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8SE - Can't get point like stars

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#26 whizbang

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Posted 23 August 2019 - 09:20 AM

Your "focuser" problem is on an artificial star 40 feet away?  If so, it's not a scope problem, it's a distance problem.  Try 400 feet.



#27 whizbang

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Posted 23 August 2019 - 09:21 AM

I would try to fine a local astronomy club so can have experiencd folks give you a hand.



#28 elwaine

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Posted 23 August 2019 - 10:43 AM

Your "focuser" problem is on an artificial star 40 feet away?  If so, it's not a scope problem, it's a distance problem.  Try 400 feet.

 

The close focusing distance of a C5 is only 20 feet (according to Celestron's specs). Researching the web I found that the close focusing distance for an 8" SE is about 25 feet. So 40 feet should be long enough for the OP to focus that donut down to a point... or at least very close to a point. 

 

The OP's posts indicated that he set up his RDF on an object 400 meters away, yet that object could not be brought into focus. 

 

The rough alignment, as shown in his photo, is good enough for him to have seen, and photographed, relatively sharp stars. Yet all stars in his image are clearly out of focus. I think something is preventing the primary mirror from its full range of travel. Getting help from a local astronomy club is an excellent idea, but I will not be surprised if, in the end, he has to send the scope in for repair.  


Edited by elwaine, 23 August 2019 - 11:25 AM.


#29 Lola Bruce

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Posted 23 August 2019 - 11:43 AM

Curious Developer where are you? I believe your problem may have a simple answer. Most likely a new set of eyes with experience will sort out your situation quickly

 

Bruce



#30 Gary Z

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Posted 23 August 2019 - 02:31 PM

Help me out guys I'm curious where you get the distance of 400 feet as the recommended distance. I looked up the artificial star info on Astronomics and it says 100 feet for a c8. But it also says further is better until you start losing contrast. Is the 400 foot mark at this distance for this scope? I'm not trying to contradict anyone, just trying to understand the best method in using the artificial star. I'd rather use an actual star.

Thanks,

Gary

#31 Astrojedi

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Posted 23 August 2019 - 03:38 PM

Help me out guys I'm curious where you get the distance of 400 feet as the recommended distance. I looked up the artificial star info on Astronomics and it says 100 feet for a c8. But it also says further is better until you start losing contrast. Is the 400 foot mark at this distance for this scope? I'm not trying to contradict anyone, just trying to understand the best method in using the artificial star. I'd rather use an actual star.

Thanks,

Gary

I am able to get an artificial star to focus at about 40 feet with my EdgeHD 8 and a little bit further for the 9.25. This is how I collimate my SCTs with excellent results.



#32 Astrojedi

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Posted 23 August 2019 - 03:44 PM

Thank you both for providing more things to think about.

 

To respond to the questions:

 

Was the first image a crop from the center of the frame or one of the corners? It was sort of from the center-ish. I have attached a full reference image with the highlighted area that was cropped to.

attachicon.gif whole.jpg

 

What about the donut? Mostly centered. I have attached the full reference image (resized and compressed)

attachicon.gif whole2.png

 

Was that star centered in the fov? yes, as best I could get it.

 

Please note that I cannot focus this to a point. This is at about the minimum the focuser allows me to go. 

 

You need to perfectly center a bright star and evaluate/collimate with the star in focus. Were you using an artificial star? If so, it needs to be at least 40 ft away from the OTA.

 

If after collimation the issue still exists then it is likely that your corrector is out of alignment. If so, I would advise you to send it back to Celestron for warranty repair.



#33 Boom

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Posted 23 August 2019 - 04:28 PM

You need to perfectly center a bright star and evaluate/collimate with the star in focus. Were you using an artificial star? If so, it needs to be at least 40 ft away from the OTA.

 

If after collimation the issue still exists then it is likely that your corrector is out of alignment. If so, I would advise you to send it back to Celestron for warranty repair.

 

He already said he used an artificial star placed 40 feet away.   He also said he can't even reach focus on this artificial star 40 feet away.   

 

I think he just needs to use a diagonal and EP.  Celestron says you should be able to focus as close as 25 feet with the 8SE.


Edited by Boom, 23 August 2019 - 04:29 PM.


#34 fcathell

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Posted 23 August 2019 - 05:02 PM

I'm beginning to wonder about something jamming/limiting the mirror movement.  I have collimated C-5s, C-8s, Meade 8s, 3.5, 4, 5, and 6 inch Maks all with an indoor artificial star 35 feet away, with and without a 90* diagonal.

