Whats up with my darks?

[Eruliaf]

Good afternoon all. I have been struggling recently with gradients across my images - which I just put down to moonlight. I managed to get some time imaging last night, and noted that my darks are coming out very strange (see image below).

Any pointers on where this issue is likely to be stemming from - light getting into the scope, camera issues, user error? Or, is this normal and I have still got lots more to learn!

 

Thanks in advance!

 

[happy-kat]

What camera are you using?

[Eruliaf]

I am using my ZWO ASI071MC Colour with the Skywatcher 130PDS. I have a skywatcher comma corrector attached to the camera, otherwise just the scope.

The darks were taken outside (at night) with no specific lights pointing in the direction of the scope, with the scope cap on (i.e. I don't think its light getting into the scope)

I have the temp of the camera set at -10 Deg C and it was holding this well all night.

[ONIKKINEN]

Light leaks, definitely. Since you mention moonlight its probably the moonlight that finds its way to the camera then. If its bright enough outside for you to see your own feet (always) your camera can definitely see this as well. And if leaks exist they will be in the darks.

If your shooting location is not perfectly dark you will get light leaks through all sorts of gaps, especially on a newtonian.

You have 4 main ways for light to get into the sensor:

Front of scope, your cap is probably not light tight even if it appears that way. Back of scope, you have an open mirror in the back and if you dont block this somehow you will get light leaks through there. Actually you want to plug this during imaging as well (as long as the scope is cooled) unless you image from perfect darkness. Gaps between the focuser drawtube and the focuser body, these gaps must exist or your focuser cannot move and some light will find its way through. The fourth possibility would be your cameras tilt plate itself. It can have a little gap from which light could get through.

Take the camera out and plug it somehow completley for darks (put it in a box or something), or they wont really work properly.

[Eruliaf]

Thank you - that all appears to make sense. I will look into this when I am imaging next.

[newbie alert]

Defo a light leak

[Budgie1]

With the likes of the ASI071MC and other cameras like it, you don't need to take your Darks frames during an imaging session because it has set point cooling, so you can recreate the sensor temperature at any time.

You can simply remove the camera from the scope and take the Dark frames when convenient (cloudy days/night), as long as you use the same sensor temperature, gain & offset settings as you use for imaging. You can even build a Darks Library and take, say, 20-30 Dark frames for every exposure setting you're likely to use, then you can just reuse them.

If your camera came with a cap for the sensor, then use that and put it in a box, as @ONIKKINEN suggested.

[Eruliaf]

Re-shot my darks with the camera out of the scope - much better result, however it did make me realise that if the darks when in the scope are impacted by stray light, the lights will be as well. I have evidenced this by inputting the new darks into my images and whilst better, I still have a gradient across them.

So, my next move will be to take dark shots with the camera in the scope whilst inside with the lights on to try and identify where the light is getting in.

Will report back once I get an opportunity to do this.

[jjosefsen]

On 17/02/2022 at 15:26, Eruliaf said:

Re-shot my darks with the camera out of the scope - much better result, however it did make me realise that if the darks when in the scope are impacted by stray light, the lights will be as well. I have evidenced this by inputting the new darks into my images and whilst better, I still have a gradient across them.

So, my next move will be to take dark shots with the camera in the scope whilst inside with the lights on to try and identify where the light is getting in.

Will report back once I get an opportunity to do this.

I found that light was getting in around my focuser tube on my old focuser, so worth checking out.

I've even seen people claim that IR leaks with some screwed adapters, even though I have no idea how that can happen.

[Eruliaf]

@jjosefsen How did you stop the light getting in around the focusser and still be able to focus?

To the wider community - It has also been mentioned that light may be getting in at the bottom of my scope - Can anyone advise how this has been effectively prevented?

Thanks

[Varavall]

On 05/02/2022 at 19:22, Eruliaf said:

Good afternoon all. I have been struggling recently with gradients across my images - which I just put down to moonlight. I managed to get some time imaging last night, and noted that my darks are coming out very strange (see image below).

Any pointers on where this issue is likely to be stemming from - light getting into the scope, camera issues, user error? Or, is this normal and I have still got lots more to learn!

 

Thanks in advance!

 

I have a SW 200pds and I had the same issue as you are having. Light was entering around the primary mirror, so I made a "shower cap" out of black butyl rubber (cut from old tractor inner tube) for the mirror end of the tube. Problem solved.

[ONIKKINEN]

1 hour ago, Eruliaf said:

To the wider community - It has also been mentioned that light may be getting in at the bottom of my scope - Can anyone advise how this has been effectively prevented?

Thanks

Showercap, DIY a cap from cardboard, dustcap that fits your tube etc. 

My VX8 came with caps to both the front and back end of the tube. Doesnt have to be fancy, nobody will notice it in the dark.

[jjosefsen]

1 hour ago, Eruliaf said:

@jjosefsen How did you stop the light getting in around the focusser and still be able to focus?

To the wider community - It has also been mentioned that light may be getting in at the bottom of my scope - Can anyone advise how this has been effectively prevented?

Thanks

I have changed the focuser (for other reasons), but I plan to check for leaks tonight on the new focuser and elsewhere. 

I have seen others make flexible shade tubes around focusers, example: