Newbie question- Flats & ZWOASI1600MM Cooled

David_Fielder · October 1, 2016, 8:44pm

I’ve just switched over from a DSLR to a mono CCD camera - ZWO ASI1600MM cooled with auto filter wheel.
SG Pro works great controlling the equipment.

Here’s a Newbie question for Flats and specifically the ZWO ASI1600MM

I’ve taken some indoor flats with as Spike-a-Fielder light box. I’m trying to determine the best setting to get good flats before going outside.

Attached are 4 files using High Dynamic Range ACSOM setting (O Gain, Offset 10); 3 sec

Lum (3 sec) 1x1 and Red (3sec) 2x2

Each one has the light box off as well as the Light setting on 1 (lowest)

The learning curve here is steep. I understand that Gain is like ISO and there for kept low to not blowout the image. Offset is like brightness and hence keeping it low too. Some other posts suggest keeping the exposures over 1 sec to avoid horizontal lines. At 2 sec exposures, they are visible.

I thought flats should show vignetting in the corners but I only see them in the top right and left corners.
The images appears relatively blown out although I can see dust motes (donuts) in the image even thought the camera and filter wheel is just out of the box.

Can someone conclude I’m going in the right direction here?

Also, What are the best setting for Flats Calibration Wizard for the ASI1600MM? - Target mean ADI and min and max exposures? How does this setting after saving does this assist in taking Flats in the Sequence table?

Any comments are most welcome! Thanks.

mads0100 · October 2, 2016, 5:17pm

Make sure you use the gain that you’re using for imaging. Otherwise it’s hard to subtract out what you’re trying to remove from the image.

Jared · October 2, 2016, 8:15pm

As mads said make sure you use whatever gain/offset for your flats as you do for imaging. It’s not AS important with flats as it is with darks and bias but it can make a difference how the data is scaled.

As far as the vignetting that can vary for many different things. My guess would be that the camera is not exactly centered or there is some tilt to the CCD relative to the objective. You can try rotating your camera 180 degrees and see if the pattern moves or if it stays in the same place to figure out what may be causing it. But overall I wouldn’t worry too much about it, provided the flats do their job correctly.

Min and Max exposure values are just used as a range to find your “good” value. Since you have an idea about what your exposures are (1 and 3 seconds) you can probably set this to a min of 1 and a max of 5. Then run the wizard and see what exposure values SGP comes up for your filters. Also for binned images you may need to decrease your lightbox brightness even more if possible.

Thanks,
Jared

David_Fielder · October 2, 2016, 10:28pm

Thanks for your comments/suggestions Mads/Jared,

Since it was raining today, I tried a few different settings. The best that seemed to work well was:

High Dynamic Range (Gain 0; Offset 50); first brightness setting on Spike-a-Fielder
5 secs for Lum (1x1); RGB and NBP (2x2). Based on the expertise of others, I’ve started with 2x2 binning of RGB and then resize then in Nebulosity before layering in PS.

For the 2x2, I could go as low as 4 sec without getting horizontal banding, except for Lum, which needed 5 seconds minimum.

If I increased the brightness of the light box past the first setting, the Lum was totally blown out. Similarly, if I use the Unity setting, the exposure time would be below 1 sec with heavy horizontal banding and hence sticking with the pre-set HDR setting.

I tried the Flats Calibration wizard and the only settings that seemed to work (all green calibration check marks) were Target ADU (31,000 +/- 500); 0.5 sec minimum; 10 sec maximum. If I used an ADU higher or lower than 31,000, one or more of the filters would fail to calibrate.

The problem after saving the calibration is that setting up my sequence for flats, SGPro would put a calibration factor in for the Exposure and all of the exposures were set below 5 seconds and significant horizontal banding occurred.

I then ditched the calibration wizard and ran the Frame and Focus until I got what appeared to be good flats without banding. (see below). I like Jared’s suggestion of rotating the camera and see if the vignetting stays in the same place. I was wondering if it had something to do with the back focus? I use a 0.7x reducer on my Celestron EDGE HD 800; a T-adapter, FW and ZWO camera. Measuring the distance, Im within 1-2 mm of the Celestron’s reference 105 mm back focus from the end of the 0.7x reducer.

Flat_5 sec_1x1_Lum_frame 10

Flat_5 sec_2x2_Red_frame 10

Flat_5 sec_2x2_Green_frame 10

Flat_5 sec_2x2_Blue_frame 10

Flat_5 sec_2x2_NBP_frame 10

David