Mysterious diagonal star spikes

Obviously there is a backfocus error, however, I took my image train apart, piece by piece, and found that my guide camera was slightly rotated by about 2-5 degrees! Now that could cause guiding errors right? I know that guide camera orientation doesn't matter because you can always recalibrate, but the guide camera's sensor should always be aligned or perpendicular to the prism/guide scope, so that guide pulses move the mount up/down or left/right, NOT diagonally. Perhaps this is my issue.

Not really. During calibration PHD2 will determine which direction is RA and which is Dec. I assume the guide camera is in focus across the field.

I used to have someting like that and eventually I connected it to thermal currents inside the tube.

Your defocused star looks really symmetric.  I agree with Andrea.  A baffle plume can easily cause the things that you are seeing.  Do you have any tube fans running?

John

John Hayes:
Your defocused star looks really symmetric.  I agree with Andrea.  A baffle plume can easily cause the things that you are seeing.  Do you have any tube fans running?

John

My OTA doesn't have ventilation nor fans .  Could it be tilt?  I ran that image through ASTAP.  The strange part is that those star spikes are the same angle across the frame.  I would think if it was back focus then they'd all point towards the center.
 

Jason Schella:
My OTA doesn't have ventilation nor fans .  Could it be tilt?  I ran that image through ASTAP.  The strange part is that those star spikes are the same angle across the frame.  I would think if it was back focus then they'd all point towards the center.

Sensor tilt will cause a field dependent focus error and that's not you asked about.  You may have a slightly tilted sensor but that won't fix your "oval" stars or the spike that you see running through them.  So, sensor tilt is not causing the problem that you asked about.

How old is your OTA?  A number of years ago, Celestron had a bad batch of glass that they used for the corrector plates.  It had what are called "pull lines" and that can be a common issue with float glass.   It creates linear striations of varying index over the corrector.  The issue is minor and initially they couldn't see any issue.  However, the striations create unidirectional, weak diffraction patters on bright stars--a little like what I see in your stars.  If you look carefully, you can see the striations in a defocused star image, but they are easy to see if you knife edge test the system on a still night against a bright star.  Celestron has since fixed that problem and I doubt that that's what you are seeing but it's worth considering.  I think that a baffle plume might be a prime suspect and I'd start by either confirming or clearly eliminating that idea from consideration.

John

John Hayes:

Jason Schella:
My OTA doesn't have ventilation nor fans .  Could it be tilt?  I ran that image through ASTAP.  The strange part is that those star spikes are the same angle across the frame.  I would think if it was back focus then they'd all point towards the center.

Sensor tilt will cause a field dependent focus error and that's not you asked about.  You may have a slightly tilted sensor but that won't fix your "oval" stars or the spike that you see running through them.  So, sensor tilt is not causing the problem that you asked about.

How old is your OTA?  A number of years ago, Celestron had a bad batch of glass that they used for the corrector plates.  It had what are called "pull lines" and that can be a common issue with float glass.   It creates linear striations of varying index over the corrector.  The issue is minor and initially they couldn't see any issue.  However, the striations create unidirectional, weak diffraction patters on bright stars--a little like what I see in your stars.  If you look carefully, you can see the striations in a defocused star image, but they are easy to see if you knife edge test the system on a still night against a bright star.  Celestron has since fixed that problem and I doubt that that's what you are seeing but it's worth considering.  I think that a baffle plume might be a prime suspect and I'd start by either confirming or clearly eliminating that idea from consideration.

John

I bought this OTA in Feb 2020.  I went through older subs before I installed the Celestron heater ring and was using a dew strap, and didn't see any spikes, stars weren't round, but no spikes.  

Jason Schella:

Jason Schella:
If you are using the Celestron Dew Ring, it could be the current is too high. I had the exact looking stars when the ring was too hot. 

Jason

I am using the Celestron heater ring.  I could lower the aggressiveness of the heat in the UBP Control Panel.

* I found that I could only run it at <0.9 amps. Any higher and the spikes would start to get larger. More heat, bigger spikes. 

Try it with the heater off, take a pic, turn the heater on (your original settings) let it heat for a couple of minutes, then take another picture. This should tell you if that's the cause. 

Good luck, 

Jason

Was .9a enough to keep the dew off?

I turned the dew ring power down, the "aggressiveness" setting from 8 to 5, and the spikes were exchanged for dew on the corrector plate.  Next time I'll try putting my dew shield on and keep the dew ring power down, see if that does anything.  I emailed Celestron about the issue.

I saw that aluminum dew shield.  Wouldn't that create even more turbulence as the entire shield will be heated?

Well I have the flexible shield so I'll use that.  Celestron oversight strikes again, they didn't really make that public knowledge available in the instructions for the dew ring, like "make sure you use this with a dew shield and low power or else the glass will get too hotamd cause image distortions. 🙄

Useful thread. My star shapes have gone to hell since I put the Celestron dew ring on my C8+Starizona. I’m controlling it from NINA with a Pegasus universal power box and am struggling to get a power management system that avoids dew while keep the stars incorrupted. Auto dew control seems to put through a significant current even when the air temperature is several degrees above the dew point even with no positive bias. Going to try manual control and keep the current down to a steady level. I’ve got the shield on. Dismayed to see comment that Celestron’s own dew controller might be needed.

Celestron should have said they "recommend" their controller. I get those spikes, too, but have found they can be reduced using the Pegasus controller by simply turning it down until the spike disappear. The Celestron box does this automatically in all conditions, but it is exorbitantly expensive in my view.