Collimation.. please humour me one last time

[barees63]

ok, there is one last remaining thing I'm not sure about re collimation and I'm hoping someone can give the answer I so desperately need.. anyway,

I have now gone thru collimation from scratch at least 3 times, this involves the following:

1.carefully centering the secondary using a sight-tube with colored paper opposite the focuser and white paper blocking tube so I just see the face of the secondary and nothing else, by sliding the sight tube down so the secondary fills the tube FOV I can get it perfectly round and perfectly centered.

2. removng the paper and tilting the secondary so the primary mirror appears centered (and I can see all 3 clips etc) and the center-spot is centered under the sight tube crosshairs (this is quite hard since the crosshairs are very fuzzy and out of focus) I then use the laser to get the spot dead center in the center marking.

3. Using the cheshire now, tilt the primary to line up the cheshire reflection with the center mark. finally double check with laser and lo and behold! it seems to agree..

BUT..

every time I do this the reflection of the spider is not quite centered on the primary center-spot.

so my question is, should it be? Given that everything else seems right why is this always slightly out in one axis (no more than a 1-2 mm, the triangle center-spot is "touching" the spider reflection it just doesn't pass thru dead center)

:help:

[asimov]

That sounds correct to me. What your seeing is secondary offset (I'm guessing here lol). Which is achieved automatically using the sight tube.

I'm no expert though....& I don't know anything about laser collimation.

[Starkler]

Its entirely possible that the spider and vanes are not perfectly mechanically centred, and for everything else to be well aligned.

If you have a large pair of dividers (like a drawers compass) its simple to check the mechanical alignment of the spider.

The worst thing that can happen is the the fully illuminated field isnt perfectly centred but thats of little consequence. In other words enjoy the views until you can be bothered centering the spider ;)

[barees63]

Guess what.. I think you're quite right, I tried measuring again with calipers (a bit hard, dividers would be more accurate) and it seems the spider is definitely off-center on the same axis that it appears out visually (maybe 2mm, no more, closer to the focuser). Since, if I read you right, it sounds like this won't really affect sharpness or detail too much I might just live with it for a while, I'm generally really happy with what I'm seeing thru the scope (sample of one though since I've never looked thru another scope) and the views have certainly improved since I got the collimation tools (really noticed on globular clusters.. much more "resolved").

[Starkler]

Reading what you say above, the views wont be affected in terms of sharpness at all, but only as I described earlier.

I had an offset spider on mine, but sideways in my case due to a manufacturing error.
Once I fitted a washer under the chromed spider retaining nut to pack it, i was able to centre it fine.

[RAJAH235]

Bruce, All sounds as it should be. Dobs need offset. see link >
http://home.att.net/~dale.keller/atm/newtonians/notes/diagonaloffset.htm
HTH. Maybe move 2ndary slightly away from focuser? :D L.
ps. My Meade has offset, built in, 'at the factory', to the 2ndary. Looks really strange from the top of the tube. Doesn't seem quite right but it is..

[davidpretorius]

hey, my 10" is the same. i look at janoskiss's 8: and every single circle was centred and concentric. we collimated mine and mine wasn't.

star test is now the next trick. defocus a good half turn of the focusser and then start to slowly foucs, with the star centred at all times. if you can hold that round shape all the way to focus and out the othe side, you are there!!!

well done, i am sure it has been worth the little bit of pain for the gain!

[janoskiss]

If it is only the spider that looks off centre but everything else is centred AND your secondary is fully illuminated, i.e., the entire primary is visible from the focal plane, then all is well. To check everything else is centred you use the cheshire collimator, as you've already done.

To check that light from the entire surface of the primary is reaching the focal plane, defocus with a low power (long focal length eyepiece) until stars start looking like the spider and secondary obstruction. If you can also see the primary mirror and three mirror clips in all defocused star images across the entire field of view, then the spider and secondary position are fine.

Fine tune primary orientation with star test as usual.

[barees63]

Cool, thanks again guys.. kind of OT but I went out tonight and had a slight panic attack.. nothing would focus and everything looked really dim - I thought, wow I've really screwed up the collimation this time! then I looked at the eyepiece with the torch and saw it was totally dewed up :P

so once I dealt with that (wiped with a cotton bud - is there a better way?) I tried one of the Feb. challenges, splitting Rigel, and was able to no problem at all with 9mm EP so I guess that bodes well.. btw, I also viewed Ghost of Jupiter for the first time - neat! (A good "Averted Vision" trainer!)

Tonight was the worst night for dew I've encountered so far, by the time I got inside the scope was literally dripping and even primary was completely fogged up.

[davidpretorius]

hairdryer is best, i don't like touching the lens much unless to clean etc

well done on the challenges, i will be keen to see how your 9mm goes on the likes of galaxies

[barees63]

ah yes.. that rings a bell, I'll have to have a look for a cheap 12v hairdryer. I've probably been a bit rude to these EPs, luckily they're cheap ones (mind you I never "scrub" them, just drag the cotton bud around under its own weight, same when I clean them.. I use 50/50 Mr Muscle Window cleaner & distilled water - sounds weird but S&T recommend 50/50 Windolene/distilled water and I found that Mr Muscle is the downunder equivalent of Windolene) If/when I get expensive EPs it will be a different story ;-)



I did look at several mag 9.x - 10 galaxies last night as well as a brighter one in Centaurus (using AN tour mode) they were visible but pretty indistinct but the dew problems had a lot to do with it, by that time I couldn't really focus and the whole image was very dim.