god i love this software, my latest image using BYN

[astro_gaz]

 NGC 6960: The Witch's Broom Nebula by Gareth Harding, on Flickr

NGC 6960: The Witch's Broom Nebula

Scope: Orion Optics VX6 with 1/10 PV upgraded optics

Guide Scope: Skywatcher ST80

Guide Cam: QHY 5 Mono

Mount: Skywatcher HQE5

Camera: Nikon D5100 Modded

Additional: Astronomik CLS CCD Filter, Baader Mark-III MPCC Coma Corrector

Exposure: 18x5 Minute Subs ISO:1600, Darks, Bias & Flats 

Technical: 750mm f/5

Software: DSS, Pixinsight, BackyardNikon, PHD2

 

taken from a light polluted city

[rsewards]

OK, so from what DSS are saying, bias frame deal with non-zero output from the sensor but so do darks as they deal with non-zero output + amp glow and non-linear response?  Is that right?  If so , why both?  

 

Dark frames are temperature dependent. The "noise signature" of your sensor will be different depending on ambient temperature and length of exposure (as the sensor heats up which acquiring data). That's why these need to be taken at the same exposure time, settings and temperature as the light frames. The average noise of the final stacked master dark frame will be very similar to the noise in your light frames generated by temperature and will cancel it out. 

 

The baseline non-zero output of the sensor aren't temperature dependent. That's why you shoot bias frames at the fastest speed. You want to get the "true" reading of the sensor without heat generated by capturing an exposure. This basically gets rid of signal created by hot pixels, not noise generated by the exposure.

 

Darks contain both dark current and bias.  You only need to take bias frames if you want to scale the darks (ie. scale a 10 minute dark to 5 minutes) to calibrate different length exposures with one dark set.  When you subtract the bias frame from the dark frame the result is the time dependent component which can then be scaled for different length exposure light frames.  Both dark and bias are temperature dependent, but the bias is so short that I don't think temperature matters much.

 

PI's bpp script scales darks this way, which is why it doesn't ask for darks for flat frames (this puzzled me for a while until I read http://www.pixinsight.com/tutorials/master-frames/index.html).

 

DSS lets you provide darks for your light frames and a different set of darks for flat frames, so you don't need to take bias frames.

 

My question about bias frames for a Nikon camera is whether or not they are true bias frames or has the camera done some preprocessing first?

 

I hope this helps,

-

Richard Sewards

 

 

[Astro_Andruce]

Astro_Gaz absolutely fantastic images pretty impressive! I was wondering though how light polluted is the region you are imaging from and is your LP filter is pretty effective at keeping the LP out of your images?  I have a similar setup and the LP is pretty much keeping me down to less than 2 minute exposures, and I'm imaging from an orange zone so I'm thinking about picking up an LP filter but not sure how effective it will be.

[andy-w-h]

Epic image!

 

Not that I'm confused over the use of darks and biases ... obviously, cough ... but why did you use both?

 

edit: and to carry on the BYN love-in (ok, I buggered up the processing somewhere ... stars with super-halos)

 

 

edit2:  another question, why DSS and  Pixinsight?  I tried a bit with DSS and hated it.  PI has just been joyful to use, batch pre-processing and a quick ctrl-a and I know if I've got a lemon or not.  (I have almost zero experience in this malarky so idiot-grade software is my level!)

 

 

 

 

 

 

[astro_gaz]

Taken from DSS website 

 

Light, Dark, Flat, Bias... What are they and how to create them?

(if you are wondering how all the files are used during the calibration process you can have a look here)

Light Frames

The Light Frames are the images that contains the real information: images of galaxies, nebula...

This is what you want to stack.

 

Dark Frames and Dark Flat Frames

The Dark Frames are used to remove the dark signal from the light frames (or the flat frames for the Dark Flat frames).

With DSLRs and CCD Camera, the CMOS or CCD is generating a dark signal depending of the exposure time, temperature and ISO speed (DSLR only).

To remove the dark signal from the light frames you use a dark frame that contains only the dark signal.

 

The best way to create the dark frames is to shoot pictures in the dark (hence the name) by covering the lens.

The dark frames must be created with the exposure time, temperature and ISO speed of the light frames (resp. flat frames).

Since the temperature is important try to shoot dark frames at the end or during your imaging session.

 

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master dark or master dark flat.

 

Bias Frames (aka Offset Frames)

The Bias/Offset Frames are used to remove the CCD or CMOS chip readout signal from the light frames.

Each CCD or CMOS chip is generating a readout signal which is a signal created by the electronic just by reading the content of the chip.

 

It's very easy to create bias/offset frames: just take the shortest possible exposure (it may be 1/4000s or 1/8000s depending on your camera) in the dark by covering the lens.

The bias frames must be create with the ISO speed of the light frames. The temperature is not important.

 

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master bias/offset frame

 

Flat Frames

The Flat Frames are used to correct the vignetting and uneven field illumination created by dust or smudges in your optical train.

 

To create good flat frames it is very important to not remove your camera from your telescope before taking them (including not changing the focus).

You can use a lot of different methods (including using a flatbox) but I found that the simplest way is to put a white T shirt in front of your telescope and  smooth out the folds. Then shoot something luminous (a flash, a bright white light, the sky at dawn...) and let the camera decide of the exposure time (Av mode), 

 

The flat frames should be created with the ISO speed of the light frames. The temperature is not important.

 

Take a few of them (between 10 and 20 is usually enough). DeepSkyStacker will combine them automatically to create and use a clean master flat frame.

[astro_gaz]

nice image by the way this was my eastern veil 

Eastern Veil Nebula by Gareth Harding, on Flickr

[andy-w-h]

OK, so from what DSS are saying, bias frame deal with non-zero output from the sensor but so do darks as they deal with non-zero output + amp glow and non-linear response?  Is that right?  If so , why both?  

[Crunchmeister]

OK, so from what DSS are saying, bias frame deal with non-zero output from the sensor but so do darks as they deal with non-zero output + amp glow and non-linear response?  Is that right?  If so , why both?  

 

Dark frames are temperature dependent. The "noise signature" of your sensor will be different depending on ambient temperature and length of exposure (as the sensor heats up which acquiring data). That's why these need to be taken at the same exposure time, settings and temperature as the light frames. The average noise of the final stacked master dark frame will be very similar to the noise in your light frames generated by temperature and will cancel it out. 

 

The baseline non-zero output of the sensor aren't temperature dependent. That's why you shoot bias frames at the fastest speed. You want to get the "true" reading of the sensor without heat generated by capturing an exposure. This basically gets rid of signal created by hot pixels, not noise generated by the exposure.