is illumination from a flat box or other nearby stuff correct for making flats ?

[rotatux]

I'm thinking about trying a bias+flats based process rather than my current darks based one. Bias is no problem. Now I'm thinking about the most simple and hopefully successful way of making flats. In case this matters, I will need to make flats for my proper scopes (130PDS and 127MAK) and also some traditional lenses.

I have searched and read a bit. I consider the following techniques would be accessible to me: white tee-shirt / plastic bag, LCD screen, and light/flat box.

However what striked me is that all those techniques make light sources which are very close to the scope's objective (whether reflector or refractor). At first this seems wrong with the intention of correcting images of light sources at a (relatively) infinite distance, because then the incoming light rays certainly aren't parallel and will also illuminate unexpected places on the camera sensor.

I didn't find discussion on that subject. So, what's the trick I missed, and how (and if, after all) can those techniques actually produce good enough flats ?

Thanks for reading me.

[Davey-T]

Something stuck on the end of the scope works fine, as does aiming camera at laptop screen from a distance and using a twilight sky which is miles away.

Dave

[vlaiv]

It depends on design of OTA if flat box is going to be good enough.

Lens / mirrors don't care whether light is coming from in front of aperture or millions of LY away - they only care about the angle of incident light. All light rays of proper angle will end up on sensor giving a flat field. All other rays will hit somewhere else, and here is the part where design / execution of OTA comes into play - if OTA is properly baffled and blackened to minimize internal reflections - flats will be good, otherwise reflected light will end up on sensor giving brighter image than it should be - flats will over correct.

I use flat box with refractor and with RC - both scopes are baffled good and control stray light, I had no problems with my flats. I suspect that newtonian design is somewhat more susceptible to over correcting flats. Owners should pay attention to minimize impact of stray light - like flocking inside of the tube - it is fairly flat with no knife edge baffles, also blackening any reflective surface like secondary holder, masking off problematic parts of mirrors, etc. Other technique that can be used (also recommended for optimizing visual performance of newtonian) is to extend tube at least 1 - 1.5 times ota diameter above focuser position, again flocked / blackened material should be used.

Only way to know really if flatbox / LCD screen is going to work is to try it out. If you execute flats properly and still end up with over correcting flats - you might want to look at how to optimize OTA rather than flatbox - alternatively you can always use daylight / sky to do flats (but it might turn out to result in similar over correcting if problematic light is few degrees out of optical axes - sky will send such light down the tube as well).

[rotatux]

Thanks vlaiv, exactly the kind of answer I was looking for. So rays of too wide an angle wrt optical axis are supposed to end absorbed by baffling or coating.

Now I can tell where and how my 130PDS has a problem for flats: the painting inside is dark but reflective, and diffusion of strong lights can be seen too just by eye through the focuser without eyepiece. I already made a DIY flocked dew shield, now I need to use it for flats too, and add flocking in the OTA at least under the focuser.