Hello! After going down a number of paths trying to identify the possible sources of image problems from my SBIG ST-402 with B,V,I,C filters, the one possibility left standing is a failing capacitor.
(As I’ve described in other posts, I am getting dark pixels in images darker than my master dark resulting in wrap error on the histogram. It takes a lot of work to reduce such images to useful data.)
The folks at the SBIG Legacy Camera forum had the following suggestion-
“I’d suggest you turn off everything else in the vicinity and then try taking a dark frame - see if it goes away. If not, then my best guess would be that C125 is failing. It’s obsolete but there are substitutes available:
https://www.digikey.com/en/products/detail/nichicon/uud1v150mcr1gs/591430”
Turning off the dew heater did not change the problem.
The SBIG technician who worked on the ST402 in the past has retired (Bill), and the SBIG folks on the forum did not suggest anyone else. My soldering skills are poor, though I can give it a try, but I thought I would ask if anyone has a suggestion for someone who might be able to replace the capacitor.
The ST402 has been a great camera, with a huge linear range and is an excellent match for my 8" SCT working with a focal reducer, so I hate to give it up, even to migrate to a new CMOS camera.
Thank you for your consideration. Best regards.
Mike
Mike,
I do hope you can repair your beloved ST-402.
But if you absolutely had to consider buying a CMOS camera, it’s not the end of the world. You probably know more about these things than I do, but for example a ZWO ASI533MM (mono) camera has a sensor 11.2x6.3mm (the ST-402 is 6.9x4.6mm), 14 bit ADC (you had 16 bits), and full well capacity of 50,000 e- (of course you had 100,000). The 3.76 micron pixels are way smaller than your 9 micron ones, but it’s not really dire to have small pixels.
There are 16 bit ZWO mono cameras but they are of course more expensive and have much larger sensors, though still with small pixels. I mention ZWO simply because I’m familiar with them.
Do you have a schematic? Is your power supply in good shape?
60Hz ripple less than 20 mV, all voltages correct etc. Any frayed cables?
How much do you trust the advice to change C125?
Suggest a local technician so that you don’t need to ship it back and forth while trying various fixes. A tech can put the power supply on a scope and confirm that it is good.
Ray
Thank you for your note!
The folks at SBIG continue to support their cameras through the various forums. They made quality products and performed good service int he past, so I trust them for guidance even though they don’t provide technical repair support at present.
The capacitor should be easy to check (even by me!). We are on a third power supply. The first through the mount did not seem to work well. The negative pixels happened a lot with the power supply that they provided and got much better when they switched to another power supply.
The camera may just be sensitive to all the surrounding electronics.
So, I am checking for local folks who might be able to check the camera. If it checks out OK, that would imply it is sensitive to the surrounding electronics, in which case the only option I might have would be to continue to work around the problem or upgrade to a new CMOS camera with photometric filters.
I’ll ask SBIG about a schematic and/or photo of the circuit board.
Thank you again and best regards. Mike
If the C125 is a discrete component anyone with soldering skills should be able to find it, desolder it, remove it and replace it with the new one. Assumes it is readily available that is to say not blocked by other components of the camera. Most CCD camera interiors are pretty crowded.
If the C125 is a surface mount you need someone with some surface mount soldering skills, not easy, might require serious disassembly of the camera. Risk of total failure or breaking something else.
I doubt the noise I’ve seen in your single science image is from an external source, could be, but a failing cap might introduce the semi-random “stuff” in the science image I last looked at, maybe… Still seems strange the structure was only in the science image and not in the dark or flat. Might be being averaged out over time in the Dark and flats.
You will be much happier with a new camera, IMHO! 
Have you searched ebay or some of the astronmy buy/trade web sites to look for a used SBIG ST-402?
Jim (DEY)
Google AI gives these ideas to explore… Many sources of RFI in the world these days and finding RFI sources is an art, wall-wart power supplies are the easiest and worst. Many others such as motors, fans, etc. etc.
