Bad optics in Seestar S30

thomas.westerhoff

I recently purchased a Seestar S30 directly from ZWO, not through a middleman. However, I have noticed that, especially when using the IRCUT filter, the stars are stretched out significantly in the lower part of the image. This also happens with the LP filter, but not as severely. What is the reason for this? In fact, you have to cut away a good third of the lower part of the image because the data in that area is unusable. I realize that a device of this price cannot deliver high-quality images, but even my cheap $70 telescope doesn't have such a severe optical flaw.

Balrog

The problem isn't exactly the quality of the S30's optics. The problem is the size of the camera sensor, which, combined with the lenses used, causes comatic aberration to appear. This problem could be seen in the first S30 images published in the Seestar app community. If the star distortion is only on one edge of the image, you could also have a tilt issue with the sensor. My first S50 had a lot of tilt, and only the store where I bought it accepted that it was a problematic unit and replaced it with another one with almost no tilt. Zwo's European distributor didn't accept that there was any problem. Returning to the S30, I bought it knowing about the comatic aberration issue, as I find it to be a much better portable device.

Lilo

Common issue. My S50 is almost Ok, slightly elongated stars in the corners.
S30 three corners are Ok, forth corner has noticably elongated stars from corner to the center.

As for picture above. Absolutely inappropriate, such units should not pass manufacturer quality control.

thomas.westerhoff

That's not chromatic aberration. It looks more like coma or an image field that has been corrected too much and is too small for the sensor.

Lilo

thomas.westerhoff
"comatic aberration"
I believe he meant exactly the coma )

Digital Don

The optics in my S30 show something similar on the bottom of the frame, though not quite as bad.
However, I've discovered a very simple procedure that completely eliminates elongated stars.

After you locate the object you wish to observe but before you start the exposure, open the 'Framing' function and rotate the field a couple of degrees. This causes the Seestar to take multiple frames and by doing so, eliminates the elongated stars.

Works for me!

Don

Lilo

Digital Don
Yep, noticed that too. It's the case when math of the stacking overcomes the optics unperfection.
The downside is "swollen" stars ))

Digital Don

Lilo

I consider that the lesser of two evils.

Don

thomas.westerhoff

Hey guys, I need to let off some steam because I'm really stressed out with ZWO Service.
I know that my Seestar has a problem. The corrected image field of the Seestar's optics is too small for the sensor. This results in elongated stars in the corners. As long as this is consistent in all corners, the alignment of the optics is correct. However, in my case, it only affects the lower part of the image field significantly, while the upper part is almost flawless. This is a clear indication of a tilt in the lenses and thus a misalignment of the optical axis in relation to the center of the sensor.
ZWO customer service sees it differently. They accuse me of incorrect handling. It should be noted here that I work at an observatory myself, used to grind mirror optics myself, and therefore consider myself to be quite knowledgeable in this area. I was advised to position the object a little higher up in the image field, where the error does not occur. I was also told that the next firmware update would include a function to correct this in the software. However, I don't think much of this, because then I might as well download “overprocessed pictures” from the internet, of which there are unfortunately far too many.
Unfortunately, I bought the device directly from ZWO in China because I was tempted by the low price. Here in Germany, I would have a two-year warranty guaranteed by law and could return the device in accordance with product liability law and demand my money back. Instead, I have a telescope that I can't really do anything halfway decent with. But that's just how it is with Chinese junk.

Support@Seestar

thomas.westerhoff
As far as I understand, it seems that we haven't actually rejected your request — perhaps there was just a misunderstanding or some miscommunication along the way.
We truly value your input and are always happy to discuss things further to make sure we're on the same page.

telfish

This is sensor tilt so the sensor is off axis to the optics My s50 is surprisingly good in this respect. There are several tools out there to measure tilt from an image. This is yours which is severe. The numbers should be close to each other in all directions .

telfish

This is my s50 tilt. Which is really surprisingly good.

pscharff

Hm, in the case of a sensor tilt I would expect that this behavior will be nearly the same at all images.

So I have checked two of my Seestar S30 images. The result is, that one image shows a sensor tilt of 11%, and the other image shows a tilt of 37%.

Patric

telfish

pscharff

A few things can effect the result.

Focus.
Seeing.
scope orientation.

So it can vary from night to night.

Plus ideally you use a starfield with no nebulosity in it.

pscharff

telfish
Thanks for the info.

I just found an interesting post of a Seestar user. He experienced a sensor tilt with the Seestar, which has got worse on the last sessions. He has solved the problem by moving the focus from one extreme to the other a few times.

Patric

telfish

pscharff

Poor focus can exacerbate tilt for sure.

gofraro

When this occurs with my S50, it helps to move the focuser manually completely in and out and back to its normal position (I can see sometimes slightly elongated stars in the upper right corner). The ASIfitsviewer has a tilt check feature, but you have to use a single fits frame to check it and not the stacked file.

But, there is a another point of failure. The Flats.
In my case, the introduction of the Flat-Shoot-function with provided factory-flats, caused the appearance of tilted halos, clearly visible on brighter stars. With IR-cut and LP-filter. And I could prove that this not happened before the update.
After some research through various sources, faulty flats can cause such aberrations, when the flats were created on a system that doesn't match the optics or, if custom flats are created once, but the optics changed in between (focuser tilt etc.).

Also, when using mosaic mode, the alignment and stacking algorithm is to consider distortions (TAN-SIP) and has to correct it accordingly. Therefore, if you see such aberrations in the sum-stack of a mosaic, try to restack it internally with the DSS or externally, using the correct algorithm.
If you see such aberrations in the single frames, the mentioned before is more likely.