Flats with a Coronado PST?

[AstroManDan]

Hi,

I took some images with my Corondao PST and my DMK21, usually when doing planets or lunar work I take a flat with my EL light panel, as I have lots of dust bunnies on my DMK21.

But how do you take a flat with a PST and a DMK21?

All I got was a black image no matter how long I exposed for.

[Psychobilly]

The PST setup only allows a tiny amount of a specific wavelength of light through and seeing as its designed to look at the sun which we all know is incredibly bright .. I doubt if any "practical" earth bound source is going to enable you to take flats ...

[Photosbykev]

As Pete commented the amount of normal light getting through the PST is very very small so it would be simpler to clean the CCD sensor and be better than taking flats which just mask a problem.

[Merlin66]

The only way is to get enough Ha diffused light to generate a flat.

This usually means de-tuning slightly on an area of the sun with no features and using that as a "pseudo" flat. Works where the field of view is the solar surface (ie with barlow) not so good for a wide full surface image......

[Nomis]

Ken is right... the only way to do it is if you can fill the frame with the sun (i.e. no black bits).

If you can then move the camera to the center of the solar disc (away from any large surface details like active region etc) then don't de-tune as Ken says but fully de-focus until you lose all the surface detail and are just left with a grey "flat" image. In mine I can usually see Newtons rings, the sweet spot and any orthogonal miss-alignment and of course all those dust bunnies.

Then take around 500 frames (remember they need to be taken at exactly the same ICCapture settings as you used to take the original images !)... stack all frames in registax and don't apply any wavelets (but you will still need to click "Do All") then save out as a .png (don't save it as a .tif as registax will only use .png or .bit !)

You can then load this into registax as a Flatfield (via the Flat/Dark/Reference menu option). As normal the flatfield will then be removed from each frame during the stacking process.

You could also make "false flats" by copying the layer then doing a gaussian blur then inverting then subtracting but I found the de-focus method was the only way to get rid of 100% of the dust bunnies !

HTH's

S.

[markt]

I've tried before using a full frame of sun, then, tape a semi translucent tupperware container over the objective and get a flat this way - it works, shows the dust bunnies and any newtons rings etc; but only if there in no movement in the fov between the flat and the actual exposed image. If the 2 images are only slightly out of sync then it can actually emphasise the newtons rings due to constructive interference.

Best bet is good chip hygeine...