You said: astronomy CCD camera repair
You said: SBIG ST-402
For the legacy SBIG ST-402 (or ST-402ME) camera, repair options are heavily dependent on the exact nature of the failure, as original manufacturer part supplies are now highly constrained. [1]
Crucial Diagnostics & Hardware Workarounds
Before sending your camera in, verify if the issue is a known power supply or software glitch, which account for a high percentage of ST-402 “failures”: [1]
- The Power Pin Compression Issue: If your camera’s Peltier Thermoelectric Cooler (TEC) is failing or dropping connection, it is often caused by the power supply socket pins compressing over time. You can gently spread the two halves of the center pin inside the camera’s power socket using a tiny flat jeweler’s screwdriver to restore a solid 12V connection. [1]
- Power Supply Degradation: Noisy images or sudden communication drops are frequently fixed simply by replacing the wall-wart power supply rather than servicing the camera body itself. [1, 2]
- Modern OS Compatibility: The ST-402 reloads its firmware from the PC upon every single boot. If Windows 10/11 fails to recognize the camera, ensure you have run the Diffraction Limited Setup Driver Checker 4.05 and updated
sbigudrv.dll. [1, 2]
Where to Seek Repair Support
If you have confirmed a physical hardware failure (like a blown board component or shutter failure), use these pathways:
- Diffraction Limited (Official SBIG Support): You can open a ticket on the Diffraction Limited Support Forum to reach their service technician. Note that the manufacturer has explicitly stated they lack replacement parts for many core components of this model. If the internal board or CCD sensor itself is dead, they may not be able to fix it. [1, 2]
- Third-Party Repair (Shoestring Astronomy): For physical damage to the external ports—such as a broken USB 2.0 port or a loose DC power jack—third-party service providers like Shoestring Astronomy (contact via
sales@shoestringastronomy.com) can perform standard component-level soldering repairs without needing specialized SBIG parts. - The “Donor Camera” Solution: Because these cameras are widely used in university labs and astronomy clubs, the most cost-effective path for a dead internal component is often purchasing a cheap, used “donor” ST-402 on forums like Cloudynights and swapping the working parts (such as the valuable internal BVIC/RGBC photometric filter wheel) into the functioning housing. [1, 2, 3]
What specific symptoms is your ST-402 showing? If it’s a cooling failure, a USB connection issue, or an internal shutter/filter wheel jam, I can guide you on the exact troubleshooting steps or whether it is salvageable.
[image][image][image]
6 sites
- ST-402 Seeing Monitor USB Device Not Recognized
Mar 8, 2023 — Colin Haig * Disconnect the device from the computer. * Launch the SBIG Driver Checker and [Remove] the device drivers. * Launch D…
[image]
Diffraction Limited
- SBIG ST402 Astro-Camera - Cloudy Nights
Page 2. Unlike the other cameras in the SBIG portfolio, the ST402 is a single sensor camera, so it can be used as an imager, or an…
[image]
[image]
Cloudy Nights
- ST-402 TEC problem | Diffraction Limited Support Forum
Aug 25, 2025 — Michael, a couple of points: When new, the ST-402 has a Delta T of approximately 25C - so it will cool at most 25 Celsius below am…
[image]
Diffraction Limited
Show all
You said: semi-random linear noise structures
Semi-random linear noise structures in a legacy CCD camera like the SBIG ST-402 are rarely caused by a permanently broken sensor. Instead, these artifacts—often appearing as horizontal or vertical ripples, herringbone patterns, static banding, or “walking noise”—are typically caused by external electrical interference or aging calibration frames. [1, 2, 3]
Because the ST-402 uses an older, slower USB 1.1 protocol wrapped inside a USB 2.0 interface, its analog-to-digital read phase is highly susceptible to electronic noise injected into the ground circuit. [1, 2]
- Ground Loops and Alternating Current (AC) Ripple (Most Common)
If the linear noise patterns shift slightly or change frequency between frames, your camera is picking up stray electromagnetic interference (EMI) or AC ripple from adjacent equipment. [1, 2, 3]
- The Culprit: Cheap or degrading switching power supplies (especially for mount drives, dew heaters, or laptops) can inject high-frequency noise through a common ground. [1, 2]
- The Fix: Unplug all auxiliary equipment (dew heaters, focuser motors, mount power) and run a test frame using only the camera plugged into a separate power outlet or a dedicated DC battery pack. If the lines disappear, isolate the offending device. Replacing an old factory power supply with a high-quality, regulated 12V DC power unit resolves this specific issue on the ST-402. [1, 2, 3]
- USB Data Line EMI
The USB cable shielding on older SBIG units acts like an antenna if it runs parallel to high-current power cables. [1, 2]
- The Culprit: Cross-talk between USB data lines and 12V/110V power lines during the pixel digitization phase.
