Black Clipping in Nikon D7000 Dark

[Rocket Pooch]

Hi,

That scope looks nice, but no good for me, I need F4 at max or I'll have image train issues without changing all my filters.

Chris

[MarkS]

To continue ...

Bracketing - the D7000 will not need bracketing - you can use the full dynamic range because the read noise is unusually low at ISO 100 & 200.  This is mainly due to the revolutionary sensor design - the A/D Converters are on the sensor so you get the same read noise (in electrons) at all ISOs.

I totally agree that DSLRs are not thermally stable.  The way to cope with this is to create a good quality master dark at an average ambient temperature and then during processing of subs a program such as IRIS or PixInsight (and probably most others) will decide what multiplier to apply to the dark before subtraction to minimise noise.  It's not ideal situation but the real issue is how much noise does this generate in the final stack - a difficult one to answer.

On the subject of darks, you asked how I calculate dark current.  The level can't be measured directly on DSLRs because some bias is always applied. However, on the Canons you shoot two successive long exposures and calculate the standard deviation in the difference frame - divide it by square root of 2 to get the standard deviation of a single frame.  Since the arrival of thermal slectrons (the dark current) is a Poisson process you can calculate the average level as the square of this because for a Poisson process the standard deviation is always the square root of the mean.  The average level is then converted to electrons by using the gain.  From the number of electrons and the exposure length you get the dark current.  For the Nikon, with its black clipped pixels, the calculation is similar but adjustments have to be made for the fact you only have part of the distribution.

Going back to cameras, I've so far been comparing the D7000 against one-shot-colour cameras.  But we all know I would get better images by using a mono camera and LRGB - broadly speaking it would halve for imaging time.  The truth is it's very much a question of convenience and of cost.  With a DSLR or OSC you get a complete colour image in one go.  Shooting LRGB there is always a risk that clouds will prevent the acquisition of all the channels required.  A good compromise might be to use a DSLR or OSC for the colour component and a mono camera to grab the Luminance and/or H-alpha.

One other thing, I said earlier that dynamic range is a bit of a red herring.  For wide band imaging around here read noise is also a red herring.  Light pollution is by far the biggest limitation on RGB imaging in all parts of Kent, Sussex and Norfolk.  It swamps the read noise and for fast F-ratios it even swamps the thermal noise of DSLRs.  

However, for short exposures, read noise tends to be the limiting factor and this is one place where a low read noise DSLR comes into it's own.

For one reason or another I've ended up specialising in RGB imaging and I further admit that using a DSLR is the lazy and cheap route to do so, but also very versatile  

[JohnP]

Well I'm waiting for input from the expert.. Mr. G where are you. ?     

[MarkS]

Well I'm waiting for input from the expert.. Mr. G where are you. ?     

No, this is a colour camera - Mr. G. is the mono afficionado.

[JohnP]

Ahhh I see...... obviously why he hasn't participated in this extremely interesting thread.. my excuse is that I haven't got a clue what you are talking about..... Wooossssshhhhhhhhh!!!!!!!

Mark - Why don't you apply for a job as technical head of Nikon & you can tell them how they should be designing their camera's.......?

[Fay]

I was going to make a substantial, relevant participation in this thread.........................but decided not to, dont want to confuse the discussion!

[MarkS]

.. my excuse is that I haven't got a clue what you are talking about..... Wooossssshhhhhhhhh!!!!!!!

That's useful feedback.  I need to summarise the pertinent information clearly, together with a list of pros and cons and then add some tips for those unfortunate enough and/or crazy enough to ever want to use one for astrophotography.

Mark - Why don't you apply for a job as technical head of Nikon & you can tell them how they should be designing their camera's.......?

I think they would boot me out as soon as I tell them to:
1) Change-over the IR filter to allow H-alpha
2) Add a peltier for set point cooling the sensor
3) Stop destroying valuable data by pre-processing the RAWs
4) Produce an optional mono version (for Tony G. of course) but with a built in filter wheel
5) All whilst maintaining the same competitive price!

[Mac]

Just a thought, does the D7000 still have the old mode 3 raw hack?

[Mike]

Yes but that's not exactly practical for multiple exposures.

[MarkS]

Just a thought, does the D7000 still have the old mode 3 raw hack?

I thought it was disabled on the D7000.  A mode 3 image gives exactly the same as a normal raw.

[Tony G]

Well I'm waiting for input from the expert.. Mr. G where are you. ?     

John, it started whoooossssshhhhing over my head at about this point in the thread.....................................

Here's an odd thing - a 300s dark at ISO 800

Tony G

[Fay]

Did you borrow the camera Mark?

[Carole]

This has been an interesting thread.  Some of it (with the statistics) went completely over my head, but I got the jist of enough of it.

I agree with Mark's comments about lack of clear skies limiting LRGB imaging which is why I am doing precisely what Mark suggests below.  

A good compromise might be to use a DSLR or OSC for the colour component and a mono camera to grab the Luminance and/or H-alpha.

I'm adding Ha or Luminance to existing DSLR images for the RGB component (blurring the DSLR images to smooth out the noise).  Or using my OSC for the colour component and using the Ha or luminance for the detail.
I also have LRGB filters which I will use when the weather gets more settled and we have clearer nights.  

Having had the experience of both DSLR and CCD cameras, I would definitely recommend a Mono camera for the fine detail, and use whatever is most convenient for the colour.  There is also the option of CCD narrowband as well.

Also having set point cooling makes life a lot easier.  

If only we could get some decent imaging time!!!!

Carole

[MarkS]

Did you borrow the camera Mark?

Mike did all the hard work, taking all the test frames and sending them to me.  Thanks Mike!

[MarkS]

I agree with Mark's comments about lack of clear skies limiting LRGB imaging which is why I am doing precisely what Mark suggests below.  

A good compromise might be to use a DSLR or OSC for the colour component and a mono camera to grab the Luminance and/or H-alpha.

In terms of overall noise reduction it is very slightly better to shoot the R,G & B through separate filters with 2x2 on chip binning.