deciding if I need to collimate my laser collimator

[moflaher]

Hi Guys:

I have recently acquired a second hand 8" f5 () and I am about to check its collimation, and I will follow the advice given by astrobaby, although I have a laser collimator as well.

My question is about the laser collimator.

This is an antares laser collimating tool (inherited) and I checked its collimation, using a variation of the v-block approach. I have traced the laser beam at 90 degrees intervals projecting it into a target about 90cm away. The laser rotates around a circle with a diameter 0f 3.7 mm . This data is a reduction of 22 measurements.

Carlin (FAQ about Collimating a Newtonian telescope) calculated that the sweetspot of an f5 mirror is about 2.8mm. So the laser circle is 1mm bigger in diameter than the sweetspot. I am tempted to say that this is good enough for visual use, but I want to hear your opinions, as I have no idea how big the tolerances are .

Thanks!

Martin

[John]

I have a similar laser collimator to you which needed collimation when it was delivered. Using the V-block method, I projected the beam onto a target 6 metres away and then made the adjustments to get the beam rotation to stay more or less on the spot (well within 10mm anyway). The laser is now reasonably accurate for collimation but I really only use it i) to do the secondary tilt and ii) to do rough adjustment of the primary. I do the final collimation using a simple cheshire eyepiece and star testing.

I guess my feeling is that 3.7mm of beam rotation over 90cm is not really accurate enough to be honest.

Perhaps I'm being picky though

[brianb]

I guess my feeling is that 3.7mm of beam rotation over 90cm is not really accurate enough to be honest.

Perhaps I'm being picky though

No you're not. You need to be accurate within 1mm when collimating an f/5 scope.

[moflaher]

Thanks! This is very helpful.

Cheers,

M

[steppenwolf]

The collimation of your laser is not accurate enough I'm afraid. 90cm range is far to short, 6 metres would be more like it and this is what I use.

Always finish up with a star test.

[Bizibilder]

I use the length of a double garage (ie about 10m) and I try to get the laser to describe a circle of less than 5mm diameter at that range. I clamp a "v" block to a windowsill (because it is solid) and put a mark on the inside of the white garage door. Rotate the laser with care and make sure you have it securely in the "v" block all the time. It can be a bit of a fiddle but once done the lser has behaved itself so far without need for re-collimation.

[Jason D]

This is an antares laser collimating tool (inherited) and I checked its collimation, using a variation of the v-block approach. I have traced the laser beam at 90 degrees intervals projecting it into a target about 90cm away. The laser rotates around a circle with a diameter 0f 3.7 mm . This data is a reduction of 22 measurements.

Carlin (FAQ about Collimating a Newtonian telescope) calculated that the sweetspot of an f5 mirror is about 2.8mm. So the laser circle is 1mm bigger in diameter than the sweetspot. I am tempted to say that this is good enough for visual use, but I want to hear your opinions, as I have no idea how big the tolerances are .

Referring to Nils Olof Carlin’s link, he describes two types of errors: Type 1A and type 1B.

When a laser collimator traces a circle, this error relates to the less critical type 1B error.

The more critical type 1A error has more to do with:

1- How centered is the primary mirror spot

2- How centered is the forward laser beam

3- How well does the returned laser beam retraces the forward beam

The 2.8mm sweet spot is for type 1A error – not type 1B.

The point of my post is that scope owners need to be more concerned about the accuracy of the primary mirror center spot and how well they can center the forward laser beam.

Jason