Collimation Accuracy Star Test

[Dennis G]

I've not read of this before and would like some one to describe it in full. eg the required magnification x? etc.
When replacing the primary mirror I've always been careful never to tighten the clamping screws too tight and although I made a laser collimator I gave it away for a 200mm length of 31.75 mm (1 1/4 inch) aluminium tube with cross hairs at one end and 2 mm hole in the closed cap. Accurately made in a lathe. Using this I assumed it had to be accurate. BUT IS IT?

[rastis95 (Scott)]

Dennis,
I was a bit like you in that i wasn't sure if everything was collimated or not. i use an orion cheshire collimator very similar to your homemade one and after a bit of trial and error have learned to trust it. The star test is simply, when you look at a star or planet adjust it out of focus and the blurry image should be a perfect circle then focus it back to a clear image. If the blurry star is oblong or in my case a semi circle, then something is a miss.
Cheers
Scott

[Dennis G]

Thanks Scott.
Next clear evening I'll test my collimation using this method.

[Screwdriverone (Chris)]

Hi Dennis,

I also have a slightly different method to achieve better collimation after aligning with a cheshire eyepiece.

I focus on Jupiter or a really bright star such as Spica, Regulus or Alpha Centaurus and then wind my focuser ALL THE WAY OUT so that you get a classic target type picture in the eyepiece ( I use the LEAST magnifying and complicated one I have - 20mm plossl).

When I look at the "shadow" of the secondary ringed by light and bisected by the secondary spider vane shadows, I can see if the secondary is not exactly concentric within the circle of light that surrounds it.

What I do then is very slightly adjust the primary tilt on the screw to where the image needs to shift, this causes the picture to shift in the eyepiece, I jog it back with the slo-mo controls into the centre, check and repeat. Make sure you DONT adjust the primary tilt too much to make the image fall off the eyepiece's view, you only want to do LITTLE adjustments in any direction.

After a little practice with TINY adjustments, I can see if my secondary shadow and the light donut around it are concentric and then I return to the focus point and check it by eye.

Sometimes I wind the focuser all the way in to double check the circles that way as well and after a while, the whole process only takes me about 2 mins to fine tune after using my cheshire to get it initally all lined up.

Its surprising just how much difference a tiny adjustment makes on my F7.7 reflector, often being the difference between chromatic abberation (red or blue flaring at the limb) on the object or not.

See if this works for you, you will get what I mean when you wind the focuser all the way out and take a look, just be mindful not to lose the star by adjusting too much otherwise its a bugger to find it again with the mirror tilted too far off centre. In this case, I return the scope to the lit garage and realign using the cheshire and then start again (this saves time and swearing in the dark)

Cheers

Chris

[peter_4059 (Peter)]

Dennis - you can also do this with your DMK on the laptop screen rather than with your eye to the eyepiece. That way you can position the lappy so you can see the screen and adjust the colimation screws at the same time. I've got an image somewhere - I'll post when I find it.

[Dennis G]

Originally Posted by peter_4059

Dennis - you can also do this with your DMK on the laptop screen rather than with your eye to the eyepiece. That way you can position the lappy so you can see the screen and adjust the colimation screws at the same time. I've got an image somewhere - I'll post when I find it.

Sounds great Peter. I'd really like to do this. Know how that is.
Dennis G

[peter_4059 (Peter)]

Dennis,

Here is the image I referred to. This shows the scope out of colimation as the circles are not concentric. The original image is an AVI file that I have stacked in registax to get the first jpeg. The others are the first few frames from the sequence so you can get an idea of what you see on the live view - not quite as sharp.

Hope this helps.

Peter

[Dennis G]

Thanks Fellas! I really appreciate your assistance and will give it ago when I can next see stars!
Have a peep at these two pics and add your comments. This is the only way I'll ever learn. I waited two whole months to get two clear evenings, but its worth the wait. I've learnt to refocus after every two captures, as each attempt is only a guess and this way one gets the odd one close. I'm able to hold the image reasonably stationary with regard to the EQ platform, but far from stationary regards to the atmosphere, which I'm sure you also experience the difficulty to get that perfect sharp point. Like one of you wrote, when played back on Windows Meadia Player you get some idea of success if you can see a few sharp images within the total.