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  #21  
Old 02-03-2018, 06:52 PM
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Camelopardalis (Dunk)
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Thanks everyone for all the great feedback :thumbup:

The scope has certainly given me a taste of its potential despite the ugly stars. Of course, Iíd like to nail it eventually but itís certainly feels like hitting a wall

Today Iíve been looking at a couple of points raised and listed here...

Iíve been collimating in a horizontal position in its rings, before transferring to the mount (NEQ6). I added 13Ē Losmandy dovetails (top and bottom) to improve rigidity of the ring structure themselves, and to separate the rings farther than the included single Vixen dovetail.

I removed the mirror cell to check the holder clips. These were really tight and I had to use some force with a couple of the screws. I loosened them enough to be able to slide a piece of paper underneath and out, before reinstalling the primary cell in the tube. I forgot to measure the primary spot was centred on the mirror, but itís a simple enough operation to remove it and check tomorrow.

I have removed the secondary mirror and the adjustment screws had been digging into the secondary holder. I picked up a stainless steel washer of the perfect size from a local hardware store and have installed this between the adjustment screws and the holder surface. This has improved the manoeuvrability of the secondary substantially. Such a simple fix, I canít see why SW donít do this from the factory.

While Iím the area, I measured the spider vanes. The vane in the axis of the focuser was dialled out 5mm more than the others (measured to the centre screw), so Iíve adjusted this to match the others. It appeared to be distorting the shape of the open end of the scope - when I noticed it, I was looking squarely at it from the front and it was quite obvious. The same vane was also not straight...it looks a bit warped. Iím a bit puzzled by this as Iíd not touched them before, the closest I got was adjusting the secondary screws.

I donít fully understand the significance of the offset. I understand why it needs to be so, and SW state in the manual that it has been offset at the factory. Since I have removed the secondary and refitted it, have I destroyed that and need to reset it? I donít yet grasp whether I need to factor this in.

Looking down the focuser tube, I tried my best to centre the secondary under the focuser. However, when I insert the collimation cap in the focuser, the clamps on the outer edge of the mirror are not then all visible without adjusting the secondary some more. Does this indicate that I havenít accommodated the offset, or that the focuser is not aligned with the secondary?

With the secondary adjusted (Iím not going to say collimated!), I then centred the dark dot from the collimation cap into the centre circle on the primary mirror relatively easily. I then racked the focuser in and out and the spot stays put.

The mirror is stuck to the stalk with a sticky pad, although it seems pretty secure.

It appears that my laser has lost collimation, as the rotation trick is no longer working. Rotating it in the focuser traced out a circle about 5cm in diameter. It has adjustment screws, so Iíll just have to learn how to do it. This could have been off the previous night, but I had worked previously.
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  #22  
Old 02-03-2018, 07:48 PM
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codemonkey (Lee)
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Ooh. Interesting. Sounds like you may have found the cause of the astigmatism given the description of the primary. Hopefully that solves the bulk of your issues.

It's a bit of a drive but you're welcome to borrow my Catseye kit if you want to go down that path Dunk. They're the only collimation tools I have now but I don't use them often 'cause I leave my newt permanently set up.

One thing to note re Catseye is they take a fair bit of learning to use well, so are a bit of an investment in time. To be honest I still don't know if I "get it".
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  #23  
Old 02-03-2018, 08:14 PM
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RickS (Rick)
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Hi Dunk,

Do you have a license for that thing?

I have a Catseye collimation kit if you want to borrow it.

Cheers,
Rick.
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  #24  
Old 02-03-2018, 08:25 PM
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peter_4059 (Peter)
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Quote:
Originally Posted by RickS View Post
Hi Dunk,

Do you have a license for that thing?

I have a Catseye collimation kit if you want to borrow it.

Cheers,
Rick.
I think the main issue is he will need the catseye spot on the primary for it to work properly. The normal paper reinforcing ring is not sized correctly for the visual cues to work.
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  #25  
Old 02-03-2018, 08:30 PM
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RickS (Rick)
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Originally Posted by peter_4059 View Post
I think the main issue is he will need the catseye spot on the primary for it to work properly. The normal paper reinforcing ring is not sized correctly for the visual cues to work.
Yeah, think I might have one of them too... or we could use a Troy 180 out of phase selfie.
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  #26  
Old 02-03-2018, 09:16 PM
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billdan (Bill)
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" I donít fully understand the significance of the offset. I understand why it needs to be so, and SW state in the manual that it has been offset at the factory. Since I have removed the secondary and refitted it, have I destroyed that and need to reset it? "

Hi Dunk,

What Skywatcher means is that the secondary mirror itself has the offset built into it. In this image attached your secondary should look like the one on the right hand side.

