Flats with ASI1600mm Pro

[Adam1234]

Hi all, 

I took images of M27 in Ha the other night (aiming to produce a HaRGB composite image, working on the Ha while the moon is up). I'm just after some advice to make sure that my flat frames with the ASI1600mm Pro are as best they can be. 

The below images are the stacked Ha images (20x 300s), the first calibrated with darks but no flats, the second calibrated with darks, flats and dark flats (with slight curves and levels adjustment applied to both). Stacked in DSS. At a first glance they do seem to have improved the vignetting, but I still feel further improvement could be made? Not sure how easy it is to see the vignetting on here so I've attached the original stacked tiff files. 

Is my master Ha flat too dark? I used the APT flats aid to take the flats, using a target ADU of 12000. Is this about right for target ADU or should I aim for higher? With this target ADU the exposure that APT decided for my flats was 1.5s. I used the white t-shirt method and white laptop screen as an illumination source. 

Darks, no flats

Darks, Flats, Dark Flats

Master Flat

TIFF files

MasterFlat_Gain200.tif

Ha_Dark_NoFlat.tif

Ha_Dark_Flat_FlatDark.tif

 

Thanks all.

Adam

[groberts]

I'm certainly no expert in the science of calibration but following advice a few years ago, I set ADU at 25,000 using APT and find it works well. 

So I would suggest ADU 12,000 is way too low and your flats are too dark.  

[inFINNity Deck]

Hi Adam,

be careful when it comes to using short exposure times for ZWO ASI1600 cameras. Depending on the type of 1600 you have and the USB interface that is used (USB2 or USB3), there is a tipping point at which the camera used global shutter or rolling shutter. You may find the following thread of interest: https://www.astroforum.nl/forum/astrofotografie/beeldbewerking-methoden-en-technieken/1438084-daar-is-dat-lastige-onderwerp-weer-flats?p=1438404#post1438404

It is in Dutch, but opening it in Chrome should provide a decent translation.

Summarizing, the Pro editions of the 1600 do not have a tipping point, but all the others (ZWO ASI1600, ZWO ASI1600 Cool, both the MM and MC models) do (2s  at USB3 and 5s for USB2). The method used causes the flats to be skewed (darker on one side than on the other).

You stated that you use a ASI1600 Pro, so you should be safe.

Nicolàs

[Spaced Out]

5 minutes ago, groberts said:

I'm certainly no expert in the science of calibration but following advice a few years ago, I set ADU at 25,000 using APT and find it works well.

Nor me but I am just going through the same learning process with the ASI 1600 and I have read up a little bit online, it generally suggested ADU 25000 for flats using APT so that is what I have used on my two imaging sessions so far !

[Adam1234]

Thanks for the advice all. I will retake my flats later and try 25000 adu and see how they look. I'll have a look at that thread as well. 

This is the version of the ASI1600 I have https://www.firstlightoptics.com/zwo-cameras/zwo-asi1600mm-pro-usb-3-mono-camera.html

What exposure time do you think I should be aiming for?

[vlaiv]

34 minutes ago, inFINNity Deck said:

Summarizing, the Pro editions of the 1600 do not have a tipping point, but all the others (ZWO ASI1600, ZWO ASI1600 Cool, both the MM and MC models) do (2s  at USB3 and 5s for USB2). The method used causes the flats to be skewed (darker on one side than on the other).

I use flat exposures in milliseconds and never had issues with my ASI1600 - it is cool model - one released before pro (I guess it was version 2 overall).

[inFINNity Deck]

52 minutes ago, Adam1234 said:

What exposure time do you think I should be aiming for?

I would recommend to stay at least well above 1 second (2 seconds for non-Pro models). Not only for the effects described above, but also to avoid interlacing as a result of the flat panel's (laptop's screen) flicker rate. When short exposures are taken interference will occur between the exposure length and update rate of the flat panel, resulting in interlacing effects (horizontal bands) in the flats.

The skewing due to low exposure times in non-Pro models will not be very obvious when doing single panel work or when using processing software that deals with uneven illuminated subs, but it is better to avoid.

Nicolàs

[Starwiz]

1 hour ago, Adam1234 said:

Thanks for the advice all. I will retake my flats later and try 25000 adu and see how they look. I'll have a look at that thread as well. 

This is the version of the ASI1600 I have https://www.firstlightoptics.com/zwo-cameras/zwo-asi1600mm-pro-usb-3-mono-camera.html

What exposure time do you think I should be aiming for?

I've been using 19000 and an exposure time between 0.5s and 5s based on a tutorial I saw.  I have the ASI1600mm-Pro.

John

[Adam1234]

50 minutes ago, inFINNity Deck said:

I would recommend to stay at least well above 1 second (2 seconds for non-Pro models). Not only for the effects described above, but also to avoid interlacing as a result of the flat panel's (laptop's screen) flicker rate. When short exposures are taken interference will occur between the exposure length and update rate of the flat panel, resulting in interlacing effects (horizontal bands) in the flats.

The skewing due to low exposure times in non-Pro models will not be very obvious when doing single panel work or when using processing software that deals with uneven illuminated subs, but it is better to avoid.

Nicolàs

 

39 minutes ago, Starwiz said:

I've been using 19000 and an exposure time between 0.5s and 5s based on a tutorial I saw.  I have the ASI1600mm-Pro.

John

 

Excellent, thanks.