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Old 22-09-2019, 07:07 AM
morls (Stephen)
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Replacing collimation screws 180 Mak

I jumped in the deep end with this (fools rush in...) because I need to know how to collimate this scope so I can squeeze every bit of performance out of the setup. I've figured out that it's 3 pairs of push-pull screws, rather than 3 sets of 1 adjustment screw and 1 locking screw. When I started I noticed all three pull screws were tightened all the way in, drawing the the mirror right back, which made me wonder if this was a setting for shipping to stop the mirror moving. The full range of travel with these pull screws is 10 turns, so I started collimating with them set at 4 full turns out. I think this has resulted in the mirror being a few millimeters further forward than before, and I think it's made an improvement. It'll be interesting to see how it all settles...
Cheers
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Last edited by morls; 22-09-2019 at 08:41 AM.
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Old 22-09-2019, 10:53 AM
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Tinderboxsky (Steve)
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Interesting results Stephen. I look forward to hearing about any further improvement you can achieve.
My wife has the smaller 127mm. Whilst a star test looks ok, I have felt for a while that there might be some improvement to be teased out of the scope.
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Old 22-09-2019, 12:08 PM
morls (Stephen)
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Hi Steve,


The big moment for me was deciding to remove one of the big countersunk screws. When the mirror didn't shift or fall out I was happy

The skywatcher documentation I got with the scope was pretty useless, so I had to look around. I'd assumed the small screws were for locking only, and all the adjustment was done with the large screws. This page put me on the right track:
http://www.ioptron.com/v/Manuals/610...ollimation.pdf
I did a rough adjustment looking through the front, to get the main alignment straight. I then used a defocussed star at 77x magnification to tune further.

It really helps to have a widefield eyepiece and diagonal. I rotated the diagonal until the areas where the defocussed star rings were closer together were in the same position as the collimation screws.

After collimating here I then went to 168x magnification. This is more challenging, and having a tracking mount really helps, as the image shift is significant with even the slightest screw adjustment. I was constantly using the direction pads on my hand controller to keep the defocussed star centred. At the moment the collimation is 90% there at this magnification. The next step is to go to higher mags and keep tuning. It's really interesting to see the defocus rings as they collapse into the focus point. I think this is the level at which the really fine tuning can take place.

I lost the star once after making too large an adjustment, and there was no easy way to get it back in the eyepiece. The finder scope quickly becomes out of alignment as the primary is shifted, so its necessary to keep readjusting the finder.

At one point the collimation got way out, and I lost touch with what was happening. I had to start all over again the next day, from the rough adjustment through the front.

The lightbulb moment was when I realised that my goal when adjusting was to move the outermost defocus ring, rather than move the centre points. After this it came together very easily.
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Old 22-09-2019, 02:57 PM
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mental4astro (Alexander)
kids+wife+scopes=happyman

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Hi Steve,

I understand why you wanted to change the bolts, but it wasn't necessary. These longer bolts may also be more of a hinderrance than a help.

There is a simple thing to do to the original bolts that is MAGIC in helping one work out the amount and direction of the bolt turning.

The bolt head and the edge of the hole on the OTA are both marked with a little line - this marks point zero. Then when you turn the bolt, you will know exactly by how much and in which direction the bolt was turned, and you don't need to worry about keeping a mental track of this, which when you are also trying to keep control of the collimation process, these marks will free your mind to stay totally focused on the collimation.

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The diagrams above show a 1/8 turn of the bolt. With the collimation process, you will be doing just 1/8th turns at a time, and even smaller. With an Allen key, this is possible, by hand turning the bolts, you won't have the same degree of fine control. This way you won't experience the star being lost outside of the FOV because you will have fantastic control of the torque and angular movement of the bolt head.

Collimation is also best done using just about the highest magnification the night allows. It is also a misnomer that you make a BIG doughnut with the star for collimation. This is ONLY for course collimation. FINE collimation is achieved using only a SMALL doughnut! It sounds counter intuitive, but man, once it was shown to me I changed the way I collimate my scopes.

