Why do my flats look like this

[Jjmorris90]

Using the white t-shirt method and the sky as a light source.

The light never seems to be directly in the centre.  No matter what I do. Perhaps a secondary mirror issue?

Also what are those donuts. The camera is in focus.

Lastly the image has been autostretched.

Asiair

Asi183mc pro

[Shimonu]

This looks like a decemt flat frame to me, what are you expecting? It's showing the vignetting and dust particles causing the donuts.

[Jjmorris90]

2 minutes ago, Shimonu said:

This looks like a decemt flat frame to me, what are you expecting? It's showing the vignetting and dust particles causing the donuts.

I don't know what I was expecting lol. Shouldn't it be more centered? 

 

How can I get rid of the dust?

[Shimonu]

1 minute ago, Jjmorris90 said:

I don't know what I was expecting lol. Shouldn't it be more centered? 

 

How can I get rid of the dust?

The point of the flat frame is to measure the uneven light across the sensor. The point is that these frames are then used to remove the dust artifacts you'll have in your picture so you don't have to remove the dust, because you're unlikely to remove all the dust everywhere and keep it clean. What is the result like when you have applied the flat frame?

[Adam J]

1 hour ago, Jjmorris90 said:

Using the white t-shirt method and the sky as a light source.

The light never seems to be directly in the centre.  No matter what I do. Perhaps a secondary mirror issue?

Also what are those donuts. The camera is in focus.

Lastly the image has been autostretched.

Asiair

Asi183mc pro

In essence yes your collimation is slightly off if the vignetting is not centred in on the sensor. However, it could also caused by a filter not being correctly centred in a wheel. Although is this is an OCS that is not likely the explanation? 

Adam

[robin_astro]

1 hour ago, Jjmorris90 said:

 

How can I get rid of the dust?

The flat is there to deal with them but if you want to track them down you can work out which surface they are on from the size using eg a calculator like this

https://astronomy.tools/calculators/dust_reflection_calculator

Cheers

Robin

[skybadger]

I don't think Ive ever seen a centred flat on all my collimated scopes. it probably indicates a small degree of camera droop and sensor misalignment in the camera. They are only positioned according to the PCB fasteners. 

The donuts are out of focus dust shadows showing umbra and penumbra if they are small and a penumbra only for further away ones. The size is determined by the focal ratio and the distance. 

 

[Jjmorris90]

So do I need to do anything about it being off centre?

[The Lazy Astronomer]

4 hours ago, Jjmorris90 said:

So do I need to do anything about it being off centre?

What do your lights look like? If all good, then no. 

Bear in mind when you auto stretch a flat frame, it greatly exaggerates the appearance of the vignetting. On the linear image, there likely won't be that much difference in brightness between the centre and the edges. 

[Jjmorris90]

This is after stacking. They are very visible

[Jjmorris90]

I just discovered my camera lens is covered with dust. I blew it off and took another flat and no sign of these donuts. Though I assume they will be present in every image taken up to this point ?

[teoria_del_big_bang]

When I first looked at my stretched flats I thought they were horrendous, several big donuts, some visible on all filters, some specific to each filter, and offset vignetting, just like yours.
But after the initial surprise when I started to look at them in Pixinsight I could see that the actual difference in ADU between the almost black after stretching in the corners of the frame and the light area in the middle was actually very small, and the difference between the donuts and the lighter areas even less. It is just the fact that flat frames when auto-stretched are actually stretched so much to differentiate between these areas.

As states in above replies the real test is whether the flats improve the final stacked image and removes the donuts and most of the vignetting. If it does then no t to worry, if it doesn't then more than more than likely either something has moved in the image train between the lights and flats, or the calibration of the lights or flats has not worked correctly.

Steve

[Jjmorris90]

30 minutes ago, teoria_del_big_bang said:

When I first looked at my stretched flats I thought they were horrendous, several big donuts, some visible on all filters, some specific to each filter, and offset vignetting, just like yours.
But after the initial surprise when I started to look at them in Pixinsight I could see that the actual difference in ADU between the almost black after stretching in the corners of the frame and the light area in the middle was actually very small, and the difference between the donuts and the lighter areas even less. It is just the fact that flat frames when auto-stretched are actually stretched so much to differentiate between these areas.

As states in above replies the real test is whether the flats improve the final stacked image and removes the donuts and most of the vignetting. If it does then no t to worry, if it doesn't then more than more than likely either something has moved in the image train between the lights and flats, or the calibration of the lights or flats has not worked correctly.

Steve

 

 

Thanks for the reply. They did not calibrate out 

[Jjmorris90]

Here is the final image