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Fast Newtonians how often are you collimating?


Cumbrianwolf

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It may be a little obsessive, but I laser collimate my scope each and every time out as I run a fast F/4, this is either for visual or photography. So do you collimate your just as much, as I am curious?

Edited by Cumbrianwolf
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I should add, I'm using a trick I picked up in another forum.  I cut a couple of disks of plastic from a milk bottle and used them as a soft washer between the secondary holder and the three collimating bolts. This seems to even out the pressure and the bolts bed-in to the plastic rather than pit the holder. It makes the laser collimation process slicker too. (It is also easier to finely rotate secondary when major collimation is required such as after a mirror clean or a focuser upgrade). I've used the same disks for several years, periodically rotating/flipping them when ever I do scope maintenance.

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1 minute ago, paul said:

I should add, I'm using a trick I picked up in another forum.  I cut a couple of disks of plastic from a milk bottle and used them as a soft washer between the secondary holder and the three collimating bolts. This seems to even out the pressure and the bolts bed-in to the plastic rather than pit the holder. It makes the laser collimation process slicker too. (It is also easier to finely rotate secondary when major collimation is required such as after a mirror clean or a focuser upgrade). I've used the same disks for several years, periodically rotating/flipping them when ever I do scope maintenance.

Thanks for that, and I will probably do just that, as I noticed the recessing / dimpling getting worse on the secondary mirror's aluminium mount from the screws that are in there, was also tempted to replace them three collimation screws for thumb adjustable ones to save needing the screwdriver.

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Whenever I take my 200mm Dob out which is not so often I do a quick collimation with the laser. It doesn't move much but it is a quick check,

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I collimate my 10 inch f4 before every imaging run, even temperature differences can change collimation and f4 has a very small margin of error 

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  • 2 months later...

 

 

I cant argue with that. Visual astronomers using high magnification where the detail you want to see is at the Dawes-limit for your scope will benefit from collimating each session to catch those moments of steady air. Those that lucky image planets at long focal lengths will benefit from it too. 

 

For me, as long as dim stars are round and spikes are central on bright stars then the collimation is good enough to do long exposure imaging. When I was tearing down after every session I was finding my scope holds collimation well enough for that. Even now, although I have a semi-permanent setup I do check collimation periodically.

 

I guess a lot will depend how well the mirrors are held in place. The clamping of the secondary holder with the plastic washers has allowed be to keep good tension and movement is minimal. Of course things do move a little bit and temperature plays a big part part in this. It helps with my gear staying outside that it is almost always at ambient when I do the collimation. As the spider is under tension things will move as the OTA cools. ( If the spider flexes significantly as the OTA orientation is adjusted then either the spider isn't secured properly or the design is not sufficiently rigid for long exposure imaging.)

 

We know that we should not stress the primary mirror by clamping them down - that distorts the image badly. Equally I believe the primary should not be flopping about  as the OTA moves on the mount. That is going too far IMO 😀. The primary mirror cell on my Orion Optics UK OTA holds the primary three ways:

 

From the side, plastic bolts lightly hold the mirror central - they are adjusted for an interference fit - not deformed against the cell.

 

The over hanging clip has some low density foam between it and the mirror - again an interference fit without real compression on the mirror (the backside of the mirror is raised on plastic bolts to level and achieve that friction fit).

 

The last securing mechanism is electrical tape across the back of each clip and sticking to the side of the mirror. That prevents the plastic bolts from moving and any tendency for the mirror to rotate as you jostle it onto a mount.

 

The result is a primary that doesn't flex appreciably and transports well and holds collimation. A bigger mirror would probably need a more sophisticated cell.

 

 If you are in any doubt that I treat collimation seriously have a look at this post 😄

 

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