Help needed – 21-panel mosaic (105 hours, Namibia Bortle 1) – visible seams, color gradients and loss of faint nebulosity

Hi everyone,

I’m currently working on a rather ambitious astrophotography project and would really appreciate some advice from experienced Astro Pixel Processor users and the developers.

The project

I captured a large 21-panel (7 × 3) mosaic of the Rho Ophiuchi region from Isabis, Namibia, under exceptional Bortle 1 skies.

The dataset consists of approximately 105 hours of total integration time, with around 5 hours per panel and 25% overlap between adjacent panels.

My imaging setup:

• Telescope: Askar SQA55
• Camera: ToupTek 24AC (full-frame OSC)
• Mosaic: 7 × 3 panels
• Overlap: approximately 25%
• Total integration: approximately 105 hours
• Location: Isabis, Namibia (Bortle 1)
• Processing: Astro Pixel Processor
• Computer: MacBook Pro M3 Max, 36 GB RAM

My processing workflow so far

I calibrated, registered and integrated each of the 21 panels individually in APP.

Afterwards, I loaded the 21 integrated panels into APP to create the final mosaic.

I’ve experimented extensively with different mosaic integration and normalization settings, including:

• Multi-Band Blending (MBB), including 25%
• Local Normalization Correction (LNC), including second-degree LNC with 5 iterations
• Different normalization approaches
• Different reference panels, including a central panel
• Full Image normalization
• Background neutralization and gradient correction
• Different mosaic integration settings

I’ve also been experimenting with Drizzle during the individual panel integrations. Due to APP’s practical image-size and memory limitations, I cannot currently process the entire mosaic at its maximum potential resolution.

The problem

Despite numerous attempts, I’m still struggling with visible transitions between the individual panels.

The attached image illustrates the problem.

There are noticeable horizontal and vertical structures along the panel boundaries, often appearing as yellowish lines or areas with different background brightness and color.

Interestingly, some of the overlapping regions appear to have a more uniform or cleaner background than the central areas of the individual panels.

This makes me wonder whether the normalization and gradient correction are treating the overlap regions differently from the rest of each panel.

Increasing the strength of LNC and other corrections can improve the visible transitions, but unfortunately it also seems to suppress faint dust and nebulosity.

And that is exactly what I want to avoid.

The Rho Ophiuchi region contains extremely faint and complex dust structures, and with 105 hours of exposure under Bortle 1 skies, preserving those structures is particularly important to me.

I’ve successfully processed smaller mosaics, including 4 × 4 panels, without encountering these problems to the same extent. The difficulties seem to become much more pronounced with this larger 21-panel project.

My questions

1. What would be the recommended APP workflow for a 21-panel mosaic with significant faint dust structures and 25% overlap?
2. Would you recommend applying gradient correction individually to all 21 integrated panels before creating the mosaic, for example using APP, GraXpert or PixInsight DBE?
3. Could normalization based primarily on overlapping regions explain why those regions sometimes look better than the central areas of the panels?
4. What combination of LNC, MBB and normalization settings would best preserve faint nebulosity while eliminating visible panel boundaries?
5. Could Drizzle integration followed by downsampling or resampling contribute to these artifacts, or is this most likely an independent normalization/gradient issue?
6. Is there a recommended strategy for processing such a large mosaic at full resolution, potentially by dividing the mosaic into smaller sections before the final integration?

My goal

I want to achieve a seamless mosaic with a consistent background, natural colors and, most importantly, without sacrificing the extremely faint dust structures captured during those 105 hours.

I’m more than willing to reprocess all 21 panels if necessary, but before investing another significant amount of processing time, I would really appreciate some guidance on the most effective workflow.

I’ve attached an image showing the current result and the visible transitions between the panels.

Any suggestions, recommended APP settings or insights from users who have successfully processed similarly large mosaics would be greatly appreciated.

Thank you very much for your help!

Best Joerg