Collimating your Newtonian Telescope

[astronomynut]

I hope this location is ok about the subject matter.  My first telescope, since coming back to astronomy is a Starblast 6i.  I really like the telescope for portability and ease of setup, as well as its ability to enable you to find objects with very simple alignment procedures.

  My problem has been that just moving the telescope gets the optics out of alignment.  I tried to use the instructions from the Orion manual, and I believe, really messed up the alignment.  I think the telescope has always been out of collimation, even though I used the laser collimator, and it showed alignment.  The planets and other objects just did not look nearly as bright, and sharp as I get with my 4" APO refractor.  I know the refractor is a better telescope, but the Starblast has 2 inches more aperture, and I would think that it's performance might approach that of the refractor

  So I searched the internet, a came along on a helpful guide by AstroBaby, which I have printed out.  The guide is very easy to understand.  I have not yet attempted to fix the mess I created.  One thing stopping me is the mention of the tools list to perform this task.  I have the pinhole cap, Phillips screwdriver, and I do not need a hex wrench, since I upgraded the hex nuts to Bob's Knobs.  But I do not have a Cheshire unit.  I've tried to find out how this unit works.  It looks very much like a laser collimator, which I do have, but does not have any source of light.  As I understand it, you need to shine a light on the side hole where the 45 degree angle is to provide the light to work with.  But I wish to have someone confirm my suspicions, or possibly provide some kind of guide to how to work with a Cheshire. 

I really wish to get this telescope back in operation, as it is my go to scope at home, when I go into the backyard to do observing.  I appreciate whatever help you can provide to resurrect my home scope.  Caley

[NGC 1502]

Maybe I can comment on this - "just moving the telescope gets the optics out of alignment".   Shouldn't be that way, except maybe for a large truss tube scope where the trusses are undersized, or similar construction that's not the best.

A Starblast 6i should hold collimation well, unless it takes a bump. If collimation shifts when the scope is moved, something is loose.

Things you can check -

Are the spider vanes tight enough ?

Are the secondary adjustment screws tightened ?

Is the primary cell to tube fixing secure ?

Is the primary mirror flopping around in the cell ?  The primary should never be secured too tightly, that can cause "pinching" and leads to bad views, but the commonly mentioned "credit card " distance between the primary clips and mirror surface is (in my opinion, for what it's worth) too much.  Just a smidge is enough.

Is the focuser properly attached to the tube ?

Any of the above could cause collimation to shift when you move the scope.

Hopefully others can comment on the rest of your post.

Hope you sort it, Ed.

[astronomynut]

I've started the redo of the collimation on my Starblast 6i. The first step set out by AstroBaby, is to get the spider vanes adjusted so that you

have the secondary mirror centered, if that is the way the design requires. Some telescopes apparently have their secondary offset.

Mine is the centered type. I tried the measuring method without success, as I have Bob's Knobs which keep getting in the way. I also tried the

cardboard circle with hole in center, but again, Bob's Knobs were in the way. So I came up with my own method, a set of Spreading Calipers. I put

one point into the Phillips head screw, and opened them up until the other point touched the side of the tube. I adjusted one set of vanes until

the were equal, then used the same spread to perform the same on the other set.

Don't know if AstroBaby is on this forum, but this might be another idea to put in your "How-To" of collimation.

Next is to get the secondary mirror directly under the focus tube, so that it appears as a circle shape. I will continue once I figure all this

out.

[astronomynut]

Ed, Thanks so much. I will check the focuser tube. I've just started the collimation process to undo the damage I did yesterday with my following

Orion instruction guide. I will check all those items you listed, as I go through the collimation process. Figure, as slow as I am understanding

some things, it will take me about a week to finish the task. Hopefully I will learn something along the way.

One other thing I remember, is when the store I purchased the telescope at put it together, and yes, they did that for me, was that one reached in and twisted the secondary mirror while using the laser collimator. I get the feeling that yes, he got the secondary and primary mirrors aligned,

but in the process, knocked the secondary every so slightly out of alignment on the other axis. That might be the reason why my visual images

seemed to be somewhat washed out. Caley

[wookie1965]

This is another very good guide i have used in conjunction with astrobaby`s

http://www.schlatter.org/Dad/Astronomy/collimate.htm

[astronomynut]

Wookie, Thanks for that link. I've bookmarked it for that just in case I need to pass the information on to another newbie performing a full

collimation procedure. Caley

[wookie1965]

Wookie, Thanks for that link. I've bookmarked it for that just in case I need to pass the information on to another newbie performing a full

collimation procedure. Caley

Your welcome i struggled the first time read through a few different ones but those two make it clear the introduction of the coloured card makes quite a bit of difference, once you have done it leave your secondary that will not need touching again unless you bang your tube all that will need doing is a quick tweak of the primary every now and then which takes 2 minutes tops.

