C11 collimation

[DrRobin]

Hi,

Okay, Hotech Advanced arrived and I had chance to have a go with it last night. Generally good with few problems, but I did notice it is very sensitive to pointing accuracy. A very small movement of the collimator alignment has the returned beams flying off the face of the collimator, but when I put all 4 lasers on, it was clear if anything was wrong.

So after a very short time, I had the secondary adjusted and the three laser spots reflected back on to the target on the same ring, so everything was good yes? Apparently not. When I checked the three small spots on the back of the eyepiece mirror they were off centre by about 5-6mm and I am guessing that they should be central? I also noticed that the laser at the top didn't reflect back to the vertical line when all three spots were on the same ring on the collimator target, another clue that something wasn't correctly aligned. I took a couple of pictures (attached), but the 3 laser dots in the eyepiece are a little indistinct, but you can see they are off centre.

Just as I was packing up, I sussed out what was wrong. I added a Baader Steeltrack crayford focusser to the rear of my C11 and it also needs collimating. I have a couple of suggestions for aligning this;

Either, put my normal laser in the eyepiece holder and adjust the Baader alignment until the returned laser dot stays in the same place when it is rotated; or,

Set the Hotech up so that the three laser dots are central in the cross on the eyepiece mirror and adjust the Baader, until the three laser dots are back on the collimator target in roughly the right place.

Bill, I am guessing that when you are correctly collimated, the three laser dots are in the centre of the eyepiece mirror?

[billhinge]

Hi

To tell you the truth I don't remember as it was done around september time. As you have already sussed the collimator will also show you if your other apparatus such as focuser is out of true.

It sounds like your steeltrack needs adjusting, If you see the bottom of the manual in section 9.0 it covers this eventuality. I must recheck mine over christmas now that you mention this

Did you check the little mirror is sitting fully in and square/tight in the eyepiece holder?

For info only, here is some unrelated info I got back from Hotech about some questions I raised (they seem good at answering support)

Q1: (What) Is this reasonable accuracy?

Yes. Within half track is acceptable.

Q2: Large diffraction rings around the dots in the dark, normal?

The image has been expanded by the secondary mirror twice (in & out of the optical train) so you are referencing the far-field image of a laser dot. As long as you use the center of the dot as the collimation reference on the track will be fine.

Q3: Three fine focused laser dots?

Yes, they are from the reflection from the corrector plate, and please ignore them.

Q4: Ideal collimation distance:

a). Three collimation dots far apart:

- If the dots are far apart outside the target, please adjust the telescope focus to bring all three closer. When the three dots is on the same track (4.5) as the laser exit dots, it means the reflector mirror is positioned on the focal point. This is the ideal collimation distance.

. Co-alignment distance:

- The co-alignment distance is when you use the crosshair projection and the diffuser technique to line up both the collimator and the telescope in the first stage (like aiming & tracking a star during star collimation).

- The farther away from the scope the higher the accuracy you can achieve but the co-alignment became too difficult to aim square and the collimation became too sensitive.

- The ideal co-alignment distance is a little shorter than one tube length in front of the scope where the crosshair or the diffuser donut on the collimator target is about the size of ring 1 or 2.

[DrRobin]

Hi,

Thanks for all your help and printing the Q&As from Hotech. I reckon that without the Hotech, I would never have spotted that the focuser was out of alignment and I had never appreciated how much difference it could make on this type of scope.

You refer to section 9 in the manual, is that a manual for a Steeltrack as I don't have a copy and couldn't find one with a quick search on the Internet? If you could point me in the right direction I would be grateful.

[billhinge]

Hi

Sorry, I meant the hotech manual