Darks? Lights? Flats? I just want to see images

First, we will address the much dreaded flats/bias/darks. It can be rather confusing... but in a nutshell this is what each consists of as far as settings required:

  • DARKS must match LIGHTS on: gain, offset, temperature, exposure time
  • BIAS must match LIGHTS on: gain, offset
  • FLATS must match LIGHTS on: gain, offset
  • DARK FLATS must match FLATS on: gain, offset, exposure time
Some say that temperature is not important for Flats, dark flats and bias (only Darks)... but with a cooled camera, it's not that hard to keep consistency. I prefer to try to keep as minimal variables in my calibration frames.
If you are not using a cooled camera, you can disregard the temp for those 3, but for Darks the same temperature is important.

Darks are just what they sound like... they are captures utilizing a cover over the end of the telescope or camera that allows no light in. They need to be of the same gain setting, offset and exposure time as the light images. They ideally need to be at the same temperature.. this is even more important when using a cooled camera like an ASI Pro series.
Some ASI cameras do better without using dark frames. The ASI533MM/MC Pro's seem to be two of them.

Bias images must match both the gain setting, offset and temperatures of the lights... the difference is that they are ran at a high shutter speed/exposure setting. For some astrophotography specific cameras, bias frames are not needed and some cameras cannot produce a reliable bias frame (ZWO ASI294MC Pro being one). Each camera used will need to be researched to see if bias frames are needed, or if it can take them. Most DLSR's have no issues, and in fact are needed. It's the devoted astrophotography cameras that you will need to research.

The method of taking bias frames consists of:
  1. Use the fastest shutter speed possible (often 1/8000″ on DLSR, I use 0.05 on my dedicated astrophotography cameras that need it).
  2. Keep the lens cap on your camera or telescope (and cover the viewfinder on a DSLR, many DSLR cameras come with a viewfinder cover)
  3. Use the same ISO (DSLR) or gain/offset as your light frames.
  4. Capture about 20-40 bias frame

Flat frames will help to magically remove any dust spots that were on your camera sensor. Flats are targeted to correct for minor vignette, sensor mottling (if any), as well as dust and other stray particles that get into your imaging train (on reducers, the sensor window, or filters).
There are several ways of taking them, but the most popular seems to be the "white t-shirt" process.
Stretch a plain white t-shirt over the telescope objective, you create an even flat field when pointed at a bright light source. I personally have a heavier white t-shirt (not an undershirt) that I use, and only need 1 layer of it. If using a lighter undershirt style t-shirt you should double the thickness up. Then take the t-shirt and put it over the end of your telescope, making sure there are NO wrinkles in it. I use 3 rubber bands to hold mine in place, spaced out evenly on the scope body with one right at the top to minimize any wrinkling.
Once you have your t-shirt on, you can then point the scope at the early morning blue sky if you are still up. Using a flashlight at night and pointing it at the t-shirt is not going to work as you want an evenly distributed light source. If you have a larger iPad Pro you can use this and an app to make the iPad screen show white light, but you don't want it on the brightest white. A more subdued will work better. You can point the telescope upward and then place the iPad Pro over the t-shirt and use it as your light source and not have to hold it in place. If you want to get "really fancy", Artesky makes a flat field panel that you can purchase (about $380). You will have to use a computer to run the software that it requires, so if going to remote sites and your capture computer is not Windows based, it's just another computer you will have to take to run their software. There have been reports that there are issues with uneven light exposure (wires interfering in the light panel) with the units... personally, I use the iPad Pro/t-shirt if I don't want to stay up until daybreak or if capturing Bode's Galaxy, since I have to stay up until almost daybreak to do it the t-shirt/blue sky method.

You will want to examine the histogram of the flats you have taken. If the data clips the right side of the histogram, that’s an immediate indicator that your light source is too bright. The solution here is to dim your light source by adding another layer of t-shirt or to point somewhere less intense. If using the iPad Pro method, lower the intensity of the app.

  1. Shoot at the same ISO/Gain as your light frames.
  2. Keep the camera connected to the scope/lens.
  3. Maintain the same focus as light frames.
  4. Shoot a minimum of 15-20 flat frames.
  5. Temperature is not important for most cameras, but if using a cooled camera, i would keep it at the same temperature your lights were taken at
Generally around 20 flat frames will be adequate to create a good master flat image.

Dark flats
Taking dark flats is very simple. After you are finished taking your flat frames, cover your aperture with your dust cap if DSLR or telescope with lens cover and proceed to take your dark flats with the same exposure length and settings as your flats.

Not all cameras (specifically astrophotography specific ones) will require some (if any) of the above. You will need to research your camera train and ascertain what you need to do.
The same way if you use filters... darks don't care which filter is used,
Flats on the other hand will should be done with each filter you used in your capture as part of the think it does is help offset dust particles that can be present during each capture, and each lens will be different. There are many that will only take them with Ha filter or Luminance filter.. which is fine, but you will get your best results utilizing each filter you captured with.

Notes on my equipment:

The ASI533MM Pro that I have doesn't really need darks, but does benefit from bias frames. I have not tested the ASI585MC yet as it's primarily used for planetary capture on the NetStar 8se.
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