- The Fix: Snap a ferrite core choke onto both ends of your USB cable. Keep data cables physically separated from power cables, and avoid using unshielded USB extension cables. [1, 2]
- Aging Electronics and Power Sag
- The Culprit: Internal capacitors on the camera’s mainboard naturally degrade over decades, reducing their ability to smooth out voltage ripples before the analog signal reaches the KAF-402ME sensor’s A/D converter. [1, 2]
- The Fix: If the camera performs fine when the TEC cooler is off, but develops heavy linear noise when the cooler ramps up to 100%, the camera’s internal power delivery is sagging. Run the cooler at a conservative setting (e.g., 50% power or 0°C instead of -20°C) to reduce current draw and stabilize the internal boards. [1, 2]
- Software Artifacts vs. “Walking Noise”
If the structures always trend diagonally across stacked images, the issue is not hardware at all. [1]
- Fixed Pattern Noise: If the pattern is strictly identical on every single raw frame, it is Fixed Pattern Noise (FPN). This must be calibrated out using a fresh stack of at least 20–30 dark frames taken at the exact same exposure time and sensor temperature. Old, reused master darks fail to calibrate out changing FPN.
- Walking Noise: If the pattern looks like linear streaks after stacking, it is caused by the telescope’s natural tracking drift dragging the sensor’s underlying FPN across the sky. To eliminate this, you must enable dithering in your capture software (like MaxIm DL or CCDSoft) to shift the telescope by a few random pixels between each exposure. [1, 2, 3, 4, 5, 6]
Are these linear structures perfectly static and identical on every single raw frame, or do they flicker, wave, and shift position from image to image? If you can describe how they behave when you toggle the cooling on and off, we can pinpoint whether it’s a software calibration fix or an AC power issue. [1, 2]
Because the ST-402 uses an older, slower USB 1.1 protocol wrapped inside a USB 2.0 interface, its analog-to-digital read phase is highly susceptible to electronic noise injected into the ground circuit. [1, 2]
- Ground Loops and Alternating Current (AC) Ripple (Most Common)
If the linear noise patterns shift slightly or change frequency between frames, your camera is picking up stray electromagnetic interference (EMI) or AC ripple from adjacent equipment. [1, 2, 3]
- The Culprit: Cheap or degrading switching power supplies (especially for mount drives, dew heaters, or laptops) can inject high-frequency noise through a common ground. [1, 2]
- The Fix: Unplug all auxiliary equipment (dew heaters, focuser motors, mount power) and run a test frame using only the camera plugged into a separate power outlet or a dedicated DC battery pack. If the lines disappear, isolate the offending device. Replacing an old factory power supply with a high-quality, regulated 12V DC power unit resolves this specific issue on the ST-402. [1, 2, 3]
- USB Data Line EMI
The USB cable shielding on older SBIG units acts like an antenna if it runs parallel to high-current power cables. [1, 2]
- The Culprit: Cross-talk between USB data lines and 12V/110V power lines during the pixel digitization phase.