Also, if the scope focuses by moving the primary mirror, the scope will typically exhibit some degree of mirror shift. Only a few Cassegrains show no mirror shift with primary mirror focus. If your scope shows mirror shift, you must only collimate AND focus the scope exclusively in the one direction of turn of the focus knob. Mirror shift WILL alter collimation, and it is impossible to collimate a scope that exhibits mirror shift as moving the mirror inside and outside of focus, the collimation will be stuffed as the mirror shifts from one position to the other.

So, to both collimate AND focus, you need to get into the habit of first turning the focus knob counter-clockwise, and then slowly turn it clockwise. This will set the primary mirror always into the same position for both collimation, and for best focus.

Now, if your scope has an external focuser, like yours Stephen, then mirror shift won't be an issue (not always though for other reasons that are beyond the scope of this discussion).

Alex.

Last edited by mental4astro; 25-09-2019 at 04:06 PM.
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Old 22-09-2019, 05:02 PM
morls (Stephen)
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Thanks for the insights Alex.

I might be going about this the wrong way, but at the moment I'm not concerned with exactly how far I move each screw. I realise there are recommendations to match the adjustments made to one pair by doing the opposite to the others, but this isn't something I've been doing.

The technique I'm exploring at the moment (and which I haven't had a chance to fully test due to lack of clear skies) is to get the screws 95% tightened at 168x magnification (16mm Nagler), and then add the powermate, push it to 336x for the final tightening. I've noticed that at this high mag - and with the small doughnut you mentioned - the slightest movement has an effect, so my goal is to combine moving the screw the last maybe 1/20th of a turn with the fingers to get that final lock with reaching good collimation.

I'm trying these finger screws so I can avoid fumbling around with the allen key while looking through the eyepiece. You're right that the fine control isn't the same, but it's much easier to feel where each pair is. I've used some moly paste on the threads, but there was issue with the point of contact between the pull screw and visual back. Initially I used flattened spring washers beneath the two nuts in the picture as I didn't have anything else with a small enough diameter to fit inside the countersink hole, but there was still a ridge in the washer that made the final adjustment too rough. I now have suitable flat washers which have solved that issue.

I've kept the original screws, so can revert back if this experiment shows the longer finger screws to be impractical. Your point about the collimation changing each time the original focuser is used is a good one. Using the external focuser goes some way towards removing that as an issue, but I still use the original focuser for the big changes necessary after removing the diagonal to attach a camera. I'll have to remember what you've said about always using the same direction of focus.

It's teaching me a lot about this scope as well as star testing and tuning. Great fun too...
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Old 24-09-2019, 02:17 PM
morls (Stephen)
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Just a quick update,
Well, it was always going to happen...curiosity got the better of me, and I opened up the mak. After a lot of looking and thinking and shining a torch inside to work out how things were put together, I saw that the front corrector cell is very solid and self-contained, as is the rear cell. Each is connected to the tube by 4 tor head screws. After setting up the scope on it's end, pointing up, I removed the screws from the front and the corrector (very heavy) lifted off. It was then just a case of removing the screws attaching the rear cell to the tube and very carefully lifting off the tube.
I wanted to see behind the primary, and was able to after moving the mirror all the way forward with the stock focuser. Attached is a photo showing the plate and tap where the pull screw attaches, as well as a pic of the primary with baffle. It's very solid! I'm satisfied now that I understand how this scope is put together. Reassembling was easy, and there is no real chance of mis-aligning the cells as the fit is good.

I took the scope down to the local park today with a home-made artificial star (LED torch, foil and pinpricks). Alex, you're 100% on the money with the allen keys being much easier to control, luckily the screws I put in have the same hex heads as the originals. I was able to get really close with the collimation, and did most of the adjustment at 338x, but pushed the mag to a ridiculous 675x for the final setting (2700mm focal length, 8mm eyepiece with 2x powermate). It'll be interesting to see how a real star looks.

The screws are now tight (stronger than finger tight but not the tightest I could go with the allen key). Having seen the mounting plate I'm comfortable that there is no risk of damaging the primary with these collimation screws. The mounting plate where the screws attach seems solid enough. I wasn't particularly careful to tighten or loosen the screws according to a system, so I hope there aren't any issues caused by this. I'm confident the screws aren't going to move, so fingers crossed the plate is thick enough that balancing the screw tension isn't necessary.
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