is being difficult to go forward.  I am attaching a hand sketch of what I have with approximate dimensions (almost to scale).

 

Hi Eddgie,

Please comment on my sketch if this is what you meant and what you think.  I surely see with A-Setup w/o Comma corrector, a cone originating. With B-Setup w/ coma corrector not sure.  Please note that the coma corrector is longer than the adapter.  Per the drawings, I made them exploded views with items numbers defined in circles.  I feel I don't think I am using the recommended adapters by Orion.  I just picked up what was supplied with the telescope that seem to work not realizing vignetting if you think that is the case.

 

Thanks.

Chandrasekhar

Attached Thumbnails

  • IMG_2413.JPG
  • IMG_2414.JPG


#21 Oberon

Oberon

    Soyuz

  • -----
  • Posts: 3,504
  • Joined: 24 Feb 2013
  • Loc: Hunter Valley NSW Australia

Posted 23 February 2017 - 11:43 PM

OK, your sensor size is (22.30mm x 14.90mm) and your smallest aperture appears to be 1 3/8" or 35mm. Your secondary is 70mm.

Comments:

1. your secondary is barely big enough. Its OK, just, but your camera needs to be as close to it as possible.
2. your adapters are far from ideal; the 35mm constraint must be closer than 70mm from the camera chip to avoid vignetting, and the adapters serve to keep the camera at a greater distance from the secondary than needed.
3. it is possible to get a 2" adaptor that fits straight onto your camera in into the focuser, or into a 2" fitting on the CC

Changing your camera adapter/s is likely to fix the problem because it addresses all 3 points. Of course you will have to lower your primary mirror to suit; presumably you have it adjusted high at the moment?

Finally, its just possible that your focuser tube is too long. Was that an option? Check it because my f/4.5 has a long focuser tube to allow me to both use a camera and eyepieces without adding adapters, and the cost of that is that I risk vignetting a wide field.

 

gallery_217007_4746_58084.jpg



 


  • cvnori likes this

#22 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 27,430
  • Joined: 01 Feb 2006

Posted 24 February 2017 - 09:31 AM

What is the size of the aperture on the front of the coma corrector?  Is the opening very big or is the element retained in a cell with a wide ring at the front?  It is possible that the vignetting source is the coma corrector, or as Oberon mentioned, it is possible that a lot of in-focus is causing the front of the focuser tube to intercept the off axis rays.

 

While I agree that your secondary mirror may be a bit small, this would not cause you to see a diffraction artifact (which I of course think this is).  It would cause illumination falloff but that would not produce the ring.

 

The clear aperture of the coma corrector is important at these speeds.   If it has a retaining ring that is rather wide, this could be an issue.

 

I think you are in the right spot.

 

And the picture thought the focuser... This view needs to be done with the same attachments or devices that you use to image minus the camara.  If you rack out to the same position that you would use for imaging, then hold your eyes behind that to the point where your camera focal plane would be (Flange to Focal Plane distance) and move your eye to the edge of the field, you should be able to see the entire primary mirror in the view.  If you cannot see the entire primary because some part of your adatper setup, focuser tube, or your Coma Corrector blocks the edge of the primary mirror, that is your vignetting source.

 

Again, I am not 100% sure that this is your problem but having see it many times over the decades, I would say that it is highly likely to be your problem.  

 

It is though I assure you nothing to do with your collimation or your spider.  Neither of these have anything to do with it because they are in no way capable of producing a curved diffraction pattern.

 

Also, don't dismiss glint.  I mentioned this earlier and should have given it more emphasis, but this should be easy to check.   Any hard anodized surface that is not matte or is not threaded can cause glint, but this looks more like diffraction to me.   When you sight though the setup though, pay careful attention to all of the surfaces inside your focuser and attachments to ensure that all of the surfaces are very flat black. 


Edited by Eddgie, 24 February 2017 - 09:32 AM.

  • cvnori likes this

#23 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 24 February 2017 - 09:54 AM

Hi Oberon,

 

Just what I came across reading the telescope manual that suggests a 2" adapter that you mentioned to be used rather that the whole combination of parts I am using just to get the image into focus not realizing the vignetting issue.  The sad thing is Orion does not provide this adapter along with the Astrograph that is made for the Astrophotography as a package.  I feel most of the other adapters provided are only useful for viewing purpose that I believe is not the main purpose of this OTA.  The scope cannot be used for astrophotography right out of the box.  I like this scope because of many reasons that includes internal baffles (I believe for stray light absorption/cooling), the Crayford focuser and f/3.9.  I found out that the 2" adapter costs about $29.00+shipping.

 

Thanks for sharing the picture.  Could you also tell me how you setup if you use the coma corrector?

 

Thanks once again,

 

C.



#24 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 24 February 2017 - 10:04 AM

Thank you Eddgie,

As soon as I go home today I will look into your notes and follow your instructions.  I am just getting used to the AP jargon and is being little hard to follow some of the instructions but I will get to it.  It is quite possible that the coma corrector being longer than the adapter could be penetrating into the light path in the normal direction causing the strange curved diffractions.  I will check my setups today and play with the trouble shooting fun, analyzing it.  I also consulted the Orion Telescopes and they were quick to point some of the issues you mentioned.  I have too much to understand and digest here because of my unusual (ignorant) setup I created.  I will keep you posted.

Best,

C.



#25 cvnori

cvnori

    Explorer 1

  • *****
  • topic starter
  • Posts: 80
  • Joined: 09 Jun 2016
  • Loc: Ellington, CT

Posted 17 January 2018 - 01:31 PM

Hello All,

Sorry for the delay in my update of how I could resolve the problem:

I sent out the scope back to Orion and they ended up replacing it for the issue I had.  I did not get any reason from them as they replaced the OTA very quickly.  When I received the replaced scope I did not see any construction differences.  The focuser drawtube did stick into the OTA that I thought was the issue at one point.  It turned out it was not.  I really do not know what the fix is but I do not have the curved diffraction spikes any more. One thing I can say for sure is that Orion Telescopes really provides a good customer support.  Now I am able to get some very good images. See attached.  Note that the 1st picture I posted in this thread is a section of M45.  Same is shown below.

Thanks

 

Attached Thumbnails

  • M45 - Pleiades Open Cluster in Taurus.jpg

Edited by cvnori, 17 January 2018 - 01:33 PM.

  • Jon Isaacs and DesperateSky like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: astrophotography



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics