SCT (Edge HD) 8" Collimation - Shifting is that ok?

[glend]

I have a new Edge HD 8 that i am attempting to set up for AP. It has a Baader Steeltrack focuser attached to the back (no reducer in the path at this point). Bench collimated seems perfect, using a Cheshire and artifical star, then Star tested to tweak. I used a Duncan mask to get precision. However, when i rack the focuser in and out the collimation shifts (this is with the primary locked). Checking with my laser the return spot is shifted to the left of centre, and moves up and down on the left side of the target as the focuser is racked in and out. The Baader Steektrack is squared at the factory and has not been touched. This says to me that the Edge Optical axis is not squared up. Reading on forums like Stargazers, suggest that precision is hard to achieve on SCTs due to the moving primary mechanism tolerances.

So suggestions? Should i use the Baader adjustments to align a return laser to target (at focal point of focuser)? Or remove the secondary, which is possible as it is a Faststar, and use a centre spotted target in its place and adjust the focuser to hit that perfectly, then reinstall the secondary and collimate from there? I am leaning towards the later as it removes reliance on the secondary for focuser alignment.
Or is it simply not worth worrying about on these scopes, as shifts are going to happen, and when i get the camera on it i would have to do it all again at that focal point?

Or put it back in the box and get the APO out, haha.

[casstony]

How does the collimation look without the external focuser?
With everything locked down I'd be suspecting the Baader focuser for the movement you're seeing.
Then again maybe it's not worth worrying about if the images turn out ok.

[glend]

Thanks Tony. I pulled the secondary and put on a target, and yes the Baader focuser was showing the laser spot with a slight left offset (2mm) through its range. I tuned out the left offset. But when racking the focuser in and out i still get a little up and down movement from dead centre at each end of the range, down 1mm fully inward and up 1mm fully out. It is perfectly dead centre at the mid-point of the focuser range. So i think i will have to leave it there for now. I will re-collimate now and test again visually tonight (if possible). If it looks ok with the Duncan mask i will stick the ASI224 on and work out the focal point for that and re-check collimation there. It is feasible with a camera on it the focuser may level out slightly. We'll see.:thumbsup:

[billdan]

Glen find the position where best focus is under the night sky, the Baader has ruler markings that you can write down.
Then only collimate from that setting, even as the temperature drops you will never move too far from that spot +/- 2mm.

Bill

[Slawomir]

And that's why I utterly prefer refractors :lol:

But more seriously, this is a very interesting topic and I look forward to reading about your experiences Glen. Hopefully you will fine-tune the scope perfectly and with ease :thumbsup:

[LewisM]

If the laser shifts in racking would this not indicate that the steeltrack is not square and not collimated in relation to the rest of the scope? Unless you collimate the focuser you will only get one sweet spot of focus.

Squared at the factory means nothing. We see factory shortcuts and errors continually, then factor in the scope bumping its way around the world...

[glend]

Still having trouble getting this scope collimated. I keep hearing how easy SCTs are but i am getting two different results with two different methods. When i bench collimate and then tweak it with a star test, the rings are perfectly centred and equal but when focused it looks like it has coma. When i use a Duncan mask, and adjust to get the bars to meet in a perfect Y and cross in focus, the star looks great when the mask is removed. However, when i rack the focuser in the standard collimation rings appearance is lobsided as if out of collimation.
Obviously the mask is giving me a better result but why the difference?
I do have the Baader Steeltrack on the back, and it has been aligned to the centre of the secondary, could this focuser be the cause of the difference i am seeing? I plan on removing the focuser and trying again with just the visual back. Any other ideas? Similiar experiences?

[Camelopardalis]

I would certainly be removing the Steeltrack and collimate with an eyepiece (or Metaguide) with the focal plane in the correct place at 133mm or thereabouts.

Also note that the star needs to be centred when you defocus otherwise it will give a false result.

[thebonz]

I found that I had a similar problem with an edge 14 Focus that was not even throughout the focal plane. It had a moonlite focuser attached. I compared this with a standard baader visual back by comparing both with a laser on a centered secondary target. The secondary was removed for this. Both landed on different spots that were not in the centre of the secondary. Puzzled with this I removed the corrector plate retaining ring and discovered that the corrector was not centered. I centered the corrector plate by aiming the laser at the secondary centre target with the baader visual back insitue This solved my problem.
To sum up, the corrector plate was not in the optical axis of the visual back.
John

[MortonH]

There's a thread on CN that talks about Edge SCT's and their variability.

Although the Edge design is capable of significantly better imaging performance than a regular SCT, they're still a mass-produced product built to a price point.

At least the issues being described here can potentially be fixed by the user who has some technical/optical knowledge. Hope you can figure it out, Glen.

[glend]

I removed the Steeltrack focuser and stuck the visual back on to check the corrector centering with the laser. Pulled the secondary and installed the target, the laser hits the target bullseye, so the corrector is right. Collimated on the bench for visual star test. Making progress, I need a clear night now. Once visually ok, the next step is to install the T adaptor and my camera and check again.
I don't know what to do with the Steeltrack.

[Camelopardalis]

Glen, check the manual for your specific model. Some of the Steeltracks can be collimated.

[glend]

Yes Dunk, i thought i had explained clearly, perhaps not. I have the instructions for collimating the focuser, and have years of experience collimating focusers. The adjustments work, i can align the focuser at any particular point on the scale, even at the Edge back focus point of 133.35mm; the issue is any movement of the focuser draw tube position ruins collimation, rack in collimation out, rack out collimation out. With a laser in the focuser i can see the dot move diagonally above and below the centre target of the corrector when racked in and out, it will not hold the centre. Nothing in the range of adjustments will allow it to hold the centre when moved. So i have removed it all together, and the scope has stable collimation now using just the standard mirror movement focuser. I don't know how to explain it any clearer.

[Camelopardalis]

Sorry Glen, I didn't intend for it to sound patronising :D

Being a SCT user I've just used the regular focuser. The back focus on the Edge 8 isn't sufficient for a r&p, oag, filter wheel and camera, less so with the reducer inline. It's probably doable but tricky.

[glend]

Yeah, no worries. As you know that focuser is of "unknown history" and when it came to me it was not in its original packaging and had been used for 'testing'. It could have been damaged before, or during shipping, and as it was a "freebie" i am not upset about it not working out. I gave it a shot, now onto a standard config with solid T tube to the camera.
That wraps this thread up i think, thanks for all the suggestions.