Hi everyone,
I’m relatively new to imaging with SCTs and recently picked up a Celestron C9.25 EdgeHD after primarily working with refractors. I’m hoping to get some help troubleshooting a couple of issues I’m running into.
Current Setup
C9.25 EdgeHD
0.7x focal reducer
Celestron T-adapter
ZWO OAG-L
ZWO 2" filter wheel
ZWO ASI2600MM Pro (recently serviced)
ZWO EAF
Guide camera:
Previously: ASI120MM
Just upgraded (haven’t used yet): ASI174MM Mini
Bob’s Knobs installed for collimation
I’ve confirmed backfocus with Celestron support, so spacing should be correct. It doesn’t help I’ve had two clear nights in the last 2 months of owning this new scope but here are my questions:
Question 1 – Collimation / Star Shape Issues
I’ve been working on getting my C9.25 EdgeHD properly collimated, but I’m struggling to produce clean, round stars. After several attempts at collimation (I’ll attach my most recent images), I spent about three hours collecting test data and still ended up with stars that have unusual or distorted shapes across the frame. The issue doesn’t appear to be isolated to one area of the image, the stars just don’t look quite right overall.
Since I’m new to SCTs, I’m not entirely confident that I’m approaching collimation correctly, even after upgrading to Bob’s Knobs to make adjustments easier. I’m unsure whether what I’m seeing is truly a collimation problem or if something else in the imaging train could be contributing, such as backfocus, tilt, or even guiding-related artifacts. At this point, I’m trying to narrow down whether this is user error on collimation, a setup issue, or something inherent to the optical configuration.
For those with more experience using EdgeHD systems, what are the most common collimation pitfalls that can lead to this kind of star distortion? Based on your experience, does this sound like a classic collimation issue, or should I be looking more closely at spacing, tilt, or another aspect of the setup?
(See images for examples)
Question 2 – Large Central Bright Spot (Not Removed by Flats)
Since getting my ASI2600MM Pro back from servicing, I’ve been dealing with a large bright spot in the center of my images that is present across all filters. This isn’t behaving like typical dust donuts, it’s more of a broad, centered illumination issue, and my flats are not correcting it. I’ve already gone through and carefully cleaned all of my filters multiple times and verified that there’s no visible dust or debris in the imaging train, but the issue remains unchanged. I’ve also taken new flats under consistent conditions, but the calibrated images still show the same problem.
Because the pattern appears across every filter and is centered in the frame, I’m starting to think it may not be related to the filters themselves. Instead, I’m wondering if it could be something on the sensor-side, such as contamination on the protective glass window, or possibly something more serious with the sensor following servicing. I’m also considering whether this could be a flat-field or illumination mismatch issue rather than a physical contamination problem.
Has anyone experienced a large central bright spot like this that flats couldn’t remove? Does this sound more like a calibration issue, or something related to the sensor or its protective window? Also, if cleaning is warranted, is it safe to clean the sensor on the ASI2600MM Pro, and what’s the best approach to do that safely?
(See images for examples)
Thanks for all your help!
Chase
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