C14 Losing Collimation

[HCR32]

So my scope loses collimation when I engage the mirror locks. I’m going to put a Crayford on the back and lock it down then collimate that way see if it works. At this stage I’ve followed the process but stars appear like they have a small comet tail after collimation is done and I try to lock the mirror. Any one no any trick I don’t with collimation or am I supposed to chase it all night long. I forgot to mention that I’ve also found that the course focuser is adding to the issue with the mirror flopping around a fair bit when trying to re focus the star. Could the focus mechanism be the issue here and if so how do you tighten the tolerances? Is it a know issue for the course focuser to put misalignment into C14s after collimation is done and refocusing the star? I’m scratching my head here not knowing what can be causing it?? Mirror locks or focuser?

[multiweb]

Is that an edge or older model?

[HCR32]

Older
Late 90s

[HCR32]

Mirror locks. Is it possible that as you screw these down the thread can tug on the steal rods and tug on the mirror. Not enough to see it physically happening but enough to minutely move the optics out?

[multiweb]

ha ok. I did something similar on my C11. I don't know the internals of your scope or how the rods rest on the back of the mirror but the back of the mirror is not flat, it's very curvy like a bell shape. Couple of things to look for:

1_ focus as best as you can close to the zenith so the mirror sits and rest in a good position with minimum tilt.

2_ while still looking up move your rods to make contact with the glass then lock them in place.

If you push one rod more than the other you're likely to tilt the mirror. If you only lightly touch the glass then the mirror will be supported by three points at 120 degrees. That's the perfect support to hold it in place. No pressure needed.

You're right in saying that pushing the rods too far will get you out of focus. So push them in until you touch something, no further.

Finally your focus will change over time. You can feel if the rods get tighter or looser. If that's the case re adjust them by releasing the pressure of push them in some more.

Those old school scopes are usually very good mechanically and optically so look after it.

PS: here's what it looks like inside of my C11 to give you an idea:

back of mirror: http://www.astropic.net/astro/C11/article.view&productid=17

primary cell:
http://www.astropic.net/astro/C11/article.view&productid=15

[HCR32]

Hmmm
I think the rods are fixed onto the mirror because when I use the course focuser the rods move in and out with the mirrors movement in relation the mirrors movement.

[multiweb]

ok, that's pretty clever. So do they slide in/out easily without any friction when you release them?
Might need to have a peek inside see if they are fitted to the back of the mirror or an intermediate plate/mounting.

[HCR32]

Why what would the two options present in this case.
I don’t think they move but I’ll double check when a get home late today.

[multiweb]

Until you know for sure how those rods are fitted to your primary it would be hard to tell.

[HCR32]

The rods are fixed to the mirror cell they are not free moving.

[multiweb]

Then as posted earlier on try to focus at the zenith with the mirror at rest, lock all the rods, check collimation then slew to your target and see if any change.

[Merlin66]

Peter,
I used various Meade SCT's 8" through 12" for many years....the mirror slop was significant due to the crap mirror/ focuser connection.
I then moved to a C11. The focuser mechanism is completely different and much (much) more capable than the Meade.
When collimating, how much movement of a centre star do you see when focusing inward then outward???
This is a key to determine the mirror slop.
An easy way of reducing it, is to run the focuser through the full travel (about 30 turns) in and out a few times. This redistributes the grease between the baffle tube and the mirror support tube. Reduces clearance and reduces slop.

Out of necessity I used mirror locks on the Meade, but never needed them on the C11. I don't use an external SCT focuser, but have used and enjoyed a Feathertouch 10:1 micro focuser.
http://starlightinstruments.com/store/index.php?route=product/product&product_id=261

HTH

[HCR32]

Weather is going to be crap the next few days so I’ll have to wait till my next outing to collimate at zenith and run the focuser in and out see if that helps. The flop in this scope is shifting the target from one side of the FOV to the other side and back.
Hopefully being an old scope that has probably been sitting around some time distributing the grease with the focuser will help which would be the easiest fix.