The Collimation Saga : Part 3 (sorry Mr Q)

[F1Bird]

Thought I'd had some success with my collimation and have been waiting for a clear night to star test. Well, it's better than it was after I first tried to collimate it but still as bad as it was to start with. I still have a flare (coma?) coming out the side of the stars only this time its in a different direction! I tried to tweak the primary while I was out. I only moved it slightly but whilst the star jumped around in the view the flare remained the same. Exactly the same. I also defocused on a bright star and the disc was lop sided in the same direction as the flare. Does this mean the problem is my secondary as tweaking the primary didn't seem to have any effect or is it more likely that I just needed to tweak more? Also, can bad seeing conditions cause a similar effect as bad collimation?

[barkis]

Your scope seems to have an artificial focal length.

If i have it right, the Astromaster has a focal length of 1000mm, in a tube less than half it's length.

That being the case, there has to be another element in the optical train that is possibly 

causing the grief you are having.

There is possibly, another lens, probably incorporated within your focuser, that extends the focal length, and may not be aligned accurately  and disrupting the light path and giving you your  collimation problems.

You need to check this out, as I might be quite wrong.

The reason this lens is used, is to allow a spherical  mirror to be used in the scope, and not a paraboloid, which are more expensive to produce.

Ron.

[todd8137]

Any of these look familiar when defocused am sure astro baby will not mind

Pat

[ronin]

If you have as pointed out above the Astromaster 114 then there is an additional lens in the focuser to "extend" the focal length up to 1000mm. The tube is only 457mm.

That extra bit of optics will make it difficult to collimate, at least normally.

You may be better just going low tech and using a collimation cap, but not sure that would be "easy" to use either.

The design used in a Astromaster 114 (Bird-Jones) is prone to difficulties, simply because it needs reasonable optics and that costs, and Synta are not going to put anything overly costly in the scope.

One thing in all the talk of collimation and collimators is this:

Is a collimator intended to set the collimation, or to check the collimation.

By rights you cannot use the same thing to do both.

[Charic]

I centred spotted my Celestron for the same reasons, but its not made any real improvement to a spherical mirror.  The Celestron's advice in the manual, is to look into the focuser tube (without eyepiece fitted) and you should see the  the 3 primary clips and your eye centred in the middle?

They also recommend a tool, but the focuser dust cap or a 35mm film cap is all that's  necessary to align your eye. The cap should have some sort of reflective surface on the inside. I just used some white paper (don't forget to hole the centre of the paper).

If everything is not central, adjustments are then made to the primary mirror at the rear of the telescope.

[itmo]

One way to collimate your scope is to find a bright star , use it as a reference. look at the stars surrounding your reference and see where the tails of the stars point, turn the collimation knobs so that the tails move to point to the center of the visible field. This will also move the stars so do it carefully. After turning the knob, recenter the bright reference star and repeat until all tails are pointing to the center of your view (or away from it ,depending on how you think about it). This will give you a roughly correct collimation, which you can then finetune using star testing. 

[Peter Drew]

It's easy to mistake coma introduced by poor collimation and the natural coma of the primary mirror. Natural coma is a function of the paraboloid, the severity increasing with the focal ratio, spherical mirrors have spherical aberration not coma, uncorrected they produce poor star images across the whole field, a corrector plate, as in a Scmidt camera or similar system corrects this over the whole field hence the great performance of such instruments. The coma in a Bird-Jones type telescope is most likely to come from a secondary or inbuilt Barlow that needs adjusting.