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Old 22-10-2007, 06:23 PM
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Omaroo (Chris Malikoff)
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Amp glow - any way to judge how long is too long?

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. What's an acceptable limit - knowing that much of the glow is able to be processed out. There doesn't seem to be much anyway.

How do you guys achieve this test? 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_...view_album.php

It seems that this forum alters the images on upload for some reason - and the glow doesn't appear as prevalent as it should - not that it's really bad. Here are direct links to the full 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

Dark frames - 1-8 minutes
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Last edited by Omaroo; 22-10-2007 at 07:58 PM.
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Old 22-10-2007, 06:33 PM
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Omaroo (Chris Malikoff)
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Dark frames - 9-15 minutes:
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Old 22-10-2007, 07:59 PM
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h0ughy (David)
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just remember, take the same length of time with the darks as your exposures, and when processing remember to use the darks and bias's and do some flat field shots too. you should be home and hosed with that. i did notice the amp glow at the top left
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Old 22-10-2007, 08:01 PM
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Omaroo (Chris Malikoff)
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Quote:
Originally Posted by h0ughy View Post
just remember, take the same length of time with the darks as your exposures, and when processing remember to use the darks and bias's and do some flat field shots too. you should be home and hosed with that. i did notice the amp glow at the top left
Thanks David

This is all new to me!

"Darks" I understand...."Biases"?

Cheers
Chris
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Old 22-10-2007, 08:08 PM
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h0ughy (David)
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like a full dark only at 1/4000 of a second
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Old 22-10-2007, 08:10 PM
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Omaroo (Chris Malikoff)
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Quote:
Originally Posted by h0ughy View Post
like a full dark only at 1/4000 of a second
Ok... so where are they used?

TIA!
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Old 22-10-2007, 08:56 PM
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ballaratdragons (Ken)
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15 minutes!!!!!

I can't get passed 80 seconds any more without Amp Glow almost destroying an image
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Old 23-10-2007, 09:34 PM
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rogerco (Roger)
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15 minutes, with my Nikon D70s about 20% of the frame starting in the top corner would be wiped out.

You seem to have a very quite camera. You need a program like Deepskystacker to process the darks along with the actual picture sub frames and flats.

Just had a look at your website, very nice, you have some fine photos there, I see you have Registack so you can use that to process your photos with dark frames.

But you seem to be doing fine. Wish I had some shot like those.

Roger
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Old 23-10-2007, 10:31 PM
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D40 is well know for low amp glow.
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Old 24-10-2007, 06:13 AM
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Omaroo (Chris Malikoff)
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Thanks everyone for your input. It looks like the general consensus is that the glow is pretty minimal and can be easily removed by subtracting appropriately-shot darks. I'm going to go up in 5-minute increments now (still at ISO400) to 20, 25 and 30 minutes. Then I'll do the whole lot over again at ISO 800 and 1600. Phew!

I have used (to a varying degree) Registax and DeepSkyStacker, and to a far greater extent Photoshop CS3. Now that Houghy's explanations have prompted me to do a bit of research, I'll try the difference layer technique in Photoshop first. It is by far the most powerful image editing software available to mortals and should do a great job.

I'm having another deep sky session with the D40 over the weekend of the 9-10th November back at Magellan.

Cheers
Chris

Last edited by Omaroo; 24-10-2007 at 06:35 AM.
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Old 24-10-2007, 09:56 AM
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20, 25 and 30 minutes:

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.
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