Stray light from LEDs - an underestimated problem in astrophotography

Hello fellow astronomy enthusiasts, or those who aspire to be,

The continuation of the story about building my own observatory has to be postponed for the time being because another issue is currently urgent and takes priority.

It concerns stray light emitted by LEDs (specifically infrared LEDs) which, when viewed from the back of the Newtonian telescope in certain positions, leads to disastrous images, as the following pictures show.

The day before yesterday, I encountered a problem during my photography that I hadn't experienced before. Photos that previously looked like this...

 

They suddenly looked like this without any changes...


    

I must preface this by saying that, for the purpose of remote monitoring, I have a video camera located in the telescope room.



Using the photo data from my full-frame camera, as well as the monitoring of the control states of the ASIair, I have set up a system of 4 mesh adapters, which allow me to monitor this data from every corner of my property....


 This system worked perfectly until the day before yesterday, March 29, 2021, when I suddenly noticed these "strange flats" in my photos.

I started investigating the cause. I inspected the Newtonian telescope – the primary mirror, shining a light into it, no dirt, leaves, or small animals on it. I also examined the secondary mirror – everything was fine there, and the mirror was secure. Sometimes similar phenomena can occur if the secondary mirror is a bit loose and then tilts due to a certain rotation or position of the tube. But as I said, that wasn't the case here.

So what to do? My friend from Munich, Karl Kloss, suggested... "Try taking some flats against a blue sky or a bright white surface..."


     
No sooner said than done, but different gain and exposure time settings didn't produce anything noteworthy.

The night of March 30th/31st, 2021, was another window of opportunity for photography. Setup was perfect, polar alignment was perfect, and focus was spot on.

I slewed to the M81 region in the Big Dipper and...

 Unbelievable! I was pretty annoyed because I had no explanation, so I called my buddy Karl, who looked at the picture and said... "That looks like stray light interference to me..." Stray light interference???

I only put a board in the clearing a few days ago because of the excessive glare from a streetlamp...


     
It can't be coming from there. But where could it be coming from then?

Karl suggested maybe an LED... I've had quite a few of those, especially power supplies, or the mount's connector, and so on. I guess I've been lucky so far that they were in a position where they couldn't pose a danger. But the danger lurked... in the unseen... infrared LEDs... surveillance cameras...


This nice cone of light is the stray light emitted by my infrared camera. It's only because I haven't focused on certain areas for a while, and because the angle of incidence and emission didn't hit the back of the telescope tube and spread inside, that I've been spared this interference so far.

I switched off the camera and checked again... all good.

So, dear astrophotography enthusiasts and those who aspire to be... take a close look at any small light sources on or near the mount that might be emitting light, as they could be ruining your images.

A quick fix is ​​to put a "winter cover" (as Karl calls it) on the back of the tube. That should eliminate the biggest problem. Covering some LEDs with tape is also an option, but in extreme cases, this could lead to them burning out if the heat radiation is too intense and directed inwards.


So, dear astrophotography enthusiasts and those who want to become one... Dear Karl, once again my heartfelt and most sincere thanks for your calm demeanor, your tireless efforts in helping, troubleshooting, and fixing problems, and for remaining the epitome of courtesy and decency despite your enormous workload, which you currently have even more than usual, and the occasional disgruntled or unfriendly person you encounter throughout the day.

Kind regards, Axel