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  #1  
Old 01-10-2018, 08:26 PM
Startrek (Martin)
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Stars blurred only on the right hand side

Iíve just noticed last night and tonight that my stars are ďblurredĒonly on one side , the other half is ok
My 6Ē f6 reflector collimated ok with Orion laser collimator ( Iíve done this hundreds of times before )
Checked my focus with BYEOS ( FWHM ) focus looks ok but stars blurred on one side only
Checked my sidereal tracking both with EQMOD on the laptop and my Synscan handcontroller , Stars still blurred on one side only using both methods
Tried unguided 5 sec, 20sec and 50 sec exposures , stars still blurred only on one side
Tried PHD2 guided 60 sec, 120sec , 240 sec , 300 sec exposures , stars still blurred only on one side
Pulled the DSLR out of the focuser and reinstalled again to confirm sitting hard against focuser and tight
Obviously the larger Stars are more noticeable than the small ones
Havenít pulled the rig apart yet as I will wait for your comments and advice

Any ideas why the stars are blurred on the right hand side ??
This is the first time itís happened to me after nearly a year using this rig
Very strange ?
Thanks in advance
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Old 01-10-2018, 09:12 PM
Startrek (Martin)
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I just had a brain wave and thought to put an eyepiece in the focuser and lock on to a star then defocus it into a round donut

So I went outside and did just that , locked on to Altair with my 10mm Delos , defocused and my donut was ovoid shaped with the hole offset from centre, not round with a hole in the centre

So am I correct in saying my optical train is not aligned , probably the primary mirror not square in its support frame ???

Note: I did have the primary mirror moved forward by 4mm to give me more in focus for my DSLR which was carried out a month ago, they used spacers made of compressed cork

Welcome any advice and comments for my frustrating issue

Thanks
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Old 01-10-2018, 09:18 PM
Startrek (Martin)
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Onother brain wave
Could my tube rings be too tight causing a bit of tube flexure ??
I donít usually overtighten them
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Old 01-10-2018, 10:12 PM
Wavytone (Nick)
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It’s mis-aligned, that much is obvious. As to the cause... you’ll have to twiddle various things till you find the culprit. The strongest suspect is indeed the primary mirror alignment so start with that.

FWIW the budget Newtonians like yours do not stay perfectly aligned for long periods - so get used to the idea of checking it visually on a bright star - and if need be, adjusting- before adding camera - or the shots will be a waste of effort. That yours lasted a year without tweaking is unusual. Thin metal bits including the tube distort, and wood/mdf/chipboard parts swell and warp as humidity changes.
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Old 01-10-2018, 11:51 PM
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mental4astro (Alexander)
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Martin, are you solely relying on the laser to collimate your Newt? If so, there's 50% of your problem solved. A laser will not help collimate the secondary mirror. With the laser, you will not be able to determine if the secondary is square with the focuser. The laser may "say" that the beam coming off the secondary is pointing to the centre spot on the primary, but because it is a point source of light, it is not telling you if the secondary is properly aligned to the focuser.

What you are seeing of the stars being blurred on one side is a tell-tale sign of a misaligned secondary mirror. I too thought that a laser was a "do it all" tool, and instead of helping over time things were getting worse and worse. And the skewed star image was one thing that just refused to improve. And it was after I exhausted everything else, I thought I'd look at the secondary mirror for no particular reason, and boy didn't I get a surprise! It was WAY off. Worst thing was I had sold my Cheshire eyepiece when I bought the laser. I've come to find out that a Cheshire will actually deal with not just the primary but also the secondary.

Flex of the OTA would be noticed as a change in shape of the stars with the OTA pointing in different directions from horizontal to vertical. You don't appear to be finding changes in star images. With a heavy camera such as your DSLR, this flex would be most obvious as the scope changes in its orientation.

Similarly, overtightened tube rings wouldn't be a problem because you "collimated" the scope after setting it up. This would not be the source of your blurred stars.

Another tell-tale sign that it is your secondary mirror is that this problem is getting more pronounced. If it was flex then this problem would have been obvious from the start.

Alex.
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Old 04-10-2018, 06:08 PM
Startrek (Martin)
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I purchased an Orion Cheshire today and got to work this afternoon on my 6" newt to check the alignment between the focuser and secondary mirror. As you mentioned Alex the secondary was not aligned to the focuser. I backed off the 3 adjustment screws and loosened the main centre screw to provide some tilt and rotation but no matter how much I tried I could not centre the secondary over the primary ( I was a quarter circle out and could only see two of the primary mirror clamp shadows in my view)
After 2 hours of cursing and swearing I thought maybe have a look at the primary mirror. When I pulled it out I noticed the gap between the mirror base and the frame under one of the clamps was at least 2mm to 3mm wider than the other two. So I pulled the mirror off, gave it a clean and reinstalled it so all three clamp locations had equal gaps after tightening ( I just nipped it up firm but not to tight )
Re installed the primary mirror and tightened up then backed off the secondary screws and found that I could move the secondary mirror to line up with the focuser tube and primary mirror ( I thought what the hell , I've nailed it ) Tightened up the centre screw and tweaked the 3 adjustment screws so everything was concentric and aligned with the crosshairs , centre bulls eye on the primary mirror etc.. Then used my laser collimator to check alignment which only required a minor tweak on the secondary and primary
So to summarise , I believe it was mainly the primary that was causing my ovoid shaped refocused star pattern as the primary was adjusted by others a month ago with some cork spacers and when I removed the mirror the gaps were not even ( 2mm to 3mm difference) in which the error would be magnified further along the optical system
It's raining for the next week or so I will have to wait to test the scope again but here's hoping I've nailed the issue
Cheers and clear skies
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  #7  
Old 04-10-2018, 07:08 PM
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mental4astro (Alexander)
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Martin, good to hear of some progress.

One thing about the secondary, don't look at it in relation to the primary mirror. Only the focuser. Center the secondary to the inside edge of the Cheshire eyepiece. The Cheshire can be inserted quite deep into the OTA, and this is to facilitate the centering of the secondary to the inside edge of the Cheshire. Part of this process involves looking at the outside edge of the primary to see that it is co-centric to the edge of the secondary.

Once you have the secondary centred, you can begin tweaking its collimation screws to get the alignment of the center spot of the primary to the crosshairs of the Cheshire. Nothing else.

Once the crosshairs and center spot are aligned, then you can swap for the laser. You may find the laser is out from the center spot, but it should be bugger all. Once the laser is centered, you can then look at tweaking the primary's collimation.

One thing to look at is the secondary mirror's holder where the collimation screw push down onto the holder. The ends of these screws will be sharp, and because they are made of steel and the holder is soft aluminium, the screws most likely will have gouged the contacting surface of the holder. Also there will be a little play between the screws and the threads they are wound through. This play may mean that when you tweak the colli screws, the gouge marks may have the colli screws ease into these marks and in turn throw out your entire collimation process, and you will be cursing and screaming wondering what the hell is going on!!!!

Have a look at the secondary holder where the colli screws push against it. Check for gouge marks. If there are no matks, you are very lucky. If there are marks, there's some things that can be done to correct this. But first check the holder.

Alex.
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