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So I've got a xt8 plus and I'm struggling with focusing the 10mm especially with deeper sky things such as Orion Nebula. I recalibrate today and the secondary mirror was off so got the calibration as per the pic. 

I've also included a video of me focusing and going out of focus on Rigel

It's definitely improved but still not as clear as I would like. Stars through the Barlow are small lines rather than dots. I'm probably at the limit of the telescope and conditions but just checking there's nothing else

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The picture of the collimation looks ok but the video of the star certainly is not. The star image looks distorted, this could be caused either by the optics being not thermally stabilised or the primary mirror being held too tightly in its cell. I have a XT8 and the images are excellent in it.  :icon_biggrin:

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Yeh that's what I thought. Regarding the distortion. That's a lot better than it was too. Not sure how to make it better. I was star gazing for a good hour before taking that video so don't think it's temperature. If I loosen the mirror I'm guessing my collimating will be out but can give it a go. In 2 minds to call orion

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Are you talking about the big mirror at the back or the small one at the front. 

I readjusted both yesterday before those pics above and the callimation view and actual view were slightly better than before. 

Also what do you mean by 'if you can't easily turn the mirror once you can get at it'?  I didn't think I should turn a mirror until I've loosened the clips anyway??

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I'm very sorry. But you have hit on one of the very reasons that swayed me to being a refractor-head.

I could not see my limited patience dealing with a reflector. 

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Is a dob a reflector? To be honest I'm wondering if I made the right decision too. I chose this to get the most bang for buck with a good aperture. I sacrificed photography but thought at least I will have good views - but I really don't. Not sure why though. Things just aren't crisp through 10mm. I can't even show you a photo of it to show how average it is lol

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Hi Sachan.  I'm lucky - my telescope seemed pretty good straight out of the box.  I came across this guide:

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0ahUKEwjLyaH4pPHQAhUiM8AKHeGsDjcQFghBMAY&url=http%3A%2F%2Fwww.contrailspotting.co.uk%2FAstro%20Babys%20Guide%20to%20Collimation.pdf&usg=AFQjCNG2aWl0LtkZozbNo3Iaf6XoVZ9GEA&sig2=ZNCvu4DFUOEyIKAWqnRG1A&bvm=bv.141320020,d.ZGg&cad=rja

(I hope the link works - it's Astrobaby's guide to collimation)

At the end there are some 'star tests' to help troubleshoot where your problem might be.

 

 

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Thanks Mikey that's a useful link, I will have another try but seems from the pics either it is overly tight primary mirror / poor conditions. I will try it next time it's clear and report back. I think carrying my scope tends to move it out of sync quite easily ( I am gentle lol)

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  • 2 weeks later...

Still struggling with this  I need help

ive tried adjusting the primary mirror more and think looks better but star collimating I think is out but don't think I can get the collimation looking much better than that. 

I tried to look at Venus and mars just now. Don't think I could tell they were a planet had I not known what I was looking at. I thought it might be the 10mm lens but tried with a brand new vixen zoom scope and similar results

it could be poor conditions and them being low in the sky. Not sure if I'm expecting too much but expect better. Does anyone have a realistic pic of mars/Venus from uk from a recent session?

thanks

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Looking at those two videos... The way the object stretches one way then the other when you're going through the focus suggests astigmatism. 

Astigmatism can result from the mirrors or the eyepiece, if you can rule out the eyepiece by trying another 10mm one of known good quality, if the mirror clips aren't too tight then the problem will be with the figuring of the mirrors, in which case the scope should probably be returned (if that's possible?)

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Thanks for the reply. Not sure if I will be able to return it now. It will ask Orion  

probably a stupid question but how do I know if the mirror is tightening or loosening? Is looser when the mirror moves away from the eyepiece end?

 

is there any other way to correct astigmatism?

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The only real way to check the tightness of the mirror clips is to remove the mirror cell, you should be able to rotate the mirror in the cell and if you can't the clips are probably too tight. 

I'm only guessing at astigmatism, it might not be though I'm not much of an expert! As far as i know astigmatism presents itself as elongation of the star either side of focus, but in opposite orientations inside and outside if focus, which is what your videos seems to show. 

I think at this point the best thing to do is to take your scope along to your nearest astro society and have someone look at it, they will be able to test different eyepieces in it and may have an artificial star to further test the mirrors.

 

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