HELP!! What are these donuts?

[EzyStyles (Eric)]

I'm currently imaging right now and noticed black donuts appear in my images? . I think these came out after I removed the IR Filter off the DSI .

Anyone know what these are? (see attached. 2 donuts on left and right)
At first i thought collimation, centre spot?, star out of focus? but it isn't as i have also tried on other objects. these black donuts appears same location.

thanks

[h0ughy (David)]

dust moats

[EzyStyles (Eric)]

phew relief. arhh i see i see. so i simply just blow onto the ccd chip to get rid of it?

[h0ughy (David)]

maybe, probably when you screwed the lens off debris fell onto the ccd. be very careful in cleaning it.

[EzyStyles (Eric)]

thanks h0ughy! will do.


edit: did the trick! blew is very gently and all gone! BUT i killeddd my horsehead neb pic with it just then. was a good one too now my neighbours house is covering it.

[[1ponders] (Paul)]

Ahhh yes... the old dust bunnie problem. If you have a ccd camera then you better start getting used to them. Better quickly start to learn how to do flat fields. You use flat field for correcting the image for defects in the optical train like vignetting and dust bunnies.

btw, be careful blowing dust off your chip. use a camera puffer if you have one. Or a big syringe, but keep it well away from the chip surface. The second last thing you want on your chip is a spit bubble. Nasty when they dry The last thing you want on your chip surface is a nice big scratch from the tip of the syringe

Goes without saying both syringe and blower need to be very clean inside.

[Vermin (Tom)]

What's the best/easiest way to get flat fields?

[iceman (Mike)]

Eddie T wrote an article in the Articles section about flats and darks.

[asimov (John)]

Your lucky they just blew off, I usually have to wash my bunnies off the toucam chip with distilled water & a cotton bud.

[Vermin (Tom)]

Thanks for the pointer Mike.

I was more interested in the methods people use for obtaining flats than the theory.

It seems to me that it would done best before or after taking the main shots (so that dust does not move, focus is the same etc...).

I can think of an easy way to make a diffuse 12V light box that will fit over the end of my 4" refractor, but building the same thing for a 16" Dob poses a few problems, not least of which are size and even field illumination. Any ideas?

[Merlin66 (Ken)]

A clean white Tee shirt over the end and pointing to a bright sky seems to work pretty well!!!!

[Vermin (Tom)]

Unfortunately bright skies are not usually available when doing astrophotography.

[iceman (Mike)]

The flats don't need to be taken at the same time as your exposures. The flats can be done any time, the dust shouldn't be moving around.

[Lester]

I had the same problem with my Neximager, and didn't know what it was. Been to busy lately to use it again.

So thanks for the post and Houghy for the answer.

I named mine onion rings, but donuts is fine, don't give you so much wind.