 

Frank


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#35 Asbytec

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Posted 23 August 2019 - 06:35 PM

Help me out guys I'm curious where you get the distance of 400 feet as the recommended distance. I looked up the artificial star info on Astronomics and it says 100 feet for a c8. But it also says further is better until you start losing contrast. Is the 400 foot mark at this distance for this scope? I'm not trying to contradict anyone, just trying to understand the best method in using the artificial star. I'd rather use an actual star.

Thanks,

Gary

Gary, I am sure Astronomics site is more accurate for the distance needed for collimation. It depends on the purpose of using the artificial star. For collimation, it has to be at a distance the scope can focus on it. With a moving mirror, the C8 can probably focus quite close. Not sure how close with or without a diagonal (preferably with).

 

The other purpose is for star testing. That distance needs to be pretty far, again not sure how far, to avoid inducing a lot of spherical aberration that can be seen in the star test. You want the mirror spacing to be pretty close to it's design while operating at infinity to get the best results.

 

For collimation, inducing some spherical aberration is not that important, you just need to see a defocused pattern (and a focused one). So, focus can be as close as the scope can get within reason, i.e., without adding a bunch of spacers and diagonals to the visual back. 

 

One potential problem with using an artificial star near the ground is the mirror my shift a bit when you rotate the scope to observing altitude. This can affect collimation a little. The OP does look a bit out of focus along with what looks like coma (miscollimation).


Edited by Asbytec, 23 August 2019 - 06:45 PM.

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#36 Auburn80

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Posted 23 August 2019 - 07:14 PM

<SNIP>

One potential problem with using an artificial star near the ground is the mirror my shift a bit when you rotate the scope to observing altitude. This can affect collimation a little. The OP does look a bit out of focus along with what looks like coma (miscollimation).


Wouldn't that be an issue when moving across zenith as well? I understand that "flop" can mess up an image at the small pixel level.

Thanks,
Clark

#37 Asbytec

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Posted 23 August 2019 - 08:26 PM

Wouldn't that be an issue when moving across zenith as well? I understand that "flop" can mess up an image at the small pixel level.

Thanks,
Clark

Yes, makes sense. I have heard folks mention it. I never paid much attention to it back when I had a run of several SCTs. 



#38 Eddgie

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Posted 24 August 2019 - 08:02 AM

For collimation, the distance is not so important but as Asbytec suggests, often when SCTs are collimated in a horizontal position, the collimation will chance when the scope is pointed at the sky.   That does not mean you can't collimate horizontally but it does mean that the collimation may not be the same as when the scope is in use.

 

The distance is important for star testing because if the star is too close, it can cause what looks like spherical aberration.  This is because unless the artificial star is very small, the telescope can actually partially resolve the star as a circle, and this is what would cause the spherical aberration (In all scopes) but in the SCT, there is the added complication of mirror spacing.

 

Last, the mirror locks on the EdgeHD scopes are there to prevent mirror flop when tracking across the meridian.  The design was updated with more emphasis on imaging, and since it was known that this was an issue for imagers, Celestron included the mirror locks.  Since the 8SE is an older design, it does not include this feature.  Best not to image past the meridian.   



#39 Curious Developer

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Posted 24 August 2019 - 04:55 PM

Again, thank you for the informative responses.

 

I measured the actual distance from the artificial star to the telescope. It turns out it is only 29' 4". 

 

I did find out that the screw on cap around the secondary mirror was very loose, and the mirror was kind of bouncing around in there, so I tightened the cap. That definitely had some impact because the image is now a lot clearer than it was before. 

 

I also found out there is a Royal Astronomical Society of Canada meeting at a local university on the third Friday of each month here. I plan to take it there with me next month - since I just missed this one.

 

Thank you again, everyone. You were all very helpful.


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#40 Gary Z

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Posted 24 August 2019 - 05:15 PM

Again, thank you for the informative responses.

 

I measured the actual distance from the artificial star to the telescope. It turns out it is only 29' 4". 

 

I did find out that the screw on cap around the secondary mirror was very loose, and the mirror was kind of bouncing around in there, so I tightened the cap. That definitely had some impact because the image is now a lot clearer than it was before. 

 

I also found out there is a Royal Astronomical Society of Canada meeting at a local university on the third Friday of each month here. I plan to take it there with me next month - since I just missed this one.

 

Thank you again, everyone. You were all very helpful.

That's all great news!  So your post helped us understand a few things better and with what you were given, take a deeper look at things and come to some great findings!  Sounds like the RASC will be a great benefit to you!

 

Gary




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