What's causing this strange pattern of doughnuts ?

[LandyJon]

I have an Edge HD 9.25 with the Celestron OAG and v2 adapter followed by a filter drawer with LPF then a few 48mm extensions to set correct back focus in to an Altair 294C  pro.

It's been collimated accurately, but I can't understand what's causing this strange pattern of doughnuts ...

 

Any ideas, much appreciated,

Many thanks, Jon.

[vlaiv]

Micro lens on pixels combined with some other reflecting element that is a bit further away - like a filter.

It will be only visible on very very bright source with very long exposure times.

[LandyJon]

2 minutes ago, vlaiv said:

Micro lens on pixels combined with some other reflecting element that is a bit further away - like a filter.

It will be only visible on very very bright source with very long exposure times.

So you think it's reflection back off the pixels bouncing back from my LPF maybe ?

I could try moving my filter drawer back up the train a bit, that might widen the spacings and remove some of them from the sensor at least.

It is only visible on bright stars when I'm focusing or when there's one in frame, but it's not restricted to long exposures, this one was only 5s.

Thanks for the help

[vlaiv]

1 hour ago, LandyJon said:

So you think it's reflection back off the pixels bouncing back from my LPF maybe ?

Yep.

1 hour ago, LandyJon said:

I could try moving my filter drawer back up the train a bit, that might widen the spacings and remove some of them from the sensor at least.

You could try calculating reflection distance using one of online calculators - by measuring diameter of reflections and using F/ratio of the system.

Maybe repositioning filter by small amount will be enough to upset interference pattern that lead to reflections?

1 hour ago, LandyJon said:

It is only visible on bright stars when I'm focusing or when there's one in frame, but it's not restricted to long exposures, this one was only 5s.

It really depends on how bright source is.

5s of magnitude 0 star is equal to 500s of exposure of magnitude 5 star (x100 intensity for 5 magnitudes of difference).

[LandyJon]

Brilliant, thanks vlaiv, I'll try some different placements for the filter and see what happens.

Thanks again for your advice.