Amp glow - any way to judge how long is too long?

[omaroo]

Hi all

Just wondering what the best way might be to be able to judge how long an exposure I could take before amp glow got the better of me. Given that summer is coming on, I've done a few dark tests on the D40 at ISO400 and ambient temperature of 22 degrees C. I've taken frames from 1 to 15 minutes long at one minute intervals and can't see hugely appreciable difference in glow - but a few noisy pixels start to creep in after 9 minutes.

How do you guys achieve this? Is 15 minutes too short for a meaningful test? Should I push the next set up to ISO1600 or even 3200 if I'm going to shoot most at 400 or 800?

Cheers

Chris

All images have noise reduction set to "off". "Mode 3" has been used (camera turned off prior to median NR taking place automatically) such that the frame is flushed & written to storage before the NR algorithms can remove noisy pixels. More or less a true RAW in Nikon parlance.

A higher resolution version can be found here: http://www.omaroo.net/index.php?set_albumName=album47&option=com_gallery&Itemid=40&include=view_album.php

Here are direct links to the full-sized images:

http://www.omaroo.net/albums/album47/1_min.jpg

http://www.omaroo.net/albums/album47/2_min.jpg

http://www.omaroo.net/albums/album47/3_min.jpg

http://www.omaroo.net/albums/album47/4_min.jpg

http://www.omaroo.net/albums/album47/5_min.jpg

http://www.omaroo.net/albums/album47/6_min.jpg

http://www.omaroo.net/albums/album47/7_min.jpg

http://www.omaroo.net/albums/album47/8_min.jpg

http://www.omaroo.net/albums/album47/9_min.jpg

http://www.omaroo.net/albums/album47/10_min.jpg

http://www.omaroo.net/albums/album47/11_min.jpg

http://www.omaroo.net/albums/album47/12_min.jpg

http://www.omaroo.net/albums/album47/13_min.jpg

http://www.omaroo.net/albums/album47/14_min.jpg

http://www.omaroo.net/albums/album47/15_min.jpg

http://www.omaroo.net/albums/album47/20_min.jpg

http://www.omaroo.net/albums/album47/25_min.jpg

http://www.omaroo.net/albums/album47/30_min.jpg

Dark frames - 1-30 minutes:

1 minute

2 minutes

3 minutes

4 minutes

5 minutes

6 minutes

7 minutes

8 minutes

9 minutes

10 minutes

11 minutes

12 minutes

13 minutes

14 minutes

15 minute

20 minutes

25 minutes

30 minutes

[blinky]

The amp glow there looks like nothing that cannot be removed with a dark frame.

[centroid]

Chris

I think you'll find that simply 'subtracting' a 'Dark Frame' from your image, taken at the same temperature and duration as your image, will remove the 'amp glow'.

Dave

[centroid]

Ooops!!, me an Craig 'posted' the answer at the same time

Dave

[omaroo]

Thanks fellas

Now - given that I use Photoshop CS3, how do I "subtract" a dark? As a difference layer?

Cheers

Chris

[centroid]

Good questio Chris , something I've not tried, and am not even sure that Photoshop can do this.

However, we do have one or two Photoshop, experts on the forum, and if they read this 'thread', will be able to let us all know how, or if it can be done.

Dave

[cygnusx1]

Chris

I think you'll find that simply 'subtracting' a 'Dark Frame' from your image, taken at the same temperature and duration as your image, will remove the 'amp glow'.

Dave

Just as an aside, does the duration of the dark HAVE to be same as the light frames?

What happens if your exposures are at differing durations like i do mine?

[Kaptain Klevtsov]

The duration of the dark frame has to be the same as the light frame so that the amp glow effect matches. You can do a standard set of darks and use the appropriate exposure length one for your image, unless the temperatures vary a lot.

I don't know how to subtract a dark frame in Photoshop, but someone will.

Kaptain Klevtsov

[Trevor]

Could try this i guess.

http://www.takegreatpictures.com/HOME/Columns/Digital_Photography/Details/Dark_Frame_Subtraction_using_Adobe_Photoshopby_Chris_Limone.fci

[Milamber]

... call me stupid, but that looks like a light leak rather than amp glow. Amp glow tends to be in one place as the chip warms, not two as shown here?

