Help needed with Darks/Temperature with DSLR

[lensman57]

Very interesting Mark. I'll read the paper later. This would change things dramatically for me as darks can be very time consuming!! 

Here is a comparison chart showing how sensor temp changes against time, as the dark current increases as the sensor  temp goes up the chart gives a good indication of how darks are unrepresentative in a camera without set point cooling ie a DSLR. The credit for compiling the chart goes to Mr Gary Honis BTW.

Regards,

A.G

[MattJenko]

When using calibration images (darks/flats/bias)  generated with a DSLR, do not convert to TIFF before creating master calibration frames in DSS, as this causes shifts in the various levels as part of the conversion and you will find they do not calibrate your lights correctly. It is fine to have DSS generate master tiff files from RAW calibration files and use those to calibrate your lights, but I had severe flat overcompensation in my light frames for months before realising what had happened, as I was converting everything to TIFF before handing them over to DSS.

Matt

[thohem]

I personally don't use darks because

- as stated above you can't match temperatures

- I only shoot while dithering - best method for eliminating hotpixels

CS

Thomas

[Kropster]

If I am guiding with PHD, is there an option to dither the guiding, or maybe another way of doing it?

[thohem]

I use the MGEN standalone Guider, but AFAIK PHD can dither too. Perhaps another User can tell you where to switch it on

[lensman57]

If I am guiding with PHD, is there an option to dither the guiding, or maybe another way of doing it?

PHD does not guide by itself, the dither instructions must be issued from your capture software. Which one are you using?

A.G

[Kropster]

PHD does not guide by itself, the dither instructions must be issued from your capture software. Which one are you using?

A.G

I am using APT.

[SkyExplorer]

The subject of darks is one that tends to generate a lot of confusion - especially when related to DSLRs - because of the lack of sensor cooling and especially the lack of set point cooling.

Here are a few pointers from my own experience.

Hot pixels are pixels that are more or less saturated in every frame.  A hot pixel map is the best way to get rid of these

There are other pixels (I call them warm pixels) which are consistently brighter than their surroundings - this is what the darks get rid of.  It is what is known as coherent noise i.e. the component of thermal noise that is constant from frame to frame.

There are other pixels that look bright occasionally - this is just the random nature of photons/electrons.  Only stacking gets rid of these.

To work well, darks need to be taken at a sensor temperature of within 1C of the lights.  Ignore the temperature recorded in the DSLR EXIF file - this never corresponds well to the dark current in my experiments.  An additional complication is that the dark current is continually changing during an imaging session - during a typical session the dark current will increase by a factor of 4 as the sensor gradually warms up.  So the later exposures will have far more thermal noise than the earlier ones.  The conclusion is that you have no hope of manually matching darks to lights - even with a "darks library" - because you need a different temperature of dark for each exposure.

So use the dark optimisation, available in most processing packages.  For each light frame, it scales the master dark frame to best subtract out the coherent noise.  In DSS, make sure "Dark Optimisation" is switched on.  It calculates a multiplier (in the range 0 to 1) for the dark frame using "Entropy-based Dark Frame Subtraction". Further details and a link to a technical article are found here:

http://deepskystacke...h/technical.htm

All DSLR users should have this DSS option switched on.

There is one other important point to get this right. DSS scales the dark frame down (and not up) during dark optimisation. Therefore, if you are using this option, the darks used must be taken at the end of an imaging session on a warmish night and not during a coldish night.  In my case I have a single set of darks that I took at room temperature.  I shoot a new set a couple of times a year as the sensor characteristics evolve.

Mark

Mark,

How many darks do you take at room temperature?

Anyone,

What is dithering ??

[sharkmelley]

Mark,

How many darks do you take at room temperature?

Anyone,

What is dithering ??

I took a few hours worth of darks - around 20-30 darks.

Dithering is where each subsequent image is taken with a random positional offset to the original i.e. the star field is shifted slightly between each image frame.  Conversely, when the images are registered before stacking, it is the hot pixels that appear to have random offsets from frame to frame.  Hence when the registered frames are summed the impact of the hot pixels is reduced.  Even better, if sigma stacking is used, the hot pixels are identified as having outlying values and are not summed into the final image at all.

