Fine Collimating A Fast Newt

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Fine Collimating A Fast Newt

Postby Dion » Mon May 28, 2012 4:06 pm

After numerous tweaking sessions on my f4 quattro I now have a fine collimation technique. For this you really need an astro modded webcam and a cheshire collimater. Without experiencing both, many people assume that an F4 will be much the same as an f5 scope for collimating, they're actually quite a way apart and at F4, perfect collimation takes some time and effort. I'm not talking about an average, standard colli here, I mean a finelely tuned lean mean imaging machine!

What I'll call 'average' collimating an F4 is just a standard collimation like you'd do with an F5, and in most cases, an average colli will behave 'averagely'. You may get a little coma, even with a coma corrector in place, and also any diffraction patterns on stars will probably be off center on the edges of the field. In fact, to be honest, you will quite possibly still get a touch of coma even when fine collimated, purely because, at f4 you really are pushing such as the baader mpcc to the edges of it's limits. So here goes.

Do a normal collimation procedure, preferably like in a couple of my videos where you've also squared the focuser. Next, on a clear night, set up the scope and find a nice, average brightness star that is high up and point the scope at it. Now at this point you really want the webcam in and you really need to have the star bang in the centre of the field of view (I'll explain why later).

Next, defocus the star until you see something like a doughnut with a black center and concentric rings, chances are, it probably wont be that concentric. Now make a not of which direction the black circle in the middle needs to go to in order to center it. Each time you alter one of the bolts on the primary, the star will move in the field of view, after EVEY adjustment, move the scope controls and re-center the star.

Why re-center? Well, have a look at that doughnut again and slew the scope backwards and forwards to the edges of the FOV in the camera preview window, you will see that the doughnut goes off center too when you move it, so to be deadly accurate, adjust primary, re'center in FOV, check doughnut, adjust and repeat until you have a perfect doughnut with a black circle dead center and a symetrical ring pattern. That's part one done.

Now, either wait until the next day or strategically position a torch so that you can use a cheshire collimater, place the cheshire in the focuser and you will see that your secondary is now out, so, re-center ONLY your secondary with the cheshire.

next, camera back in again and do the same process with the doughnut, you will notice that now, you don't have to adjust as much as last time.

Now repeat the procedure with the cheshire, again you will notice that a lot less adjustment is needed.

Keep repeating the steps until you have both a concentric doughnut AND a centered secondary, it may take 4 or 5 goes at each stage. When done, as long as your secondary is nice and tight it should never need further adjustment. HOWEVER, it's a good idea, especially if imaging to repeat just the doughnut star test at the beginning of every session.

I think I can safely say that no matter how good you are with lasers, cheshires, whichever, with an F4 scope it will NOT pass the star test on first use.

Regarding the torch and cheshire, it's quite doable so you can complete the whole process in one evening as opposed to waiting till the next day to do the cheshire part.

Discussion/questions on this here :-

viewtopic.php?f=42&t=10170

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