Hi cataclysm,
Thanks for the tips. While I know the purpose of dark frames, what is the purpose of flats/bias frames? How to take such frames?
Astrosiao
M8 Lagoon Neb
Re: M8 Lagoon Neb
Astrosiao:
Dark frames basically represent the profile of noise in a given chip, and is temperature dependent. Basically the fundamental premise of taking dark frames (at least 18 for a given ISO setting) is to have an equivalent exposure duration taken as close a temperature to those of light (data) frames. However, assuming we are usually taking light frames in a tropical climate where night and day temperatures will usually be less than 5 degrees difference, the adherence to the "same temperature" principle will not be as marked as engaging DSLR photography in temperate countries. Unless you are taking from a modified DSLR with cooling, you do not have to be overly fussy about when one should take dark frames or waste time accumulating dark frames right after light frames. We do not have frequently clear skies to spend these precious time consuming the production of dark frames. Usually one will not just take one dark frame for a given light frame (same duration and ISO), but take a couple of dark frames of the same setting and average them out through software (DeepSkyStacker, Nebulosity, IRIS, etc).
Bias Frames basically represent the readout differences of each pixel in the chip and is taken using the shortest possible length of exposure that your DSLR can achieve. These frames are basically registering the profile of noise that occurs during the downloading process, making the zero-readout of each pixel vary (ever so) slightly. But usually, you do not need to take bias frames when you ensure that your dark frame library has a collection variety based on exposure and ISO. It is when you have no time for dark frame library accumulation that you utilize bias frames to "extrapolate" the behaviour of dark frames from one exposure to another (under same ISO).
Flat frames on the other hand represent the optical imperfections in the system (both telescope and camera, and everything in between (filtering behaviour, etc)). Flat frames are actually very necessary since no telescope out there can have 100% perfect even illumination across the FOV, or have very very clean (no dust motes, etc) illumination profiles. Flat frames are taken (more than 6) and then averaged (don't forget to minus the dark frame duration equivalent), and mathematically, it is divided by the light (data) frames so as to eliminate the sources of optical distortion or dirt/dust profile. Try to take flat frames that are not too short (less than 1 second). If you can, an exposure of 1-5 seconds should be sufficient (longer if narrowband channels), but exact "ideal" exposure of flat frames can be studied based on the resultant histogram profile (peak at 1/3 to 2/3 along the x-axis measured from far left).
You will notice that after once each light frame is darks subtracted as well as flats subtracted, it will result in a much more pleasant light frame that is easy for processing later.
Hope this helps!
Dark frames basically represent the profile of noise in a given chip, and is temperature dependent. Basically the fundamental premise of taking dark frames (at least 18 for a given ISO setting) is to have an equivalent exposure duration taken as close a temperature to those of light (data) frames. However, assuming we are usually taking light frames in a tropical climate where night and day temperatures will usually be less than 5 degrees difference, the adherence to the "same temperature" principle will not be as marked as engaging DSLR photography in temperate countries. Unless you are taking from a modified DSLR with cooling, you do not have to be overly fussy about when one should take dark frames or waste time accumulating dark frames right after light frames. We do not have frequently clear skies to spend these precious time consuming the production of dark frames. Usually one will not just take one dark frame for a given light frame (same duration and ISO), but take a couple of dark frames of the same setting and average them out through software (DeepSkyStacker, Nebulosity, IRIS, etc).
Bias Frames basically represent the readout differences of each pixel in the chip and is taken using the shortest possible length of exposure that your DSLR can achieve. These frames are basically registering the profile of noise that occurs during the downloading process, making the zero-readout of each pixel vary (ever so) slightly. But usually, you do not need to take bias frames when you ensure that your dark frame library has a collection variety based on exposure and ISO. It is when you have no time for dark frame library accumulation that you utilize bias frames to "extrapolate" the behaviour of dark frames from one exposure to another (under same ISO).
Flat frames on the other hand represent the optical imperfections in the system (both telescope and camera, and everything in between (filtering behaviour, etc)). Flat frames are actually very necessary since no telescope out there can have 100% perfect even illumination across the FOV, or have very very clean (no dust motes, etc) illumination profiles. Flat frames are taken (more than 6) and then averaged (don't forget to minus the dark frame duration equivalent), and mathematically, it is divided by the light (data) frames so as to eliminate the sources of optical distortion or dirt/dust profile. Try to take flat frames that are not too short (less than 1 second). If you can, an exposure of 1-5 seconds should be sufficient (longer if narrowband channels), but exact "ideal" exposure of flat frames can be studied based on the resultant histogram profile (peak at 1/3 to 2/3 along the x-axis measured from far left).
You will notice that after once each light frame is darks subtracted as well as flats subtracted, it will result in a much more pleasant light frame that is easy for processing later.
Hope this helps!
Re: M8 Lagoon Neb
Hi Remus, thanks. It is only today that upon re-reading this that I manage to finally understand it!rcj wrote:Astrosiao:
Dark frames basically represent the profile of noise in a given chip, and is temperature dependent. Basically the fundamental premise of taking dark frames (at least 18 for a given ISO setting) is to have an equivalent exposure duration taken as close a temperature to those of light (data) frames. However, assuming we are usually taking light frames in a tropical climate where night and day temperatures will usually be less than 5 degrees difference, the adherence to the "same temperature" principle will not be as marked as engaging DSLR photography in temperate countries. Unless you are taking from a modified DSLR with cooling, you do not have to be overly fussy about when one should take dark frames or waste time accumulating dark frames right after light frames. We do not have frequently clear skies to spend these precious time consuming the production of dark frames. Usually one will not just take one dark frame for a given light frame (same duration and ISO), but take a couple of dark frames of the same setting and average them out through software (DeepSkyStacker, Nebulosity, IRIS, etc).
Bias Frames basically represent the readout differences of each pixel in the chip and is taken using the shortest possible length of exposure that your DSLR can achieve. These frames are basically registering the profile of noise that occurs during the downloading process, making the zero-readout of each pixel vary (ever so) slightly. But usually, you do not need to take bias frames when you ensure that your dark frame library has a collection variety based on exposure and ISO. It is when you have no time for dark frame library accumulation that you utilize bias frames to "extrapolate" the behaviour of dark frames from one exposure to another (under same ISO).
Flat frames on the other hand represent the optical imperfections in the system (both telescope and camera, and everything in between (filtering behaviour, etc)). Flat frames are actually very necessary since no telescope out there can have 100% perfect even illumination across the FOV, or have very very clean (no dust motes, etc) illumination profiles. Flat frames are taken (more than 6) and then averaged (don't forget to minus the dark frame duration equivalent), and mathematically, it is divided by the light (data) frames so as to eliminate the sources of optical distortion or dirt/dust profile. Try to take flat frames that are not too short (less than 1 second). If you can, an exposure of 1-5 seconds should be sufficient (longer if narrowband channels), but exact "ideal" exposure of flat frames can be studied based on the resultant histogram profile (peak at 1/3 to 2/3 along the x-axis measured from far left).
You will notice that after once each light frame is darks subtracted as well as flats subtracted, it will result in a much more pleasant light frame that is easy for processing later.
Hope this helps!
Hehe
Re: M8 Lagoon Neb
Still a very good try in taking the M8. The fact that you took the trouble to learn drift alignment, do some processing work is already very good considering the kind of skies we have.
Hi Remus, thanks. It is only today that upon re-reading this that I manage to finally understand it!
Hehe
I am pretty sure that you will be able to take better shots given time.
Keep it up!