What is this dark circle?

[Damien2904]

Hello everyone,

I hope you can help me, I'm just getting started with Deep Sky imaging from scratch and recently I've been practising using my Canon EOS 5D Mark III with my scopes.

At first I tried using my SW SkyMax 180PRO as it was the only scope I had at the time (I'm aware that this scope isn't ideal for Deep Sky), and after taking various images I noticed that the images were coming back with a dark circle around the centre.

I've very recently purchased the SW Evostar 80ED PRO with a .85x reducer, I did a couple test shots using the Canon & noticed the same thing but further out. Originally I thought this maybe my dew shield on the SkyMAX, or possibly the telescope barrel...but when practising DSS I noticed that stars are appearing outside the dark circle so presume it can't be either of those things. Is this vignetting? Ultimately...can I stop this happening? 

I've included some images for you to see what I'm referring to...

SkyMAX images 

 

Evostar 80ED - image of Vega on a night with lots of high cloud looking in a heavily light polluted direction.

Please let me know if you know what's happening here.

Thanks for reading. 

Damien 🙂

[vlaiv]

That is vignetting.

Telescopes can illuminate only limited part of focal plane. At some distance from the center - amount of light reaching it from objective simply starts to fall and ends up at 0 (due to construction of telescope and internal baffling).

This is normal thing. You are using very large sensor - it is full frame sensor with diagonal ~43mm. Most telescopes can't fully illuminate such a large sensor.

You need to do two things here:

1. Use flats

2. Crop to sensible size.

Even with flats that correct vignetting - you don't really want to use far corners of the frame. In fact - you probably want to limit your image to area that receives 80% or more light. Anything lower than that will have lower SNR and it will start showing as increased noise after flat fielding.

 

[alacant]

13 minutes ago, Damien2904 said:

SW Evostar 80ED PRO with a .85x reducer

Hi

I doubt whether either combination will cover full frame very well. You could try flat frames and/or lose a margin around the edges.

Cheers