calibration frames

[RobK (Rob)]

Hi all,
I am looking for some advice on calibration frames.
I intend to try and image M83 over several nights later this month.
This will be the first time that I have tried multiple sessions and am wondering whether I need to take calibration frames at the end of each night.

I will be imaging with a SW Esprit 80mm and a ZWO ASI533mc.

Any advice greatly appreciated
regards
Rob

[sil (Steve)]

It depends really, putting together an image only requires Lights, but the extra calibration frames help eliminate all the factors that will make your image poor quality and hence your next set of questions about how to fix it. If you are spending the time and effort to capture Lights do yourself and everyone a favour and take darks and flats at least too each time. We dont take them to be annoying. And if you cant be bothered then dont bother with any of it The more you capture the better your data set will be to process from. Ideally you want a full set each session , process each session to a linear result then combine those for multiple sessions. As I said you CAN get away with less but the result will be poor and unfixable.

[AdamJL]

Originally Posted by RobK

Hi all,
I am looking for some advice on calibration frames.
I intend to try and image M83 over several nights later this month.
This will be the first time that I have tried multiple sessions and am wondering whether I need to take calibration frames at the end of each night.

I will be imaging with a SW Esprit 80mm and a ZWO ASI533mc.

Any advice greatly appreciated
regards
Rob

Hi Rob

If you have a permanent setup, and all you do is put the cap on the front of your scope, then you don't need to take flats. A flat is an image of the "imperfections" in your image train.. dust spots, vignettes, whatever. Dust is what moves/changes when you setup manually as the scope gets knocked about.

Darks, well that depends on the camera. I don't know a lot about the 533MC, but I hear it does require dark frames. For my cameras, all that's needed are Flats and Bias frames. YMMV depending on the camera, but from memory (not always accurate!) I think the 533 requires these and also Darks.

Good thing is that both generally just need to be taken once, so long as you're shooting at the same temperature.

[Startrek (Martin)]

Hi Rob,
Here’s some info on Calibration Frames I put together a while back
In most cases the type of Calibration frames required are dependent on your telescope image train and camera type ( imaging set up ) In most cases trial and error will get the results you need ( took me 2 months to sort out Calibration frames for my 2600MC through research, trial and error , and now it’s just a routine part of my imaging )
I didn’t use them for years and now I realise they are an extremely important part of your imaging and processing to improve your final images.

Calibration Frames

What are Calibration Frames ?
Calibration frames are specific photos or images captured in addition to the light frames which are stacked with your light frames to improve the quality of your final image

Types of Calibration Frames
Calibration frames include Darks, Flats , Dark Flats and Bias frames

Darks
Darks are frames which record the electronic noise or thermal noise your camera produces during an exposure. Darks will also eliminate stuck pixels in an image which can be both hot and cold pixels.
Darks are captured with the lens cap or lid fitted to the end of the telescope and are the same exposure length, same ISO or same Gain and same camera temperature as your light frames.
Generally 20 to 50 Darks are sufficient for Calibration purposes

Flats
Flat frames are used to create a uniform evenly illuminated or flat field across your image.
Flats help in minimising vignetting, dust donuts and other artifacts or anomalies in your image.
Flats are usually captured using a white t shirt stretch over the end of the telescopes OTA and pointed to an evenly illuminated light source like the early morning sky ( sky flats ) or a dimmer able Led light panel ( light flats ) Flat frames are usually short in exposure time ( 0.5 sec to 15 sec ) and must have the same focus and ISO or Gain settings on your camera as your light frames. Flats may differ from camera to camera but generally should be exposed 1/3 to 1/2 way across on your Histogram. Generally 20 to 40 Flat frames should be sufficient for Calibration purposes.

Bias Frames
Bias frames are used to reduce noise in your image in particular dark fixed pattern noise.
Bias frames are captured with the lens cap or lid on the end of the telescope with the shortest possible exposure available from your camera ( <1 sec ) Bias frames must have the same camera temperature, ISO and Gain settings as your light frames.
Generally 50 to 100 Bias frames should be sufficient for Calibration purposes.

Dark Flats
Dark Flat frames are captured to calibrate your Flat frames and reduce noise.They are captured with the lens cap or lid on the end of the telescope. Dark flats must have the same camera temperature, focus , ISO or Gain as your light frames but most importantly the same exposure time as your Flat frames. Generally the quantity of Dark flats should match your Flat frames to keep things simple.

Capturing Calibration Frames
There many brands of Astro capture software on the market that are designed for use with both DSLR and dedicated Astro Cmos cameras cooled and uncooled.Some examples for DSLR’s are Backyard EOS , Backyard Nikon and APT
For dedicated Astro cameras there are many like APT , NINA , Sharpcap , Sequence Generator Pro and many more ....
They all do a great job at capturing your image or light frames and your calibration frames

Capturing Calibration Frames
Many cameras including various DSLR , cooled CMOS OSC and Mono cameras require different Calibration stacking requirements. Calibration frames from cameras sometimes produce better images after stacking from different combinations but in every case Flats frames must be used to create an evenly illuminated field.
Examples:
Lights ,Flats, Darks
Lights , Flats , Dark Flats
Lights , Flats , Bias
Lights, Flats , Darks , Bias
It’s best to research your camera type on forums or from the manufacturer to find out what Calibration frames work best.
The quantity of Calibration frames stated above should be used as a guide only. Final required quantities of Calibration frames are based on individual equipment requirements and outcomes

Stacking Calibration Frames
There are many Astro software programs on the market available for stacking your lights with calibration frames.
Deep Sky Stacker is very good stacking software and popular worldwide. It’s also open sourced so it’s a free download. There are many other good stacking programs to choose from like Astro Pixel Processor , PixInsight , Siril but most require purchasing.

Hope the above offers some help

Cheers
Martin

[RobK (Rob)]

Hi Steve, Adam and Martin,
Thanks for getting back with your sound advice.

Martin, your explanation of the various types of calibration frames is excellent.

I am hoping to achieve several hours of data, over multiple nights, in bortle 1 skies on the banks of the Darling.

cheers
Rob