cant collimate astromaster 130eq (help)

[Roobi]

Hi there everyone.
I've been trying to collimate this scope on and off since i got it last year, and it's been impossible. I've read as many tutorials as i a could find on the subject, but no matter what i do, the mirrors never seem to align up. Now after more searching i've just found this info:

"With a fast scope there is alkways an offset in collimation which causes the theoretically set of concentric circls to be pushed off centre. This means either the sceondary cannot be centred under the focuser OR the primary reflections cant be made concentric to the secondary."

so all this time it seems that i've been doing it wrong anyway.
Is there anyone out there that knows how to collimate this properly. Any help would be greatly appreciated. Thanx

Roobi

[erick (Eric)]

You are in Melbourne, Roobi? Which suburb?

Don't get concerned about offsets of the secondary mirror. Let's just start and see if we can get some basic collimation.

Maybe I can visit, bring a few collimation tools, and we'll see what is possible.

But I need to check that it is not a Jones-Bird design which has a built-in lens at the bottom of the focusser tube. They are a pain to collimate. The lens should be visible, but another check is whether the length of the tube is less than half the reported focal length, rather than a more convention 80% or so of the focal length for a regular newtonian design.

I also read that the focusser on one sample was so wobbly that collimation was difficult.

[JethroB76 (Jeff)]

Originally Posted by erick

check that it is not a Jones-Bird design

This was my first thought, but the Celestron website suggests this one isn't; the fl is similar to the tube length.

[Roobi]

hey guys thanx for the replies.
Erik, I'm in werribee. I dont think the astromaster is a jones bird design, the focal length is 650mm, measuring the actual length of the tube (roughly) it looks to be about 600mm. The focuser doesn't seem wobbly.
To be honest though i really dont have much idea of what im actually doing though. I just keep reading through the tutorials i find and following them as much as i can, but im pretty sure the collimation is way off now.
Thanx guys

Roobi

[multiweb (Marc)]

Hi Roobi, I have one of these, although mine is called a Celestron Nexstar 130 SLT but it looks exactly like what you've got (only older). Here's a quick step by step procedure that should get you started.

First be aware than in a collimated newtonian it's ok if the reflections are not concentric. It's not like a SCT. Trying to make everything concentric is not the right way to do it.

1_ Center the secondary axis in the tube as best as you can. Measure it. Then move it oaway from the focuser by let's say 1mm. (spider arms)

2_ Center your secondary in the draw tube as best as you can. Ignore any reflections. Twist, tilt and up and down the tube until you get an apparent circle centered in the draw tube.

3_ Stick a laser in it and adjust the secondary tilt so the dot hit the primary in the center.

4_ Stick a cheshire and tilt the primary until you get your center dot aligned with the cheshire. Doublecheck with the laser.

That should get you very close to the ball park. There is a very good tutorial and tools here.

[erick (Eric)]

Send me a PM if you want me to drop by, Roobi. Eric

[Roobi]

Ok i think i might have it reasonably close now, but i was a little confused by your instructions multiweb as i dont have a laser or a cheshire, so heres what i did do.

I used my handmade collimator cap (photo cannister) and popped it in the focuser.
I then adjusted the secondary until i could see the three little mirror holders on the primary mirror evenly.
I then adjusted the primary mirror until the secondary was centred.

Once i took the collimator cap off and looked through aagin, everything is pretty much good except for where i can see the reflection of my eye in the secondary mirror... It now looks like this:

http://i1102.photobucket.com/albums/...ollimation.jpg

Is this ok, or do you have any idea what i need to do now if not?

Cheers

[multiweb (Marc)]

A cheshire is cheap and will help you get a better alignment. Have you center spotted the primary. Ideally the initial secondary centering should be done without looking at any reflections. You need to center the outline of the secondary in the draw tube, not in relation to the primary.

[Visionoz (Bill)]

Roobi

It looks like that you have to "rotate" the secondary on its own axis to get rid of that "off-center" alignment

Yep getting a cheapo cheshire will certainly help like Marc said

HTH
Cheers
Bill

[Roobi]

cool thanx guys, il look into the cheshire tomorrow.

[Jason D (Jason)]

That little secondary mirror dark offset is normal

[Roobi]

cheers jason, that makes it easier. wish it could have said that in my scopes manual.

[Jason D (Jason)]

If you are interested in the reason, refer to the attachment. It all comes down to perspective. We see the secondary mirror from two different perspectives. The first perspective is through the focuser where it would typically look circular and concentric. The second perspective is through the bottom of the mirror. The secondary mirror will look dark from that perspective. This is the image that we see shifted.

[Jason D (Jason)]

You can't make the secondary mirror concentric from both perspective. It can only be concentric from only one perspective.

[multiweb (Marc)]

Great drawings Jason. Always very good info from you.
PS: Just implemented the milk container washer at the back of my secondary last night. Smooth as ... might even do that on my SCT secondary screws now.

[Jason D (Jason)]

Thank you Marc
Glad to hear the secondary mirror milk jug mod is working for you
Jason