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Could use some help figuring out cause of extra flare

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#1 Whichwayisnorth

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Posted 06 August 2014 - 05:19 PM

Attached File  spikey.jpg   34.54KB   0 downloadsI have an 8" F 3.8 Newt that I'm playing with.  I was setting up the collimation on it and when on a bright star I am very happy with the nice concentric rings through focus and while I am no expert at collimating reflectors, I *think* it looks pretty good.   HOWEVER, when I tighten up the focus I notice an extra flare between the normal four spikes from the spider.  Any bright star in the image shows this extra flare.   My first thought is that I just need to tweak the collimation a bit to bring this "flare" into the middle but tweaking collimation ends up making everything else bad.

 

Using a Hotech laser collimator I made sure that the laser is landing dead center on the mirror (the mirror is marked) but other than that I don't know how to make sure the secondary mirror is physically and properly centered.  I am looking for something that could be obstructing the light path but the only thing that comes to mind are the nuts that hold on the finder scope bracket but they seem to be hidden below the line of the tube support ribs.  

 

Well anyways, please take a look at this video and let me know what I'm doing wrong.  

 

Youtube http://youtu.be/vMD10bBxwrE

 

attached is an example of a quick image I did last night.  I cropped out the bright star to show you how it looks very similar to what i see when using a laser to collimate.  


Edited by Whichwayisnorth, 06 August 2014 - 05:49 PM.


#2 choran

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Posted 06 August 2014 - 06:08 PM

I don't know the answer, but given the photograph you have wisely posted, I don't doubt that someone will come along with a satisfactory answer.  I wonder if perhaps your focuser somehow intrudes into your light path?  Or that there is something not properly blackened that is reflecting light?  Or that the mirror is being pinched somehow and thus is producing a less-than-ideal image?  These are simply wild guesses.  I am curious to see how this works out, however.



#3 MitchAlsup

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Posted 06 August 2014 - 08:05 PM

The focuser draw tube intercepting the light cone will cause such flare.

 

Focus on a star, then bring the focuser inward until the star is defocused 20-30 waves. The focuser will be silhouetted on the stellar image if the draw tube is at fault.



#4 Whichwayisnorth

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Posted 06 August 2014 - 08:09 PM

The focuser draw tube intercepting the light cone will cause such flare.

 

Focus on a star, then bring the focuser inward until the star is defocused 20-30 waves. The focuser will be silhouetted on the stellar image if the draw tube is at fault.

 

The focuser draw tube is in line with one of the veins.  Should that not cause the flare to be right over one of the spikes?

 

I'll try it tonight.  I also want to take a few defocused images to post up here tomorrow.


Edited by Whichwayisnorth, 06 August 2014 - 08:10 PM.


#5 azure1961p

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Posted 06 August 2014 - 08:10 PM

Im wondering if this image was with the star far from the center of field.

 

Pete



#6 Whichwayisnorth

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Posted 06 August 2014 - 09:27 PM

 

The focuser draw tube intercepting the light cone will cause such flare.

 

Focus on a star, then bring the focuser inward until the star is defocused 20-30 waves. The focuser will be silhouetted on the stellar image if the draw tube is at fault.

 

The focuser draw tube is in line with one of the veins.  Should that not cause the flare to be right over one of the spikes?

 

I'll try it tonight.  I also want to take a few defocused images to post up here tomorrow.

 

 

Im wondering if this image was with the star far from the center of field.

 

Pete

 

Hi there Pete.

No actually the star was towards the center of the field.    Not dead center but reasonably close.  On my first alignment star of the night I am going to center a star and take a few photos to show what it looks like.  I'll get it as close to perfectly focused as possible and in the center. Then I'll do a few pictures of out of focus to show the rings.      I also need to do this anyways to test for coma in the corners so I might as well do it all at once.



#7 Whichwayisnorth

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Posted 06 August 2014 - 10:51 PM

Attached File  starflare1.jpg   295.78KB   0 downloads



#8 Whichwayisnorth

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Posted 06 August 2014 - 10:57 PM

Attached File  rings1.jpg   210.72KB   0 downloads



#9 Whichwayisnorth

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Posted 06 August 2014 - 10:58 PM

Ok those last two are of Arcturus just a moment ago.   Any thoughts?  Does my collimation look bad?



#10 Stephen Kennedy

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Posted 06 August 2014 - 11:10 PM

The inner ring on the lower right quadrant is very bright compared to the rest of the defocused image.  Maybe that is a clue to what is causing the flare.



