more collimation queries and info

[kniclander]

http://www.skyandtelescope.com/howto/diy/3306876.html

When i had a 6" f8 Newt I wondered what all the fuss was about. Now I have 10" f4.8 dob I wonder how the b***dy hell anyone get's it right!

I found the above article which I thought was very interesting. In particular, it says that a small error in secondary alignment is not critical - is that right? It also says that a small error in secondary alignment is critical if you are also using the laser collimator to adjust the primary. So, I guess what I take away from it is that a laser's ok for adjusting the secondary but maybe a cheshire is better for the primary?

Also, my laser collimator has got very faint (mine's a baader). I tried changing the batteries but it made it even fainter (ie I can't test its collimation because if I shine it on a wall,say 10 fet away, I can't see it at all!). Do I need any special batteries other than the regular 1.4v hearing aid type ? If not, is there anything else I can do or do lasers just die after a while (mine's only about a year old)?

thanks

Dan

[Doc]

Imagine how I feel I have a F4.5 mirror and like you can't seem to get it spot on.

Stars are not pin points.

From what I've read use a collicap to get the primary mirror centred first.

Then use the laser to collimate by turning the primary screws until the beam enters the whole on the etched grid.

But when I centre my primary with a collicap then check with a laser the beam does not hit the donut.

So I'm a bit confused.

[kniclander]

From what I've read use a collicap to get the primary mirror centred first.

do you mean primary or secondary?

I can never get the beam to go inside the donut - it seems to wander around the edge....

I think that if you can't exactly centre the seondary then you can't adjust the primary accurately with the laser.

Life was much easier with a 6" F8 when I just wandered outside and looked up.... :?

[Doc]

Place the collicap in the focuser and adjust the secondry screws until you see the primary exactly centred you should see all mirror clips evenly spaced.

[Doc]

Thats strange as I can get the laser bang in the middle of the donut if I ignore the collicap way to do it and just do as per baader instructions.

[kniclander]

... if the laser "works" and the collicap doesn't then either the laser or the collicap is inaccurate?

I've spent ages chasing the little red light around the edge of the hole...

[Doc]

How close to the hole do you actually get this red dot as I can get mine inside the hole but due to the brightness it illuminates the rim of the hole. This makes it look like it isn't exactly centered in the hole.

[kniclander]

I can get it sort of smeared on the inside of the black ring. With my old 6" scope I definitely saw it as a red dot completely inside the ring and I don't get that with the 10". I wonder if it's something to do with the laser being very faint? Maybe the beam has got too broad? Can that happen? I have no idea.

[timetraveller]

hiya guys

could i pass on a hint as both of you seem to be having big problems with trying to use a laser collimator. The problem with you not being able to get the stars as pinpoints may well be because your secondary mirror is not at the actual focus point it may need moving either up the tube away from the primary or opposite way round and your laser Will not show you this as long as its at right angles to the focus tube. as that is all the laser really does your first job has to be to get your secondary to the point were you can see the primary mirror clips equally spaced and fully in view only then can you properly collimate the secondary and primary. I wont use a laser i use a Cheshire collimator instead as i think its far more accurate also you need to be sure that the doughnut is dead centre of the primary mirror hope this helps

regards Pete

[kniclander]

thanks Pete.

A third thing to adjust :shock: this is getting scary! I thought that I would only see all the clips on the primary after everything was collimated properly. Are you saying that if the secondary is at the right height in the tube, I will see all of the clips, even if the primary and secondary are not collimated?

thanks again

Dan

[Astronut]

You seem to be getting yourself tied up in knots over something that really isn't that difficult. Yes, it's fiddly and time consuming but you have to take your time, understand what's happending at each step and do things in the correct order.

You could do a lot worse than read Mel's (Astro_Baby) collimation guide. It covers each part of the process in great detail with piccies as well

Click This

I'm not keen on lasers myself and neither is Astro_Baby but there's a good video here, part of which covers laser collimation:

Click This

[kniclander]

Astronut, thanks - I am reading Astrobaby's collimation guide. Looks good.

[kniclander]

well, a very useful evening. I made a collicap and collimated the secondary using Astrobaby's instructions (if you read this Astrobaby, many thanks - it was really really helpful).

At the end, I checked the positioning of the secondary with the laser. The laser showed it as a little off centre but when I centred the secondary with the laser and then checked through the collicap, it clearly wasn't centred.

So, is the problem with the laser or the collicap? I don't know. Possibly my collicap hole isn't exactly exactly in the middle - I can't think what other error there could be in the colicap method? More likely, I suspect the laser but then Baader go to such lengths to check their instruments so I don't know.....(but Cheshire collimator is on its way...)

Anyway, I feel that I've learnt a great deal so thanks again.

Dan

[Astro_Baby]

Hmmm - question for you - is the focuser a compresion style or one with a screw that holds the laser/EP in. If its a screw then its more than likely that the laser is being pushed out of line perhaps.

One of the BIG advantages I found with the compression fit focuser is collimation is a more reliable process.

[kniclander]

i have two screws that hold the ep/ laser so I guess screw type not compression type. So that would suggest the collicap method might be more accurate? Once I get my cheshire I will go through all of the various different ways I can do it and compare/contrast (yes, I am that sad :oops: )

Dan

[Doc]

Ian has it got a brass ring in the focuser that moves when the thumbscrew is turned.

Mines a compression eyepiece holder.

[kniclander]

no brass ring - cheap screw type not comprssion type

Dan (not Ian )

[Doc]

Sorry dan, you talk to so many you get confused. Well I do anyway

Maybe like Mel stated the barrel of the laser is a slight sloppy fit in your focuser which is leading to the fault you have.

I like the idea that was mentioned about the focal length being a tad off.

I'm sure my primary is adjusted to far out and needs backing off and re collimating.

So am going to give that a go.

[kniclander]

I like the idea that was mentioned about the focal length being a tad off.

I didn't really get that, ie how do you tell if the problem is your focal length being a bit off and how do you tell if it isn't a bit off (other than get everything else right and if the image still isn't right start playing with the "focal length" ) ie there does not seem to be a "looking down the scope" method of telling if the focal length is off or not (at least the poster did not specify one..) I'm sticking with Astrobaby's instructions now cos they've worked so far and I've understood them.

I'll report back when i've finished