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Topic starter

1 day ago

Hi guys.

I've hit a wall with persistent bad flats on my Quattro 250P with the TS GPU coma corrector. After a lot of trial and error i think i am getting closer since vignetting appears to be corrected but there is a large ring remaining in the middle of the image. I will provide image examples.

Here is what i did to fix this:

Flocked and blacked out every reflective surface inside the tube, confirmed no light leaks (10-min darks in daylight are clean).
Narrowband flats calibrate perfectly. The problem only appears in broadband.
Tried: panel flats, panel + T-shirt diffuser, twilight sky flats (with and without T-shirt).
Tried: bias-only calibration and dark-flats — no difference.
Collimation seems solid; I don't think that's the cause, though I know it's often blamed for this.
Fixed a similar issue on a previous Newtonian just by flocking — this time flocking alone isn't enough.
Bonus problem: bright stars (e.g. Sadr) throw rainbow-like arcs across the frame.
I am also using a dew shield.

After all these i was unsuccesful.

I suspect the coma corrector itself, but plenty of AstroBin users run this exact scope/corrector combo successfully. Anyone know what's going on, or what they did differently?

Also considering the TeleVue Paracorr (known for better reflection control) as a replacement, but it's expensive — not sure it'd actually fix this.

📷 Before.jpg📷 after.jpg📷 off axis reflection.jpg

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Kristof Vandebeek

1 day ago

I use the TS GPU with a Sky-Watcher 250PDS at f/4.8, and previously used the same corrector with a 130PDS. I have not experienced this issue with either telescope.

My 250PDS also has a dew shield, a three-vane spider, and a mask around the primary mirror. I additionally cover the rear of the telescope because some stray light can otherwise enter there. However, since your daylight dark frames are clean, a major light leak seems unlikely.

My collimation is currently good, but it has not always been perfect. Even when it was noticeably off, I never had problems calibrating my broadband images, so I would also be hesitant to blame collimation for this.

Since you have already inspected and ruled out most causes on the telescope side, could the reflections be coming from the camera, filters, filter wheel, or one of the adapters? Does the ring appear during every imaging session and with every broadband filter?

I have seen similar problems when there was even a very small amount of dew somewhere in the optical system. I store my 250PDS vertically with the primary mirror at the bottom, and I suspect it does not always dry properly. As a result, dew appears more frequently than I would like, sometimes without being immediately obvious.

Check as follows:
1) shoot without a filter: issue gone? filter or filter wheel are involved
2) shoot without the CC: issue gone (you will have coma, not important for this testà): TS GPU issue
3) rotate the camera: issue rotates along? then probably something in the camera
4) bright star issue: what if you move the star to different corners of the image?

Also: what filters do you use? And how fast do you cool the camera?

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andrea tasselli

Frequent contributor

1 day ago

The arc and the other nuisances lead me to believe that it’s the chamfer on the secondary that’s the issue, coupled with the GPU with its long barrel. You can try masking out the outer portion of the GPU, say by few mm from the edge and see what happens (black non-reflective cardboard shall do for the while). The second needs a more complex masking of the secondary, tailored to your secondary size, a sleeve the covers the chamfer and not much more. 3D printing should do it nicely.

mkbn

New contributor

1 day ago

Being a fellow large newt user (gso 12” f4) and having the dreaded ring issue, I will make a hasty bet that it is shifting primary/secondary with sky position especially if paired with bad light pollution, meaning the illumination and reflection profile (esp with the CC) is not consistent between your flats and the mirror positions on the sky for the lights.

Characteristic of some of the above and that I have noticed in my own images and many threads is often a profile with a darkened part of the ring on one side and a raised on the other.

I once read a thread where someone mentioned testing and artificially shifting their well corrected flats over a few pixels before calibration and being able to reproduce this ring issue from a previously flat and well calibrated frame.

You can test what is moving by presenting a laser collimation tool in drawtube and potentially hanging a weight similar to your imaging train off that laser. Slew or move the scope around the sky. If the laser centering on the primary moves around, it’s the secondary changing angles. If the return to the laser collimator moves around but the center laser on the primary remains fixed, the primary is shifting. Edit: another thing you can do is take 3x light frames in L (if mono, just a sensitive channel) and IMEDIATELY take some flats at the exact same sky position, calibrate, see how the stack comes out. It may be better corrected than previous attempts, which is what I saw. Can link a cloudy thread I made if it’s helpful.

It’s something I suggest exploring or ruling out, as I have spent hours looking online to find out about the issue and so many of the threads end up there. I also considered upgrading to the Paracorr from the nexus to fix the issues, and I am sure there would be an easier illum profile to correct, but I can’t convince myself it would solve every issue immediately as I found people with the ring issue and the paracorr online. Aside from that, flocking the inside of the tube both for increased contrast and to cut out on reflections as you already did. Adding an m48, flocked extension tube or similar to the end of many CCs to cut out on stray light entering the CC can help, as is ensuring a dew shield or half sheild prevents light from directly entering the CC from not the direct sky on the short newts is useful. I have done all of that, yet still have issues (tho reduced, and need to fashion the more permanent dew shield). I am moving to a 24” dovetail and additional ring, potentially reinforcing the drawtube to OTA region, replacing primary squishy pads with delrin, etc to combat the issue and finally have an ocal to image the relationship between the mirrors while at different sky positions in the next few weeks.

also, my issue is severely reduced under bortle 4 skies versus my home location of bortle 8. It’s still present for me currently, but much closer to correct without excessive local LP

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