Mark

[SkyExplorer]

Thanks mark. So 30 darks at room temperature each at say 3 minutes, so more than the lights duration.

Do I need apt for dithering ? I currently use the eos software to control my 450d.

Thanks

[thohem]

With dithering the mount 'hops' just a few pixels (4 - 10) after each photo in a square like (or snake) formation. So the hotpixels on the lights are always on different locations - DSS then eliminates them without the need for darks. For my setup the MGEN takes care of dithering and I never ever shoot without it. As for dslr photography, dithering is much better than taking darks (especially with that weather at the moment, cause you save alot of time)

[PeterCPC]

There is an option for dithering in BYE - I use it. Top right on the screen. You must have the server enabled in PHD to do so.

Peter

[sharkmelley]

Do I need apt for dithering ? I currently use the eos software to control my 450d.

Yes, APT or some similar acquisition software must be used because the random offset applied to the guide star must take place between exposures.  If you are using PHD then the acquisition software will communicate with PHD to achieve this.

I use darks to improve the quality of each frame together with dithering to remove the more serious outlying values caused by satellite trails, cosmic ray hits etc.

Mark

[SkyExplorer]

Brilliant,

I do notice a lot of vignetting in my images once stretched, but that should be another post.

Just been reading a few articles, and it seems darks are not all they are cracked up to be for DSLR imaging as the temperature of the chip is not constant, so you cannot really get a good match when taking them during an imaging session.

I took a few hours worth of darks - around 20-30 darks.

Mark

So your exposures were long then, did you leave any time between exposures when taking your darks?

Thanks

[sharkmelley]

Just been reading a few articles, and it seems darks are not all they are cracked up to be for DSLR imaging as the temperature of the chip is not constant, so you cannot really get a good match when taking them during an imaging session.

So your exposures were long then, did you leave any time between exposures when taking your darks?

Ignore those gainsaying articles and instead switch on dark optimisation as I explained earlier.  This completely copes with the fact that CCD temperature is not constant and it provides a good match for the darks.

I don't leave any time between exposures for lights nor for darks.  It's not an effective use of imaging time.  Yes the CCD might run slightly warmer but it also means you collect more data during your imaging session.

One other thing I forget to mention is to switch off the rear LCD display while imaging.  The LCD circuitry adds a lot of unwanted heat to the CCD.

Mark

[SkyExplorer]

Thanks, I've setup and taken 100 flats using the same equipment I use for lights, with the same focal point, and used a white t-shirt (the histogram for these is between 1/4 and 1/2 way along from the left (black side) and am now doing 20 x 5min darks.

Thanks for your help.

[SkyExplorer]

One other thing I forget to mention is to switch off the rear LCD display while imaging. The LCD circuitry adds a lot of unwanted heat to the CCD.

Hi, not sure how to do that on my 450d, just looked through the menus and cannot see any options for it. ??

[lensman57]

I am using APT.

Enable the sever in Phd guiding, launch APT, in the GEAR tab shift click on the GUIDE (D) and the dithering dialog opens, here you can set the parameters but with a DSLR as large a dither that is possible is the way to go. Allow long enough pause time after a Dither for the guiding to settle.

A.G

[sharkmelley]

Hi, not sure how to do that on my 450d, just looked through the menus and cannot see any options for it. ??

It's not an option in the menus. Try looking for a button named DISP - it will switch the display on and off.  I'm not familiar with the 450D but the 400D, 550D and 600D certainly have it.

Mark

[Nathan_Pembs]

I've just dabbled with dithering in byeos, think I set the movement quite low at 3 pixels?

A stacked image I got out of DSS contained a few red and blue "worm" like artefacts, I'm assuming they're the hot pixels travelling?

What settings would get rid of them?

[SkyExplorer]

It's not an option in the menus. Try looking for a button named DISP - it will switch the display on and off.  I'm not familiar with the 450D but the 400D, 550D and 600D certainly have it.

Mark

Ah, yes the the big button on the back that reads "DISP"    Not sure why I missed that   Thanks.