#11 Jon Isaacs

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Posted 07 August 2014 - 05:07 AM

 

 

 
Using a Hotech laser collimator I made sure that the laser is landing dead center on the mirror (the mirror is marked) but other than that I don't know how to make sure the secondary mirror is physically and properly centered. 

 

 

How are you making sure that the primary is properly adjusted..  If you are using a laser, then some form of the Barlowed laser is necessary.  Also, have you checked the collimation of your laser?

 

Jon



#12 Asbytec

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Posted 07 August 2014 - 07:32 AM

The inner ring on the lower right quadrant is very bright compared to the rest of the defocused image.  Maybe that is a clue to what is causing the flare.

It does and might be a clue. Good catch.

 

In fact, the entire ring along the bottom is brighter and the Poisson spot is displaced below center, as well.

 

The shadow looks displaced toward the top opposite the bright ring and Poisson spot. The upper part of the pattern is actually thinner. Is this an offset artifact? Is the diagonal offset in toward the top (whichever direction that is in the tube, away from the focuser?) 

 

But the fact the lower part of the pattern appears thicker, the inner bright ring peeks out from the shadow below center, and the Poisson spot not being dead center of the pattern seems like collimation might be a bit off.

 

Something appears not straight, but it may be due to any focuser offset. Still, the Poisson spot should be dead center, right? What about the other side of focus, is the image reversed with the Poisson spot above center?

 

I've seen a similar effect after disassembling my own CAT. The central shadow was off center on opposite sides of focus and my stars flared a bit as the first ring was a bit brighter on one side. It took a little mechanical adjustment so the central shadow stayed centered through defocus on both sides.

 

I believe there maybe a mechanical misalignment causing the optical axis to not be squared up the focuser. This was true with mine. It cause me to "look" at the pattern from "above" on one side of focus and from "below" on the other as the focuser axis deviated from the optical axis. It's almost as if one side of the pattern is a tiny bit closer to focus than the other causing some flare at best focus.

 

Here's an illustration of what I mean (exaggerated) on the left, and a closer look at yours on the right. That may not be the answer, but something seems amiss.

Attached Files



#13 Whichwayisnorth

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Posted 07 August 2014 - 11:45 AM

Ok so it looks like I need to spend some time today re-aligning the optics :) 

 

The OTA shipped with the primary loose to prevent damage and so I had to pull it out and tighten up the holder on it.  Also one or two of the screws holding the spider in place (outside of the tube) were not very tight.

 

I need a clever way to verify the spider is properly centered in the tube. I think I'll make a paper template and mark the center, fold it in half and use it to see if the center bolt is actually center.  Then I will re-collimate and see what happens.  I guess the primary mirror may not be centered either?  Sounds like a big project to get this all sorted.  I need a local expert to come over and drink beer and help. 



#14 timmyt101

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Posted 07 August 2014 - 01:42 PM

Would you please share how you took the photos of Arcturus?  Looks like maybe a webcam and software?  Thanks.

 

Tim



#15 labmand

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Posted 07 August 2014 - 04:03 PM

"I neeed a local expert to come over and drink beer and help. "

If I were nearby I would drink beer and watch  :waytogo:



#16 Asbytec

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Posted 07 August 2014 - 06:41 PM

North, if you mean you clamped down on the mirror retaining clips, they should be loose. Well, they should not be resting on the mirror's surface as being tight they will distort the mirror enough to pinch it and disturb the in focus image. You should be able to slip a post card or even a credit card between the clip and the mirror. 

 

The diagonal can be in the center of the tube, but it can also be offset away from the focuser and toward(?) the primary a tiny bit. It looks like, in your video above looking down the focuser, your diagonal is offset - which is preferred because (IIRC) it only affects the position of the fully illuminated field (centered in the eyepiece.) So, that may be why your diagonal shadow is off center, I'd leave it or at least verify the amount of offset.

 

So, I am not sure I nailed down the cause of the flare nor centering the diagonal will help. I'd really be curious to have a beer (not being an expert) and roll back and forth through focus to see how that ring brightening and the Poisson spot (faint spot below center in your image) behaves either side.

 

Something looks slightly weird in the image above (bright ring below along with an off center central bright spot within the shadow. I've seen something similar in a different design that had some flaring, it took some mechanical tweaking of the focuser axis and re-collimation to fix it...in my scope's design there is no secondary offset.

