Collimation

[jimmyjamjoejoe]

So i got my 200p last week, arrived straight from the importer as opposed to via FLO, and the collimation was WAY out.

No matter what i did i couldn't get a round secondary central to the focuser/sight tube. It was always "too high" leaving a larger gap around the bottom of the secondary when looking down the tube. Eventually, I tried adjusting the spider vanes, moving the secondary into a position where it was round, central, and pointing at the primary.

The rest of the collimation went textbook easy. Secondary is round, all 3 primary clips are in view, and the central circle on the primary is dead on the crosshairs.

I'm just wondering if there is any ill effect from having the secondary not quite central. It's about 5mm off central, my only guess as to why this was necessary is the focuser may be mounted a bit squiffy.

I suppose technically it's fine, since the light path is intact, i get a full view of the primary, via a secondary that is angled straight into the focuser. Maybe the only effect i can think of is diffraction spikes being slightly off centre, but since i'm not imaging, that isn't a worry for me really.

What we all think?

[Jason D]

There are two ill effects:

1) Diffraction spikes (one set) will be thicker since you have at least two spider vane segments out-of-line -- which translates to poorer contrast. HOWEVER, what have mentioned is theoretical. You might hardly notice it in practice -- if at all.

2) If the secondary mirror is way off-center, then you would potentially introduce what is known as front-end aperture vignetting. That is, the edge of the OTA will obstruct the light path of stars. If you can't see the edge reflection of the OTA via the primary mirror while looking though the center of the focuser, then you are OK.

Check the "squareness" of your focuser. That could be part of the problem. Or you can just leave things as is and enjoy your scope.

Jason

[jimmyjamjoejoe]

Pretty much as i thought. Visually, i haven't noticed anything weird with the diffraction spikes, and certainly don't see the edge of the OTA in the reflection.

I'l give it another once over tomorrow just to make sure i'm not being an idiot, but this will be the 4th time i'd have slackened everything off and started from scratch, and each time so far has come to the same.

Out of interest, how could i check the squareness of the focuser?

[Jason D]

Remove the secondary mirror then do one of the following:

1- If you have a laser, insert it in the focuser then adjust the focuser either by adjusting its base screws or by shimming it until the laser hit a pre-marked point on the other side of the OTA. You need to set that mark to be precisely opposite to the focuser axis.

2- Use a long stick if you do not have a laser

3 - Or place your OTA horizontal on a table then have the focuser on the top. Lower a small weight hooked to a string until it hits the opposite side of the OTA. Use a level to ensure the OTA is horizontal and the focuser is located vertically on the top.

Jason

[Moonshane]

do note the position of the secondary/primary reflection in the centre of Jason's avatar. with fast scopes there will always be this apparent misalignment but this is correct and called secondary offset. this may not be what you are seeing of course.

[jimmyjamjoejoe]

Yeah it's not that kind of misallignment shane. It's apparent in the first stage of collimation (according to astrobabys guide), when just centering the secondary beneath the focuser. It can either be round, or central, but not both. I'm very much leaning towards the squareness of the focuser, since i could achieve a round secondary whilst it was central in the vanes, but the focuser would need to be tilted slightly, literally a couple of bits of paper would probably do it.

Not to scale or anything, but the general idea of what's going on is like this -

http://imageshack.us/photo/my-images/864/collimation.jpg/

It was actually a little more severe - to the point where the top 10% or so of the secondary was cut off by the focus tube.

Any amount of adjustment to bring it central and round leaves it looking at the side of the OTA, with just a glimpse of the primary.

The only way i could get it round, central and with 3 primary clips in view was by offsetting the vanes.

[Tinker1947]

I am wondering if your problems are due to the Seconday Mirror being to far into the Tube passed the center point of the Focuser, or not far enough in..??

I lined my spider vanes up using a large compass, one end in the center of the Secodary Mirror screw, the other end just touching the spider vanes ends.

[jimmyjamjoejoe]

I can get it central in the vanes no problem using a tape measure. The problem is when it IS central in the vanes, it isn't central to the focuser, and results in the image above (where left is towards the objective and right towards the primary).

I have had all screws loosened, and literally sat there rotating and tilting the secondary by hand, and no matter what, if it was central via the vanes, no combination of tilting/rotating would have it central and round. It ~has~ to be the focuser.

But either way, i don't think i'm losing light or anything, so i'm not even sure i ~need~ to ****** about with it.

[Tinker1947]

The secondary mirror moves in and out of the tube by undoing or doing up the centre screw thats in the middle of the spider vanes, taking care that the scope is level so anything dropped does fly down and hit the mirror, and holding the edge of the Secondary should you unloosen it so much that it becomes detached, then just follow the tutorial videos ect to get it set in the right place, i used a cap fitted into the focusor with a very small hole in the centre then move the focusor in and out and the Secondary Mirror back and forth until it is in the centre of the focus tube, take a bit of time but its a one off job.

[jimmyjamjoejoe]

Thats what i'm saying. I DID loosen it off, to the point where i could rotate it, tilt it, and move it up or down the tube with my hand as i saw fit, and no matter what, it won't be round and central and pointing at the primary, it is literally impossible.

It only becomes possible if the vanes are adjusted. Once they're adjusted 5mm off center, it's easy to centre the secondary, get it round, and pointing at the primary etc.

The view down my cheshire is textbook BANG ON perfect collimation, anybody looking through it would say it is perfect, but niggling away at the back of my mind is the fact that the secondary isn't central in the tube, even though it isn't noticable.

[Tinker1947]

Try a Star test when the clouds clear, high mag EP and move it right out of focus, you should end up with a round'ish web shape...if its right...

[Moonshane]

looks like your secondary bolts might not be even (ie the bottom one longer or shorter (by a lot) than the others)). use a mirror to check - this might be it.

I can never get the view right down my Cheshire so use a collicap for this aspect.

[sunshine185]

My 200p is exactly the same, if I move the secondary until it is central, then measuring the spider veins shows it to be out, when the secondary is bang on with the veins, then looking down the focusser shows it to be off, slightly bigger gap at the bottom. I trusted my instinct and left the veins central, lined up the primary and did a star test, which showed it to be perfect. Was up at 3am this morning to check out Saturn, it was fantastic. So, either the 200p are built this way, or we have got a bad batch, but judging from my polaris star test I think we are ok. Let us know how you get on.

[jimmyjamjoejoe]

The 3 adjustment bolts on the secondary are all roughly the same. And a star test shows everything is fine...

Think i'm gonna put it back central in the veins like sunshine, get everything lined up and sorted, then just pack out the focuser with a shim so that the secondary will be centered.

Nice and cloudy tonight anyway, nothing better to do

[jimmyjamjoejoe]

OK so it's all sorted now, just a minor adjustment since the focuser was indeed out. Luckily, there are 3 adjustment screws for such a situation. What i thought was going to take hours actually took 10 minutes, and that includes a full collimation with my new, central secondary.

Thanks to everyone for the advice, and Sunshine, take a look at the focuser, it's easy, there are three screws plus three locking screws, much like the primary screws.

Wootwoot clear skies tonight too, if i weren't on 12 hour shifts for the rest of the week i'd wait up for saturn... maybe next week...

[Jason D]

Glad to hear you managed to resolve your issue.

Jason