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Weird Collimation Question

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#1 gravy11

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Posted 29 June 2020 - 10:07 PM

I've read a ton on this so I know I'm doing it right. Lol. I focused on a star and then put in A Medium EP and defocused. The Circle looked to be in the middle but here's the weird thing. A part of it was missing or blocked. Picture a donut with a piece missing. The center was in the middle. Perfectly. But again.... There was a missing piece. Any ideas?

#2 Richard O'Neill

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Posted 29 June 2020 - 10:17 PM

There may be an obstruction somewhere in the light path. Does it occur with other eyepieces?



#3 gravy11

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Posted 29 June 2020 - 10:22 PM

There may be an obstruction somewhere in the light path. Does it occur with other eyepieces?

Grest question. Yes. Any idea where to start?



#4 msl615

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Posted 29 June 2020 - 10:29 PM

Try looking through the scope without any eyepiece at all....do you see the same thing?

 

I assume this is a Newtonian; is it new, Dobsonian, home built, purchased used?

 

How big is the slice and is the slice parallel, or pie shaped? 

 

Lots of things could be possible.  



#5 gravy11

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Posted 29 June 2020 - 10:45 PM

Try looking through the scope without any eyepiece at all....do you see the same thing?

 

I assume this is a Newtonian; is it new, Dobsonian, home built, purchased used?

 

How big is the slice and is the slice parallel, or pie shaped? 

 

Lots of things could be possible.  

SCT Celestron 6SE. It’s definitely something because while I was moving it around, it grew. The “circle” when defocused is concentric but the bottom half is blocked by something. It’s a semicircle now. I have no idea where to start. Never messed with this.



#6 TOMDEY

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Posted 29 June 2020 - 10:46 PM

Yeah, please relate more detail. It could be anything, as you have described. If it's a big arch, that could be a badly decentered folding flat or such. If it's a square thing, it could be your focuser poking into the incoming light path. Things like these are generally easy to investigate and correct.    Tom



#7 TOMDEY

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Posted 29 June 2020 - 10:49 PM

SCT Celestron 6SE. It’s definitely something because while I was moving it around, it grew. The “circle” when defocused is concentric but the bottom half is blocked by something. It’s a semicircle now. I have no idea where to start. Never messed with this.

Your scope is therefore badly out of whack. Will need a complete alignment. Is the scope new or used? ... aka ... who has been messing around with it?!    Tom



#8 KTAZ

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Posted 29 June 2020 - 11:03 PM

Three photos would help:

 

1) use your cell phone to try and get a snap of that defocused ring

 

2) Take a shot looking into the front of the the corrector with the baffle as centered as possible from about 3 or 4 feet away

 

3) Remove the visual back and take a shot looking into the rear of the baffle at the secondary mirror.

 

Post those and lets see what folks think.



#9 ButterFly

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Posted 29 June 2020 - 11:04 PM

The defocused star is centered in the eypiece when it does this?

 

"Missing or blocked" is exactly right.  Stick your finger over the tube and rotate it around when the star is defocused in the center of the eypiece to gauge which side of the tube the blockage is on.



#10 gravy11

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Posted 30 June 2020 - 07:17 AM

Update: So I walked away from the scope then went back, determined.....I moved it around to Antares and......the blockage was completely gone. Clearly there was something in there (which isn’t good) that moved. Looking at all collimating examples, the defocused star looks exactly like it should. I have to figure out an approach if it happens again. Thx for everyone’s help!



#11 Cames

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Posted 30 June 2020 - 07:53 AM

I suspect a thermocline inside the telescope.

 

The cold air and the warm air inside the telescope forms into layers.   Air at different temperatures refracts light to different degrees. What you saw will dissipate as the system temperature homogenizes more thoroughly.    

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C



#12 Asbytec

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Posted 30 June 2020 - 07:54 AM

Were you looking over the roof of a house? 


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#13 Andrekp

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Posted 30 June 2020 - 10:40 AM

Yeah, I get that when things get near the roof and half the tube is looking at the roof, and the other half the moon or whatever.



#14 spereira

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Posted 30 June 2020 - 11:00 AM

Moving too Reflectors, for a better fit.

 

smp



#15 gravy11

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Posted 30 June 2020 - 11:19 AM

Were you looking over the roof of a house? 

OK, I’m new and this is An excellent point. I was at certain times (The Moon was just over my roof line) but it appeared when I was checking out other stars that weren’t as high. I’m curious if it happens again tonight, not sure my heart can take it. :-/


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#16 SteveG

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Posted 30 June 2020 - 01:03 PM

Moving too Reflectors, for a better fit.

 

smp

The OP’s scope is an SCT.


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