 

So, my post was just a thought about what it might be by looking at what should be. The secondary shadow will be off set if the diagonal is, and it should be. The kicker is the Poisson spot should be at the center of the concentric diffraction pattern. It's not. And one side of the inner ring is brighter. It does not appear to be an optical issue, maybe mechanical in nature (then requiring slight re-collimation to bring it to perfect.)



#17 dave brock

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Posted 08 August 2014 - 09:58 PM

North, if you mean you clamped down on the mirror retaining clips, they should be loose.

 

I think Norme's right and the cell needs further investigation.

 

I'd have thought if it was a collimation issue the flares would be more in one direction only, i.e. coma.

The defocused image shows the first signs of the flares on the outer edge at 12-2 o'clock, 5.30-7 o'clock and 8-9 o'clock.

 

Is the exposure 1 second? The bright ring area on the lower right of the secondary shadow could be an effect of turbulence that just happened to be in that area at the time of the exposure. Any miscollimation doesn't seem to be aligned with that bright area.

 

Dave


Edited by dave brock, 08 August 2014 - 10:00 PM.


#18 dave brock

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Posted 08 August 2014 - 10:27 PM

I just looked at the collimation video and I'm pretty sure the cross on the laser spot is not from the spider vanes as no part of the laser light goes past them, that is, they are not in play. If you rotate the laser in the focuser do the spikes and flare rotate as well? If so, they're from the laser and not related to the star image problem.

 

Dave

 

Edit: I mean if the spikes and flare rotate in relation to the scope. Have you looked at the spot on a smooth flat wall?


Edited by dave brock, 08 August 2014 - 10:39 PM.


#19 dave brock

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Posted 08 August 2014 - 10:56 PM

Assuming your scope was well cooled so the flare isn't from tube currents, could you try an experiment? Cut a 7 1/2" circle in a piece of card and carefully center it over the front end of the tube (stop the mirror down by 1/4" all round) and check a well centered star again.

 

Dave



#20 Whichwayisnorth

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Posted 09 August 2014 - 02:24 PM

North, if you mean you clamped down on the mirror retaining clips, they should be loose. Well, they should not be resting on the mirror's surface as being tight they will distort the mirror enough to pinch it and disturb the in focus image. You should be able to slip a post card or even a credit card between the clip and the mirror. 

 

The diagonal can be in the center of the tube, but it can also be offset away from the focuser and toward(?) the primary a tiny bit. It looks like, in your video above looking down the focuser, your diagonal is offset - which is preferred because (IIRC) it only affects the position of the fully illuminated field (centered in the eyepiece.) So, that may be why your diagonal shadow is off center, I'd leave it or at least verify the amount of offset.

 

So, I am not sure I nailed down the cause of the flare nor centering the diagonal will help. I'd really be curious to have a beer (not being an expert) and roll back and forth through focus to see how that ring brightening and the Poisson spot (faint spot below center in your image) behaves either side.

 

Something looks slightly weird in the image above (bright ring below along with an off center central bright spot within the shadow. I've seen something similar in a different design that had some flaring, it took some mechanical tweaking of the focuser axis and re-collimation to fix it...in my scope's design there is no secondary offset.

 

So, my post was just a thought about what it might be by looking at what should be. The secondary shadow will be off set if the diagonal is, and it should be. The kicker is the Poisson spot should be at the center of the concentric diffraction pattern. It's not. And one side of the inner ring is brighter. It does not appear to be an optical issue, maybe mechanical in nature (then requiring slight re-collimation to bring it to perfect.)

 

Right, when I mentioned the retainers on the mirror I meant along the outside so that the mirror can not slide.  My understanding is that you want the retainers to barely touch (sheet of paper thickness) where the retainers touch the reflective surface. 

 

I also verified that the collimator itself is collimated by placing it snug but not tight inside the focuser and then rotating it while looking at the point of light at the center of the mirror.  Looks like it doesn't move at all.

 

So at this point I wonder if the secondary needs to be offset.  A little google searche http://www.skyandtel...condary-mirror/ I learned that the formula is Offset = (secondary size)/(4*focal ratio).  I think it would be trial and error for me without the proper collimating tools and the experience in how to use them.  My secondary appears to be 2.5" and my focal ratio I believe is F/3.8   So.. 2.5/15.2 = .1645" or 11/64" or 4.18mm.   That seems legit.  I'll give it a try.

 

 

Using a barlow, I only have a 2x barlow, diffuses the laser to the point where I can't get a nice round dot. 

 

I am using my ASI 120mm with a barlow and an extension tube to try and fine tune the collimation.  I thought I made some good progress the other day but doing a quick star test that night revealed that the in focus and out focus were opposites. 

