Halos and diffuse glow around bright stars with iTelescope T5

My first guess would actually be a high haze or very thin cloud layer in at least a few of the subs, have you blinked them? If it is atmospheric you’ll see a drop in the star count for the effected subs.

andrea tasselli · Apr 19, 2026 at 04:55 PM

Are they still running an SBIG camera?! I hope not for pay!

Ehm :-) yes they do… it is the cheapest one, though!

And back to the question: thanks a lot for the hint! I think you are right, I am just too inexperienced to understand the problem…. here is the plot of the number of stars ….
📷 Screenshot 2026-04-19 alle 19.20.00.png
So thank you @andrea tasselli and @Tony Gondola I believe this is the explanation. The subs with few stars are the ones with the more evident halo…

Maybe I should have rejected based on the number of stars, instead of looking at the FWHM.

Thank you very much for the suggestions. I tried to test the “thin haze / cirrus” hypothesis by rejecting the worst subframes.

Originally I had 27 Clear frames. I restacked the image using only 17 of them, excluding the frames with the lowest star count and/or worst FWHM. The result is attached here. Somewhat surprisingly, the diffuse halos / glow around the brighter stars are still present, even though the stack should now contain only the better frames.

I also attached the Siril plots for star count and FWHM. The selection was not perfect science, of course, but I tried to remove the most suspicious frames. The remaining stack still shows the same kind of broad light excess around the brighter stars.

As an additional check, I made a strongly enlarged intensity profile across two of the affected stars. The profile shows that this is not just a very local star-core effect: around the brighter star there is a broad, low-level pedestal extending well beyond the stellar core. That seems consistent with some kind of halo / scattered light / atmospheric diffusion, but I am no longer sure that it was caused only by a few bad frames.

So at this point I am wondering whether this could be a more general feature of the data from this session, perhaps due to a persistent thin haze, scattered light, or the optical/camera system itself, rather than only a few individual bad subframes.

Any further diagnostic suggestions would be very welcome. It is not a big problem, but I just like to understand things as good as possible :-)

📷 r_clear_stacked_only 18.png
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Claudio Pedrazzi · Apr 20, 2026, 09:46 AM

Originally I had 27 Clear frames. I restacked the image using only 17 of them, excluding the frames with the lowest star count and/or worst FWHM. The result is attached here. Somewhat surprisingly, the diffuse halos / glow around the brighter stars are still present, even though the stack should now contain only the better frames.

Hi Claudio.

Even if you took only 17 photos, you can't be sure whether there were any cirrus clouds in the sky. Also the number of stars found on the frames does not show you a clarity of night. Only wheather stability - you can have still qute constant thin layer of clouds. I do not suppose the internal reflection, fog on the mirror, etc. in the Takahashi Epsilon scope.

Best regards, Marcin.

Thats cloud/smoke or haze in the air, or potentially a dew affected mirror/corrector…

With my fast newtonian I have to run a long dew shield or my corrector/secondary will suffer dew quite early in the evening.

Having owned a ST10XME in my time (aging my self with this…) I can absolutely agree with you that that’s not blooming. In blooming situations, you see the spill of saturated pixels into surrounding pixels in vertical rows, you can see the very start of blooming on some of the brighter stars, with a vertical diffraction spike between the normal ‘X’ shaped spikes… that vertical mini-spike is likely electron spill-over…

Alex Nicholas · Apr 21, 2026, 01:47 AM

Thats cloud/smoke or haze in the air, or potentially a dew affected mirror/corrector…

Alex. The thelescope is placet on Utah Desert with night humidity of about 60% and dew point 20 Faranhait degrees lower than ambient temperature (c.a 10 Celsius degrees). It is very hard to expect dew on the optical elements even you cooling down the camera very fast and to very low temperature.

Spacetime Pictures · Apr 20, 2026, 07:50 PM

In this configuration, the corrector is quite exposed due to the short OTA and is extremely sensitive to dust.

Yes. Dust is a second potential problem. But if yes I suppose rather the dust on the main mirror instead of corrector. Even it is extreme sensitive to dust. Dust is rather heavy and likely fall down to the mirror instead of the corrector (90 degrees to the OTA). Also the haze is rather constant on the picture. I am not sure if this effect is observable when the dust covers the corrector not uniformly (what should be expected). But yes, I think the dust may play a huge role.

Only what make me diffused is the detected stars numbers whitch is different from frame to frame. Dust does not make such effect.

IMHO the only way to solve the problem is to take some pictures from a good and clear night.

Best regards. Marcin.

Thank you very much to everyone for the very interesting comments, suggestions, and diagnostic ideas.

There are several good points here that I would really like to investigate properly. Unfortunately, I am not able to follow up on them immediately, but I hope to have some time in the next few days to look more carefully at the data and try some of the proposed tests (though I do Not have Pixinsight)

If I manage to understand something more, I will certainly come back to this thread and report what I find.

Thanks again — this has been very helpful.

Claudio Pedrazzi · Apr 26, 2026 at 10:23 AM

Sorry to come back to this topic again, but given the many thoughtful replies, I felt it was only fair to do a bit more checking and report back.

I do not have PixInsight, so I tried to investigate this in Siril.

One thing I realized is that the glow / diffuse halo is present on all four nights. To test this, I made four separate nightly stacks, using exactly 4 frames for each night, just to keep the comparison uniform and avoid differences caused simply by stack depth.

I am attaching those four nightly stacks, all shown in Siril Autostretch.

📷 r_clear_04_10_stacked.png📷 r_clear_04_11_stacked.png📷 r_clear_04_14_stacked.png📷 r_clear_04_17_stacked.pngI am also attaching three Siril plots for the full sequence:

• FWHM

• background

• number of stars

In these plots I added vertical separators between the different nights, since I verified that in this sequence the frame order is chronological.

📷 FWHM.png📷 background.png📷 stars.pngFinally, I am also attaching the frame list / conversion list, to show the chronological order of the frames across the four nights.

📷 list.pngSo, at least from this additional Siril check, the glow does not seem to belong only to a few isolated bad subs, or to only one night. It seems to be present throughout the whole dataset, although with varying strength. This seems less consistent with only a few isolated bad subs, although I cannot rule out a more persistent atmospheric cause affecting multiple nights.

At this point, I also intend to ask iTelescope support for their opinion, in case this is a known characteristic of the system or of this particular dataset.

Thanks again to everyone for the many useful suggestions.

Best regards, Claudio

Interesting update here. To be honest, the only time I’ve ever encountered or seen something similar was when I used to try imaging with my old Celestron C8-N and dew would partially form no matter how I tried to address it. Dew would not totally form but it would be enough to cause hazy images on different nights and different targets.