laidman
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Reged: 04/17/12
Loc: Jeju City, Jeju Island, South ...
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Making a dark frame library
#5557182 - 12/06/12 10:00 AM
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Hi folks,
I've recently read Jerry Lodriguss's excellent
A Beginner's Guide to DSLR Astrophotography, and have a few questions.
I'd like to start building a dark frame library like is mentioned on the CD, and this seems like a pretty darn good idea given how you can shoot a bunch of frames on cloudy nights when there's nothing else to do.
Shooting the bias frames takes a matter of seconds, so those aren't a big deal at all, even when you're cold and cranky in the early hours...
I know that aperture, ISO, and temperature are all important (and I hope to be able to use the bias frames to scale the darks, that's why I didn't list exposure time).
I was playing with my Astrotrac the other night and could manage 5-5.5 minute exposures at 200mm on a crop Canon body with nice round stars.
Assuming that'll be my longest exposure time with any lens, can I just go ahead and start shooitng darks when I have the time at night, with the appropraite ISO, and keeping track of the temperature?
How important is focal length? And focus? DEpeiding on the seeing and temperature it will change slightly...is that minute difference at all important for the darks? If my longest exposure with any lens will be five minutes, do I need to shoot variable ISOs and focal lengths at roughly 5 degree (Celsius) intervals as Jerry recommends? Or can I just base my darks on temperature and ISO, using bias frames shot with the lights to scale the darks to the correct exposure length?
Thanks everyone,
Mike
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Mike Wiles
scholastic sledgehammer
Reged: 02/04/09
Loc: Goodyear, AZ
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Re: Making a dark frame library
[Re: laidman]
#5557225 - 12/06/12 10:26 AM
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Because it's a dark frame, focal length and focus do not matter. It's time and temperature that you're most interested in with dark frames. With a non-cooled DSLR like you have...it's important to match darks at the temperature of the sensor itself - which can be significantly different than the ambient temperature outside. If you have bias frames shot at the same ISO, you should be good going on temperature/ISO only and then scaling the dark frames. That's essentially what I've done with my CCD.
Mike
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jgraham
Postmaster
Reged: 12/02/04
Loc: Dayton, Ohio
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Re: Making a dark frame library
[Re: Mike Wiles]
#5557299 - 12/06/12 11:16 AM
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Yep, the important factors are time, temperature, and ISO. As an added precaution I'd mount the camera in approximately the same way as it is on the scope so the heat losses to the air closely match what you'd get on the scope. If you do this in daylight also make sure that the camera is well shadowed. Back when I maintained a library of darks I'd make good use of a cloudy night by setting my gear out on my patio and grab a set of darks.
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SMigol
super member
Reged: 07/30/10
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Re: Making a dark frame library
[Re: jgraham]
#5557332 - 12/06/12 11:36 AM
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Doing a few of these during the daytime is a great way to check for light leaks. You can stretch a dark frame or two to see if you're getting light from around seals or the viewfinder.
I've also put my camera into the refrigerator with varying layers of ice packs and cloth padding to manufacture a variety of temperatures. I've also let it run for days in a dark shed so that it gets the full range of day and night temperatures.
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Barry E.
super member
Reged: 12/27/08
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Re: Making a dark frame library
[Re: laidman]
#5557626 - 12/06/12 02:13 PM
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If your DSLR camera includes sensor temp in the exif data, you could use the (free) DarkMaster program that I wrote that helps match up your lights and darks based on temperature, ISO and exposure time.
DarkMaster
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laidman
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Reged: 04/17/12
Loc: Jeju City, Jeju Island, South ...
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Re: Making a dark frame library
[Re: Barry E.]
#5558243 - 12/06/12 08:44 PM
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Thanks for the replies, everyone!
So I can just stick on my most used lens, aim it in such a way as to approximate the kind of heat loss I'd get while imaging, choose an ISO I'll use, and then let it run for 5 minute exposures (since that's what I figure my longest exposure will be)?
I assume DarkMaster or DarkLibrary then scane the dark frame library and picks those frames which most closely match the lights.
I downloaded ExifTools, which showed the tempratures of all my RAW files (I'm using a 40D), and I've downloaded DarkMaster as well. Cheers, Barry!
Will DSS automatically scale the darks to the right time once they've been added?
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SMigol
super member
Reged: 07/30/10
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Re: Making a dark frame library
[Re: laidman]
#5558586 - 12/07/12 01:11 AM
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DSS will make an attempt to scale the darks if possible within a few degrees C. It will not know the temperature of the darks vs lights. That's where library management programs like the one mentioned help. They can organize lists of groups to assist in using DSS.
For my use, I have a windows 7 bat file that runs exiftool and writes the ccd temp out to a tag which I can use to sort my lights and darks. Makes it easy to see the progress over the course of a night.
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laidman
member
Reged: 04/17/12
Loc: Jeju City, Jeju Island, South ...
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Re: Making a dark frame library
[Re: SMigol]
#5559872 - 12/07/12 07:44 PM
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Ah, I understand. The library programs will do things behind the scenes, whereas the batch program you describe lets you include the temperature in the file name itself? That would be quite handy.
The weather here has been totally overcast for the past few weeks, and looks to stay that way for a few more, but as soon as I have a chance I'll have to give all of this a try.
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SMigol
super member
Reged: 07/30/10
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Re: Making a dark frame library
[Re: laidman]
#5560402 - 12/08/12 04:19 AM
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The bat file I have just sends a command to exiftool to read the temp in the EXIF data and then writes it to a tag. That tag is then sortable in the windows 7 file system - just like you can sort by duration and ISO setting. The filename could be changed. I prefer the tag as I can sort in the folder view easily.
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laidman
member
Reged: 04/17/12
Loc: Jeju City, Jeju Island, South ...
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Re: Making a dark frame library
[Re: SMigol]
#5561834 - 12/09/12 03:02 AM
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Sounds like a useful thing to have, SMigol. Any chance there's a download link to the file? Or is it something you wrote yourself?
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SMigol
super member
Reged: 07/30/10
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Re: Making a dark frame library
[Re: laidman]
#5561859 - 12/09/12 04:02 AM
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It's something I banged together once looking at the pages on EXIFTOOL.
Here's what it looks like:
Code:
@echo off
exiftool -r -P "-cameratemperature+>keywords" -ext .JPG -ext .DNG .
I put a copy of EXIFTOOL.exe in the same folder and double click the bat file. As written, it will scan recursively through sub directories, which is nice to index a whole collection of shots. Be aware that it will keep a backup of the original file so there's some need of extra storage space.
Note that your camera maker/model would have a tag for "cameratemperature" that may not be the same as mine - or may not even be used. You may wish to check what the tags are by using PhotoMe - another piece of EXIF reading software.
Hope this helps!
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laidman
member
Reged: 04/17/12
Loc: Jeju City, Jeju Island, South ...
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Re: Making a dark frame library
[Re: SMigol]
#5563098 - 12/09/12 08:37 PM
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Hmmm, I'm reading a bit about bat files now. I'll play around with it a little more and see if I can't figure something out. I've only used the ExifTool GUI program so far to get at the temp. data, but maybe it's easier to have the bat file go to the original EifTool program from the get-go. I really have no experience with this stuff!
Mike
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