Good flats?

[mgutierrez]

Hi all,

I would like to know your opinion about the following flats.

Taken with a tablet with an uniform and white background, covered with a stretched white shirt:
https://www.dropbox.com/s/q4fguzi15eqv6 ... 2.nef?dl=0

Same config, but without white shirt (directly with the tablet):
https://www.dropbox.com/s/0256qpau0btbk ... 2.nef?dl=0

I don't know if both are ok, or wrong, or one is better than the other. To be honest, I don't notice many difference between them, even in the stacked result process.

Thanks for your help.

[admin]

It's sort of hard to say whether a flat is "good". It is "good" if it adequately describes any uneven lighting happening in your imaging train.

Does the master flat correct uneven lighting correctly in your stacks?

[mgutierrez]

It's sort of hard to say whether a flat is "good". It is "good" if it adequately describes any uneven lighting happening in your imaging train.

Does the master flat correct uneven lighting correctly in your stacks?

Thanks for the reply Ivo. To be honest, I don't notice a big difference whichever combination I use. Maybe because I got only a few lights. I thought that a good/bad flat was easy to identify, but you're right, it makes sense. I will try again with a larger imaging session. Thanks!

[Stefan B]

Hi everyone!

During the last imaging/processing sessions I encountered a problem with my flats. Applying the flats resulted in a very bright stack and in heavy over correcting of the vignetting. Here's what the first autodev looks like:

overcorrecting.JPG (148.22 KiB) Viewed 6127 times

Using a different set of flats from September 2020 and stacking with the same paramters in DSS gave a nice result, i.e. a rather flat image:

oldflats.JPG (193.37 KiB) Viewed 6127 times

Both sets of flats were shot at ISO1600 on the same telescope, using the same filter (l-eNhance), with the same light pad and the same camera (EOS 1100Da). Stretching the Master flats I can even see that the dust spots are still in the same places (fortunately...). The exposure time for the single flats was 0.03 sec. I know that this is pretty short, but that was what the flat wizard in APT came up with in September 2020. And it worked well for me so why not? The second set of flats that did not work was generated with NINA. But the hardware was the same (same filter, telescope, camera, light pad) as were the parameters (ISO1600, 0.03 sec).

The Master flats from APT and NINA, respectively, are provided here:
APT (working flats): https://www.dropbox.com/s/rk79ipi7l3dbt ... T.tif?dl=0
NINA (not working): https://www.dropbox.com/s/wyvh4orujhkxp ... A.tif?dl=0

Can anyone see what is wrong with the NINA flats? I now had this problem reproducibly with the NINA flats being somehow deficient (and dark flats making it even worse by the way) and APT flats being fine.

Of course I can always turn to APT for the calibration frames after the imaging session, but that's not very comfortable (disconnect/connect again, set up new flat plans, etc.). Alternatively, I have to use my old flats forever and can never again clean the sensor

Advice would be greatly appreciated!

Best regards
Stefan

[almcl]

I'll be very interested in replies to this because I've got a very similar problem, but with an ASI 2600 MC. My flats (taken with an EL panel using APT's flats aid) are also over correcting.

I have found that stacking with Flats, Dark Flats and Bias frames in DSS greatly reduces the problem. Unfortunately you can't do that in ASTAP because it doesn't provide for Bias frames, instead suggesting that either Bias or Dark Flats be used. That doesn't work for me at the moment so I can't use ASTAP.

[mgutierrez]

Hi guys,

I would like to take the opportunity and ask about how are you taking flats with l-enhance. Mine shows an histogram with the channels quite separated:

post-352016-0-19468500-1614198992_thumb.jpg (17.98 KiB) Viewed 6110 times

is this normal?

[Carles]

Hi,

I wonder if you are using the same Bias frames (so, old Bias frames) with the new flats ?
I had similar problems with this. I think Bias frames and Flat frames are very connected
so when using Bias, the Bias frame gets calibrated using Flats (I think ) and when using new
flats with old bias, it can overcompensate. O at least that seemed to happen to me.

For me the solution was to start using Dark Flats (i.e, same exposure as Flats, same temp
as Darks), and no Bias.

Had no problems since. In case you're using the whole new set of Calibration frames, forget
what I've said! haha But if not, try with new Bias frames, fresh Flat frames and do the stacking
of them.

btw, I also use NiNa, and usually the Flat frame Wizzard works good with default values.

Regards

Carles.

[Stefan B]

so when using Bias, the Bias frame gets calibrated using Flats (I think ) and when using new
flats with old bias, it can overcompensate. O at least that seemed to happen to me.

Hi Carles,

not sure if bias frames get calibrated with flats or rather the other way round...?

For my session with the problem no bias frames were used at all. Only flat darks, which was the solution for you, right? But I used bias for the flats which worked. I definitely should look into that!

The flat wizard in NINA didn't work for me. It said it couldn't reach enough way up the histogram. But it had 20 sec of exposure on a light pad with the strongest illumination possible. Something wrong there...

I will try taking different combinations of calibration frames with different software at the next opportunity. Thanks!

[mgutierrez]

Well, actually my problem is that I don't know how to take flats with this filter. I was told that good flats should have an equilibrated histogram, quite centered. Without filter I managed to get that. But with the filter the channels are quite separated so I don't know how to center them in the histogram

[Carles]

so when using Bias, the Bias frame gets calibrated using Flats (I think ) and when using new
flats with old bias, it can overcompensate. O at least that seemed to happen to me.

Hi Carles,

not sure if bias frames get calibrated with flats or rather the other way round...?

For my session with the problem no bias frames were used at all. Only flat darks, which was the solution for you, right? But I used bias for the flats which worked. I definitely should look into that!

The flat wizard in NINA didn't work for me. It said it couldn't reach enough way up the histogram. But it had 20 sec of exposure on a light pad with the strongest illumination possible. Something wrong there...

I will try taking different combinations of calibration frames with different software at the next opportunity. Thanks!

Hi Stefan,

not sure if bias get calib by flats or the other way around, but certainly the dark flats and flats have to be taken with same exposure time.
if you used the old dark flats they're not gonna work with the new flats. But I guess you take that into account.
Many people mention that is better off using dark flats than bias. And for me it did the trick.. worth trying, but always with same temp and exposure times as flats and darks .

And is weird about the NiNa problems you're encountering. Did you try playing with the % of brightness needed or the minimum and maximum exposure times?

Hope you can sort it out

Carles