Star Test

[BlueBird]

I saw in another thread about collimation, and I am having problems with collimating an 200mm F4 scope.

Just to clarify.. the star collimation test you did is the one to check collimation, not the optics (i.e. the 'airy disc') test?


I was wondering if the star test could tell me which adjustment is out?
Or how to adjust it?
Or what can it tell me?

[programmer]

Hi BB that was me.

The Star collimation test is just the final tweak. It tells you if you primary is in perfect collimation. You need to have gone through the normal collimation steps first and just fine tune it with this test.

[Kokatha man]

Hi BB - actually, you can collimate your scope entirely from "star-tests": there are numerous articles on this procedure but there a reasons not to contemplate adopting this procedure. In it you basically choose a bright star well above the horizon, and with it centred in the scope's FOV at medium magnification, take the image well out of focus - a uniform "donut" image should result with a central "dark hole." If this hole is not centralized within the dim out-of-focus star "disk," adjustment is necessary: remembering that if you take the fous too far out this central "dark hole" will materialize into the secondary mirror/spider assembly.

The next step is using a dimmer star (2/3 mag.) - again high above the horizon (this, hopefully, reduces the risk of atmospheric disturbances obscuring the delicate/difficult to see diffraction rings that you'll be hoping to detect with the higher magnification you're now using: ideally 300+)

Again you de-focus either side of focus and these diffraction rings should appear as concentric circles around the central star image - the ones further out (ie with the greater diameter) will be increasingly difficult to detect/discern clearly. If the "gaps" between the rings is compressed towards one side this indicates collimation adjustment - in fact, you may even find only half of each of the DR's visible and an incomplete, distended central star if you have major misalignments! Also, depending upon "seeing" conditions these rings may appear more like "broken" or "dotted" lines rather than complete circular rings.

Having said all that, I have only used the star-tests to "tweak" my optics a few times to familiarize myself with the basics of this process: I believe it is far better/easier to use something like a sight tube/chesire collimating tool to get a very good approximation; use the star-test to check, and go back to the collimating tool for any refinements.

For many people getting the hang of a collimating tool is enough of an issue; dealing with DR's that can be very hard to discern, working in the dark and getting the hang of "which screw?" turns things into a minefield!

Having described my observations/experience with the above, I should confess that I've never done this with a badly out of collimation scope - I shudder to think of tackling that "cold" - deliberately encountering a scope with, or throwing everything way out of alignment, and using conventional methods in the daytime with a helper can be an ordeal, as many on IIS will attest!!! Using star-testing to achieve this would, imho, be utterly masochistic....!

Regards, Darryl.

[Satchmo]

If you don't centre spot your primary and use a cheshire sight tube or laser to collimate the diagonal first, all the primary mirror on a star will get nowhere.

[Kokatha man]

That should read "the DR's closer in....." - unless ideal seeing conditions etc are present, I have found that several DR's are readily discernable, but the inner ones become more difficult to discern individually.

[Kokatha man]

Originally Posted by Satchmo

If you don't centre spot your primary and use a cheshire sight tube or laser to collimate the diagonal first, all the primary mirror on a star will get nowhere.

I suspect you're possibly right there: that's why I commented that I've never done it with a badly out of alignment scope - meaning the necessity to adjust the secondary mirror, which should not be a regular occurrence once set: unlike the primary.

[BlueBird]

Thanks guys for your responses.

I am very much the beginner when it comes to collimating and looking to find a usable technique that works for me.

I lashed out and bought an Orion cheshire/site tube tool, but still haven`t got that collimation of a friend`s 200mm f4 scope.

I keep looking for information that may turn the corner and can get usable images.

Bluebird.

[Kokatha man]

A good choice (the chesire collimator/sight tube) in my opinion BB: there are innumerable threads on IIS re collimation (perhaps too many!!!) and some good video clips like Andys shotglass etc.

Just keep at it, but if you can get along to the local astro society viewing etc night you'll find people to give you hands-on practical assistance - nothing like having things shown to you.

Without going into the whole process again word by word - and therein lies the problem for many beginners trying to learn off of posts etc; meaning the level of articulation in these written instructions: the first thing to ensure is that the secondary mirror is centred (ie concentric) within the bottom end of the sight tube/focussing tube.

This usually should only entail looking down the orion tube with the focusser racked right in, the orion fully inserted into the focusser, and observing the actual appearance/shape of the secondary mirror. NOT looking at/into the sec mirror or any reflections within it, but just the actual physical outline of the sec mirror!

If the sec mirror does NOT appear circular and perfectly framed with the surrounding orion tool/focusser tube (meaning that there is a uniform "ring" of space surrounding the circular appearance of the sec mirror with the sight tube tool/focusser encircling this - the "concentricity" mentioned before) THEN you will have to adjust the sec mirror.

This is done by loosening the 3 screws surrounding the central screw/bolt of the sec mirror and physically rotating the sec till it does appear concentric as per above. Keep your fingers off the sec mirror surface, and when it's concentric looking down the orion tool (at which time it will also appear circular, even though it is actually an oval mirror) nip up those 3 screws to hold it in this position.

JUST REMEMBER THAT YOU ARE NOT LOOKING INTO THE SEC MIRROR AND YOU ARE NOT PAYING ANY ATTENTION AT ALL TO THOSE CONFUSING REFLECTIONS WITHIN REFLECTIONS WHEN YOU ARE DOING THE ABOVE!

Then it's time to adjust the primary.....

The proviso to the above is if the secondary mirror is not positioned in the tube directly under the focusser tube you will have to first adjust that central bolt/screw on the secondary mirror to move the sec assembly up or down the tube to get it directly under the focusser, before doing any of the above. And for this you will have to loosen those 3 screws on the sec if you need to turn the central bolt/screw in the direction that moves the whole sec assembly up the tube away from the primary.

Hope this helps, counter-reading between several detailed descriptions often uncovers the missing bits/understandings needed to "crack" the methodology.

Cheers, Darryl.