Possible Astigmatic C8?

[Kim2010]

I have discussed this at length over with folks at CN, but I am still troubled by this.

So I bought an 8SE NEW last Aug 2022, and for a few months I don't remember seeing this issue, but then around April this year, I started to visually see cross-shaped stars at high power, which sometimes vanished and focused into a proper Airy disc and diffraction ring. 

 

Some of the videos I took are below. Is this astigmatism in the optics? And if so how can it suddenly appear out of nowhere apparently? 

 

 

 

 

 

[AstroKeith]

I'm going to say, based on what I can see, pinched optics. Perhaps your correctorplate is being distorted by its clamp ring.

[davidc135]

It's certainly astigmatism, hopefully caused by pinched optics as Astrokeith says.

You would surely have noticed it before April if it was there? It could be obscured by poor seeing but hardly for such a length of time. In moments of great seeing the diffraction ring is temporarily formed from the four points.

I assume that it's now out of warranty and you are free to investigate which component is responsible. I'm also assuming that the problem doesn't lie with the Barlow etc. Why not loosen the corrector retaining ring and check if that helps. How is the corrector centered in its cell? Does the scope use shims or lateral screws?

If the problem persists, take note of original corrector plate position, rotate the corrector/secondary assembly and see if the axes of astig move, or if the astigmatism is improved or worsened. If no effect then the problem lies with the primary. If they do the problem is either in the corrector plate or (less likely) the secondary. It might be that the secondary housing connects too stiffly with the corrector.

David

[Kim2010]

Thanks for the responses so far.

I am not sure if the seeing was obscuring the problem, but what I am 100% sure was that there was a night sometime in March which I remember so clearly because the seeing was so stable that I was looking at Antares at 400 and 500x and it looked like a mini-solar system with the bright Airy disc surrounded by a semi-bright first ring and then fainter rings with Antares B sitting on one of the fainter rings. I then looked at another random star with a 3mm Zoom and OMG, the Airy disc/Fresnel pattern was still as pretty. This is why I know it presented a non-astigmatic image before April.

 

Now since April, I was shocked when I saw the crosses at high power. I remember the only thing I changed was that I added a new finder mounting and replaced the red dot finder and also used a 6.3 reducer corrector-- which I since have removed, but the astigmatic images remain.

 

Now my wife and I travel a lot, and so maybe it got bumped along the way and that caused the issue?

 

As to what I have done so far since finding the issue, I have rotated the corrector although I did not take note of the change in orientation of the cross but it seemed not to do too much so I returned it to the original orientation. I have not yet checked the secondary and primary. I have also removed the corrector screws and just tightened them "finger tight" so to speak. 

 

I am curious with the thing regarding the secondary housing-- how do I check this?

 

And yup, this is brand new, but I don't think I can get warranty because I imported it from the US to the Philippines, and I think the warranty from the US store only applies if I were in the US.

[dweller25]

How is the primary held in place around the central baffle - is it a circular screw down disk ?

If so I would check if it is too tight

It’s also worth checking that the secondary housing is not too tight.

Have you tried viewing straight through with no diagonal and different  eyepieces ?

Also are any of the finder screws contacting the mirror ?

Good luck 👍
 

Edited September 18, 2023 by dweller25

[davidc135]

I would rotate the corrector again and if the astig doesn't rotate then you know it must be at the mirror end. I assume eyepieces, diagonal etc are in the clear.

You could try easing off the finder mounting screws just in case.

In some models of secondary housing the front plate and baffle screw together and trap the corrector plate with shims being used to centre the housing. In my ancient C8 it looks like the front plate and baffle are glued together.

David

[Elp]

New scope with fastar, likely the secondary housing is keyed so it can't rotate, unless the whole corrector is rotating.

Edited September 18, 2023 by Elp

[Oldfort]

I think a hard knock could have done this.  I had a 10" SCT which took a tumble, due to me not tightening the tripod legs enough. On one side of focus, the image was elongated vertically and the other side of focus the image was elongated horizontally.

I don't know wht the fix was because I took it back to the retailer (when they had an after sales service) but it was sorted out quickly and cheaply.

[Paz]

Something else to consider is the collimation screws on the secondary and if any of those has ended up over tight.

[Kim2010]

39 minutes ago, Oldfort said:

I think a hard knock could have done this.  I had a 10" SCT which took a tumble, due to me not tightening the tripod legs enough. On one side of focus, the image was elongated vertically and the other side of focus the image was elongated horizontally.

I don't know wht the fix was because I took it back to the retailer (when they had an after sales service) but it was sorted out quickly and cheaply.

This sounds promising, but when we travel, it is always in its hard case. But still, I wish I knew what that aftersales folk did..

[Oldfort]

14 minutes ago, Kim2010 said:

But still, I wish I knew what that aftersales folk did..

The work was done by Steve Collingwood, then at Telescope House but he now spends his time building and installing observatories (Pulsar).

[Peter Drew]

Check the screws holding the finder mount.  They are directly over the edge of the moving primary mirror.  If too long they can catch on the edge and impart enough pressure to cause astigmatism.  It has been known to happen!     🙂

[Kim2010]

Here are the latest star video and stacked image I was able to do a few weeks ago with my C8.

 

Raw video:

 

Stacked images of the video as well as an image of Jupiter are attached.

 

Is this level of astigmatism significant enough to affect visual and/or images with this scope?

 

 

[dweller25]

Did you try any of the suggestions that were made in September ?

[Kim2010]

4 hours ago, dweller25 said:

Did you try any of the suggestions that were made in September ?

Yup.

[Kim2010]

No matter what, this seems to be the best I can get to with the C8.

[dweller25]

I am wondering how the primary on your scope is attached to the focuser, it may be glued and there have been reported issues with this method.

https://www.cloudynights.com/topic/759150-c8-edgehd-with-heavy-astigmatism-mirror-glue/

Edited October 29, 2023 by dweller25

[dweller25]

PS - in your stacked image of Jupiter the two moons are clearly elongated so something is affecting image quality.

Edited October 29, 2023 by dweller25

[dweller25]

Is your secondary central to the corrector and the OTA ?

[davidc135]

A cemented component could explain the time lag in the issue showing. Maybe even the secondary but it would be unusual.

David

[Peter Drew]

I have several SCT's including a 8se, the video image of the star is quite commonly seen in all of them due to either less than perfect seeing or insufficient cooling to ambient.  I would say that the Jupiter image shows little wrong with the overall performance.    🙂 

[Kim2010]

Hmm looks like the elongated moon is just seeing or maybe tracking artifacts. The below is Jupiter same camera, but using my 100ED 4" apo with 2X Barlow. I am sure this scope has zero astigmatism.

[Kim2010]

The moons are elongated as well on the 100ED image.

[Kim2010]

Or focusing issue on the moons?