 

I am going to make a video in a few minutes and   made a video to show you folks what is happening.    I've never owned a reflector that wasn't super easy to collimate and I've even added new focusers, spiders (curved 3 vein) and never had trouble getting everything lined up and ready for use before.  I'm glad you guys are here to help. :)   http://youtu.be/lQKngFZZV8M


Edited by Whichwayisnorth, 09 August 2014 - 04:43 PM.


#21 Whichwayisnorth

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Posted 09 August 2014 - 04:24 PM

Question about adjusting offset.  Do I adjust the spider by backing off the screw on one side and tightening the other side?  Seems the only way to do it since the secondary mirror is centered exactly in the secondary holder.  If I could even move the secondary mirror it would create overlapping circles :(   Right now, moving the mirror by adjusting the spider screws on the outside of the tube my OTA resembles the attached photo. :(Attached File  offset.png   11.14KB   0 downloads



#22 Whichwayisnorth

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Posted 09 August 2014 - 04:35 PM

Would you please share how you took the photos of Arcturus?  Looks like maybe a webcam and software?  Thanks.

 

Tim

 

 

 

Tim sorry I never caught your question.  This was with my CCD camera.  I believe it was a 1 second exposure in focus mode which is also using fast readout. Then I just increased the magnification.



#23 dave brock

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Posted 09 August 2014 - 05:09 PM

When I click on your second video it says it's unavailable.

 

Looking at your first video, the secondary is already offset.

 

I'll be very surprised if the flare is from collimation. As you say in the first video "it looks good to me".

 

Dave


Edited by dave brock, 09 August 2014 - 11:36 PM.


#24 tazer

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Posted 09 August 2014 - 08:01 PM

How far does your drawtube extend into the OTA? I get some sort of flare in my Mak-New that seems to have radial striations like yours. My drawtube is just at the edge of the light path so, in my case at least, I may be getting some flare from the end of the tube (which is black/shiny).

 

Have you looked into the Catseye collimation tools? They're pricey but well worth it. I could never get my f/4.8 collimated with a laser and after purchasing the Catseye tools/spotting kit I discovered the factory placed center spot on my primary was off by 3-4mm. Collimation has been much easier, producing excellent results, ever since. I can only imagine that at f/3.8 collimation would be even more difficult.



#25 Asbytec

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Posted 09 August 2014 - 09:35 PM

I agree with Dave, the secondary looks to be offset (the reflection of your eye is nearer to one edge.) I did mine just as you described, loosening one and tightening the opposite vane support screw. I'd leave it as is, for now. It actually does not have to be off set for collimation, it only affects the position of the fully illuminated FOV.

 

You can measure the draw tube extension into the OTA and see if it is longer than the space between the mirror and the tube. That may be the problem, but really only so long as the draw tube is extended into the OTA. 

 

It looks to me you are collimated well enough. But I am wondering if the focuser tube is parallel to the optical axis causing a bit of a skew as you defocus and some flare at focus. That does not mean the scope is not collimated, it just means the eyepiece is "looking" at the light cone from a slightly different perspective rather than straight on. As you defocus to one side, the focuser axis deviates to one side of the light cone bringing the eyepiece with it. You're actually looking at the image slightly from one side.

 

If you scroll through focus, the bright ring and I believe the central spot will change position. At focus the central disc will be centered in the rings, but the in-focus image will be slightly out of focus on one side. And on either side of focus, one side of the image will be closer to focus than the other - just ever so slightly. This was true with my own scope (CAT design). I was collimated well, but the in focus image flared to one side. When I scrolled through focus, the shadow changed position and did not look concentric on both sides.

 

This was a mechanical alignment problem - in this case. I am just throwing that out there because your system might have the same problem. Much less than my own, but maybe enough to cause some flaring. My secondary is not adjustable, so to make the focuser axis and optical axis parallel, I had to mechanically align the focuser to the optical axis so the eyepiece focal plane sat squarely on the scope's focal plane.

 

As I understand it, the focuser tube itself does not need to be square to the tube, but the optical axis needs to be pretty well centered and coincide with the focuser axis. This is part of the collimation process, so I guess your diagonal can be tilted a tiny bit to bring the optical axis in line with the focuser draw tube axis. Doing so should scroll the eyepiece through focus squarely to the optical axis removing any fare it may cause even at focus. So, after thinking about it, I think your diagonal might need a slight tilt adjustment.








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