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What's causing this spike in my SCT image?

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19 replies to this topic

#1 pianoman2

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Posted 05 October 2016 - 12:44 AM

Hi there'

 

The scope is a C6XLT and the camera used was a Cannon 450D modded. No reducers or barlows.

I'd like to make sure nothing is wrong with the optics before I sell this puppy.

Thanks for any advice, Victor



#2 futuneral

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Posted 05 October 2016 - 12:51 AM

I'm guessing you forgot the picture


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#3 steveward53

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Posted 05 October 2016 - 01:22 AM

I'll take a punt and say that you will post a photo of a defocused star that has a spike in the diffraction ring.

 

Something like this ... ?

 

4907_14_43.jpg

 

If so it's a heat plume caused by warm air rising off the end of the internal baffle tube , let the 'scope cool down further and it will disappear.



#4 pianoman2

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Posted 05 October 2016 - 11:39 AM

LIGHT_10s_1600iso_+25c_20161004-20h31m53s277ms.jpg The pic finally showed up, thanks



#5 anismo

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Posted 05 October 2016 - 11:51 AM

That looks like a thread or something in the light path causing diffraction. I have seen these in hyperstar config with cables crossing light path. If you look down from the front with the camera connected, do you see clear path all the way to the sensor?



#6 pianoman2

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Posted 05 October 2016 - 02:36 PM

Thanks, i'll check that as soon as I can.

Ok, I looked down the scope and nothing out of the ordinary.There are also no spikes in my defocused stars. I just cleaned the mirror, it was very messy  and maybe that may be the problem.

I was wondering if I tightened the collimation screws too tight in order to hold collimation, and something got tweaked. panic!  thanks, victor


Edited by pianoman2, 05 October 2016 - 03:23 PM.


#7 futuneral

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Posted 05 October 2016 - 04:06 PM

Try this - take a picture. then rotate the camera like 30-45 degrees and take another pic.

 

If in both pictures the orientation of those spikes is the same - it's something in the camera itself.


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#8 pianoman2

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Posted 05 October 2016 - 10:24 PM

Ok, I turned the camera to different positions and the spikes ( yes more than one now).changed positions. and I now noticed a few hairline scatches on the mirror and I think that's probably the cause. If so,where can I send the scope to get the scratches out.  

 

Thanks for your time guys, victor


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#9 schwim

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Posted 06 October 2016 - 12:30 AM

More than one spike? i see two in your picture in post #4. Are the scratches you see on the mirror centered that uniformly within the aperture that they would create such a clear opposition?

 

Maybe try this - grab some monofilament (fishing string or similar) and tape it across the aperture. Take an image and see where that creates a new diffraction spike relative to the other. Move it, trying to get the spikes lined up with each other. This will tell you where the "defect" creating the spike you're presently seeing is within the optical train. Start looking there. If it lines up with the scratch you may be on to something.

 

If the mirror is indeed scratched my guess is you might be better off getting a new (to you) C6 or even a C8 for that cost. Check the classifieds.



#10 pianoman2

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Posted 06 October 2016 - 12:48 AM

Thanks Greg,

.

That sounds like a good idea. Tomorrow night i'll try that.

Is it possible my secondary is to tight because of my collimation?

And those spikes only appear on the brightest stars.If that means anything.

And the scratches are all around the mirror and not just in one place, about ten of them about 3/4 of an inch long.

Also, more spikes appeared when I rotated the camera.

 

victor


Edited by pianoman2, 06 October 2016 - 12:57 AM.


#11 schwim

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Posted 06 October 2016 - 02:53 AM

Thanks Greg,

.

That sounds like a good idea. Tomorrow night i'll try that.

Is it possible my secondary is to tight because of my collimation?

And those spikes only appear on the brightest stars.If that means anything.

And the scratches are all around the mirror and not just in one place, about ten of them about 3/4 of an inch long.

Also, more spikes appeared when I rotated the camera.

 

victor

Re: the secondary, I don't know. You can loosen it and see what happens.

 

Diffraction spikes on brighter stars only is typical. 

 

More spikes appeared on rotating the camera? I can't think of a plausible way that would happen. I'm at a loss. Maybe return to the previous position and see what happens, then move again?



#12 Swatter

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Posted 06 October 2016 - 09:06 AM

Just some Ideas I had. Just in case you did not think about it. I am assuming this is a SCT and you already thought about this.

 

1. Anything in the light tube or T-ring? (spider web, hair ect...)

2. Using a flatter? (cracked)

3. Using a diagonal? (cracked mirror, hair, spider web)

4. Camera sensor dirty or cracked? ( Hair, dust, spider web, fingerprint, ect..)

 

I would borrow and try a different camera.

Just my 2 cents



#13 wargrafix

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Posted 06 October 2016 - 09:14 AM

Too tight can cause pinched optics. I hope you get it resolved.



#14 pianoman2

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Posted 06 October 2016 - 11:49 AM

Ok, here's my next plan of attack. I am going to loossen my Bobs Knobs at least a half turn each and see if the spike changes even a little bit. That should tell me something. 

 

I also get the same spike using my ZWO 224MC camera screwed dirrectly to the back of the scope. 

Also while using the celestron 6.3 flattener/ reducer or without.

 

Tonight's another night, we'll see. thanks, Victor



#15 pianoman2

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Posted 07 October 2016 - 11:58 PM

Hi there,

 

Tonight I loosened the collimation knobs to make sure  nothing was too tight but that didn't change anything. I was able to get a couple shots of a clear star but then realized clouds were dimming the star and therefore no spike.

Thought I had something for a while. I'd like to think that spikes on a bright star with an SCT might be typical but not at all too sure. Thanks for everything, victor



#16 Wisotzkey

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Posted 08 October 2016 - 04:28 AM

Those look like the spikes I've had when shooting through overhead utility lines.  Any sky obstructions?



#17 pianoman2

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Posted 08 October 2016 - 12:41 PM

Yes, unfortunately the only fairly bright stars I can see are Altair and Vega from where I live. Tonight i'll try to wait until one of them pass by those lines and cables. victor



#18 Thirteen

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Posted 08 October 2016 - 01:04 PM

Some info on another possible cause of SCT spikes

http://interferometr...tml?view=mosaic
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#19 rice224534

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Posted 09 October 2016 - 07:42 AM

Hi there'

 

The scope is a C6XLT and the camera used was a Cannon 450D modded. No reducers or barlows.

I'd like to make sure nothing is wrong with the optics before I sell this puppy.

Thanks for any advice, Victor

 

Microlens on a camera sensor may cause small spike, especially on bright stars. 

 

http://archive.sbig....s/KAF3200ME.htm

 

 

Koji



#20 pianoman2

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Posted 09 October 2016 - 12:14 PM

Hello Koji in Tokyo, I get the spike whether I use the dslr or the ZWO 224MC camera so i'm going to have to rule that out.

Arigatou Gozaimasu.

 

Jason, I'm going to go with that article you posted. That sounds reasonable and it also gives me a good selling point that

my C6 has some similar characteristics as the C11 Edge.(smirk)

 

I only bought this scope to get some images of the 3 planets that were out a while ago, did that and in my opinion the C6 is too much work for DSO'S, for me anyway.

 

Nothing drives a person quiker to a nice APO then an SCT.

 

victor  




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