Arthur

[centroid]

... call me stupid, but that looks like a light leak rather than amp glow. Amp glow tends to be in one place as the chip warms, not two as shown here?

Arthur

Good thinking, it could be as you suggest.

Dave

[omaroo]

... call me stupid, but that looks like a light leak rather than amp glow. Amp glow tends to be in one place as the chip warms, not two as shown here?

Arthur

Not as far as i understand. Amp glow is an electroluminescent effect caused by the IR noise emitted from the readout amplifiers situated at the corners of the chip, and is not caused by heat. Pixel noise or "hot pixels" are caused by heat.

[Milamber]

But it *is* a graduated thing whereas yours is in two maybe three distinct areas in one place.

Arthur

[omaroo]

Hi Arthur

I suspect that it's a graduated effect as a function of time rather than heat. It couldn't be light leakage because I had the body cap on snugly and the camera was placed in a dark cupboard while the exposures were running.

Cheers

Chris

[Astroman]

Amp glow is a general term and is explained well by Omaroo. The internal arrangement of the electronics will cause the 2 sources, regardless of exactly the component causing it. Dark frame subtraction will work. Not sure if Photoshop does this.

[omaroo]

Just found this: http://www.takegreatpictures.com/HOME/Columns/Digital_Photography/Details/Dark_Frame_Subtraction_using_Adobe_Photoshopby_Chris_Limone.fci

Open your dark-frame image and the original image in Adobe Photoshop. With the dark-frame image window active, select all (Ctrl-A) and copy (Ctrl-C). Then go to the original image window and paste it in (Ctrl-V). This should place it directly over the original image onto a separate layer so both are aligned correctly.

subtraction1

Select the dark frame layer and change the blending mode to “Difference”. This will get rid of the hot-pixels by inverting the image only where the dark frame is not black. You can change the opacity of the dark frame layer to find a better balance if the difference is too much. Usually 50% opacity works well.

Seems pretty simple!

Cheers

Chris

[Milamber]

Well, if you say so. What do *I* know about ccd's eh?

Arthur

[Kaptain Klevtsov]

I'll bet one glow is the amp. and the other glow is something else thats like an amp.

Kaptain klevtsov

[Astroman]

I'll bet one glow is the amp. and the other glow is something else thats like an amp.

Kaptain klevtsov

That's what I said!

[Kaptain Klevtsov]

You did, but I didn't read it because I was having a blonde moment. I have black hair that is going blonde, so I blame the blonde hairs that I have so far.

Sorry AM, didn't see your post.

Kaptain klevtsov

[Astroman]

NP. I thought you were making fun of my overblown language. Wouldn't be the first time... 8)

[Milamber]

Well, if it's a function of time then the frames are in the wrong order...

Arthur

[omaroo]

Arthur - I think I see where you're trying to go. The order looks "out" because some frames exhibit less glow than the previous.

What I probably really need to do was to take a frame and then turn OFF the camera to let it cool down COMPLETELY between shots. I've realised that what I've done is take some of the shorter frames here, waited a minute or two while I reset my countdown timer and then exposed the next one. The longer frames (up beyond 10-11 minutes) I did make sure to turn off the camera between shots to cool down to a constant.

Hmm.. re-do time?

[omaroo]

20, 25 and 30 minutes (added above as well):

Full-sized versions here -

http://www.omaroo.net/albums/album47/20_min.jpg

http://www.omaroo.net/albums/album47/25_min.jpg

http://www.omaroo.net/albums/album47/30_min.jpg

Really starting to get some glow going now, although nowhere near as much as I was expecting still. Starting to also get quite a number of hot pixels. Each of these frames were taken and then the camera turned off for 10 minutes to cool - the idea being that there would be a stable reference point to start from each time.

[Kaptain Klevtsov]

Any of these would allow a dark frame subtraction, so I don't think you need worry. BTW Mode 3 should have NR turned on.

Kaptain Klevtsov