A worthwhile short cut to flats?

[tomato]

Another query on calibration frames:

With no observatory and most likely work looming in the morning, I always struggle to take flat frames at the end of the session. If I leave the optical train assembled and the focuser locked, would the flat frames I take indoors be any good, or would there be sufficient movement to render them useless?

How about if I unscrewed the camera and flattener then reassembled them, presumably that's definitely  a step too far?

 

[Alien 13]

I process flats in two ways, the optical defects like vignetting etc I just take a couple remove any obvious sensor defects with PS (dust bunnies) then use them forever, the dust bunnies I take out using the actual subs again in PS.

Alan

[daz]

1 hour ago, tomato said:

Another query on calibration frames:

With no observatory and most likely work looming in the morning, I always struggle to take flat frames at the end of the session. If I leave the optical train assembled and the focuser locked, would the flat frames I take indoors be any good, or would there be sufficient movement to render them useless?

 

No, that would be fine - as long as you don't move any of the components.

1 hour ago, tomato said:

How about if I unscrewed the camera and flattener then reassembled them, presumably that's definitely  a step too far?

Yes!

The whole point of flats is to remove optical defects - vignetting, dust bunnies, etc. If you move part of the optical train, those defects will no longer match the position of where they occur in the light frames. If you try and process the light frames with flats taken in a different position, you'll end up with the original problems PLUS new introduced by the calibration process.

Hope this helps!

[Zakalwe]

Flats should only take at most a few minutes to do. With my camera I have to use an exposure of a fraction of a second, so rattling off 30 or 40 takes less than a couple of minutes.

 

Darks are another matter entirely.

[ollypenrice]

If you're using a mono camera you can usually get away with luminance flats for every filter, so that's a big time saver.

If you don't change anything you can do the flats later, as you suggest, unless a big dust mote displaces itself, which would be unlucky.

It's impossible to overstate the importance of flats, though.

Olly

[tomato]

Thanks a lot for the replies, Zakalwe makes a very good point, if I have my act together taking the flats at the end of the session would not add lot of time at all.

Here is another thought, the camera takes 10-15 minutes to warm back up to ambient temperature, could I take the flats during this period given they are very short exposures compared to the lights, even though the sensor temperature would be changing during this period?

[ollypenrice]

8 hours ago, tomato said:

Thanks a lot for the replies, Zakalwe makes a very good point, if I have my act together taking the flats at the end of the session would not add lot of time at all.

Here is another thought, the camera takes 10-15 minutes to warm back up to ambient temperature, could I take the flats during this period given they are very short exposures compared to the lights, even though the sensor temperature would be changing during this period?

My answer would be no. Any noise in your flats will find its way into the final image. We can de-noise our flats by subtracting a master bias as a perfectly good surrogate dark and by taking a lot of flats to average out residual noise. Don't add noise by turning off the cooler!

Olly

[AlistairW]

Just wondering - does dust on the objective make that much of a difference to flats, or are dust motes more of a factor the nearer the sensor the dust artifact is?

[tomato]

Thanks Olly, your advice is duly noted.

AlistairW, Dust on the objective I think gives the diffuse circles on a flat, dust on the sensor gives a much darker, more distinct spot, but both however, are not diserable when processing your data.