Flats

[Benjamin]

Just wondering if I need to take full aperture flats on my 8" Newtonian or if I could just stop down the aperture (via the dust cover) and use my ed80 lightbox and expose longer until hitting the 50% mark on the histogram? Any issues I'm not aware of in doing this? Wouldn't get any effects from spider vains I guess (which might change something...)?

[RickS]

At the very least, I think you'll find that flats taken with an aperture stop won't correct for vignetting correctly.

Cheers,
Rick.

[Benjamin]

My goal is to avoid trying to make a light box, which with my skills, would be a disaster. I'm often setting up and packing down in darkness or near darkness so working out how else to get flats? My laptop screen is a little too small.

[Benjamin]

Thanks for this as I wasn't really sure where the vignetting would come from in a Newtonian. I had assumed it would be more at the camera end. Cheers :-)

[RickS]

An EL panel is an option. I have one from http://www.gerdneumann.net that I used to use to take flats for my 300mm astrograph.

These days I mostly use dawn and dusk flats. They seem to do a better job of calibration and I can take them when the scope is idle. Having software to automate the process (ACP in my case) certainly helps though...

[Benjamin]

It seems elec2go do an A3 EL panel kit that might do the job. Thinking of just sandwiching the panel between some sheets of corflute, with a whole for the aperture of the scope on the luminous side (a flat lightbox that should stay on the scope). I'm thinking I won't need some kind of light diffusing panel, but guess I could overly that if required? Thanks for all the advice :-)

[StuTodd]

Or use the sky for "twilight" flats...free and less messy..

[LightningNZ]

Do you have a 22"+monitor around? That's what I use. I just set it to white and adjust my exposure accordingly. I move the scope around to avoid any fixed pattern effects from the screen.

[RobF]

Point it at a white dimly lit wall inside with a white t-shirt stretched over the top Ben. Even better, white T-shirt and dawn or dusk flats (even no T-shift if the sky is dark enough, but you'll have to avoid bright stars.

[Benjamin]

No 22" screens sadly, and mostly I felt it better to take flats (at least with my ed80) post imaging so all dust bunnies are accounted for and focus and camera orientation are correct. I do this with a lightbox while I pack down other visual equipment. Quite often I can't get out until it's dark, thus missing that wonderful twilight period. In anticipation of using my 8" Newt for imaging I see much the same situation arising. However if I could establish a rough focus and rough camera orientation would this be enough to get good flats at twilight or is more exactitude required? My setup is non-permanent.

[RickS]

Works well enough for me.

Cheers,
Rick.

[Benjamin]

Great. When the option is available I'll give the twilight flats a crack. Love your images too Rick. Astounding.

[skysurfer]

This also works fine.

Put a white cloth in front of the objective, tightly (i.e. without wrinkles or folds) over it and point at a white wall and focus the scope the same as you were taking astrophotos. Take the pictures using the metering plus two stops.