ASI533MC-Pro Calibration frames

[Pitch Black Skies]

Can someone recommend what calibration frames I should use with the above camera please?

So far I have just made up some darks. I have processed two stacked images (30s exposures, -10°C and at Unity Gain 100). One image is just lights and the other is with lights and 100 darks. I can't see any difference in both of the pictures side by side. Are dark frames needed with this camera? I can upload a stretched dark if someone wants to have a look.

I've read somewhere that bias frames are not needed as darks contain both dark noise and read noise. Is that correct and if so, should I not use them?

Do I need to subtract bias frames from my flats as they contain read noise or should I just include dark flats?

Edited November 29, 2021 by Pitch Black Skies

[Richard_]

This camera has very little dark noise so it'll be difficult to see the difference between your images. Also, if you are dithering during the imaging sequence and you're using hot/cold pixel detection during stacking, then you'll likely remove most if not all the hot pixels found in the master dark frame anyway. 

There are others who do not use dark frames at all, but I use them with my ASI533 purely out of good practice and that I don't want to solely rely on the hot pixel detection. 

I do not use bias frames with this camera as it feels like they make it worse.  Instead, I use dark flats, which are dark frames of the same exposure time as my flats. This seems to work well for me so it'll be worth giving them a try. If you use pixinsight WBPP script, simply put the dark flats in as dark frames. Go into "control panel" and "calibration diagram" which should show your flats being calibrated with master dark flats, and your lights being calibrated with master dark and master flat. See example diagram below which includes bias (it wouldn't appear in the above flow). 

[Pitch Black Skies]

Cheers Richard, that's really helpful.

I haven't mustered up the courage to venture over Pixinsight yet. I'm using DSS and Startools atm.

Coming from a DSLR, I suppose I'm just not used to seeing no dark noise.

How do recommend I should take the flats?

I only have access to the t-shirt method or a laptop screen with an A4 page.

[Richard_]

Ah fair enough! You'll need to look up if DSS asks for dark flats to be presented as "darks" or "bias". I have a book at home which may explain it, so I'll have a look when I get home (if I remember!). 

For my flats, I too use the t-shirt method but I use a light like below:

https://www.amazon.co.uk/gp/aw/d/B07TYLGFD5?psc=1&ref=ppx_pop_mob_b_asin_title

I can't give you the exposure time as it depends on the brightness of your light source. If I aim for ~1/3 of the histogram, I get great results! For a 16 bit image, the maximum ADU is 65,536 whilst my flats are around 22,000-24,000. Going halfway (~30,000) was over correcting. 

I made a previous thread about issues with my flats when using a tablet screen and l-extreme filter targeting ~30,000 ADU (see below). I haven't updated the thread since I bought the video screen, but it seems that this light source and lower ADU value has fixed my over correcting issue. 

Apologies if you're new to this approach and this is all gobbledegook. Happy to explain anything which doesn't make sense! 

 

 

[Pitch Black Skies]

Cheers mate,

I've just found an option on NINA called flat wizard. It seems pretty good. Going to give that a try now.

[Pitch Black Skies]

Luckily DSS has a dropdown for dark flats.

I've done a little experimentation. You're right about the bias frames, as is @CCD-Freak who I was chatting with also. Thank you both for your help btw. The bias frames indeed add to the noise.

There isn't a significant difference when using darks but there is definitely some data there so best to use them I agree.

Flats made the biggest difference for me, and more so when combined with the dark flats.

I have found the best result using darks, flats and dark flats (just like both of you recommended).

Here is some data I have pulled from a master dark, flat and dark flat. Included is a stretched and unstretched version.

MasterBias_Gain100 Stretched.tifMasterBias_Gain100.tifMasterDark_Gain100_30s Stretched.tifMasterDark_Gain100_30s.tifMasterDarkFlat_Gain100_1s Stretched.tifMasterDarkFlat_Gain100_1s.tifMasterFlat_Gain100 Stretched.tifMasterFlat_Gain100.tif

[Richard_]

Awesome, sounds like DSS has an intuitive way of adding dark flats! It'll be interesting to see how your calibrated lights turn out when you get some.

Question, I just looked at your flat frames, and there's some weird spotting on the right side of the images (visible in the regular and stretched versions). Are these specs of dust on the sensor or lens, or something else?

[Pitch Black Skies]

I had some lights from last week so I chanced calibrating them.

Just lights stacked and processed ⬇️

 

The lights then calibrated and processed the same ⬇️

 

I noticed those specs too. I have no idea what they are. They look a bit worrying. I can't see any negative impact on the final image when the master flat is integrated though. The camera lens looks pretty clean. I'm thinking it might be the primary mirror.

[Richard_]

Very nice! Your calibrated image seems to have produced a more neutral background as there's no blue cast. There's a big difference here with some lovely detail, well done! 

If the spots in the flat are caused by dust on your mirror, then it's fine to stay there. If you do try and remove the dust, use one of those rubber hand blowers, don't use compressed air from a can, chemicals, cloth etc as you'll potentially  do more harm than good to the mirror. There was a thread on here sometime ago where an unfortunate individual caused smearing on his mirror due to chemicals in the compressed air landing on the mirror which they weren't able to fully remove. Your final image looks great after calibration in my opinion, so I wouldn't worry about it! 

[Pitch Black Skies]

Thanks very much mate.

If it's not having any major impact I'll live with it for the time being.

Now just waiting for another few clear nights.