Please don't skimp on your flats - Forums

Topic starter

Apr 18, 2022 · Edited: Apr 18, 2022

Two days ago I was trying out my new OAG and decided to do a quick capture of M81/M82. I thought: oh well, I haven't changed that much, so I just used old flats and darks. What was I thinking? I ended up with weird dust bunnies, vignetting and gradients everywhere. I'm actually too embarassed to show you the results.

Bottom line: last night, at the end of my session, I took some new flats. Plonk the LED tracing board on the top of my scope which I pointed straight up, set it to its lowest light level, press start on my APT flats routine and within 6 1/2 minutes I had 250 flats covering LRGB and Ha. That's only a little bit more time than taking one Ha frame!

The result: almost all vignetting, gradients and dust bunnies immediately disappeared.

Note to self: take flats every night even if you don't end up using them.

As for the darks: maybe every 3 months or so.

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Christopher Davenport

Apr 18, 2022

Agreed, although i only take flats per session. (i.e if the weather holds for 3 or 4 days and i don't tear down, then i don't retake them.)
Quick question though, what benefit is there for taking say 50 flats vs say 10 flats?

Thanks in advance.

Andy Wray

Topic starter

Apr 18, 2022

Christopher Davenport:
Quick question though, what benefit is there for taking say 50 flats vs say 10 flats?

Probably not that much, but they take so little time to take and they only get processed once into masters that I guess I feel it can't harm to have a few more.

GalacticRAVE

Apr 18, 2022

Well, one learns over and over again (and for myself: guilty as charged) there are no shortcuts in our hobby!

Matthias

Stuart Taylor

Apr 18, 2022 · Edited: Apr 18, 2022

Andy Wray:
Two days ago I was trying out my new OAG and decided to do a quick capture of M81/M82. I thought: oh well, I haven't changed that much, so I just used old flats and darks. What was I thinking? I ended up with weird dust bunnies, vignetting and gradients everywhere. I'm actually too embarassed to show you the results.

Bottom line: last night, at the end of my session, I took some new flats. Plonk the LED tracing board on the top of my scope which I pointed straight up, set it to its lowest light level, press start on my APT flats routine and within 6 1/2 minutes I had 250 flats covering LRGB and Ha. That's only a little bit more time than taking one Ha frame!

The result: almost all vignetting, gradients and dust bunnies immediately disappeared.

Note to self: take flats every night even if you don't end up using them.

As for the darks: maybe every 3 months or so.

Re-using flats? really?

I always take flats after every sequence because there is always something different in the optical train (slight smudges on the camera window, different focus etc etc etc. Nothing is ever the same)

But darks are only a function of the camera, so I just use those from a library (they can be made during the day, when you have nothing better to do. Just make sure you create them at all the gains and exposure times you will need). Definitely no need to do those each session.

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andrea tasselli

Frequent contributor

Apr 19, 2022

Christopher Davenport:
Agreed, although i only take flats per session. (i.e if the weather holds for 3 or 4 days and i don't tear down, then i don't retake them.)
Quick question though, what benefit is there for taking say 50 flats vs say 10 flats?

Thanks in advance.

In calibrating the light frames the division by the master flat (renormalized) is the one that brings in noise the most. So, having a very large set of flats to create a master flat makes sense in order to minimize that contribution.

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kuechlew

Apr 19, 2022

For those who are not blessed with a permanent rig it's good practice to arrive at location well before the stars get out. Gives your equipment time to adjust to the temperature and to take some calibration frames, mainly flats. Dark libraries are difficult for non-cooled cameras where you may struggle to reproduce the same temperature of the sensor. With my mirrorless camera I let it take darks on my way home while it's sitting in my backpack. It helps to have an intervalometer in your camera. So usually I have a full set of fresh calibration frames for each session. And yes I learned that the hard way too …

Clear skies
Wolfgang

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Robert Winslow

Apr 30, 2022

I think we all did this, heck when I first started I did not even bother wit any calibration frames. Now, I take flats each session, this can be a bit of a pain when you are using filters, as you have to have flats for each image train.

Flats do not take tat much time, and now I take Flats, Dark Flats, as well as Bias.

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