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does this look collimated ok ?


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Hi all

I bought a Hotech collimator but am struggling.

I get the laser inside the donut ring on the primary then center the laser on the Hotech target using the collimation screws.

Problem is, if I remove the collimator then re-insert it, it shows collimation is off again :D

Is it possible to tell if collimation is ok with a star image ?

Here's one I just took of Arcurus

Cheers in advance

post-13805-133877451513_thumb.jpg

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Hi there Greenkat. If I were you, Id be looking at my focuser draw tube mechanism to see if there isn't excessive "slop". You might have a couple of screws under the main focusing knob.. refer to the manual and ensure they are tightened correctly.

The next thing to look at, is making sure you insert the Hotech in the same way every time to reduce any fitting errors to a minimum.

Finally, have you got a cheshire collimator ? It might be worth doing a quick side by side comparison - and don't forget, the final test [if you are a visual observer] is a star test.

Locate Polaris, put in a high maginifaction EP and defocus the image. You are looking for the shadow of the secondary to be perfectly centralised in the image of the defocused star.

The Hotech is a very good laser - better than most - so you should get perfectly acceptable results with it.

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Cheers Albedo

Its a Skywatcher 200mm with one screw under the focuser which I nip up but not too tight.

I do have a cheshire & will check with that tomorow, but bought the Hotech because I thought it would be more accurate & easy to use.

Cant see Polaris where I am as the house is in the way, though when viewing Arcturus visualy I did notice that when out of focus most of the star image was to one side of the shadow. Heres an image of Vega unfocused

post-13805-133877451518_thumb.jpg

Edited by greenkat
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A simple star test is the best way to check collimation IMHO. This Youtube video shows what a collimated 8" F/4 star test looks like as the scope is taken from one side of focus to the other:

If the collimation is off, the dark shadow, showing when the star is defocussed, will be off centre.

Edited by John
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Many do not perform the star collimation properly.

For example, star collimation in the video link and the attached photo are not showing the proper star collimation method. Attempting to center the secondary mirror shadow within the star test image is not the right way to do it. The proper way is to concentrate only on making the rings concentric – only the rings and nothing else. For best results, you need to use high magnification with a slightly defocus star – enough to see only few rings. If you see the spider vanes diffraction as shown in the attachment of post#3 then you have defocused too much. One more tip, evaluate the star collimation image only when the star is in the middle of the FOV. If the star is closer to the edge then the eyepiece optical aberrations might interfere with the evaluation.

In addition, keep in mind that star collimation only coincide the eyepiece and primary mirror focal points. It does not evaluate or correct focal plane tilt between the eyepiece and the primary mirror.

Jason

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If you've checked for tightness of parts and followed the above suggestions then try using a bhatinov mask to double check the focus. This may help reveal the true state of collimation. Your diffraction spikes look very acceptable to me :D

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