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Thread: Triangular stars

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    Default Triangular stars



    My Newtonian is showing stars as severe triangles rather than points. My first thought was that the three supports for the primary mirror were over-tightened and causing the mirror to flex but after backing off all of the screws, the images still show stars as small triangles. I had a go at collimating with an inexpensive laser collimator but this hasn't helped things. Any ideas?

    It's late now but tomorrow I will upload a sample picture of some defocused stars to demonstrate the problem.

    Thanks,
    Jonathan
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    Default Re: Triangular stars

    The usual culprit is pinched optics, either the primary (most often the case), or depending on how your secondary is mounted, more rarely.

    The next possibility is excessive coma, exacerbated by the combination of a fast primary and poor collimation.

    If the stars appear triangular uniformly across the field, a pinched primary is most likely the cause. Otherwise, I would suspect collimation or a related problem.

    If your primary uses edge supports, or edge clips (more likely), these may be pinching the primary rather than the collimation screws. The edge clips need to be completely loose - they don't actually clamp the primary down, they are intended only to prevent it from tipping out of the cell.
    Last edited by AustinPSD; 11-13-2013 at 02:01 AM. Reason: missing part of post
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    Default Re: Triangular stars

    Thanks. I released all 3 of the primary's adjustment screws and lock screws and this didn't help. I also backed off all of the secondary's screws and this didn't help either. The stars are triangular right across the field so it does sound like a pinched mirror so I guess my next port of call is to remove the primary mirror cell and check that it's OK.

    As promised, here's a quick photo of Jupiter and its moons, deliberately defocused to show the problem. Triangular stars everywhere, although worse towards the left edge, so I guess collimation is also a bit wonky.

    IMG_0822b.resized.JPG
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    Default Re: Triangular stars

    I just removed the mirror cell. Given that most of the guides I've read say the mirror clamps should not pinch it, the ones in my telescope were pretty well screwed down. The clamps had foamy rubber between themselves and the mirror which I guess would alleviate the pressure a bit, but it seemed tight nonetheless. I backed off each of the screws about half a turn. The mirror is still being held in place but not so hard.

    I've re-collimated the scope using a laser collimator but it's cloudy out, so no stars to look at and see if they are still triangular.
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    Default Re: Triangular stars

    Try rotating the primary and collimate again. See if the triangularity rotates too. Could be a manufacturing problem. Could also be a cooling problem, if it isn't acclimated. Try pointing it straight up (and give the primary a little wiggle, in case it is stuck between two edge clips) to alleviate "edge support" and see if the triangles disappear. It might be stuck against the edge clips, but it should be slightly loose not snug.. Getting it unstuck could fix this. Try these tests and report your findings, please.

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    Default Re: Triangular stars

    Quote Originally Posted by dj_judas21 View Post
    I just removed the mirror cell. Given that most of the guides I've read say the mirror clamps should not pinch it, the ones in my telescope were pretty well screwed down. The clamps had foamy rubber between themselves and the mirror which I guess would alleviate the pressure a bit, but it seemed tight nonetheless. I backed off each of the screws about half a turn. The mirror is still being held in place but not so hard.

    I've re-collimated the scope using a laser collimator but it's cloudy out, so no stars to look at and see if they are still triangular.
    You should be able to fit a business card between the foamy rubber and the mirror surface.
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    Default Re: Triangular stars

    Quote Originally Posted by pederv View Post
    You should be able to fit a business card between the foamy rubber and the mirror surface.
    Really! If the mirror is that loose, won't it move around in its cradle and spoil the imaging as the telescope is moved about on its mount?
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    Default Re: Triangular stars

    The clips are only intended to prevent the mirror from tipping out of its cell. They must be loose, barely in contact, if at all, otherwise they will create astigmatism in the mirror, and distort the image.
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    Default Re: Triangular stars

    So if the clips are loose on the mirror's surface, what holds the mirror in its cell? Is it glued etc? Sorry if this is a silly question - I just can't see what else is holding the mirror in place other than the 3 clips.

    If the mirror really is free to move around and may rest in different positions or at different angles, surely it's impossible to collimate it properly - and that sounds unlikely to me
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    Default Re: Triangular stars

    These guys know what they're talking about. You have to be able to slip an index card or business card between the mirror surface and the clips, or you run the distinct risk of pinched optics. The clips are there and only there to keep the mirror from flopping out of the cell in the event you get down to and below a horizontal position. They are tightened down for shipment to keep the mirror from bouncing around. But when you are using the scope you want it have a small amount of play. As long as the clips remain attached and you don't have a huge space, the mirror will be fine. Remember the collimation screws are putting forward pressure from the rear of the mirror. You will be fine.
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