Feedback from C9.25 XLT + Starizona SCT Corrector IV + APS-C users: poor corner stars

With a diagonal on the chip of 28.7mm, you are a bit outside the advertised design image circle of the corrector which is 27mm. Depending on how loose they are with the definition it could be that stars in the extreme corners of your image circle are starting to degrade.

How bad are your corners?

I have this set up, but I use it for galaxies at a dark site. I don't need the wide field of my QHY268, so I use a CCD SX694 with 4.5 nm pixels. Much smaller chip also.

Those stars were pretty good and BlurX will take care of the rest.

It doesn't take much camera tilt to get wacky corner stars. If there is no tilt then the corner star distortion should be similar.

Lynn K.

Thank you all for your feedback.

I understand that achieving good corner performance with the Starizona SCT Corrector IV and an APS-C sensor is probably a bit ambitious.

I've attached three subframes analyzed with Aberration Inspector.

As you can see, the corner stars are quite poor.

Of course, for small targets, after cropping and using BXT, I'm very happy with the results. However, it's disappointing not to be able to take full advantage of the entire field of view.

If any of you have untreated subframes, I would really appreciate it if you could share them, as final processed images rarely reflect the actual capture quality.

AberrationInsp1.jpg

AberratoionInsp1.jpg

Abinsp2.jpg

Or could this simply be sample variation? How consistent are these reducers?

Tony Gondola · Aug 1, 2026 at 02:47 PM

mmmmm, have you really refined your collimation?

Well, the optical center isn’t in the center of your aberration inspector image and I don’t think it’s tilt.

check your spacing between the corrector and your camera’s chip.

Marc Bonnet · Aug 1, 2026, 10:42 PM

Thanks everyone for your valuable feedback!

I’ll keep investigating. I completely understand that my setup is probably still not fully optimized.

My next step will be to revisit the tilt, perhaps with the help of the Wanderer Astro ETA if I eventually get tired of the manual adjustment process. I’ll also check the collimation again, although I’m a bit less suspicious of that at the moment.

The entire imaging train is fully threaded, but it’s also quite heavy (Starizona reducer, ZWO OAG-L, 7×2” filter wheel, and ASI2600), so a small amount of flexure or tilt seems plausible.

I’ll let you know if I finally manage to solve the problem.

Thanks again, and clear skies!

I think 7×2’’ filter wheel is a little too much for 2’’ tube, this may cause some drops. If possible, you could try 7×36mm filter wheel and filters instead.

Hey everyone,

A warm thanks for your help once again. After many iterations and quite a bit of trial and error, I think I have finally managed to get a decent result across the APS-C sensor. I’ve attached a single 180 s exposure together with an Aberration Inspector view.

Abinsp.jpg

📷 Abinsp.jpg

Here is some feedback from my experience:

1/ Backfocus was the main issue.

As suggested by some of you, the aberration pattern initially looked very much like the sensor was too far from the corrector, despite my imaging train being set close to the theoretical 90.3 mm backfocus.

I progressively shortened the spacing and eventually found a sweet spot with a nominal mechanical backfocus of approximately 83 mm, based on the sum of the individual components:

20 + 5 + 0.8 + 2 mm spacers/adapters + 5 mm tilt plate + ZWO OAG-L + EFW + ASI2600MM (without the original ZWO tilt plate).

I was not able to directly measure the complete imaging train with sufficient accuracy, so this value is based on the nominal thickness of each individual component. Nevertheless, the overall spacing is clearly significantly shorter than the recommended 90.3 mm.

I would be very interested to know whether anyone else using the Starizona SCT Corrector IV has had a similar experience and ended up with an optimal spacing significantly shorter than the theoretical backfocus.

Collimation was also checked during the process and was found to be acceptable, so I did not make any further adjustments.

2/ A very interesting point for Celestron users: the dew-heater ring.

Despite the considerable improvement obtained by correcting the backfocus, on some nights I was still seeing rather unpleasant asymmetrical spikes/distortions around the stars.

I initially suspected the remaining backfocus error and changed the spacing again, but this made essentially no difference.

Eventually, I turned the Celestron dew-heater ring down to its minimum setting — and the spikes simply disappeared.

This was quite a surprise. I assume that thermal effects from the heater ring were somehow inducing a local deformation or thermal gradient affecting the optical path. In any case, this may be worth keeping in mind for Celestron users seeing unexplained asymmetric star shapes while using the heater ring.

3/ Tilt was the final adjustment.

Finally, I played around with the tilt. This provided some additional improvement, but it was clearly a relatively minor factor in my case compared with backfocus and the dew-heater ring.

