Newt collimation and secondary offset (split from Local help rqd)

[janoskiss]

While I cannot offer you "local help", I think it is the secondary offset that confuses you as does many newbies. What it means is that the reflection of the spider and secondary mirror will not look centred once you are finished. Collimation instructions usually do not mention or illustrate this and can be very confusing. So if you follow the basic steps you'll be right:

1. Centre secondary under focuser (no need for super-high precision)
2. Adjust secondary tilt of secondary to bring primary to centre (X-hairs on centre spot).
3. Adjust primary tilt to bring Cheshire reflection to centre: coincident with primary centre spot.

Let me stress again: After you do this the reflection of spider + 2ndry mirror will not look centred. That is okay. That is how it is supposed to be. As long #1,2,3 are right, you are done.

See more detailed instructions from S&T here: http://skytonight.com/howto/diy/3306876.html (which does mention 2ndry offset btw).

[EzyStyles]

Steve, that comment you made have solved my issue!!!!!!! why all these time i try to make "everything" inside the focuser all lined up. i still read the newbie section for into like this!. thanks dude.

[EzyStyles]

Can someone confirm if this is right? pic taken looking down the peep hole of a cheshire.

As you can see, the X-hair of the cheshire (red) doesnt line up with the spider vain (green). Nor does it line up with the secondary mirror reflection (blue).

[ving]

looks pretty out to me. i'd probably attack the secondary.

[EzyStyles]

thanks ving. hmmm but according to steve:

"What it means is that the reflection of the spider and secondary mirror will not look centred once you are finished. Collimation instructions usually do not mention or illustrate this and can be very confusing. "

[janoskiss]

That looks fine Eric. The only thing we cannot tell from the photo is if the secondary is centred well enough in the focuser for the entire primary to be visible.

Ving, stop being so violent. Don't listen to him Eric. There is no need to be attacking anything.

[EzyStyles]

cheers steve. otherwise i'll get confuseddd :)

[gel]

confused you must be joking,im all away with the faries on this and my first comment still applies"local help rqd" im sure something will happen-like a knock on the door?

[EzyStyles]

hi gel. can you take a pic like mine looking down the focuser or cheshire?

[gel]

ezystyles hang loose ill see what i can do,just bieng out mooning/jupitering things were reasonable

[RAJAH235]

Eric. Your red X & green X don't line up. They should. Your 2ndary is out. As Ving, (the 'Tweaker' said), 'adjust it'.
The reflection of the main is not in the centre. ie; not fully seen.
With it set like that, you'll find that the 'striations' will go from one side to the other when going from 'inside' focus to 'outside' focus..... & when 'in focus', the star image will not be a pinpoint.
They should all look centred thru the focuser tube BUT not when viewed down the top of the t'scope tube. This is the required '2ndary offset'.
HTH. :D L.
ps. It's 'Andy's Shotglass'.
pps. Steve mentioned that it's diff to see if the 2ndary is in the centre of the focuser tube too. Make sure it is by placing a sheet of white paper inside the tube, opposite the focuser. Stand back a little, so that you can see a 'white circle' around the 2ndary. You may have to 'shim' the focuser to get it right.
pps. See > http://www.fpi-protostar.com/collim.htm

[janoskiss]

I disagree with Rajah here. The photo looks about right to me. The faster the scope the more off centre the spider will look. Erik's is an f/4, which is getting pretty damn fast. The S&T collimation guide I linked above mentions that the spider reflection will look offset when finished, so I think you are probably allright, Eric. To confirm that secondary is fully illuminated, throw a medium power eyepiece in the focuser and check that you can see all three mirror clips in a defocussed star image (rack focuser out rather than in to be sure). I'll see if I can take a picture through the focuser of my 8" f/4.


Well you are the one in need of help, so if there is no one in your immediate area willing and able to oblige, you might have to do the door knocking yourself. Mike already suggested a couple of options. I'm sure the Bintel guys would be happy to help too if you get your scope over to them.

[RAJAH235]

FWIW. Here's a couple of quik pics of mine, which gives near perfect pinpoint stars & diff rings.
Shows the 2ndary offset that you end up with. (It's built into the mount by Meade).
ps. The 'thicker' spider spike has my 2ndary heater wire running along it.
pps. 10" f4.5

[ving]

lol steve! you are so disagreeable sometime :P
:lol:

i guess eric the proof is in teh tasting... or seeing. i have never worked with such a fast scope and steve has so he is probably right :)
how does a star test look either side of focus. can you post a pic of that?

[EzyStyles]

now im really confused..

thanks for the feedback and apologise to Gel for highjacking your post . Rather than creating a new post, I have the same issue.

So the cheshire x hair has to line up with the reflection of the spider??

[janoskiss]

For those of you still confused about secondary offset but too lazy :poke: to follow the link in my post above to the Sky and Telescope website's page on collimating Newtonians and read through the article, here is a relevant excerpt:

For an f/4 you could change "slightly" to "considerably".

[EzyStyles]

through a cheshire the secondary reflection now looks better as seen in pic 1. but the cheshire cross hair (green line) isn't on it. To me everything seem to line up. pic 2 shows a front view of the scope with a laser. As you can see from the laser pointer, it is not hitting the center of the primary mirror??????

[ving]

does this provide you with nice defraction rings?

[EzyStyles]

thanks steve.


heres a shot of antares from last night 20sec. as you can see the diff spikes are all even but look at the stars in the background. they are either elongated or doubling up.

[ving]

antares is enlongated too. :confuse3: hmm...
i am guessing that insade and outside focus doesnt give you nice rings either?
do you happen to live near one of our experts?

[EzyStyles]

thats the main issue im having after using the coma corrector. Im not blaming the coma corrector. I think it helps me to determine to find the correct collimation point. I can get circle stars, but the diff spikes will be doubled meaning that im not focused. :(

[bird]

Eric, try this site for more info on collimation:

http://web.telia.com/~u41105032/

Also seriously consider buying a copy of "New Perspectives in Collimating a Newtonian" by Vic Menard and Tippy D'Auria. It's about $15 U.S. and is widely considered to be "the" reference.

regards, Bird