Collimation of Newtonian Telescopes using SharpCap

[metastable]

I gave this a shot tonight and found it extremely difficult to find and keep track of the center dot of the primary. I followed all the instructions and thought I had it spot on but after I checked it with my laser collimator and found it to be a bit off. I think there was too much "slop" from using a 1.25" adapter plus the inherit wiggle room of the camera into the adapter. The line tools themselves worked flawlessly.

[admin]

Hi,

it's an interesting situation... If any slop in the mounting of the camera is also happening when you attach your actual imaging camera or eyepiece then you are going to struggle to get truly good collimation. Correct collimation needs to take into account any tilt in the mounting of the camera or eyepiece in the drawtube.

cheers,

Robin

[metastable]

I'm not exactly certain that was the cause, I was tightening things down as I was going through the steps. I think the biggest downfall on my part was the difficulty of keeping track of the center dot. I was removing the camera many times adjusting the lens focus in an attempt to bring it out and while it did help, it of course caused me to move things around which likely increased my errors. If I was a braver soul, I would consider making the center dot black which likely would help. Either way, I don't think it's an issue with the tool and instructions, but more an observation on my part.

[Borodog]

Robin,

Gave this a shot and after a few hours (!) I think the scope is as collimated as I can get it. The main difficulty was that it's simply too dark in the scope to see the black center mark on the primary in the shadow of the secondary, and much too dark in the draw tube to see the white ring around the lens. Ultimately I had to fall back on a cheshire collimator where I could see the ring on the primary.

collimation image

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A friend of mine uses a trick with a laser collimator in a Barlow. I have to figure that one out.

[admin]

Hi,

good to hear that it mostly worked out. I used a torch at times shone from odd angles to get the illumination level right to see the things I needed to see.

The barlowed laser trick is good - since the barlow is a diverging lens, the laser going through it is turned into a diverging fan of light coming from the focal point. This will reflect off the primary as a parallel beam of light (the primary working the opposite way around to usual, which will go back up the telescope. Part of that is reflected back into the barlow by the secondary, and shows the center mark of the telescope as a shadow. For good collimation, that shadow should be central. The technique is very sensitive to the tilt of the primary, less so to the tilt of the secondary and insensitive to the alignment of the barlow.

cheers,

Robin

[Borodog]

I think to make this easier, I am going to paint the inside flat surface (the one facing the optical axis) of the 2"-1.25" adapter that I used to make the camera fixture with glow in the dark paint. I won't be using it for anything else so it isn't going to be polluting any views or images. SharpCap should easily be able to see that.

[Fir Chlis]

I've just tried this myself on my Skywatcher 200PDS, and (pending a clear night) managed to get what I think is a very good collimation.

I'd already spent a considerable amount of time using a collimation cap, but what I was struggling with was the secondary mirror position and alignment. Judging concentric circles while looking down a dark tube isn't easy (though I was using pieces of coloured card in the main tube for contrast), and each adjustment tended to throw out previous adjustments. In addition, I don't think that my focuser was quite aligned, as I could not get the secondary mirror centred laterally simply by adjusting the three adjustment screws - or at least if I did that, then the centralised position was lost when I had to readjust to get the primary mirror centred.

My first task was to get the draw tube, the secondary mirror, and the reflection of the primary mirror all concentric. Using the multiple reticule tool, I placed a coloured circle on each of the three objects, and I could see from the crosses at the centre point that they were all slightly off, although not too bad, I think, from just using a collimation cap and judging concentricity by eye.

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I then moved the centre point of the primary and secondary reticules so that they were all concentric with the draw tube.

Over 2-3 iterations, I adjusted (where needed):

* axial position of the secondary with the middle screw
* lateral position of the secondary using the three adjustment screws on the Crayford focuser
* reflection of the primary using the three adjustment screws on the secondary

until the secondary, and reflection of the primary, were all aligned with the appropriate circle reticule.

After this, I followed the procedure described by Robin earlier in this thread. As it turned out, the adjustments needed after my concentric circle alignment were minuscule and, after this, the camera showed no noticeable movement when rotated 180deg. The two photos show the two rotations after I'd finished. Although not obvious on these photos, the very short reticule line remained centred on the camera reflection.

Hopefully, with the secondary now accurately aligned, it will just be an occasional check of the primary collimation with a star test.

So if you happen to have a camera with a lens (I used an ASI120MM with the all sky lens that came with it), this seems to be a good and, once you've done it a couple of times, fairly straightforward procedure.


Geoff

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