Secondary mirror offset

Lads,
I've always used a laser to center dot the secondary mirror and the the primary mirror. It has always seemed to work ok and give decent images.
Recently I looked down the focuser draw tube in daytime and imagine my shock when it seemed to me that the image of the primary mirror was not entered in the secondary mirror, it wasn't just slightly out, it was so far out I suspected that one side of the primary mirror was been missed by the secondary!
It turned out it was catching the entire primary mirror but there was 5 minutes of "oh crap, this cant be !!" but the Cheshire eyepiece did prove that the secondary position was wrong even though the laser aligned up perfectly.

I had heard of secondary mirror offset years ago and Kreige talks about it in the obsession manual but says its not worth the effort, well, after abit of searching I was able to confirm that the secondary mirror for my particular scope should be about 3.6mm towards the primary and 3.6mm off center away from the focuser, this means a shift of about 5.5mm of the center dot on the secondary towards the spider.

I was amazed at the difference this made in the Cheshire eyepiece, all mirrors and the new center dots seems to line up perfectly but the real test is under the stars. The next clear night I brought out the scope and the star points were perfect where as in previous times, I wasn't absolutely certain about them at very high powers.
For those who haven't heard of this before, this only applies to Newtonian scopes.
Has anyone else tried offsetting their secondary or experienced something similar

The offset should not significantly effect the image at the eyepiece if the scope is otherwise properly collimated. The offset simply ensures that the secondary mirror is getting fully illuminated by the primary. What it will have the single greatest effect on image sharpness and clarity is making sure the optical axis of the light cone from the secondary is co-incident with the optical axis of the eyepiece. If the light cone does not hit the eyepiece exactly on axis, but is slightly offset in any direction, then you will experience significantly softer images.

As you've just found out - lasers are great for tweaking the collimation but are crap for general collimation. A sight-tube and Cheshire eyepiece are the only tools you can rely on for getting it 100% right.

Tectron sell a set of three collimating eyepieces and an excellent short book on collimating - "Perspectives in Collimation" by Tippy d'Auria. This lot is worth more than its weight in gold!!!

Phil.

Hi Phil,
I'm not certain it helped the view through the scope, but I did it anyway thinking what harm can it do, the obsessions are setup in a way that makes it quite easy to adjust the secondary.
My first concern was seeing an offcenter shadow in the defocused image, but when I saw it was only ~3.5mm, you're not going to notice that in an aperture of 500mm, a value of ~0.6% of the diameter.

Didn't I hear that by centering the secondary in a sight tube you are in effect setting an offset towards the primary.

Certainly the tectron kit and autocollimator are the last word in collimation with a cheshire close behind (is the techtron a fancy cheshire?) but what I found is that any tool is only as good as its fit in the drawtube/1.25" adapter. As I posted before not only where my cheap 50-80 euro lasers losing collimation every other night but their and the cheshires fit in the drawtube/1.25" adapter left a lot to be desired.

Thats why I invested in a quality focuser (besides wanting to go motorised) in the form of a moonlite with precision milled 2" drawtube apeture and 1.25" adapter, with 3 screw compression rings....to be used in conjunction with a premium laser in the form of a Howie Glatter with perfect collimation out of the box and designed to stay that way with precision milled 2" and 1.25" barrels.

Now I have perfect fit and register with the laser and drawtube/adapter.

I believe you have a premium kendrick laser Dave and did you get a premium jmi EV1 with your obsession, so pretty much all of the above is moot advice in your case but I thought I would post for future browsers.

What I have read with regards to the Howie Glatter and Blug which I am sure applies to your kendrick is that when everything else is right like whats posted above...that invariably people were finding that after collimating with the laser and then double checking wth a good auto collimator, that both agreed perfectly. So a good laser can be right up there in accuracy terms with the likes of Tectron and autocollimator.

With the Glatter I also got the 1mm apeture stop that screws into the laser apeture and creates a much smaller laser dot to more accuratly center the laser in the centre of the primary donut. I also got the Holographic attachment which is a little diffraction lense that screws into the laser apeture that projects a grid pattern. By placing a sheet of white card under the secondary and spider vanes blocking the primary one can accurately center the secondary under the focuser with the aid of the grid and the outline of the apeture of the drawtube

web.telia.com/~u41105032/holographic/holographic.htm

How this relates to the topic is that what I found was that cumulative tweaking of the secondary when aligning the laser dot had begun to rotate the secondary, I could still get the dot centered by adjusting the tilt without realising that the rotation was now off. I was shocked to find this one night after deciding to check secondary centering again. So now I check centering/rotation every 5 or 6 sessions/collimations to make sure I can see the whole primary reflection centered.

Its ironic that after the expenditure on the focuser and laser to make sure everything was right, now it takes so short a time to do the entire process (including checking/adjusting centering and rotation) that I almost paradoxically feel I am not getting value for money because I only use this expensive laser for 3 or 4 minute a night when in actual fact this is the very reason I went to the expense, to get accurate collimation that I am confident in down to 3 or 4 minutes

I wish you guys had posted this about four months ago.
The secondary mirror of my 16" fell off (luckily the Dob was disassembled) onto a nice padded car seat, as the previous owner (not saying who, now!) had almost completely unseated the retaining screw by overuse of the three tilting knobs at the front.
I ended up getting it back with a Cheshire, as you've done, but it was a long and painful process, as clear nights are few and far between.
However,the end result was worth it, was a marked improvement from the initial images pre-fall off.

I ended up getting it back with a Cheshire, as you've done, but it was a long and painful process, as clear nights are few and far between.

Um... It's actually easier to collimate in daylight - you don't need darkness or stars to do the deed. Collimating the top cage of a truss-dob can be done without assembling the rest of the scope. Once the spider / focuser / secondary mirror are all collimated then you can work on the primary collimation without touching the top cage again... ideally!

Phil.