Overall, I’m now getting what I consider a reasonably clean and usable field across the APS-C sensor. It is certainly not perfect at the extreme edges, particularly on the right upper corner, but this is by far the best result I have obtained so far. I’ll try to further improve the field, but I’m quite happy with the result at this stage.

I hope this feedback can be useful to others struggling with a similar setup, and I’d be interested to hear about your own optimal backfocus measurements.

A warm thanks to everyone for your guidance and recommendations along the way — they have been extremely helpful.

Clear skies to all !

Dear Marc

Seems you are nailing the sweet spot.

Some years ago, I had the classic SCT C8 with the first starizona SCT Corrector (i still have it) I did quite a lot of imaging with a QSI 583 WS

Despite the spacing I had two things to considered which drove me crazy. That was because I had good days with stars shapes ok and other days I had similar aberrations as you described.

1st Always keep in mind when focussing to push (one direction) the primary mirror while focussing. If the focus spot was overpassed I went backwards and pushed again. The thing is the mirror had a tilt while pushing and it also had effects when you change the meridian or the inclination of the telescope, so was also wise to keep the mirror pushing in one direction to keep it steady. Kind of difficult if you are using N.I.N.A. focusing routine…

This issue was typical for classic SCT and was resolved in the HEDGE version with the locking knobs.

2nd. Give some time to cool your SCT by pointing down the telescope and letting the air (from the inside) catch the same temperature from the outside. After 20 minutes or more (probably more because you a a 9.25 “ mirror), I place the image train. I remember some guys use a fan to speed the process

This was also corrected in the new HEDGE version letting some grates in the back side for refrigeration

So these two last corrections I believe are still an issue in the classic SCT…though engineers at Celestron addressed the problem in the HEDGE version

PS

Back then I switched to Newtonians, but the truth is that i still miss the classic SCT Celestron I had… 😞

Good luck !!

Sergio

Corentin Martine · Sep 16, 2026, 06:16 PM

Salut Marc,

I have a similar setup, except that I'm using the color version of the 2600 on this telescope.

I'm facing the same issue with elongated stars, even though I bought the specific spacer that is supposed to provide the correct backfocus for the Starizona once you add the standard 55 mm backfocus of the ZWO components. So, in theory, I should be at 90.3 mm, yet I still get elongated stars in the corners. I usually fix them with BlurX 🫣, but I'll try shortening the imaging train as a test.

A few questions:

How do you attach your optical train? I'm using a 2" Baader ClickLock. The issue I have with it is some uncontrolled tilt. I've circled the Starizona with a parfocal ring, but even with that, I feel that the alignment of the imaging train relative to the telescope axis is somewhat random.

Out of curiosity, I saw your image of NGC 7331 (which is great, by the way). As it happens, I just imaged the same target because of the supernova. Did you crop your image? I noticed that the field of view you're covering is much smaller than mine, which could be due to the shorter backfocus. According to the AstroBin plate solve, mine is 0.88° × 0.58°, whereas yours is 0.58° × 0.37°.

Finally, regarding the dew ring: when you say to set it to the lowest power, do you mean completely off, or do you still apply a small amount of power?

Thanks!

Corentin

Hi Corentin,

Yes, I cropped the image, but only for framing purposes. The goal was not to exclude bad stars in this case. I also stretched the stars quite a bit in this image — probably a little too much.

With the issues I was facing and the many tests required, I switched to a Baader ClickLock, and I really like it. Before that, I had a fully threaded imaging train, but I wasn’t able to easily adjust its orientation, and I frequently had issues with threaded adapters getting locked together.

Of course, the Baader ClickLock may introduce a little more tilt, but in my case I would say that 95% of the problem was related to backfocus rather than tilt.

I don’t know if this will help, but with the ClickLock threaded onto the SCT and the Starizona inserted into the ClickLock, my Celestron focuser position is around 29,200 steps. I’m not sure how reproducible this value is from one C9.25 to another, though.

My imaging train is described above if you want to try a similar backfocus. I’d be very interested to know whether shortening yours improves your field as well. I shortened mine progressively, roughly millimeter by millimeter, checking the stars after each adjustment until I was satisfied with the result.

Regarding the dew heater ring, this is something I noticed only recently. Here is a picture showing the spike I was getting. It was asymmetrical — extending in one direction only, towards the center — and particularly prominent in the upper-right corner (no issue on the left side of the frame). It completely disappeared when I turned the dew heater down to 20% on the PPBADV. So I think the heater is still perfectly usable, but you may want to check for this kind of artifact and keep the power as low as possible.

Spike1jp.jpg

📷 Spike1jp.jpgHope this helps, and good luck optimizing this fantastic setup. Don’t hesitate to keep me updated if you try shortening your backfocus!