[Dubhar]

I just properly aligned my telescope a few weeks ago after (shamefully) never having done it before. I had more trouble than its worth with a laser and just went with a Cheshire collimator and dust cap with a 3-4 mm hole drilled in the centre. Took a couple of goes to figure everything out but once i figured it out it was actually pretty easy. Amoung other internet resources I found this guide useful : http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

The site wookie1965 suggested looks good, and more detailed (ive bookmarked it ). I also agree with the contrasting colour, I used some white paper taped down with some masking tape, it helped a lot.

[AndyWB]

I've just started the collimation process to undo the damage I did yesterday with my following

Orion instruction guide. I will check all those items you listed, as I go through the collimation process. 

Forum member Moonshane did a wee talk about collimation at a star party I was at, and it made the whole thing clearer to me. I may have these rules in the wrong order though.

Rule number 1 - Don't Panic. You can undo any mistakes you make, and as long as you don't tighten anything too tight, you can't damage your scope.

Rule number 2 - Collimate with the scope near level, so you don't risk dropping something down the OTA.

Rule number 3 - For visual astronomy, don't worry about "perfect" collimation, particularly of the secondary. You won't notice a difference between that and 'pretty good' collimation. (This certainly seems true in my experience). The primary is the important one.

My own experiences:

Laser collimators seem useless for secondary alignment. There may be a way of using them, but to be honest, I'd suggest a Cheshire. It's far simpler.

What we call a Cheshire is usually a combined Cheshire collimator and sight tube. That will do all the tasks of collimating a scope.

To position the secondary, I pull the cheshire out until the end of the site tube is a diameter just larger than the secondary when you look down it. I then use it's edge as a guide for adjusting the spider, and rotation of the secondary so it's flat (i.e. a nice round even circle, though both my secondary mirrors have flat spots on the side, as though from manufacturing). 

Don't focus on the spider vanes being equal, but rather that the secondary is under the focusser. It turns out that the tube on my 10" is slightly egg shaped...

The first few times I did this it took a long time. Since seeing someone else do it in the flesh, it's much easier. My first attempt with my 5" took 3 evenings. I did a better job of it last night in 15 minutes - and that was all faffing with the secondary.

[astronomynut]

Sorry it has taken so long to get back. I got kind of depressed with the constant clouds, and saying to myself, "Why bother if the clouds are never going to go away?". So far we've only had snippets of what could be called high haze, which isn't even good enough to do viewing. It is what is

commonly known as the monsoonal flow that we in the south and western US are so fortunate to experience between late May and the middle of September. This has just been more so than past years.

Well, I finally got back on collimating my Newtonian today, and actually finished. I was totally surprised at how quickly the rest went.

First was inserting white cardstock into the OTA to block the primary mirror, then another piece of yellow cardstock to place directly under the

secondary mirror. This procedure definitely speeds up centering your secondary mirror under the focusing tube. It blocks out the confusing primary mirror which is difficult to separate from the secondary, and provides a contrast under the secondary, which is not provided by the black insides of the telescope.

I loosened up the "Bob's Knobs" that I use to adjust the secondary mirror tilt, and then loosened up the center screw to adjust the mirror. This

central screw allows you to move the secondary mirror forward and backward in the tube, as well as rotate the whole mirror assembly. I found that

my secondary mirror was about 1/8th inch too far forward. How it got that far forward I have no idea.

The whole procedure in this step is to get the mirror centered in the focuser tube so that you get the secondary mirror looking like a shiny circle, which is equidistant from the edges of the focusing tube. Like I said, this went fairly quickly.

The next step was to first remove the cardboard I used for the previous step. Next I needed to get the collimation cap centered in the secondary

mirror, or so I understand the directions pointing to. I used the "Bob's Knobs" (you may be using hex screws), to make this adjustment, but I also

had to use the central screw to make very minor adjustments. This took me about half an hour to get the collimation cape, secondary mirror and spider vanes all lined up.

I finally decided to just skip ahead to the primary mirror alignment. I know I missed a few steps, which I will go back and complete, but I just

wanted something to be able to take out when I get a short break in the clouds.

What surprised me was that I needed very little adjustment to the primary mirror, using a laser collimator. I am a whole lot closer than I was

before, but I do know that things will only improve when I finish a few intermediate steps.

I trained the telescope with a low power eyepiece on the Joshua Tree mountains to see just how things looked. I was actually able to get a pretty

sharp image, though I am still not quite happy with it. But it is now useable. What surprised me was seeing something I have never noticed before, a rainbow fringe in the outer area of the eyepiece FOV. I am wondering if this is the aberration people talk about. Or is it because I still need work on my collimation?

Anyway, I will come back with the steps following the secondary mirror collimation that I missed to see just how much of a difference they will

make in the overall picture through the eyepiece. Thanks for your inputs and listening to a real collimation amateur. Caley