- The Fix: Snap a ferrite core choke onto both ends of your USB cable. Keep data cables physically separated from power cables, and avoid using unshielded USB extension cables. [1, 2]
- Aging Electronics and Power Sag
- The Culprit: Internal capacitors on the camera’s mainboard naturally degrade over decades, reducing their ability to smooth out voltage ripples before the analog signal reaches the KAF-402ME sensor’s A/D converter. [1, 2]
- The Fix: If the camera performs fine when the TEC cooler is off, but develops heavy linear noise when the cooler ramps up to 100%, the camera’s internal power delivery is sagging. Run the cooler at a conservative setting (e.g., 50% power or 0°C instead of -20°C) to reduce current draw and stabilize the internal boards. [1, 2]
- Software Artifacts vs. “Walking Noise”
If the structures always trend diagonally across stacked images, the issue is not hardware at all. [1]
- Fixed Pattern Noise: If the pattern is strictly identical on every single raw frame, it is Fixed Pattern Noise (FPN). This must be calibrated out using a fresh stack of at least 20–30 dark frames taken at the exact same exposure time and sensor temperature. Old, reused master darks fail to calibrate out changing FPN.
- Walking Noise: If the pattern looks like linear streaks after stacking, it is caused by the telescope’s natural tracking drift dragging the sensor’s underlying FPN across the sky. To eliminate this, you must enable dithering in your capture software (like MaxIm DL or CCDSoft) to shift the telescope by a few random pixels between each exposure. [1, 2, 3, 4, 5, 6]
Are these linear structures perfectly static and identical on every single raw frame, or do they flicker, wave, and shift position from image to image? If you can describe how they behave when you toggle the cooling on and off, we can pinpoint whether it’s a software calibration fix or an AC power issue. [1, 2]
Here is a used one on ebay. There is another one too. Buyer beware tho!
Thanks! Everything is new except the ST-402 itself (though the Ioptron HEM44 is 5 years old).
I’ll try some darks with the cooler off and see how the darks look compared to those in my Dark library taken with the cooler from 5 to -10 dedgrees.
At present, Reductions seem to work fine by converting images to 32-bit float and then processing normally with MPO Canopus. Data is consistent with historical data when I check the light curves. So, if it is the capacitor, I’ll likely give it more time before sending if off to check the capacitor. Adding a ferrite core to the USB line would be easy. Nothing changed when I turned off the dew heater. There are lots of other electronics and systems around it at Starfront, so if any of those are causing electrical interference, there is nothing to do about it except continue with 32-bit float processing.
There are a few used ST-402s that come onto the market periodically. However, their ages are similar to my STY-402, and would run into the capacitor life time concern as does mine. They are not very expensive, and it should be easy to swap out the BVIC filter wheel if I purchase one.
Thank you again for the feedback. Best regards. Mike
Here are three raw darks at 15 seconds. The -10 deg was taken in March (about 85% power, if I remember correctly)and I just took two - one at 5 deg and 50% power and one at ambient.
I don’t see any pattern noise in anything, so it appears the capacitor and power supply are fine, though I’m not good at telling the difference.
When stacking 25 darks, the darkest pixels in the histogram rise from about 700 to close to 1000 ADUs while an image may have ADUs closer to 750 or 800 ADUs, resulting in negative pixels after calibration. Best regards. Mike
Target_DARK_C_15.00_32.63_2026-07-01_0000.fits (767.8 KB)
Target_DARK_C_15.00_10.24_2026-07-01_0000.fits (767.8 KB)
DARK_C_15.00-10.20_2026-03-26_0046.fits|attachment (767.8 KB)
Mike,
I’m curious to know why you averaged over 700 darks. You would have negligible additional noise reduction long before you were anywhere that number.
These uploaded darks are single raw darks. For my masters, I averaged 25 raw darks at each temperature (5,0,-5,-10 deg) and integration time that I might use. Best regards. Mike
Perhaps I misunderstood. You wrote about “stacking 756 darks”.
My apologies! Misclick that I did not see with proof reading. I corrected it. Thanks for the heads up. Mike