Cheers
Bill
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  #27  
Old 02-03-2018, 10:20 PM
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peter_4059 (Peter)
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Quote:
Originally Posted by RickS View Post
Yeah, think I might have one of them too... or we could use a Troy 180 out of phase selfie.
That might work ok with the Brounaye colimation tools.
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  #28  
Old 02-03-2018, 11:18 PM
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Camelopardalis (Dunk)
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Thanks for the offers and explanations, guys - much appreciated! Hopefully all this instruction will advance me to a “P” before the winter goodies arrive

I’m hoping loosening the mirror clips will help...but with the weather forecast I might be in for a long wait
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  #29  
Old 03-03-2018, 08:35 AM
gaseous (Patrick)
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Quote:
Originally Posted by Camelopardalis View Post
Thanks everyone for all the great feedback :thumbup:



It appears that my laser has lost collimation, as the rotation trick is no longer working. Rotating it in the focuser traced out a circle about 5cm in diameter. It has adjustment screws, so Iíll just have to learn how to do it. This could have been off the previous night, but I had worked previously.
I donít know how your laser got so badly out of whack, but 5cm over the length of your tube is pretty substantial. I wish you luck in colllimating your laser - in my experience this was possibly one of the most frustrating things Iíve ever tried to do. A proper V-block would possibly be a better guide to how aligned it is - I found that anything less than a perfectly snug fit in the focuser will present apparent mis-collimation. Good luck!
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  #30  
Old 03-03-2018, 11:27 AM
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Camelopardalis (Dunk)
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yeah the problem multiplies
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  #31  
Old 03-03-2018, 01:31 PM
glend (Glen)
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Yeah, laser fitting into focuser tubes, we could have a thread on that alone (and have in the past). Not all focuser tubes, or adaptors, are the same diametre, and the various type of compression fittings can push the laser to one side. I suppose they allow some slack in the bore for various EPs that might all be the same diametre, and for EPs that might not matter too much. I found on my GSO laser that I had to shim it by wrapping a thin piece of plastic film around it to get it fit snug in the focuser and not push the beam off to one side when I tightend the knob. The self-centering adaptors are great but of course scopes don't come standard with those.
My Moonight focusers can't be faulted but stock ones are just too loose.

This slack in the bore can also be a factor in lasers showing collimation changes as scopes are moved on their axis.
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  #32  
Old 04-03-2018, 09:30 PM
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Shiraz (Ray)
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Hi Dunk. lots of good advice here. FWIW, maybe a little more?

1. If the mirror assembly is like the 250f4, these scopes cannot be collimated while horizontal. The mirror is free-floating on cork pads and can also slide very slightly sideways - it can flop around alarmingly if the OTA is horizontal. It can hold collimation OK, but only if you initially do it with the mirror held back against the pads by gravity - suggest that you collimate on the mount with the scope pointing above the horizon by say 30+ degrees.

2. Collimation through the CC seems to work best - the CC can have enough wedge to throw off the collimation, so leave it in.

3. a simple jig to collimate your laser can be made with a bit of wood and 4 nails (image). Project on the wall and adjust the offset screws to fix any misalignment when you rotate the tube. A well aligned laser is plenty accurate enough to get these scopes working properly - if the centre spot is right and you clamp the laser tightly in the drawtube adapter.

3. your weird stars suggest that the light column is being truncated by the OTA, because the secondary is misaligned in-out. This might just be due to collimation varying when you put the scope up on the mount, but it could also be a more fundamental problem with the secondary alignment. It is nigh on impossible to get the secondary in the right place by looking down the focuser - your eye needs to be in the focal plane, which means that you cannot see the edges of the mirror (the drawtube gets in the way) and you must guess how well it is aligned. If you find that the stars still look crook after you have fixed the collimation problem, the back-projection method woks OK to get the secondary tidied up. Will put a summary in another thread, rather than clog this one up. Cheers Ray
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Last edited by Shiraz; 05-03-2018 at 10:48 AM.
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  #33  
Old 05-03-2018, 01:12 PM
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Camelopardalis (Dunk)
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Thanks Ray, great little jig btw. I think my level of handiness might even be able to manage that

The mirror is held in the inner metal cell by 3 rubber/plastic clamps, then the inner cell attaches to the outer metal cell by the 3 pairs of collimation push/pull screws. This whole unit is attached to the tube by half a dozen screws.

I'm going take the cell out again to check the centre spot is actually centred...

Certainly lots to learn with this scope
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  #34  
Old 05-03-2018, 10:53 PM
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andyc (Andy)
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I think Richard (and possibly others) mentioned the primary mirror clamps - I'd definitely be checking those, as I had star shapes quite like yours when I tightened mine too much after cleaning the mirror one time - as Richard said these need to be barely touching the mirror with minimal pressure. If it's not that, then I'm not sure, others will know more than me on other issues. And collimate at roughly the tube elevation of your target as it can shift around a bit (I learned that the hard way on planets after over a year of lazy horizontal Newt collimation).
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  #35  
Old 06-03-2018, 11:56 PM
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Camelopardalis (Dunk)
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Thanks Andy, I need to take the mirror cell out again anyone so will see if I can find a sweet spot.

Clouds and rain have stopped play, however
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  #36  
Old 07-03-2018, 11:44 PM
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Camelopardalis (Dunk)
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So I've just been idly poring over one of my seemingly futile captures from that night and having centred the secondary shadow in the doughnut, I took a short capture of the Moon in video mode.

With the FOV of the scope/camera, the Moon was occupying about 1/4 of the frame.

Stacked and lightly sharpened. I expected worse...
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  #37  
Old 08-03-2018, 12:04 PM
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Good one Dunk none of the stars are elongated☺
Alex
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  #38  
Old 08-03-2018, 01:39 PM
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Camelopardalis (Dunk)
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