collimation...(again)

[Madhatter]

Im sorry guys, i know there is countless collimation questions and im going to have to add another:(

When i bought my dob i opted to buy a baader mkIII laser colli with it and have been using that until now.

For some reason i decided to make a colli cap using an old ep cover just to see how well the secondary and primary were lined up.

I was quite horrified to see that on the reflection i can see some of the black insides of the tube reflected of the mirror! yet the laser collimater hits the center donut on the mirror and bounces back on the cillimater fine.

so i tryed aligning just using the colli cap and got everything looking much better..yet the laser now said i was way out.

under the stars it acctualy looked better using the laser..but im now acctualy thinking i may be losing a significant amount of the scopes light gathering capability!.

Any ideas what i might be doing wrong or how to fix this?.

Thanks guys.

[x6gas]

Have you checked the collimation of the laser collimater?

Find or make something that will hold the laser collimater in position horizontally and aim it at a wall a few meters away. Mark where the laser hits the wall (the wife won't mind you putting ink marks on the new wallpaper - this is all in the name of science!) and then rotate the collimater through 180 degrees. The laser dot should still be in exactly the same place, but I've a hunch it won't be...

Incidentally, I've seen cradles for this made from push-fit plastic plumbing pipe. You basically need two v-shapes to lie the collimater in... if you follow...

[ecosse]

The baader mkiii is an odd shape so may be hard to collimate, I have one but wish I never bought it now, Cheshire is more accurate and simple to use once you get the hang of it.

[Parus major]

I've got one of them as well and it's also a large whack out.

[Madhatter]

Thanks for the suggestions, i will give this a go:).

[Jason D]

Madhatter,

Your issue has nothing to do with the laser accuracy but rather with understanding the capability of the tools you are using.

When a laser beam hits the secondary mirror then the primary and back, where does the secondary mirror edge or shape come to the picture? It does not. Only a few millimeters area of the secondary surface is used by the laser without referencing the edge. On the other hand, when you used the colli cap, you are referencing the secondary mirror edge.

I made the attached animation few years ago to explain the issue you have experienced which many others before you have also experienced. Every frame in the animation is axially aligned -- that is, for every frame the forward laser beam hits the primary center then retraces its path in the way back. But only one of these frames is optimally positioning the secondary mirror. The point is that the laser collimator can’t be used to optimally position the secondary mirror. The colli cap can be used but it is not the optimal tool. The optimal tool is called a sight-tube. You can build your own or you can purchase one.

I recommend following the instructions outlined in AstroBaby collimation document.

Jason

[Moonshane]

Good point by Jason (who I consider to be FAR more knowledgeable than I am about collimation) and I agree that collimation cap is one of the best ways to initially align the secondary.

as you have a barlow you could try the barlowed laser method for the primary. put the barlow in as normal and the laser in it. this makes the alignment or otherwise of the laser irrelevant as it spreads the beam and you are collimating the shadow of the donut not the laser dot.

if lucky, you'll see the shadow of the donut on the screen of the baader collimator. if so centre it on the hole and you are sorted (assuming the secondary was set properly first).

if not then google the process and you'll see the options.

[Madhatter]

Thanks for all your help everyone, that animation is brilliant:) i cant wait to get it properly aligned now!.