A question of flats

[LewisM]

Is there ANY benefit to running flats for EACH filter, or does it not really matter? I was thinking perhaps any surface imperfections/motes could be more actively eliminated using filter specific flats.

Auto-performing 5hrs worth of darks now (600 sec and 1200 sec) and a run of bias frames (poofteenth of a minute total :) ), then deciding on flat run (still haven't finalised my lightbox, so will do perhaps T-Shirt ones tonight)

[Amaranthus]

Dust motes are the potential issue, but I've found that if I keep the filters clean, it's not really worth taking filter-specific flats. I no longer do it.

[RickS]

The best way to figure this out is to try it, Lewis. Build a master flat for each filter then use one of them to calibrate all the others. If you get a nice featureless result every time then the flats are as good as identical.

Whether dust on the filters will be visible in flats depends on the speed of the scope and the distance between the filters and the focal plane. It's not a problem with my FSQ-106ED but with the Ceravolo at f/9 dust on the filters is all too visible.

Cheers,
Rick.

[gregbradley]

I agree. It depends on dust motes. You may have dust on one colour filter and not another that may show up and wreck an image. They are very hard to remove in post processing.

The best policy with regards to flats and filters is to keep your optics and filters clean and try not to change them over. I am wanting to swap from a 5 position filter wheel to a 7 position filter wheel for this very reason. Every swap potentially lets dust into the filter wheel and onto filters.

Photographic lens cleaning clothes are great for wiping filters and then a blast from a hurricane bulb is usually enough to keep them clean.

So if they are all clean then no usually rgb flats look the same and I have often used one for all 3 colour filters.

Luminance though probably should be its own and so should Ha O111 etc.
Luminance shows up dust motes the most.

Greg.

[LewisM]

Thanks.

I do keep the filters and ccd faceplate spotless and the LP filter screwed onto the FW face keeps dust out too.

Wish I could change to a 7 position FW carousel, but fairly certain SBIG didnt make one for the CFW9... (I am sure Capt. Ward knows for certain)

[multiweb]

Yes, definitely a set of flats per filter. New filter, new flat.

[gregbradley]

Certainly new filter new flat is best practice in case there are differences but if you keep your filters clean you should be able to get away with one flat for rgb and one for luminance. I have done that at times when I couldn't get or didn't have every filter's flat.

I generally though take flats for each filter. If you use dusk flats you need to increase the exposure time a bit for each one as the light fails.

Greg.

[Paul Haese]

I do flats for each filter. I have found that some dust motes can be seen from the filter. It takes only half an hour or so to get those all done with a light box from EXFSO (Peter).

[multiweb]

We spend hours and nights capturing light frames and to get the best results you need to calibrate properly otherwise you might as well dump the lot. A fresh set of flats takes minutes. So I just do them with an EL panel. I understand about keeping optics/filters clean, etc... In real life it never happens. There is uneven illumination, dust motes on filters, on correctors, on lenses, on filters, on CCDs... If you rotate your camera or even refocus then your flats are useless.

They're so easy and quick to do. Why not?

[Paul Haese]

Pretty much my assessment too. Rotation can have a significant effect on flats application. It all depends on tip, tilt, if you are truly on the optical centre with your sensor and the collimation of the scope.

[Amaranthus]

Point is Marc, of all the things you list, only dust motes on filters makes a difference to the original question - whether to do a different one for each filter. The rest of the aberrations etc. you list are dealt with via any flat (which definitely IS essential).

I now typically use a TS filter slider, for various reasons, and so I can't guarantee that the dust motes don't move unless I take my whole filter series in one run (which I sometimes do, most often don't). So in that case, it makes no sense for me personally to do separate flats for each filter. However, I do make sure I give the filter a quick, fresh burst from my air-bulb before I replace it each time! All about compromises... ;)

[LewisM]

Finished my dark and bias master yesterday, then set about fixing the damned column defect (a LONG one). Now that's done, getting NICE calibration, now only needing me to finish the lightbox (using strip white LED's and milk-plastic acrylic sheet from a sign maker) to do the flats.

I ended up with 5 hours worth of darks, and 200 bias frames (maybe overkill :)). Will do another run of darks tonight, and then recombine into a master AGAIN (and remove the column no doubt AGAIN! - one day I will get a Class 1 sensor:) )

The resulting overall effect seemed worth it to me -so much so I reprocessed ALL my paltry IC2944 data again: http://www.astrobin.com/176758/D/