CatsEye Collimation question

[peter_4059]

I'm looking for some input from people experienced with collimating Newtonians with Catseye Collimation equipment. I've been tweaking the collimation on my Meade LXD55 SN10 but can't seem to get the hotspots to stack properly in the Infinity XLP offset pupil view.

This all started because I wanted to replace the stock plastic focuser with a Moonlite. As I had to remove the corrector plate cell/secondary to install the new focuser, I went to a lot of trouble to collimate the focuser by carefully determining (measuring around the tube) the point opposite the centre point of the focuser and adjusting the collimation using the Teletube cross hairs. While the scope was in pieces I also respotted the primary mirror with a Catseye Hotspot. I also checked the secondary offset - checked measurements according to this:
http://www.mikeblaber.org/sn10collimation.htm

I've managed to take some photos this morning that show where I've got to:

View through Teletube XL: pictures 1-3
View through Blackcat: picture 4
View through Infinity XLP centre pupil: picture 5
View through Infinity XLP offset pupil: picture 6

To me it appears the secondary rotation is ok (all mirror clips visible in image 3 however I can't seem to get the centre and offset pupil images to stack properly. I'm also wondering about the silver reflection at the left hand edge of the secondary mirror in the Blackcat image 4.

Any advice greatly appreciated.

Peter

[Peter.M]

The first thing I would make sure of is that the collimator is at the focal point of the telescope. If that is not the case it does not matter what you do, the hotspots will never look right in the central and offset pupils.

[peter_4059]

Peter, how do you define the focal point in the context of the autocollimator vs eyepiece vs camera? Do I focus with an eyepiece then replace that with the autocollimator?

[peter_4059]

Thanks for the advice Peter. That seemed to work a lot better and was much easier to see the four images.

[Peter.M]

That's looking much better. It is something that is not well defined I'm their instructions, but needed to get everything lined up. The reason I knew is because the offset image in your first go the hotspots are different sizes. In your second images they are the same!

[peter_4059]

Good tip - I don't recall it being mentioned in their instructions either.

[Jason D]

Spot on...

Here is more technical details:

When the autocollimator mirror is at the focal plane, reflection 2 image is located on the primary mirror surface -- same level as reflection P. Therefore, both will have the same apparent size and there will be no parallax as you view them from both pupils.

When the autocollimator mirror starts to move below the focal plane, reflection 2 will start to rise above the mirror surface. Not only it will appear larger but parallax will be introduced. The opposite is true.

If you look at the last photo of the first post, you will see the following:
1- Reflection 2 is larger than reflection P
2- You will also see reflection 6 which can only be seen when the autocollimator mirror is below the focal plane.

One more observation. Center spot reflection will appear slightly larger when the Blackcat is below the focal plane.

How critical is it to get the autocollimator mirror precisely at the focal plane?
NOT THAT CRTITICAL. As long as reflection 2 size appears about the same size as reflection P then the location of the autocollimator is good. I found that as long the the autocollimator mirror is within 1% of the scope's focal length from the focal plane, both reflections will appear about the same size. So, for a scope with 1000mm focal length, the autocollimator mirror location is OK if it stays within 10mm from the focal plane.

To be fair to Catseye instructions, it does state at the top:

"*Equal-sized images as illustrated & best results are achieved with the focuser/autocollimator racked in/out appropriately to place the autocollimator mirror at or very near the focal plane."

Jason

[peter_4059]

Thanks for the explanation Jason. I had failed to recognise the significance of the camera backfocus but now it makes perfect sense.

[multiweb]

Incredible illustrations as usual, Jason. How do you work all this stuff out. Seems like second nature to you. I just get a brain freeze thinking about it.

[Jason D]

Thanks for the compliment, Marc
I enjoy putting these diagrams and gifs together to help others.

Jason

[multiweb]

Yeah, they're great. I always save them up. I think I've got every single one you've done on my HD.

[tlgerdes]

I went through the same issues 4 years ago. Took a few emails back and forth with them before I saw the light.